JPH08285822A - Ultrasonic inspection apparatus - Google Patents

Abstract

PURPOSE: To provide an ultrasonic inspection apparatus by which the existence of a cavity, impurities and other flaws can be inspected even in a sample in which ultrasonic waves at a high frequency are attenuated immediately. CONSTITUTION: The exciting energy of the output of a laser 10 for ultrasonic generation is lowered by setting a pulse width at about 100 to 200ns. A laser beam 11 is amplified by a laser amplifier 12 in a state that the pulse width has been maintained, and it is output. Consequently, the amplified laser beam 11 has a wide pulse width and a sufficient intensity. When the laser beam 11 is emitted to a sample surface 16, ultrasonic waves at a low frequency of about 1 to 2MHz are generated. The ultrasonic waves at the frequency of this extent are propagated easily even in a sample such as a slab or a molten ore in which ultrasonic waves at a high frequency of about 10 to 20MHz are hard to propagate, and the sample can be examined as an object.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser ultrasonic inspection for detecting the internal state of a sample in a non-contact manner by generating an ultrasonic wave in the sample using a laser beam and optically detecting it. It relates to the device.

[0002]

2. Description of the Related Art The so-called laser ultrasonic method is one of the methods for detecting internal defects in various materials. About this, for example, ultrasonic TECHNO May issue (vol.5, N
o.5, p38 (1993) Nihon Kogyo Shuppan). Since this method detects ultrasonic waves using laser light, it can be used for non-contact inspection for examining the internal state of a material or the like that cannot be inspected by contact.

FIG. 5 is a block diagram showing a configuration of an example of a laser ultrasonic device using the laser ultrasonic method. In the figure, an ultrasonic wave generation laser 70 is a laser for generating ultrasonic waves on the surface of a sample to be inspected, and is a laser capable of emitting a pulsed laser beam having a relatively large output, such as a Q switch YAG laser. Used. A laser beam 71 emitted from the ultrasonic wave generating laser 70 is a beam splitter 72 and a mirror 7.
The surface of the sample 75 to be inspected is irradiated via 3, 74. When the surface of the sample 75 is irradiated with the laser light 71, the surface of the sample is instantaneously evaporated, and thermal stress or evaporation reaction force at that time generates a wide-band ultrasonic wave of about 10 to 20 MHz. This ultrasonic wave propagates inside the sample, reaches the back surface of the sample, is reflected here, and then returns to the sample surface as an echo again. The laser 70 for ultrasonic wave generation
A part of the laser light from is guided to the photodetector 76 by the beam splitter 72, is detected, and is used as a trigger signal of the oscilloscope 77.

On the other hand, a probe laser 80 provided separately from the ultrasonic wave generating laser 70 is a laser for detecting ultrasonic echoes returning to the surface of the sample 75, and for example, He- having a stable frequency. Ne laser or the like is used. The laser light 81 emitted from the probe laser 80 is reflected by the mirror 82, the beam splitter 83, 1 /
The surface of the sample 85 is irradiated via the four-wave plate 84.
This reflected light is transmitted through the quarter wavelength plate 84 and the beam splitter 8
The light enters the Fabry-Perot interferometer 86 through the lens 85. The light transmitted through the Fabry-Perot interferometer 86 is converted into an electric signal by the photodetector 87 including a photodiode. This signal is amplified by an amplifier 88, passed through a high pass filter 89 to remove low frequency noise, and then supplied to an oscilloscope 77.

FIG. 6 is a schematic diagram showing the result of measuring the optical frequency of He-Ne laser light by sweeping the resonator length (horizontal axis) of the Fabry-Perot interferometer, and FIG. It is the schematic which expanded and showed the horizontal axis. That is, the cavity length at which the transmitted light intensity reaches its peak is the wavelength ν of the He-Ne laser light.
Corresponding to (632.8 nm), when the resonator length is changed, the wavelength at which the transmitted light intensity of the interferometer peaks also changes, so the transmitted light intensity at the wavelength ν decreases. Utilizing this characteristic, He-
The transmitted light intensity with respect to the wavelength ν of the Ne laser light is A ′ in FIG.
The resonator length of the Fabry-Perot interferometer 86 is adjusted in advance so that the value becomes a value indicated by a point, and the reflected light from the sample 75 is incident on the Fabry-Perot interferometer 86.

