JPH0771725B2 - Breakout prediction method in continuous casting - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for predicting a breakout that occurs during casting by utilizing the temperature change of a continuous casting mold, and more specifically, the above temperature during casting. It is intended to provide a method capable of predicting breakout with high accuracy even when the change is large.

[Prior art]

If breakout occurs in continuous casting equipment and unsolidified molten steel leaks inside the slab, stop casting and discharge the slab that caused the breakout or replace equipment such as rolls with molten steel It is necessary, and the operation must be stopped for a considerable period of time. For this reason, breakout is the largest of the operational problems in continuous casting, and it has been desired to establish preventive measures against it.

By the way, if the solidified shell of the cast slab being pulled out adheres to the mold and ruptures, and molten steel leaks out from it and exits from the lower end of the mold before it is sufficiently cooled, a breakout occurs, As shown in FIG. 6, it is known that in the mold portion where the fractured portion of the solidified shell passes, the mold temperature gradually rises before passing the fractured portion and gradually drops after passing the fractured portion.

For this reason, a temperature measuring element such as a thermocouple is embedded in the copper plate of the mold and the temperature of the copper plate of the mold (hereinafter referred to as the mold temperature)
Is measured, and the rate of change of the measured mold temperature per unit time is calculated to monitor the magnitude of that value and the reference value (Japanese Patent Laid-Open No. Sho 61-96).
57-115962), or by calculating the difference between the measured mold temperature and the moving average value of the mold temperature before that, and monitoring the difference between that value and the reference value (JP-A-57-115959).
No.), breakout prediction is possible.

[Problems to be solved by the invention]

However, the mold temperature is not always stable during continuous casting, and fluctuations in the molten metal level in the mold, large and small drawing speeds, non-uniform inflow of lubricating powder introduced into the mold, and contact between the mold and the slab. It changes depending on the size of the area.

In particular, when medium carbon steel or low carbon steel is continuously cast, the change appears remarkably, and as shown in FIG. 8, the unit time (t)
When the rate of change of mold temperature (T) per unit (hereinafter, simply referred to as rate of change of mold temperature) dT / dt is monitored, the mold temperature change rate caused by the above causes is, for example, 4.5 ° C / sec. It may be equal to or larger than the threshold value (see FIG. 7). Also, when monitoring the difference (T-) between the mold temperature (T) and the mold temperature moving average value (), a threshold value, for example, 27 ° C (see Fig. 7)
Can be equal to or greater than.

For this reason, in the case of the conventional method, the breakout of the solidified shell is often predicted as a breakout even when the breakage of the solidified shell does not actually occur, resulting in lack of reliability. If a breakout is predicted, the drawing is generally stopped or the drawing speed is considerably slowed, so that the operation stability is poor and the quality of the slab is deteriorated.

The present invention has been made in view of such circumstances, and an object thereof is to provide a method capable of predicting breakout with high accuracy even when the mold temperature during casting is not stable.

[Means for solving problems]

The breakout prediction method in continuous casting according to the present invention measures the mold temperature at each of one or two or more positions of the continuous casting mold, and determines the mold for a predetermined period before the measurement time point and near the measurement time point. The standard deviation of temperature and the average temperature are calculated for each position, the difference between the mold temperature at the time of the measurement and the calculated average temperature is obtained, and the difference between the mold temperature difference and the threshold value proportional to the standard deviation is compared. It is characterized by predicting a breakout in.

That is, the breakout prediction method in continuous casting according to the present invention measures the mold temperature at each of one or two or more positions of the continuous casting mold, and the standard deviation of the mold temperature in a predetermined period before the measurement time and Calculate the average temperature for each position,
A feature is characterized in that the difference between the mold temperature at the time of the measurement and the calculated average temperature is calculated, and the breakout is predicted by comparing the mold temperature difference with a threshold value proportional to the standard deviation.

〔Example〕

The present invention will be specifically described below with reference to the drawings. FIG. 1 is a schematic diagram showing an embodiment of the present invention, in which a molten metal 1 such as molten steel stored in a tundish (not shown) vibrates up and down at a constant cycle through a submerged nozzle 2 attached below the molten metal 1. It is loaded into the mold 3. The molten metal 1 in the mold 3 is
The input powder 6 for lubrication flows along the inner wall of the mold 3 and is primarily cooled through the powder film formed to solidify the solidified shell 5.
The cast slab 4 which forms the wall and is used as a peripheral wall is pulled downward by a pinch roll (not shown).

