JPS63203260A - Method for predicting breakout in continuous casting - Google Patents

Abstract

PURPOSE:To improve the predicting accuracy by calculating variation rate of mold temp. per unit time at plural points of a mold and a standard deviation and an average of the former mold temp. and judging based on compared with each setting threshold value. CONSTITUTION:Molten metal 1 is charged into the mold 3 through a submerged nozzle 2 and also tip parts of temp. measuring elements 11-13 of thermocouples, etc., are buried along drawing direction of cast slab 4 below the molten surface level in the mold 3 at these positions. The measured mold temp. T are inputted to each differentiation circuit 20, 30, 40, subtracter 15, 25, 35, average temp. calculating circuit 16, 26, 36 and standard deviation calculating circuit 17, 27, 37 by digitalizing through A/D converter 14, and the difference between the mold temp. T and the former average temp. is found, and the mold temp. variation rate, the difference of the mold temp. and each threshold value are compared by a comparators 19, 29, 39. As the variation of the molten temp. based on the variation of the molten surface, drawing velocity, etc., is considered, the predicting accuracy is improved.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for predicting breakout occurring during casting by utilizing temperature changes in a continuous casting mold. The present invention provides a method that can predict breakout with high accuracy even when the temperature fluctuation is large.

[Prior art]

If a breakout (BO) occurs in continuous casting equipment and unsolidified molten steel inside the slab leaks, stop casting, drain the slab that has caused the breakout, and remove equipment such as rolls to which the molten steel has adhered. It would be necessary to replace it, forcing the suspension of operations for a considerable period of time. For this reason, breakout is the biggest operational trouble in continuous casting, and it has been desired to establish measures to prevent it.

By the way, if the solidified shell of the slab that is being drawn sticks to the mold and breaks, and molten steel leaks out from there and comes out from the lower end of the mold before it is sufficiently cooled, a breakout occurs. As shown in FIG. 7, it is known that in the part of the mold through which the broken part of the solidified shell passes, the temperature of the mold gradually increases before passing the broken part, and gradually decreases after passing the broken part.

For this reason, temperature measuring elements such as thermocouples are embedded in the copper plate of the mold to measure the temperature of the copper plate of the mold (hereinafter referred to as mold temperature).
(JP-A-57-115962), or determine the rate of change of the measured mold temperature per unit time and monitor the magnitude of that value and the reference value (JP 57-115962), or compare the measured mold temperature with the previous mold temperature. By determining the difference between the moving average value of
115959), it is possible to predict a breakout.

[Problem that the invention seeks to solve]

However, the mold temperature is not always stable during continuous casting, and there are fluctuations in the scene inside the mold, large and small drawing speeds, uneven inflow of lubricating powder into the mold, and contact between the mold and slab. Variations occur due to factors such as the size of the area.

In particular, when continuously casting medium carbon steel or bottom carbon steel, the mold temperature per unit time (1) (T
) Rate of change (hereinafter simply referred to as mold temperature change rate) d
When T/dt is monitored, the mold temperature change rate caused by the above causes should be equal to or greater than the breakout prediction threshold of, for example, 4.5°C/sec (see Figure 7). There is. Also, when monitoring the difference (T-T) between the mold temperature (T) and the moving average value (〒), it may be equal to or greater than the threshold value of 27°C (see Figure 7). .

For this reason, when using the conventional method, breakout is often predicted even when no breakage of the solidified shell has actually occurred, resulting in a lack of reliability. Furthermore, if a breakout is predicted, the drawing process is generally stopped or the drawing speed is considerably slowed down, resulting in poor operational stability and deterioration in slab quality.

The present invention has been made in view of the above circumstances, and it corrects the breakout prediction threshold according to the amount of change in mold temperature around the time when it is measured, and uses the measured mold temperature and the previous average. The size of the mold temperature difference with the mold temperature and the corrected first threshold value, the size of the rate of change per unit time of the measured mold temperature with the predetermined second threshold value, the mold temperature difference and the predetermined value. By monitoring the magnitude of the third threshold and the time rate of change of the mold temperature difference between the two measurements and the predetermined fourth threshold, it is possible to detect cases where the mold temperature during casting is not stable. The purpose of this study is to provide a method that can predict breakouts with high accuracy.

