JPH0862181A - Method and apparatus for measurement of transformation rate - Google Patents

Abstract

PURPOSE: To obtain a transformation-rate measuring apparatus of a steel plate on the basis of the attenuation factor of a magnetic-flux density. CONSTITUTION: In the transformation-rate measuring apparatus, a magnetic-flux generation means 10 is arranged on one side of a steel plate 12 to be inspected, a magnetic-flux detection means 3 which detects a magnetic flux leaked when the magnetic flux is passed through the steel plate is arranged on the other side so as to sandwich the steel plate, and the transformation rate of the steel plate is measured by a processing unit 8 on the basis of a magnetic- flux density detected by the magnetic-flux detection means. Regarding the steel plate which has been selected, the plate thickness t1 of the steel plate, the attenuation factor Ya of the magnetic-flux density in the completion of a transformation and, in addition, a characteristic curve wherein the product μ.t of the permeability μ by the plate thickness (t) of the steel plate is expressed on the horizontal axis and the attenuation factor Y of the magnetic-flux density is expressed on the vertical axis are stored in advance in the processing unit. A value on the horizontal axis at a time when the vertical axis Y becomes Ya in the curve is designated as Xa, and a value Xb on the horizontal axis corresponding to Yb at a time when the vertical axis Y becomes the attenuation factor of a magnetic-flux density flund from the measured output of a transformation rate in the running operation of a steel plate in the same plate thickness t1 is found from the characteristic curve F so as to be computed by (Xb/Xa)×100%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transformation rate measuring method and an apparatus for measuring the transformation rate of a steel sheet after hot rolling by electromagnetic means online.

[0002]

2. Description of the Related Art In order to maintain high quality, it is very important to maintain a constant final structure that greatly affects various mechanical properties and physical properties of steel products manufactured in steel mills. This is an important matter. For example, of the magnetic properties, the magnetic permeability has a high correlation with mechanical properties such as hardness and grain size. Therefore, if the correlation between magnetic permeability and mechanical properties (for example, toughness) is measured in advance, it is possible to estimate the mechanical properties of the steel sheet to some extent by measuring the magnetic permeability. Also in terms of manufacturing process control, during the heat treatment process of hot steel, the steel transforms from the austenite (γ) phase to the ferrite (α) phase at high temperature, but it is important to monitor the transformation timing and transformation rate. It is extremely important to control the material of the steel sheet, and it is desired to develop a sensor that measures the above transformation rate online.

In measuring the phase transformation, the fact that the γ-α phase transformation of steel is accompanied by a physical phenomenon of changing from non-magnetic (γ phase) to ferromagnetism (α phase) is utilized by using a magnetic detector. There is a method of detecting transformation behavior. According to this method, the magnetic properties of steel largely change near the Curie point, but the Curie point of most commercial steels is as high as 750 ° C. or higher, whereas the γ-α phase transformation of the steel during the cooling process is high. Since it progresses at a lower temperature, it suffices to detect a change in the magnetic characteristics accompanying it. In addition, in order to measure the magnetic properties of a steel sheet that actually flows continuously during the manufacturing process, it is necessary to perform these measurements in a non-contact and online state.

Conventionally, when measuring the magnetic characteristics of such a steel sheet in a non-contact and on-line state, a transformation rate measurement utilizing the property that the attenuation rate of the magnetic flux passing through the steel sheet changes depending on the magnetic characteristics of the steel sheet. A device has been proposed, and one example is disclosed in Japanese Patent Laid-Open No. 56-82443. FIG. 3 is a configuration block diagram showing a conventional example of the transformation rate measuring device disclosed in the document. In FIG. 3, a steel plate 21 to be inspected is provided with a U-shaped iron core 22 and an exciting coil 23 on one surface side (the lower surface side in the drawing) of a predetermined distance d ₁ and has the U-shaped shape. A magnetizer 24 constituted by an electromagnet having a pair of magnetic poles 24 a, 24 b constituted by the ends of the iron core 22 is arranged facing the steel plate 21. That is, the pair of magnetic poles 24a,
The magnetizer 24 is arranged in the vicinity of the steel plate 21 so that the end face of 24 b faces the steel plate 21 with a predetermined distance d ₁ .

