JPH0474839B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a method for controlling induction heating, and particularly to a method for controlling the temperature of molten steel in a tundish in continuous casting.

B. Summary of the Invention The present invention, in induction heating control of the temperature of molten steel in a tundish, performs program control during unsteady operation and feedback control during steady operation, and aims at the reference power and proportionality constant that determine the input power in this feedback control. By switching according to the difference between the temperature and the molten steel temperature and the time rate of change in temperature, the temperature fluctuations of the molten steel are reduced over the entire period between unsteady operation and steady operation in continuous casting.

C. Prior Art The surface quality of a slab in continuous casting is inferior to that during steady operation during unsteady operation, such as at the beginning and end of casting, or when changing pots (joints) in continuous casting. One of the causes of surface quality deterioration during unsteady operation is a decrease in the temperature of the molten steel in the tundish.
The temperature of molten steel in the tundish drops significantly in the early stages of casting, recovers in the middle, and then drops again at the end or when the pot is replaced.

Therefore, conventional methods have been used to continuously measure the temperature of the molten steel in the tundish, monitor the casting temperature, and adjust the heating amount of the induction heating furnace to keep the molten steel temperature constant. In order to avoid heating problems (inability to heat due to heating), methods are being used to adjust the input power using a specific pattern.

D. Problems to be Solved by the Invention Conventional heating power adjustment involved comparing the measured temperature with the target temperature and manually switching the taps to adjust the input power, which was a complicated operation. In addition, even in a simple feedback control method using an inverter as a power source and a PID controller that compares the actual measured temperature with the target temperature, the temperature sensor inside the tundish may be in contact with the molten steel during unsteady operation at the beginning of casting or when changing the ladle. The problem is that automatic control cannot be performed because it is far from the molten metal surface, and if simple feedback control is used even during steady operation, if the molten steel temperature is lower than the target temperature, the method becomes zero, and the molten steel in the groove of the induction furnace cools down. Including the problem of hardening, it is difficult to improve control followability to the target temperature, suppress hunting, and reduce temperature deviation, and it is difficult to maintain the molten steel temperature constant.

An object of the present invention is to provide an induction heating control method that reduces temperature fluctuations in molten steel over the entire range during unsteady operation and steady operation.

E Means for Solving the Problems In order to achieve the above object, the present invention performs feedback control during steady operation, and performs program control to adjust the input power according to changes in the amount of molten steel in the tundish during unsteady operation. , the reference power and proportionality constant for determining input power during feedback control are switched according to the difference between the target temperature and molten steel temperature and the time rate of change of temperature.

F Effect During unsteady operation, the temperature is stabilized by program control that adjusts the input power based on the relationship between the rise and fall of temperature due to changes in the amount of molten steel in the tundish for the same input power, and during feedback control during steady operation. The reference power is changed depending on whether the temperature difference is positive or negative to improve responsiveness, and hunting is suppressed by changing the proportionality constant depending on whether the temperature difference exceeds a predetermined positive or negative value and whether the temperature difference is about to become large. do.

G. Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an apparatus configuration diagram. Tandaisuyu 1
The internal molten steel 2 is heated by AC power supplied to the inductor 3 of the induction furnace 1A, and this AC power is subjected to power control or voltage control by a power converter 4 consisting of an inverter INV and a converter CON. The control device 5 performs feedback control of the output power or voltage of the power converter 4 according to the input power command Es.

The input power command Es is switched by a changeover switch 6 into a command Ep for program control and a command Eo for temperature feedback control. Program control is used for molten steel 2 during unsteady operation such as the initial stage of pouring when pouring from the ladle 7, the ladle replacement stage, and the final stage of casting.
Temperature feedback control is performed during a period when the molten steel level (molten steel amount) changes significantly, and temperature feedback control is performed after the set time of program control or during steady state when the molten steel temperature reaches the target temperature, and the temperature feedback control is performed by comparing the measured temperature signal and the target temperature. This is done using a comparison operation.

During unsteady operation in which the amount of molten steel in the tundish changes significantly, changes in the amount of molten steel significantly affect the temperature of molten steel for the same input power. The input power command Ep is obtained by sequentially reading out data groups selected from the program data table 9 according to this detection signal and the target temperature θs.