Means for Solving Problem C] The breakout prediction method in continuous casting according to the present invention measures the mold temperature at each of a plurality of positions of the continuous casting mold, and measures the temperature of the mold per unit time in the vicinity of the measurement point. Calculate the rate of temperature change, the standard deviation and average temperature of the mold temperature for a predetermined period before the measurement time for each position, find the difference between the mold temperature at the measurement time and the calculated average temperature, and calculate the difference between the mold temperature at the measurement time and the calculated average temperature. A magnitude comparison between the mold temperature difference and a first threshold proportional to the standard deviation, a magnitude comparison between the mold temperature change rate and a predetermined second threshold, and a magnitude comparison between the mold temperature difference and a predetermined third threshold. A breakout is detected by comparing the magnitude with a threshold value and the rate of change per unit time of the mold temperature difference between any two points among the plurality of positions with a predetermined fourth threshold value. Characterized by prediction.

[Effect]

In the method of the present invention, the mold temperature is determined at each of a plurality of positions in a continuous casting mold, and the mold temperature change rate per unit time near the measurement point and the temperature of the mold during a predetermined period before the measurement point are determined. The standard deviation and average temperature of the temperature are calculated for each position, and the temperature difference between the mold temperature at this measurement point and the calculated average temperature is determined.

Then, the magnitude of the mold temperature difference and a first threshold proportional to the standard deviation, the magnitude of the mold temperature change rate and a second predetermined threshold, and the magnitude of the mold temperature difference and a third predetermined threshold. The rate of change per unit time of the difference in mold temperature between multiple measurement points is compared with the fourth predetermined threshold value, and each value exceeds the corresponding threshold value. If so, predict a breakout.

〔Example〕

The present invention will be specifically explained below based on the drawings.

FIG. 1 is a schematic diagram showing the state of implementation of the present invention, in which molten metal 1 such as molten steel stored in a tundish (not shown) passes through a submerged nozzle 2 installed below it, and passes through a mold which is vibrating up and down at a constant period. 3. The molten metal 1 in the mold 3 is primarily cooled through the powder film formed by the lubricating powder 6 flowing along the inner wall of the mold 3 to form a solidified shell 5, which is used as the surrounding wall of the mold. The piece 4 is pulled out downward by a pinch roll (not shown).

Temperature measuring elements 11.1 such as thermocouples are installed at three locations below the hot water level of the mold 3 along the drawing direction (arrow direction) of the slab 4.
The tip of 2.13 is buried. This temperature measuring element 11.
12. Regarding the burial position of 13, in order to reduce the influence of uneven inflow of powder and @ surface fluctuation,
A position below 0 is preferable.

Mold temperature T measured by each temperature measuring element 11, 12, 13
are analog/digital converted by the A/D converter 14, and are then outputted to a differentiating circuit 20, 30, 40, a subtracter 15, 25° 35, an average temperature calculating circuit 16, 26, 36, and a standard deviation calculating circuit 17, 27, respectively. Given to 37. Average temperature calculation circuit 16.26.36 and standard deviation calculation circuit 17.27
．． 37 each takes in the input signal from the A/D converter 14 at a predetermined pinch (Δt) of, for example, 0.5 seconds to 1 second, and stores and updates the previous m human input signals including the latest input signal. The average temperature calculation circuit 16.26.36 calculates the average temperature 〒 of n signals from the lowest storage order among the stored signals, and calculates the average temperature 〒 of the stored signals, and calculates the average temperature 〒 of the stored signals, and calculates the average temperature 〒 of the stored signals, and calculates the average temperature 〒 of the n signals starting from the lowest storage order among the stored signals.
7.37 and subtractor 15.25.35.

The subtractor 15, 25, 35 calculates the difference (T - 〒) between the input mold temperature T and the average temperature 〒, and this is sent to the comparator 19゜29.
, 39.

