JP6859919B2 - Breakout prediction method - Google Patents

Description

The present invention relates to a breakout prediction method, and more particularly to a method of predicting the occurrence of breakout due to redissolution of a coagulated shell.

In continuous casting of steel, the molten steel injected from the tundish into the mold through the dipping nozzle is cooled in the mold and the portion in contact with the mold solidifies to form a solidified shell. The slab on which the solidified shell is formed is drawn from below the mold and cooled in the secondary cooling zone. However, if there is a location where the flow velocity is locally high at the solidification interface due to the flow abnormality of the molten steel in the mold, the solidification shell is redissolved by the heat supply from the molten steel, and the solidification shell thickness becomes thin. If this thin solidified shell reaches the mold outlet, there is a risk that the solidified shell will break and molten steel will blow out, so-called breakout.

Since operation stoppage is unavoidable when a breakout occurs, it is necessary to select operating conditions that do not cause a breakout. It causes deterioration and is not preferable.
Against this background, it is desired to develop a method that can accurately predict the occurrence of breakout, and various methods have been proposed so far.

For example, in Patent Document 1, the local heat flux of each part of the mold is measured by a large number of heat flux meters arranged on the outer surface of the mold, and the wave height of the heat flux waveform representing the temporal change of the heat flux suddenly increases. A technique for preventing the occurrence of breakout by reducing the casting speed when the value exceeds a predetermined value and performing low-speed casting until the wave height returns to the original value is disclosed.

In Patent Document 2, a plurality of thermocouples for measuring the mold wall temperature are embedded in the mold wall, the solidification shell thickness at the mold outlet is calculated based on the mold wall temperature measured by the thermocouple, and the calculated solidification shell thickness is calculated. A technique for preventing the occurrence of breakout by controlling the casting speed and the molten steel flow velocity in the mold when the value falls below the threshold value is disclosed.

Further, in Patent Document 3, the temperature of the inlet and outlet sides of the cooling water flowing in the opposite mold wall and the amount of cooling water are measured, the amount of heat extracted from the molten steel to the mold is calculated, and the calculated amount of heat is higher. A technique for determining that a breakout is signaled when the lower limit is exceeded is disclosed.

Special Publication No. 63-53903 JP-A-2010-194548 Japanese Unexamined Patent Publication No. 57-39068

The technique disclosed in Patent Document 1 is a method of preventing the occurrence of breakout by measuring the local heat flux of each part of the mold and detecting the change of the heat flux, but the local heat flow of each part of the mold is prevented. Monitoring changes in the bundle is not always sufficient to predict the occurrence of breakouts. This is because even if there is an abnormality in heat flux in the initial stage of the solidification shell formation process in the mold, solidification proceeds in the subsequent stage of the solidification shell formation process, and a solidification shell of a predetermined thickness is formed at the mold outlet. This is because it may be judged that there is no risk of breakout. That is, the occurrence of breakout is over-detected.

The technique disclosed in Patent Document 2 estimates the thickness of the solidified shell from the temperature measurement result only at the position where the thermocouple is installed. It is difficult to grasp everything. In addition, adding thermocouples to improve breakout prediction accuracy requires a major modification of the mold wall structure because it is necessary to change the cooling water flow path laid on the entire surface of the mold wall.

On the other hand, the technique disclosed in Patent Document 3 does not require a significant modification of the mold wall structure, and can remove heat that correlates with the solidification shell thickness on the mold exit side regardless of the local heat flux change in the mold. It is a method of monitoring and predicting breakout, and although it is rational, the amount of heat extracted from the molten steel to the mold is the contact area between the mold and the solidified shell due to fluctuations in the height of the molten metal that occur during casting. It is greatly affected by fluctuations. Therefore, it can be said that it is difficult to accurately detect abnormalities in molten steel flow.

The present invention has been made in view of such circumstances, and a method capable of predicting the occurrence of breakout due to the remelting of a solidified shell with high accuracy without requiring a significant modification of the mold wall structure. The purpose is to provide.

The present inventors have calculated the amount of heat removed from the mold wall by the molten steel in the mold from the molten metal surface to the mold outlet, the size of the mold wall, and the height of the molten metal near the mold wall. We have found that the average heat flux is an index for the occurrence of breakout, and have arrived at the following invention.

