JPH06201659A - Apparatus for measuring crystal grain size - Google Patents

Abstract

PURPOSE:To provide an apparatus wherein an object, to be inspected, whose crystal grain size is larger than a sheet thickness can be measured quickly in a production line out of apparatuses wherein the crystal grain size is measured nondestructively by an ultrasonic method. CONSTITUTION:A probe 1 is formed to be of a convergence type, and the convergence diameter of ultrasonic pulses is made sufficiently narrower than a crystal grain size. An object 5 to be inspected is scanned fine by a scanning mechanism 2. A sound-velocity operation device 4 measures a longitudinal-wave sound velocity continuously, and a difference in the sound velocity by a crystal orientation is analyzed. Even when a horizontal distance is changed a little, the sound velocity can be measured. As a result, the object to be inspected can be measured accurately not only in a case where it stands still but also in a case where it is moved at high speed while it is being vibrated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grain size measuring apparatus for metallic materials, and more particularly to an ultrasonic grain size measuring apparatus suitable for measuring a thin plate having a relatively large grain size.

[0002]

2. Description of the Related Art In order to measure the crystal grain size of a metal material by an ultrasonic method, there are an ultrasonic microscope method and an ultrasonic attenuation measurement method, both of which are based on an ultrasonic pulse reflection method. This is because the transmitter sends an electric high frequency pulse to the probe at a constant cycle (pulse repetition frequency), the probe emits an ultrasonic pulse, and then the pulse is reflected from each part and received by the probe. In this method, a signal (echo) is amplified by a receiver and various signal processing is performed. In addition, since there is usually water as a transmission medium of ultrasonic waves between the probe and the subject, this distance is hereinafter referred to as “water distance”.

FIG. 6 shows an example of a crystal grain size measuring device using an ultrasonic microscope. The transmitter / receiver 3 normally sends a high frequency pulse of 100 MHz or more to the probe 1, and the wave emitted from the probe is converged finer than the crystal grains on the surface of the object 5 by the function of the acoustic lens inside the probe. It becomes a sound wave pulse. The ultrasonic waves entering the subject propagate mainly on surface waves due to the strong focusing effect of the acoustic lens and propagate on the surface of the subject. If the time length (pulse width) of the high-frequency pulse is made sufficiently long,
The surface waves reflected at the grain boundaries and returned to the probe and the waves reflected inside the probe can be observed simultaneously. Since the phase of the reflected surface wave changes depending on the propagation distance, when there is a grain boundary at a specific position from the incident point, the grain boundary reflected wave and the probe internal reflected wave become negative interference and the echo height decreases. There is a part where the phenomenon occurs. While the scanning mechanism 2 sends the probe linearly, the echo height calculator 4 takes out the echo height of the part where this interference occurs, and displays it on the arbitrary two-dimensional output device 6 together with the position signal of the scanning mechanism. Then, the position of the crystal grain boundary is known as the point where the echo height decreases, and the number of crystal grains whose scanning line is cut is obtained.

The grain size n is 1 mm ^{2 on the} measuring surface. It is defined as the number of crystal particles per unit. The JIS standard for particle size measurement with an optical microscope is that each length L ₁ when scanning in two orthogonal directions,
From L ₂ and the number of particles n ₁ and n ₂ cut in each length, (n ₁ · n ₂ ) / (L ₁ · L ₂ ) is multiplied by a coefficient to obtain n.
Stipulates the method of asking for. In addition, to convert to a crystal grain size, when the particle shape is isotropic, n ^1/2 The value is inversely proportional to. These coefficients are determined by the nature of the material.

The phase of the wave reflected inside the probe of the ultrasonic microscope is constant regardless of the position of the probe, but the phase of the wave reflected from the grain boundary depends on the distance from the incident point to the grain boundary and the water distance. Affects in units of wavelength. Therefore, in order to correctly measure the interference effect, it is necessary to keep the change in water distance during scanning sufficiently smaller than the wavelength of ultrasonic waves in water. Since this value is about 15 μm depending on the longitudinal sound velocity of water (about 1,500 m / s) and the pulse frequency (100 MHz or more), it is possible to measure a stationary subject, but the vibration of the subject is usually 2-3 mm. There is a problem that is not applicable to online measurement.

