JPS61127810A - Method for controlling electrode heating electric power of vacuum degassing tank - Google Patents

Abstract

PURPOSE:To prevent throwing of excessive electric power so as to reduce power consumption, by burying a temperature detector in the refractory body at the inner wall surface of a degassing tank and continuously detecting temperature, and then, controlling the quantity of electric power supplied to graphite electrodes in accordance with the quadratic differential of the detected temperature with respect to the time by successively calculating the quadratic differential. CONSTITUTION:A thermocouple 3 is buried in the refractory body 2 constituting the inner wall surface of a degassing tank 1 and temperature inside the refractory body 2 is continuously detected. The output of the thermocouple 3 is converted into a signal representing temperature by means of a temperature converter 4 and supplied to an automatic throwing electric power calculating/setting unit 10. The unit 10 successively quadratically differentiates the temperature detecting values with respect to the time and finds temperature acceleration, and then, sets the optimum throwing electric power quantity in accordance with the found temperature acceleration. The optimum throwing electric power signal is supplied to an electric power controlling section 5 through a manual-automatic change-over switch 11 and electric power supplied to graphite electrodes 6 and 6' is automatically controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a graphite electrode inserted into a degassing tank of an RH vacuum degassing device or a DH vacuum degassing device for degassing molten steel. This invention relates to a method for controlling the amount of power supplied to a graphite metal when heating the inside of a degassing tank by generating resistance heat by supplying electricity to the graphite metal.

As is well known in the art, when molten steel is vacuum degassed using an RH vacuum degassing device or a DH vacuum degassing device, molten steel adheres to and solidifies on the surface of the refractory that constitutes the inner wall surface of the degassing tank. Even after degassing treatment, the solidified steel (so-called base metal) often remains attached, and in that case, it may be necessary to heat and melt the base metal before the next degassing treatment. It is desirable to supplementally heat the degassing area in order to stabilize the temperature inside the tank.

Methods for heating the inside of the degassing tank include an electrode heating method, a C gas heating method, a refractory energization method, and the like, but the electrode heating method is considered to be the most advantageous in terms of operating costs and operability. This electrode heating method involves inserting a graphite electrode into the degassing tank and passing a large DC current through the graphite electrode to generate resistance heat.
The inside of the degassing tank is heated by radiant heat from red-hot graphite electrodes.

By the way, in the operation of a degassing device using such a graphite electrode heating method, the temperature inside the tank (
In order to control the refractory surface temperature) to an optimal temperature, it is necessary to appropriately control the amount of power supplied to the graphite electrode. Conventionally, therefore, a temperature sensor such as a high-temperature thermocouple is buried in the refractory that makes up the inner wall of the degassing tank. The amount of electric power is controlled by operating an output setting knob and changing an output setting command signal. A control system diagram to which such a conventional general heating power control method is applied is shown in FIG.

In FIG. 5, reference numeral 1 denotes a degassing tank, and a high-temperature thermocouple 3 is inserted from outside the tank into a refractory z that forms the inner wall surface of the degassing tank 1. The output of this thermocouple 3 is applied via a temperature converter 4 to a power controller 5 such as a thyristor rectifier. On the other hand, the DC power set by the power controller 5 is applied to the graphite electrodes 6, 6' inserted in the degassing tank 1 through the bus bars 7, 7' and the water-cooled cable 8.
, 8'. Then, by operating an operating section 9 such as a variable resistor provided in the power controller 5 such as the thyristor rectifier, the power to be supplied to the graphite 16°6' is set.

Problems to be Solved by the Invention The conventional heating power control method for the graphite electrode of a degassing tank as described above has the following problems.

That is, first of all, since the operator must operate the device while constantly monitoring the temperature instruction value, it is difficult to achieve labor savings.

Second, operations are performed according to the readings from the temperature detectors embedded in the refractories, but refractories have extremely high thermal resistance, so the readings from the temperature sensors buried in the refractories do not match the degassing tank. The temperature in the tank (refractory surface temperature) changes with a significant delay, and the range of change in the indicated value is smaller than the range of change in the refractory surface temperature, so the tank changes rapidly. It is difficult to accurately and immediately grasp the internal situation from the indicated value of a temperature sensor embedded in the refractory, and therefore it is not possible to immediately and accurately respond to the internal situation of the tank.