The following well-known equations (11) are set in the differentiating circuits 20, 30, and 40 to obtain the mold temperature change rate dT/dt per unit time by numerical differentiation.

dT/dt= (-To +87+ -8T
3 + 74) - (1) 12・Δt This T1) formula is the mold temperature at the current measurement point ('ro) and 1
, 3. Mold temperature taken four times previously ('rI+73 *
74) is used to calculate the mold temperature change rate.

In addition, for To,..., T4 in the above +11 formula, instead of using the measured value itself for each capture pinch, it is also possible to set multiple periods in which multiple measured values can be obtained and use the average value for each period. good. In addition, the mold temperature change rate dT/dt is the above (11
In addition to the formula, other formulas for calculating differential coefficients may be used.

The differentiating circuits 20, 30, 40 determine the mold temperature change rate dT/dt from the input signal and the above equation (11) and provide this to the comparators 19, 29, 39.

The standard deviation calculation circuit 17.27.37 calculates the standard deviation σ of the n signals as before, and calculates the standard deviation σ of the n signals and sends it to the integrator 18.28゜3.
Give to 8. A constant of 1 is input to the integrator 18, 28, and 38 from an input setting device (not shown).
The integrator 18.28.38 calculates 1·σ as the product of the constant Kl and the standard deviation σ, and outputs it to the comparator 19.29.39.

In addition, the mold temperature T measured by each temperature measuring element 11, 12, 13 converted from analog to digital by the A/Dfi converter 14.
Among a + T b + T c, Ta + T
b is sent to the subtractor 45, and Tb is sent to the subtracter 45.

Tc is supplied to a subtractor 55, and Ta and Tc are supplied to a subtracter 65. The subtracters 45, 55, 65 take in the two input signals from the A/D converter 14 at a predetermined pitch (Δt) of, for example, 0.5 seconds to 1 second. In order to eliminate the influence of noise caused by an electromagnetic stirring device etc. provided around the mold 3, this input signal is sent from the A/D converter 14, for example, by several tens of seconds.
The average value of multiple signals output at millisecond pitch is used.

Then, the subtractor 45, 55, 65 calculates the difference ΔT between the two mold temperatures (=Tb −Ta, =Tc −Tb
, =Tc - Ta), and stores this, and also divides the latest stored signal and the stored signals for 4 consecutive pitches before it, that is, the stored signals for a total of 5 pinches, into the differentiating circuit 50.
Give to 60.70.

The differentiating circuits 50, 60, and 70 calculate the mold temperature difference ΔT per unit time at the intermediate point of the signal for 5 pitches, that is, when the acquisition pitch is 0.5 seconds, 1 second before the current measurement point. In order to find the rate of change dΔT/dt, the following well-known formula (2) is set.

dΔT/dt-d (TL (2)-TO+2)) /
dt=1/(12・Δt) ・((TL (4
1-TIJ (4))-8(TL (31-T(1(3)
)) +9 (TL (1) - TU (1)) - (TL (0) - TO (01) ) )... (2) However, TL is the temperature on the lower white side of the 2 measurement points TU is 2 measurements Temperature on the white clay side of the point TL (0)-TO(01, TL (ll-TU (1
), ・.

TL (4) -TIJ (4): The difference differential circuit 50, 60, and 70 between TL and TU for each of the 5 consecutive years including the recall from the current measurement time calculates dΔT/dt calculated based on equation (2). Provided to comparator 49.59.69.

It goes without saying that dΔT/dt may be calculated using a general differential coefficient calculation formula instead of the above equation (2).

The comparators 19, 29, and 39 have two different predetermined threshold values. Niko and T31 below. Equations (4) and (5) are set, and the comparators 19, 29, and 39 input three types of signals (31, (4), (51) for each acquisition pitch.
Determine whether each expression is satisfied. On the other hand, comparator 4
9.59.69 at the predetermined threshold, and (6) below.
A formula is set, and the comparators 49, 59, and 69 determine whether the input signal satisfies the formula (6) for each acquisition pitch.

