KR101008028B1 - Method and apparatus for measuring crystal grain size distribution - Google Patents

Abstract

The present invention provides a method and apparatus capable of measuring the crystal grain size distribution with good accuracy even if the frequency band of the measurable ultrasonic wave is limited.

Ultrasonic waves are generated in a measurement object made of a polycrystalline material, the attenuation rate of the ultrasonic waves is measured to obtain an actual attenuation rate. In comparison with the above, the grain size distribution of the measurement object is determined based on the comparison result.

Grain size, laser, ultrasonic wave

Description

Crystal particle size distribution measuring method and apparatus {METHOD AND APPARATUS FOR MEASURING CRYSTAL GRAIN SIZE DISTRIBUTION}             

1 is a schematic diagram of an apparatus for measuring grain size distribution according to the present invention.

FIG. 2 is a schematic configuration diagram of a head of the crystal grain size distribution measuring apparatus of FIG.

3 shows an example of waveform data.

4 is a graph showing the measured attenuation rate.

5 is a graph illustrating a relationship between attenuation rate and average grain size.

6 is a graph showing an example of grain size distribution.

7 is a graph showing a comparative example of the theoretically obtained attenuation rate and the measured attenuation rate.

8 is a graph showing another comparative example of the theoretically obtained attenuation rate and the measured attenuation rate.

9 is a graph showing another comparative example of the theoretically obtained attenuation rate and the measured attenuation rate.

10 is a flowchart showing an overview of the method of the present invention.

<Code Description of Main Parts of Drawing>

10: laser source for ultrasonic generation 20: laser source for ultrasonic detection

30: head 31: optical coupler

32: condenser lens 33: half mirror

34: molded lens 50: interferometer

60: photodetector 70: computer (computation means)

91a, 91b, 91c: Optical fiber 92: Condensing lens

The present invention relates to a method and apparatus for measuring the grain size distribution of a polycrystalline material.

By miniaturizing the steel structure, properties such as high strength and toughness of steel can be obtained. Thus, if the grain size can be measured online in the steel manufacturing process, there is a great advantage in steel manufacturing. For example, by measuring the grain size online and monitoring the current grain size, it helps to reduce the non-uniformity of the material with high accuracy.

The average grain size measurement of steels is described, for example, in the article "Direct measurements of grain size in low-carbon steels using the laser ultrasonic technique" (Transaction of Metallurgical and Materials, A, Vol. 33A, March 2002, P. 687-691). Although it is conventionally known as described, in this method, only the average grain size is measured in the path through which the ultrasonic waves propagate, for example, if there is a grain size distribution in the thickness direction of the steel, the distribution information cannot be obtained. there is a problem.

In addition, the article "Determination of grain-size distribution from ultrasonic attenuation: Transformation and inversion" (Denise Nicoletti and Aran Anderson, Journal of Acoustic Society of America 101 (2), February 1997, p.686-691), Although a method for estimating the grain size distribution has been described, this also requires ultrasonic attenuation information in an infinitely wide frequency range, and when the measurable frequency band is limited in reality, the grain size distribution obtained by calculation is problematic. do.

It is an object of the present invention to provide a method and apparatus capable of measuring the crystal grain size distribution to a good degree, even if the frequency band of the measurable ultrasonic wave is limited, as a technical problem to solve the above-mentioned problems of the prior art.

In order to achieve the above technical problem, the present invention, in the method for measuring the grain size distribution of the polycrystalline material, generating an ultrasonic wave in the measurement object made of the polycrystalline material, and measuring the attenuation rate of the ultrasonic wave to obtain an actual attenuation rate; Calculating the theoretical attenuation rate by theoretically calculating the ultrasonic attenuation rate based on the assumed grain size distribution, comparing the measured attenuation rate with the theoretical attenuation rate, and determining the crystal grain size distribution of the measurement object based on the comparison result. The method of measuring the grain size distribution is described.

According to another aspect of the present invention, there is provided an apparatus for measuring grain size distribution of a polycrystalline material, comprising: means for generating an ultrasonic wave in a measurement object made of a polycrystalline material, means for measuring the attenuation rate of the ultrasonic wave, and obtaining an actual attenuation rate; Means for calculating the theoretical attenuation rate by theoretically calculating the ultrasonic attenuation rate based on the grain size distribution, and for comparing the measured attenuation rate with the theoretical attenuation rate, and determining the grain size distribution of the measurement object based on the comparison result. A grain size distribution measuring apparatus is provided.

First, with reference to FIG. 10, the grain size distribution measuring method by this invention is demonstrated.

In the present invention, an ultrasonic wave is first generated using a laser beam in the measurement object, and then the average crystal grain size of the measurement object is measured by measuring the attenuation rate at a specific frequency of the ultrasonic wave (S10). Subsequently, based on the measured average grain size, the distribution of crystal grains is changed according to a specific distribution, and the embodiment described below assumes that the logarithmic normal distribution is used. Create the curve data shown (S12). Next, the attenuation rate change measured value for the frequency change in a specific frequency region, for example, 0 to 20 MHz, is compared with the operation data obtained in step S12 (S14). Next, as a result of the comparison in step S14, a case is determined in which the degree of agreement of the curve is the best, and the grain size distribution is determined (S16).

