JP4439363B2 - Online crystal grain size measuring apparatus and measuring method using laser ultrasonic wave - Google Patents

Description

  In the production line of various steel plates produced at steelworks, the present invention uses a pulsed laser to generate ultrasonic waves on the steel plates and transmits the ultrasonic waves propagating through the steel plates to a laser interferometer (confocal Fabry-Perot). By using an interferometer to calculate the attenuation coefficient of ultrasonic waves, and to provide an apparatus and method capable of measuring the crystal grain size of a steel sheet using this attenuation coefficient, and to produce at an ironworks The present invention relates to an on-line crystal grain size measuring apparatus and method that enables non-contact on-line crystal grain size measurement in a production line for various steel sheets and facilitates control of the crystal grain size of a steel sheet during hot rolling.

  Ultrasonic inspection is usually used for mechanical properties and microstructures such as metals and composite materials. Such an ultrasonic inspection is to grasp the mechanical characteristics and microstructure using the propagation characteristics of ultrasonic waves in the measurement object, and is basically a non-destructive inspection method. It is used very widely in the field. In the ultrasonic inspection, a piezoelectric probe and an EMAT (electromagnetic ultrasonic probe) have usually been used. Among these, the piezoelectric probe has a disadvantage in that an ultrasonic transmission medium is required between the measurement object and the transducer, and the function of the piezoelectric probe is reduced at a high temperature. And EMAT has the fault that it must usually be used in proximity to a measuring object to about several mm. Due to such disadvantages, the conventional ultrasonic inspection using the apparatus as described above cannot be applied on-line in a production environment, particularly in a poor environment such as a hot rolling process. On the other hand, a method of generating ultrasonic waves using a pulsed laser and measuring the ultrasonic waves propagating through the measurement object using a laser interferometer (hereinafter referred to as laser ultrasonic method) is basically In particular, since this method is a non-contact type method, ultrasonic flaw detection of a high-temperature measurement object is possible, and it has an advantage that it can be easily applied on the production line.

  In general, industrial production lines are very poor compared to the laboratory environment. Therefore, although the laser ultrasonic method as described above is in principle suitable for online application in a production line, due to environmental factors in the industrial field, the industrial application of a measuring device is generally not It's not easy. For example, in the rolling process of a production line, the steel sheet is generally thick at the initial stage of rolling, and the thickness decreases as the rolling progresses. Therefore, ultrasonic measurement can be easily performed on steel sheets of various thicknesses. Must be. Therefore, high intensity ultrasonic waves must be generated compared to laboratory tests. When ultrasonic waves are generated using a laser pulse beam as in the contents of this patent, ultrasonic waves are generated by the thermoelastic effect or ablation (ablation) of the measurement target surface by the laser pulse beam (for example, (Refer nonpatent literature 1.). Among these, ablation of the surface to be measured occurs when the intensity of the laser pulse beam is large. In this case, the surface material is ionized and vaporized, and jumps out in a direction perpendicular to the surface of the specimen, whereby a reaction force acts on the surface of the specimen, and ultrasonic waves are generated by the reaction force. In general, the ultrasonic wave generated by ablation is larger in intensity than the ultrasonic wave generated by the thermoelastic effect, and is propagated in a direction perpendicular to the measurement target surface on which the laser pulse is incident (longitudinal direction). Wave) is generated efficiently.

  In general, the upper and lower surfaces of steel plates being rolled in a production line are parallel to each other. Therefore, in the method of obtaining the crystal grain size from the attenuation coefficient using the attenuation of the ultrasonic wave due to the scattering generated at the boundary surface of the crystal grain when the ultrasonic wave propagates inside the measurement object, the generation and detection of the ultrasonic wave are performed. When performing on the same surface of the measurement object, an ultrasonic wave propagating perpendicularly to the measurement object surface is generated, and the ultrasonic wave reflected on the opposite surface of the measurement object surface is the same point where the ultrasonic wave is generated. It is most efficient to measure at. In other words, when ultrasonic waves that propagate at a certain angle with respect to the measurement target surface are used, a loss of ultrasonic intensity due to mode conversion occurs when the ultrasonic waves are reflected on the opposite surface of the measurement target surface. In addition, since the ultrasonic generation point and the measurement point are different, the calculation of the propagation path and propagation distance of the ultrasonic wave necessary for deriving the ultrasonic attenuation coefficient becomes very complicated.

  For the technical reasons as described above, when crystal grain size is measured online on a production line, it is necessary to generate ultrasonic waves by the ablation effect. However, when an ultrasonic wave is generated by such an ablation effect, plasma that emits light having the wavelength of the laser beam is generated at the position where the ultrasonic wave is generated. The light emitted from the plasma in this way enters the laser interferometer for ultrasonic measurement and acts as a major obstacle to ultrasonic measurement.

  Yet another obstacle in on-line crystal grain size measurement in production lines is the rapid crystal grain size calculation by real-time signal processing. That is, the intensity of the ultrasonic wave propagating through the measurement target is attenuated by scattering by crystal grains, absorption by the measurement target substance, and diffraction. When the crystal grain size is measured using ultrasonic attenuation, the influence of effects other than the scattering by the crystal grains must be removed from the measured ultrasonic attenuation coefficient. It is not easy to derive the ultrasonic attenuation coefficient resulting from only the scattering by the crystal grains from the ultrasonic signal measured in this way, and it usually requires a lot of time. Therefore, it is necessary to apply a signal processing method capable of calculating the crystal grain size of the measurement object at high speed in real time from the measured ultrasonic signal.

  Still another obstacle in the on-line crystal grain size measurement in the production line is the stabilization of the laser interferometer for ultrasonic measurement. A laser interferometer is mainly used for non-contact measurement of ultrasonic waves that reach the surface of the measurement object. Such characteristics are usually sensitive to external vibration. That is, if there is vibration at the position where the measuring device is installed, the performance of the laser interferometer will be degraded. From this aspect, among various laser interferometers, the confocal Fabry-Perot interferometer is less affected by external vibration in principle, and is most suitable for online measurement in production lines where external vibration exists. ing. This is because the confocal Fabry-Perot interferometer is insensitive to signals that are relatively low in frequency compared to ultrasound, such as external vibration. In addition, the confocal Fabry-Perot interferometer has a high collection efficiency of scattered light from the surface of the measurement object, so it is easy to use ultrasonic waves even in the case of a measurement object having a rough surface such as a steel plate located on the production line. It has the advantage that can be measured. However, such a confocal Fabry-Perot interferometer also requires additional stabilization when applied on-line to a production line. In other words, the resonator constituting the confocal Fabry-Perot interferometer is composed of curved partial reflecting mirrors (hereinafter referred to as curved mirrors) at both ends, and the distance between the two curved mirrors is always kept constant. Must. However, the distance between the two curved mirrors is affected by changes in the ambient temperature and the alignment state of the two curved mirrors, and the change in the distance between the two curved mirrors is minute. Even in such a case, the interference condition changes, which greatly affects the efficiency of ultrasonic measurement. Therefore, in order to apply the confocal Fabry-Perot interferometer for efficient use in a production line in a poor environment, the ultrasonic measurement is always high by keeping the distance between the two curved mirrors constant. Stabilization is required to maintain efficiency.

  Conventional techniques that apply the laser ultrasonic method to the measurement of the mechanical properties and microstructure of the object to be measured include porosity measurement of composite materials (Patent Document 1), physical property measurement of specimens utilizing ultrasonic diffusion ( Patent Document 2), measurement of physical properties of a plate material using ultrasonic resonance (Patent Document 3), and the like.

  Among them, the porosity measurement of a composite material (Patent Document 1) relates to a method for measuring the porosity of a test piece by utilizing attenuation of ultrasonic waves. Although the purpose of the invention is different from this patent for calculating the particle size, it emphasizes that the generation and measurement of ultrasonic waves by a laser beam is performed on the same surface (same position) of the measurement object, but in the steel plate production line Problems that may occur during online measurement and solutions to this are not presented.

