KR100663916B1 - Method for continuous casting - Google Patents

Abstract

A method of continuous casting is provided to control the thickness of a solidified shell formed in a mold by measuring the temperature change value for same cooling water. A method of continuous casting comprises a step of measuring the flowing time for cooling water to flow in an inlet of a cooling water slot of a mold and to flow out of an discharging outlet of the cooling water slot of the mold; a step of computing the temperature change value by using the temperature of the present time as the temperature of the cooling water discharged from the mold, and using the temperature of the time excluding the flowing time rather than the present time at the inlet unit; a step of computing the electrothermal flow velocity discharging from a molten steel through the mold by using the temperature change value.

Description

Continuous casting method {Method for continuous casting}

1 is a schematic cross-sectional view showing a typical continuous casting machine.

2 is a conceptual cross-sectional view for explaining the flow of cooling water in the mold.

3 and 4 are a cut perspective view and a plan view showing the mold installation and the solidification cell formation.

Fig. 5 is a graph showing temperature change values ΔT for the calculation of the coolant temperature of the inlet and the outlet and the heat transfer flow rate according to the prior art and the present invention.

Figure 6 is a graph showing the heat transfer flow rate of the embodiment according to the present invention.

Figure 7 is a graph showing the heat transfer flow rate of the comparative example according to the prior art.

<Explanation of symbols for the main parts of the drawings>

15: tundish nozzle 20, 30: mold copper plate

40: coolant slit 50: water jacket

The present invention relates to a continuous casting method, and more particularly, to a continuous casting method for controlling the formation of a solidification cell by accurately calculating the amount of heat exiting the mold during continuous casting, that is, the heat transfer flow rate.

1 shows a typical continuous casting machine.

Referring to FIG. 1, a continuous casting machine has a tundish 2 positioned at a lower portion of the ladle 1 for transporting molten steel, and a tundish nozzle 3 for flowing out molten steel at the bottom of the tundish 2. Is installed. In addition, a mold 4 for producing molten steel as a slab having a predetermined thickness and width is provided below the tundish nozzle 3, and a plurality of pinch rolls for guiding the slab at the lower end of the mold 4. (5) is installed. This continuous casting machine is a method in which molten steel injected from above solidifies into a cross-sectional shape of the mold 4 as it flows down through the mold 4 and continuously produces.

The mold 4 forms the molten steel injected from the tundish 2 into an appropriate size, and forms a solidified layer of the cast through cooling so as not to be ruptured by the pulling force and the molten steel positive pressure when drawn to the lower portion of the mold 4. . The cooling is performed by circulating cooling water in the mold 4 copper plate, and removes heat from the molten steel inside the solidification layer of the cast steel to form a sound solidification layer. That is, the molten steel grows by forming a solidification cell in contact with the mold 4, and the solidification cell is pulled downward under the mold 4 and continuously moves. Generally, the solidification cell thickness when exiting the mold 4 is about 10 to 20 mm, and then passes between the rolls 5 supporting the solidification cell, and the molten steel inside the solidification cell is completely solidified by the sprayed cooling water. . Here, in the case where the solidification cell exiting the mold 4 has a defect, a break-out occurs in which unsolidified molten steel flows out of the solidification layer, which can no longer proceed with continuous casting. There is a problem that interferes with the stability of the operation and causes damage to the continuous casting equipment.

Therefore, it is desirable to control the thickness of the solidification cell growing in the mold to be maintained in a predetermined range in order to prevent the occurrence of breakout and stable casting operation. The thickness of the coagulation cell can be estimated by the amount of heat escaped from the molten steel into the cooling water of the mold.

2 is a conceptual cross-sectional view illustrating the flow of coolant in the mold. The coolant for cooling the mold 4 flows into the coolant slit 6 below the mold 4 and then through the coolant slit 6. Flows out and flows out. While the cooling water flows through the cooling water slit 6, the mold 4 heated by the molten steel is cooled by the cooling water, while the temperature of the cooling water rises. Here, the amount of heat escaped from the molten steel per unit time through the mold, that is, the heat transfer flow rate, may be calculated using the temperature increase amount of the cooling water and the amount of cooling water flowing per unit time.