EMBODIMENT OF THE INVENTION Hereinafter, the crystal grain size distribution measuring apparatus which concerns on preferred embodiment of this invention with reference to an accompanying drawing.

First, referring to Fig. 1, the crystal grain size distribution measuring apparatus according to the present embodiment includes an ultrasonic wave generating laser source 10, an ultrasonic wave detecting laser source 20, a head portion 30, and an interferometer 50. And a photodetector 60 and a computer (computing means) 70. The crystal grain size distribution measuring apparatus is provided with optical fibers 91a, 91b, 91c, a condenser lens 92, and the like as optical components.

The ultrasonic wave generation source 10 irradiates a laser for exciting an ultrasonic wave in the measurement object 2. As the ultrasonic wave generating laser, for example, a high energy pulse laser such as a YAG laser or a CO ₂ laser is used. The laser beam irradiated from the laser source 10 for ultrasonic generation is guided to the head part 30 via the optical fiber 91a.

Ultrasonic detection laser source 20 is a laser for detecting ultrasonic waves generated in measurement object 2 and propagating in measurement object 2 by laser beam irradiation from ultrasonic generation laser source 10. . As an ultrasonic wave detection laser, a single frequency continuous laser beam is used. The laser beam irradiated from the laser source 20 for ultrasonic detection is guided to the head part 30 via the optical fiber 91b.

As shown in Fig. 2, the head portion 30 is connected to the optical fibers 91a, 91b and 91c, and has an ultrasonic wave laser beam L ₁ and an ultrasonic wave detection laser L ₁ guided through the optical fibers 91a and 91b. ₂₎ a laser beam (L _1, L ₂₎ that is emitted from the optical coupler 31, optical coupler 31 having an injection port (31a) for measuring emitted toward the surface (2a) of the object (2) to the measurement Half-mirror reflecting the condenser lens 32 condensing on the surface 2a of the object 2 and the reflective laser beam L ₃ from the surface 2a of the measurement object 2 toward the optical fiber 91c ( 33) and, a half mirror 33 having a reflection laser beam (L ₃₎ of the optical fiber (lens molding (34 to create a parallel light beam towards 91c)) of from.

The interferometer 50 may comprise, for example, a Fabriferro interferometer. The Fabri Ferro interferometer 50 detects a frequency change of the reflective laser beam L ₃ generated due to ultrasonic vibration, and has two reflecting mirrors facing each other. These two reflectors constitute a resonator and function as a band pass filter by multiple reflection of the reflected laser beam L ₃ between the two reflectors. By adjusting the distance between the two reflectors, the frequency of light passing through the resonator can be adjusted.

The transmitted light intensity output from the interferometer 50 is sent to the photodetector 60. The photodetector 60 has a photodiode for converting transmitted light intensity into an electrical signal, and ultrasonic vibration generated in the measurement object 2 is finally captured as an electrical signal. The signal from the photodetector 60 is sent to a computer 70 via a suitable A / D converter (not shown) and recorded as waveform data.

Hereinafter, the operation of the present embodiment will be described.

First, the continuous laser beam is irradiated from the ultrasonic wave detection source 20 to the ultrasonic wave detection laser beam L ₂ and transmitted to the head portion 30 via the optical fiber 91b. In the head part 30, the ultrasonic beam for laser measurement L ₂ is emitted from the injection port 31a of the photocoupler 31 toward the condenser lens 32, and the measurement object 2 is condensed by the condenser lens 32. The light is focused to focus on the surface 2a of the surface. Although the laser beam L ₂ for ultrasonic measurement continuously irradiated toward the surface 2a of the measurement object 2 is diffusely reflected at the surface 2a, a part of the half mirror 33 is reflected laser beam L ₃ . Is reflected by the shaped lens 34 to be a parallel beam and guided to the optical fiber 91c.

While the ultrasonic wave generating laser beam L ₂ is continuously irradiated toward the surface 2a of the measurement object 2, the high energy laser beam is emitted from the ultrasonic wave generating laser source 10 and the ultrasonic wave generating laser beam L ₁ . As a pulse, it is irradiated in a pulse shape, and is transmitted to the head part 30 via the optical fiber 91a. In the head part 30, the ultrasonic wave generation laser beam L ₁ is mixed with the ultrasonic measurement laser beam L _{2 in the} photocoupler 31, and a condensing lens () is formed from the injection port 31a of the photocoupler 31. It is emitted toward 32, and focused by the condenser lens 32 to focus on the surface 2a of the measurement object 2. When the ultrasonic wave laser beam L ₁ is irradiated to the surface 2a of the measurement object 2, a portion of the surface 2a (hereinafter referred to as a measurement point) of the object 2 to be irradiated is locally heated. And thermal expansion. As a result, acoustic waves or ultrasonic waves are generated on the surface 2a of the measurement object 2, which is transmitted to the measurement object 2. This acoustic wave or ultrasonic wave is transmitted from the measuring point to the inside of the measurement object 2 in various directions. In this embodiment, in particular, the thickness direction (up and down direction in FIG. 2) of the measurement object 2, that is, the laser beams L ₁ , L A longitudinal wave propagating in a direction parallel to the incident direction of ₂ ) is measured.

