JPH07197135A - Temperature controller - Google Patents

Abstract

PURPOSE:To accurately control the heating temp. of an object to be heated to a set upper limit value or below by detecting a Curie tap. and calculating the end point of heating at the time of induction heating. CONSTITUTION:The high-frequency current value flowing in an induction heating coil 3 is detected by an output ammeter 5 at the time of subjecting a steel bar 4 to high-frequency induction heating. The change in the detected high-frequency current is calculated by an arithmetic control section 6 and the oscillation operation of an oscillator 1 is shut off or controlled, by which the upper limit temp. for heating of the steel bar 4 is controlled. At this time, the impedance of the coil 3 is changed by magnetic conversion of the steel bar 4 heated up to the Curie temp. to paramagnetism if the circuit constant of an impedance matching circuit 2 is previously so set as to maximize the current flowing in the coil 3. The impedance matching conditions are then broken and the current flowing in the coil 3 changes. As a result, excessive overheating is prevented without using a sensor, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to induction heating as a pre-process in order to facilitate the rolling and shaping of steel bar analysis samples,
The heating temperature is accurately controlled to be equal to or lower than a preset upper limit value by detecting the Curie temperature.

[0002]

2. Description of the Related Art A heating method includes a combustion gas method and an electric power method, and a resistance heating furnace is the most general electric furnace. A thermometer is used to measure the temperature of the object to be heated, regardless of whether the heating method is combustion gas or electric power. In accordance with this common sense, contact thermometers such as thermocouples and resistance temperature detectors, and radiation thermometers that are non-contact thermometers have been used for a long time. In the case of a high frequency heating furnace, which is a relatively new heating method, it has been customary to prepare a thermometer separately as well.
Only in the high frequency heating furnace, the electrical contact type thermometer such as thermocouple and resistance thermometer is completely measured because the electric conductors forming its components are affected by strong induction electromagnetic waves in the high frequency heating coil. Wen cannot achieve his original purpose. Therefore, a radiation thermometer is often used in the high frequency heating furnace.

[0003]

In the radiation thermometer, it is a major premise of accurate temperature measurement that the optical emissivity of the surface of the object to be measured is accurately known. However, in metals such as steel bars where an oxide film is easily formed, the optical emissivity changes with time and with temperature, so in most cases temperature correction is performed by relying on an approximate virtual emissivity that is assumed in advance. Therefore, the heating upper limit temperature control accuracy equivalent to that of the contact type thermometer cannot be obtained at all.

[0004]

SUMMARY OF THE INVENTION The present invention advantageously solves the above problems associated with the prior art by detecting the so-called Curie temperature T _{C at} which ferromagnetism is transformed into paramagnetism and detecting the high frequency heating object. It is intended to control the heating state.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an embodiment of the present invention. Reference numeral 1 is an oscillator, which receives commercial power source power and outputs high frequency power. Reference numeral 2 is an impedance matching circuit, which normally matches impedance so as to set the high frequency energy output applied to the steel bar 4 inserted in the induction heating coil 3 to the maximum at normal temperature. 4 is a steel bar, and the high frequency induction heating temperature is controlled to the target temperature by the method of the present invention. An output ammeter 5 detects a high frequency current value flowing in the induction heating coil 3. Reference numeral 6 denotes a calculation control unit which calculates the change in the high frequency current detected by the output ammeter 5 and cuts or controls the oscillation operation of the oscillator 1 to control the upper heating temperature of the steel bar 4.

FIG. 2 is a diagram for explaining the principle of the present invention.
The temperature on the abscissa axis is the temperature of the steel bar 4. The ordinate axis is
It is a high frequency current value that flows through the induction heating coil 3 and is detected by the output ammeter 5. Considering that the steel bar 4 is ferromagnetic (ferromagnetism) at room temperature, if the circuit constant of the impedance matching circuit 2 is set in advance so that the current flowing through the coil 3 becomes maximum, the Curie temperature is set. The steel bar 4 heated to T _C changes its magnetism to paramagnetism, whereby the impedance of the coil 3 changes, the impedance matching condition is broken, and the current flowing through the coil 3 decreases.

[0007]

The feature of the present invention is that the high-frequency current detected by the output ammeter 5 changes dramatically, that is, the so-called Curie temperature T _{C at which} the steel bar 4 is changed from ferromagnetic to paramagnetic at room temperature. Focusing on the origin, the time when the coil current, which is an induction heating energy source, changes is determined to have reached the heating temperature reference point, that is, the Curie temperature T _C , and an external error such as a surface state or a shape is determined. It is to provide a method for reliably preventing overheating without the use of a sensor that involves the risk of detection.

Incidentally, the Curie temperature T _C of pure iron is 7
The temperature is 68 ° C, and the Curie temperature T _{C of} steel hardly changes even if a plurality of other elements are present in the composition, regardless of whether the steel bar, which is an industrial product, is an intentional additive element or an impurity such as carbon. The Curie temperature T _C is a physical effect that is extremely stable once the composition is determined and has little disturbance. This means that the emissivity decreases sharply due to the reduction of the oxide film at high temperature, which is caused by the radiation thermometer.
Considering the complexity of the emissivity correction for compensating for this and estimating the true temperature is one of the features sufficient to understand the superiority of this method.

Moreover, the components contained in the steel bar, oxygen, nitrogen,
The upper limit of the temperature of the high frequency induction heating for facilitating the shaping of a steel bar sample for analyzing the concentration of elements such as sulfur and carbon is usually 800 ° C. As is clear from the iron-carbon binary alloy phase diagram, this is the temperature at which iron transforms from the face-centered cubic system to the body-centered cubic system, that is, the α1 transformation point [738
It is closely related to the movement of trace elements such as oxygen nitrogen, sulfur and carbon (migration) at the temperature of [° C] to cause the degassing phenomenon. Considering even diffusion in the solid phase and degassing due to grain boundary migration, 800 ° C is the upper limit of the pretreatment heating temperature, which obviously does not affect the component analysis. Also, even if the α1 transformation point [738 ° C] is exceeded and the Curie temperature [768 ° C] is exceeded, the high-frequency heating process is a heating for a short time of several seconds to several tens of seconds, so that the element transfer rate is a rate-determining condition. It has been experimentally proved that the degassing amount of volatile elements from the inside of a steel bar is small.

From the above description, it is possible to reliably heat the steel material to a sufficient softening temperature while surely avoiding dangerous overheating by surely detecting the Curie temperature T _C. Easy to understand.

[0011]

As described above, according to the present invention,
The heating upper limit temperature control accuracy equivalent to that of the contact type thermometer can be achieved without the thermometer.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram illustrating the principle of the present invention.

[Explanation of symbols]

 1 Oscillator 2 Impedance matching circuit 3 Induction heating coil 4 Steel bar 5 Output ammeter 6 Arithmetic control unit

Claims (2)

[Claims] 1. A temperature control device comprising a calculation control unit for detecting a Curie temperature T _{C at} which a ferromagnetism is converted to a paramagnetism and setting the heating end point of a high-frequency heating object. 2. A temperature control device comprising a calculation control unit for detecting the Curie temperature T _C and maintaining the heating temperature of the high-frequency heating object at a target temperature.