JPS61203590A - Heating of metal strip - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Technology] The present invention heats a metal strip having an irregular shape in the longitudinal direction to a constant temperature by induction heating, or selectively heats each shape part in the longitudinal direction to a required temperature. It is about the method.

[Conventional technology]

When continuously heating a metal strip in its length direction, a method is widely adopted in which the metal strip is passed through an annular heating coil and heated by induction.

In this induction heating method, if the strip has a uniform shape in the longitudinal direction, the heating conditions are usually constant, so it is only necessary to detect the heating temperature and make slight adjustments. strips, e.g.

Steel tube with upset, finned tube with land.

When continuously heating a stepped strip or the like, in order to keep the heating temperature substantially constant, it is necessary to adjust the heating power according to its shape.

In general, it is relatively easy to detect the heating temperature and control the temperature to a constant value when the temperature fluctuates over a long period of time, such as in an electric furnace, or in the case of induction heating or fixed-position heating. In mobile heating, in which the object to be heated or the heating coil is heated while being moved, it is difficult to control the heating temperature at a constant level because detection position shifts and hunting may occur depending on the surface condition. In moving heating of objects, it is extremely difficult to control the heating temperature to be constant.

In such cases, it is common practice to set or detect the shape and position of the object to be heated and then switch to a predetermined power. Satisfactory effects have not been obtained except when things are repeatedly heat treated.

[Problem that the invention seeks to solve]

Therefore, the problem to be solved by the present invention is that metal strips having irregular shapes in the longitudinal direction, such as steel pipes with upsets, fin tubes with lands, and strips with steps, can be easily made into substantially the same shape by induction heating. In addition, as in the case of heating the fin portion of a fin tube with a land as the main purpose, for example, heating to different required heat treatment temperatures depending on the shape in the longitudinal direction.

[Means for solving problems]

The present invention has been made with the aim of solving the problems of the prior art as described above, and its configuration is to heat a metal strip such as a tube that has an irregular shape or a change in thickness in the longitudinal direction by induction heating. In the heating method, the metal strip and the heating coil are moved relatively, and the impedance or phase angle 9 frequency displacement of the heating coil based on the change in shape of the metal strip is detected from the power source side of the heating coil, The main feature is to use these displacement amounts to control the heating temperature to a constant temperature or to a required temperature for each shape, and more specifically,
In a method of heating a metal strip such as a tube having an irregular shape or a change in thickness in the longitudinal direction by induction heating, the metal strip and the heating coil are moved relative to each other, while the heating coil is Separately, a detection coil that detects the shape of the object to be heated or a sensor that detects the shape state is placed at an appropriate position in front of the heating coil to detect changes in the shape of the unheated portion of the strip, and the detected value is temporarily stored. Memorize and measure or calculate one position, and when the detected position comes within the heating coil, control the heating temperature by supplying the necessary heating power to the heating device based on the detected value. The heating temperature can be controlled more accurately by comparing the displacement output of the detection coil and the displacement amount of the heating coil and making corrections, if necessary. .

cPrinciple of the present invention] Induction heating is widely applied with a capacity of several kilowatts to several thousand kilowatts, and since a high frequency power supply supplies power to a heating coil, that is, an inductor, it is necessary to stably generate the required voltage. It is normally operated using constant voltage control (AVR), and is no different from a general commercial power source.

However, in the case of the same heating coil, the impedance seen from the coil terminal changes depending on the shape of the object to be heated as a load. Therefore, the high frequency power supply is controlled by constant voltage (AV
R), the coil input, ie, the output of the power supply, will change.

FIG. 2a shows an example of a heating coil, which is an electrical circuit diagram of the heating coil, and the impedance is determined by the shape of the object to be heated and the coupling factor with the heating coil.

In addition, in Fig. 3a, R2 is the resistance of the heated object, x
Similarly, l is the reactance, R1 is the resistance of the heating coil itself, xl is the reactance, Xg is the gap reactance between the heating coil and the object to be heated, and Xe is the reactance outside the heating coil. , 82' is the resistance of the object to be heated in primary terms, and xO is the combined reactance in primary terms. Figure 3a is represented by an equivalent circuit as shown in the same figure, and the load impedance from the input terminal of the heating coil, that is, This shows that it is possible to know the shape of the object to be heated.

The relationship between the heating coil and the object to be heated in induction heating is equivalently explained using the same principle as that of a normal double-wound transformer. However, whereas a transformer is formed with an iron core that can be said to be ideal for a magnetic path, a heating coil has a magnetic path with many air gaps, and there is a gap between the coil and the object to be heated, which corresponds to the secondary coil. The difference is that the reactance due to the air gap greatly affects the impedance seen from the coil side.

