Property measurement system for a metal material

Abstract

Property measurement system for a metal material, comprising: a laser oscillator designed to emit a pulsed laser beam; a lens grouping having a plurality of small lenses of the same shape, wherein the small lenses are placed in a matrix on a plane perpendicular to an optical axis of the pulsed laser beam, and arranged such that a part of a cross-section of the pulsed laser beam can be made incident on each of the plurality of small lenses; a collective lens designed to overlap and concentrate emitted rays coming from the multitude of small lenses of the lens grouping onto the same region of a surface of a metallic material as a measurement target, such that a luminous intensity of the concentrated emitted beam ensures ablation on a concentrated spot; a laser interferometer designed to detect, as an electrical signal, a pulsed ultrasound wave excited by the pulsed laser beam concentrated by the collective lens and propagating through the interior of the metal material; and a signal processing device configured to process the electrical signal, wherein a spot size of the concentrated beams incident on the metal material is greater than a predetermined threshold and the threshold is predetermined as a function of a wavelength of the pulsed ultrasound wave.

Description

Technical field

[0001] The present invention relates to a property measuring system for metal materials using a laser beam. Background technology

[0002] In recent years, there has been a remarkable improvement in metal material properties, such as strength and formability. Consequently, the need to measure such properties of metal materials and a variety of other properties that have strong correlations with the aforementioned properties of metal materials has increased in the manufacturing and inspection processes.

[0003] For example, there is a strong correlation between the strength and grain size of such a metal material, and it is known that if the grain size is reduced to a suitable range of several tens of microns to 1 micron, a high-performance metal material can be obtained that not only has high strength but also excellent processability and reusability. It is common for quality control of the metal material described above to be performed through a destructive test. For example, a test piece is cut from a product roll that has already undergone a rolling process, and its strength is confirmed by a tensile test. However, in such a process, the time required to obtain a measured value is as long as several hours to several days, and it cannot be prevented that defective products continuously occur during this time.

[0004] To solve this problem, property measurement of metal materials by a laser ultrasonic wave method is attempted (see, for example, PTL 1). The laser ultrasonic wave method is a method for measuring metal material properties, such as grain size, in a non-contact manner by irradiating a pulse laser beam onto a surface of the metal material and analyzing the propagation behavior of a pulse-like ultrasonic wave (hereinafter referred to as "pulse ultrasonic wave") in the metal material, the ultrasonic wave having a high frequency generated at the time the pulse laser beam is irradiated. According to this method, measurement results are obtained quickly.

[0005] The properties of metal materials to be measured using pulsed ultrasonic wave propagation characteristics are diverse, including grain size, elasticity, formability (r-value), elastic wave propagation velocity, and the like. Furthermore, it is known that there are strong correlations between the grain size of metal materials and tensile strength, yield strength, and yield strain. Thus, values ​​for these properties can also be obtained using a measurement method similar to that used to measure the grain size of metal materials. Citation listPatent literature PTL 1: JP 2008- 116 209 A PTL 2: US 5 748 318 A

[0006] US 5,748,318 A describes a system for characterizing thin films and interfaces between thin films by measuring their mechanical and thermal properties. In the system, light is absorbed in a thin film or in a structure consisting of multiple thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to obtain information about the ultrasonic waves generated in the structure.The information obtained by applying the measurement methods and apparatus of the invention may include: (a) a determination of the thickness of thin films with a speed and accuracy improved over previous methods; (b) a determination of the thermal, elastic, and optical properties of thin films; (c) a determination of stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects. Summary of the inventionTechnical problem

[0007] When measuring the properties of a metal material using the laser ultrasonic wave method, during the period when the pulse ultrasonic wave propagates through the interior of the metal material, the amplitude of the pulse ultrasonic wave is reduced not only by "(i) attenuation of the pulse ultrasonic wave by dispersion thereof at a grain boundary" used for property measurement of the metal material, but also by "(ii) diffusion of the pulse ultrasonic wave in directions other than a straight direction." When the amplitude of the pulse ultrasonic wave is reduced, the signal-to-noise ratio deteriorates, and the measurement accuracy of the property measurement system is lowered.

[0008] Therefore, in order to more clearly detect the attenuation of the pulse ultrasonic wave due to its dispersion at the grain boundary, it is necessary to suppress the diffusion of the pulse ultrasonic wave. To suppress the diffusion of the pulse ultrasonic wave, there is a method of increasing the spot size of the pulse laser beam. That is, on an outer peripheral portion of the spot, vibrations are confined by the metal material on its boundary surface, and thus the deflection direction of the pulse ultrasonic wave is changed, and diffusion occurs. However, if the spot size is increased, the influence of the outer peripheral portion can be relatively reduced, and the diffusion of the pulse ultrasonic wave in directions other than the straight direction can be suppressed.

[0009] However, there has been a problem that a sufficient effect cannot be obtained by simply amplifying and irradiating a pulse laser beam emitted from a general laser oscillator. That is, the light quantity distribution (beam profile) in the beam cross section of the pulse laser beam emitted from the laser oscillator is generally uneven, and the light quantity near the center of the beam is larger than at its periphery. Therefore, in the case where the pulse laser beam is simply amplified to thereby increase the spot size, the amplitude of the pulse ultrasonic wave to be excited is large at the central portion of the spot in the metal material, and the amplitude of the pulse ultrasonic wave to be excited at the peripheral portion of the spot therein is small.

[0010] Due to such an amplitude difference between the pulse ultrasonic waves, the propagation direction of the pulse ultrasonic wave is not aligned with the direction perpendicular to the emitting surface of the pulse laser beam, and the pulse ultrasonic wave is scattered to the periphery, as in the case where the spot size is small. Therefore, the amplitude of the pulse ultrasonic wave is significantly reduced with respect to the propagation distance, and the signal-to-noise ratio of the pulse ultrasonic wave to be detected is deteriorated. As a result, there has been a problem of lowering the measurement accuracy.

[0011] Considering the problems described above, it is an object of the present invention to provide a property measuring system for a metal material using a pulse laser beam in which the lowering of the measurement accuracy is suppressed. Solution to the problem

[0012] According to one aspect of the present invention, there is provided a property measuring system for a metal material, comprising: (A) a laser oscillator that emits a pulse laser beam; (B) a lens array having a plurality of small lenses having the same shape, the small lenses being laid in a matrix on a plane perpendicular to the optical axis of the pulse laser beam and arranged so that a part of the cross section of the pulse laser beam can be made incident on each of the plurality of small lenses; (C) a condensing lens that overlaps and condenses emitted beams coming from the plurality of small lenses of the lens array onto the same region on a surface of a metal material as a measurement target; (D) a laser interferometer that detects, as an electrical signal, a pulse ultrasonic wave generated by the light condensed by the condensing lens.concentrated pulsed laser beam is excited and propagates through the interior of the metal material; and (E) a signal processing device that processes the electrical signal. Advantageous effects of the invention

[0013] According to the present invention, the property measuring system for a metal material using a pulse laser beam can be provided in which the lowering of the measurement accuracy is suppressed. Short description of the drawings Fig. 1 is a schematic view showing a configuration of a property measuring system for a metal material according to a first embodiment of the present invention. Fig. 2 shows schematic views showing a configuration example of a lens array in the property measuring system for a metal material according to the first embodiment of the present invention: Fig. 2(a) is a plan view; Fig. 2(b) is a cross-sectional view along a IIb-IIb direction of Fig. 2(a); and Fig. 2(c) is a cross-sectional view along a IIc-IIc direction of Fig. 2(a). Fig. 3 is a photograph showing an example of a concentrated spot shape produced by the property measurement system for a metal material according to the first embodiment of the present invention. Fig. 4 is a graph showing an example of a detection waveform of a pulse ultrasonic wave to be excited in an interior of a metal material as a measurement target. Fig. 5 is a schematic view showing a state where a pulse ultrasonic wave transmitted from a piston noise source is scattered. Fig. 6 is a graph showing an example of a light quantity distribution of a pulse laser beam: Fig. 6(a) shows a Gaussian distribution; and Fig. Figure 6(b) shows a unit distribution. Fig. 7 is a flowchart for explaining an example of a waveform analysis method by the metal material property measuring system according to the first embodiment of the present invention. Fig. 8 is a graph showing an example of a detection waveform of the pulse ultrasonic wave detected by the property measuring system for a metal material according to the first embodiment of the present invention. Fig. 9 is a schematic view showing another configuration of the property measuring system for a metal material according to the first embodiment of the present invention. Fig. 10 is a schematic view showing a configuration of a property measuring system for a metal material according to a second embodiment of the present invention. Fig. 11 shows photographs showing a chronological change of a beam profile of the pulsed laser beam: Fig. Figure 11(a) is a photograph showing the beam profile before the chronological change; Fig. Figure 11(b) is a photograph showing the beam profile after the chronological change. Description of the embodiments

[0014] Next, a description will be given of first and second embodiments of the present invention with reference to the drawings. In the following description, the same or similar reference numerals are assigned to the same or similar parts with reference to the drawings. The first and second embodiments to be shown below are only an illustration of an apparatus and a method for embodying a technical idea of ​​this invention, but the embodiments of this invention are not limited to the specific structures, arrangements, and the like of constituent elements to be described below. The embodiments of this invention can be modified in various ways within the scope of the claims. (First embodiment)

