JP3052789B2 - Method and apparatus for measuring crystal grain size of subject by ultrasonic wave and heat treatment method and apparatus using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring the crystal grain size of a subject by ultrasonic waves, which measure the frequency characteristics of ultrasonic attenuation of the subject and measure the crystal grain size based on the characteristics.
In addition, the present invention relates to a method and an apparatus for heat treating a steel sheet or a steel strip using the same.

[0002]

2. Description of the Related Art For example, in ultrasonic flaw detection or ultrasonic thickness measurement of a metal material, ultrasonic waves propagating in a subject are scattered in the propagation process under the influence of crystal grain boundaries in the subject. The degree of this scattering increases as the crystal grain size increases,
The smaller the crystal grain size, the smaller. Therefore, the crystal grain size can be determined by measuring the degree of scattering at the crystal grain boundaries. In addition, since the crystal grain size has a close relationship with various material properties including the strength of the material, the material properties can be evaluated by obtaining the crystal grain size, so that the crystal grain size can be accurately obtained. This is of great industrial importance.

Conventionally, there have been proposed a number of methods for measuring the crystal grain size of an object using ultrasonic waves. For example, an ultrasonic probe is attached to the surface of a subject via a delay material such as a buffer, and transmits an ultrasonic pulse to the subject, and the surface (S) echo reflected from the subject surface and the bottom surface of the subject The amount of echo attenuation is determined from the degree to which the intensity of the bottom (B) echo that is multiple-reflected many times attenuates. Then, the ultrasonic diffusion attenuation included in the attenuation of the ultrasonic pulse is subtracted to determine the ultrasonic attenuation of the subject.

In general, the attenuation amount α (f) of an ultrasonic wave transmitted through an object is Rayleigh scattering when the wavelength of the ultrasonic wave is larger than the crystal grain size in the object. It is approximated by a theoretical equation such as equation (1). α (f) = s ・ D3 ・ f4 + c ・ f (1) where the first term represents scattering attenuation by crystal grains and the second term represents energy absorption in the material. The crystal grain size D in the theoretical equation can be obtained by approximating the ultrasonic attenuation expressed as described above to the ultrasonic attenuation obtained above.

[0005] Here, the ultrasonic diffusion attenuation is determined by using a contrast test specimen whose ultrasonic attenuation is measured in advance, or by ultrasonic diffusion attenuation in a long-distance sound field. There is a method that utilizes the relationship that the distance is approximately proportional to the logarithm of the distance (Japan Non-Destructive Inspection Association Standard, NDIS
2415-87, Measurement and display method of ultrasonic attenuation coefficient of solid by ultrasonic pulse reflection method). However, in the method using the contrast test piece, it is necessary to prepare a contrast test piece having the same shape as that of the subject, so that it takes a lot of time and effort to prepare the contrast test piece, and is not practical.

Further, since the measurement of the attenuation by the ultrasonic wave varies considerably depending on the acoustic contact state of the ultrasonic probe, it is necessary to make the acoustic contact state of the contrast test piece and the subject the same, and accurate measurement is required. Is difficult. In addition, the method of measuring the ultrasonic diffusion attenuation in a far field requires a large selection of the probe diameter and the distance to the subject, which has many limitations, and is not practical because the degree of freedom of the measurement is small. . Also, the ultrasonic attenuation of the subject is defined by a specific frequency,
Since the actual ultrasonic pulse contains various frequency components, the ultrasonic attenuation measurement method does not represent the ideal single-frequency ultrasonic attenuation, but the ultrasonic attenuation of the subject. Cannot be determined with high accuracy.

[0007] In order to solve such inconveniences, there has been proposed a method of obtaining the amount of ultrasonic attenuation of a subject in consideration of ultrasonic diffusion attenuation, attenuation on the surface of the subject, and the frequency of ultrasonic waves (particularly). JP-A-58-160865). In this method, an ultrasonic pulse is applied to a subject, three or more ultrasonic echoes from the subject are detected, each ultrasonic echo is frequency-analyzed, and each of the three From the amount of ultrasonic attenuation, considering the frequency characteristics of ultrasonic diffusion attenuation and the frequency characteristics of attenuation on the subject surface,
The ultrasonic attenuation, ultrasonic diffusion attenuation, and attenuation on the surface of the subject are independently obtained by solving simultaneous equations. Here, the ultrasonic attenuation of the subject is proportional to the second or fourth power of the frequency, the ultrasonic diffusion attenuation is approximately proportional to the −1 power of the frequency, and the attenuation at the surface is approximately １ ／ of the frequency. It uses a relationship that is proportional to the power.

However, the above-mentioned method (Japanese Patent Laid-Open No.
JP-A-160865) is an approximation method in a far-field sound field with respect to the treatment of ultrasonic diffusion attenuation, and the obtained calculation result includes an error. There are various factors as to the attenuation of the ultrasonic wave in the subject, but only the scattering attenuation is considered. Also, the attenuation on the surface of the subject is not necessarily proportional to the half power of the frequency, but includes various factors, and the calculation result includes an error. Therefore, even with this method, it is impossible to accurately determine the amount of ultrasonic attenuation of the subject.

In order to solve such a problem, a measuring method as disclosed in JP-A-5-333003 has been proposed. This is based on the reflection echo of the ultrasonic pulse on the bottom surface of the subject, the first bottom reflection echo (B1 echo) and the second reflection on the bottom surface of the subject as ultrasound pulses before and after transmission of the subject. The bottom surface reflected echo (B2 echo) is frequency-analyzed to determine the frequency characteristics of ultrasonic pulse attenuation, and the ultrasonic diffusion decay frequency characteristics are calculated using the positional relationship between the measurement system and the subject. The frequency characteristics of the attenuation correction amount are calculated by adding the ultrasonic diffusion attenuation and the ultrasonic attenuation at the subject boundary and the ultrasonic attenuation at the subject surface, and the above-described calculation is performed from the frequency characteristics of the ultrasonic pulse attenuation. The final frequency characteristic of attenuation in the subject is obtained by subtracting the frequency characteristic of the attenuation correction amount.

Further, the ultrasonic attenuation obtained by the above-mentioned method is based on the following theoretical equation since Rayleigh scattering is established when the wavelength of the ultrasonic wave is larger than the crystal grain size in the subject. Can be approximated by α (f) = s ・ D3 ・ f4 + c ・ f By fitting this approximate expression α (f) to the calculated ultrasonic attenuation, the crystal grain size of the subject can be obtained. Further, the frequency range in which the above-mentioned approximation formula α (f) is fitted to the ultrasonic attenuation is observed every time a person who measures the frequency characteristic state of the calculated ultrasonic scattering attenuation repeats the measurement, Determine the range. Note that the positional relationship between the subject and the probe is such that the measurement is performed in a place with a good measurement environment such as a laboratory, and the subject and the probe are fixed after adjusting the distance horizontally and performing horizontal adjustment. I do.