If we look at these relational expressions from Figure 2 a, b and Figure 3 a, b, we can see the following.

R+・Or ・・・・・・・・・・・・・・・(1)R
2ψD1 ・・・・・・・・・・・・・・・(2) Also, X2 # Rz y X 1 ” R+Xg ”
(D+”ox”)・・・・・・・・・・・・(3)
Xs sori.

Zo=(R+ +R2')+jXo・・・・・・
...(6) R1+R2' R1+
R2'The tendency of the input conditions of the heating coil to change due to the change in the shape of the object to be heated is shown in Table 1. (However, if the high-frequency power source has a constant frequency, the power factor will change,
In a thyristor inverter power supply, the frequency changes. Furthermore, a change in heating temperature also appears as a change in impedance.

Therefore, in the present invention, the changes shown in Table 1 are due to changes in impedance and phase angle, and the changes caused by these alone or in combination are amplified to match the shape and temperature characteristics, and the output of the high frequency power source is The temperature was controlled by feeding back the control. When the outer diameter shape is the same, the resistance R2 of the object to be heated changes depending on the heating temperature, although the rate of change in impedance is small. In other words, if the heated part is a non-magnetic material and is above the magnetic transformation point, the resistance value changes purely based on the thermal expansion coefficient (
This is expressed as changes in impedance and phase angle in equations (6) and (7). It is true that induction heating has a bad power factor and cos θ is about 0.3 when viewed from the coil side, so changes in resistance have little effect on reactance XO, but in moving heating, impedance displacement due to shape changes A change in the resistance R2 of the heated object based on a change in heating temperature is added.

Ultimately, we will take measures to correct these factors while controlling the temperature. That is, since the phase angle displacement due to temperature change is in the opposite direction to the impedance displacement tendency due to shape change, correction is applied based on the ratio of these.

In addition, when it is necessary to provide temperature differences for each shape rather than constant temperature control, a device is used to bias a portion of the control displacement amount positively or negatively.

Furthermore, by placing a detection coil in front of the heating coil, the detected changes in impedance etc. can be stored once, and when the detection point comes to the heating coil, the heating power source can be controlled. Since it can be used as a detection power source ranging from direct current to a heating power source frequency in excess of 1,000 yen, it is possible to detect and control changes in the shape of a cylindrical body, etc., which has the same outer diameter but changes in wall thickness.

Therefore, when the heating coil diameter is constant, if the diameter of the heated object changes, the resistance R2 of the heated object and the air gap reactance Xg between the heating coil and the heated object naturally change, so the impedance and phase seen from the coil This allows the displacement of corners and other angles to be extracted as output.

Furthermore, if the shape is simply determined, it is also possible to use a radioisotope or the like as a sensor to detect the thickness or weight of the object to be heated and control the heating power. However, providing a detection coil not only serves as a shape sensor, but also enables accurate control by comparing and correcting the amount of displacement of the same type obtained from the detection coil and the heating coil.

〔Example〕

Next, an example of implementation of the method of the present invention will be explained with reference to the drawings.

Figure 4a shows switching the constant voltage control circuit of the high frequency power supply,
This is an example circuit diagram that performs output control at a constant temperature.
Reference numeral 1 denotes a high frequency power source (MG or inverter), and power is supplied to a heating coil 4 through a matching transformer 3 to inductively heat an object 9 to be heated. Since a large current flows through the heating coil 4, the circuit cannot be made long, so the state of the object to be heated 9 is detected from the voltage transformer PT and the current transformer C on the primary side of the heating coil 4. Therefore, the voltage converter 5a,
An impedance converter 5b, a phase angle (or frequency) converter 5c, and a current converter 5d are connected, and the outputs of the converters are selected or combined as necessary and operationally amplified by an operational amplifier 6, and compared with a setting device 8. The configuration is such that the output of the power source 1 can be controlled by inputting it to the exciter or output control section 7.

Although not shown, an indicator and a protection relay are installed.

Furthermore, Fig. 4 is a system diagram when a detection coil is provided.Low voltage and low current are supplied to the detection coil 4' from the detection power supply 1', and the output of the impedance converter 5' is connected to the signal delay circuit. 12, the output is delayed according to the signal from the feed rate converter 11, and the output of the power source 1 is controlled, or
The output of the impedance converter 5' and the output detected by the heating coil 4 are corrected by an operational amplifier 6 for control.

Even if a detection output is obtained in proportion to the shape of the object to be heated, it is not necessarily necessary to apply heating power proportional to the shape. In other words, the weight and material of the heated object.

It is necessary to modify it due to differences in heat capacity or heat dissipation conditions. Therefore, the operational amplifier 6 includes a material setting device 6a.