In general, the heat transfer flow rate is calculated as follows using the amount of cooling water, the temperature increase, and the effective area of the copper plate. The heat transfer flow rate is a concept of an average of the entire area of the copper plate and is expressed in terms of unit area and amount of heat escaping per second.

HF (MW / m ² ) = 7 × 10 ^-5 × QW × (T _out -T _in ) × 1 / A _eff

In the above formula, QW (l / min) represents the amount of cooling water per unit time flowing through the mold copper plate, T _out represents the temperature of the cooling water flowing _out of the mold, and T _in represents the temperature of the cooling water flowing into the mold. The constant 7 × 10 ⁻⁵ contains the specific heat of water and the unit conversion coefficient. In addition, A _eff (m ² ) represents the effective cross-sectional area of the copper plate, in which the solidification cell is in contact with the mold in the mold.

In general, it is possible to determine the heat transfer flow rate by the above formula to determine the thickness of the solidification cell in the mold, it is preferable to satisfy the value of the appropriate heat transfer flow rate for each casting speed for stable operation of the continuous casting. In order to control the value of the heat transfer flow rate, the amount of cooling water or the components of the mold flux are mostly adjusted.

Referring to FIG. 2, the sensors 7 and 10 are respectively installed at the portions 8 and 9 of the cooling water flowing in, and the temperature of the cooling water is measured. Calculate the flow rate and adjust the operating conditions accordingly to cast continuously. That is, the temperature value measured by the temperature sensor 7 of the inlet part is substituted into T _in of the above formula, and at the same time, the temperature value measured by the temperature sensor 10 of the outlet part is substituted into T _out of the formula to calculate the heat transfer flow rate. do.

During casting, the cooling water flows from the bottom to the top of the mold and rises in temperature by the heat from the molten steel. Moreover, since the coolant flows in the circulation system, the temperature of the coolant flowing into the mold continuously rises. In order to prevent the temperature of the coolant flowing into the mold excessively high, the continuous casting machine is equipped with a cooling tower as an auxiliary equipment. When the temperature is high, the cooling tower operates to lower the temperature of the cooling water.

If the cooling tower is running, it is introduced into the mold while the temperature of the cooling water is lowered. Before the coolant is introduced, the temperature sensor of the inlet detects the temperature of the coolant, and after passing through the mold, the temperature sensor of the outlet detects the temperature of the coolant again. At this time, it takes some time for water whose temperature is lowered due to the operation of the cooling tower to reach the outlet temperature sensor via the inlet temperature sensor. That is, the coolant flowing into the mold immediately after the cooling tower is operated is relatively low in temperature, but at the same time, the coolant flowing out of the mold has already flowed into the coolant of relatively high temperature. Not only does the temperature change increase, but also includes the temperature change according to the operation of the cooling tower.

In addition, when the cooling tower is not operating, as mentioned above, the temperature of the incoming coolant is continuously increased due to the circulation of the coolant. Therefore, the temperature change between the two measured points simultaneously indicates only the increased temperature change in the mold. Rather, it includes a change in temperature as the cooling water circulates.

Therefore, calculating the heat transfer flow rate by simultaneously measuring the coolant temperature of the inlet and outlet, involves many errors and is accurate, including the effects of temperature rise due to the circulation of the coolant and temperature drop due to the operation of the cooling tower, in addition to heat exchange with the mold. Not. Accordingly, in the related art, the thickness of the solidification cell in the mold is incorrectly determined due to the value of the heat transfer flow rate having a large number of errors, and the operating conditions are unnecessarily changed to deteriorate the stability of the operation and the quality of the product.