[0011]

When the above-mentioned measuring method (Japanese Patent Application Laid-Open No. 5-333003) is applied to an on-line system such as a rolling or heat treatment line in which a sample flows continuously and continuously, it is difficult to use the measuring method. It is necessary to ensure measurement accuracy for fluctuations such as deflection and undulation of the sample and external electric noise, and to simplify maintenance and maintenance. For example, the transducer of the probe has a certain finite area, and transmission and reception of ultrasonic waves are performed on the entire transducer surface. Therefore, it is necessary to keep the transducer surface of the probe and the surface of the subject horizontal. However, in online measurement, the undulation and deflection of the subject, changes in the horizontality with the probe due to the movement of the subject, and changes in the thickness of the subject over time are cited. Can be Further, in order to ensure the measurement accuracy in consideration of the installation space of the device and the ease of maintenance, the following problems are pointed out in the conventional technology.

First, when the horizontality between the surface of the subject and the transducer surface of the probe fluctuates, the approximate polynomial fitting of the ultrasonic scattering attenuation α (f) shown above results in A large variation occurs (for example, see FIG. 17). Second, repeated automatic on-line automatic measurement requires repeated measurements.However, since it is impossible for a measurer to constantly observe the frequency characteristics of the ultrasonic scattering attenuation, the above-described approximate polynomial and approximate It is necessary to uniquely determine the frequency range to be used. Third, in continuous on-line quality control of the subject, it is also important to measure the thickness of the subject simultaneously with the measurement of the crystal grain size of the subject. In such a case, it is necessary to install separate hardware such as an ultrasonic thickness gauge, which results in excessive equipment.

Fourth, in measuring the crystal grain size of the test object, the first bottom reflection echo (B1 echo) and the second bottom reflection echo (B2 echo) are each subjected to frequency analysis, and the difference is obtained. By determining the amount of attenuation during the reciprocation of the ultrasonic pulse in the subject by doing this, in the case where the continuously moving subject is automatically measured continuously online,
Subjects vary from thin to thick.
The crystal grain size of the subject also varies in various ways. Here, assuming that the crystal grain diameter of the subject is constant, the ultrasonic scattering attenuation increases as the thickness of the subject increases, and the difference between the frequency characteristics of the B1 echo and the B2 echo increases. . Also, when the thickness is reduced, the amount of ultrasonic scattering attenuation is reduced, and the difference in amplitude is reduced. On the other hand, even when the wall thickness is constant, as the crystal grain size increases, the amount of ultrasonic scattering attenuation increases, and the difference in frequency characteristics increases.

In general, in online measurement, since the waveform of the bottom surface multiple reflection echo obtained by a change in the properties of the subject slightly changes, the state of the frequency characteristics of the B1 echo and B2 echo obtained by analyzing the frequency of the waveform is also delicate. And the difference between the two frequency characteristics also has a slight variation. Here, the echo for which the frequency analysis of the waveform is performed is uniquely determined as the B1 echo and the B2 echo because the ultrasonic scattering attenuation is small when the thickness of the subject is small and the crystal grain size is small. Becomes smaller, the difference between the two frequency characteristics falls within the range of the variation in the frequency characteristics, and the difference between the frequency characteristics cannot be calculated, or becomes smaller than the frequency characteristic of the attenuation correction amount calculated in advance, In some cases, it becomes impossible to calculate the crystal grain size.
In addition, when the thickness of the specimen is large and the crystal grain size is large, the echo having a large number of reflections has a small amplitude due to the attenuation of the material, thereby deteriorating the SN ratio, affecting the frequency analysis, and deteriorating the measurement accuracy. Resulting in.

Fifth, when the probe is fixed against the movement of the subject, the measurement results are affected by changes in the level of the probe and the surface of the subject due to the undulation or bending of the subject. Will affect you. In such a case, a sensor or the like is attached to a jig for fixing the probe to monitor the state of the surface of the subject, and a follow-up mechanism for keeping the vibration surface of the probe and the surface of the subject parallel constantly. Must be provided.

FIG. 18 is a diagram showing an example of the configuration of a measuring jig having the above-mentioned conventional tracking mechanism. Subject 1 and probe 3
Between them, water 20 is supplied as delay material 3. The probe 3 is fixed to a fixing jig 21, and a distance sensor 22 is attached to the fixing jig 21. The distance between the probe 3 and the subject 1 is measured, and the probe 3 It is output to the horizontality calculating device 23 for calculating the horizontality with the child. The horizontality calculation device 23 performs a predetermined calculation process based on the output of the distance sensor 22 and sends a control signal to the cylinder control device 24. The cylinder controller 24 adjusts the stroke of the cylinder 25 based on the control signal, and
Is held horizontally with respect to the subject 1, so that the vibration surface of the probe 3 and the surface of the subject 1 are kept horizontal. Then, a signal from the probe 3 is extracted via the signal cable 26. However, the provision of such a mechanism makes the structure of the jig large and complicated, requires a large space for its installation, and causes problems such as a large remodeling of existing equipment.

Sixth, in the case where two bottom echoes are selected from the bottom multiple reflection echoes of the subject and frequency analysis is performed, adjacent echoes must be clearly recognized.
In other words, if the number of waveforms in the time axis direction of the bottom surface reflected echo is large, it is difficult to recognize each of the adjacent bottom surface reflected echoes, and there is a restriction that the thickness of the object to be measured must be large. In addition, a large number of echo waveforms results in a narrow probe band for transmitting and receiving ultrasonic waves, and a narrow frequency range in which an approximate polynomial can be fitted with respect to its frequency characteristics, resulting in a large variation in measurement accuracy. turn into. Seventh, regarding the material of the transducer of the probe used for measurement, a probe using a piezoelectric element as the transducer generally has poor electric-vibration conversion efficiency and low sensitivity. Therefore, the SN ratio of the bottom surface reflection echo tends to deteriorate. Further, there is a problem that the size of the probe is increased and the life is short.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is simple and highly accurate to measure the crystal grain size of a subject, and at the same time, the thickness of the subject is determined by the ultrasonic thickness. It is an object of the present invention to provide a method and an apparatus for measuring the crystal grain size of a subject by using an ultrasonic wave capable of performing measurement without requiring hardware such as a meter. Another object of the present invention is to provide a method and an apparatus for heat treatment of a steel sheet or a steel strip using the above-described method and apparatus for measuring the crystal grain size of a subject by ultrasonic waves.

[0019]

According to the method for measuring the crystal grain size of an object by ultrasonic waves according to the present invention, an ultrasonic pulse is transmitted to the object, and a multiple reflection echo train from the bottom surface of the object is formed. The frequency characteristics of the two reflected echoes are analyzed and subtracted to obtain a frequency characteristic due to the attenuation of the ultrasonic pulse, and a predetermined correction process is performed on the frequency characteristic to obtain the frequency characteristic of the ultrasonic scattering attenuation of the subject. In the method of obtaining the crystal grain size of the subject by applying the Rayleigh scattering equation to the frequency characteristic of the ultrasonic scattering attenuation, only the scattering attenuation term is applied when the Rayleigh scattering equation is applied. The frequency range in which the amount of ultrasonic scattering attenuation is approximated is defined as the peak frequency in the frequency characteristic of the second reflected echo of the two reflected echoes on the bottom surface of the subject, and the two frequencies that take the half value of the peak. It is set between the frequency of the high-frequency side of the two values of numbers. For example, the frequency range for fitting an approximate polynomial to the ultrasonic scattering attenuation amount α (f) is set between the peak frequency in the frequency characteristic of the second reflection echo on the bottom surface of the subject and the upper frequency range in the range of −6 dB. I do. Further, the ultrasonic scattering attenuation amount α
A polynomial approximating (f) is defined as shown in equation (2). α (f) = sD3f4 (2)

Further, according to the method for measuring the crystal grain size of an object using ultrasonic waves according to the present invention, when the ultrasonic wave scattering attenuation caused by two reflected echoes is less than the permissible measurement level in the above-described measuring method, Two different bottom surface reflection echoes satisfying the measurement allowable level are automatically selected from the multiple reflection echo train of the sample. For example, the frequency characteristic of B1 echo and B2
In the case where the frequency characteristic of the ultrasonic pulse calculated by the difference with the frequency characteristic of the echo is smaller than the frequency characteristic of the attenuation correction amount calculated in advance in the frequency range defined above or does not reach the measurement allowable level. Calculates the crystal grain size again for another two echoes that satisfy the measurement allowable level.

In the method for measuring the crystal grain size of an object by ultrasonic waves according to the present invention, a time difference between a reflected echo from the surface of the object and a reflected echo from the bottom surface of the object is measured in the above-described measuring method. The thickness of the subject is measured based on the time difference. For example, the time difference between the start point of the echo reflected on the surface of the subject and the start point of the echo reflected on the bottom surface of the subject is measured, and the thickness of the subject is determined based on the time difference and the sound speed of the subject stored in advance. Measure the length. In the method for measuring the crystal grain size of a subject by ultrasonic waves according to the present invention, the subject is a steel plate or a steel strip in the above-described measuring method.

Further, the apparatus for measuring the crystal grain size of an object using ultrasonic waves according to the present invention comprises: an ultrasonic transmitting / receiving means for transmitting an ultrasonic pulse to the object via a probe and receiving a reflected wave;
The gate means for extracting two reflected echoes from the multiple reflected echo trains from the bottom surface of the subject included in the output of the ultrasonic transmitting / receiving means, and the two reflected echoes are subjected to frequency analysis and subtraction, respectively, thereby obtaining an ultrasonic wave. The frequency characteristic due to the pulse attenuation is obtained, and a predetermined correction process is performed on the frequency characteristic to obtain the ultrasonic scattering attenuation frequency characteristic of the subject. By applying the scattering attenuation term of the formula, the frequency range in which the ultrasonic scattering attenuation is approximated is defined as the peak frequency in the frequency characteristic of the reflection echo on the bottom surface of the subject and the two values of the two frequencies at which the peak is reduced by half. A crystal grain size calculating means for calculating a coefficient of the equation by setting the coefficient between the high frequency side and the crystal grain size of the subject based on the coefficient.

Further, according to the present invention, there is provided an apparatus for measuring the crystal grain size of an object using ultrasonic waves, wherein the ultrasonic scattering attenuation caused by the two reflected echoes is less than an allowable measurement level. And an ultrasonic attenuation calculator for controlling the gate means to select a reflected echo until two different reflected echoes satisfying the measurement allowable level are obtained. Further, the apparatus for measuring the crystal grain size of an object by ultrasonic waves according to the present invention is the same as the above-mentioned measuring apparatus, wherein the fixing jig is fixed via a fixing jig to which the probe is fixed and a flexible joint rotatable in the horizontal direction. And three or more wheels respectively connected to the tool, and the plurality of wheels are configured to be able to freely change their directions. Further, according to the apparatus for measuring the crystal grain size of an object by ultrasonic waves according to the present invention, in the above-mentioned measuring apparatus, the probe is made of a polymer material, and the upper and lower frequency difference with respect to the intensity 6 dB lower than the center frequency intensity. Has a wide band such that the characteristic becomes 80% or more of the center frequency.

Further, in the apparatus for measuring the crystal grain size of an object by ultrasonic waves according to the present invention, in the above-mentioned measuring apparatus, the object is a steel plate or a steel strip. In addition, the continuous annealing apparatus for a steel sheet or a steel strip according to the present invention includes the above-described crystal grain size measuring apparatus for the specimen at a stage subsequent to the annealing furnace.

Further, in the method for heat-treating a steel sheet or a steel strip according to the present invention, in the continuous heat-treatment step of a hot-rolled or cold-rolled steel sheet or a steel strip, the steel sheet or the steel strip before the heat treatment is subjected to ultrasonic scattering by the above-described method. Measure the amount of attenuation and thickness, further measure the grain size and thickness of the heat-treated steel sheet or steel strip by the above-described method, and, based on these measured values, heat-treated steel sheet or steel strip. The temperature in the furnace of the heat treatment furnace and / or the passing speed of the steel sheet or the steel strip is controlled so that the crystal grain size of the strip falls within a predetermined range.

The heat treatment apparatus for a steel sheet or a steel strip according to the present invention comprises: the above-described apparatus for measuring the crystal grain size of a specimen, which is installed before and after a heat treatment furnace for a hot-rolled or cold-rolled steel sheet or a steel strip; A furnace internal temperature measuring device, a steel sheet or steel strip passing speed measuring device, and a heat treatment furnace internal temperature and / or a steel sheet or steel strip passing speed control device, further comprising a crystal grain size The crystal grain size and sheet thickness signal from the measuring device, and the calculation based on the in-furnace temperature signal of the heat treatment furnace and the passing speed signal of the steel plate or the steel strip, the crystal grain size of the steel plate or the steel strip after the heat treatment is An arithmetic unit is provided for generating a signal for operating the furnace internal temperature of the heat treatment furnace and / or the sheet passing speed control device for the steel plate or the steel strip so that the value falls within a predetermined range.

[0027]

Next, a description will be given of the background in which the measurement accuracy is improved by using the above-described method and apparatus for measuring the crystal grain size of the subject using ultrasonic waves. << Frequency approximation range >> From the calculation result of the ultrasonic scattering attenuation amount α (f), regarding the frequency range to be used for fitting the approximate polynomial shown in the equation (1), the fitting range is determined by the frequency characteristic of the echo. Since the frequency characteristics greatly vary depending on the crystal grain size of the test object, the determination must be made based on the measured values. The optimal frequency range is considered to be a range in which the approximate polynomial shown in Expression (1) holds. FIG. 19 shows the result of examining the relationship between the frequency range in which the ultrasonic scattering attenuation amount α (f) of various subjects is a quartic function of frequency and the frequency characteristic of the B2 echo. Here, as for the frequency range having a quartic function, α (f) is expressed by log-logarithm as shown in FIG.
Was determined by visual observation. FIG.
The range of the frequency from the peak frequency of the frequency characteristic of the B2 echo to −6 dB on the high frequency side is 4% in almost all the subjects.
It can be seen that this corresponds to the range where the following function holds.
From this result, the frequency range for calculating the crystal grain size is B2
It can be seen that the frequency should be within a range of −6 dB on the high frequency side from the peak frequency of the echo.

<< About Approximate Polynomial >> FIG. 21 shows the fitting of the approximate polynomial (1) and the crystal grain size visually determined by a microscope of a microphotograph to the ultrasonic scattering attenuation amount α (f) in various sample samples. FIG. 22 shows the relationship between the visual grain size and the crystal grain size obtained by fitting the approximate polynomial (2). From these results, it is understood that the use of the equation (2) ignoring the energy absorption term does not affect the measurement accuracy of the crystal grain size.

<< Automatic Selection Function of Echo Performing Frequency Analysis >> Particularly when the crystal grain size and thickness of the subject are small, the effect of ultrasonic scattering attenuation is small. The difference between the amplitudes of the B2 echoes is small. If this difference falls within the range of variation in the amplitude of the echo, the particle size cannot be calculated even if the difference between the frequency characteristics of the B1 echo and the B2 echo is obtained (FIG. 2).
3). Similarly, if the frequency characteristic of the difference becomes smaller than the frequency characteristic of the attenuation correction amount calculated in advance, the particle size cannot be calculated. In such a case, it is necessary to ensure the measurement accuracy by making the influence of the ultrasonic scattering attenuation larger, such as B2 echo and B4 echo or B3 echo and B6 echo, than the adjacent echoes. However, the crystal grain size of the subject is large,
When the wall thickness is large, it is sufficient to take the difference between the normal B1 echo and the normal B2 echo (FIG. 24). Therefore,
For online automatic measurement, a function for automatically selecting an echo to be subjected to frequency analysis is required.

<< Thickness Meter >> In the present invention, two reflected echoes on the bottom surface of the subject for which frequency analysis is performed are shown in FIG.
Has a gate setting function for selecting from multiple reflected echo trains from the subject as shown in FIG. In the process of setting the gate, it is necessary to recognize the start position of the first reflection echo from the bottom surface of the subject, and the time difference between this start position and the rising position of the reflected echo from the subject surface recognized in advance, The thickness can be easily obtained, and no additional hardware is required for measuring the thickness.

<< Measurement Head >> Usually, in a rolling line or a heat treatment line in a factory or the like, the subject is formed into a steel strip shape and is continuously passed. There are factors such as subtle undulations and deflections that disturb the horizontality of the probe and the surface of the subject in the subject in the passing plate. For such disturbances, a sensor or the like that detects the level of the surface of the subject on the probe side is installed, and a tracking mechanism that allows the probe to follow a delicate tilt of the subject is installed. It must be provided (see FIG. 18). However, its installation space,
When it is difficult to install the probe fixing jig having the above functions due to the cost, a jig having a simple and small mechanism is required. Here, a plurality of wheels are attached to a jig as shown in FIG. 7 described later, so that the jig travels on the surface of the subject, and the wheels and the jig body are connected by a joint that can be rotated horizontally. Accordingly, it is possible to follow even when a force in the vertical direction is applied to the traveling direction. In addition, the probe and the surface of the subject are constantly kept horizontal by fixing the jig body with something like a chain or a coil spring so that the jig body does not move far from the measurement point. Then, the traveling position of the measuring jig can be freely changed (FIG. 8).

<< Types of Probe >> A probe using a vibrator material as a polymer material is smaller and lighter in a high ultrasonic frequency range than a probe using a conventional ceramic material. It is efficient in terms of electrical signal-to-vibration conversion efficiency, has high sensitivity in transmitting and receiving ultrasonic waves, and is excellent in SN. Further, it is advantageous because the life is long. << About the Band of the Probe >> In the present invention, the frequency analysis of the adjacent B1 and B2 echoes is performed from the bottom surface multiple reflection echo train of the subject, so that the echo for which the frequency analysis is performed is accurately extracted from the waveform of the echo train. Have to do it. For this purpose, it is desirable that the interval between adjacent echoes is wide. In other words, in order to increase the interval between echoes, the smaller the wave number of the echo, the smaller the thickness of the subject can be measured. Further, as the number of sampling data in the range where the approximate polynomial is fitted to the ultrasonic scattering attenuation is increased, the variation in the measurement accuracy can be suppressed.

In the present invention, by providing the above-mentioned particle size measuring device for the specimen at the subsequent stage of the annealing furnace, the average crystal grain size and the sheet thickness in the sheet thickness direction of the steel strip immediately after passing through the annealing furnace are determined. The measurement can be performed continuously, and it can be determined whether or not an accurate heat treatment has been performed. This makes it possible to guarantee mechanical properties by the crystal grain size over the entire length of the steel strip. Further, when a heat treatment defective portion occurs, the defective portion can be grasped very accurately, and the heating range and temperature at the time of the re-heat treatment can be accurately determined.

In the present invention, the above-described crystal grain size measuring device for the subject is disposed before and after the heat treatment furnace.
The ultrasonic attenuation and thickness of the steel strip before entering the heat treatment furnace and the average grain size and thickness in the thickness direction of the steel strip immediately after passing through the heat treatment furnace are continuously measured, and the measurement information is obtained. Based on the above, if the furnace temperature of the heat treatment furnace and / or the steel sheet passing speed, or one of them, is corrected in advance by the material of the material to be heat-treated, the surface texture, the dimensions, etc. The crystal grain size falls within a predetermined range. Specifically, the set value of the combustion temperature of the heat treatment furnace was corrected by the ultrasonic attenuation in the thickness direction of the steel strip before entering the heat treatment furnace and the measured value of the sheet thickness, and the steel immediately after passing through the heat treatment was corrected. The set value of the sheet temperature is corrected based on the average crystal grain size and the sheet thickness of the strip in the sheet thickness direction. Since the control of the sheet temperature is performed by adjusting the sheet speed of the steel strip passing through the heat treatment furnace,
As a result, the set value of the sheet speed is corrected by the average crystal grain size and the sheet thickness in the sheet thickness direction of the steel strip immediately after passing through the heat treatment furnace.

[0035]

【Example】

Embodiment 1 FIG. FIG. 1 is a block diagram showing a schematic configuration of an apparatus for measuring a crystal grain size of an object using ultrasonic waves according to one embodiment of the present invention. An ultrasonic probe 3 is mounted on a surface 1 a of a subject 1 via a delay member 2. Delay material 2 has a thickness of 10
mm, water having a sound speed of 1480 m / s, and the ultrasonic probe 3
Is fixed to a fixture (see FIG. 17) that can travel on the subject 1. The ultrasonic probe 3 is a broadband type having a nominal frequency of 20 MHz.

The ultrasonic transmission / reception unit 4 is composed of, for example, a pulsar receiver and the like, sends a pulse signal to the ultrasonic probe 3, receives an echo signal from the ultrasonic probe 3, and converts it into an electric signal. To the gate section 5. The gate unit 5 is composed of, for example, a digital oscilloscope for converting an electric signal from analog to digital and an echo waveform extractor. It has a sampling frequency of 100 MHz and the number of sampling points of one echo waveform is 512. Then, as shown in FIG. 2, the bottom (B1) echo reflected on the bottom surface 1 b of the subject 1 included in the electric signal from the ultrasonic transceiver 4, reflected on the bottom surface and reflected on the subject surface again The bottom (B2) echo reflected by the bottom is extracted. The specific method of extracting the echoes is between the surface reflection (S) echo of the subject and the B1 echo, and between the B1 echo and the B2 echo.
Since there is a portion where a certain signal level is not detected between the echoes, each echo is separated and extracted using this portion. Each read echo data is sent to the next ultrasonic pulse attenuation calculating unit 6.

The ultrasonic pulse attenuation calculating section 6 has, for example, F
A built-in FT (high-speed frequency converter) calculates the frequency characteristics of the input B1 echo and B2 echo as shown in FIG. 3, and calculates the difference between the frequency characteristics at each frequency f as shown in FIG. Attenuation of ultrasonic pulse αm
The frequency characteristic of (f) is calculated. Subject correction amount calculation unit 7
Calculates the ultrasonic attenuation at the boundary surface of the subject 1 and the diffusion attenuation due to the spread of the ultrasonic beam, and adds the respective attenuation amounts to calculate the attenuation correction amount αc (f). The calculated attenuation correction amount αc (f) is stored in the next attenuation correction amount storage unit 8.
FIG. 5 shows the frequency characteristics of the calculated attenuation correction amount αc (f), and FIG. 4 shows the frequency characteristics of the ultrasonic pulse attenuation amount αm (f) calculated based on the measurement.

The ultrasonic attenuation check unit 9 checks whether the particle diameter can be calculated based on the ultrasonic pulse attenuation calculated by the measurement. Specifically, when calculating the ultrasonic scattering attenuation amount to be performed next, the difference between the attenuation amount αm (f) of the ultrasonic pulse and the attenuation correction amount αc (f) is calculated. Am (f) of B
Attenuation correction amount αc in a frequency range of −6 dB from the peak frequency to the high frequency side in the frequency characteristic of the two echoes
When the level is lower than (f), it is determined that the particle size cannot be calculated because the ultrasonic scattering attenuation is small, and the function is performed again. When retrying, the gate unit 5 extracts the B2 echo and the B4 echo, and again calculates the attenuation amount αm (f) due to the ultrasonic pulse. And
It is checked whether the particle diameter can be calculated from the attenuation correction amount αC (f) of the B2 and B4 echoes stored in the attenuation correction amount storage unit 8. Also, if this is no good, B3 again
-Extract the B6 echo and repeat the above process.

If the ultrasonic attenuation check unit 9 determines that the particle size can be calculated, the ultrasonic attenuation calculation unit 10
The process proceeds to. Here, the attenuation amount of the ultrasonic pulse αm
By subtracting the attenuation correction amount αc (f) from (f), the frequency characteristic of the attenuation amount α (f) due to the scattering attenuation of the subject 1 as shown in FIG. 6 is calculated. The crystal grain size calculation unit 11 fits the approximate polynomial defined by the above equation (2) to the ultrasound scattering attenuation amount α (f) calculated by the ultrasound attenuation amount calculation unit 10 in the above frequency range. As a result, the coefficient s · D4 of the equation (2) is obtained. Since the coefficient s is known in advance by the configuration, the crystal grain size D can be obtained. At this time, s was 0.8324.

The thickness calculating unit 12 has a built-in storage device for storing the sound speed of the subject 1, and among the echoes from the gate unit 5, the starting point of the echo reflected on the surface of the subject 1, A time difference between the start point of the echo reflected from the bottom surface of the subject 1 and the sound velocity of the subject 1 are measured, and the thickness of the subject 1 is continuously measured.

FIG. 7 is a view showing an example of the configuration of a measuring jig having a mechanism for following the ultrasonic probe shown in FIG. 1, and FIG.
Is a side view, and (B) is a front view. The probe 3 is attached to the fixing jig 21, and a plurality of (for example, three or four)
) Wheels 30 are respectively mounted via joints 31. The joint 31 has a mechanism capable of rotating horizontally, and accordingly, each wheel 30 is supported so that its traveling direction can be changed. In order to prevent the probe 3 from moving away from the measurement point, the fixing jig 21 is connected by a chain 32, and the transducer surface of the probe 3 and the surface of the test piece 1 are constantly horizontal. The running position of the measuring jig can be freely changed so as to maintain the degree.

FIG. 8 is a diagram for explaining the operation of FIG. 7, and FIG. 8A shows the operation when the surface of the test piece 1 has a concave portion when viewed from the side, and FIG. () Shows the operation in the case where the surface of the test material 1 forms a projection when viewed from the front. In each case, it can be seen that the transducer surface of the probe 3 and the surface of the test material 1 are parallel.

FIGS. 9 and 10 show a measurement in which the measuring apparatus of the above-described embodiment is applied to a continuous annealing and pickling line for thin sheets, and the crystal grain size of a continuously passed specimen is continuously measured. It is a figure showing a result. In FIG. 9, the threading speed is 20
m / min, and the thickness of the subject was 1.5 mm. In this case, since the size of the crystal grain size varies depending on the furnace temperature of the annealing furnace and the number of times of annealing, a different grain size value is obtained for each lot of the test object, and it can be seen that the measured value is in accordance therewith.

Also, in FIG. 10, the thickness of the plate as the subject 1 changes rapidly, and when the furnace temperature of the annealing furnace is constant, the subject becomes thicker, so that the heat capacity of the subject increases and the crystal becomes larger. It can be observed that the crystal grain size value becomes smaller due to the smaller grain growth, and thereafter, the measured value of the crystal grain size becomes larger to increase the furnace temperature. FIG. 11 is a diagram showing a result of continuously measuring the thickness of the subject by the thickness calculation unit 12 in the above line. According to this measurement result, it is possible to observe a state in which the thickness of the object to be passed changes.

Embodiment 2 FIG. FIG. 12 is a block diagram showing a configuration of a continuous annealing furnace according to another embodiment of the present invention. , 21 is a steel strip, 22 is a heat treatment furnace, 23 is a cooling device,
24 is a bridle roll, 25 is a crystal grain size measuring device, 2
6 is an acid pickling device, 29a, 29b and 29c are combustion burners 30a, 30b and 30c, an in-furnace thermometer, 31 is a plate thermometer,
32 is a plate speed meter, 33a, 33b and 33c are flow rate control valves, 34a, 34b and 34c are fuel flow rate meters, 35
a, 35b, 35c are temperature controllers, 36 is a plate temperature controller,
37 is a plate speed controller, and 38 is an arithmetic and control unit. The crystal grain size measuring device 25 used here is the crystal grain size device of the first embodiment (FIGS. 1 and 7) and the cooling device 23.
Steel strip 2 that is located downstream of
The average crystal grain size and the thickness in the thickness direction of No. 1 are continuously measured.

FIG. 13 is a view showing the result of measurement of the crystal grain size by the crystal grain size measuring device 25. FIG. 7A shows an example of a normal operating section, and FIG. 8B shows an example in which an annealing failure occurs. B1 and B2 in this (B) are portions where the annealing failure occurred. Re-annealing treatment is performed on these two points. As a result, a uniform particle size is obtained as shown in FIG.

As described above, according to the present embodiment, slight annealing defects, which have not been recognized until now, can be recognized, and variations in mechanical properties in the steel strip can be suppressed to an extremely low level. Now you can. In addition, even for the apparently poorly-annealed material, the re-heat treatment of only the defective portion became possible instead of the conventional full-length re-heat treatment, so that it was possible to reduce the unit fuel consumption and homogenize the entire length of the steel strip. For this reason, equipment costs such as a conventional loop car for providing a measuring time are not required in terms of equipment, which is very economical and easy to install. Further, the time and labor for sampling and the like are not required, the process is shortened, and the coil can be smoothly transferred to the next process. In addition, since there is no need to collect a sample from a non-defective part of the coil,
mm, about 0.3% for a steel strip with a width of 1000 mm
The yield improvement was achieved.

Embodiment 3 FIG. FIG. 14 is a block diagram showing a configuration of a steel strip heat treatment apparatus according to another embodiment of the present invention.
In the figure, reference numeral 42 denotes a crystal grain size measuring device arranged on the side of the heat treatment furnace, which also has the same configuration as that of the first embodiment. 43 is a drying device, 44 is a loop car, and 45 is a winding device. Other configurations are the same as those of the second embodiment. FIG. 14 shows in detail a processing line from the crystal grain size measuring device 42 arranged on the entrance side to the crystal grain size measuring device 25 arranged on the exit side. It is omitted in the range without.
Further, in the combustion control system of the heat treatment furnace 22, a combustion burner and an in-furnace thermometer are actually installed at the upper and lower portions of the steel strip 21, but they are omitted. As for the air flow rate, a signal obtained by receiving a fuel gas flow rate signal and calculating an air-fuel ratio based on the signal is controlled as a flow rate setting signal, but this is also omitted. In the flow control system, the flow measurement signal is fed back to the flow controller, but is omitted.

First, the flow of the steel strip 21 will be described. Steel strip 2
Reference numeral 1 denotes a drying device for measuring the thickness and ultrasonic attenuation before heat treatment by a crystal grain size measuring device 42 located on the entrance side, and for removing water as an acoustic medium used in the crystal grain size measuring device 42. After passing through 43, it enters the heat treatment furnace 22 where a predetermined heat treatment is performed. After passing through the heat treatment furnace 22, the cooling device 2
3, cooled, passed through a bridging roll 24 for controlling the speed and tension of the steel strip 21, and subjected to a heat treatment with a grain size measuring device 25 disposed on the exit side. Measure the crystal grain size. Then, the oxidized scale on the surface is removed by the pickling device 26, and the coil after being wound by the winding device 45 is passed through the loop car 44 for preventing the continuous line from being stopped when the coil is cut off. Taken.

Next, the combustion control of the heat treatment furnace 22 for setting the crystal grain size of the steel strip 21 to a value within a predetermined range will be described. The heat treatment furnace 22 includes three zones A, B, and C, and performs independent combustion control. Control computer 3
8 determines the furnace temperature and the sheet temperature for each of the zones A, B, and C of the heat treatment furnace 22 according to the steel type, the nominal sheet thickness, and the sheet width of the steel strip 21, and determines the temperature based on these determined values. A temperature setting signal is output to the controllers 35a, 35b, 35c. The temperature controllers 35a, 35b, 35c output flow rate setting signals to the fuel flow controllers 34a, 34b, 34c, and the fuel flow controllers 34a, 34b, 34c are set by feedback signals of the in-furnace thermometers 30a, 30b, 30c. The setting signal is increased or decreased so as to reach the temperature. The fuel flow controllers 34a, 34b, 34c are the same as the temperature controllers 35a, 35a,
Flow control valve 3 based on setting signals from 35b, 35c
An opening signal is output to 3a, 33b, and 33c. In addition to the fuel control of the heat treatment furnace 22 described above, the control computer 38 determines the sheet temperature of the steel strip 21 on the exit side of the heat treatment furnace, and outputs a sheet temperature setting signal to the sheet temperature controller 36 based on the determined value. The plate temperature controller 36 increases or decreases the speed setting signal to the plate speed controller 37 so that the feedback signal of the plate temperature meter 31 becomes the set temperature. The plate speed controller 37 outputs a speed command to the drive system of the bridle roll 24 so that the speed detection signal of the plate speed meter 32 becomes a set value.

The above is a description of the combustion control of the heat treatment furnace which does not reflect the crystal grain size measurement information. Here, the crystal grain size measurement device 42 arranged on the entrance side and the crystal grain size measurement device arranged on the exit side are described. A method of correcting the combustion control of the heat treatment furnace 22 based on the 25 measured values will be described. Crystal grain size measuring device 42
When the measured values of the thickness of the steel strip 21 and the amount of ultrasonic attenuation obtained by the above are taken into the control computer 38, the control computer 38
1 was determined, and the deviation between the nominal plate thickness and the actual plate thickness of the steel strip 21 was determined.
A difference from a deviation predicted in advance based on a crystal grain size before heat treatment obtained from past operation data of the heat treatment furnace is obtained. The temperature difference in the zone A of the heat treatment furnace 22 is corrected based on the difference between the difference in the crystal grain size and the difference between the nominal thickness and the actual thickness. Is corrected by a coefficient obtained by multiplying the correction coefficient for the temperature set value of the above by a value in the range of 0 to 1. Further, the correction coefficient for the temperature set value of the zone B 0 to
Correction is made by a coefficient obtained by multiplying a value in the range of 1. When the measured values of the thickness and crystal grain size of the steel strip 21 by the crystal grain size measuring device 25 are taken into the control computer 38, the control computer 38 obtains the target crystal grain size after the heat treatment of the steel strip 21. And the deviation between the nominal plate thickness and the actual plate thickness of the steel strip 21 are determined, and the temperature setting of the plate temperature controller 36 is determined based on the deviation of the crystal grain size and the deviation between the nominal plate thickness and the actual plate thickness. Correct the value.

FIG. 15 shows the measurement of the grain size in the longitudinal direction of the steel strip on the inlet side and the outlet side of the heat treatment furnace when the combustion control of the heat treatment furnace is not corrected by the values measured by the thickness and crystal grain size measuring device. FIG. 16 shows the fluctuation of the values, and FIG. 16 shows the lengths of the steel strips on the inlet side and the outlet side of the heat treatment furnace when the combustion control of the heat treatment furnace was corrected with the values measured by the plate thickness and grain size measuring device. It shows the fluctuation of the measured value of the crystal grain size in the direction. However, the crystal grain size value on the entry side is a numerical value estimated based on the amount of ultrasonic attenuation. FIG.
According to 5, when the combustion control of the heat treatment furnace is not corrected by the measured values of the thickness and the grain size of the steel strip, the variation of the grain size in the longitudinal direction of the steel strip before the heat treatment is changed after the heat treatment. According to FIG. 16, when the combustion control of the heat treatment furnace is corrected with the measured values of the steel strip thickness and the crystal grain size, the crystal in the longitudinal direction of the steel strip before the heat treatment is obtained. It can be seen that the change in the particle size does not appear after the heat treatment, and the value is also within a predetermined range.

[0053]

As described above, according to the present invention, the following effects are obtained. (1) The frequency characteristics of the ultrasonic scattering attenuation of the subject are determined, and when the Rayleigh scattering equation is applied to the frequency characteristics of the ultrasonic scattering attenuation to calculate the coefficient of the equation, the Rayleigh scattering is calculated. Since only the scattering attenuation term in the equation is used, the calculation is simplified,
Further, the frequency range in which the ultrasonic scattering attenuation is approximated is set between the peak frequency in the frequency characteristic of the reflected echo on the bottom surface of the subject and the frequency on the high frequency side of the two frequencies at which the peak is halved. With this setting, it is possible to carry out appropriate measurements according to the actual properties of the test material, and therefore the measurement accuracy is high and continuous measurement is possible. It is possible in real time on such a line. (2) When the ultrasonic scattering attenuation is less than the measurement allowable level, two different bottom surface reflection echoes satisfying the measurement allowable level are automatically selected from the multiple reflection echo train of the subject, so that the measurement is performed. A decrease in accuracy is avoided. (3) The time difference between the reflected echo from the subject surface and the reflected echo from the subject bottom surface is measured, and the thickness of the subject is measured based on the time difference. Can be measured, so that a separate apparatus for measuring the thickness is not required. Therefore, by measuring over the entire length of the coil-shaped specimen, the crystal grain size value and the thickness over the entire length can be known, and the material characteristic values such as mechanical test values can be easily and quickly known. .

(4) A fixing jig to which the probe is fixed, and a plurality of wheels connected to the fixing jig via flexible joints, respectively. Can be always kept parallel to the surface of the test material. (5) The probe is made of a polymer material, has a long life, and has a wide band characteristic in which an upper and lower frequency difference with respect to an intensity 6 dB lower than the center frequency intensity is 80% or more of the center frequency. / N is good. (6) In a heat treatment line such as annealing, the furnace temperature was conventionally controlled by a radiation thermometer based on the difference in the thermal emissivity of the test object, so that the furnace temperature varied due to the variation in the emissivity due to the difference in surface properties. Although large, the application of the present invention makes it possible to control the furnace temperature from the crystal grain size of the material, which greatly contributes to quality control of products and improvement of yield. (7) An apparatus for measuring the grain size of the specimen is provided at the subsequent stage of the annealing furnace, and the average grain size and the thickness in the sheet thickness direction of the steel strip immediately after passing through the annealing furnace are continuously measured. Therefore, it is possible to judge whether or not an appropriate heat treatment has been performed, and it is possible to guarantee mechanical properties by the crystal grain size over the entire length of the steel strip. Further, when a heat treatment defective portion occurs, the defective portion can be grasped very accurately, and the heating range and temperature at the time of the re-heat treatment can be accurately determined. (8) Even if the crystal grain size of the steel strip before the heat treatment fluctuates in the length direction, by using the measured values of the plate thickness and the crystal grain size measured continuously, the fine-grained heat treatment furnace can be used. Since combustion control or plate speed control can be performed, a steel strip having a uniform grain size in the length direction can be manufactured, and quality can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of an apparatus for measuring a crystal grain size of a subject using ultrasonic waves according to one embodiment of the present invention.

FIG. 2 is a timing chart showing an electric signal of an ultrasonic reception echo in the embodiment of FIG.

FIG. 3 shows two reflection echoes (B) in the embodiment of FIG. 1;
FIG. 2 is a diagram illustrating frequency characteristics of (1, B2).

FIG. 4 is a diagram showing a frequency characteristic of an attenuation amount of an ultrasonic pulse in the embodiment of FIG. 1;

FIG. 5 is a diagram illustrating a frequency characteristic of an attenuation correction amount in the embodiment of FIG. 1;

FIG. 6 is a diagram illustrating a frequency characteristic due to scattering attenuation of a subject in the embodiment of FIG. 1;

7 is a diagram showing a configuration of a measuring jig having a follow-up mechanism in the embodiment of FIG.

FIG. 8 is an operation explanatory diagram of the follow-up mechanism of FIG. 7;

FIG. 9 is a diagram showing an example of crystal grain size measurement (No. 1) measured by applying the embodiment of FIG. 1 to a continuous annealing and pickling line for a thin plate.

FIG. 10 is a diagram showing an example of crystal grain size measurement (part 2) measured by applying the embodiment of FIG. 1 to a continuous annealing and pickling line for a thin plate.

FIG. 11 is a diagram showing an example of a thickness measurement of a specimen measured by applying the embodiment of FIG. 1 to a continuous annealing and pickling line for a thin plate.

FIG. 12 is a block diagram showing a configuration of a continuous annealing furnace according to another embodiment of the present invention.

FIG. 13 is a view showing a result of measuring a crystal grain size by the crystal grain size measuring device of FIG. 1;

FIG. 14 is a block diagram showing a configuration of a steel strip heat treatment apparatus according to still another embodiment of the present invention.

FIG. 15 is a diagram showing a change in a measured value of a crystal grain size in a length direction of a steel strip on an inlet side and an outlet side of a heat treatment furnace when no correction is applied to combustion control of the heat treatment furnace.

FIG. 16 shows the measured values of the crystal grain size in the longitudinal direction of the steel strip on the inlet side and the outlet side of the heat treatment furnace when the combustion control of the heat treatment furnace is corrected by the measurement values of the thickness and crystal grain size measuring device. It is a figure showing a change.

FIG. 17 is a characteristic diagram showing variations in measurement results due to changes in the horizontality of the probe and the subject.

FIG. 18 is a diagram showing a configuration of a measuring jig having a conventional follow-up mechanism.

FIG. 19 is a diagram showing a comparison of frequency ranges to be fitted.

FIG. 20 is a diagram showing a logarithm of ultrasonic scattering attenuation.

FIG. 21 is a view showing a result of measurement of a crystal grain size for each sample by the equation (1).

FIG. 22 is a view showing a result of measurement of a crystal grain size for each sample by the equation (2).

FIG. 23 is a diagram showing each echo waveform and frequency characteristics when ultrasonic scattering attenuation is small.

FIG. 24 is a diagram showing each echo waveform and frequency characteristics when ultrasonic scattering attenuation is large.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tadayuki Sakai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. Nippon Kokan Co., Ltd. (72) Inventor Toshio Takano 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 29/00-29 / 28

Claims (12)

(57) [Claims] An ultrasonic pulse is transmitted to a subject, and two reflected echoes of a multiple reflected echo train from the bottom surface of the subject are subjected to frequency analysis and subtraction, respectively, so that a frequency due to attenuation of the ultrasonic pulse is obtained. The characteristics are obtained, and a predetermined correction process is performed on the frequency characteristics to obtain the frequency characteristics of the ultrasonic scattering attenuation of the subject. By applying the Rayleigh scattering equation to the frequency characteristics of the ultrasonic scattering attenuation, the subject is obtained. In the method for determining the crystal grain size of the above, only the scattering attenuation term is used when applying the Rayleigh scattering equation, and the frequency range in which the ultrasonic scattering attenuation is approximated is set as the second of the reflected echoes on the bottom surface of the subject. Characterized in that it is set between a peak frequency in the frequency characteristic of the reflected echo and a frequency on the high frequency side of two values of the two frequencies at which the half value of the peak is obtained. Subject grain size measuring method according to. 2. When the amount of ultrasonic scattering attenuation caused by the two reflected echoes is less than the measurement allowable level, two different bottom surface reflection echoes satisfying the measurement allowable level are automatically determined from the multiple reflection echo train of the subject. 2. The method for measuring the crystal grain size of an object by ultrasonic waves according to claim 1, wherein the method is selected. 3. The method according to claim 1, wherein a time difference between a reflection echo from the surface of the object and a reflection echo from the bottom surface of the object is measured, and the thickness of the object is measured based on the time difference. A method for measuring the crystal grain size of a subject using the ultrasonic waves described in the above. 4. The method for measuring a crystal grain size of an object by ultrasonic waves according to claim 1, wherein the object is a steel plate or a steel strip. 5. An ultrasonic transmitting / receiving means for transmitting an ultrasonic pulse to a subject via a probe and receiving a reflected wave, and multiplexing from the bottom surface of the object included in an output of the ultrasonic transmitting / receiving means. Gate means for extracting two reflected echoes from the reflected echo train; frequency analysis of each of the two reflected echoes and subtraction to obtain a frequency characteristic due to attenuation of the ultrasonic pulse, and a predetermined correction to the frequency characteristic By performing the processing, the frequency characteristic of the ultrasonic scattering attenuation of the subject is obtained, and the Rayleigh-type scattering attenuation term is applied to the frequency characteristic of the ultrasonic scattering attenuation to obtain a frequency at which the ultrasonic scattering attenuation approximates. Range
The coefficient is determined by setting the coefficient between the peak frequency in the frequency characteristic of the reflection echo on the bottom surface of the subject and the higher frequency of the two frequencies at which the peak is reduced by half, and based on the coefficient. An apparatus for measuring a crystal grain size of a subject by ultrasonic waves, comprising: a crystal grain size calculating means for calculating a crystal grain size of the subject. 6. When the ultrasonic scattering attenuation due to the two reflected echoes is less than the measurement allowable level, the gate unit is controlled to control the reflection until two different reflected echoes satisfying the measurement allowable level are obtained. 6. An ultrasonic wave attenuation calculating means for selecting an echo.
An apparatus for measuring the crystal grain size of a subject using the ultrasonic waves described in the above. 7. A chain or spring comprising a fixing jig to which the probe is fixed, and three or more wheels connected to the fixing jig via a flexible joint rotatable in a horizontal direction. The apparatus according to claim 5 or 6, wherein the apparatus is fixed at a fixed position by a member. 8. The probe according to claim 1, wherein the probe is made of a polymer material, and has a characteristic in which an upper and lower frequency difference with respect to an intensity 6 dB lower than the center frequency intensity is 80% or more of the center frequency. An apparatus for measuring the crystal grain size of a subject by ultrasonic waves according to claim 5, 6, or 7. 9. The apparatus according to claim 5, wherein the test object is a plate or a steel strip. 10. A continuous annealing apparatus for a steel sheet or a steel strip, comprising the apparatus for measuring the crystal grain size of an object by ultrasonic waves according to claim 9, after the annealing furnace. 11. A continuous heat treatment step for a hot-rolled or cold-rolled steel sheet or steel strip, wherein the steel sheet or the steel strip before the heat treatment is subjected to the ultrasonic scattering attenuation and the thickness of the specimen by the method according to claim 1 or 3. The crystal grain size and thickness of the test object are measured by the method according to claim 1 or 2 and claim 3 for the steel sheet or the steel strip after the heat treatment, and based on these measured values, Controlling the temperature inside the heat treatment furnace and / or the passing speed of the steel sheet or the steel strip so that the crystal grain size of the steel sheet or the steel strip after the heat treatment becomes a value within a predetermined range; Heat treatment method. 12. The apparatus according to claim 4, wherein said apparatus is provided before and after a heat treatment furnace for hot-rolled or cold-rolled steel sheet or steel strip. A device for measuring the temperature inside the furnace of the heat treatment furnace, a device for measuring the passing speed of the steel plate or the steel strip, and a device for controlling the temperature inside the furnace of the heat treatment furnace and / or the speed of passing the steel plate or the steel strip; The crystal grain size and thickness signal from the crystal grain size measuring device, and the calculation based on the temperature signal in the furnace of the heat treatment furnace and the passing speed signal of the steel sheet or the steel strip, the crystal of the steel sheet or the steel strip after the heat treatment A steel plate or a steel strip having an arithmetic unit that generates a signal for operating a furnace internal temperature of a heat treatment furnace and / or a sheet passing speed control device for the steel plate or the steel strip so that the particle diameter falls within a predetermined range. Heat treatment equipment.