[0015] As in Fig. 1, a property measuring system 1 for a metal material according to a first embodiment of the present invention includes: a laser oscillator 10 that emits a pulse laser beam Lp; a lens array 20 having a plurality of small lenses Ls having the same shape laid in a matrix on a plane perpendicular to the optical axis of the pulse laser beam Lp; a condensing lens 30 that overlaps and condenses emitted beams Ld emitted from the plurality of small lenses Ls onto the same region of a surface 101 of a metal material 100 as a measurement target; and an analyzer 40 that analyzes a pulse ultrasonic wave SW propagated through the interior of the metal material 100 as a result of the emitted beams Ld being irradiated onto the surface 101.On the plurality of small lenses Ls, a part of the cross section of the pulse laser beam Lp perpendicular to the optical axis thereof is individually made incident, and the plurality of small lenses Ls individually emits the emitted beams Ld in which shapes of beam cross sections perpendicular to the optical axis are mutually the same.

[0016] In the Fig. In the property measurement system 1 shown in FIG. 1, the pulse laser beam Lp is irradiated onto the surface 101 of the metal material 100, causing small-scale explosions thereon, and the pulse ultrasonic wave SW is excited in the metal material 100. This pulse ultrasonic wave SW is attenuated as it propagates through the interior of the metal material 100 and appears as minute vibrations on a back surface 102 of the metal material 100, which is opposite to the surface 101 onto which the pulse laser beam Lp is irradiated.

[0017] A detection device of the analysis device 40 detects the tiny vibrations as an electrical signal. Fig. 1 shows an example of using a laser interferometer 41 as the detection device. The laser interferometer 41 detects, as the electrical signal, the pulsed ultrasonic wave excited by the pulsed laser beam Lp being condensed by the condensing lens 30 and propagating through the interior of the metal material 100. Specifically, the laser interferometer 41 radiates a weak detection laser beam Lf onto the back surface 102 of the metal material 100. This creates a reference illuminant and reflected light Lr of the detection laser beam Lf, which come from the metal material 100 and interfere with each other. The minute vibrations that have occurred on the back surface 102 of the metal material 100 are thereby detected as a voltage change.A signal processing device 42 of the analysis device 40 processes the electrical signal detected by the laser interferometer 41, thereby measuring properties of the metal material 100. For example, the detected electrical signal is captured by a digital oscilloscope 421 and sent to a signal processing computer 422, and the properties of the metal material 100 are determined by waveform analysis.

[0018] A description is given below of details of the Fig. 1 shown property measurement system 1.

[0019] For the laser oscillator 10, for example, it is possible to use a Q-switched solid-state pulse laser with a large and stable output power. Specifically, a Q-switched solid-state pulse laser with a pulse width ranging approximately from a few nanoseconds to a few tens of nanoseconds, and using neodymium-doped yttrium aluminum garnet (Nd:YAG) as an oscillation solid medium, which is widely used for industrial purposes, is suitable for the laser oscillator 10.

[0020] Generally, the pulse laser beam Lp to be emitted from the laser oscillator 10 has a small divergence angle. Therefore, the pulse laser beam Lp is converted into substantially parallel light by using the magnification system 11 in which a concave lens 111 and a convex lens 112 are combined, so that the beam is incident on the lens array 20.

[0021] Fig. 2(a) to Fig. 2(c) shows a configuration example of the lens array 20. The lens array 20 has a structure in which a large number of the small lenses Ls having the same shape are laid on the plane perpendicular to the optical axis of the pulse laser beam Lp to be made incident thereon. Fig. Figure 2 shows an example where n small square lenses Ls1 to Lsn are arranged in the matrix. The region that Fig. 2(a) is a region to be irradiated with the pulse laser beam Lp, and the beam diameter of the pulse laser beam Lp is Db.

[0022] Besides a square, it is possible to use any shape such as a rectangle, a hexagon, and a circle for the small lenses Ls. Note that if there are gaps among the small lenses Ls, then a concentration efficiency is lowered, and accordingly, it is preferable that the small lenses Ls are arranged without any gaps in between in the lens array 20. A lens pitch w of the small lenses Ls arranged in Fig. 2(c) is determined so that, as will be described later, a sufficient number of the small lenses Ls can be included in the beam cross section of the pulse laser beam Lp to be made incident thereon. Here, the "lens pitch of the small lenses Ls" is the distance between the centers of the adjacent small lenses Ls.

[0023] When the pulse laser beam Lp is made incident on the lens array 20, a portion of the cross section of the laser pulse beam Lp is made incident on each of the small lenses Ls constituting the lens array 20. The small lenses Ls are arranged to intercept a portion of the beam cross section of the pulse laser beam Lp that is affected, so that each of the small lenses Ls forms an image of the shape (e.g., a square) of the small lenses Ls. Whatever the distribution of the light quantity (beam profile) of the beam cross section of the pulse laser beam Lp to be made incident on the lens array, the emitted beams Ld of the respective small lenses Ls, created from an extremely small portion of the pulse laser beam Lp, have a uniform light distribution.

