JP2668872B2 - Breakout prediction method in continuous casting. - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for predicting a breakout occurring during casting by using a temperature change of a continuous casting mold. [Prior art] When a breakout (B0) occurs in a continuous casting facility and unsolidified molten steel leaks from the inside of the slab, the casting is stopped to discharge the slab that caused the breakout and the roll to which the molten steel adhered. It is necessary to replace equipment such as the above, and the operation must be stopped for a considerable period of time. For this reason,
Breakout is the largest operation trouble in continuous casting, and establishment of preventive measures has been desired. By the way, the solidified shell of the cast slab that has been drawn adheres to the vertically vibrating mold and breaks, and the molten steel leaks out of it and exits from the lower end of the mold before it is sufficiently cooled. When it occurs, it is known that the temperature of the mold gradually increases before passing through the fractured portion and gradually falls after passing through the fractured portion in the mold portion where the fractured portion of the solidified shell passes. For this reason, a temperature measuring element such as a thermocouple is embedded in a copper plate of a mold, and the temperature of the mold copper plate (hereinafter, this is referred to as a mold temperature) is measured therewith, and a rate of change of the measured mold temperature per unit time is obtained. The magnitude of the value and the reference value are monitored (JP-A-57-115962), or the difference between the measured mold temperature and the moving average value of the previous mold temperature is determined, and the difference between the value and the reference value is determined. By monitoring the size (Japanese Patent Application Laid-Open No. 57-11595)
9) It is possible to predict a breakout. [Problems to be Solved by the Invention] By the way, at the time of high-speed casting with a high drawing speed, the mold temperature at the time of stable casting and at the time of occurrence of an abnormality, that is, at the time of occurrence of restraint breakout, becomes high. Both the mold temperature at the time of occurrence and at the time of occurrence of an abnormality become low. For this reason, when a breakout is predicted by the above monitoring method, if a low threshold suitable for prediction at a low speed casting is used at a high speed casting, the breakage can be performed even when the fracture of the solidified shell does not actually occur. The frequency of erroneously predicting out is increased, and if a high threshold suitable for prediction during high speed casting is used during low speed casting, even if a solidified shell fracture occurs, it cannot be detected and sometimes overlooked. In addition, when a breakout is predicted, the drawing is generally stopped or the drawing speed is slowed considerably, so that the operation stability is poor and the quality of the cast product is deteriorated. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method capable of predicting a restrictive breakout with high accuracy even when the drawing speed is different. [Means for Solving the Problems] The method for predicting breakout in continuous casting according to the first invention is to measure a mold temperature in time series at one or more positions of a continuous casting mold that vibrates up and down, While calculating the mold temperature change rate per unit time in the vicinity of each measurement time and the standard deviation and average temperature of the mold temperature in a predetermined period before the measurement time for each position, the drawing speed of the continuous cast slab or First, second, and third thresholds are determined based on this and the vibration period of the mold, and the difference between the mold temperature at each measurement point and the calculated average temperature is determined. Breakout by performing a magnitude comparison between the product of the threshold value and the standard deviation, a magnitude comparison between the mold temperature change rate and a second threshold, and a magnitude comparison between the mold temperature difference and a third threshold. It is characterized by predicting. In the method for predicting breakout in continuous casting according to the second invention, the mold temperature difference is larger than a product of a first threshold value and a standard deviation, and the mold temperature change rate is larger than a second threshold value. It is characterized in that a breakout is determined when at least one of the following is satisfied, or whether the mold temperature difference is greater than the third threshold value. The method for predicting breakout in continuous casting according to the third invention is characterized in that at two or more vertical positions of the mold, the mold temperature difference is larger than a product of a first threshold value and a standard deviation, respectively. A breakout is determined when at least one of a change rate is larger than a second threshold value and the mold temperature difference is larger than a third threshold value is satisfied. . EXAMPLES Hereinafter, the present invention will be specifically described with reference to the drawings. FIG. 1 shows that the drawing speed (Vc) and the oscillation cycle (c) are predetermined functions. FIG. 1 is a schematic view showing an embodiment in which the present invention is applied to continuous casting satisfying the following condition. A molten metal 1 such as molten steel contained in a tundish (not shown) is passed through a dipping nozzle 2 attached thereunder at a constant period. Into the mold 3 which is oscillated up and down. The molten metal 1 in the mold 3 is primarily cooled through a powder film formed by pouring the lubricating powder 6 along the inner wall of the mold 3 to form a solidified shell 5, which is used as a peripheral wall. The piece 4 is pulled downward by a pinch roll (not shown). The pulling speed Vc is detected by the pulse generator 9 and provided to the threshold value setting device 10. Below the molten metal level of the mold 3, there are three temperature measuring elements 11, 1 such as thermocouples along the drawing direction (arrow direction) of the slab 4.