When the surface of the sample 75 is ultrasonically vibrated by the ultrasonic echo reflected on the back surface of the sample, the spatial position of the surface of the sample is displaced in the cycle of this ultrasonic wave. Therefore, when the laser light from the probe laser 80 is reflected by the sample surface, it undergoes a Doppler shift due to this displacement, and the wavelength changes. As shown in FIG.
When the reflected light from No. 5 undergoes a frequency change of ± Δν 'due to the Doppler shift, the intensity of the transmitted light also undergoes a change of ± ΔI' as shown in the figure. That is, the change in wavelength can be converted into the change in intensity by passing the reflected light from the sample 75 through the Fabry-Perot interferometer.

Therefore, the output signal of the photodetector 87 greatly changes every time the ultrasonic echo repeatedly reflected on the front and back surfaces of the sample 75 returns to the front surface. When this signal is displayed on the oscilloscope 77 with time on the horizontal axis and signal intensity on the vertical axis, if there is no defect in the sample, the signal is output at constant time intervals determined by the speed of sound in the sample and the thickness of the sample. The intensity changes. On the other hand, if there is a defect such as a cavity or an impurity inside the sample, the ultrasonic wave is reflected by this defect portion, and the signal intensity changes in a time shorter than the above time. Therefore,
By examining this time interval, it is possible to examine the presence or absence of defects inside the sample.

[0008]

By the way, in the steel manufacturing process, pores and inclusions such as silica and alumina may exist in the slab immediately after coming out from the tundish or continuous casting. Such defects remain inside as they are even if they are thinned by repeating rolling with a mill, and when a steel material is formed, the strength of these defective parts is reduced, which also causes breakage.

For this reason, in the steel manufacturing process,
There is a demand for non-contact, non-destructive inspection of slabs before rolling for inclusions such as voids, silica, and alumina inside, and for this purpose use the laser ultrasonic device. Is being considered. However, at the slab stage, the temperature of the plate is still high, and since it has not been rolled, the ultrasonic waves with a high frequency of about 10 to 20 MHz generated by an ordinary laser ultrasonic device are immediately attenuated and propagated to the inside. It is difficult to do. Therefore, it is not suitable to directly apply the conventional laser ultrasonic inspection apparatus to the defect inspection of the slab.

The present invention has been made on the basis of the above-mentioned problems, and even a sample in which a high frequency ultrasonic wave is immediately attenuated can be inspected for the presence or absence of voids, inclusions and other defects. The purpose is to provide a device.

[0011]

The invention according to claim 1 for solving the above-mentioned problems is directed to a laser beam irradiating means for generating a laser beam having a wide pulse width and irradiating the sample with the laser beam, and to the sample. A probe light irradiating means for irradiating a probe light of a single wavelength, and a Doppler shift detecting means for detecting a Doppler shift received by the probe light reflected from the sample and converting it into a light intensity are provided. It is a thing.

According to a second aspect of the present invention, in the first aspect of the present invention, the laser light irradiating means is a laser light oscillator for generating a laser light having a wide pulse width by reducing excitation energy in the Q switch laser, A laser light amplifier that amplifies this laser light while maintaining its pulse width is provided.

According to a third aspect of the present invention, in the first or second aspect of the invention, the pulse width of the laser beam corresponds to the frequency at which the ultrasonic waves generated in the sample effectively propagate in the sample. It is characterized by being.

The invention according to claim 4 is characterized in that, in the invention according to claim 1, 2 or 3, the sample is a slab in a steel manufacturing process.

The invention according to claim 5 is characterized in that, in the invention according to claim 1, 2 or 3, the sample is a molten ore in a steel manufacturing process.