[0024] The respective emitted rays Ld emitted from the lens array 20 are condensed onto the surface 101 of the metal material 100 by the condensing lens 30, which is a convex lens. Note that the focal length of the condensing lens 30 is substantially equal to the standoff distance SD (the interval between the condensing lens 30 and the surface 101 of the metal material 100).

[0025] Then, the respective images from the plurality of small lenses Ls are superimposed and projected onto the focal position of the condensing lens 30, which is a point on the surface 101 of the metal material 100. To achieve this, the pulse laser beam Lp may need to be reshaped so that a concentrated spot SP, to be irradiated with the plurality of pulse laser beams with the uniform light quantity distribution and the same shape (for example, a square) as that of the small lenses Ls, can be formed on the surface 101 of the metal material 100. The shape of the concentrated spot SP is the same as the shape of the small lenses Ls, and its size is determined by the focal length of the condensing lens 30.

[0026] In the concentrated spot SP, the images projected by the plurality of small lenses Ls are superimposed on each other, and accordingly, brightness irregularities caused by interference fringes occur. However, if the number of images to be superimposed is sufficiently increased, the interval between the interference fringes can be reduced to a negligible size, and a substantially uniform light quantity distribution can be obtained on the concentrated spot SP. If the number of images to be superimposed falls below 15, then in some cases the influence of the interference fringes from the waveform of the pulse ultrasonic wave SW propagating through the interior of the metal material 100 cannot be ignored, and in addition, the ability to uniform the light quantity becomes insufficient.Therefore, the lens pitch of the small lenses Ls in the lens array 20 is set so that 15 or more small lenses Ls can be included in the beam cross section of the pulse laser beam Lp to be made incident.

[0027] The focal length of the lens array 20 is decided by experiment or ray tracing simulation calculation so that the size of the concentrated spot SP to be formed on the surface 101 of the metal material 100 can have a desired value. Practically, the focal length of the lens array 20 is within a suitable range from a value equivalent to the pitch distance SD to one-tenth of the pitch distance SD. Since there are various limitations in manufacturing the lens array 20, such small lenses Ls with the desired focal length cannot be obtained in some cases. In this case, a plurality of the lens arrays 20 may be arranged in combination with each other.

[0028] In the pulse laser beam Lp, the peak value of the light density is high. Accordingly, a material made of glass, such as quartz glass, in which the damage threshold is high, is suitable as a raw material of the lens array 20. However, a lens array 20 made of plastic or the like is also applicable, depending on the wavelength and output power required for property measurement of the metal material 100.

[0029] Fig. 3 shows an example where the shape of the concentrated spot SP on the surface 101 of the metal material 100 is printed and recorded on burn paper. Fig. Figure 3 shows the shape of the concentrated spot SP formed by allowing the pulse laser beam Lp with the beam diameter Db of 14 mm to be incident on the lens array 20 in which the small lenses Ls are arranged with a pitch of 1.1 mm in a matrix of 18 rows and 18 columns. Therefore, Fig. 3 shows an example where images of approximately 127 small lenses Ls are superimposed on each other. The shape of the concentrated spot SP is a square, which is the same as the shape of the small lenses Ls of the lens grouping 20, and it is understood that the shapes of the small lenses Ls are projected while being superimposed on each other in a good way. In the Fig. In the example shown in Figure 3, the size Ds of the concentrated spot Sp is 4.5 mm.

[0030] By irradiating the pulse laser beam Lp onto the concentrated spot Sp, the pulse ultrasonic wave SW is excited in the metal material 100, and this pulse ultrasonic wave SW propagates through the interior of the metal material 100. As shown in Fig. 4, the pulse ultrasonic wave SW to be excited in the metal material 100 has components of a variety of vibration modes. For example, to measure the grain size of the metal material 100, multiple reflection echoes (emphasized portions of the waveform indicated by arrows in Fig. 4) of longitudinal wave components. In Fig. 4, waveforms E1, E2 and E3 are a first echo, a second echo and a third echo of the longitudinal wave, respectively.

[0031] Note that in Fig. 4, the ordinate axis represents a relative amplitude (unit: bE (arbitrary unit)), and the abscissa axis represents an elapsed time from the emission of the pulse laser beam Lp. The first echo is detected at the time when the pulse ultrasonic wave SW first reaches the back surface 102 of the metal material 100 after the pulse laser beam Lp is emitted. Subsequent echoes are detected each time the pulse ultrasonic wave SW returns to the back surface 102 after being reflected from the back surface 102 and surface 101. Therefore, the time until the first echo is detected corresponds to the thickness of the metal material 100, and intervals between which the respective subsequent echoes are detected correspond to multiples of twice the thickness of the metal material 100.