(T-T)≧1 ・σ ... (3) d
T/dt≧2...(4)
(T-T)≧3...(5)d
ΔT/dt>K4... (6) For example, if 5 seconds is the 180 judgment period, (3
), +41. (5) and (6) Even if the points at which each equation is satisfied are different in terms of timing, if all of them exist, the alarm 41 issues an alarm and an abnormality occurrence signal is output to a control device (not shown). The BO determination period is set to move at a pitch for each capture pinch.

However, the constants K1*K2*K3 may each have different values depending on the mold position where the temperature is measured, and the constants may have different values depending on the combination of the two 2!111 fixed points.

When the control device (not shown) receives an abnormality occurrence signal, it drives the sliding nozzle section 7 provided midway through the immersion nozzle 2 with a hydraulic cylinder 8, closes one end of the immersion nozzle 2, and operates a pinch roll (not shown). Stop rotation. In this regard, the immersion nozzle 2 may be left slightly open and the drawing speed may be considerably reduced.

The method of the present invention using the prediction device configured as described above will be explained below.

First, 4 is determined for m, n and Kl*K2+K3 as follows. When the steel type to be continuously cast is medium carbon steel or low carbon steel, the mold temperature has a periodic temperature change as shown in Figure 2 (time is plotted on the horizontal axis and mold temperature is plotted on the vertical axis). , the period is about 20-30 seconds. Note that FIG. 2 shows mold temperatures Ta+Tb+Tc at three different positions in the vertical direction of the mold. For this reason, n is set to a number that can be predicted with high accuracy among the signals measured in 30 seconds, for example, about 60, assuming that the signals are stored every 0.5 seconds.

In addition, when measuring the part where the solidified shell is broken, the time from when the mold temperature reaches the peak value until it returns to the original temperature just before rising is 5 to 15 seconds, as shown in FIG. Therefore, m should be set so that the temperature change period equivalent to 5 to 15 seconds is not included in the period necessary for prediction, and m is 5 to 15 seconds.
The number of pinches that can be predicted with high accuracy among the signals continuously measured for 35 to 45 seconds (5 seconds plus the above 30 seconds) is determined to be 70 to 90, for example, to be stored every 0.5 seconds.

Also, K, 2. 3. The values of and differ depending on mold dimensions, drawing speed, etc., but are determined to be the critical temperature change amount and change rate at which solidification shell rupture occurs, based on the results of the present invention described below.

For example, 1 is 5 to 10, K2 is 2 to b, 5 to lO ℃, K is 1.5 to b. When such preparation is completed, continuous casting is started, and then when drawing starts, the prediction device is activated. .

When the mold temperature T at each position is measured by the temperature measuring element 11.12.13, the average temperature calculation circuit 16.26.36 and the standard deviation calculation circuit 17.27.37 store the mold temperature T signal, No calculation is performed or output until the number of stored signals reaches m, and when the m-th signal is stored,
The average temperature T and standard deviation σ of the signals of n11 minutes from the youngest memory order are calculated and output.

Subtractor 15, 25, 35 is the m-th input mold temperature T
and the average temperature T (T-T).

Also, the product calculator 1B, 28.38 calculates the product (Kl·σ) of the constant 1 and the standard deviation σ.

When the differential circuits 20, 30, and 40 receive a signal related to the mold temperature from the A/D converter 14, they calculate the time rate of change dT/dt based on equation (1), and calculate the time rate of change dT/dt using the comparator 19.
Give to 9, 39.

Comparator 19.29.39 receives the input signal of 311, namely T
−〒+1 ・σ, dT/dt is above (31, (4
), (Determine whether each formula satisfies formula 51 or not.

Next, the m+1th and subsequent signals are sent to the average temperature calculation circuit 16.
etc. and stored in the standard deviation calculation circuit 17, etc., the same procedure as before is repeated.

Subtractors 45, 55, and 65 obtain the mold temperature difference ΔT between the two input measurement points. Further, when the differential circuit 50.60.70 receives a signal related to the temperature difference from the subtractor 45.55.65, it calculates dΔT/dt based on the above equation (2), and provides this to the comparator 49.59.69. .

While performing signal processing in this manner, (31, (4t).