  In another conventional technique for measuring physical properties of a specimen using ultrasonic diffusion (Patent Document 2), ultrasonic pulses generated by a pulsed laser beam are diffused by continuous scattering inside the specimen. When this occurs, the temporal change of the spread signal is measured. According to the diffusion signal measured in this way, the above-mentioned various physical properties of the specimen are measured by comparing with the theoretical diffusion equation using various physical properties (ultrasonic absorption coefficient, crystal grain size) as parameters. A method is provided. According to this prior art, in principle, the crystal grain size of the specimen can be measured as in this patent. However, since the intensity of the diffusing ultrasonic waves is generally very small, the signal-to-noise ratio (S / N ratio) is generally used in a production line in a poor environment (large steel plate thickness, various noise factors). ) Is very low, and in order to derive the diffusion equation that most closely approximates the measured diffusion signal, it is necessary to derive the diffusion equation while continuously changing the parameters and compare this with the measurement signal. However, it has a disadvantage in that an evaluation criterion for determining a theoretical diffusion equation that is most approximate to the measurement signal must be provided. Because of these disadvantages, this prior art is not suitable for on-line measurement in a production line, even though it can be applied in a laboratory environment. In addition, problems that may occur during on-line measurement in a production line and solutions to the problems are not presented.

  Furthermore, according to another conventional technique for measuring physical properties of a plate material using ultrasonic resonance (Patent Document 3), the thickness and mechanical properties of a specimen are measured using the resonance frequency of longitudinal waves and / or transverse waves. Although a measurement method is provided, in order to apply this technique, a large number of continuous ultrasonic reflected wave signals must be acquired. Have. Further, it is necessary to analyze the resonance frequency from a large number of measured continuous ultrasonic signals, and to calculate the thickness and physical properties of the measurement object through this analysis. In this prior art (Patent Document 3), the resonance frequency analysis is performed. In addition, since a method for automatically performing physical property calculation has not been presented, there is a problem that it is difficult to apply online in a production line that requires real-time signal processing.

  A conventional technique relating to a laser interferometer for ultrasonic measurement is disclosed in Patent Document 4. In this patent, a method for measuring an ultrasonic wave reaching the surface of a specimen using a confocal Fabry-Perot interferometer is provided, and a method for stabilizing the interferometer as described above is also presented. . However, according to the method presented in this patent, in order to stabilize the interferometer, a device having a large scale, an expensive and complicated circuit configuration such as a lock-in amplifier or an electro-optic cell must be used. Don't be. In this prior art (Patent Document 4), the frequency of the ultrasonic measurement laser must always be adjusted in order to maintain the optimum condition of the confocal Fabry-Perot interferometer. It also has the disadvantage of having to configure the circuit. In recent years, when configuring a measurement device using a laser, there has been a tendency to use a commercialized laser in order to facilitate the configuration of the device. In the case of such a commercialized laser, It is not easy to change the internal configuration. According to this conventional technique (Patent Document 4), it is impossible to use a commercialized laser as it is, and the internal configuration of the commercialized laser is changed, or according to the purpose of the measuring apparatus. It is very difficult to realize the content of the patent because the laser must be manufactured by itself.

Scruby, C.I. B. et al, "Laser-Ultrasonics: Techniques and Applications", Adam Hillger, Bristol, UK, 1990. US Pat. No. 6,684,701 US Pat. No. 6,532,821 US Pat. No. 6,057,927 US Pat. No. 4,659,224

  The present invention has been devised in order to solve the above-mentioned problems of the prior art and stably measure the crystal grain size of various steel plates produced in steelworks on-line. It is possible to easily install the measuring device on the production line by performing the generation and measurement of ultrasonic waves on the same surface of the measuring object surface so that the measuring device exists only above the steel plate being produced. It is possible to reduce the influence of the plasma present when generating high-intensity ultrasonic waves, enable efficient ultrasonic measurement, and calculate the crystal grain size in real time from the measured ultrasonic signal To provide an on-line crystal grain size measuring apparatus and method that facilitates online crystal grain size measurement using a signal processing algorithm.

  The present invention utilizes a pulsed laser to provide in real time crystal grain size information necessary for efficient rolling operations in the hot (or cold) rolling process of various steel plates produced at steelworks. The present invention relates to an on-line crystal grain size measuring device that generates ultrasonic waves on the surface of a steel sheet, measures the ultrasonic waves propagated inside the steel sheet using a laser interferometer, and measures the crystal grain size of the steel sheet in real time. More specifically, by performing ultrasonic wave (longitudinal wave) oscillation and measurement on the same surface of the steel sheet, online application to the steel sheet during hot (or cold) rolling is facilitated, and the optimum super Improve longitudinal wave measurement efficiency by applying sound wave oscillating position and measurement position, and at the same time reduce plasma interference, stabilize the ultrasonic measurement signal through the application of an efficient laser interferometer, Using calibration curves corrected for the effects of wave diffraction, ultrasonic oscillation direction and temperature, it is possible to derive the crystal grain size from the measured ultrasonic attenuation coefficient. By selecting only the optimum frequency and applying only this frequency to the attenuation coefficient analysis and the crystal grain size measurement, it is possible to measure the crystal grain size quickly and easily. The present invention relates to an on-line crystal grain size measuring apparatus that enables crystal grain size measurement.

The gist of the present invention is as described below.
(1) A first invention is an on-line crystal grain size measuring apparatus for measuring a crystal grain size of a metal plate on-line using a laser ultrasonic method in a metal plate production line.
Pulsed laser 10 for generating ultrasonic waves on the surface of the measuring object 60; ultrasonic measuring laser 30 for detecting reflected ultrasonic waves of the measuring object 60; light beam generated by the ultrasonic measuring laser An optical fiber 33 for guiding 35 to the vicinity of the measurement object; an optical fiber 41 for guiding the scattered light of the light beam 35 reflected by the surface of the measurement object 60 to the ultrasonic stable measurement unit 20; The surface of the measurement object 60 is irradiated with the light beam 13 to generate ultrasonic waves in the measurement object, and the ultrasonic measurement laser 30 transmitted by the optical fiber 33 to the same surface as the generation surface of the ultrasonic waves. The optical head 70 irradiates the light beam 35, and collects the scattered light of the light beam 35 reflected by the reflected ultrasonic wave and makes it incident on the optical fiber 41. An ultrasonic stability measuring unit 20 for measuring the intensity of the reflected ultrasonic wave by detecting an intensity change of the scattered light transmitted by the optical fiber 41; and a crystal from an electrical output signal of the ultrasonic stable measuring unit 20 Consists of a signal processing unit 40 for calculating the particle size,
The stable ultrasonic measurement unit 20 uses the frequency shift of the scattered light transmitted by the optical fiber 41 from the frequency of the ultrasonic measurement laser due to the reflected ultrasonic wave as a change in the intensity of the transmitted interference light and the reflected interference light. A laser interferometer configured by a confocal Fabry-Perot interferometer 116 to be detected; a part of the light beam of the ultrasonic measurement laser 30 is separated into the confocal Fabry-Perot interferometer 116 by an optical fiber 52; An interferometer stabilizing unit that transmits and stabilizes the interference of the confocal Fabry-Perot interferometer 116; plasma radiation that is present when generating ultrasonic waves on the surface of the measurement object; 2 is an on-line crystal grain size measuring device including a high-speed shutter 110 that prevents the light from being incident on the crystal.
(2) In a second aspect of the invention, the ultrasonic measurement laser 30 is a continuous wave laser, the optical fiber 33 is a polarization-maintaining optical fiber, and the optical fiber 41 is a multimode optical fiber. It is an on-line crystal grain size measuring apparatus of description.
(3) A third invention is a light beam of the pulsed emission laser 10 on one surface of the measuring object 60 by condensing Irradiation to generate ultrasonic waves, a position substantially the same position where the ultrasonic wave is generated wherein a light beam 35 of ultrasonic measurement laser 30 and the condenser-irradiation, of the measuring object 60 is a longitudinal wave reflected and returned to the light beam at the opposite surface of the one side of the condenser-irradiated reflection an online grain size measurement device according to, characterized in that for measuring the ultrasonic wave (1) or (2).
(4) In a fourth aspect of the invention, the light beam of the ultrasonic measurement laser 30 is transmitted by being incident on the optical fiber 33 via a half-wave plate 53 whose rotation plane is rotated. A polarizing beam splitter 82, a quarter-wave plate 83, a first condensing optical system 1 and a second condensing optical system, and a light beam incident from the optical fiber 33 is converted into the polarizing beam splitter. After being reflected at 90 ° by 82, the light is condensed on the surface of the measurement object 60 by the first condensing optical system through the quarter-wave plate, and the scattered light 36 reflected by the surface is reflected on the first collection light. After being collected by the optical optical system, the light is transmitted through the quarter-wave plate 83 and the polarizing beam splitter 82, and further incident on the multimode fiber 41 by the second condensing optical system (1). Online as described in one of (3) It is a crystal grain size measuring device.
(5) In a fifth aspect of the invention, the first condensing optical system includes a concave lens 84 and one or two convex lenses (85 and / or 86) installed closer to the object to be measured than the concave lens. The cross-sectional area of the scattered light from the surface of the measuring object 60 is reduced and then incident on the second condensing optical system, and incident on the multimode optical fiber 41 from the second condensing optical system. The on-line crystal grain size measuring apparatus according to (4), wherein the incident efficiency of the scattered light into the multimode optical fiber is increased by reducing the angle.
(6) In a sixth invention, the laser interference unit transmits the scattered light transmitted by the multimode optical fiber 41 in the order of the polarization beam splitter 92 and the quarter-wave plate 93, and then The light is incident on a focal Fabry-Perot interferometer 116, and the reflected interference light of the confocal Fabry-Perot interferometer 116 is reflected by the polarization beam splitter 92 by 90 degrees to be separated from the scattered light by the photodetector 100. The on-line crystal grain size measuring apparatus according to any one of (1) to (5), wherein measurement is possible.
(7) In a seventh aspect of the invention, the laser interfering unit includes a quarter-wave plate 97 and a polarizing beam splitter 98 at the rear of the confocal Fabry-Perot interferometer 116 opposite to the incident side of the scattered light. Are arranged in order, and the transmitted interference light of the confocal Fabry-Perot interferometer 116 is transmitted through the polarization beam splitter 98, and the intensity of the transmitted interference light is measured by the photodetector 99. The on-line crystal grain size measuring apparatus according to one of (1) to (6).
(8) In an eighth aspect of the invention, the interferometer stabilization unit separates a part of the output beam of the ultrasonic measurement laser 30 from the light beam that irradiates the measurement object, and the confocal Fabry-Perot interferometer 116, the interference light (102) is formed, and the resonance of the confocal Fabry-Perot interferometer 116 is stabilized by utilizing the intensity change of the interference light. The on-line crystal grain size measuring apparatus as described in one of them.
(9) In a ninth aspect of the invention, the interferometer stabilization unit separates a part of the output beam of the ultrasonic measurement laser 30 from the light beam that irradiates the measurement object, and transmits the separated beam using a multimode optical fiber. After 90 degree reflection by the polarization beam splitter 98 and incidence on the confocal Fabry-Perot interferometer 116, the transmitted interference light is reflected by 90 degrees by the polarization beam splitter 92, and further reflected by the prism to transmit the transmitted interference light. The on-line crystal grain size measuring apparatus according to (7), wherein the intensity change of the light is detected by the photodetector 104 and the resonance of the confocal Fabry-Perot interferometer 116 is stabilized.
(10) In the tenth aspect of the invention, the interferometer stabilization unit performs interference signal processing using the output signal of the photodetector 104 so as to keep the interference condition of the Fabry-Perot interferometer at a predetermined interference condition. A stabilization control circuit 105; a piezoelectric actuator 107 installed on a resonator mirror 95 of the confocal Fabry-Perot interferometer; and a piezoelectric drive adjuster 106 that drives the piezoelectric actuator by an output signal of the interference stabilization control circuit 105. The on-line crystal grain size measuring apparatus according to (9), characterized in that:
(11) In an eleventh aspect of the invention, the laser interference unit detects the intensity of transmitted interference light and reflected interference light of the scattered light from the surface of the measurement object 60 in the confocal Fabry-Perot interferometer 116, respectively. And (1) to (99), comprising an ultrasonic signal detection device 39 for obtaining a reflected ultrasonic signal by differentially amplifying the two electric signals after being converted into electric signals by the detector 99 and the optical detector 100. The on-line crystal grain size measuring apparatus according to one of (10).
(12) In a twelfth aspect, the signal processing unit obtains a Fourier frequency component of the reflected ultrasonic signal output from the ultrasonic signal detection device 39 by FFT, and obtains an ultrasonic attenuation coefficient for each frequency. The on-line crystal grain size measuring apparatus according to any one of (1) to (11), wherein the crystal grain size to be measured is calculated.
(13) In a thirteenth aspect, the signal processing unit has information related to a correlation equation between a crystal grain size of a measurement object and an ultrasonic attenuation coefficient, which are derived in advance through a measurement experiment using a specimen of the measurement object, The online crystal according to any one of (1) to (11), wherein a crystal grain size is rapidly calculated by substituting an ultrasonic attenuation coefficient measured at the time of online measurement into the correlation equation. Particle size measuring device.
(14) The fourteenth invention is an on-line crystal grain size measuring method for measuring a crystal grain size of the metal plate on-line using a laser ultrasonic method in a metal plate production line,
The surface of the measurement object 60 is irradiated with the light beam 13 generated by the pulsed laser 10 to generate ultrasonic waves in the measurement object, and is transmitted by the optical fiber 33 to the same surface as the generation and generation surface of the ultrasonic waves. Then, the scattered light of the light beam 35 reflected by the reflected ultrasonic wave of the ultrasonic wave is collected by using the optical head 70 and irradiated to the optical fiber 41. The intensity of the reflected ultrasonic wave is measured by the ultrasonic stable measuring unit 20 by detecting the intensity change of the scattered light transmitted by the optical fiber 41; the crystal is obtained from the electric output signal of the ultrasonic stable measuring unit 20; An online crystal grain size measuring method for calculating a grain size in a signal processing unit,
The stable ultrasonic measurement unit 20 uses the frequency shift of the scattered light transmitted by the optical fiber 41 from the frequency of the ultrasonic measurement laser due to the reflected ultrasonic wave as a change in the intensity of the transmitted interference light and the reflected interference light. A laser interfering unit including a confocal Fabry-Perot interferometer 116 to detect; a part of the light beam of the continuous wave laser 30 for ultrasonic measurement is separated, and the confocal Fabry-Perot interference is separated by a multimode optical fiber An interferometer stabilizing unit that transmits to the meter 116 and stabilizes the interference of the confocal Fabry-Perot interferometer 116; plasma radiation light that is present when the ultrasonic wave is generated is incident on the confocal Fabry-Perot interferometer 116. This is an on-line crystal grain size measuring method including a high-speed shutter 110 that suppresses the above.

  As a result, according to the on-line crystal grain size measuring apparatus according to the present invention, an ultrasonic wave is remotely generated using a pulsed laser, and the ultrasonic wave propagating through the inside of the specimen is converted into a confocal Fabry-Perot interferometer. By using the measuring device used to measure remotely, it is possible to perform ultrasonic testing of the measurement target being transferred in the production line, and to generate and measure ultrasonic waves on the same surface and at the same position on the measurement target surface. By making the measuring device exist only on one side of the steel plate being produced, it is possible to easily install the measuring device in the production line, and by generating high-intensity ultrasonic waves by the ablation effect, Even in the case of a thick measuring object, the crystal grain size can be measured.

  Also, by generating and measuring ultrasonic waves on the same surface and at the same point of the measurement object, loss of ultrasonic energy due to mode conversion that occurs when ultrasonic waves (longitudinal waves) are reflected on the surface of the measurement object This facilitates the calculation of the ultrasonic propagation distance and consequently the calculation of the ultrasonic attenuation coefficient, and the plasma present when generating high-intensity ultrasonic waves due to the fusion effect is used for ultrasonic signal measurement. By reducing the influence on the photodetector, it has an advantage that a more accurate attenuation coefficient can be measured through efficient ultrasonic measurement.

  In addition, after selecting one specific frequency (Φ) having the best correlation with the crystal grain size for a specific measurement object in advance, only the attenuation coefficient (α (Φ)) of this frequency (Φ) is used. It also has the advantage that real-time online crystal grain size measurement is facilitated by quickly calculating the crystal grain size.

  Yet another advantage of the present invention is that in the measurement of ultrasonic waves using a laser, the scattered light reflected at the measurement point can be confocal with maximum efficiency without loss by an efficient scattered light collecting device. Ultrasonic signal measurement is facilitated by transmitting to a measuring device using a Fabry-Perot interferometer.

  In addition, the configuration of the measuring apparatus using an efficient confocal Fabry-Perot interferometer firstly causes no loss in the output light of the interferometer used for ultrasonic measurement, and secondly, the interferometer. It is possible to stabilize the interferometer stably at all times regardless of the intensity change of the scattered light incident on the light source. Third, the two types of output light of the interferometer through the two photodetectors, that is, It also has the advantage that the intensity of the ultrasonic signal can be increased by measuring all reflected interference light and transmitted interference light.

  Hereinafter, embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

First, FIG. 1 shows an overall drawing of an on-line crystal grain size measuring apparatus according to the present invention. The pulse beam generated from the pulse laser 10 is condensed by the lens 11 and then incident on the surface of the steel plate 60 to be measured by the total reflection mirror 12 to generate various ultrasonic waves (longitudinal wave, transverse wave, and surface wave). (Reference: Non-Patent Document 1). As described above, the generation efficiency of various ultrasonic waves generated by the incidence of the pulsed laser beam is determined by the intensity density of the incident pulse beam, and when the intensity density of the pulse beam increases (on the surface to be measured, When it becomes approximately 5 × 10 ⁸ W / cm ² or more), ultrasonic waves are generated mainly by ablation of the surface material. As described above, since ultrasonic inspection with high intensity is necessary for ultrasonic flaw detection in a production line, generation of ultrasonic waves by the ablation effect is required. In order to generate ultrasonic waves by the fusion of the surface to be measured in this way, a Q-switched laser that generates a laser beam with a short pulse width of several nanoseconds to several tens of nanoseconds is mainly used.

Even when the same laser pulse is used, the intensity density of the laser pulse beam on the surface to be measured varies depending on the size of the focused pulse beam. Usually, since the cross section of the laser pulse beam is circular, the pulse beam focused on the surface to be measured is also circular, and the intensity density increases as the size of the pulse beam, that is, the diameter (S ₁ ) decreases. . Usually, when realizing the generation conditions of the ablation effect, the size (S ₁ ) of the pulse beam is adjusted. The size (S ₁ ) of the pulse beam focused on the measurement target surface is determined by the focal length (F ₁ ) of the lens (11) and the propagation distance (D ₁ ) of the pulse beam between the lens and the measurement target surface. Determined by. That is, when D ₁ and F ₁ coincide with each other, the size (S ₁ ) of the focused pulse beam is the smallest, and S ₁ increases as D ₁ becomes smaller than F ₁ . In the present invention, D ₁ and F ₁ are appropriately adjusted so that ultrasonic waves are generated mainly by the ablation effect.

  As described above, various ultrasonic waves (longitudinal waves, transverse waves, and surface waves) are generated by the ablation effect, and the longitudinal waves generated at this time are mainly measured as shown in FIG. It propagates around the direction perpendicular to the target surface. In a production line, steel plates being rolled usually have upper and lower surfaces that are parallel to each other. Therefore, as in the present invention, in order to facilitate online measurement, the generation and measurement of ultrasonic waves are performed on the same surface of the measurement target, and the crystal grains generated when the ultrasonic waves propagate through the measurement target are used. When trying to obtain the crystal grain size using the ultrasonic attenuation coefficient, an ultrasonic wave propagating perpendicularly to the surface to be measured is generated as shown in FIG. It is most efficient to measure the ultrasonic wave reflected and returned from the surface at the same point where the ultrasonic wave is generated.

  When the generation point of ultrasonic waves is different from the measurement point, ultrasonic waves that propagate at a certain angle with respect to the surface to be measured must be used as shown in FIG. In this case, when the ultrasonic wave is reflected on the opposite surface of the surface to be measured, a part of the longitudinal wave energy is converted into a transverse wave by the mode conversion, so that the longitudinal wave used for measuring the crystal grain size is used. It has the disadvantage of loss of strength. In addition, when the ultrasonic generation point and measurement point are different as shown in FIG. 2- (c), the calculation of the ultrasonic propagation path and propagation distance necessary for calculating the ultrasonic attenuation coefficient becomes very complicated. There are also disadvantages. For the above technical reasons, when measuring the grain size online on the production line, ultrasonic waves (longitudinal waves) are generated vertically by the fusion effect, and at the same point as the ultrasonic generation point. It is most efficient to measure ultrasound.

  As shown in FIG. 1, ultrasonic measurement is performed by an ultrasonic stable measurement unit 20, a multimode optical fiber (33, 41) for transmitting a laser beam and scattered light, and a scattered light collecting unit 34. It has been realized. The linearly polarized laser beam generated from the ultrasonic measurement laser 30 of the ultrasonic stable measurement unit 20 passes through the half-wave plate 53 and the polarization beam splitter 31, and then is polarized by the condensing lens 32 with a polarization plane maintaining optical fiber (PMF). Then, it is transmitted to the scattered light collecting section 34. Then, the laser beam transmitted to the scattered light collecting unit 34 by the polarization plane holding optical fiber 33 is condensed on the measurement target surface via the optical element inside the scattered light collecting unit 34, and reflected on the opposite surface. The reflected longitudinal wave returned to the measurement surface is used for measurement.

As described above, such an ultrasonic measurement laser beam 35 is displayed as shown in FIG. 3A in order to facilitate the measurement of the reflected longitudinal wave and the calculation of the attenuation coefficient in the production line. The laser pulse beam 13 for ultrasonic wave generation and the laser beam 35 for ultrasonic wave measurement are incident on the same position on the surface of the measurement object and are focused on the surface of the measurement object as shown in FIG. It is condensed by the size of the diameter S ₂ in the center of the light spot 14 of the ultrasonic generation laser pulse beam. As shown in FIG. 3B, normally, as the diameter (S ₂ ) of the light spot 37 by the ultrasonic measurement laser beam is smaller, the scattered light collecting unit 34 is used to measure the surface of the measurement object. After collecting the scattered light, the efficiency of transmission to the laser interference unit 38 through the optical fiber 41 is increased. The transmission efficiency of the scattered light 36 depending on the size of the light spot 37 of the ultrasonic measurement laser beam will be described in more detail as follows. As shown in FIG. 4, the ultrasonic measurement laser beam transmitted through the optical fiber 33 is collimated by the collimator 80 and then reflected by the polarization beam splitter 82.

  The reflectance of the laser beam by such a polarizing beam splitter 82 varies depending on the polarization direction of the laser beam. The half-wave plate 53 shown in FIG. 1 is an optical element that rotates the polarization direction of a linearly polarized laser beam. Therefore, by appropriately rotating the polarization direction of the laser beam transmitted through the polarization-maintaining optical fiber 33 by the rotation of the crystal axis of the half-wave plate 53, the reflectance by the polarization beam splitter 82 is maximized. . The laser beam thus efficiently reflected by the polarization beam splitter 82 passes through the quarter-wave plate 83 and is then condensed on the surface of the measurement object 60 through the concave lens 84 and the convex lenses (85, 86). The At this point, the laser beam that was originally linearly polarized becomes circularly polarized after passing through the quarter-wave plate 83. The scattered light 36 reflected on the surface of the measurement object 60 again passes through the convex lenses (85, 86) and the concave lens 84 to become parallel light, and then passes through the quarter-wave plate (83) and is again linear. Become polarized. Thus, the polarization direction of the light linearly polarized again by the quarter-wave plate (83) is rotated by 90 degrees with respect to the original polarization direction, and is reflected by the polarization beam splitter 82 according to the above principle. The rate is minimal. Based on such a principle, the scattered light 36 reflected on the surface of the measurement object 60 passes through the polarization beam splitter 82 most efficiently, and is further condensed on the core cross section of the multimode optical fiber 41 by another convex lens 81. The

Usually, the core of a multimode optical fiber has a diameter of about several tens to several hundreds of μm, and light incident on such a core is transmitted. Accordingly, the smaller the size of the light spot focused on the core cross section of the multimode optical fiber 41 by the convex lens 81, the greater the amount of light incident on the optical fiber. Thus, the size of the light spot collected on the core cross section of the multimode optical fiber 37 is proportional to the size of the light spot (S ₂ ) collected on the surface of the measurement object 60 by the optical principle. For such technical reasons, the smaller the diameter (S ₂ ) of the light spot 37 by the ultrasonic measurement laser beam, the more scattered light is collected on the surface to be measured using the scattered light collecting unit 34. Later, the efficiency of transmission to the laser interference unit 38 through the optical fiber 41 increases. Yet another factor that affects the transmission efficiency of scattered light by the optical fiber 41 is the incident angle of the light collected on the cross section of the optical fiber by the convex lens.

  That is, as shown in FIG. 5, when the incident angle (θ) of light from the condenser lens is large, the light incident on the optical fiber core is absorbed by the optical fiber cladding and is not transmitted. is there. According to the scattered light collecting unit 34 shown in FIG. 4 presented in the present embodiment, the scattered light 36 reflected on the surface of the measurement object 60 has a cross-sectional size due to the convex lens (85, 86) and the concave lens 84. After the collimated light is reduced, it is condensed on the cross section of the optical fiber 41 by another convex lens 81, so that the incident angle (θ) displayed in FIG. 5 is small. Therefore, according to the scattered light collection device (34) of FIG. 4, an efficient optical fiber transmission of scattered light is possible. As described above, according to the scattered light collecting unit 34 of FIG. 4 presented in the present invention, the laser beam transmitted by the polarization-maintaining optical fiber (33) can be transmitted without loss at maximum efficiency. The scattered light 36 condensed on the surface of the measuring object 60 and reflected on the surface of the measuring object 60 can be transmitted to the laser interference unit 38 through the multimode optical fiber 41 with the maximum efficiency.

  The scattered light 36 transmitted to the multimode optical fiber 41 is analyzed for the frequency by the laser interference unit 38, and an optical signal resulting from the ultrasonic signal is output. FIG. 6 shows the configuration of the laser interference unit 38 presented in the present invention. In FIG. 6, the scattered light transmitted by the multimode optical fiber 41 is converted into parallel light by the collimator 91, and then passes through the high-speed shutter 110, the polarization beam splitter 92, and the quarter-wave plate 93. The incident light enters a confocal Fabry-Perot interferometer (referred to as an interferometer) 116. Here, the operation of the high-speed shutter 110 is controlled by the shutter adjuster 111. That is, the trigger signal is applied to the shutter adjuster (11) at the moment when the pulsed laser for ultrasonic oscillation 10 oscillates the pulse beam, and the shutter adjuster 111 has passed a specific time since the trigger signal was applied. Later, during that time, the high-speed shutter 110 that was closed is operated to allow the scattered light to pass through. An example of the configuration of the high-speed shutter 110 is shown in FIG. 16, and high-speed light is opened and closed by an EO modulator. As a result of this operation of the high-speed shutter 110, the scattered light that has passed through the high-speed shutter 110 enters the interferometer, and forms interference light 96 by multiple reflection inside the resonator of the interferometer. The interference light 96 that interferes with the multiple reflections inside the resonator is reflected or transmitted from both ends of the interferometer. As shown in FIG. 7, the intensity of the reflected or transmitted light is determined by the interference condition. Is done. At this time, the interference condition for determining the intensity of the transmitted or reflected light is expressed by the light frequency (v) and the distance (d) between the two resonant mirrors, as shown in FIG.

  That is, when the frequency (v) of the scattered light or the length (d) of the resonator changes due to the ultrasonic wave reaching the surface to be measured, the intensity (A) of the light reflected by the interferometer is transmitted. The intensity of light (B) changes. Accordingly, when the length (d) of the resonator is constant, the change in the intensity (A) of the light reflected by the interferometer and the intensity (B) of the transmitted light is used for ultrasonic measurement. be able to. The stabilization region of the interferometer displayed in FIG. 7 shows the change in the intensity of reflected light (A) and the intensity of transmitted light (B) with respect to a constant change in the frequency (v) of scattered light. This indicates the largest area, and when the interferometer is located in this area, the ultrasonic measurement efficiency is highest. Therefore, it is required that the interference condition of the interferometer is always in this region. Since the center frequency of the scattered light is always constant, the change in the interference condition of the interferometer is mainly caused by the change in the length (d) of the resonator, and when stabilizing the interferometer, The piezoelectric actuator 107 displayed in FIG. 6 is used. That is, the length of the resonator is adjusted by the piezoelectric actuator 107 so that the interferometer is always located in the stabilization region of the interferometer displayed in FIG. The intensity (A) of the light reflected by the interferometer in FIG. 7 is measured by the photodetector 100 in FIG. That is, since the scattered light reflected by the interferometer passes through the quarter-wave plate 93 again, the polarization direction rotates, and is reflected by the polarization beam splitter 92 and enters the photodetector 100. Its intensity is measured efficiently.

  Interfering light that passes through the interferometer passes through the quarter-wave plate 97. Since the polarization direction can be adjusted by appropriately rotating the quarter-wave plate 97, the polarized beam A splitter 98 can be passed. Therefore, the intensity (B) of the transmitted light can be measured by still another photodetector 99. The crystal axes of the two quarter-wave plates (93, 97) in FIG. 6 are appropriately adjusted, and the light passing through the quarter-wave plate 93 twice has a polarization direction of 90 degrees. In the case of light that has been rotated and passed through two quarter-wave plates (93, 97) in sequence, there is no change in polarization direction. In this way, the proper arrangement of the quarter wave plates (93, 97) and the polarizing beam splitters (92, 98) allows the reflected and transmitted interference light to pass through the two photodetectors (99, 99, without loss). 100).

  The automatic stabilization procedure of the interferometer of FIG. 6 is as follows. That is, as shown in FIG. 1, a part of the output beam of the ultrasonic measurement laser 30 is reflected by the polarization beam splitter 31 and then transmitted to the multimode optical fiber 52 using the condenser lens 51. To do. As shown in FIG. 6, the laser beam transmitted by the multimode optical fiber 52 is reflected by the polarization beam splitter 98, passes through the quarter-wave plate 97, enters the interferometer, and is multiplexed. The interference light 102 formed by reflection is transmitted, and the transmitted interference light that has passed through the interferometer passes through another quarter-wave plate 93 and is reflected by the polarization beam splitter 92 and the prism 103, and is detected by the photodetector 104. Intensity is measured. At this time, the output signal of the photodetector 104 is as shown in the lower chart of FIG. 7. The interferometer stabilization control circuit 105 and the piezoelectric actuator 106 always output the output of the photodetector 104 as shown in FIG. ?) To be located in the interferometer stabilization region shown. Thus, the method of generating the separate interference light 102 other than the interference light 96 for ultrasonic measurement and using it for stabilizing the interferometer has the following advantages. First, there is no loss in the interference light 96 used for ultrasonic measurement, and second, the interferometer is always stabilized stably regardless of the intensity change of the scattered light incident on the interferometer. Third, the two photodetectors (99, 100) can measure all the reflected interference light and transmitted interference light of the interferometer.

  As described above, the change in the intensity of the reflected interference light and the transmitted interference light of the interferometer indicates an ultrasonic signal that has reached the surface of the measurement object 60. At this time, the phases of the ultrasonic signals measured through the two photodetectors (99, 100) are opposite to each other. Therefore, if the differential amplifier 108 is used as shown in FIG. 8, the ultrasonic signal can be increased by a factor of two. Such differential amplification also has the effect of removing electrical noise of the same phase applied to the two photodetectors (99, 100). FIG. 9 shows an embodiment in which two photodetector signals are differentially amplified. In FIG. 8, the signal of the differential amplifier (108) is applied to the signal processing unit 40 shown in FIG. 1 through the frequency filter (bandpass filter) 109, and the crystal grain size () to be measured is Will be calculated.

  As described above, the high-speed shutter 110 in FIG. 6 passes or blocks the scattered light transmitted to the multimode optical fiber 41. Such a high-speed shutter 110 is used for the purpose of reducing the influence of plasma radiation generated when ultrasonic waves are generated. That is, as described above, when measuring the crystal grain size online in the production line, it is necessary to generate high-intensity ultrasonic waves due to the ablation effect, and in order to easily calculate the ultrasonic attenuation coefficient, It is most efficient to measure the ultrasonic wave reflected and returned from the surface opposite to the sound wave generation surface at the same point where the ultrasonic wave is generated.

  However, at the time of generation of ultrasonic waves due to such an ablation effect, generation of plasma that emits light of a continuous wavelength is accompanied at the generation position of the ultrasonic waves, and the generation and measurement of ultrasonic waves are the same (or When performed at a close point), the light emitted from the plasma enters the laser interferometer for ultrasonic measurement and acts as an obstacle factor for ultrasonic measurement. As shown in FIG. 10, the plasma radiation accompanying the generation of the ultrasonic wave as described above is an initial stage (several microseconds) from the time when the ultrasonic pulse for laser generation is incident on the surface to be measured. Within). Compared to this, since the minimum propagation distance (L) of the ultrasonic wave used for the crystal grain size measurement corresponds to twice the thickness of the steel plate to be measured, for example, in the case of a measurement target having a thickness of 10 mm The arrival time of the ultrasonic wave (longitudinal wave) is approximately 3.3 μs or more, and is the time when the intensity of the plasma radiation is lowered as shown in FIG. The generation of high-intensity ultrasonic waves due to the fusion effect is mainly required when measuring the crystal grain size of a thick measurement object. As a result, the arrival time of ultrasonic waves is 3 to 3. Most cases are 4 μs or more. Therefore, the intensity of the plasma radiation generally reaches the highest point before the ultrasonic wave reaches.

  However, since an extremely sensitive photodetector such as APD (avalanche photo diode) is used for ultrasonic measurement, the APD is saturated by plasma radiation. Then, the photodetector once saturated in this way requires a certain amount of time to restore to a normal operating state even if the intensity of the plasma radiation light decreases thereafter. Therefore, even if the ultrasonic wave arrives when the intensity of the plasma radiation light decreases, it is difficult to measure normal ultrasonic waves because the photodetector is not completely restored from the saturated state. In general, optical noise generated by the incidence of light other than the measurement laser beam is generally solved by installing an optical filter that allows only light of a specific wavelength to pass through at the input of the photodetector. However, since the plasma radiation has a continuous wavelength spectrum, it has the same wavelength component as the wavelength of the measurement laser beam. Therefore, the effect of plasma radiation cannot be solved by a normal optical filter. The high-speed shutter 110 displayed in FIG. 6 is for solving such an existing problem, and very strong plasma generated light is generated at the initial stage of ultrasonic generation by a pulsed laser beam. Efficient ultrasonic measurement is enabled by preventing the light from entering the measurement photodetector.

  That is, the pulse signal laser 10 for ultrasonic measurement applies a trigger signal to the shutter adjuster 111 while oscillating a pulse beam. As a result, after a certain time (ΔT) has elapsed from the moment when the shutter adjuster (111) receives the trigger signal, the high-speed shutter 110 is opened to allow the scattered light to pass, so that the scattered light enters the interferometer. Then, ultrasonic measurement starts. That is, as shown in FIG. 10, the high-speed shutter 110 is operated immediately before the first reflected ultrasonic wave (longitudinal wave) reaches the lateral position by the shutter adjuster 111 and allows the scattered light to pass therethrough. The operation time (ΔT) of the high-speed shutter 110 is calculated using the thickness of the measurement target and the propagation speed of the ultrasonic wave (longitudinal wave). As a result of this action, when the intensity of the plasma radiation light decreases, the inflow of the scattered light for ultrasonic measurement into the interferometer begins, and if the ultrasonic wave is measured, a certain amount of the plasma radiation light together with the scattered light is detected by the photodetector. Even if it flows into the light source, saturation of the photodetector due to plasma radiation does not occur, and normal ultrasonic measurement becomes possible.

  Hereinafter, a method for automatically calculating the crystal grain size using the ultrasonic signal efficiently measured by the operation of the present invention as described above will be described in detail as follows.

The attenuation coefficient of the ultrasonic wave propagating inside the substance changes depending on the crystal grain size of the substance. The correlation between the ultrasonic attenuation coefficient and the crystal grain size differs depending on the frequency of the ultrasonic waves. Therefore, the crystal grain size (D) to be measured is usually measured by the following procedure. As a first step, a continuous reflected ultrasound signal is measured as shown in FIG. In the ultrasonic signal waveform shown in FIG. 11, a continuous series of ultrasonic signals displayed as R _i (i = 1, 2, 3,...) Is as shown in FIG. This is a signal that is reflected on the surface of the measurement object 60 and continuously reaches the measurement point. As a second step, FFT (Fast Fourier Transform) is performed on each continuous reflected ultrasound signal as shown in FIG. 11 to obtain a continuous reflected ultrasound signal as shown in FIG. Obtain the frequency spectrum of the sound wave signal. As a third step, the frequency attenuation factor (α (φ)) for each frequency is obtained by using the frequency spectrum of the continuous reflected ultrasonic signal in FIG.

α (φ) = (l / 2l) ln (I _Ri (φ) / I _{Ri + 1} (φ)) (Formula 1)
In Equation 1, l is the thickness of the specimen, and I _Ri (φ) indicates the ultrasonic intensity of the i-th reflected wave (R _i ) for the ultrasonic frequency φ. Then, as a final step, the ultrasonic attenuation coefficient (α (φ)) for each frequency obtained by the above procedure is analyzed, and the crystal grain size of the measurement object is calculated.

  However, in order to calculate the information on the crystal grain size by analyzing the frequency-specific ultrasonic spectrum obtained through the FFT of the continuous reflected ultrasonic signal and the FFT, it usually takes a lot of time. Since complex comparative analysis algorithms are required, it is not suitable for online measurement in a production line that requires real-time crystal grain size derivation. In order to solve such a problem, in the present invention, after a specific frequency (Φ) having the best correlation with the crystal grain size is selected in advance for a specific measurement object, this frequency (Φ) Presents a method for rapidly calculating the crystal grain size using only the damping coefficient (α (Φ)). That is, as shown in FIG. 14, from the steel plates or steel materials being produced in the production line, one or many measurement objects whose crystal grain size is important for quality and whose crystal grain size needs to be controlled are selected. After selecting the correlation equation between the crystal grain size and the ultrasonic attenuation coefficient (α) of the specific frequency (Φ) for each of these measurement objects through selection and measurement experiments, this is calculated during online measurement. It presents a method used to calculate

As described above, the configuration of the measurement apparatus used when deriving the correlation equation between the crystal grain diameter and the ultrasonic attenuation coefficient (α) of the specific frequency (Φ) experimentally in advance before performing the online measurement is shown in FIG. The same measuring device as shown in This is because the ultrasonic wave generated by the laser pulse beam varies in the degree of diffraction depending on the size (S ₁ ) of the light spot of the pulse beam. FIG. 15 shows an example in which the correlation between the optimum frequency (Φ = 8.8 MHz) selected by the method presented in FIG. 14 and the crystal grain size is experimentally obtained. In the final stage in the procedure presented in FIG. 14, the following correlation equation is calculated through the correlation analysis of data as shown in FIG.

D _Mi = f (α (Φ)) (Formula 2)
In Equation 2, D _Mi is the crystal grain size of the measuring object M _i . By using the correlation equation obtained in this way for a specific measurement target, the crystal grain size of this measurement target is automatically determined by measuring the attenuation coefficient of a specific ultrasonic frequency for the specific measurement target online. Can be calculated.

  Hereinafter, the crystal grain size to be measured is calculated in real time during online measurement using the correlation equation between the crystal grain size calculated by the method presented in FIG. 14 and the ultrasonic attenuation coefficient of the specific (optimum) frequency. A method will be described.

The continuous reflected ultrasonic signals (R ₁ , R ₂ , R _2, etc. of FIG. 11) measured by the above-described technical procedure for the measurement object 60 being produced in the production line are displayed in FIG. As described above, a signal having a specific frequency is input to the signal processing unit 40 through the frequency filter (bandpass filter) 109. At this time, the frequency filter 109 is adjusted to pass the specific (optimum) frequency (Φ) calculated by the method presented in FIG. Then, the signal processing unit 40 obtains the intensity of the input ultrasonic signal having the specific (optimum) frequency (Φ), and calculates the attenuation coefficient using Equation 1. Subsequently, an attenuation coefficient is substituted into a correlation equation such as Equation 2 input in advance to the arithmetic processor of the signal processing unit 40 to calculate a crystal grain size. Thus, by applying the crystal grain size calculation method devised by the present invention, it is possible to quickly measure the crystal grain size to be measured in the production line by a very simple signal processing procedure.

It is the schematic of the on-line crystal grain size measuring device by this invention. It is drawing which shows the mode conversion which generate | occur | produces at the time of reflection of the longitudinal wave produced | generated by the laser pulse. It is drawing which shows the spot for ultrasonic generation, and the spot for ultrasonic measurement. It is the schematic which shows the scattered light collection part by this invention. It is drawing which shows the incident angle of the scattered light by a condensing lens, and the cross section of an optical fiber. It is the schematic which illustrated the laser interferometer for ultrasonic measurements by this invention. It is drawing which shows the relationship between the reflected interference light and transmission interference light of a confocal Fabry-Perot interferometer. 1 is a schematic diagram illustrating an ultrasonic signal detection apparatus according to the present invention. It is drawing explaining increase of the ultrasonic signal by differential amplification. It is drawing which shows the intensity | strength change in the time-axis of the plasma radiation light accompanying at the time of the production | generation of an ultrasonic wave. It is the Example which measured the reflected ultrasonic signal measured by continuously reflecting on the upper and lower surfaces of a measurement object. It is drawing which showed the longitudinal wave which propagates by multiple reflection in the inside of a measuring object. It is drawing which shows the frequency spectrum of the reflected ultrasonic signal measured continuously. 3 is an algorithm for deriving a correlation formula between a crystal grain size of an object to be measured and an ultrasonic attenuation coefficient presented by the present invention. It is the Example which derived | led-out the correlation between a crystal grain diameter and an ultrasonic attenuation coefficient by this invention. It is an Example of a high-speed shutter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Pulse type laser 11 Condensing lens 12 Total reflection mirror 13 Laser pulse beam for ultrasonic wave generation 14 Light spot of laser pulse beam for ultrasonic wave generation 20 Ultrasonic stable measurement part 30 Laser for ultrasonic measurement 31 Polarizing beam splitter 32 Condensing Lens 33 Polarization plane holding optical fiber 34 Scattered light collecting unit 35 Laser beam for ultrasonic measurement 36 Scattered light 37 Light spot of laser pulse beam for ultrasonic measurement 38 Laser interference unit 39 Ultrasonic signal detecting unit 40 Signal processing unit 41 Multi Mode optical fiber 51 Condensing lens 52 Multi-mode optical fiber 53 Half-wave plate 60 Measurement object 70 Optical head 80 Collimator 81 Condensing lens 82 Polarizing beam splitter 83 Quarter wave plate 84 Concave lens 85 Condensing lens 86 Condensing Lens 91 Collimator 92 Polarized Bee Splitter 93 4 minutes resonator mirror 96 interfering light wave plate 94 confocal Fabry-Perot interferometer resonator mirror 95 confocal Fabry-Perot interferometer
97 Quarter-wave plate 98 Polarizing beam splitter 99 Photo detector 100 Photo detector 101 Collimator 102 Interfering light 103 Prism 104 Photo detector 105 Interferometer stabilization control circuit 106 Piezoelectric actuator regulator 107 Piezoelectric actuator 108 Differential Amplifier 109 Frequency filter 110 High speed shutter 111 Shutter adjuster 112 Bandpass filter 113 Polarizer 114 Crystal of EO modulator such as KDP, ADP 115 Polarizer 116 Confocal Fabry-Perot interferometer

Claims (14)

In an on-line crystal grain size measuring apparatus that measures the crystal grain size of a metal plate on-line using a laser ultrasonic method in a metal plate production line,
Pulsed laser 10 for generating ultrasonic waves on the surface of the measuring object 60; ultrasonic measuring laser 30 for detecting reflected ultrasonic waves of the measuring object 60; light beam generated by the ultrasonic measuring laser An optical fiber 33 for guiding 35 to the vicinity of the measurement object; an optical fiber 41 for guiding the scattered light of the light beam 35 reflected by the surface of the measurement object 60 to the ultrasonic stable measurement unit 20; The surface of the measurement object 60 is irradiated with the light beam 13 to generate ultrasonic waves in the measurement object, and the ultrasonic measurement laser 30 transmitted by the optical fiber 33 to the same surface as the generation surface of the ultrasonic waves. The optical head 70 irradiates the light beam 35, and collects the scattered light of the light beam 35 reflected by the reflected ultrasonic wave and makes it incident on the optical fiber 41. An ultrasonic stability measuring unit 20 for measuring the intensity of the reflected ultrasonic wave by detecting an intensity change of the scattered light transmitted by the optical fiber 41; and a crystal from an electrical output signal of the ultrasonic stable measuring unit 20 Consists of a signal processing unit 40 for calculating the particle size,
The stable ultrasonic measurement unit 20 uses the frequency shift of the scattered light transmitted by the optical fiber 41 from the frequency of the ultrasonic measurement laser due to the reflected ultrasonic wave as a change in the intensity of the transmitted interference light and the reflected interference light. A laser interferometer configured by a confocal Fabry-Perot interferometer 116 to be detected; a part of the light beam of the ultrasonic measurement laser 30 is separated into the confocal Fabry-Perot interferometer 116 by an optical fiber 52; An interferometer stabilization unit that transmits and stabilizes the interference of the confocal Fabry-Perot interferometer 116; and plasma radiation that is present when ultrasonic waves are generated on the surface of the object to be measured. An on-line crystal grain size measuring device comprising a high-speed shutter 110 that prevents the light from being incident on the screen.   The on-line crystal grain size measuring apparatus according to claim 1, wherein the ultrasonic measurement laser (30) is a continuous wave laser, the optical fiber (33) is a polarization maintaining optical fiber, and the optical fiber (41) is a multimode optical fiber. The light beam of the pulsed emission laser 10 on one surface of the measuring object 60 by condensing Irradiation to generate ultrasonic waves, light of the ultrasonic measurement laser 30 at a position substantially the same position where the ultrasonic wave is generated characterized in that the beam 35 and the condenser-irradiation, of the measuring object 60, measuring the light beam reflected ultrasonic sound waves are longitudinal waves reflected and returned by the opposite surface of the one side of the condenser-irradiated The on-line crystal grain size measuring apparatus according to claim 1 or 2.   The light beam of the ultrasonic measurement laser 30 is transmitted by being incident on the optical fiber 33 through a half-wave plate 53 that rotates its polarization plane. A first wavelength plate 83, a first condensing optical system 1 and a second condensing optical system, and the light beam incident from the optical fiber 33 is reflected by the polarizing beam splitter 82 after 90 degrees; After condensing on the surface of the measurement object 60 with the first condensing optical system through the quarter-wave plate and collecting the scattered light 36 reflected by the surface with the first condensing optical system, 4. The light beam transmitted through the quarter-wave plate 83 and the polarizing beam splitter 82 and further incident on the multimode fiber 41 by the second condensing optical system. 5. Online crystal grain size described in one item measuring device.   The first condensing optical system includes a concave lens 84 and one or two convex lenses (85 and / or 86) installed on the measurement object side from the concave lens. By reducing the cross-sectional area of the scattered light and making it incident on the second condensing optical system, and reducing the incident angle from the second condensing optical system to the multimode optical fiber 41, the scattering 5. The on-line crystal grain size measuring apparatus according to claim 4, wherein the incident efficiency of light into the multimode optical fiber is increased.   The laser interference unit transmits the scattered light transmitted by the multimode optical fiber 41 in the order of the polarization beam splitter 92 and the quarter-wave plate 93 and then enters the confocal Fabry-Perot interferometer 116. The reflected interference light of the confocal Fabry-Perot interferometer 116 is reflected by the polarization beam splitter 92 by 90 degrees so as to be separated from the scattered light and measured by the photodetector 100. The on-line crystal grain size measuring apparatus according to claim 1.   In the confocal Fabry-Perot interferometer 116, the laser interfering unit sequentially installs a quarter-wave plate 97 and a polarizing beam splitter 98 on the rear side opposite to the incident side of the scattered light. The intensity of the transmitted interference light is measured by the photodetector 99 so that the transmitted interference light of the Fabry-Perot interferometer 116 is transmitted through the polarizing beam splitter 98. The on-line crystal grain size measuring apparatus according to one item.   The interferometer stabilization unit separates a part of the output beam of the ultrasonic measurement laser 30 from the light beam that irradiates the measurement object, and enters the confocal Fabry-Perot interferometer 116 for interference light (102). And the resonance of the confocal Fabry-Perot interferometer 116 is stabilized using the intensity change of the interference light. measuring device.   The interferometer stabilizing unit separates a part of the output beam of the ultrasonic measurement laser 30 from the light beam that irradiates the measurement object, transmits it through a multimode optical fiber, and reflects it by 90 degrees with the polarization beam splitter 98. Then, the incident light is incident on the confocal Fabry-Perot interferometer 116, and then the transmitted interference light is reflected by 90 degrees by the polarization beam splitter 92 and further reflected by the prism, and the change in the intensity of the transmitted interference light is detected by the photodetector 104. 8. The on-line crystal grain size measuring apparatus according to claim 7, which detects and stabilizes the resonance of the confocal Fabry-Perot interferometer.   The interferometer stabilization unit uses the output signal of the photodetector 104 to perform signal processing so as to maintain the interference condition of the Fabry-Perot interferometer at a predetermined interference condition, the confocal point 10. A piezoelectric actuator 107 installed in a resonator mirror 95 of a Fabry-Perot interferometer, and a piezoelectric drive controller 106 for driving the piezoelectric actuator by an output signal of the interference stabilization control circuit 105. The on-line crystal grain size measuring apparatus described in 1.   The laser interference unit electrically transmits the scattered light from the surface of the measurement object 60 and the intensity of the transmitted interference light and the reflected interference light in the confocal Fabry-Perot interferometer 116 by the light detector 99 and the light detector 100, respectively. 11. An ultrasonic signal detection device 39 for obtaining a reflected ultrasonic signal by differentially amplifying the two electric signals after conversion into a signal, according to one of claims 1 to 10. The on-line crystal grain size measuring apparatus described.   The signal processing unit obtains a Fourier frequency component of the reflected ultrasonic signal output from the ultrasonic signal detection device 39 by FFT, obtains an ultrasonic attenuation coefficient for each frequency, and calculates a crystal grain size to be measured. The on-line crystal grain size measuring apparatus according to any one of claims 1 to 11, wherein   The signal processing unit has information related to a correlation equation between the crystal grain size of the measurement object and the ultrasonic attenuation coefficient derived in advance through a measurement experiment using a specimen of the measurement object, and the ultrasonic attenuation measured at the time of online measurement. The on-line crystal grain size measuring apparatus according to any one of claims 1 to 11, wherein a crystal grain size is quickly calculated by substituting a coefficient into the correlation equation. In the on-line crystal grain size measuring method for measuring the crystal grain size of the metal sheet on-line using a laser ultrasonic method in a metal plate production line,
The surface of the measurement object 60 is irradiated with the light beam 13 generated by the pulsed laser 10 to generate ultrasonic waves in the measurement object, and is transmitted by the optical fiber 33 to the same surface as the generation and generation surface of the ultrasonic waves. Then, the scattered light of the light beam 35 reflected by the reflected ultrasonic wave of the ultrasonic wave is collected by using the optical head 70 and irradiated to the optical fiber 41. The intensity of the reflected ultrasonic wave is measured by the ultrasonic stable measuring unit 20 by detecting the intensity change of the scattered light transmitted by the optical fiber 41; the crystal is obtained from the electric output signal of the ultrasonic stable measuring unit 20; An online crystal grain size measuring method for calculating a grain size in a signal processing unit,
The stable ultrasonic measurement unit 20 uses the frequency shift of the scattered light transmitted by the optical fiber 41 from the frequency of the ultrasonic measurement laser due to the reflected ultrasonic wave as a change in the intensity of the transmitted interference light and the reflected interference light. A laser interfering unit configured by a confocal Fabry-Perot interferometer 116 to detect; a part of the light beam of the continuous wave laser 30 for ultrasonic measurement is separated, and the confocal Fabry-Perot interference is separated by a multimode optical fiber 52; An interferometer stabilizing unit that transmits to the meter 116 and stabilizes the interference of the confocal Fabry-Perot interferometer 116; plasma radiation light that is present when the ultrasonic wave is generated enters the confocal Fabry-Perot interferometer 116. An on-line crystal grain size measuring method comprising a high-speed shutter 110 for suppressing the above.

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