The present invention is to solve the above problems, by accurately calculating the heat transfer flow rate, which is the amount of heat flowing out of the molten steel from the molten steel to the cooling water through the mold during continuous casting, to accurately determine the thickness of the solidification cell in the mold during continuous casting to increase the productivity It is characterized by providing a continuous casting method which can improve process stability and improve quality.

In order to achieve the above object, the present invention is a continuous casting method, the step of measuring the time taken for the coolant to flow into the inlet of the cooling water slit of the mold and outflow to the outlet of the cooling water slit of the mold, outflow from the mold The temperature of the cooling water to be used is the temperature of the current time at the outlet portion, the temperature of the cooling water flowing into the mold is a temperature change value using the temperature of the time earlier than the current time at the inlet portion by the moving time It provides a continuous casting method comprising the step of calculating the heat transfer flow rate and the amount of heat exiting the cooling water through the mold from the molten steel using the temperature change value.

The following formula

HF (MW / m ² ) = 7 × 10 ^-5 × QW × [T _out (t)-T _in (t-Δt)] × 1 / A _eff

The heat transfer flow rate (HF) can be calculated from the equation, where the constant 7 × 10 ^-5 contains the specific heat of water and the unit conversion coefficient, QW represents the amount of cooling water per unit time (l / min), and T _out ( t) represents the temperature (° C) of the cooling water flowing out of the mold at time (t), and T _in (t-Δt) represents the temperature (° C) of the cooling water flowing into the mold at time (t-Δt). ,? T represents the travel time taken for the coolant to flow into and out of the mold, and A _eff represents the effective cross-sectional area (m ² ) of the mold copper plate. Δt is inversely proportional to the amount of cooling water per unit time.

And controlling the coagulation cell thickness in the mold by adjusting the operating conditions according to the calculated heat transfer flow rate value.

Hereinafter, with reference to the accompanying drawings will be described for the continuous casting method according to the present invention.

In the continuous casting method, it is intended to accurately determine the thickness of the solidification cell in the mold by accurately calculating the heat transfer flow rate, which is the amount of heat flowing out of the molten steel into the cooling water through the mold.

3 and 4 show a cut perspective view and a plan view showing the mold installation and the solidification cell formation.

Referring to the drawings, the mold installation includes a coolant slit 40 to allow the coolant to flow through the mold copper plates 20 and 30 in contact with the molten steel A. In addition, a backup plate or a water jacket 50 formed outside the mold copper plates 20 and 30 to prevent deformation of the mold copper plates 20 and 30 due to heat and to supply cooling water to the mold copper plates 20 and 30. , water jacket).

The coolant flows into the coolant slit 40 in the lower part of the mold and then flows upward through the slit 40. While the cooling water flows through the slit 40, the mold copper plates 20 and 30 heated by the molten steel A are cooled by the cooling water, and the molten steel A in contact with the mold copper plates 20 and 30 is solidified. The solidification cell B is formed. In addition, the cooling water flows from the bottom of the mold to the top, and the temperature rises due to the heat released from the molten steel (A). Here, the amount of heat escaped through the mold per unit time, that is, the heat transfer flow rate, may be calculated using the temperature increase amount of the coolant and the amount of the coolant flowing per unit time. Accordingly, the coagulation cell (B) thickness is grasped and the operating conditions are controlled to maintain the appropriate coagulation cell (B) thickness for preventing the breakout phenomenon.

As mentioned above, the conventional formula of the heat transfer flow rate is as follows.

HF (MW / m ² ) = 7 × 10 ^-5 × QW × (T _out -T _in ) × 1 / A _eff

In the above formula, QW (l / min) represents the amount of cooling water per unit time flowing through the mold copper plate, T _out represents the temperature of the cooling water flowing _out of the mold, and T _in represents the temperature of the cooling water flowing into the mold. The constant 7 × 10 ⁻⁵ represents the specific heat of water. In addition, A _eff (m ² ) represents the effective cross-sectional area of the copper plate, in which the solidification cell is in contact with the mold in the mold.

However, the heat transfer flow rate according to the conventional formula is calculated by simultaneously measuring the temperature of the coolant at the inlet or outlet, and includes not only heat exchange with the mold, but also effects of temperature rise due to circulation of the coolant and temperature drop due to the operation of the cooling tower. In other words, the amount of heat escaping from the molten steel into the cooling water through the mold cannot be accurately calculated, and the thickness of the solidification cell formed in the mold cannot be accurately determined.

Thus, the present invention calculates the heat transfer flow rate by measuring the temperature of the inlet or outlet for the same coolant by taking into account the travel time of the coolant between the two points.

The temperature change value ΔT (t) of the same cooling water between two points can be expressed as follows.

ΔT (t) = T (outlet, t)-T (inlet, t-Δt)

In the above formula, Δt represents the time taken for the coolant to reach the inlet to the outlet of the mold. That is, the temperature of the coolant flowing out of the mold is calculated using the temperature of the current time, and the temperature of the coolant flowing into the mold is calculated using the temperature of the time earlier than the current time by the moving time. By measuring the cooling water temperature of the inlet at the time t-Δt and the cooling water temperature of the outlet at the time t traveled from the inlet to the outlet during the time t, the pure water mold The amount of heat released can be calculated.

The time Δt that the cooling water takes from the inlet to the outlet of the mold is inversely proportional to the amount of cooling water flowing per unit time. When the travel time is Δt ₀ when the amount of coolant per unit time is L in one continuous casting machine, for example, when the travel time (Δt ₀ ) is 50 seconds when the amount of coolant L per unit time is 3400 l / min. In addition, the movement time (DELTA) t with respect to the cooling water quantity QW per arbitrary unit time can be expressed as following Formula.

Δt = Δt ₀ × L / QW

    = 50 × 3400 / QW

In consideration of the movement time Δt of the cooling water between these two points, the temperature change value ΔT (t) when moving from the inlet to the outlet for the same cooling water may be expressed as follows.

ΔT (t) = T (outlet, t)-T (inlet, t-Δt)

= [T _out (t)-T _in (t- (50 × 3400 / QW))]

Using the temperature change value, the heat transfer flow rate may be calculated as follows.

HF (MW / m ² ) = 7 × 10 ^-5 × QW × [T _out (t)-T _in (t- (50 × 3400 / QW))] × 1 / A _eff

Using the above equation, it is possible to accurately calculate the heat transfer flow rate, which is the amount of heat drawn out through the mold for the same cooling water. That is, by measuring the temperature change value [Delta] T for the same cooling water in consideration of the movement time [Delta] t of the cooling water between the two points, the heat transfer flow rate can be obtained regardless of the temperature of the cooling water flowing into the mold.

The above equation is applied to the case where the moving time Δt is 50 seconds when the cooling water amount per unit time is 3400 l / min in one continuous casting machine. The moving time Δt for the cooling water amount varies for each continuous casting machine. For example, when the amount of cooling water per unit time is 4000 l / min, the continuous casting machine in the case where the moving time Δt is 40 seconds can calculate the heat transfer flow rate as in the following formula.

HF (MW / m ² ) = 7 × 10 ^-5 × QW × [T _out (t)-T _in (t- (40 × 4000 / QW))] × 1 / A _eff

5 is a graph showing the temperature of the coolant temperature of the inlet and the outlet and a temperature change value ΔT for calculating the heat transfer flow rate according to the prior art and the present invention. Here, (a) and (b) of Figure 5 shows the coolant temperature measured by the inlet temperature sensor and the outlet temperature sensor, respectively, Figure 5 (c) is the inlet and outlet for the calculation of the conventional heat transfer flow rate The temperature change value, which is the difference between the cooling water temperatures measured at the same time, is shown in FIG. 5, and FIG. 5 (d) shows the temperature of the same cooling water in consideration of the movement time of the cooling water between two points for the calculation of the heat transfer flow rate according to the present invention. The change value is shown.

Referring to (a) of FIG. 5, which shows the coolant temperature of the inlet part over time, the cooling water temperature decreases as the cooling tower equipment operates at about 1055 seconds, and the cooling water temperature is stopped at about 1140 seconds. You can see it increase again. In addition, referring to FIG. 5B, which shows the coolant temperature of the outlet portion over time, it can be seen that the coolant temperature decreases by about 1110 seconds and then increases by about 1195 seconds. In such a case, it can be seen from the temperature transition described above that the movement time of the cooling water flowing from the inlet to the outlet of the mold is 55 seconds.

Referring to (c) of FIG. 5, the temperature change value is calculated by simultaneously measuring the cooling water temperature of the inlet or the outlet, and the temperature change is changed as the temperature of the incoming cooling water changes, and the heat exchange with the mold is performed. In addition, it can be seen that many errors are included due to the temperature rise due to the circulation of the cooling water and the temperature drop due to the operation of the cooling tower. Due to this error, the thickness of the solidification cell may be incorrectly determined and control such as casting speed may be frequently performed. As a result, the operation may be deteriorated, and the breakout phenomenon may occur in which unsolidified molten steel flows out of the solidification cell. This no longer permits the continuous casting operation and causes damage to the continuous casting plant. In addition, unnecessarily changing the operating conditions or severely causing an accident such as break-out may reduce the productivity of continuous casting and adversely affect the quality of the product.

Referring to (d) of FIG. 5, the temperature change value for the same cooling water, that is, the temperature of the cooling water at the inlet at (t-55 seconds) and (t seconds) in consideration of the travel time of the cooling water between the two points. By calculating the cooling water temperature difference in the outlet, it can be seen that the temperature change value maintains a stable value of about 4.7 ° C regardless of the temperature of the cooling water flowing into the mold.

As described above, the present invention measures the temperature change value ΔT for the same cooling water in consideration of the movement time Δt of the cooling water between the inlet and the outlet, and thus the heat transfer flow rate regardless of the temperature of the cooling water flowing into the mold. Can be calculated accurately. That is, even if the cooling water temperature of the inlet rises due to the circulation of the cooling water or the cooling water temperature of the inlet decreases due to the operation of the cooling tower, the amount of heat escaped from the molten steel into the cooling water through the mold can be accurately calculated. The thickness of the coagulation cell can be controlled. In addition, by precisely knowing the thickness of the solidification cell to control the operating conditions, it is possible to prevent the occurrence of the break out and improve the process stability. In addition, there is an advantage that can increase the productivity and quality by reducing the change of unnecessary operating conditions.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

For continuous casting according to the present invention, when the amount of cooling water per unit time was 3400 l / min, a continuous casting machine was used when the travel time Δt was 35 seconds. When continuously casting at a casting speed of 1.1 m / min, the mold is cooled by cooling water flowing into the cooling water slit of the mold to form a solidification cell. The amount of cooling water per unit time of cooling water is 3800 l / min, and the temperature of the incoming cooling water is The temperature varied continuously in the range of 28 to 45 ° C. Figure 6 shows the heat transfer flow rate of the embodiment according to the present invention, was calculated by the following equation.

HF (MW / m ² ) = 7 × 10 ^-5 × QW × [T _out (t)-T _in (t- (35 × 3400 / QW))] × 1 / A _eff

Referring to Fig. 6, molten steel is used by using the same temperature change value (ΔT) of the same cooling water between the inlet and outlet as described above, even under the condition that the temperature of the incoming cooling water is severely changed in the range of about 8 ° C. It can be seen that the heat transfer flow rate, which is the amount of heat exiting the cooling water from the mold, is maintained in a narrow range of 1.25 to 1.3 MW / m ² . According to the present invention, it is possible to accurately calculate the amount of heat escaping into the cooling water, and to recognize the thickness of the solidification cell formed in the mold without any error, so that proper operation control can be performed when necessary, thereby preventing operation accidents such as breakout. You can prevent it.

[Comparative Example]

For continuous casting according to the prior art, a continuous casting machine in which the travel time Δt was 35 seconds when the amount of cooling water per unit time was 3400 l / min was used. When continuously casting at a casting speed of 1.1 m / min, the mold is cooled by cooling water flowing into the cooling water slit of the mold to form a solidification cell. The amount of cooling water per unit time of cooling water is 3800 l / min, and the temperature of the incoming cooling water is The temperature varied continuously in the range of 28 to 45 ° C. Figure 7 shows the heat transfer flow rate of the comparative example according to the prior art, it was calculated by the following equation.

HF (MW / m ² ) = 7 × 10 ^-5 × QW × (T _out -T _in ) × 1 / A _eff

Referring to FIG. 7, it can be seen that as the temperature of the incoming cooling water changes, the heat transfer flow rate, which is the amount of heat exiting the cooling water from the molten steel to the cooling water, varies significantly in a wide range of 1.0 to 2.0 MW / m ² . Thus, the heat transfer flow rate calculated includes errors including temperature change values in addition to heat exchange with the mold, and deteriorates process stability due to the above unstable heat transfer flow rate values. In addition, in the case where the heat transfer flow rate is 2.0 MW / m ² when the casting speed is 1.1 m / min, the rate of breakout is high, so the casting speed must be lowered unconditionally. That is, the productivity can be lowered and the quality can be deteriorated due to unnecessary casting speed change.

As mentioned above, although this invention was demonstrated in detail using the preferable Example, the scope of the present invention is not limited to a specific Example and should be interpreted by the attached Claim. In addition, those skilled in the art should understand that many modifications and variations are possible without departing from the scope of the present invention.

The present invention can control the thickness of the solidification cell formed in the mold by measuring the temperature change value (ΔT) for the same cooling water in consideration of the movement time (Δt) of the cooling water flowing from the inlet to the outlet of the mold. . In addition, by precisely knowing the thickness of the solidification cell to control the operating conditions, it is possible to prevent the occurrence of the break out and improve the process stability. In addition, there is an advantage that can increase the productivity and quality by reducing the change of unnecessary operating conditions.

Claims (4)

In the continuous casting method, Measuring a travel time taken for the coolant to flow into the inlet of the coolant slit of the mold and to the outlet of the coolant slit of the mold; The temperature of the coolant flowing out of the mold uses the temperature of the current time at the outlet, and the temperature of the coolant flowing into the mold uses the temperature of the time earlier than the current time at the inlet. Calculating a temperature change value; And And using the temperature change value, calculating a heat transfer flow rate, which is a quantity of heat flowing out of the molten steel into the cooling water through the mold. The method according to claim 1, The following formula HF (MW / m ² ) = 7 × 10 ^-5 × QW × [T _out (t)-T _in (t-Δt)] × 1 / A _eff Calculate the heat transfer flow rate (HF) by Where the constant 7 × 10 ^-5 contains the specific heat of water and the unit conversion coefficient, QW represents the amount of cooling water per unit time (l / min), and T _out (t) flows out of the mold at time (t) Represents the temperature of the cooling water (° C.), T _in (t−Δt) represents the temperature of the cooling water (° C.) flowing into the mold at time (t−Δt), and Δt represents the cooling water flowing into the mold and outflow. The movement time taken to become, and A _eff represents the effective cross-sectional area (m ² ) of the cast copper plate, characterized in that the continuous casting method. The method according to claim 2, The Δt is inversely proportional to the amount of cooling water per unit time. The method according to any one of claims 1 to 3, And controlling the solidification cell thickness in the mold by adjusting the operating conditions according to the calculated heat transfer flow rate value.

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