The breakout prediction method according to the present invention
The temperature of the cooling water entering and exiting the mold wall that flows inside the mold wall facing the discharge hole of the immersion nozzle, the flow rate of the cooling water per unit time, and the height of the molten metal near the mold wall. Steps to measure and
The average heat flux of the mold wall is based on the measured cooling water temperature on the side entering the mold wall, the temperature of the cooling water on the exit side of the mold wall, the flow rate of the cooling water per unit time, and the height of the molten metal in the vicinity of the mold wall. With the step of calculating using Eq. (1)
It is characterized by including a step of determining that a breakout may occur when the calculated average heat flux of the mold wall exceeds the threshold value.

q = QC _WATER (T _OUT- T _IN ) / (A × B) (1)
here,
q: Average heat flux of the mold wall (J · s ^-1 · m- ² ), Q: Flow rate of the cooling water per unit time (L (liter) · s ^-1 ), C _WATER : Specific heat of water (J)・ Kg ^-1・ K ^-1 ), T _OUT : Cooling water temperature (K) _{on the outside of the mold wall, T IN} : Cooling water temperature (K) on the inside of the mold wall, A: From the lower end of the mold wall Distance to the surface of the water (m), B: Average width of the mold wall (m)

The amount of heat that the mold wall removes heat from the molten steel during the period from the molten metal surface to the mold outlet in the mold increases or decreases depending on the contact area between the mold wall and the solidified shell. On the other hand, the contact area between the mold wall and the solidified shell is greatly affected by the fluctuation of the height of the molten metal that occurs during casting. That is, the amount of heat extracted from the molten steel by the mold wall varies depending on the height of the molten metal. Therefore, in the present invention, _{the average heat flux q of the mold wall obtained by dividing the amount of heat QC WATER} (T _OUT −T _IN ) per unit time for the mold wall to remove heat from the molten steel by the contact area A × B between the mold wall and the solidified shell is obtained. It is used as an index for breakout occurrence.

When a resolubility breakout occurs, there is a location where the molten steel flow velocity is locally high at the solidification interface due to an abnormality in the molten steel flow, and the solidified shell is redissolved by the heat supply from the molten steel. Specifically, breakout occurs when the average heat flux of the mold wall where the molten steel discharge flow discharged from the discharge hole of the immersion nozzle collides is high. Therefore, in the present invention, the temperature of the inlet cooling water, the temperature of the outlet side cooling water, the flow rate of the cooling water per unit time, and the height of the molten metal near the mold wall facing the molten steel discharge flow are measured.

In the breakout prediction method according to the present invention, the average heat flux of the mold wall in consideration of the fluctuation of the molten metal surface height that occurs during casting is used as an index of breakout occurrence, so that the mold wall structure does not need to be significantly modified. , It is possible to accurately predict the breakout caused by the remelting of the solidified shell and prevent the breakout from occurring.

It is a block diagram of the breakout prediction system which executes the breakout prediction method which concerns on one Embodiment of this invention. It is a graph which showed an example of the average heat flux of a mold wall with and without breakout calculated from the past results.

Subsequently, an embodiment embodying the present invention will be described with reference to the attached drawings, and the present invention will be understood.

FIG. 1 shows a configuration of a breakout prediction system that executes a breakout prediction method according to an embodiment of the present invention. The mold 10 for continuous casting includes a pair of long-side mold walls (not shown) arranged to face each other, and a pair of short-side mold walls 11 and 12 sandwiched between the pair of long-side mold walls and facing each other. ing. The figure schematically shows a cross section of the mold 10 cut in a virtual vertical plane parallel to the long side mold wall.

The immersion nozzle 13 is made of a cylindrical tube having a bottom, and the upper end of the flow path 13a formed inside is an inflow port of molten steel. On the other hand, a pair of discharge holes 13b communicating with the flow path 13a are formed to face each other on the lower side surface portion of the tubular body. The lower end of the immersion nozzle 13 is inserted into the molten steel 14 in the mold 10, and the mold is provided so that the discharge holes 13b face the short side mold walls (hereinafter, simply referred to as "mold walls") 11 and 12. It is located in the center of 10.