+51. +6) If it is determined that all the points that satisfy each equation are present even if the timing is different, the corresponding comparator predicts a breakout, causes the alarm 41 to issue an alarm, and also alerts the control device (not shown) to an abnormality. Outputs the generated signal.

As described above, the control device controls the sliding nozzle section 7 and the pitch roll (not shown) to temporarily stop charging and withdrawing.

Thereby, even if the solidified shell is broken and unsolidified molten steel leaks from the broken part, breakout can be prevented.

In addition, in this example, the breakout prediction is determined according to the above (
3), but the present invention is not limited to this, but the following (
Of course, formula 7) may also be used.

(T-T)/σ≧, ...(
7) In addition, in this example, the set number of temperature measuring elements is three, so there are three combinations of temperature differences between two measurement points, but the set number of temperature measuring elements is limited to 21H or more. For example, if n temperature measuring elements are provided spaced apart in the drawing direction, there are nC2-%·n (n-1) combinations.

Furthermore, the setting position of the temperature measuring element is not limited to the drawing direction, but the temperature measuring element may be provided at a distance in the width direction or thickness direction of the mold 3. However, it is preferable to measure the temperature of the mold in the drawing direction at a position where there is enough time to detect breakage of the solidified shell and change the operating conditions, thereby preventing breakout.

〔effect〕

Figure 4 shows the inner diameter of a continuous caster for round slabs: 187ta.

Length: 120° in the circumferential direction on a 900mm molded copper plate
200.300.400 from the top of the mold in 3 directions of the pitch
Temperature measuring elements 11, 12, 13 are placed at each position of 5 m from the inner wall surface.
Fig. 2 is a diagram summarizing the results for about 6 minutes when the solidified shell did not break during the present invention when the solidified shell was buried at a depth of fi and the present invention was carried out at a pulling speed of 2.0 m/min, and the prediction accuracy of the present invention is shown. This is what is shown. In the figure, (a) is the drawing speed, (1))
is the mold temperature, (C1 is (T-T) and (T-〒)/σ for Ta, +d+ is the same (dT/ for Ta)
Each transition of dt is shown.

Here, when determining by dT/dt 1. In other words, when using the conventional method, the threshold value of 5°C/sec is
If the judgment is made by (T-T), that is, if the conventional method is used, the threshold value of 1
False alarms were issued when the temperature exceeded 0°C twice. On the other hand, in the case of the present invention, a false alarm is not issued even once when 1 is 5 (℃), and even if the aforementioned uneven inflow of powder occurs, it is not affected by this. It is possible to detect rupture of the solidified shell, that is, predict breakout.

Even when predicting a breakout based on the magnitude relationship in equation (3) above, if the mold temperature changes due to a change in the scene or a change in the drawing speed, as shown in FIG. - ding)/σ are respectively 10.3 and 7.7, and both are Kl (-5)
It gets louder and causes a false alarm. However, even in such a case, if we compare the magnitude relationship in equation (6) above, we get
Does not output alarm.

Therefore, breakout can be reliably predicted without being affected by changes in the mold temperature due to changes in the scene inside the mold, the magnitude of the drawing speed, uneven inflow of powder, changes in the contact area between the mold and the slab, etc. can.

FIG. 6 is a diagram summarizing the mold temperatures Ta, Tb, and Tc together with other operating conditions when breakout is predicted according to the present invention. ) shows the changes in mold temperature Ta, tb, and Tc. In this case, the prediction accuracy was changed from the case in Figure 4, and the threshold value was increased from 1 to 7. In this case as well, there was uneven inflow of powder, etc. No false alarm was issued even when the mold temperature changed, and an alarm was issued only when the solidified shell actually ruptured and the mold temperature changed.

In response to this alarm, the drawing speed was temporarily stopped, and the part where the solidified shell was broken was cooled for a long time in the mold to make the solidified shell thicker, that is, in a state where no breakout occurred, and drawing was started again.

After casting was completed, inspection of the area revealed leakage of molten steel, confirming that breakouts could be predicted with high accuracy.