[0016]

According to the present invention, with the above construction, the frequency of the ultrasonic wave generated in the sample can be lowered by widening the pulse width of the laser beam. Further, by amplifying the laser light generated by reducing the excitation energy of the Q-switched laser light oscillator in order to widen the pulse width by the laser amplifier, the amplified laser light has high intensity and wide pulse width. Will be things. The pulse width of the laser light is such that an ultrasonic wave of a frequency capable of propagating in the sample is generated based on the characteristics of the sample to be inspected.

When the laser light irradiation means irradiates the surface of the sample with the laser light having a wide pulse width, the frequency of the ultrasonic wave generated in the sample is low and the laser light is amplified by the amplifier. The strength is high enough.
The low-frequency ultrasonic waves thus generated can be propagated even in a sample in which high-frequency ultrasonic waves, such as a slab or a molten metal, are hard to propagate, and return to the sample surface again as an echo. Therefore, the ultrasonic echo can be captured by irradiating the sample surface with the probe light and detecting the Doppler shift received by the reflected light by the Doppler shift detection means.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic block diagram of an ultrasonic inspection apparatus according to an embodiment of the present invention. Laser for ultrasonic wave generation 1
Reference numeral 0 is a laser for irradiating the surface of the sample to be inspected with laser light to generate ultrasonic waves, and for example, a pulse laser such as a Q-switch YAG laser is used. However, the excitation of the oscillator is suppressed lower than that of the ultrasonic wave generation laser 70 shown in FIG. Moreover, in the ultrasonic wave generating laser 70 shown in FIG.
The pulse width of the laser light is set to about 10 ns in order to generate ultrasonic waves of a certain degree. On the other hand, in the ultrasonic generating Q-switched laser 10 of this embodiment, the pulse width of the laser light is set to about 100 to 1000 ns. The reason and method for doing this will be described later.

Laser light 11 emitted from the Q-switched laser 10 for generating ultrasonic waves enters a laser amplifier 12, where it is amplified and output. Laser amplifier 1
2 simply increases the excitation intensity of the laser light,
The pulse width does not change before and after amplification. The amplified laser light 11 passes through the beam splitter 13, is focused by the lens 14, and is reflected by the dichroic mirror 15.
It is reflected by and is irradiated on the surface of the sample 16. Sample 16
Laser evaporation occurs on the surface of No. 1 at the moment when the laser light 11 is irradiated, and ultrasonic waves are generated by thermal stress or evaporation reaction force at that time. This ultrasonic wave propagates inside the sample 16, is reflected on the back surface of the sample, and returns to the surface of the sample as an ultrasonic echo again. In addition, a part of the laser light 11 is
The photodetector 17 is reflected by the beam splitter 13.
Be led to. The output of the photodetector 17 is used as a trigger signal for the oscilloscope 18.

On the other hand, the probe laser light 21 is emitted from the probe laser 20 provided separately from the ultrasonic wave generating laser 10. Laser for probe 20
For example, an Ar ⁺ laser is used. The probe laser light 21 passes through the dichroic mirror 15, illuminates the surface of the sample 16, and is reflected as scattered light. When an ultrasonic echo appears on the sample surface, the reflected light of the probe laser light 21 undergoes Doppler shift, and its optical frequency changes. The reflected probe laser light 21 is reflected by the mirror 22, focused by the lens 23, and incident on the confocal Fabry-Perot interferometer 24. When the probe laser light 21 undergoes a Doppler shift to change the optical frequency, this change is converted into a change in the light intensity in the confocal Fabry-Perot interferometer 24 and is captured by the photodetector 25. The output signal of the photodetector 25 is amplified by the amplifier 26,
It is supplied to the oscilloscope 18 through the filter 27. Incidentally, as will be described later, the cutoff frequency of the high-pass filter 27 is set to about 0.5 MHz corresponding to the low frequency of the ultrasonic wave generated in the sample.