Below the surface level of mold 3, there is a drawing direction (arrow direction).
The tips of temperature measuring elements 11, 12, 13 such as thermocouples are embedded along three points along the line. The mold temperature T measured by each temperature measuring element 11, 12, 13 is stored in the A / D converter 14. And analog / digital converted to subtractor 15,25,35, average temperature calculation circuit 16,26,36 respectively
And standard deviation calculation circuits 17, 27 and 37.

Average temperature calculation circuit 16, 26, 36 and standard deviation calculation circuit 17, 27,
37 denotes the input signals from the A / D converter 14, for example, 0.5 to 1
Captured at a predetermined pitch of seconds, stores and updates the previous m input signals including the latest input signal, and the average temperature calculation circuits 16, 26, 36 have the smallest storage order among the stored signals. The average temperature of n signals is obtained from the other side, and this is given to the standard deviation calculating circuits 17, 27, 37 and the subtracters 15, 25, 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 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 constant K is input to the integrators 18, 28, 38 from an input setting device (not shown). The integrators 18, 28, 38 calculate the product K · σ of the constant K and the standard deviation σ. Output to the comparators 19, 29, 39.

The following formula (1) is set in the comparators 19, 29, 39, and when the two input signals satisfy the formula (1), the alarm device 40 gives an alarm. At the same time, the abnormality generation signal is output to a control device (not shown).

(T−) ≧ K · σ (1) However, as the constant K, different values may be used depending on the mold position for temperature measurement.

When the control device (not shown) inputs an abnormality occurrence signal, the sliding nozzle portion 7 provided in the middle of the dipping nozzle 2 is driven by the hydraulic cylinder 8 to close the dipping nozzle 2 once and to move the pinch roll (not shown). Stop rotation. 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, n and K are defined as follows. When the type of steel to be continuously cast is medium carbon steel or low carbon steel, the mold temperature has a cycle of temperature change as shown in Fig. 2 (the horizontal axis represents time and the vertical axis represents mold temperature). , Its cycle is about
20 to 30 seconds. Note that FIG. 2 shows the mold temperatures T ₁ , T ₂ , and T ₃ 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 for taking in and storing every 0.5 seconds.

Further, when measuring the broken portion of the solidified shell, the time from when the mold temperature reaches the peak value to when it returns to the original temperature immediately before the rise is about 5 to 15 seconds, as shown in FIG. For this reason, m should not include the temperature change period corresponding to 5 to 15 seconds in the period required for prediction, and it should be continued for 5 to 15 seconds plus the above 30 seconds for 35 to 45 seconds. The number of pitches that can be predicted with a high degree of accuracy of the measured signal, for example, 70 to 90 is determined as being stored every 0.5 seconds.

The value of K varies depending on the mold size, drawing speed, etc., but is set to an appropriate value of 5 to 10, that is, a critical temperature change amount at which solidified shell rupture occurs based on the results of the present invention described below.

When such preparation is completed, continuous casting is started, and when drawing is started thereafter, the prediction device is operated. Temperature measuring element 11,12,
When the mold temperature T at each position is measured at 13, the average temperature difference calculation circuit 16, 26, 36 and the standard deviation calculation circuit 17, 27, 37 store the mold temperature T signal, and the number of stored signals is m. The calculation is not performed and output is not performed until the number is reached.

Then, when the m-th signal is stored, based on the n-th signal from the younger one in the memory order, that is, the n-piece mold temperatures obtained in the area A or A ′ shown in FIG. ,
The average temperature and the standard deviation σ are calculated and output.

Subtractors 15, 25 and 35 find the difference (T-) between the m-th inputted mold temperature T and the average temperature. Further, the integrators 18, 28 and 38 obtain the product (K · σ) of the constant K and the standard deviation σ.

The comparators 19, 29, 39 have two input signals, namely T- and K.
It is determined whether or not the above expression (1) is satisfied by comparing the magnitude with σ. That is, it is determined whether the amount of change (T−) in the mold temperature is smaller or larger than the threshold value K · σ which is proportional to the standard deviation σ even if there is a temperature change due to the above cause.

Next, when the (m + 1) th and subsequent signals are stored in the average temperature calculating circuit 16 and the standard deviation calculating circuit 17, the same operation is repeated.

When any one of the comparators 19 and the like obtains a result satisfying the expression (1) while performing the signal processing in this manner, the corresponding comparator predicts that the breakout occurs, and outputs an alarm.
At 40, an alarm is issued and an abnormality occurrence signal is output to a control device (not shown). As described above, the control device controls the sliding nozzle portion 7 and a pinch roll (not shown) to temporarily stop the charging of molten steel and the withdrawal of cast slab.

Thereby, even if the solidified shell breaks and the unsolidified molten steel leaks from the broken portion, breakout can be prevented in advance.

In the above embodiment, the breakout prediction is determined by the above equation (1), but the present invention is not limited to this and the following equation (2) may be used.

(T −) / σ ≧ K (2) FIG. 4 is an explanatory diagram showing the relationship between the threshold value K · σ determined as described above and the mold temperature. FIG. 4 (a) shows time on the horizontal axis and mold temperature on the vertical axis, and FIG. 4 (b) shows time on the horizontal axis and T− on the vertical axis. .

In FIG. 4 (a), the variation of the mold temperature is large in the B region, while the variation of the mold temperature is small in the C region, and the mold temperature sharply rises and falls in the C region. The symptom signal D appears.

In such a case, the threshold value K · σ becomes as shown by the alternate long and short dash line in FIG. 4 (b). That is, in the portion corresponding to the A region where the mold temperature greatly fluctuates, the threshold value is kept substantially constant and maintained at a stable level exceeding T-. On the other hand, the threshold value becomes small in the portion corresponding to the C region where the change of the mold temperature becomes small, and as a result, the sharp T-change that occurs in the region corresponding to the C region, that is, the signal corresponding to the breakout symptom signal is detected. It will be possible to capture it with certainty.

On the other hand, for example, when the threshold is constant, the fourth
As shown by the broken line in Fig. (B), it is not possible to maintain a stable level with respect to changes in T-, frequent false detections are made, and even if a provisional breakout symptom signal can be captured, it is falsely detected. It can be seen that there is a risk of being overlooked because it cannot be distinguished from the signal.

FIG. 4 (c) is an explanatory view showing a histogram showing the frequency for each value of T− in the form of a graph, in which the horizontal axis represents the mold temperature,
Also, the vertical axis shows the frequency. The portion corresponding to the B region where the temperature variation is large shown in FIG. 4A is shown by a broken line, and the portion corresponding to the C region where the temperature variation is small is shown in practice.

As is clear from this, the threshold value in the region where the temperature fluctuation is large is Kσ ₂ , and the threshold value in the region where the temperature fluctuation is small is Kσ ₁ , and it can be seen that the threshold value changes in accordance with the degree of fluctuation of the mold temperature. .

Fig. 5 is a continuous casting machine for round cast slabs. Inner diameter: 187mm, length 900mm on a copper plate, 120 ° pitch in the circumferential direction in three directions, 200,300,400mm from the top of the mold. Five
It is a figure that summarizes the results of about 6 minutes when the present invention was carried out at a drawing speed of 2.0 m / min while being buried in a depth of mm, and during that time the solidified shell rupture did not occur. Is compared with that of the conventional method. In the figure, (a) is the drawing speed, (b) is the mold temperature, (c) is (T-) / σ.
The changes of the above are shown, and for comparison, the conventional rate of change per unit time is also shown in (d), and T- is shown in (c).

As can be seen from this figure, 200,300,400 from the top of the mold
When the mold temperature by the thermocouple provided at the position to _{T 1, T 2, T 3} , the temperature change was changed as shown in each (b).
At this time, in the case of the conventional rate of change per unit time, the threshold value of 5 ° C / sec is exceeded eight times within 6 minutes, and an erroneous alarm is issued. In the case of (T-), the threshold value is set. Value of 10
A false alarm was issued twice above ℃. On the other hand, in the case of the present invention, a false alarm is not issued even once when K is 5, and even if the above-mentioned uneven flow of powder occurs, the solidified shell rupture is not affected by this. Can be detected, that is, breakout can be predicted.