The mold temperature T measured by each of the temperature measuring elements 11, 12 and 13 is converted from analog to digital by the A / D converter 14 and differentiated by the differentiating circuits 20 and 30, respectively. 40, subtractor 15,2
5,35, average temperature calculation circuit 16,26,36 and standard deviation calculation circuit
17,27,37. Average temperature calculation circuits 16, 26, 36 and standard deviation calculation circuits 17, 2
7 and 37 fetch the input signal from the A / D converter 14 at a predetermined pitch (Δt) of, for example, 0.5 to 1 second. Regarding this acquisition signal, in order to eliminate the influence of noise due to an electromagnetic stirring device provided around the mold 3, for example, a few tens
An average value of a plurality of signals output at a millisecond pitch is used. Then, the average temperature calculating circuits 16, 26, 36 and the standard deviation calculating circuits 17, 27, 37 store and update m input signals including the latest input signal and the previous m input signals. , 36
Calculates the average temperature of n signals from the stored signal with the lowest storage order (in other words, the oldest one), and calculates the average temperature of the standard deviation calculation circuits 17, 27, 37 and the subtractor 15. ,twenty five,
Give to 35. Subtractors 15, 25 and 35 find the difference (T-) between the input mold temperature T and the average temperature, and apply this to comparators 19, 29 and 39. The well-known formula (1) is set in the differentiating circuits 20, 30, and 40 in order to obtain the mold temperature change rate dT / dt per unit time by numerical differentiation. This equation (1) is obtained by using the mold temperature (T ₀ ) at the current measurement time out of the mold temperatures taken at the pitch Δt and 1,3,4
Using the four mold temperatures (T ₁ , T ₃ , T ₄ ) taken before
And calculates a mold temperature change rate at time T _2. Note that T ₀ , ..., T ₄ in the above formula (1) uses a mean value in each period by setting a plurality of periods in which a plurality of measurement values are obtained without using the measurement value itself for each acquisition pitch. May be.
Further, the mold temperature change rate dT / dt is not limited to the above equation (1), and another equation for calculating a differential coefficient may be used. The differentiating circuits 20, 30, 40 obtain the mold temperature change rate dT / dt from the input signal and the equation (1), and apply this to the comparators 19, 29, 39. The standard deviation calculation circuits 17, 27, 37 obtain the standard deviation σ of n signals as in the previous case, and apply this to the integrators 18, 28, 38. The threshold value setting device 10 is connected to Vc and the following (2), (3),
Based on equation (4), the first, second, and third thresholds K ₁ (constant),
Calculate K ₂ [℃ / sec] and K ₃ [℃], and use K ₁ as the integrator
, And K ₂ and K ₃ to comparators 19, 29 and 39. However, A ₁ : 0.1 to 1 [(min / m) ^1/2 ] B ₁ : 3 to 15 A ₂ : 0.1 to 1 [° C / sec. (Min / m) ^1/2 ] B ₂ : 0.5 to 3 [° C / sec] A ₃ : 0.1 to 1 [° C (min / m) ^1/2 ] B ₃ : _{3 to} 15 [° C] The accumulators 18, 28, 38 are the product K of K ₁ and the standard deviation σ. ₁ · σ is calculated and output to comparators 19, 29, and 39. The following equations (5), (6), and (7) are set in the comparators 19, 29, 39, and the comparators 19, 29, 39 receive the three types of input signals at each acquisition pitch ( 5), (6), (7) is determined separately whether or not satisfied, for example (2-10) seconds 1B
An alarm device if all of the equations (5), (6), and (7) are satisfied at different timings during the O determination period, or if any one of the equations is satisfied. At 41, an alarm is issued, and an abnormality occurrence signal is output to a control device (not shown). The BO determination period is provided so as to move at the pitch for each capture pitch. (T−) ≧ K ₁ · σ (5) dT / dt ≧ K ₂ (6) (T−) ≧ K ₃ (7) Note that for K ₁ , K ₂ , and K ₃ , The vertical length (l) of the break opening differs depending on the oscillation mark pitch (P). That is, if P is large, l is large,
Conversely, if P is small, l becomes small, and the opening area of the solidified shell fracture is determined based on such l, and since the mold temperature is further determined based on this area, P (= Vc / c) is determined. A function based on the specified pulling speed Vc and the oscillation cycle c is used. However, as in this embodiment In the case of continuous casting with the relationship Therefore, the above equations (2), (3), and (4) are functions of Vc only, which does not include c as a term. When the threshold value is expressed using the drawing speed V _C and the oscillation cycle c of the mold, the following (2) ′,
(3) ', (4)'. Of course, the threshold value is K = AP +
It is represented by the pattern of B, and since P is P = V _C / c as is clear from the above equation, the threshold values K ₁ , K ₂ and K ₃ are the drawing speeds.
It goes without saying that it can be expressed as in the following expressions (2) ', (3)', and (4) 'using V _C and the oscillation cycle c. K ₁ = A ₁ × V _C / c + B ₁ (2) ′ K ₂ = A ₂ × V _C / c + B ₂ (3) ′ K ₃ = A ₃ × V _C / c + B ₃ (4) ′ A ₁ , A ₂ , A ₃ , B ₁ , B ₂ , B ₃ are (2), (3), (4)
The same as in the formula. here In the case of, the expressions (2) ', (3)', and (4) 'are the above-mentioned expressions (2), (3), and (4), but for a constant c, A ₁ : [minute / m] B ₁ : Constant A ₂ : [° C / sec.min / m] B ₂ : [° C / sec] A ₃ : [° C / min / m] B ₃ [° C] In addition, A ₁ , A ₂ , A ₃ , B ₁ , B ₂ , B of the expressions (2), (3), and (4) are
_{As for 3,} different values may be used depending on the embedding position and depth of the temperature measuring element. When the control device (not shown) inputs the abnormality occurrence signal, the sliding nozzle unit 7 provided in the middle of the immersion nozzle 2 is driven by the hydraulic cylinder 8 to close the immersion nozzle 2 once and to pinch it (not shown). Stop the rotation of the roll. For this, the immersion nozzle 2 may be slightly opened and the drawing speed may be reduced considerably. The method of the present invention using the prediction device configured as described above will be described below. First, the above m and n are determined as follows. When the steel type to be continuously cast is medium carbon steel or low carbon steel, the mold temperature has a period of temperature change as shown in Fig. 2 (time is taken on the horizontal axis and mold temperature is taken on the vertical axis). , Its cycle is about 20
~ 30 seconds. Note that FIG. 2 shows the mold temperatures Ta, Tb, and 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 to be stored every 0.5 seconds. When measuring the portion where the solidified shell is broken,
As shown in the figure, the time required for the mold temperature to reach the peak value from the original temperature immediately before the rise is 5 to 15 seconds. For this reason, m
It is better not to include the temperature change period corresponding to this 5 to 15 seconds in the period necessary for prediction,
The number of highly predictable pitches of the signals continuously measured in 35 to 45 seconds including 30 seconds, for example, 70 to 90, is stored for every 0.5 seconds. The values of K ₁ , K ₂ , and K ₃ are respectively described in the above (2),
(3) and (4) are calculated based on the formulas, but when the drawing speed and the oscillation stroke are set to predetermined values according to the slab size, such as when continuously casting round slabs. Alternatively, the critical temperature change amount and change rate at which solidified shell fracture occurs may be set according to the drawing speed based on the mold size. In this case, a host computer is provided, and the mold size or the threshold value determined based on the mold size is supplied to the threshold value setting device 10 as shown by a broken arrow in FIG. When the mold size is given, a table is set in the threshold value setting device 10 and the threshold value is read based on this table. When such preparation is completed, continuous casting is started, and when drawing is started thereafter, the prediction device is operated. Temperature measuring element 11,1
When the mold temperature T at each position is measured at 2 and 13, the average temperature calculation circuits 16, 26 and 36 and the standard deviation calculation circuits 17, 27 and 37 store the mold temperature T signals, and the number of stored signals is No calculation is performed until the number becomes m, and no output is made. Then, when the m-th signal is stored, the average temperature and the standard deviation σ of the n-th signal are calculated and output from the n-th signal having the smaller storage order. Subtractors 15, 25 and 35 find the difference (T-) between the m-th inputted mold temperature T and the average temperature. Integrators 18, 28, 38
Calculates the product (K ₁ · σ) of the constant K ₁ and the standard deviation σ. When the signals relating to the mold temperature from the A / D converter 14 are input to the differentiating circuits 20, 30, 40, the time change rate is calculated based on the equation (1).
Calculate dT / dt and give it to comparators 19, 29 and 39. The comparators 19, 29 and 39 have five types of input signals, that is, T−, K ₁
It is determined for each equation whether σ, dT / dt, K ₂ , K ₃ satisfy the above equations (5), (6), (7). Next, the (m + 1) th and subsequent signals are output to the average temperature calculating circuit 16.
And the like, and stored in the standard deviation calculation circuit 17 or the like, the same is repeated as before. While performing the signal processing in this way, the comparator 19
In any one of the above, during a certain BO judgment period, (5),
(6), (7) If it is determined that all of the points satisfying the respective expressions are different even if the timings are different, or if it is determined that any one exists, the corresponding comparator predicts a breakout. The alarm device 41 issues an alarm and outputs an abnormality occurrence signal to a control device (not shown). As described above, the control device controls the sliding nozzle unit 7 and the pitch roll (not shown) to temporarily stop the loading and the withdrawal. Thereby, even if the solidified shell breaks and unsolidified molten steel leaks from the broken portion, breakout can be prevented beforehand. In the above embodiment, one of the breakout prediction determinations is performed.
However, the present invention is not limited to this, and it goes without saying that the following equation (8) may be used. (T −) / σ ≧ K ₁ (8) Although not described in detail, the threshold value setting device 10
The K ₄ below (9) defined by an alarm of breakout in time following the equation (10) satisfies showing the comparison between the measured mold temperature T in comparator 19,29,39 from the alarm 41 in Is also good. A ₄ : constant B ₄ : constant T ≧ K ₄ (10) Note that K ₄ [° C.] is given in the same manner as K ₁ , K ₂ and K ₃ described above. Table 1 shows the number of predictive median times, the number of false alarms and the number of missed breakouts when the present invention is applied to the casting of round slabs with various slab sizes, and the slab size (R ₁
<R ₂ <R ₃ <R ₄ <R ₅ , R ₅ 1.5R ₁ ), reference drawing speed (V _c1 > V
_c2 > V _c3 > V _c4 > V _c5 , V _c1 2V _c5 ) together. For comparison, the results in the case of the conventional method I in which the threshold value is set as an appropriate constant high threshold value at the time of high-speed casting at the same ingot size and the standard drawing speed, and the breakout is predicted. Tables 2 and 3 show the results of the conventional method II for predicting breakout by setting the threshold value as an appropriate constant pulling threshold value during low speed casting. As can be understood from these tables, in the case of the conventional method I (Table 2), the predictive predictive value [= predictive predictive count / (predictive predictive count + false alarm count)] is 38/43 = 90.5 ( %), Missed rate [= number of missed times / (number of times of intellectual intelligence + number of missed times)]
However, 5/43 = 11.6%, and the oversight rate was high. In the case of the conventional method II (Table 3), the predictive predictive value is 50.8%.
%, The oversight rate was 0%, and the hit rate was poor. On the other hand, in the case of the present invention (Table 1), the predictive predictive value is 8
The miss rate was 7.1% and the miss rate was 0%, and it was possible to obtain a high hit rate without overlooking the occurrence of fracture of the solidified shell. In the above embodiment, the mold temperature is measured at three positions of the mold different in the drawing direction. However, the present invention is not limited to this.
Needless to say, the breakout can be predicted by measuring the mold temperature at any position of 1 or 2 or 4 regardless of the drawing direction and the direction orthogonal thereto. However,
It is preferable that the position where the mold temperature is measured in the drawing direction is set to a position where the operating conditions are changed by detecting solidified shell breakage, thereby allowing breakout to be prevented beforehand. Further, in the present invention, when two or more temperature measuring elements are provided in the vertical direction of the mold, the breakout can be more reliably predicted by the following method. When a solidified shell fracture is detected with a time difference at each of a plurality of temperature elements provided in the vertical direction of the mold, the transition time
It is generally known that tB (second) is represented by the following equation (9). However, L: distance between temperature measuring elements separated in the vertical direction e: constant (0.5 to 0.9) Therefore, each of the comparators 19, 29, and 39 that processes signals from each temperature measuring element has a timer function on the output side. Provide a calculator,
A detection signal of solidification shell fracture (a signal that is output under the output condition of the abnormality occurrence signal and is different from the abnormality occurrence signal) is input from the comparator for the upper temperature measuring element, and after about tB seconds, it is immediately below the signal. When a similar detection signal of solidification shell rupture is input from a comparator relating to a temperature measuring element, a breakout is predicted, whereby an alarm is issued and an abnormality occurrence signal is output to the control device. Thereby, a breakout can be predicted more reliably. [Effects] As described in detail above, the present invention measures the mold temperature at one or more positions of the continuous casting mold, and determines the mold temperature at the measurement time and the average mold temperature during a predetermined period before that. Comparison of temperature difference and product of first threshold value and standard deviation, comparison of mold temperature change rate and second threshold value, and comparison of mold temperature difference and third threshold value Therefore, even if the drawing speed changes, the restraint breakout can be reliably predicted without being affected by the change and the number of false alarms can be reduced, thereby improving the reliability. It has an excellent effect such that it can prevent the quality of the slab from being deteriorated by changing the operating condition.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of a cycle of mold temperature change, and FIG. 3 is a standard deviation of the present invention.
FIG. 4 is an explanatory diagram of a period for calculating an average temperature. 3 ... mold, 4 ... slab, 10 ... threshold value setting device 11, 12, 13 ... temperature measuring element, 15, 25, 35 ... subtractor 16, 26, 36 ... average temperature calculation circuit 17,27,37 …… Standard deviation calculation circuit 18,28,38 …… Integrator, 19,29,39 …… Comparator 20,30,40 …… Differentiation circuit

Claims (1)

(57) [Claims] The mold temperature is measured in time series at each of one or more positions of the continuous casting mold that vibrates up and down, and the mold temperature change rate per unit time near each measurement time point and a predetermined time period before the measurement time point. While calculating the standard deviation and the average temperature of the mold temperature for each position, the first, second, and third thresholds are determined based on the drawing speed of the continuous cast slab or the vibration cycle of the mold, and The difference between the mold temperature at the time of measurement and the calculated average temperature is determined, the magnitude of the mold temperature difference is multiplied by the product of the first threshold value and the standard deviation, and the mold temperature change rate and the second threshold value are compared. A method for predicting breakout in continuous casting, wherein a breakout is predicted by comparing the size of the mold temperature difference with a third threshold value. 2. Whether the mold temperature difference is greater than the product of the first threshold value and the standard deviation, the mold temperature change rate is greater than the second threshold value, and the mold temperature difference is the third threshold value. The breakout prediction method in continuous casting according to claim 1, wherein a breakout is determined when at least one of the threshold values is satisfied. 3. At two or more vertical positions of the mold, whether the mold temperature difference is greater than the product of the first threshold value and the standard deviation, and whether the mold temperature change rate is greater than the second threshold value, The breakout prediction method in continuous casting according to claim 1, further comprising the step of judging a breakout when at least one of the mold temperature differences is greater than or equal to a third threshold value.