In addition, when calculating the descending speed of the solidified shell fracture from the thermocouple detection time difference of the peak mold temperature around the time when the breakout alarm was issued and the separation distance between the thermocouples, it was found that the drawing speed was less than 2 m/min. It is slow, 1 mZ minute. Assuming that the fracture moves at this speed, the breakout would have been predicted approximately 42 seconds before it occurred, which means that when continuous casting is performed at a faster drawing speed of 3.5 m/min, Breakout can be predicted about 24 seconds in advance, allowing time to deal with rupture of the solidified shell and reliably preventing breakout.

Further, in the present invention, when two or more temperature measuring elements are provided in the vertical direction of the mold, breakout can be predicted more reliably by the following procedure.

When a solidified shell fracture is detected with a time difference by each of the temperature elements provided in the vertical direction of the mold, the transition time t
It is generally known that B (seconds) is expressed by the following equation (8).

However, L: distance a between temperature measuring elements separated in the vertical direction; constant (0.5 to 0.9) vc: drawing speed (m/min) Therefore, each comparator that processes the signal from each temperature measuring element 19.29.39.49.59.69 A computing unit with a timer function is provided on the output side, and a detection signal of solidified shell rupture (outputted under the output condition of the abnormality signal) is output from the comparator related to the upper temperature measuring element. If a signal different from the abnormality occurrence signal) is input, and then after about tB seconds, a similar detection signal of solidified shell rupture is input from the comparator for the temperature measuring element directly below, a breakout is predicted, and this causes a breakout. It issues an alarm and also outputs an abnormality signal to the control device. This allows breakouts to be predicted more reliably.

As described in detail above, the present invention measures the mold temperature at multiple positions of a continuous casting mold, and compares the mold temperature difference between the mold temperature at the time of measurement and the average mold temperature in the previous predetermined period, and A comparison between the mold temperature difference and a predetermined second threshold value, a comparison between the mold temperature change rate and a predetermined second threshold value, and a comparison between the mold temperature difference and a predetermined third threshold value. and the temporal change rate of the mold temperature difference between two points with a predetermined fourth threshold. Breakouts can be reliably predicted without being affected by changes in mold temperature due to changes in the contact area between the mold and the slab, improving reliability. This has excellent effects such as preventing deterioration in slab quality due to leverage.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the implementation state of the present invention, Fig. 2 is an explanatory diagram of the cycle of mold temperature change, Fig. 3 is the standard deviation of the present invention,
Explanatory diagram of the period for calculating the average temperature, 4th. 5. FIG. 6 is a detailed explanatory diagram of the present invention, and FIGS. 7-8 are explanatory diagrams of problems in the prior art. 3... Mold 4... Slab 11.12.13...
Temperature measuring element 15, 25.35.45.55.65...Subtractor 16.26.36...Average temperature calculation circuit 17
．． 27.37...Standard deviation calculation circuit 18.28.3
8... Integrator 19.29.39.49.59゜69
... Comparator 20.30.40.50.60.70・
... Differential circuit 41 ... Alarm device patent Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent attorney Noboru Kono Jinkai 5 Time distribution Honmoku 212 + 89 3 Zugin FI! (minutes) 6th 71st) Arithmetic 80. 1 0 + 2 3
4 5 gin (min) Q+ 2 34 5 gin t
ill (minutes) 0 + 2 3
4 5 till('d> Fig. 4

Claims (1)

[Claims] 1. Measure the mold temperature at each of multiple positions of the continuous casting mold, and calculate the mold temperature change rate per unit time near the measurement point, and the standard deviation and average of the mold temperature for a predetermined period before the measurement point. The temperature is calculated for each position, the difference between the mold temperature at the time of measurement and the calculated average temperature is determined, and the magnitude of this mold temperature difference is compared with a first threshold proportional to the standard deviation. Comparing the mold temperature change rate with a predetermined second threshold value, comparing the mold temperature difference with a predetermined third threshold value, and comparing the mold temperature between any two points among the plurality of positions. A breakout prediction method in continuous casting, characterized in that a breakout is predicted by comparing the rate of change of temperature difference per unit time with a predetermined fourth threshold.