When an exciting current is supplied from the exciting power supply 25 to the exciting coil 23 of the magnetizer 24, the magnetic poles 24a,
During 24b, magnetic flux is generated in proportion to the exciting current. In addition to the magnetic flux passing through the inside of the steel plate 21, this magnetic flux also has a magnetic flux 29 that leaks to the outside.
(In the figure above) of the other surface side is detected by the magnetic detecting element 26 which are spaced apart by a distance d _2. Other,
The surface temperature of the steel plate 21 is measured by the thermometer 27, and this temperature measurement signal is transmitted to the arithmetic unit 28. Magnetic detection element 2
The magnetic flux detected by 6 is also transmitted to the arithmetic unit 28. Further, the information on the steel type and the plate thickness is also transmitted to the arithmetic unit 28, and correction calculation is performed on each. In the measurement shown in FIG. 3, when the transformation rate of the steel sheet 21 changes, the magnetic characteristics such as the magnetic permeability of the steel sheet 21 also change. Then, since the amount of magnetic flux passing through the inside of the steel plate 21 changes, the magnetic flux density detected by the magnetic detection element 26 also changes accordingly. Further, by continuously measuring the density of the magnetic flux 29 leaked to the opposite side of the steel sheet 21 by the magnetic detection element 26 while moving the steel sheet 21 in a certain direction, the point where the magnetic flux density greatly changes is determined by the transformation rate of the steel sheet. It can be regarded as a change point.

[0006]

The above-mentioned conventional method for measuring the transformation rate of a steel sheet and its measuring apparatus, for example, utilize the change in the magnetic permeability as the transformation of the steel sheet progresses. The present invention provides a means for quantitatively measuring the transformation rate of a steel sheet traveling in a hot rolling line by correcting the sheet thickness and the steel sheet temperature from the start to the end of transformation. However, no specific calculation method of the steel plate transformation rate from the attenuation amount of the magnetic flux and the plate thickness and temperature correction method for each steel type are disclosed, and therefore, the description of the above-mentioned conventional technology is not enough. It cannot be said that this method is insufficient for constructing an accurate transformation rate measuring method or a measuring apparatus therefor.

The present invention has been made to solve the above-mentioned problems, and it is a method of calculating the transformation rate of a steel sheet from the attenuation rate of the magnetic flux density, and the sheet thickness correction and temperature correction of the sensor output for each steel type. By establishing the method, it is an object of the present invention to provide a highly accurate transformation rate measuring method for a steel sheet and a measuring apparatus therefor based on these correction methods.

[0008]

According to the transformation rate measuring method of the present invention, a steel sheet to be tested is sandwiched between the magnetic flux generating means for generating a magnetic flux on one side, and the magnetic flux penetrates the steel sheet on the other side to leak. In the transformation rate measuring method using a transformation rate measuring device for measuring the transformation rate of the steel sheet from the magnetic flux density detected by the magnetic flux detecting means, the magnetic flux detecting means for detecting the magnetic flux is provided. The plate thickness t _{1 of the} steel plate, the attenuation factor Ya of the magnetic flux density at the completion of transformation, the horizontal axis represents the product μ · t of the magnetic permeability μ of the steel plate and the plate thickness t, and the vertical axis represents the magnetic detection element of the magnetic flux detection means. A characteristic curve F, which is the attenuation rate Y of the magnetic flux density at the position, is obtained, and the value of the horizontal axis when the vertical axis Y is Ya in the characteristic curve F is Xa, and the vertical axis Y is the same as the characteristic curve F. transformation rate when the steel plate of the steel grade of plate thickness t ₁ is traveling Value Xb of the horizontal axis corresponding to Yb1 when the attenuation rate of the magnetic flux density obtained from the output of the constant device
1 is calculated, and the steel plate transformation rate at the transformation rate measuring device installation site is calculated and displayed as (Xb1 / Xa) * 100%. In this case, when a steel plate having a plate thickness t _n different from the plate thickness t ₁ travels, a value Xbn on the horizontal axis when the Y-axis becomes the attenuation rate Ybn of the magnetic flux density is obtained, and the steel plate at the transformation rate measuring device installation location is obtained. the transformation ratio _{{Xbn / (Xa × t n} / t 1)} × 100
It can also be calculated and displayed as a percentage.

The transformation rate measuring device according to the present invention is
Magnetic flux generating means for generating magnetic flux on one side of the steel sheet to be inspected and magnetic flux detecting means for detecting magnetic flux leaking through the steel sheet on the other side are respectively provided and detected by this magnetic flux detecting means. In a transformation rate measuring device that measures a transformation rate of a steel sheet by a calculation processing device based on the magnetic flux density, a plate thickness t _{1 of the} steel sheet and a decay of the magnetic flux density at the completion of transformation are calculated in advance in the calculation processing device for a certain selected steel sheet. Rate Ya
And the horizontal axis is the product of the magnetic permeability μ of the steel plate and the plate thickness t μ · t
The characteristic curve F having the vertical axis as the attenuation rate Y of the magnetic flux density at the position of the magnetic detection element of the magnetic flux detecting means is stored, and the horizontal axis when the vertical axis Y is Ya in the characteristic curve F is stored. The value is Xa, and the horizontal axis corresponds to Yb when the vertical axis Y is the attenuation rate of the magnetic flux density obtained from the output of the transformation rate measuring device when the steel sheet of the same steel type and the plate thickness t ₁ travels from the characteristic curve F. The value Xb is calculated and the steel plate transformation rate at the transformation rate measuring device installation location is calculated and displayed as (Xb / Xa) × 100%. Here, determine the abscissa value Xbn when Y axis when the steel different thickness t _n is traveling is the attenuation factor Ybn of the magnetic flux density and the thickness t _1, in transformation rate measuring device location the steel sheet transformation ratio _{{Xbn / (Xa × t n} / t 1)}
× 100% can be calculated and displayed.

In the above-mentioned transformation rate measuring device,
Using an experimental device consisting of the same structure as the transformation rate measuring device, after heating a steel plate having a known plate thickness to a temperature range below the transformation point by a heating furnace, while cooling it, the magnetic flux of the detection part of the magnetic flux detection means after the transformation is completed. Data Ya (T), which was obtained by measuring the temperature characteristics of the density decay rate, was stored, and when online measurement was performed at the measurement point in the cooling zone after hot rolling, the data Ya (T) was measured at the same location as the online measurement device. By inputting the steel sheet temperature T1 and setting the obtained result as the magnetic flux density attenuation rate Ya (T1) at the time of the transformation at the measurement location, the magnetic flux density attenuation rate obtained from this value and the output of the transformation rate measuring device Therefore, the steel plate temperature may be corrected by deriving the steel plate transformation rate at the measurement point. In addition, in the current solid transformation rate measuring device, for the steel types that require transformation rate measurement in the cooling zone after hot rolling, the experiment with the above-mentioned experimental device was conducted, each result was stored, and online measurement was performed. Sometimes, the steel type may be corrected by inputting the steel type of the traveling steel plate.

[0011]

Now, the method of calculating the transformation rate from the sensor output and the method of correcting the transformation rate based on the plate thickness and temperature will be described in detail. FIG. 2 shows the magnetic flux density when the product μ · t of the relative permeability μ as a parameter of the transformation progress of the steel plate and the plate thickness t of the steel plate under the actual machine model is increased by the magnetic field analysis using the finite element method. It is a simulation diagram which calculated | required the attenuation rate of.
As shown in this figure, the relationship between μ and μ · t is approximated by a single curve F. As can be seen from the figure, if the magnetic flux density attenuation rate at 100% transformation is obtained in advance, the transformation rate in the middle can be derived from the magnetic flux density attenuation rate obtained from the sensor output. For example, in FIG. 2, the magnetic flux density attenuation rate Ya at the time of 100% transformation is known for the steel type A having a certain plate thickness t ₁ whose transformation rate is to be determined, and the magnetic flux density attenuation rate Yb at the measurement point is calculated from the transformation rate sensor. Suppose that you have been asked.
Then, by reading the points Xa and Xb on the horizontal axis corresponding to these Ya and Yb, these X and
From a and Xb, the transformation rate can be expressed by (Xb / Xa) × 100%.

Next, in the same steel type A, for a steel plate having a plate thickness different from the plate thickness t _{1 of} , for example, t ₂ , the plate thickness is t ₁
If Ya at the time of is known, Xa obtained from Ya is calculated as (t
_{If the} value obtained by multiplying by ₂ / t ₁ ) is used as the denominator of the transformation rate formula, the transformation rate in the middle can be obtained, and thus the plate thickness can be easily corrected. The attenuation rate characteristic curve F of the magnetic flux density depending on the product of the relative permeability and the plate thickness shown in FIG. 2 is obtained by using an actual machine in addition to the method obtained by the magnetic field analysis using the finite element method under the actual machine model. In offline experiments, the size effect is large enough to be ignored, the steel type and material are the same,
That is, it may be determined by increasing the plate thickness while stacking room temperature sample materials having the same magnetic permeability and measuring the sensor output change with respect to the plate thickness increase.

By the way, the magnetic permeability of the steel sheet after the transformation has a temperature characteristic (Steel Manual, Vol. 1, Basic [The Iron and Steel Institute of Japan: June 20, 1981], page 322), and the above-mentioned 100. The magnetic flux density attenuation rate Ya at the time of% transformation also has temperature dependence. That is, the transformation rate is 100% for a certain steel type and plate thickness.
For example, the steel sheet of No. 6 was transformed at 700 ° C, and
The sensor output is different from that transformed at 00 ° C. Therefore, in the off-line experiment, it is necessary to heat the steel sheet once using a heating furnace or the like, and obtain the temperature characteristic while cooling. However, the point to be noted here is that in the process of cooling after heating above the transformation point,
Since the rolling operation in the actual line is not applied and the cooling rate is different, the material is different from the material of the steel sheet manufactured in the hot rolling line, which affects the transformation rate sensor output. Therefore, in order to prevent the material of the online material from being damaged by the heating and cooling operations during the offline experiment, the above Ya (T) heats the sample to a temperature range below the transformation point (about 730 ° C.) and then excites the sensor. It is better to take out between the coils and measure while cooling.

The magnetic flux density decay rate Y when the transformation rate is 100% for the steel type having the known plate thickness t ₁ sampled in this manner
a (T) is stored as temperature correction data during online measurement of the same steel type. Then, for example, an indication value T of a radiation thermometer installed at the same place as the transformation rate measuring device
When 1 is input, the magnetic flux density attenuation rate Ya (T1) of the online material at the temperature T1 at the transformation rate of 100% is obtained. From this value and the transformation rate sensor output value, the plate thickness correction is performed as necessary by the method as described above, and the transformation rate at the transformation rate sensor installation location is obtained as described above.
The transformation rate measurement is performed for each required steel type, and therefore, the temperature correction data is stored only for the required steel type, and the steel type of the flowing plate is input during the online measurement.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic structural explanatory view showing one embodiment of a transformation rate measuring and measuring apparatus according to the present invention. In FIG. 1, 1
Reference numeral 2 is a steel plate as an object, 10 is a magnetizer, and is composed of an iron core 10a and an exciting coil 10b.
The exciting coil 10b is connected to the exciting power supply 13. In this case, the magnetizer 10 is a steel plate 12 that runs online.
It is installed on the lower side by a predetermined distance d1. The excitation power supply 13 has a built-in low frequency oscillator 14 that oscillates at a frequency f, and an alternating current of the low frequency f is passed through the excitation coil 10b to alternately excite the magnetizer 10. Also,
Reference numeral 1 is a high frequency power supply, 2 is a resistance, 3 is a magnetic sensor for detecting the leakage magnetic flux of the magnetizer 10, and is composed of a sensor core 3a and a detection coil 3b wound around the sensor core 3a, and its output is a magnetic detection circuit 4 Connected to and sent to. The magnetic detection circuit 4 outputs a voltage signal V2 proportional to the detected magnetic flux density based on an input signal from the magnetic sensor 3, and a low pass filter 5 (hereinafter referred to as LPF) high pass filter 6 (hereinafter referred to as HF).
After passing through PF), its output V3 is transmitted to the lock-in amplifier 7.

Here, the magnetic measuring device composed of the high frequency power source 1, the resistor 2, the magnetic sensor 3 and the magnetic detecting circuit 4 is disclosed in Japanese Patent Application Laid-Open No. 1-3089 previously filed by the present applicant.
The same device as that disclosed in "Magnetic measuring method and magnetic measuring device" of Japanese Patent No. 82 is used. That is, the magnetic sensor 3 is configured by winding the detection coil 3b around the sensor core 3a made of a ferromagnetic material, and the constant voltage signal output from the high frequency power supply 1 is supplied to the detection coil 3b through the resistor 2. As a result, the exciting current is constantly flowing. The sensor core 3a is in a state of being excited up to the saturation region. In addition, the magnetic detection circuit 4
Is a positive voltage detector and a negative voltage detector which individually detect a positive voltage and a negative voltage, which are alternately generated based on the high frequency exciting current, at both ends of the detection coil 3b, and output voltages of these two detectors. Is added to obtain the difference voltage.

In the above-mentioned configuration, in the state where there is no magnetic flux in the installation region of the magnetic sensor 3 which is excited to the saturation region, the peak value Vp of the positive voltage and the negative voltage alternately generated at both ends of the detection coil 3b. And -Vn are equal. Therefore, the output obtained by adding the positive and negative DC voltages obtained by detecting the two peak values by the adder becomes zero. However, in this state, when an external magnetic flux is applied to the magnetic sensor 3, the sum of the peak values of the positive voltage and the negative voltage generated at both ends of the detection coil 3b does not change, but the positive and negative peak values Vp and -Vn. There is a difference in the value of. Therefore, if positive and negative DC voltages obtained by detecting these two peak values are added by an adder, the obtained difference voltage (Vp-Vn) becomes a value corresponding to the external magnetic flux density applied to the magnetic sensor 3. In this way, the detection signal V2 corresponding to the magnetic flux density leaked from the traveling steel plate 12 and applied to the magnetic sensor 3 is sequentially output from the magnetic detection circuit 4. The response frequency of the detection signal V2 output from the magnetic detection circuit 4 based on the detection signal of the magnetic sensor 3 is sufficiently higher than the frequency f of the oscillator 14 described above.

The output signal of the magnetic detection circuit 4 is LPF5, HPF6 so as to remove noise components other than the detection signal.
After passing through, it is input to the lock-in amplifier 7. The output signal of the low frequency oscillator 14 is also input to the lock-in amplifier 7 as a synchronization signal. The lock-in amplifier 7 has a built-in synchronous detector and an amplifier, synchronously detects an input signal from the magnetic detection circuit 4 by an output signal of the low-frequency oscillator 14, and outputs an electric signal obtained by amplifying the detected signal. . Then, the voltage value of this output signal becomes a value corresponding to the magnetic flux density attenuation rate of the detection unit, and is transmitted to the arithmetic processing unit 8. Then, in the arithmetic processing unit 8, the attenuation characteristic curve F (see FIG. 2) of the magnetic flux density depending on the product of the above-mentioned steel plate permeability μ and the plate thickness t, and each magnetic flux density attenuation for each steel type A, B, ... The temperature characteristic curves Ya _A (T), Ya _B (T), ... Are stored. Since these parameters can be utilized as needed, the steel type, the plate thickness, the lock-in amplifier output V4, and the steel plate temperature instruction value 11 from the radiation thermometer 9 are input, so that the sensor installation location is finally reached. The steel plate transformation rate of is output.

As described above, in the measuring method and apparatus for measuring the transformation rate of the steel sheet by determining the attenuation rate of the magnetic flux density, by using the saturable magnetic sensor 3 as a magnetic detector, high sensitivity is obtained. A magnetic sensor has been realized, and by using the magnetic sensor, a transformation rate calculation method from a sensor output and a plate thickness and temperature correction method have been easily performed. Therefore, the distance d ₂ between the steel plate 12 and the magnetic sensor 3 can be measured with a distance of 1 m or more, and the influence of the pass line fluctuation of the steel plate 12 can be removed by separating with a distance of 1 m or more. Because,
It has become possible to measure the transformation rate of steel sheets with a high S / N ratio.

[0020]

As described above, according to the present invention, magnetic flux generating means for sandwiching a steel sheet to be tested to generate magnetic flux on one side thereof and detecting magnetic flux on the other side of the magnetic flux leaking through the steel sheet. In the transformation rate measurement in which each of the magnetic flux detection means is provided and the transformation rate of the steel sheet is measured by the arithmetic processing device based on the magnetic flux density detected by the magnetic flux detection means, 100% transformation is performed for the steel sheet whose plate thickness is known for each steel type. Regarding the attenuation rate of the magnetic flux density at that time, the attenuation characteristic of the magnetic flux density with respect to the product of the plate thickness and the magnetic permeability is experimentally measured off-line in advance, so the transformation rate in the middle of 0 to 100% is the sensor output. It has become possible to easily derive from the magnetic flux density attenuation rate that can be obtained more. And applying this measurement method,
In actual online measurement, 100% obtained as a result of inputting the steel plate temperature at the sensor installation place for the steel plate having a known plate thickness that runs in the cooling zone after hot rolling
From the magnetic flux density attenuation rate during transformation and the above-mentioned magnetic flux density attenuation characteristic curve, it is possible to correct the transformation rate with respect to temperature and the sheet thickness. Was given.

[Brief description of drawings]

FIG. 1 is a structural explanatory view showing an embodiment of a transformation rate measuring device according to the present invention.

FIG. 2 is a simulation diagram explaining the operating principle of the transformation rate measuring device of the present invention.

FIG. 3 is a schematic configuration explanatory view of a conventional transformation rate measuring device.

[Explanation of symbols]

 1 high-frequency power supply 2 resistance 3 magnetic sensor 3a sensor core 3b detection coil 4 magnetic detection circuit 5 low-pass filter (LPF) 6 high-pass filter (HPF) 7 lock-in amplifier 8, 28 arithmetic processing unit 9 radiation thermometer 10, 24 magnetizer 10a, 22 iron core 10b, 23 excitation coil 11 thermometer indication value 12, 21 steel plate 13, 25 excitation power supply 14 low frequency oscillator 24a, 24b magnetic pole 26 magnetic detection element 27 thermometer 29 magnetic flux

Claims (6)

[Claims] 1. A magnetic flux generating means for sandwiching a steel sheet to be tested and generating a magnetic flux on one side thereof, and a magnetic flux detecting means for detecting a magnetic flux leaking through the steel sheet by the magnetic flux on the other side, respectively. In a transformation rate measuring method using a transformation rate measuring device for measuring the transformation rate of the steel sheet from the magnetic flux density detected by the magnetic flux detecting means, for a selected steel sheet, the sheet thickness t _{1 of the} steel sheet and completion of transformation Decay rate Ya of the magnetic flux density at that time, the horizontal axis is the product μ · t of the magnetic permeability μ of the steel sheet and the sheet thickness t, and the vertical axis is the decay rate of the magnetic flux density at the position of the magnetic detecting element of the magnetic flux detecting means. The characteristic curve F is defined as Y, and the value of the horizontal axis when the vertical axis Y of the characteristic curve F is Ya is Xa, and the vertical axis Y is the plate thickness of the same steel type from the characteristic curve F. the transformation in the case of steel sheet of t ₁ is traveling Value Xb1 of horizontal axis corresponding to Yb1 when the attenuation rate of the magnetic flux density obtained from the output of the measuring device obtains, the steel transformation rate in the transformation rate measuring device location (Xb1
/ Xa) × 100%, which is calculated and displayed. 2. A transformation rate measuring device for determining a value Xbn on the horizontal axis when the Y axis is the attenuation rate Ybn of the magnetic flux density when a steel sheet having a thickness t _n different from the thickness t ₁ is running. The steel plate transformation rate at the installation location is {Xbn
The transformation rate measuring method according to claim 1, wherein the transformation rate measuring method is calculated and displayed as / (Xa × t _n / t ₁ )} × 100%. 3. A magnetic flux generating means for sandwiching the steel sheet to be tested and generating a magnetic flux on one side thereof, and a magnetic flux detecting means for detecting a magnetic flux leaking through the steel sheet by the magnetic flux on the other side, respectively. In a transformation rate measuring device for measuring a transformation rate of the steel sheet by an arithmetic processing device based on the magnetic flux density detected by the magnetic flux detecting means, for a certain selected steel sheet, the arithmetic processing device previously has a thickness t _{1 of the} steel sheet. And the attenuation rate Ya of the magnetic flux density at the completion of transformation, and the horizontal axis is the product μ of the magnetic permeability μ of the steel plate and the plate thickness t.
A characteristic curve F, where t is the vertical axis, and the vertical axis is the attenuation rate Y of the magnetic flux density at the position of the magnetic detection element of the magnetic flux detection means, and the vertical axis Y is the ya in the characteristic curve F. The value of the horizontal axis of X is Xa, and the vertical axis Y is the attenuation rate of the magnetic flux density obtained from the output of the transformation rate measuring device when a steel sheet of the same steel type with the sheet thickness t ₁ travels from the characteristic curve F. Y
The value Xb on the horizontal axis corresponding to b is obtained, and the steel plate transformation rate at the transformation rate measuring device installation location is given by (Xb
/ Xa) × 100% for calculating and displaying the transformation rate measuring device. 4. A transformation rate measuring device for obtaining a value Xbn on the horizontal axis when the Y axis is the attenuation rate Ybn of the magnetic flux density when a steel sheet having a thickness t _n different from the thickness t ₁ is running. The steel plate transformation rate at the installation location is {Xbn
The transformation rate measuring device according to claim 1, wherein the transformation rate measuring device is calculated and displayed as / (Xa × t _n / t ₁ )} × 100%. 5. An experimental apparatus having the same structure as the transformation rate measuring apparatus is used to heat a steel sheet having a known sheet thickness to a temperature range below the transformation point in a heating furnace, and then cool the magnetic flux after the transformation is completed. The data Ya (T) obtained by measuring the temperature characteristic of the magnetic flux density attenuation rate of the detection part of the detection means is stored, and when online measurement is performed at the measurement point of the cooling zone after hot rolling, the data Ya (T) is online measured. The steel plate temperature T1 measured at the same place as the device is input, and the obtained result is the magnetic flux density attenuation rate Ya at the time of completion of transformation at the measurement place.
The steel plate transformation rate at the measurement point is derived from this value and the magnetic flux density attenuation rate obtained from the output of the transformation rate measuring device by setting (T1). Transformation rate measuring device. 6. For steel grades for which transformation rate measurement is required in the cooling zone after hot rolling, experiments are conducted by the above-mentioned experimental apparatus, each result is stored, and during online measurement, the traveling steel sheet is tested. The transformation rate measuring device according to claim 3, wherein the steel type is corrected by inputting the steel type.