Temperature feedback control uses continuous thermometer 10.
The average calculation unit 11 detects the average temperature θa from the detected temperature θr, and converts this average temperature θa into the consumable thermometer 1.
The correction calculating unit 13 performs correction as needed according to the detected temperature θ _T according to No. 2, and the input power calculating unit 14 calculates the input power command Eo from the corrected average temperature θa ₁ and the target temperature θs.

The calculation in the input power calculation section 14 is performed by the following formula.

Eo=Ef (1+ΣKΔθ/θs) Here, Ef is the reference power, K is the proportional constant, Δθ is the deviation between the target temperature θs and the average temperature θa ₁ , and has the ratio of the deviation Δθ between the molten steel temperature and the target temperature θs. The input power command Eo is obtained by correcting the reference power Ef at the sampling period.

The input power commands Ep and Eo by such program control and temperature feedback control are set by the limiter section 1.
5 is set as the command Es, and the pinch effect is suppressed.

FIG. 2 is a diagram showing the temperature control mode in FIG. 1. Start of casting into tandate 1 (time
At t ₁ ), program control is performed to read input power command Es commensurate with the weight of molten steel from table 9 to control input power and prevent a drop in molten steel temperature at the initial stage of casting (period T ₁ ). When the specific time T ₁ (or the molten steel temperature reaches the target temperature θs) from the start of pouring (time t ₂ ), the changeover switch 6 is switched to the Eo side so that feedback control is performed by continuous temperature measurement of the molten steel. Next, a specific time before the end of pot 7 (time t ₃ )
When the temperature reaches 1, the program control is switched again and power is supplied in accordance with the weight of the molten steel (period T ₃ ).
In this program control, the input power starts to decrease when the ladle ends (time t ₄ ), and the weight of the molten steel begins to decrease, and the input power follows this, lowering the input power to prevent overheating, and starts pouring from the next ladle (time t ₅ ). As the weight of the molten steel increases, the input power is increased, and after a predetermined time or when the molten steel temperature reaches the target temperature (time t ₆ ), the control is returned to feedback control.

The program control period for replacing the ladle is used to prevent the molten steel temperature from decreasing when the ladle is replaced, and the program control is similarly performed before the end of casting (time _t7 ).
In this case, the weight of the molten steel begins to decrease at the end of pouring (time t ₈ ), and as all of the molten steel in the tundish 1 is discharged, the input power is also reduced.

As mentioned above, during unsteady operations such as starting pouring or replacing a pot, program control compensates for temperature fluctuations that are difficult to respond to with feedback control, and during steady operations, feedback control provides accurate temperature control. I do. In Figure 2, ΔT shows the measured deviation between the target temperature and the molten steel temperature as a solid line, and the broken line shows the conventional measured value using only feedback control, which shows that the temperature fluctuation is smaller compared to the conventional method.

Here, in this embodiment, the reference power Ef and the proportionality constant K are adjusted in the calculation of the input power calculation unit 14 through the processing described below, thereby improving responsiveness during the feedback control period and heating with less hunting. Enabling control. The basic idea is that the difference between the molten steel temperature and the target temperature Δθ (θs−θa ₁ )
The reference power Ef is determined, and the relationship between the time rate of change of Δθ (dθ/dt) and Δθ takes into account the amount of adjustment to the reference power Ef to improve responsiveness and accuracy.

The reference power Ef is adjusted as shown in FIG.
Deviation Δθ between target temperature θs and molten steel temperature θa ₁ (= θs − θa ₁ )
The reference power Ef is switched as follows according to three regions in which the voltage is negative, positive, less than or equal to a certain value Δθ ₁ , and greater than Δθ ₁ .

When Δθ<0, Ef=Ef ₁ When 0≦Δθ≦+Δθ _1, Ef=Ef ₂ −Ef ₁ /Δθ ₁ Δθ
+ Ef ₁ + Δθ ₁ When < Δθ, Ef = Ef ₂ However, Ef ₁ and Ef ₂ are determined by experiment and experience based on the scale of the tundish and the heating efficiency of molten steel with respect to the input power, etc., as well as the relationship with the temperature difference 0 and Δθ ₁ . This is a predetermined value, and Ef ₁ <Ef ₂ .

On the other hand, the proportionality constant K is adjusted as shown in FIG. The switching is performed as follows according to the three regions in which Δθ is bordered by +Δθ ₁ and −Δθ ₂ and depending on the sign or negative of the time rate of change of the molten steel temperature θ in each region.

When Δθ>+Δθ ₁ , dθ/dt<0, K=K ₁ When Δθ>+Δθ ₁ , dθ/dt≧0, K=0 Δθ<−Δθ ₂ , When dθ/dt≧0, K=K ₂ Δθ< K=0 when −Δθ ₂ , dθ/dt<0 −K=0 when −Δθ ₂ ≦Δθ≦+Δθ ₁ However, K ₁ and K ₂ are the temperature difference −Δθ ₂ as in the above-mentioned Ef ₁ and Ef ₂ . , +Δθ ₁ are predetermined values based on experiments and experience, and K ₁ >K ₂ .

By adjusting the reference power Ef and the proportionality constant K, the input power command output Eo is determined as follows according to the difference Δθ between the target temperature θs and the molten steel temperature θ.

(a) When Δθ>+Δθ ₁ and dθ/dt<0 Eo=Ef ₂ (1+ΣK ₁ Δθ/θs) In this case, the molten steel temperature θ is higher than the target temperature θs.
Since Δθ is lower than ₁ (molten steel temperature θ is lower than target temperature θs) and molten steel temperature θ is decreasing with time, the molten steel temperature can be lowered by performing feedback control using high reference power Ef ₂ and high proportionality constant K ₁ . θ
attempts to quickly raise the temperature toward the target temperature θs.

(b) When Δθ > + Δθ ₁ and dθ/dt ≥ 0 Eo = Ef ₂ In this case, the molten steel temperature θ is increasing over time toward the target temperature, so no adjustment is made in proportion to the temperature difference Δθ. , performs fixed control using only the high reference power Ef ₂ , and throttles the output Eo so that the rise in molten steel temperature toward the target temperature is somewhat gradual.

(c) When 0≦Δθ≦+Δθ ₁ Eo=Ef ₂ −Ef ₁ /Δθ ₁ Δθ+Ef ₁ In this case, the molten steel temperature θ is within a small temperature difference of the set value + Δθ ₁ with respect to the target temperature θs. Therefore, as the temperature difference Δθ becomes smaller, the output Eo is reduced from Ef ₂ to Ef ₁ at a constant slope, and the feedback control is fixed so that the molten steel temperature θ does not protrude from the target temperature θs, that is, does not exceed hunting.

(d) When −Δθ ₂ ≦Δθ < 0 Eo=Ef ₁ In this case, the molten steel temperature θ exceeds the target temperature θs, but the temperature difference is small enough that the difference Δθ is within the set value −Δθ ₂ . Therefore, the feedback control is fixed, and the molten steel temperature θ is gradually lowered toward the target temperature θs using the low reference power _Ef1 .

(e) When Δθ<-Δθ ₂ and dθ/dt≧0 Eo=Ef ₁ (1+ΣK ₂ Δθ/θs) In this case, the molten steel temperature θ is lower than the target temperature θs.
Since Δθ is higher than ₂ and the molten steel temperature θ is increasing with time, it is even higher than the low reference voltage Ef ₁ .
The molten steel temperature θ is quickly lowered toward the target temperature θs by feedback control that lowers it by K ₂ ·Δθ/θs (negative).

(f) When Δθ<-Δθ ₂ and dθ/dt<0 Eo=Ef ₁ At this time, the molten steel temperature θ increases with time to the target temperature θs
The lower reference power Ef ₁
, and the decrease in the molten steel temperature θ toward the target temperature is made somewhat gradual.

As is clear from the above explanation, when the difference between the molten steel temperature θ and the target temperature θs is within the range of the set value +Δθ ₁ and −Δθ ₂ , the molten steel temperature θ is close to the target temperature θs, so feedback is The control is fixed so that the melting temperature θ does not exceed the target temperature θs, that is, does not exceed hunting. And the molten steel temperature θ
When there is a large difference exceeding +Δθ ₁ , −Δθ ₂ from the target temperature θs, the feedback of the input power command Eo is determined depending on whether or not the temperature change dθ/dt at that time is in the direction toward the target temperature. The molten steel temperature θ quickly reaches the target temperature θs by switching whether or not to perform the control.
In other words, while increasing the responsiveness of the control of the molten steel temperature θ to the target temperature θs, the temperature change is made gradual when the temperature approaches the target temperature. Setting value to obtain such control + Δθ ₁ ,
−Δθ ₂ gives the threshold value for determining whether or not to fix the feedback control when the molten steel temperature θ approaches the target temperature θs, and as mentioned above, K ₁ , K ₂ and
Together with Ef ₁ and Ef ₂ , it is determined in advance by experiment and experience.

Note that the control of the input power command Es by calculations of each part in the embodiment is not limited to sampling control by a computer, and it is of course possible to obtain the same effect by using analog calculation means.

H. Effects of the Invention As described above, according to the present invention, during unsteady operation, which is difficult to handle with feedback control, the molten steel temperature is stabilized by programmatically controlling the input power according to the amount of molten steel, and the molten steel temperature is stabilized during steady operation. In some cases, feedback control is performed by switching the reference power and proportionality constant that determine the input power according to the temperature difference and the rate of temperature change over time to improve responsiveness, reduce hunting, and reduce molten steel temperature fluctuations. Temperature fluctuations in the molten steel are reduced throughout the entire range during steady-state operation and steady-state operation, making it possible to obtain a high-quality structure.

[Brief explanation of drawings]

FIG. 1 is a device configuration diagram showing an embodiment of the present invention;
Figure 2 is a diagram showing the control mode in Figure 1;
1 is a reference power switching characteristic diagram of the calculation unit 14 in FIG. 1, and FIG. 4 is a proportional constant switching characteristic diagram of the calculation unit 14 in FIG. 1. DESCRIPTION OF SYMBOLS 1... Tandyshi, 2... Molten steel, 4, Power converter, 5... Control device, 6... Changeover switch,
7...Ladle, 8...Weight detection section, 9...Program data table, 10...Continuous type thermometer, 11
...Average calculating section, 12...Consumable thermometer, 13...
...Compensation calculation section, 14...Input power calculation section, 15...
...Limitsuta Club.

Claims (1)

[Scope of Claims] 1. The power input to the induction furnace of the continuous casting tundish is controlled by a program that adjusts the input power Ep according to the change in the amount of molten steel in the tundish during unsteady operation, and the amount of molten steel in the tundish during steady operation. Deviation Δθ between temperature θ and target temperature θs and reference power Ef
Feedback control is performed using the input power Eo obtained from the following equation Eo = Ef (1 + ΣKΔθ/θs) from the proportionality constant K, and the reference power Ef is set to a positive set value where the deviation Δθ is predetermined to be negative + Δθ ₁ or less and the corresponding Setting value +
The following conditions apply depending on the three regions exceeding Δθ ₁ : When Δθ<0, Ef=Ef ₁ When 0≦Δθ≦+Δθ ₁ , Ef=Ef ₂ −Ef ₁ /Δθ ₁ Δθ+Ef ₁ +Δθ ₁ <Δθ, Ef= Ef ₂ However, Ef1 and Ef2 are predetermined values, and Ef1<Ef2. The proportional constant K is set to a negative set value where the deviation Δθ is predetermined as set value + Δθ ₁ .
From the three regions bounded by Δθ ₂ and the time rate of change dθ/dt of the molten steel temperature θ, the following conditions are established: When Δθ>+Δθ ₁ and dθ/dt<0, K=K ₁ When Δθ>+Δθ ₁ and dθ/dt≧ When 0, K=0 When Δθ<-Δθ ₂ and dθ/dt≧0, K=K ₂ When Δθ<-Δθ ₂ and dθ/dt<0, K=0 When −Δθ ₂ ≦Δθ≦+Δθ ₁ , K =0 However, K ₁ and K ₂ are predetermined values, and K ₁ >K ₂ . An induction heating control method in continuous casting, characterized in that the input power Eo is adjusted by switching according to the following.