Thirdly, in the operation of a degassing tank, the temperature distribution inside the refractory does not reach a steady state with a constant temperature distribution, so the refractory surface temperature is determined from the reading from a temperature sensor embedded in the refractory. It is difficult to correctly recognize the temperature inside the tank and set the optimal amount of power.

As a result of the above, in the conventional method, the indicated value of the temperature sensor embedded in the refractory is only used as a rough guide, and in actual operation, the amount of electricity must be kept constant, or the In this case, it is inevitable that there will be a tendency to provide excessive power on the safe side of operation, resulting in large power loss and the use of refractories and electric poles. The reality is that this has no choice but to have a negative impact on the lifespan of people.

This invention was made against the background of the above-mentioned circumstances, and it is possible to automatically set and control the optimum amount of electricity according to the internal situation of the degassing tank (temperature change inside the tank), thereby saving labor. At the same time, the purpose is to reduce wasteful loss of stain power through optimal power consumption control, and at the same time, to extend the life of graphite electrodes and refractories through optimal power supply. .

Means for Solving the Problems As a result of various experiments and studies to achieve the above-mentioned object, the inventors of the present invention have developed a second-order time differential value of the temperature detected by a temperature detector embedded in the refractory, That is, by introducing the acceleration of the detected temperature change over time as a parameter, the inventors discovered that it is possible to quickly and accurately grasp the change in the temperature inside the tank, leading to the present invention.

That is, in the power control method of the present invention, a graphite electrode is inserted into a vacuum degassing tank and the graphite electrode is energized to generate heat to heat the inside of the degassing tank. A temperature sensor is embedded in the refractory to continuously detect the internal temperature of the refractory, the second time differential value of the detected temperature is calculated one after another, and the amount of electricity to be applied to the graphite electrode is calculated according to the second time differential value. It is characterized by controlling.

DETAILED DESCRIPTION OF THE INVENTION In the method of the present invention, as described above, the temperature detected by a temperature detector such as a thermocouple embedded in the refractory inside the tank is not directly used as a control parameter, but the secondary value of the temperature detected value is used as a control parameter. Time differential values are calculated one after another, and the amount of electric power supplied to one cylinder of graphite is controlled according to the value of the second-order time differential value.

The effectiveness of using the second-order time differential value of the temperature detection value as a control parameter will be explained next.

Figure 2 (9) shows 5+ on the inner wall of the tank at the end of the degassing process.
Assuming that there is a +a++ thick base metal adhesion, and assuming that the base metal will be melted in 30 minutes by electrode heating before the next degassing treatment, here is an example of a trial calculation of temperature, etc. when operating at a constant power value. It's something to show.

Figure 2 (4) shows the change in the surface temperature of the refractory inside the tank over time; for 30 minutes from the start of heating, the temperature on the surface of the inside wall of the tank gradually decreases due to the amount of heat consumed in melting the metal.
After the melting of the metal is completed, the temperature of the inner wall of the tank immediately starts to rise because the amount of heat required for melting the metal is no longer needed. On the other hand, FIG. 2(B) shows, corresponding to FIG. As is clear from a comparison of Figure 2 (B), the refractory has a large thermal resistance, so the internal temperature of the refractory fluctuates with a significant delay in response to changes in the inner wall surface temperature. That is, about 25 minutes elapsed after the metal melting was completed and the inner wall surface temperature started to rise until the internal temperature of the refractory began to rise, and the rising tendency was extremely gradual. As mentioned above, the time delay and slowness of the change in the internal temperature of the refractory with respect to the change in the inner wall surface temperature is due to the optimization of the electric power supplied to the refractory based on the indicated value of the temperature sensor embedded in the refractory. This is the reason why automatic control is difficult. For example, as shown in Figure 2 (4), if it is possible to detect when the metal melting is complete and the temperature on the inner wall surface of the tank rises rapidly, the subsequent power consumption can be reduced to prevent an excessive rise in the temperature inside the tank. However, such control is difficult only by directly using the indicated value of a temperature sensor embedded in the refractory. Note that thermocouples, which are normally used as temperature detectors, are difficult to install to directly detect the temperature on the inner wall surface of the tank due to problems such as melting and damage. It is necessary to bury it in refractory material.

Therefore, the inventors of the present invention indirectly used the detected value from a temperature detector embedded in the refractories in the degassing tank to find various parameters that could immediately grasp the change in the surface temperature of the inner wall of the degassing tank. As a result of repeated studies, we found that the second-order time differential value of the value detected by the temperature detector inside the refractory, which is closely related to the instantaneous change in the heat flow rate in the refractory inner wall of the degassing tank, closely follows the change in the surface temperature of the inner wall of the tank. I discovered that.

In other words, Fig. 2 (C''l) is understood from the refractories shown in Fig. 2 (B), and Fig. 2 (D) is understood from the inspection of the inside of the refractories shown in Fig. 2 (B). As shown in the figure, when the input power is kept constant, the first time differential value of * (hereinafter referred to as the first time differential value of *) at the detected temperature shows an increasing tendency after a slight delay from the rapid temperature rise after the completion of melting of the surface temperature. When the temperature detection value T* starts to show an upward trend, it becomes O, and the subsequent rising and falling trends are not a barometer for determining whether the temperature on the inner wall surface of the tank is on an upward trend or a downward trend. However, quantification of the internal situation in the tank based on its absolute value immediately responds and shows the value according to the change in the inner wall surface temperature.For example, the inner wall surface temperature after the completion of metal melting responds sensitively, so temperature acceleration can be sequentially measured. It becomes possible to automatically optimally control the amount of electric power based on the calculated value.

As mentioned above, the reason why the temperature acceleration detected by the temperature sensor embedded in the refractory inside the tank responds sensitively to the inner wall surface temperature is because the temperature acceleration inside the refractory enters the inside of the refractory from the surface of the refractory. This is thought to be in response to instantaneous changes in heat flow rate.

In other words, the heat flow rate incident from the surface of the refractory to the inside is q C
ka4'rrl·hr], the relationship with the detected temperature T* inside the refractory can be easily expressed by the following equation (1).

Here, ρ: density, C3: specific heat, V2 volume, A: inner wall area, τ: waiting time 0 This means that if the equation is further differentiated with respect to time, the following equation (2) can be expressed. Therefore, by using a meter inside the refractory, it becomes possible to control the electric power supplied to the graphite electrode to an optimum value according to changes in the internal situation of the tank.

FIG. 1 shows an example of a control system diagram for implementing the control method of the present invention. In FIG. 1, the same elements as the conventional elements shown in FIG. 5 are given the same reference numerals, and their explanations will be omitted.

In FIG. 1, the temperature inside the refractory 2 is continuously detected by a thermocouple 3 serving as a temperature detector inserted into the refractory 2 constituting the inner wall surface of the degassing tank 1. The thermocouple 3
The output is converted by the temperature converter 4 into a signal representing the temperature and then applied to the automatic input power calculation/setting unit IO. This unit IO sets the warm input power detected by the thermocouple 3. Here, specific methods for setting the optimal power input based on the temperature acceleration value include, for example, determining a relational expression between the two in advance through experiments, and calculating the optimal power input using that formula; It is sufficient to create a graph or table representing the relationship between the two, store the graph or table in a storage device, and determine the optimum input power according to the detected temperature value according to the graph or table. The input power signal thus set is applied to a power controller 5 such as a thyristor rectifier via a manual/automatic changeover switch 11, and the power supplied to the graphite electrodes 6, 6' is automatically controlled. In the actual device, the signal of the amount of power supplied from the power controller 5 such as a thyristor rectifier to the graphite electrodes 6, 6' is fed back to the automatic input power calculation/setting unit 1o, and the unit 1O performs so-called feedback control. will be done. Further, the manual/automatic changeover switch 11 is for appropriately switching from the above-described automatic input power setting mode to the manual input power setting mode, and when this switch 11 is switched to the manual setting mode, As in the conventional case, the input power can be set using the manual operation section 9.