Amplification factor setting device 6b and correction factor setting device 6c for specifying temperature
will be established.

What is shown in Fig. 1a is a finned tube with a land, which has been frequently seen in recent years as an object to be heated, but when the fin portion of this finned tube with a land is heat treated in the same way as the land portion, the usual method is used. In the heating method, the fin part has a smaller unit weight than the land part and is a protruding part, so it tends to be thought that the heating temperature is higher, but due to the increased impedance and heat dissipation, As shown, the temperature of the fin portion decreases. If the power is increased to provide a satisfactory heat treatment to the fin portion, the land portion will overheat.

However, according to the method of the present invention, when the fin portion reaches the heating coil, this is detected as an increase in impedance, and the combined output of these displacements is increased and the heating temperature is controlled to increase. As shown in FIG. 5, the fin portion can be heated to the same or approximately the same temperature as the land portion. Furthermore, it is also possible to make the heating temperature of the fin portion higher than that of the land portion by amplifying or correcting the amount of displacement.

In addition, in the method of the present invention, in order to properly maintain constant control of the heating temperature, it is necessary to apply the input change of the heating coil with the necessary combination of the above-mentioned elements detected and their respective gains, and in addition to the above. It is also possible to detect the temperature and add it as a - element.

Furthermore, with the control according to the method of the present invention, it is possible to emphasize the trends shown in Table 1, or to control the opposite trends to the above, and furthermore, it is possible to control the trends shown in Table 1 to the contrary, and furthermore, it is possible to control the tendency that is the opposite of that shown in Table 1. It is also possible to control the temperature.

〔effect〕

Since the present invention is as described above, for example, when heat treating a finned tube with lands, this heat treatment is performed as work hardening due to fin processing or as a post treatment for the material. It is necessary to ensure that the heating temperature is not too high compared to the fin part, but as mentioned above, this is not possible with normal means, so heating the land part when heating the fin part following the land part is necessary. When heating the fin part, a larger heating power is supplied to the heating coil to heat it, and when the land part following the fin part is heated, a smaller heating power is supplied to the heating coil so that the heating temperature does not become higher than the heating temperature of the fin part. By heating, it is possible to perform homogeneous heat treatment on the entire surface, and therefore, it is suitable for use in heat treatment of metal strips such as tubes that have an irregular shape or a change in thickness in the longitudinal direction.

[Brief explanation of drawings]

Fig. 1 shows a metal strip to be heated by the method of the present invention, in which a is a fin tube with a land, and d to d in the same figure.
Figure 2a is a partial front view of another irregularly shaped tube, Figure 2a is a diagram showing the relationship between the heating coil and the object to be heated, Figure 2 is its equivalent circuit diagram, and Figure 3a is a circuit diagram of the heating coil during heating. , FIG. 3 is an equivalent circuit diagram thereof, FIG. 4 a is a control system diagram of the high frequency power supply according to the present invention, FIG. 4 is a system diagram similarly equipped with a detection coil, and FIG. FIG. 5 is a temperature chart when heating is performed by ordinary induction heating, and FIG. 5 is a temperature chart when induction heating is also performed by the method of the present invention. l... High frequency heating power supply, 1'... Power supply for detection, 2.
... Matching capacitor, 3... Matching transformer, 4.
- Heating coil, 5a... Voltage converter, 5b... Impedance converter, 5c... Phase angle (or frequency) converter, 5d... Current converter, 6... Operational amplifier, 6a.・Material setting device,
6b...Amplification factor setting device. 6c... Correction factor setting device, 7... Exciter, 8... Output setting device, 9... Heated object. lO...Pinch roller, 11...Speed converter, 12
...Signal delay circuit

Claims (1)

[Scope of Claims] 1. A method of heating a metal strip such as a tube having an irregular shape or a change in thickness in the longitudinal direction by induction heating, in which the metal strip and the heating coil are relatively moved, The displacement of the impedance, phase angle, and frequency of the heating coil based on changes in the shape of the strip is detected from the power supply side of the heating coil, and these displacements are used to control the heating to a constant temperature or to the required temperature for each shape. Characteristic heating method for metal strips. 2. In a method of heating a metal strip such as a tube with an irregular shape or a change in thickness in the longitudinal direction by induction heating, the metal strip and the heating coil are moved relative to each other, while the heating coil is Separately, a detection coil that detects the shape of the object to be heated or a sensor that detects the shape state is placed at an appropriate position in front of the heating coil to detect changes in the shape of the unheated portion of the strip, and the detected value is temporarily stored. control the heating temperature by memorizing the position, measuring or calculating the position, and when the detected position comes within the heating coil, supplying necessary heating power to the heating coil based on the detected value; A method for heating a metal strip, characterized by: