JP2011047623A - Induction heating method in continuous heating furnace, and continuous heating furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous heating furnace capable of heating a billet from an ordinary temperature to a prescribed temperature, and shortening its temperature rising cycle even in the continuous heating furnace using induction heating coils to which non-resonant inverters are connected. <P>SOLUTION: A billet heater 10 provided with the induction heating coils 20, 22 includes the induction heating coils 20, 22 disposed along a transporting passage 12 for transporting the billet 34, inverters 24, 26 independently connected to the induction heating coils 20, 22, and having load circuits of which coil inductance is changed accompanied by the transfer of properties of the billet 34, and a control section 28 for independently controlling the inverters 24, 26. The control section 28 detects electric currents and voltages charged to the induction heating coils 20, 22, calculates a frequency to have the set electric current when the plurality of inverters 24, 26 lead the electric current with rated voltages, and an operation is performed while keeping the frequency of the output electric current in each of the inverters 24, 26 at the calculated frequency. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a continuous heating furnace that continuously heats an object to be heated, such as a billet heater, and particularly relates to an induction heating method in a continuous heating furnace that performs heating using induction heating, and an induction heating furnace. .

  In a billet heater that continuously heats a billet of an iron-based material that is an object to be heated, the billet is heated in a continuous feed state from room temperature to about 1250 ° C. Conventionally, in such a billet heater, when feeding is stopped due to trouble or heating is stopped, there is a lot of material that is wasted and burned until the billet is heated to the desired temperature and sent out. Shortening the time until recovery of the temperature raising cycle has been an issue.

  Since a heating device using induction heating is suitable for rapid heating of an object to be heated, it is also employed in a billet heater (see Patent Document 1). The billet heater disclosed in Patent Document 1 includes a first induction heating coil, a second induction heating coil, a first inverter, a second inverter, temperature measuring means, and a control device. The 1st induction heating coil arrange | positioned on the conveyance path | route where a billet is conveyed is connected to a 1st inverter, and is heated even to the temperature slightly lower than the desired heating temperature. A second induction heating coil connected to the second inverter is disposed after the first induction heating coil. The second induction heating coil plays a role as an auxiliary heating unit for suppressing variations in the heating state of the billet. A temperature measuring means is arranged between the first induction heating coil and the second induction heating coil, measures the temperature of the billet discharged from the first induction heating coil, and based on the measured temperature. Control power by the second inverter is determined.

JP-A-7-249478

  Since the inverter disclosed in the cited document 1 is a resonance type inverter, the space occupied by the resonance capacitor is large, and it is desirable that the inverter be a non-resonance type in order to reduce the size of the device. However, the billet, which is an iron-based material, undergoes property transfer from ferromagnetism to paramagnetism by straddling the Curie point. Here, when a non-resonant inverter is employed, the heating state below the Curie point is more induced than the heating state as a ferromagnetic material below the Curie point and the heating state as a paramagnetic body above the Curie point. The coil inductance in the heating coil is about twice as high. For this reason, a high voltage is required to flow a predetermined current through the induction heating coil, and it may not be possible to flow a predetermined current at the rated voltage, resulting in a decrease in input power and poor heating efficiency. In some cases, the recovery of the heating cycle is delayed, or the temperature cannot be raised to a predetermined temperature.

  Therefore, in the present invention, even in a continuous heating furnace using an induction heating coil connected with a non-resonant type inverter, the billet can be heated from room temperature to a predetermined temperature, and the heating cycle can be shortened. An object of the present invention is to provide an induction heating method in a continuous heating furnace and a continuous heating furnace.

  In the induction heating method in the continuous heating furnace according to the present invention for achieving the above object, the induction heating coil is connected to an inverter having a load circuit whose inductance changes along with the property transfer of the object to be heated. The frequency is controlled so that the current when the inverter flows current at the rated voltage becomes the set current.

Further, in the induction heating method in the continuous heating furnace having the above-described characteristics, the control of the frequency may be set to a maximum frequency at which each inverter can pass the rated current when the set current is a rated current.
By adopting such a configuration, the highest power can be obtained in a realizable range at the rated voltage and rated current.

Further, in the induction heating method in the continuous heating furnace having the above-described characteristics, the inverters may synchronize the frequency of the current flowing through the induction heating coil.
By adopting such a configuration, the inverter connected to the induction heating coils arranged adjacent to each other can perform current synchronous control and adjust the current supplied to each induction heating coil while avoiding the influence of mutual induction. Will be able to do.

  In addition, the continuous heating furnace according to the present invention for achieving the above object includes a plurality of induction heating coils arranged close to each other along a passage through which an object to be heated is conveyed, and each of the plurality of induction heating coils. And an inverter having a load circuit whose inductance changes as the property of the object to be heated changes, and a control unit that controls each of the plurality of inverters. The control unit is input to each induction heating coil. Each of the plurality of inverters calculates a frequency at which the current becomes a set current when each of the plurality of inverters flows a current at a rated voltage, and a frequency of an output current in each inverter is calculated. It is characterized by operating as a frequency.

Further, in the continuous heating furnace having the above-described characteristics, the control unit calculates the maximum frequency at which each inverter can pass the rated current when the set current is the rated current, and the output current of each inverter is calculated. It can be operated as a frequency.
By setting it as such a structure, the highest electric power can be obtained in the realizable range in a rated voltage and a rated current.

Moreover, the continuous heating furnace which has the above characteristics WHEREIN: The said control part is good to make it synchronize the frequency of the electric current which each said inverter flows into the said induction heating coil.
By adopting such a configuration, the inverter connected to the induction heating coils arranged adjacent to each other can perform current synchronous control and adjust the current supplied to each induction heating coil while avoiding the influence of mutual induction. Will be able to do.

According to the induction heating method in the continuous heating furnace having the above characteristics, the billet is heated from room temperature to a predetermined temperature even in a continuous heating furnace using an induction heating coil connected to a non-resonant type inverter. And the temperature raising cycle can be shortened.
Moreover, according to the continuous heating furnace which has the above characteristics, the said method can be implemented and the effect of the said method can be show | played.

It is a figure which shows the structure of the continuous heating furnace which concerns on embodiment.

Hereinafter, an induction heating method in a continuous heating furnace according to the present invention and an embodiment related to the continuous heating furnace will be described in detail with reference to the drawings. In the following embodiments, a billet heater will be described as an example of a continuous heating furnace.
FIG. 1 is a block diagram illustrating a configuration of a billet heater 10 according to the embodiment. The billet heater 10 according to the embodiment is configured based on the conveyance path 12, induction heating coils 20 and 22, inverters 24 and 26, an outer shell 32, and a control unit 28.

  The conveyance path 12 plays the role of a furnace in which the billet 34 that is an object to be heated is continuously charged. A skid rail 14 for transporting the billet 34 is disposed in the transport path 12, and the skid rail 14 extends from the entrance to the exit of the transport path 12. A pinch roller 16 is disposed on the entrance side of the transport path 12, and the billet 34 transported to the entrance of the transport path 12 by the conveyor 18 or the like is slid on the skid rail 14 and pushed into the transport path.

  With this configuration, the billet 34 slides on the skid rail 14 by the pushing force of the subsequent billet 34 and is pushed out to the outlet side. It is desirable that the skid rail 14 has a hollow structure so that a coolant such as cooling water or cooling gas is circulated therein. With such a configuration, it is possible to prevent the skid rail 14 from being damaged or deformed by heat.

  The induction heating coils 20 and 22 are coils wound around the outer periphery of the conveyance path 12 in a solenoid shape. The billet heater 10 according to the embodiment includes a first induction heating coil 20 provided on the inlet side of the conveyance path 12 and a second induction heating coil 22 provided on the outlet side of the conveyance path 12. Each induction heating coil (the first induction heating coil 20 and the second induction heating coil 22) is preferably made of a copper tube and has a structure in which a coolant such as cooling water or cooling gas is circulated. By setting it as such a structure, it can avoid that the induction heating coils 20 and 22 themselves are heated to high temperature.

  The inverters 24 and 26 are power adjusting means for adjusting the power input to the induction heating coils 20 and 22 described above. In the billet heater 10 according to the embodiment, the inverter 24 is individually provided for each induction heating coil 20 and 22. , 26 are connected. Accordingly, the first inverter 24 is connected to the first induction heating coil 20, and the second inverter 26 is connected to the second induction heating coil 22. The billet heater 10 according to the embodiment employs an inverter having a load circuit whose inductance changes in accordance with a property change of an object to be heated, that is, a property transfer in which the property of the billet 34 is transferred from ferromagnetism to paramagnetism. As a result, the resonant capacitor can be omitted from the control unit, and since it is not necessary to form a resonant circuit, it is possible to easily switch the current frequency in the operating state. An example of such a non-resonant type inverter is a PWM inverter. In the present embodiment, the first inverter 24 and the second inverter 26 are connected to a single power supply unit 30.

The outer shell 32 is an exterior covering the transport path 12 and the induction heating coils 20 and 22 described above, and bears actions such as heat insulation and magnetic flux shielding.
The control unit 28 calculates the amount of power supplied to each induction heating coil 20, 22 and outputs a control signal to each inverter 24, 26 in order to supply the calculated power to each induction heating coil 20, 22. Take a role. The control unit 28 receives detection signals a from ammeters (not shown) and voltmeters (not shown) provided in the induction heating coils 20 and 22.

  Based on the detection signal from the voltmeter, the output voltage of each inverter 24, 26 is compared with the rated voltage. If the inverter 24, 26 is below the rated voltage, the output voltage of the inverter 24, 26 is reduced to the rated voltage. A signal for raising is output to each inverter 24 and 26. The current value obtained by the detection signal from the ammeter in the inverters 24 and 26 operated at the rated voltage by the output of the voltage control signal is compared with the current value (set current value) set to obtain a predetermined amount of heat. To do. As a result of comparison, when the current value obtained from the detection signal is smaller than the set current value, control is performed so that the current value matches the set current value.

ここで、ｆは周波数、Ｌはコイルインダクタンスである。従ってＬは数式上においては定数として取り扱う事ができるため、Ｉを設定電流にまで向上させるには、周波数ｆを低減させれば良い。これによりインピーダンス（２πｆＬ）の値が低下し、数式１の関係を満たすには、低下したインピーダンス分、他の変数を向上させる必要が生じ、変数である電流Ｉを向上させることができることとなる。 The relationship between the voltage V and the current I can be expressed by Equation 1.

Here, f is a frequency and L is a coil inductance. Therefore, L can be treated as a constant in the mathematical formula, and the frequency f can be reduced to improve I to the set current. As a result, the value of the impedance (2πfL) is lowered, and in order to satisfy the relationship of Formula 1, it is necessary to improve other variables by the reduced impedance, and the current I that is a variable can be improved.

と示すことができる。μｓは比透磁率、ρは固有抵抗であり、いずれも被加熱物の温度帯域を定めた場合には定数として扱うことができる。このような関係から、電流Ｉが設定電流であっても、周波数が低下した場合には電力は大幅に低下してしまうことが解る。具体的には、周波数が１／２になった場合、電力は１／√２になってしまうということである。 The relationship between power, current, and frequency is

Can be shown. μs is a relative magnetic permeability, and ρ is a specific resistance, both of which can be treated as constants when the temperature range of the object to be heated is determined. From this relationship, it can be seen that even if the current I is the set current, the power is greatly reduced when the frequency is reduced. Specifically, when the frequency is halved, the power is 1 / √2.

  For this reason, when it is desired to obtain the maximum power during rapid heating or the like, it is necessary to operate each inverter 24, 26 at the maximum frequency at which the maximum current can flow at the rated voltage. As can be seen from Equation 2, when the current I is halved, the power becomes ¼. Therefore, lowering the frequency than lowering the current I has no effect on the power. It can be said that there are few.

  Regarding the heating control, the inverter 24 is synchronized so that the frequencies of the currents supplied to a plurality (two in the present embodiment) of induction heating coils (the first induction heating coil 20 and the second induction heating coil 22) are synchronized. , 26 is sent a control signal b. By synchronizing the frequencies, it is possible to control the current waveforms so that they match or keep a predetermined amount of deviation. Such control can be performed by instantaneously changing the frequency of the output current, and the influence of mutual induction between the first induction heating coil 20 and the second induction heating coil 22 arranged in proximity to each other can be controlled. It can be avoided. Thereby, in each inverter 24 and 26, it becomes possible to control the electric current input into the induction heating coils 20 and 22 with high accuracy, and it is possible to realize heating control that draws an arbitrary temperature rise curve.

Next, heating of the billet 34 by the billet heater 10 having the above configuration will be described. First, a heating process when charging the billet 34 from the inlet side to the outlet side of the conveyance path 12 will be described.
When the billet 34 is put into the conveyance path 12, the billet 34 is first heated by the first induction heating coil 20. The billet 34 thrown into the transport path 12 has a property as a ferromagnetic material because it is at room temperature. For this reason, the value of the coil inductance L in the load circuit is high, and it may be difficult to input the set current to the first induction heating coil 20 during rated voltage operation.

  In such a case, the frequency of the current supplied to the first induction heating coil 20 is set low (for example, 400 Hz) so that the set current can flow. As a result, even when the low temperature billet 34 is heated via the non-resonant type first inverter 24, it is possible to efficiently heat and raise the temperature.

Since the billet 34 heated to the Curie point (about 770 ° C.) or more by the first induction heating coil 20 becomes a paramagnetic material, the value of the coil inductance L in the load circuit of the second inverter 26 becomes small. Therefore, the second induction heating coil 22 can be operated at a rated voltage and a rated current, and the billet 34 can be heated to a desired heating temperature (about 1250 ° C.). Here, as shown in Formula 2, a decrease in frequency causes a decrease in power supplied to the induction heating coil. For this reason, the first inverter 24 and the second inverter 26 are operated at the highest frequency at which the set current can flow when the first inverter 24 is operated at the rated voltage. Since the coil inductance L of the second inverter 26 is low, the second inverter 26 can be operated at the rated voltage and the rated current even when the frequency is adjusted to the operating frequency of the first inverter 24. .
As a result, the inverter can be miniaturized and the billet 34 can be raised to a desired temperature in a short time.

  Secondly, a description will be given of the heating process in the case where the transport and the supply of electric power to the induction heating coils 20 and 22 are stopped and the reheating is performed while the billet 34 is put in the transport path. In such a case, the billet 34 left in the transport path 12 returns to room temperature. For this reason, all the properties of the billet 34 arranged in the conveyance path 12 at the time of reheating are ferromagnetic, and the coil inductance L in the inverters 24 and 26 shows a high value. The problem at the time of reheating is to reduce the waste baking material as much as possible. For this purpose, it is necessary to recover the heating cycle in the normal heating state as soon as possible.

  Therefore, the first inverter 24 and the second inverter 26 need to input a rated current at a rated voltage to the first induction heating coil 20 and the second induction heating coil 22, respectively. For this reason, the first inverter 24 and the second inverter 26 start operation at the maximum frequency that can be operated at the rated voltage and the rated current, and the temperature of the billet 34 carried into the second induction heating coil 22 is increased. When the temperature becomes equal to or higher than the Curie point, the first inverter 24 operates by reducing the current supplied to the first induction heating coil 20 to the set current.

By passing through such a heating process, even in the billet heater 10 that has been heated again, the waste baking material can be reduced as much as possible, and the heating cycle can be recovered in a short time.
In the above description, the output current frequency can be switched at the Curie point, but the output power from the inverters 24 and 26 has the maximum output power at the set current or the rated current. As described above, it is desirable to operate by sequentially changing.

DESCRIPTION OF SYMBOLS 10 ......... Billet heater, 12 ......... Conveyance path, 14 ......... Skid rail, 16 ...... Pinch roller, 18 ...... Conveyor, 20 ...... 1st induction heating coil (induction heating coil), 22 ......... Second induction heating coil (induction heating coil), 24 ......... First inverter (inverter), 26 ......... Second inverter (inverter), 28 ......... Control unit, 30 ... ... power supply, 32 ... outer shell, 34 ... billet.

Claims (6)

An induction heating method in a continuous heating furnace in which a plurality of induction heating coils are arranged,
Connected to the induction heating coil is an inverter having a load circuit whose inductance changes with the property transfer of the object to be heated,
An induction heating method in a continuous heating furnace, characterized in that the frequency is controlled so that a current when each inverter passes a current at a rated voltage becomes a set current.   2. The induction heating method for a continuous heating furnace according to claim 1, wherein the frequency is controlled to a maximum frequency at which each inverter can pass the rated current when the set current is a rated current.   3. The induction heating method in a continuous heating furnace according to claim 1, wherein each inverter synchronizes the frequency of a current flowing through the induction heating coil. 4. A continuous heating furnace with an induction heating coil,
A plurality of induction heating coils arranged close to each other along a path through which the object to be heated is conveyed;
An inverter having a load circuit that is individually connected to each of the plurality of induction heating coils, and whose inductance changes with property transfer of the object to be heated;
A controller that controls each of the plurality of inverters,
The control unit detects a current and a voltage input to each induction heating coil, respectively, and calculates a frequency at which the current becomes a set current when each of the plurality of inverters flows a current at a rated voltage, A continuous heating furnace that operates with the frequency of the output current in each inverter as the calculated frequency.   5. The control unit according to claim 4, wherein when the set current is a rated current, the control unit calculates a maximum frequency at which each inverter can pass the rated current, and operates as a frequency of an output current in each inverter. The continuous heating furnace described.   6. The continuous heating furnace according to claim 4, wherein the control unit synchronizes a frequency of a current that each inverter passes through the induction heating coil.

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