By the way, for example, F. Alan McDonald, App
As shown in l. Phys. Lett., Vol. 56, (3) (15 Jan. 1990), when the pulse width of the laser light for ultrasonic wave generation is widened, the frequency of the ultrasonic wave generated in the sample is increased. Is known to be low. FIG. 2 is a diagram schematically showing the difference in the frequency spectrum of ultrasonic waves generated when the pulse width of laser light is narrowed (a) and when the pulse width is widened (b). Assuming that the pulse width of the laser beam for ultrasonic wave generation is, for example, about 10 ns as in the conventional case, the generated ultrasonic wave is as shown by the curve a in FIG.
It has a wide frequency spectrum with a peak between 10 and 20 MHz. On the other hand, the pulse width of laser light is 1
When it is set to about 00 to 1000 ns, the frequency spectrum of the generated ultrasonic wave is limited to a narrow band of low frequency up to about 1 to 2 MHz as shown by the curve b in FIG.
With such low frequency ultrasonic waves, the attenuation in the slab is less than that of ultrasonic waves of 10 to 20 MHz.
It is easy to propagate even in the slab.

One of the methods for increasing the pulse width of the laser light for ultrasonic wave generation is JBDeaton, Jr., ADWMcKie, JBSp.
icer, and JWWagner, Appl. Phys. Lett., Vol.56, (2
4) (11 June 1990). That is, it is possible to widen the pulse width by forming a resonator having a long path by using a large number of mirrors and running a laser beam during this. According to this method, the pulse width of the laser light can be widened while maintaining the high excitation intensity of the laser. However, this method requires a resonator having a path of 10 meters or more, and such a long resonator path not only increases the size of the device, but also
In addition to requiring accurate alignment of many mirrors,
Maintaining this alignment is difficult and impractical.

It is also known that the pulse width of the laser beam can be widened by lowering the excitation energy of the oscillator of the Q-switch laser. For this, see, for example, A.YARIV, "Basics of Optoelectronics."
(3rd edition, Maruzen Co., Ltd.) can be referred to. Therefore, if the excitation energy of the oscillator of the ultrasonic wave generating Q-switched laser is lowered, the pulse width of the laser light becomes wider, and it is possible to generate an ultrasonic wave having a low frequency that easily propagates in the sample. However, if the excitation energy of the oscillator of the Q-switched laser for ultrasonic wave generation is simply lowered, the pulse energy of the laser will be reduced and the intensity of the generated ultrasonic wave will also be reduced. Difficult to detect acoustic echo.

Therefore, in this embodiment, as shown in FIG. 1, the excitation intensity of the Q-switch laser oscillator 10 is lowered to widen the pulse width to about 100 to 1000 ns, and the laser amplifier 12 is provided in the next stage of the laser oscillator 10. In this case, the intensity of the Q-switched laser light for ultrasonic wave generation is increased while maintaining a wide pulse width. By doing so, it is possible to obtain laser light having a sufficient intensity and a wide pulse width. When the surface of the sample is irradiated with the laser light 11 obtained in this way, ultrasonic waves with a low frequency of about 1 to 2 MHz, which easily propagate even in a sample such as a slab, can be detected as the Doppler shift of the laser light for probe. Can be generated with sufficient strength. Moreover, in order to widen the pulse width of the laser light, a long resonator structure that requires a large number of mirrors is not required, the structure is simplified, difficult alignment is not required, and the device can be downsized.

Further, the configuration of this embodiment has the following advantages. That is, the power of noise increases in proportion to the width of the frequency band, but the pulse width of the laser light generated by the Q switch laser for ultrasonic wave generation is widened so that the spectrum of the ultrasonic wave generated in the sample has a low frequency. Limiting to a narrow band can keep the overall noise power low. Therefore, the S / N ratio at the time of detecting the ultrasonic echo by the Doppler shift of the probe laser light can be improved.

Next, an embodiment for inspecting defects inside the slab using this ultrasonic inspection apparatus will be described. FIG. 3 is a schematic diagram showing a step of obtaining a slab from molten ore in a steel manufacturing process. In the figure, when the molten ore 31 in the tundish 30 is poured into the mold 32 in a molten state, the molten ore 31 gradually descends while cooling and solidifying from the surroundings. Underneath the mold, the water is cooled from the surroundings and the solidifying slag is supported below by pinch rolls 33. The method of solidification gradually thickens from the surrounding solidified shell,
The unsolidified part is just an inverted cone. Slab 3
4 is conveyed for a while, and is completely solidified immediately before reaching the cutting device 35. Then, it is cut into a predetermined size by the cutting device 35 and is conveyed by the conveying device 36.

In this embodiment, when the slab is obtained in this way, the presence or absence of defects such as internal holes and inclusions is inspected. For this purpose, an ultrasonic inspection device 40 is provided on the side of the transportation line of the slab 34. The configuration of the ultrasonic inspection device 40 is similar to that shown in FIG. That is, a wide laser beam 11 having a pulse width of about 100 to 1000 ns in the Q-switched laser 10 for ultrasonic wave generation.
Is generated and amplified by the amplifier 12, the slab 3
The surface of No. 4 is irradiated to generate low frequency ultrasonic waves of about 1 to 2 MHz inside the slab. At this stage of the slab,
Ultrasonic waves with a high frequency of about 10 to 20 MHz do not easily propagate, but ultrasonic waves with a low frequency of about 1 to 2 MHz propagate sufficiently, so that ultrasonic waves can be captured by the laser light for the probe, and based on this It is possible to detect defects inside the slab. If defect inspection is performed at an early stage of the steel manufacturing process in this way, the slab in which a defect is found does not need to be subjected to subsequent rolling or other processes, and therefore work is more difficult than in the case where a defect is found in the final stage of the process. Waste is eliminated and overall work efficiency is improved.

Next, an embodiment in which the above ultrasonic inspection apparatus is applied to the inspection of inclusions in the molten ore 31 in the tundish 30 will be described. FIG. 4 is a schematic sectional view showing the tundish and the molten ore therein. The smelt is extremely hot, and the ultrasonic inspection apparatus shown in FIG. 1 cannot be directly placed near the smelt. Therefore, as shown in FIG. 4, an optical waveguide 45 of sapphire is provided inside the molten ore, and the laser light 11 for ultrasonic wave generation and the laser light 21 for probe of FIG. Lead to.

In this case, since the object to be inspected is a liquid molten ore, the frequency of ultrasonic waves that can propagate through the molten ore is about 100 KHz, which is lower than that of the slab. Ultrasonic waves having such a low frequency can be generated by the same idea as described with reference to FIG.
That is, the pulse width of the laser beam is expanded to about 1 to 10 μs by further lowering the excitation intensity of the laser oscillator 10. By setting the pulse width to this level, the frequency of ultrasonic waves generated during smelting becomes approximately 100 KHz. Then, in order to compensate for the decrease in the ultrasonic wave generation intensity due to the reduction in the excitation intensity of the Q-switched laser light, the laser amplifier 12 increases the intensity of the laser light to a predetermined intensity while maintaining the pulse width. As a result, it is possible to generate ultrasonic waves having sufficient strength and propagating in a liquid ore without easily attenuating.

When the laser light of the Q switch for ultrasonic wave generation is guided to the optical waveguide 45 and reaches the tip 45a, the evaporation phenomenon similar to that described with reference to FIG. Sound waves are generated. This ultrasonic wave propagates through the molten ore in the tundish and is attenuated if nothing reflects it. On the other hand, if there are inclusions such as silica and alumina in the molten ore and the ultrasonic waves hit these, the ultrasonic waves are reflected and part of them return to the tip 45a of the optical waveguide 45. Tip 45a of optical waveguide 45
In addition to the laser light for ultrasonic wave generation, the laser light for probe is also guided to the laser light. When the ultrasonic wave reflected from the inclusion returns to the tip 45a of the optical waveguide 45, the laser light for probe is reflected there. Undergoes frequency changes due to Doppler shift. The probe laser beam that has received this frequency change is detected in the same manner as described with reference to FIG. Therefore, when such a frequency change is detected, it can be seen that inclusions are present in the smelt.

The present invention is not limited to the above embodiment, but various modifications can be made within the scope of the invention. For example, in the example shown in FIG. 4, the inspected object was a molten ore, but other high-temperature liquids that require detection of inclusions, for example, melted before being single-crystallized by the pulling method It can also be applied to the inspection of impurities in semiconductors such as silicon. Further, in the above embodiment, the confocal Fabry-Perot interferometer was used as the Doppler shift detecting means, but instead, in the patent application dated April 13, 1995 (“laser ultrasonic inspection apparatus”) As proposed by the applicant, it is also possible to use a cell in which a transparent container is filled with iodine gas or the like having a predetermined absorption characteristic for the wavelength of the probe light. Particularly, in the iron making process where the temperature changes drastically, it may be difficult to adjust and maintain the Fabry-Perot interferometer, and for such an application, it is convenient to use a cell filled with a predetermined gas. In that case, as described in the above patent application, the probe light does not necessarily have to be laser light.

[0032]

As described above, according to the present invention,
A laser amplifier for amplifying a laser beam having a predetermined pulse width emitted from a laser light source is provided, and the laser amplifier amplifies the laser beam without changing the pulse width to obtain a high intensity and a pulse width. A wide laser beam can be obtained. By irradiating this on the surface of the sample to be inspected, a low frequency ultrasonic wave is generated in the sample compared to the conventional one, and such a low frequency ultrasonic wave has a high frequency like slabs and molten ores. Since ultrasonic waves propagate even in a sample in which ultrasonic waves are difficult to propagate, it is possible to provide an ultrasonic inspection apparatus capable of inspecting slabs, molten ores, and other materials that were conventionally difficult to be inspected.

[Brief description of drawings]

FIG. 1 is a schematic block diagram of an ultrasonic inspection apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram schematically showing the difference between the frequency spectra of ultrasonic waves generated when the pulse width of laser light is narrowed and when the pulse width is widened.

FIG. 3 is a schematic diagram showing a step of obtaining a slab from molten ore in a steel manufacturing process.

FIG. 4 is a schematic cross-sectional view showing a tundish and molten metal therein.

FIG. 5 is a block diagram showing a configuration of an example of a laser ultrasonic device using a laser ultrasonic method.

FIG. 6 is a schematic diagram showing the results of measuring the optical frequency of He—Ne laser light by sweeping the resonator length of a Fabry-Perot interferometer.

FIG. 7 is a schematic view showing the horizontal axis of FIG. 6 in an enlarged manner.

[Explanation of Codes] 10, 70 Ultrasonic wave generation Q-switch laser 11 Laser light 12 Laser amplifier 13, 15, 72, 83 Beam splitter 14, 23, 85 Lens 16 Sample 17, 25, 76, 87 Photodetector 18, 77 Oscilloscope 20, 80 Laser for probe 21 Laser light for probe 22, 73, 74, 82 Mirror 24 Confocal Fabry-Perot interferometer 26, 88 Amplifier 27, 89 High-pass filter 30 Tundish 31 Molasses 32 Template 33 Pinch roll 34 Slab 35 Cutting Device 36 Conveying Device 45 Optical Waveguide 75 Sample 84 Quarter Wave Plate 86 Fabry-Perot Interferometer

Claims (5)

[Claims] 1. A laser light irradiating means for generating a laser beam having a wide pulse width and irradiating the laser light to a sample, a probe light irradiating means for irradiating the sample with a probe light having a single wavelength, and a laser beam reflected from the sample. And a Doppler shift detecting means for detecting a Doppler shift received by the probe light and converting it into a light intensity. 2. The laser light irradiating means comprises a laser light oscillator for generating a laser light having a wide pulse width by reducing excitation energy in a Q-switch laser, and a laser for amplifying the laser light while maintaining the pulse width. The ultrasonic inspection apparatus according to claim 1, further comprising an optical amplifier. 3. The pulse width of the laser light is a pulse width corresponding to a frequency at which an ultrasonic wave generated in the sample effectively propagates in the sample.
Or the ultrasonic inspection apparatus according to 2. 4. The ultrasonic inspection apparatus according to claim 1, wherein the sample is a slab in a steel manufacturing process. 5. The ultrasonic inspection apparatus according to claim 1, wherein the sample is molten ore in a steel manufacturing process.