FIG. 6 is a diagram summarizing the mold temperatures T ₁ , T ₂ and T ₃ in the case of predicting breakout according to the present invention together with other operating conditions. (A) is a drawing speed and transition of the molten metal level in the mold, Further, (b) shows changes in the mold temperatures T ₁ , T ₂ , and T ₃ . In this case, the prediction accuracy is changed from that in the case of FIG. 5, and concretely, K is set to 7 and the threshold value is increased. Also in this case, there is uneven flow of powder, etc. No false alarm was given even if the temperature changed, and the alarm was given only when the solidified shell actually broke and the mold temperature changed. And
With this alarm, the drawing speed was once stopped, and the broken portion of the solidified shell was cooled in the mold for a long time to make the solidified shell thicker, that is, in the state where breakout did not occur, and the drawing was restarted.

After the completion of casting, inspection of that part revealed a leaked part of molten steel, and it was confirmed that breakout could be accurately predicted.

Also, when the falling speed of the solidified shell rupture part was calculated from the thermocouple detection time difference of the mold temperature peak near the time when the breakout alarm was issued and the separation distance between the thermocouples, the extraction speed was calculated.
1m / min, slower than 2m / min. Assuming that the fracture part moves at this speed, a breakout will occur.
42 seconds ago, the breakout prediction was made,
Approximately 24 when continuously casting at a faster drawing speed of 3.5 m / min
The breakout can be predicted in seconds, the solidified shell rupture can be dealt with with sufficient time, and the breakout can be reliably prevented.

Although the mold temperature is measured at three positions of different molds in the drawing direction in the above-mentioned embodiment, the present invention is not limited to this, and 1 or 2 or 4 or more regardless of the drawing direction and the direction orthogonal thereto. Needless to say, the breakout can be predicted by measuring the mold temperature at any position in However,
As the mold temperature drawing direction measuring position, it is preferable to detect a solidified shell rupture and change the operating condition so that there is a time margin to prevent breakout.

〔effect〕

As described in detail above, the method of the present invention measures the measured mold temperature, the standard deviation and the average temperature of the mold temperature for a predetermined period before and close to the measurement time, and the measured mold temperature and average mold temperature. Since it was decided to compare the difference between and the threshold value proportional to the standard deviation of the mold temperature obtained in the predetermined period, the threshold value should reflect the temperature fluctuation situation near the mold temperature measurement time point. Even if the mold temperature changes due to fluctuations in the molten metal level in the mold, the magnitude of the drawing speed, non-uniform inflow of powder, changes in the contact area between the mold and the slab, etc. False detection can be avoided, breakage can be reliably predicted by accurately detecting solidified shell rupture, reliability can be improved, and slab quality deteriorates because the operating condition is changed by a false alarm in the conventional method. I was doing Etc. The present invention can prevent the excellent effects.

[Brief description of 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, FIG. 3 is a standard deviation of the present invention,
FIG. 4 is an explanatory diagram of a period for calculating the average temperature, FIG. 4 is an explanatory diagram showing a change in threshold temperature used in the method of the present invention with respect to mold temperature change, and FIGS. 5 and 6 are explanatory diagrams of effects of the present invention. 7 and 8 are explanatory views of the problems of the conventional technique. 3 ... Mold, 4 ... Slab, 11, 12, 13 ... Temperature measuring element, 15, 2
5,35 …… Subtractor, 16,26,36 …… Average temperature calculation circuit, 17,2
7,37 …… Standard deviation calculation circuit, 18,28,38 …… Integrator, 19,2
9,39 …… Comparator, 40 …… Alarm

Claims (1)

[Claims] 1. A mold temperature is measured at one or two or more positions of a continuous casting mold, and a standard deviation and an average temperature of the mold temperature are shown for a predetermined period before the measurement time and near the measurement time. Calculate for each position, find the difference between the mold temperature at the time of the measurement and the calculated average temperature, and predict the breakout by comparing the mold temperature difference and the threshold value proportional to the standard deviation. Prediction method for breakout in continuous casting.