[0032] Here, with reference to Fig. 5, a description is made of the pulse ultrasonic wave SW propagating through the interior of the metal material 100. Fig. 5 is a schematic view showing a state where the pulse ultrasonic wave SW transmitted from a flat piston sound source 500 is scattered.

[0033] In the case where the pulse ultrasonic wave SW is transmitted from the piston sound source 500, the region where the pulse ultrasonic wave SW travels straight as a plane wave Wp is called the near field (Fresnel zone), and the distance (near field limit distance x) over which the pulse ultrasonic wave SW travels straight is represented by the following expression (1): x=d2 / (4λ)

[0034] In expression (1), d is the diameter of the piston noise source 500, which is equivalent to the spot size of the pulse laser beam Lp irradiated onto the metal material 100. λ is the wavelength (mm) of the pulse ultrasonic wave SW.

[0035] Meanwhile, the region beyond the near-field limit distance x is called the far field, and here the pulsed ultrasonic wave SW is scattered or diffused as a spherical wave. Regardless of material properties such as grain size, in the far field, the amplitude of the pulsed ultrasonic wave SW rapidly decreases in proportion to the square of the propagation distance. Therefore, to clearly capture the pulsed ultrasonic wave SW for use in waveform analysis, it is preferable to increase the spot size and further extend the near-field limit distance x.

[0036] In the exemplary event of measuring the grain size of the steel material with a thickness of 2 mm, in the case of detecting the waveforms of the first to fifth echoes in the multiple reflections from the back of the steel material, the propagation distance of the fifth echo is represented by the following expression (2): 2(mm)+(1+4×2(rounds)=18(mm)

[0037] The propagation velocity v of a longitudinal ultrasonic wave of the steel material is approximately 5900 m / s, and accordingly, the wavelength λ = v / f of a longitudinal ultrasonic wave of 30 MHz is 0.197 mm. Therefore, according to Expression (1), it is understood that in the case of using a frequency component of 30 MHz or more for waveform analysis, the spot size diameter is preferably set to approximately 4 mm or more.

[0038] The numerical values ​​described above are related to the design, and in fact, there is also a case where no problem occurs even if a smaller spot size is adopted, depending on the extent to which the deterioration of the signal-to-noise ratio can be tolerated, given the measurement purpose. Furthermore, there is also a case where, even in the case where the spot size is smaller, the ultrasonic wave echoes can be clearly detected in such a way that the ultrasonic waveform can be corrected by using a well-known expression representing a reduction in amplitude due to diffusion, etc.

[0039] Meanwhile, regarding the suppression of diffusion of the pulse ultrasonic wave SW, restrictions on the upper limit of the spot size are not particularly strict. However, in response to the output of the laser oscillator 10, there is an upper limit of the spot size at which the light quantity density can ensure ablation on the surface 101 of the metal material 100. Accordingly, the spot size only needs to be equal to or smaller than the upper limit concerned. Therefore, it is not particularly necessary to adjust the upper limit of the spot size in response to the thickness and the like of the metal material 100.

[0040] The above-described study for the diffusion of the pulse ultrasonic wave SW is based on the flat-bulb sound source 500. However, the light quantity distribution (beam profile) in the beam cross section of the pulse laser beam Lp to be emitted from the laser oscillator 10 is not necessarily uniform. In particular, the light quantity distribution of the pulse laser beam of the laser oscillator, which is widely used for industrial purposes, is a Gaussian distribution (normal distribution), in which the light quantity near the beam center is large, and thus the laser concentrability is good. Any distribution approximating this is also suitable. In the case where the beam profile is the Gaussian distribution, as in Fig. 6(a), the difference between the amount of light near the beam center and the amount of light on its periphery is larger compared to that shown in Fig. 6(b) where the beam profile is uniform. Note that the axes of the ordinates of Fig. 6(a) and Fig. 6(b) represent the amount of light, and that axes of abscissas represent positions in the concentrated sport.

[0041] In the case where the pulse laser beam with the Gaussian distribution and a sufficiently large spot size is condensed or concentrated on the surface 101 of the metal material 100 as mentioned above, condensation is generally performed by an optical reduction system made of several lenses. Since the light distribution in the concentrated spot is controlled by lenses, the light quantity distribution of the pulse laser beam to be emitted from the laser oscillator can be projected as it is. Therefore, in the central portion of the concentrated spot, the amplitude of each ultrasonic wave echo to be excited is large, while in the peripheral portion of the concentrated spot, the amplitude of each ultrasonic wave echo to be excited is small. Thanks to this amplitude difference, as mentioned above, the pulse ultrasonic wave is diffused to the peripheral region, as in the case where the spot size is small.As a result, the signal-to-noise ratio of the waveform of the ultrasonic wave to be detected by the laser interferometer is deteriorated, and a measurement accuracy is lowered.

[0042] According to the Fig. However, in the property measuring system 1 shown in Fig. 1, the plurality of small lenses Ls constituting the lens array 20 superimpose and project the images having a uniform light quantity distribution onto the focal position of the condensing lens 30. Therefore, the pulse laser beam Lp is reshaped so that a concentrated spot having the same shape as that of the small lenses Ls and having a light intensity as shown in Fig. 6(b) can be formed on the surface 101 of the metal material 100.

[0043] Therefore, the diffusion of the pulse ultrasonic wave SW to be excited in the metal material 100 by the pulse laser beam Lp can be suppressed to be small. As a result, the amplitude of the pulse ultrasonic wave SW is less likely to be reduced with respect to the propagation distance.

[0044] The pulse ultrasonic wave SW excited by the pulse laser beam Lp propagates through the interior of the metal material 100, and the minute vibrations occurring on the back surface 102 of the metal material 100 are detected as electrical signals by the laser interferometer 41. As described above, the amplitude of the pulse ultrasonic wave SW is suppressed from decreasing, and accordingly, the signal-to-noise ratio of the ultrasonic waveform to be detected by the laser interferometer 41 is improved, making it possible to obtain good measurement accuracy.

[0045] For the laser interferometer 41, it is possible to use a photorefractive interferometer capable of measuring high-frequency vibrations even if the surface 101 of the metal material 100 is a rough surface. Alternatively, it is also possible to use a Fabry-Perot interferometer in a similar manner, since the Fabry-Perot interferometer can measure the high-frequency vibrations on the rough surface. Furthermore, it is also possible to use a Michelson interferometer if the surface 101 of the metal material 100 is a mirror surface.

[0046] A voltage waveform detected by the laser interferometer 41 is recorded in, for example, the digital oscilloscope 421 and is analyzed by the signal processing computer 422, whereby the property measurement of the metal material 100 can be performed.

[0047] Due to the attenuation of the longitudinal ultrasonic wave by its scattering in the grains, the longitudinal ultrasonic wave propagating through the interior of the metal material 100 differs depending on the frequency of the longitudinal ultrasonic wave and the grain size of the metal material 100. For example, Rayleigh scattering, stochastic scattering, and the like are known.

[0048] Below, a description is given of an example of a waveform analysis method based on such a scattering attenuation, with reference to the Fig. Flowchart shown in Figure 7.

[0049] In step S1 of Fig. 7, the waveform data read is decomposed into the respective frequency components by means of a Fourier transform or the like in step S2. Next, in step S3, each of the longitudinal wave echoes is extracted into each of the respective frequency components. In step S4, a change in amplitude of the extracted longitudinal wave echo is approximated by a logarithmic function, and an attenuation factor α(f) of each frequency component is calculated.

[0050] The damping factor α, the frequency f and the grain size D have a correlation relationship of the following expression (3): α=K×Dn−1×fn

[0051] In expression (3), α is the attenuation factor (dB / mm), D is the grain size (mm), f is the frequency (MHz), and K and n are coefficients.

[0052] When expression (3) is transformed, the following expression (4) is obtained: D={α / (K×fn)}1 / (n−1)

[0053] Based on the attenuation factor α(f) of a frequency f, the grain size is determined in step S5 based on Expression (4). Note that values ​​of the coefficients K and n are predetermined by an experiment of measuring a test material in which the grain size is already known. Theoretically, the value of the coefficient n is determined in response to a ratio D / λ of the grain size D and the wavelength λ. That is, n is equal to 4 (n = 4) in the Rayleigh scattering region (0.03 ≤ D / λ ≤ 0.3), and n is equal to 3 (n = 3) in the stochastic scattering region (0.3 ≤ D / λ ≈ 1).

[0054] Fig. Fig. 8 is an example of the waveform of the ultrasonic wave detected by the property measuring system 1 for a metal material according to the first embodiment. Fig. 8, the ordinate axis represents a relative amplitude (unit: b. E. (arbitrary unit)). The abscissa axis represents the elapsed time since the emission of the pulse laser beam Lp. The metal material 100 as a measurement target is a steel plate with a plate thickness of 1.35 mm, and its grain size is approximately 4 μm. Such an ultrasonic waveform detected by the laser interferometer 41 can be clearly detected from the first echo (waveform E1) of the longitudinal wave to the 13th echo (waveform E13) thereof.

[0055] As described above, according to the property measurement system 1 for a metal material according to the first embodiment of the present invention, the light quantity distribution in the beam cross section of the pulse laser beam to be irradiated onto the metal material 100 as the measurement target can be made uniform. Therefore, the diffusion of the pulse ultrasonic wave SW to be excited in the metal material 100 is suppressed. As a result, the property measurement system 1 for a metal material using the pulse laser beam is provided, in which the reduction in measurement accuracy is suppressed.