Next, FIG. 7 shows an example of an apparatus for measuring the attenuation of ultrasonic waves. The probe 1 is a standard type that does not converge the ultrasonic pulse, and makes the ultrasonic pulse enter the subject 5 vertically. Most of the ultrasonic waves that have entered the subject are trapped in a so-called multiple reflection state in which they are repeatedly reflected between the surface and the bottom of the subject. It is known that when an ultrasonic wave propagates through a crystal, the ultrasonic wave is attenuated due to scattering by a grain boundary. When this multiple reflection state is expressed by an equation,
If the attenuation rate of ultrasonic sound pressure per time is α and the height of the first bottom reflected echo is P1, the nth bottom reflected echo height Pn appearing t hours after P1 is Pn =
P1 · exp (-αt). Using the echo height calculator 4, find the envelope of the echo height of the ultrasonic pulse that was only partially leaked to the probe side, and convert it to the attenuation rate α per unit time,
The grain size is obtained by utilizing the correlation between the size of the crystal grain size and the attenuation rate, which has been obtained in advance with an optical microscope.

Since this method does not use interference, changes in water distance do not directly affect the measurement, and it can be applied online. However, since the magnitude of the attenuation greatly changes depending on the frequency of the pulse, it is necessary to set the wave number of the ultrasonic wave contained in one pulse to 5 to 6 waves or more in order to accurately measure the attenuation. The frequency of the pulse that is normally used is 5 MHz, and the longitudinal sound velocity of metallic materials is about 6,000 m / s, so the wavelength of ultrasonic waves in the subject is about 1.2 mm. If the distance for one round trip of the plate thickness is less than the length of the pulse (6-7.2 mm) or less, the echo height cannot be detected accurately because the preceding and following pulses interfere with each other. There is a problem that it cannot be applied to the following thin plates. If the frequency of the pulse is increased, a thinner plate thickness can be applied. But,
The degree of attenuation has a good correlation with the crystal grain size when the crystal grain size is sufficiently smaller than the wavelength of the ultrasonic wave in the subject (Rayleigh scattering region), and the crystal grain size approaches the wavelength. And cannot be measured accurately. For example, set the pulse frequency to 50
If it is increased to MHz, it can be applied to thin plates with a thickness of 0.3 mm or more, but the crystal grain size that can be measured at this time is limited to 0.1 mm or less.

[0008]

Some thin plates having a plate thickness of about 0.3 mm with improved electrical characteristics have a crystal grain size larger than the plate thickness. Since the particle size serves as an index of quality, it is necessary to control the quality not only by measuring the particle size with a test piece cut out from the subject but also by measuring the entire length on a production line where the subject vibrates. It is an object of the present invention to provide an ultrasonic crystal grain size measuring device capable of performing online measurement on such a thin plate.

[0009]

[Means for Solving the Problems] The above-mentioned problem is caused by a probe that emits an ultrasonic pulse that converges finer than the crystal grain size, and the probe is oriented perpendicularly to the subject while maintaining a constant distance. A scanning mechanism that moves, and a calculator that calculates the sound velocity value at each point of the subject by the time difference of multiple reflection echoes and the plate thickness, and the place where the sound velocity value changes when the probe moves a certain length. Is defined as a crystal grain boundary, and the problem can be solved by using a crystal grain size measuring device that determines the number of crystal grains. This device can be combined with a spatial frequency calculator, a synchronous scanning mechanism, etc. to automatically evaluate the particle size distribution.

[0010]

It is known that the crystal orientation differs depending on the crystal grain, and the sound velocity of ultrasonic waves varies depending on the propagation direction.
A probe that emits an ultrasonic pulse that converges sufficiently finer than the crystal grain size is directed perpendicularly to the subject, and the surface of the subject is scanned. The ultrasonic pulse is multiple-reflected at the ultrasonic wave incident point, and the time difference between the echoes returned to the probe is measured. This time difference is proportional to the product of the sound velocity at each point of the subject and the plate thickness. Since the plate thickness is known, the change in the sound velocity can be obtained. The sound velocity value at each point is displayed corresponding to the scanning distance of the probe, and the spread of crystal grains on the scanning line is read as a uniform range of the sound velocity value. By linearly scanning the probe in parallel and dividing the value of the sound velocity into an appropriate range, a method of displaying the shape of the grain suitable for visual observation can also be used. For normal quality control, it is easier to control the direction and length of the line segment with respect to the subject and evaluate the size distribution with the above-mentioned grain size.

[0011]

FIG. 1 shows an example of the structure of a grain size measuring apparatus according to the present invention. This is a configuration suitable for viewing the basic characteristics of the device off-line, and includes a convergent probe 1 oriented vertically to the subject 5, a linear scanning mechanism 2, a transceiver 3, and a sound velocity calculator. It consists of 4. This calculation result is multiplied by an appropriate magnification such as a plate thickness value and displayed on the output device 6 such as a recorder together with the position signal of the scanning mechanism. The following is a plate thickness of 0.3 m
Subject a with a crystal grain size of 0.42 mm observed by an optical microscope at m, and subject b with a plate thickness of 0.5 mm and a crystal grain size of 0.71 mm.
A configuration suitable for the above will be described as an example.

The probe 1 has a pulse frequency of 50 MHz and a diameter of 6
mm, the focal length in water is 25 mm, and the water distance is 23 mm so that the focal point is located in the center of the plate thickness of the subject.
With this setting, the water distance is 2-3mm near the focal point.
Even if it fluctuates, it is possible to obtain an ultrasonic pulse that is always smaller than the crystal grain size and converges. Convergence diameter φ of ultrasonic pulse in water
Is expressed as φ = λ · f / D, where λ is the wavelength of ultrasonic waves in water, f is the focal length, and D is the aperture of the probe. In this case, d ~ 0.12 mm Become. If the variation range of the crystal grain size of the sample is known in advance, it is advisable to select a probe with a convergent diameter that is about 1/5 of the minimum grain size. As will be described later, depending on how the particle size is evaluated,
Since it can be measured even if the convergent diameter is equal to the particle diameter, the smaller the convergent diameter is not necessarily the better.

The speed of sound calculator 4 is basically the surface of the subject.
The time difference between any two consecutive pulses that are multiply reflected between the bottom surfaces is measured, and the sound velocity is calculated by dividing the value of twice the plate thickness by the time difference. Multiple reflection does not appear when the test object and the probe are not perpendicular. On the contrary, if it was set so that multiple reflections would appear long and many times, it would have been set correctly vertically. When the pulse continues for a long time, more accurate sound velocity may be obtained by averaging using continuous time differences of 5 to 6 pulses or more. When the plate thickness changes depending on the location, use the measured value of the plate thickness measured separately. If the plate thickness is uniform, a constant may be used. Further, in the present invention, since the relative value representing the difference in sound velocity due to the crystal grains is significant, not the absolute value of sound velocity, the sound velocity value may be obtained by subtracting the reference value from the time difference. For the transceiver 3 and the sound velocity calculator 4, a broadband ultrasonic flaw detector and gate device, or an ultrasonic thickness gauge can be used. The scanning mechanism 2 may be one that moves relative to the subject at a constant speed. When the pulse repetition frequency is 200Hz (5ms cycle), a length of about 10mm of the subject is scanned at a speed of 4mm / s, and a sound velocity value of 512 points is obtained every 0.02mm. These values may be sufficiently smaller than the size of the crystal grain (0.42 mm) (0.02 mm) and sufficiently large scanning line length (10 mm). When the scanning distance and the sound velocity value are recorded at an appropriate magnification that is easy to observe, the position of the crystal grain boundary can be found as a portion where the sound velocity value has changed significantly, and the number of crystal grains cut by the scanning line can be obtained. Having a function to calculate the moving average of sound velocity values in the sound velocity calculator,
There is also a method of integrating the number of crystal grains that becomes a crystal grain boundary when the average is shifted up and down by a constant value during scanning. The calculation of the crystal grain size can be performed by the method defined in JIS, for example, according to the number of cut crystal grains and the scanning length. Further, when it is necessary to convert the crystal grain size measured by other optical microscopes such as the comparison method and the calculation method, the coefficient is determined by correlating the material with many measurements for each property.
Since this method does not use interference, the difference in sound velocity can be accurately measured even if the water distance changes slightly. The speed of change of water distance is usually negligible compared to the speed of sound in water, so it does not matter. The wave number of the ultrasonic wave in one pulse does not use the echo height, so 1-2
I want the waves. Therefore, the longitudinal wavelength of 2 to 2
4 times the plate thickness (0.24-0.48 when the pulse frequency is 50MHz)
(mm) interference does not occur, and even if there is some interference, it is possible to measure the accurate time difference by using the known waveform processing method of the thickness gauge, so it is possible to measure even a sufficiently thin plate thickness. The sound velocity distribution obtained by this linear scanning is shown in FIG. 2a corresponds to a crystal grain size of 0.42 mm, and b corresponds to 0.71 mm.
, And all show good correspondence.

If the positions of the grain boundaries are known, several methods can be considered for evaluating the distribution state of crystal grains. For example, statistical processing such as maximum value / minimum value of particle diameter, standard deviation, and moving average can be carried out by various arithmetic units. An example of a configuration using a frequency analyzer suitable for automatic evaluation is shown in FIG. Convergence type probe oriented vertically to the subject 5
1, the transmitter / receiver 3, and the sound velocity calculator 4 have the same configuration as in FIG. In this example, the probe 1 is fixed, and the scanning mechanism 2 moves the subject 5 linearly. Specifically, the probe may be attached to a line that conveys the subject at a constant speed and direction by a conveyor or rollers. The scanning mechanism outputs a position signal for each 0.02 mm of the amount of movement of the subject, which is one input of the frequency analyzer 7 for automatically evaluating the particle size distribution.
On the other hand, the sound velocity calculator 4 is
The time difference of the ultrasonic pulse reflected on the bottom surface is calculated, the appropriate coefficient correction is performed, and the longitudinal wave sound velocity value of the subject is obtained, which is used as another input of the frequency analyzer.

The frequency analyzer 7 stores each scanning position and the sound velocity value in association with each other, and the sound velocity value is reciprocal every distance of 512 points (every 10 mm of the subject), that is, the spatial frequency region. The spatial frequency distribution of the crystal grain size is obtained by performing a Fourier transform on. This result can be displayed on the output device 6 having the spatial frequency as the other axis each time the conversion is performed, and the peak position and spread of the spectrum can be obtained. If the crystal grain size is constant, the peak of the spectrum appears in the reciprocal of the moving distance of the object where the sound velocity value changes twice. Therefore, if the spectrum peak of the frequency analysis is obtained, 1/2 of the reciprocal of the spatial frequency It is appropriate to use This coefficient (1/2) can be set to any value because it is related to the value obtained by another measurement method. If another 10 mm portion is stored separately during the Fourier transformation and the transformation is repeated, the representative particle size of the entire length of the subject can be continuously and automatically measured every 10 mm.

FIG. 4 shows an example of the spatial frequency analysis output by this method. 4a corresponds to FIG. 2a, and FIG. 4b corresponds to FIG. 2b, and shows good agreement with the data of the microscope observation. According to this method, it is possible to perform an accurate evaluation using all the measurement data. If the convergent diameter of the probe is smaller than the crystal grain size, the same peak frequency appears even if it is extremely small or equal, so that the probe can be selected from a wide range. Further, according to this apparatus, there is an advantage that even when the grain pitch is the same and the grain boundary width is changed, the representative grain size can be evaluated without variation.

The pulse repetition frequency cannot be increased indefinitely because it is restricted by the size of the water distance and the reverberation echo in the subject. Maximum pulse repetition frequency is 10 kHz (0.1
When the sound velocity value for each 0.02 mm is used as the reference, the total length can be measured with the probe fixed up to a line speed of 0.2 m / s (0.02 mm / 0.1 ms). FIG. 5 shows an example of an apparatus configuration in which a synchronous scanning mechanism is combined in order to apply the same condition to a line higher in speed than this. Convergence type probe 1, which is oriented vertically to the subject 5, transceiver 3, sound velocity calculator
4 has the same configuration as that in FIG. The subject 5 linearly moves at a constant speed by another driving mechanism (not shown). Subject
A velocity detector 8 for detecting the velocity of 5 and a position detector 9 for detecting the position of the subject from a reference point (for example, the tip) are arranged. Frequency analyzer 7 for evaluating the grain size of the object
The conversion speed of is 1,024 points and is an example configured using a 3 ms fast Fourier transformer.

The synchronous scanning mechanism 10 scans the probe with reference to the output of the velocity detector 8 so that the probe has a sliding velocity of 0.2 m / s with respect to the subject. If the line speed is 1 m / s, for example, there is a method of reciprocating scanning at a constant speed of 0.8 m / s in the forward and reverse directions of the line. From the perspective of the probe being measured, the subject is moving at a velocity of 0.2 m / s. It takes about 100 ms to obtain 1,024 points of data for every 0.02 mm, and the moving distance of the probe is about 80 mm during this time, so reciprocal scanning can be realized using a pulse motor or the like. Although it is a non-measurement time while returning the probe to its original position,
A value measured every 0.2 m (200 ms) over the entire length of the subject is usually sufficient for the entire length of the subject. As a result, the crystal grain size of the entire length of the subject can be measured using the representative grain size subjected to the spatial frequency analysis even in a high-speed line.

[0019]

【The invention's effect】

1. Since a probe that emits an ultrasonic pulse that converges finer than the crystal grain size, a scanning mechanism, and a sonic velocity calculator are combined, it is possible to accurately measure an object with a crystal grain size larger than the plate thickness even if the water distance changes. Can measure grain size. Therefore, it is possible to perform online measurement in which the subject vibrates, not limited to the stationary state of the subject. 2. Furthermore, since it is combined with a frequency analyzer, it is possible to automatically perform accurate statistical processing of the particle size distribution. 3. Furthermore, the synchronous scanning mechanism enables full-length measurement of particle size distribution on a high-speed production line.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a grain size determining apparatus according to the present invention.

FIG. 2 is a diagram showing a measurement result of crystal grain size according to the present invention.

FIG. 3 is a diagram showing an example of a crystal grain size measuring device according to the present invention.

FIG. 4 is a diagram showing the measurement results of crystal grain size according to the present invention.

FIG. 5 is a diagram showing an example of a grain size measuring apparatus according to the present invention.

FIG. 6 is a diagram showing an example of a conventional crystal grain size measuring device using an ultrasonic microscope.

FIG. 7 is a diagram showing an example of a conventional crystal grain size measuring device by attenuation measurement.

[Explanation of symbols]

1 ... Probe 2 ... Scanning mechanism 3 ... Transceiver
4 ・ Calculator 5 ・ ・ Subject 6 ・ ・ Output device 7 ・ ・ Frequency analyzer 8 ・ Velocity detector 9 ・ ・ Position detector 10 ・ Synchronous scanning mechanism

Claims (3)

[Claims] 1. A probe that emits ultrasonic pulses that have a parallel front and back surface and that converges finer than the crystal grain size of the subject, and that probe is oriented perpendicular to the surface of the subject and is constant with the subject. A scanning mechanism that relatively moves while maintaining a distance, and a calculator that calculates the sound velocity value of each point of the subject by the time difference and the plate thickness of the plurality of ultrasonic pulses reflected from the subject and returned to the probe And a calculator for determining the number of crystal grains by defining a portion where the sound velocity value changes while the scanning mechanism scans a certain distance as a crystal grain boundary. 2. A position signal of the scanning mechanism and a sound velocity value of the computing unit corresponding to the position signal are input, and the position / sound velocity value distribution is converted into a spatial frequency domain to obtain a peak value of the spectrum. The crystal grain size measuring apparatus according to claim 1, further comprising a spatial frequency calculator that outputs the reciprocal of the peak frequency as a representative grain size. 3. The crystal grain size measuring apparatus according to claim 1, wherein the scanning mechanism includes means for moving the probe at a sliding speed that is constant with respect to the moving speed of the subject.