Inside the mold 10, a water passage through which cooling water for cooling the wall surface of the mold 10 flows is formed over the entire circumference. Water passages 11a and 12a are also provided inside the mold walls 11 and 12 with the lower ends of the mold walls 11 and 12 as the entrance side and the upper ends of the mold walls 11 and 12 as the exit side.

The molten steel 14 injected into the mold 10 by the dipping nozzle 13 is solidified from the outside by being cooled in contact with the water-cooled inner wall while staying in the mold 10. The outer side of the molten steel 14 in the mold 10 is covered with the solidified shell 15 as it moves downward, is pulled downward from the lower opening of the mold 10, and then cooled in a secondary cooling zone (not shown).

The water passages 11a and 12a provided inside the mold walls 11 and 12 pass through the thermometers 16 and 17 for measuring the cooling water temperature on the inlet side of the mold walls 11 and 12 and the inside of the mold walls 11 and 12. Flow meters 20 and 21 for measuring the flow rate of the flowing cooling water per unit time are installed on the inlet side of the mold walls 11 and 12, and thermometers 18 and 19 for measuring the cooling water temperature on the outlet side of the mold walls 11 and 12 are installed. It is installed on the outlet side of the mold walls 11 and 12. Further, in order to measure the height of the molten metal in the vicinity of the mold walls 11 and 12, a plurality of thermocouples 22 and 23 are embedded in the upper part of the inner wall surface of the mold walls 11 and 12.

Furthermore, the breakout prediction system calculates the amount of heat per unit time at which the mold walls 11 and 12 remove heat from the molten steel 14 based on the outputs of the thermometers 16, 17, 18, 19 and the hygrometers 20 and 21. It is provided with heat removal amount calculation units 31 and 32, and molten metal height calculation units 33 and 34 that calculate the height of the molten metal in the vicinity of the mold walls 11 and 12 based on the outputs of the plurality of thermocouples 22 and 23.
The calculation results of the heat extraction amount calculation units 31 and 32 and the molten metal height calculation units 33 and 34 are output to the average heat flux calculation units 35 and 36, and the average heat flux calculation units 35 and 36 average the mold walls 11 and 12. The heat flux is calculated. The calculation results of the average heat flux calculation units 35 and 36 are output to the evaluation unit 37, and the evaluation unit 37 determines whether or not the average heat flux of the mold walls 11 and 12 exceeds the threshold value.

Next, a procedure for predicting a breakout using the breakout prediction system will be described. In addition, (STEP-2A) to (STEP-3) shown below are executed for each of the mold wall 11 and the mold wall 12.

_{(STEP-1) First, the threshold value q max} of the average heat flux of the mold walls 11 and 12, which is an index of breakout occurrence, is set based on past results and the like.
FIG. 2 shows an example of the average heat flux of the mold wall when the breakout occurs and when the breakout does not occur, which is calculated from the actual results of the past 3650 charges. In this example, breakout occurs when the average heat flux of the mold wall ^{exceeds 150 J · s -1} · m- ^2.

(STEP-2A) Per unit time for the mold walls 11 and 12 to remove heat from the molten steel 14 based on the outputs of the thermometers 16, 17, 18, 19 and the flow meters 20 and 21 by the heat removal amount calculation units 31 and 32. The calorific value q ₀ (J · s ^-1 ) is calculated using Eq. (2).
q ₀ = QC _WATER (T _OUT- T _IN ) (2)
here,
Q: Flow rate of cooling water per unit time (L (liter) · s ^-1 ), C _WATER : Specific heat of water (J · kg ^-1 · K ^-1 ), T _OUT : Cooling of mold walls 11 and 12 Water temperature (K), T _IN : Cooling water temperature (K) on the mold walls 11 and 12

(STEP-2B) The molten metal level calculation units 33 and 34 calculate the molten metal surface height in the vicinity of the mold walls 11 and 12 based on the outputs of the plurality of thermocouples 22 and 23. The mold 10 being cast is vibrated up and down with a constant amplitude, and the height of the molten metal with reference to the mold 10 changes accordingly. Therefore, the height of the molten metal is continuously increased by several vibrations, for example, 5 vibrations. Find the value and calculate the average value.

(STEP-3) The average heat flux calculation units 35 and 36 calculate the average heat flux q (J · s ^-1 · m- ² ) of the mold walls 11 and 12 using the equation (3).
q = q ₀ / (A × B) (3)
A is the distance (m) from the lower ends of the mold walls 11 and 12 to the molten metal surface, B is the average width (m) of the mold walls 11 and 12, and the size and the molten metal surface height of the mold walls 11 and 12 are calculated. It is calculated from the height of the molten metal calculated from the parts 33 and 34.

(STEP-4) The evaluation unit 37 checks whether _{the average heat flux of the mold wall 11 exceeds the threshold value q max} and whether the average heat flux of the mold wall 12 _{exceeds the threshold value q max.} If the average heat flow velocity of the mold wall 12 _{exceeds the threshold value q max} , it is determined that breakout may occur.

Although one embodiment of the present invention has been described above, the present invention is not limited to the configuration described in the above-described embodiment, but is within the scope of the claims. It also includes other possible embodiments and variations. For example, in the above embodiment, a thermocouple group is installed on the upper part of the mold wall, and the height of the molten metal near the mold wall is calculated based on the measurement results. However, a vortex type level meter or the like is used. The height of the molten metal near the mold wall may be obtained. Further, in the above embodiment, the flow meter is installed on the water passage on the side of entering the mold wall, but it may be installed on the water passage on the side of exiting the mold wall.

The verification test carried out for verifying the effect of the present invention will be described.
In order to verify the breakout prediction accuracy, it is necessary to reliably predict and prevent the occurrence of breakouts, and to confirm that breakout predictions are not over-reported when there is no risk of breakouts. is there. Table 1 shows the results of the number of breakouts and the total number of breakout predictions issued during the operation of 3700 charges in the example and 3650 charges in the conventional example using the continuous casting equipment.

In the conventional example, a patent document that predicts breakout by monitoring the amount of heat extracted from the molten steel by the mold wall, which is calculated by the difference in water temperature between the inlet side and the outlet side of the cooling water flowing through the inside of the mold wall. 3 is the technique described.

The following can be seen from Table 1.
The number of breakouts is 3 in the conventional example and 0 in the embodiment, and the breakout prediction method according to the present embodiment can reliably predict and prevent the occurrence of breakouts.
On the other hand, the total number of breakout prediction reports is 201 in the conventional example and 53 in the embodiment, and the breakout prediction method according to the present embodiment does not excessively report breakout prediction.
From the above results, it was confirmed that the breakout prediction method according to the present embodiment can detect breakouts more accurately than the conventional technique.

10: Mold, 11, 12: Mold wall (short side mold wall), 11a, 12a: Water passage, 13: Immersion nozzle, 13a: Flow path, 13b: Discharge hole, 14: Molten steel, 15: Solidified shell, 16 , 17, 18, 19: Thermometer, 20, 21: Flow meter, 22, 23: Thermocouple, 31, 32: Heat removal amount calculation unit, 33, 34: Hot water level calculation unit, 35, 36: Average heat flow Bundle calculation unit, 37: Evaluation unit

Claims (1)

The temperature of the cooling water entering and exiting the mold wall that flows inside the mold wall facing the discharge hole of the immersion nozzle, the flow rate of the cooling water per unit time, and the height of the molten metal near the mold wall. Steps to measure and
The average heat flux of the mold wall is based on the measured cooling water temperature on the side entering the mold wall, the temperature of the cooling water on the exit side of the mold wall, the flow rate of the cooling water per unit time, and the height of the molten metal in the vicinity of the mold wall. With the step of calculating using Eq. (1)
A breakout prediction method comprising a step of determining that a breakout may occur when the calculated average heat flux of the mold wall exceeds a threshold value.
q = QC _WATER (T _OUT- T _IN ) / (A × B) (1)
here,
q: average heat flux of the mold wall ^{(J · s -1 · m -2} ), Q: the unit time per flow rate of the cooling water ^{(L · s -1), C} WATER: water specific heat (J · kg ^{- 1} · K ^-1 ), T _OUT : Cooling water temperature (K) _{on the outside of the mold wall, T IN} : Cooling water temperature (K) on the inside of the mold wall, A: From the lower end of the mold wall to the molten metal surface Distance (m), B: Average width (m) of the mold wall

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