[0056] Note that the positional relationship between the collective lens 30 and the lens group 20 differs from that shown in Fig. 1. That is, the lens array 20 may be arranged between the laser oscillator 10 and the condenser lens 30, or alternatively, the condenser lens 30 may be arranged between the laser oscillator 10 and the lens array 20.

[0057] Fig. 9 shows an example where the collective lens 30 is arranged between the laser oscillator 10 and the lens array 20. Also by a Fig. 9, the emitted rays Ld to be individually outputted by the plurality of small lenses Ls of the lens array 20 can be condensed or concentrated to the same region of the surface 101 of the metal material 100 as the measurement target by the condensing lens 30.

[0058] As in Fig. 1 and Fig. 9, an aperture plate 51 is further placed between the metal material 100 and the lens array 20 and the condenser lens 30. In order to emit the pulse laser beam Lp to the exterior of a casing 50 in which the laser oscillator 10, the lens array 20, and the condenser lens 30 are housed, the aperture plate 51 is placed in the casing 50.

[0059] Optical instruments, such as lenses and reflecting mirrors, as well as the laser oscillator, are often made of special raw materials and subjected to special coatings to improve their optical efficiency. If dust and the like adhere to the optical instruments, they are easily damaged and their optical efficiency is reduced. Accordingly, all of the instruments are generally housed in a casing with strong sealing capability, and the laser beams are emitted from there through the aperture plate.

[0060] However, in the case where the aperture plate is placed in an actual production line, liquid, powder, and the like sometimes adhere to this aperture plate. In this case, the transmittance of the affected portion is reduced. When the laser beam is incident on the portion where the transmittance is reduced, the portion absorbs the energy of the laser beam and is heated; and the affected portion of the aperture plate is damaged. If the laser beam continues to be irradiated, the damage progresses and expands. When the damage described above occurs, in the conventional compaction system, such a damaged portion acts as a light-shielding mask, so to speak, and the light quantity distribution in the form of a concentrated spot becomes uneven.

[0061] In each of the Fig. 1 and Fig. In contrast, in the property measurement systems 1 shown in FIG. 9, the light quantity distribution of the pulse laser beams can be made uniform in the concentrated spot SP of the surface 101 of the metal material 100. That is, even if the beam profile of the pulse laser beam Lp passing through the aperture plate 51 is deteriorated by the damaged portion of the aperture plate to which the liquid, powder, and the like adhere, the diffusion of the pulse ultrasonic wave SW to be excited in the metal material 100 is suppressed to be small. As a result, the amplitude of the pulse ultrasonic wave SW is less likely to be reduced with respect to the propagation distance, and the signal-to-noise ratio of the waveform of the ultrasonic wave to be detected by the laser interferometer 41 is hardly changed.This means that even if the orifice plate 51 is damaged, it is possible to maintain good measurement accuracy. Therefore, even if the orifice plate 51 is damaged, it is not necessary to replace the orifice plate 51 immediately, and the orifice plate 51 only needs to be replaced in accordance with an idle period in the production schedule. Therefore, it is possible to prevent a decrease in the service frequency of the property measurement system 1. (Second embodiment)

[0062] As in Fig. 10, a property measuring system 1 for a metal material according to a second embodiment of the present invention differs from the property measuring system 1 for a metal material according to the first embodiment shown in Fig. 1, in that it further includes a wavelength conversion crystal 60 that converts the wavelength of the pulse laser beam Lp to be emitted from the laser oscillator 10. Other configurations are similar to those of the first embodiment.

[0063] The absorption capacity of the laser beam on the polished surface 101 of the metal material 100 increases as its wavelength becomes shorter. Therefore, the pulse laser beam Lp to be emitted from the laser oscillator 10 is converted into a short-wavelength beam by the wavelength conversion crystal 60 and then irradiated onto the metal material 100. Thus, in such a highly reflective metal material with the polished surface, a pulse ultrasonic wave SW with a larger amplitude can be excited with respect to the same output power of the pulse laser beam.

[0064] In accordance with the Fig. In the property measurement system 1 shown in Figure 10, the pulse laser beam Lp to be emitted from the laser oscillator 10 is converted into a beam with a shorter wavelength by the wavelength conversion crystal 60 and is then irradiated onto the metal material 100. As the wavelength is shorter, the absorption capacity of the laser beam by the surface 101 of the metal material 100 becomes greater, and accordingly, the pulse ultrasonic wave SW with a larger amplitude can be excited on the polished surface and the like with respect to the same output power of the pulse laser beam. For example, the pulse laser beam Lp from the Nd:YAG laser with a wavelength of 1064 nm is converted into a pulse laser beam Lp with a wavelength of 532 nm or a wavelength of 355 nm by the wavelength conversion crystal 60, such as LBO(LiB 3 O 5 ) and KTP(KTiOPO 4 ), and is then radiated onto the surface 101 of the metal material 100.

[0065] To prevent damage to the wavelength conversion crystal 60, the light quantity density of the pulse laser beam Lp to be incident on the wavelength conversion crystal 60 is usually set to a damage threshold or less. Therefore, if the output power of the pulse laser beam Lp is large, it is necessary to increase the beam diameter of the pulse laser beam Lp to be incident by using such a large-size wavelength conversion crystal 60.

[0066] However, if the wavelength conversion crystal of large size 60 is used, there is a problem that the beam profile of the laser beam to be emitted from the wavelength conversion crystal 60 is deteriorated due to the temperature non-uniformity in the crystal and a chronological change of the affected crystal. For example, Fig. 11(a) and Fig. 11(b) shows an example of a chronological change of the beam profile in the case where the wavelength of the YAG laser beam is converted to 532 nm. Fig. 11(a) is a beam profile immediately after use of the wavelength conversion crystal 60 is started, and Fig. 11(b) is a beam profile after 10 months have passed since the start of use.

[0067] In Fig. 11(b), a region D1 with a large amount of light and a region D2 with a small amount of light are mixed. As described above, when Fig. 11(a) and Fig. 11(b) are compared with each other, it is understood that the beam profile is greatly changed chronologically. The deterioration of the beam profile as described above affects the diffusion status of the pulse ultrasonic wave SW to be excited in the metal material 100 to a non-negligible extent. That is, the amplitude of the ultrasonic wave is significantly reduced with respect to the propagation distance, and the signal-to-noise ratio of the ultrasonic waveform to be detected by the laser interferometer 41 is deteriorated. As a result, there is a problem that the measurement accuracy is lowered.

[0068] In contrast, according to the Fig.According to the property measurement system 1 shown in FIG. 10, even if the deterioration of the beam profile occurs due to a chronological change, temperature non-uniformity, and the like of the wavelength conversion crystal 60, the light quantity distribution of the pulse laser beam in the concentrated spot SP of the surface 101 of the metal material 100 can be made uniform. Therefore, the diffusion of the pulse ultrasonic wave SW to be excited in the metal material 100 can be suppressed to be small. As a result, the amplitude of the pulse ultrasonic wave SW is less likely to be reduced with respect to the propagation distance, and the signal-to-noise ratio of the waveform of the ultrasonic wave to be detected by the laser interferometer 41 is improved, thereby making it possible to obtain good measurement accuracy.

[0069] Other parts of the system are substantially similar to those of the first embodiment, and duplicated description is omitted.

[0070] As described above, the present invention has been described through the first and second embodiments; however, the description and drawings, which form a part of this disclosure, should not be construed as limiting this invention. The technical scope of the present invention is determined only by the scope of the claims. Commercial applicability

[0071] The property measuring system for a metal material according to the present invention is usable in manufacturing industries that measure properties of metal materials in a non-contact manner. Reference symbol lists 1 PROPERTY MEASURING SYSTEM 10 LASER OSCILLATOR 11 MAGNIFICATION SYSTEM 20 LENS GROUPING 30 COLLECTIVE LENS 40 ANALYSIS DEVICE 41 LASER INTERFEROMETERS 42 SIGNAL PROCESSING DEVICE 50 HOUSINGS 51 APERTURE PLATE 60 Wavelength Conversion Crystal 100 METAL MATERIAL 101 SURFACE 102 BACK 111 CONCAVE LENS 112 CONVEX LENS 421 Digital Oscilloscope 422 SIGNAL PROCESSING COMPUTERS

Claims

[1] Property measurement system for a metal material, comprising: a laser oscillator configured to emit a pulse laser beam; a lens array having a plurality of small lenses having a same shape, the small lenses being laid in a matrix on a plane perpendicular to an optical axis of the pulse laser beam, and arranged so that a part of a cross section of the pulse laser beam can be made incident on each of the plurality of small lenses; a condensing lens configured to overlap and condense emitted beams coming from the plurality of small lenses of the lens array onto a same region of a surface of a metal material as a measurement target, such that a light quantity density of the condensed emitted beam ensures ablation on a condensed spot; a laser interferometer configured to detect, as an electrical signal, a pulse ultrasonic wave excited by the pulse laser beam concentrated by the condensing lens and propagating through an interior of the metal material; and a signal processing device configured to process the electrical signal, wherein a spot size of the concentrated beams incident on the metal material is larger than a predetermined threshold, and the threshold is predetermined depending on a wavelength of the pulse ultrasonic wave. [2] The property measuring system of claim 1 further comprising: a wavelength conversion crystal configured to convert a wavelength of the pulse laser beam emitted from the laser oscillator, wherein the wavelength conversion crystal is provided between the laser oscillator and the lens array. [3] The property measuring system of claim 1, wherein the laser interferometer detects multiple reflection echoes of longitudinal wave components of the pulse ultrasonic wave.

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