Example An example of automatic control of electrode input power using temperature acceleration is shown below. The device configuration shown in FIG. 1 was used. In addition, input power calculation/setting unit
In case of O, the relational expression of temperature acceleration-input power setting value change as shown in Fig. 3 is set in advance, and the thermocouple 3
Based on the temperature acceleration value obtained by second-order time differentiation of the detected temperature inside the refractory, the amount of change in the input power set value was determined according to the relational expression shown in FIG. 3, and the optimum input power was determined from that value.

The changes in the inner wall surface temperature and the amount of input power when degassing operation is performed by automatically controlling the input power in this way are as follows:
Fig. 4 shows a comparison with the case of operation with constant power input (700 kW) according to the conventional method.

As is clear from FIG. 4, in the case of the method of the present invention in which automatic power supply control is performed using temperature acceleration as a control parameter, the situation inside the cypress due to the degassing operation pattern changes, that is, the heating standby period including the metal melting period. , electric power is efficiently input in response to changes in the situation inside the tank during the degassing treatment period, and as a result, the total amount of input power is significantly reduced compared to the case of constant power input using the conventional method, and the power consumption is reduced by approximately 16%. Consumption reduction was achieved. It is also clear that the average temperature on the surface of the refractory has been reduced by about 40°C compared to the conventional method, and the range of its fluctuation has also become smaller.

Effects of the Invention As is clear from the above explanation, according to the method of this invention,
Setting and controlling the amount of electricity supplied to the heating graphite electrode in the degassing tank eliminates the need for operator judgment and operation, which saves labor and automatically adjusts the amount of power supplied to the heating graphite electrode in the degassing tank according to changes in the tank conditions. Since the optimum power is input,
As in the case of operator operation, an extremely excessive amount of electric power is estimated to be on the safe side, and therefore it is possible to significantly reduce electric power costs. In addition, since excessive power input is avoided in this way, the surface temperature of the refractories that make up the inner wall of the tank is prevented from becoming excessively high, and the fluctuation range of the refractory surface temperature is also small. It is possible to significantly extend the life of refractories. Furthermore, even if metal adheres to the inner wall of the tank or the surface of the electrode during degassing, the metal is quickly melted, preventing electrode breakage due to falling metal or adhesion of molten steel splash. This prevents cracking and deterioration of the electrode rod, and extends the life of the electrode rod.

[Brief explanation of the drawing]

FIG. 1 is a block diagram schematically showing an example of an apparatus for carrying out the control method of the present invention, and FIG. This is a diagram showing the results of trial calculations of the changes in the temperature, (4) shows the changes in the surface temperature of the tank inner wall, (B) shows the changes in the temperature detected by the temperature detector embedded in the refractory inside the tank, and (C) (D) shows the change in the first time differential value (!variable speed) of the detected temperature, and (D) shows the change in the second time differential value (temperature acceleration) of the detected temperature. Figure 3 shows the temperature acceleration and input in the example. FIG. 4 is a graph showing the relationship with the amount of change in power setting, and FIG. 4 is a diagram showing the transition of the degassing tank inner wall surface temperature and input power amount in the example in comparison with the conventional method. FIG. It is a block diagram showing an example of a device for carrying out the operation. 1... Degassing tank, 2... Refractory material, 3... Thermocouple (
temperature detector), 5... Power controller such as thyristor rectifier, 6.6'... Graphite electrode, 10... Automatic input power calculation/setting unit.

Claims (1)

[Claims] When a graphite electrode is inserted into a vacuum degassing tank and the graphite electrode is energized to generate heat to heat the inside of the degassing tank, temperature is detected in the refractory material that forms the inner wall surface of the degassing tank. The internal temperature of the refractory is continuously detected by burying the refractory, the second-order time differential value of the detected temperature is calculated one after another, and the amount of electricity supplied to the graphite electrode is controlled according to the second-order time differential value. A method for controlling electrode heating power for a vacuum degassing tank, characterized in that: