JPH0790343B2 - 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 utilizing temperature change of a continuous casting mold.

[Prior art]

If breakout occurs in continuous casting equipment and unsolidified molten steel leaks inside the slab, stop casting and discharge the slab that has broken out and 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, the solidified shell of the cast slab being pulled out adheres to the mold and ruptures, and the molten steel leaks from the slab and exits from the lower end of the mold before it is sufficiently cooled, causing a so-called restrained breakout. In this case, it is known that in the mold portion where the fractured portion of the solidified shell passes, the mold temperature gradually rises before passing through the fractured portion and gradually drops after passing through 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)
It is possible to predict the breakout by measuring the change rate of the measured mold temperature per unit time and monitoring the difference between the measured value and the reference value (JP-A-57-115962). .

[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, the magnitude of the drawing elementary degree, the non-uniform inflow of the lubricating powder introduced into the mold, and the mold and the slab. It changes depending on the size of the contact area.

Therefore, in the case of the conventional method, the breakout of the solidified shell is frequently predicted even when the breakage of the solidified shell does not actually occur, and the reliability is lacking. 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 the circumstances, and an object of the present invention is to provide a method capable of predicting a breakout with high accuracy.

[Means for solving problems]

The present invention predicts a breakout by measuring the mold temperature at two positions, calculating the temperature difference, calculating the rate of change of the temperature difference per unit time, and comparing the calculated value with a reference value. .

That is, the breakout predicting method in continuous casting according to the present invention measures the mold temperature at two positions of the continuous casting mold, calculates the difference between the two measured mold temperatures, and calculates the rate of change per unit time, The feature is that the breakout is predicted by comparing the calculated value with the reference value.

[Action]

In the present invention, when one mold temperature rises once when passing through the solidified shell rupture portion, and when the other mold temperature rises, the other mold temperature rises, so a change in the difference between both mold temperatures per unit time. The rate will be greater than each mold temperature drift. Also, when a temperature change occurs due to non-uniform inflow of powder, the moving speed does not decrease due to sticking of the fractured part to the mold in the case of solidified shell fracture, and the time difference for measuring the temperature of the same ingot part Is small, the temperature difference between the two molds changes gently. Therefore, the breakout can be accurately predicted.

〔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 the temperature measuring elements 11 and 12 such as thermocouples are embedded in two locations along the A / D converter 13 for the mold temperatures T _U and T _L measured by the temperature measuring elements 11 and 12, respectively. Is analog-to-digital converted at and is given to the subtractor 14. The buried positions of the temperature measuring elements 11 and 12 are preferably below 50 mm below the molten metal level in order to reduce the effects of uneven flow of powder and fluctuations in the molten metal level.

The subtractor 14 takes in the two input signals from the A / D converter 13 at a predetermined pitch (Δt) of 0.5 second to 1 second, for example. About this acquisition signal, in order to remove the influence of noise due to the electromagnetic stirring device provided around the mold 3, the average value of a plurality of signals output from the A / D converter 13 at a pitch of, for example, several tens of milliseconds. To use.

Then, the subtractor 14 obtains a difference ΔT (= T _L −T _U ) between the two taken-in mold temperatures T _U and T _L, and stores the difference ΔT (= T _L −T _U ), and stores the latest memory signal and four consecutive pitches before that. The memory signal of, that is, the memory signals for a total of 5 pitches are given to the differentiating circuit 15.

In the differentiating circuit 15, the rate of change d of the mold temperature difference (ΔT) per unit time at the intermediate point of the signals for 5 pitches, that is, at the point of 1 second before the current measurement point when the capture pitch is 0.5 seconds. In order to obtain (ΔT) / dt, the following known formula (1) is set.

d (ΔT) / dt = 2 {T _L (2) −T _U (2)} / dt = 1 / (12 · Δt) · [{T _L (4) −T _U (4)} − 8 {T _L (3) -T _U (3)} +8 {T _L (1) -T _U (1)}-{T _L (0) -T _U (0)}] (1) where T _L (0 ) -T _U (0)}, T _L (1) -T _U (1), ...,
T _L (4) -T _U (4): Difference between T _L and T _U for five consecutive pitches including this from the current measurement time. Differentiating circuit 15 calculates d (ΔT) based on equation (1). ) / D
The t is supplied to the comparator 16.

It is needless to say that d (ΔT) / dt may be calculated using a general differential coefficient calculation formula instead of the above formula (1).

The following equation (2) is set in the comparator 16, and the comparator 16
When the input signal relating to d (ΔT) / dt satisfies the equation (2), the alarm device 17 issues an alarm and
An abnormality occurrence signal is output to a control device (not shown).

d (ΔT) / dt> K (2) However, K: Positive constant When the above-mentioned control device (not shown) inputs an abnormality occurrence signal, the sliding nozzle portion 7 provided in the middle of the immersion nozzle 2 is moved to the hydraulic cylinder. By driving at 8, the immersion nozzle 2 is once closed and the rotation of a pinch roll (not shown) is stopped. For this, the immersion nozzle 2 may be slightly opened and the drawing speed may be reduced considerably.

In the present invention using such a device, the temperature difference between the upper and lower molds is as follows, so that breakout can be predicted with high accuracy. This will be described below.

In Fig. 2, the horizontal axis represents time and the vertical axis represents mold temperature.
It is the figure which showed the transition of the mold temperature in each of upper and lower two positions when a solidified shell fracture | rupture part moves below, and a solid line shows an upper mold temperature, and a broken line shows a lower mold temperature, respectively. As can be seen from this figure, the time point at which the temperature change occurs due to the passage of the solidified shell rupture portion shifts between the upper and lower positions.

Therefore, the temperature difference of T _L, by subtracting at T _U descending at the time of rise increases with a delay as shown in FIG. 3 (dashed line), the
The change amount d (ΔT) / dt when the temperature rises is significantly larger than the rate of change dT / dt (see FIG. 2) when T _U and T _L rises. At this time, d (ΔT) / dt is a positive value.

Therefore, in the present invention, d (ΔT) is calculated by the equation (1).
By calculating / dt and determining whether or not the constant K and the calculated value, which are empirically determined threshold values so as to predict breakout with high accuracy, satisfy the above equation (2), Breakout can be predicted with higher accuracy. K
The value of is different depending on the mold position of the temperature measuring element, the burial depth, the operating conditions of continuous casting, etc., but a value of 1.5 to 15 (° C / sec) is suitable.

Further, it is known that when a constrained breakout occurs, the descending speed of the solidified shell rupture portion in the mold is significantly lower than the drawing speed by the pinch roll, and it is possible to obtain the temperature difference as described above. It is possible. However, even if the temperature changes due to the above-mentioned uneven flow of powder, the descending speed of the temperature changing portion is the same as the drawing speed,
As shown in Fig. 4 (a), the mold temperatures T _U and T _L detected by the upper and lower temperature-measuring elements have a small time difference
As shown in FIG. 6B, the mold temperature difference ΔT is also small, and therefore the differential value of the mold temperature difference is extremely small.

As described above, in the case of the present invention, the behavior of the mold temperature change when the solidified shell rupture, which is the cause of the breakout, is taken into consideration, and the temperature change caused by the uneven flow of the powder occurs. Also, it is possible to accurately detect only the solidified shell rupture without falsely predicting it, and it is excellent in detection accuracy.

In the above embodiment, the differential value of (T _L −T _U ) is K (> 0).
The break-out is predicted when the value is larger than the above, but the present invention is not limited to this, and when the differential value of (T _U −T _L ) is calculated and the calculated value is smaller than −K, the break-out occurs. It can be implemented even if it is predicted to be out.

Further, in the above-mentioned embodiment, the two temperature measuring elements are provided in the mold with the positions separated in the drawing direction, but the present invention is not limited to this, and the temperature measuring elements are separated in the width direction or the thickness direction of the mold. It can be implemented even if installed. That is, in the case of restraint breakout, when solidified shell rupture occurs, molten steel leaks from the rupture and sticks to the mold. Even in this state, the slab is pulled out by the pinch rolls, so that a new fractured part is formed in the width direction or the thickness direction side of the slab of the solidified shell portion that is fixed. Therefore, the breakout can be predicted with high accuracy according to the present invention even if the temperature measuring elements are installed in the slab width direction or the thickness direction in a separated manner or in an oblique direction in which the direction and the drawing direction are respectively provided. .

Furthermore, in the above description, two temperature measuring elements are installed separately in the vertical direction, the width direction, and the thickness direction, but the present invention is not limited to this, and a large number of temperature measuring elements are installed in advance in the mold. Alternatively, the two temperature measuring elements to be used may be arbitrarily selected and the mold temperature measured by them may be used.

However, regarding the position where the lower temperature measuring element (which may be at the same height position) is installed, as described above
Not only the position below mm, but even if you predict the breakout and then change the operating conditions such as the withdrawal speed, you can actually prevent the breakout from occurring. The position where the broken portion of the strong shell falls from the temperature measuring element to the bottom of the mold is longer than the time required to change the operating conditions.

〔effect〕

When using the conventional method, the frequency of false alarms is high,
The warning predictive value, which is a ratio of the number of warnings based on solidified shell rupture to the total number of warnings, was low at about 40%. On the other hand, in the case of the present invention, the warning hit rate is doubled to about 80%.

As described above in detail, in the case of the present invention, the mold temperature at two positions of the continuous casting mold is measured, the difference between the two measured mold temperatures is calculated, and the rate of change per unit time is calculated. Since the breakout is predicted by comparing the magnitude with the reference value, it is possible to predict with high accuracy, which can reduce the number of times of stopping drawing or lowering the drawing speed, and suppressing the occurrence of low quality cast pieces, The present invention has excellent effects such as improvement in yield and productivity.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing changes in upper and lower mold temperatures, FIG. 3 is an explanatory diagram of a breakout prediction principle according to the present invention, and FIG. It is a figure which shows the transition of the upper and lower mold temperature and the mold temperature difference when a mold temperature changes by uniform inflow etc. 3 ... Mold, 5 ... Solidification shell, 11, 12 ... Temperature measuring element 14 ... Subtractor, 15 ... Differentiation circuit, 16 ... Comparator

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-60-61151 (JP, A) JP-A-61-46362 (JP, A) JP-B-63-47545 (JP, B2)

Claims (1)

[Claims] 1. A mold temperature at two positions of a continuous casting mold is measured, a difference between the two measured mold temperatures is calculated to calculate a rate of change per unit time, and the calculated value is compared with a reference value. A breakout prediction method in continuous casting, which is characterized by predicting a breakout.