When the acoustic wave or ultrasonic wave reaches the bottom surface 2b from the surface 2a of the measurement object 2, the acoustic wave or the ultrasonic wave reflects from the bottom surface 2b and propagates within the measurement object 2 toward the surface 2a. The acoustic wave or ultrasonic wave is attenuated and repeated. When the acoustic wave or ultrasonic wave reaches the surface 2a of the measurement object 2, the surface 2a is locally displaced, and this displacement is the reflection laser beam L _{3 of the} laser beam L ₂ for ultrasonic measurement. As a Doppler effect, it is detected by the interferometer 50, converted into an electrical signal by the photodetector 60, and sent out to the computer 70. It is recorded by the computer 70 as waveform data.

An example of the waveform data thus obtained is shown in FIG. Fig. 3 shows the change in the amplitude A of the waveform with respect to time t, and the acoustic waves or ultrasonic waves generated on the surface 2a of the measurement object 2 are the surface 2a and the bottom surface 2b of the measurement object 2; It reflects periodically between the surfaces of the measurement object 2, and is periodically observed.

The computer 70 performs fast Fourier transform on the received waveform data.                     

Acoustic waves or ultrasonic waves transmitted in the measurement object 2 are attenuated in the measurement object 2 and gradually decreased according to the following equation. For reference, d denotes a number indicating the thickness of an object to be measured, α denotes an ultrasonic attenuation rate, f denotes a frequency, and i denotes the number of reflected signals among the multi-reflection signals measured as shown in FIG.

A (f) _i = A (f) _i-1 × exp (-2dα) ... (8)

In this manner, the computer 70 performs fast Fourier transform on the waveform data to calculate the measured value α _exp of the ultrasonic attenuation rate for each frequency (FIG. 4).

On the other hand, as already mentioned, it is experimentally known that there is a constant correlation between the ultrasonic attenuation rate α and the average grain size. For example, referring to FIG. 5, an experimental result showing the relationship between the average grain size and the attenuation rate when the frequency of ultrasonic waves propagating in the measurement object made of austenitic steel is 4.7 MHz is shown. Therefore, the relationship between the ultrasonic attenuation rate (α) and the average crystal grain size for various materials or steels is experimentally obtained for each of a plurality of specific frequencies, stored in a computer 70 in a table or an empirical form, and the ultrasonic attenuation rate measured at the specific frequency (α _exp By referring to the table above or substituting into the empirical formula, the average crystal grain size (μ) of the measurement target (2) can be obtained.

Next, a method of creating curve data showing the relationship between the attenuation rate and the frequency from the measured average crystal grain size µ will be described.

The above-mentioned paper "Direct measurements of grain size in low-carbon steels using the laser ultrasonic technique" describes the attenuation of ultrasonic waves propagating in polycrystalline materials. According to another paper "Determination of grain-size distribution from ultrasonic attenuation: Transformation and inversion" described above, the grain size distribution in the polycrystalline material and the attenuation rate of the ultrasonic wave propagating through the polycrystalline material are represented by the following equation (1).

here,

f (x): some function to get

α: ultrasonic attenuation rate

D: grain size

λ: the wavelength of the ultrasonic pulse

to be.

If equation (1) is true for all wavelengths, the attenuation factor can be rewritten by the following equation (2) to integrate the effects of all grains.

here,

N (D): Number of crystals of particle size D

to be.

Here, by converting into N (D) = x (1nD) and f (λ / D) = y (1n (λ / D)), equation (2) can be converted into the following equation (3).

Here, by converting the variable into t = 1n (λ) and τ = 1n (D), equation (3) can be converted into the following equation (4).

Further, by converting the variable into z (t) = α (e ^t ), the following equation (5) is finally obtained.

In formula (5), particle size distribution x (t) is represented by following formula (6).                     

Where FFT and FFT- ¹ are fast Fourier transforms and fast Inverse Fourier transforms.

In addition, the paper "Determination of grain-size distribution from ultrasonic attenuation: Transformation and inversion" shows a particle size distribution model according to the lognormal distribution according to Equation (7) below.

here,

μ: average grain size

σ: parameter for determining distribution width

to be.

As described above, the measured average crystal grain size (μ) is fixed, and σ, which is a parameter related to the distribution width, is changed to several values in Equation (7), and the grain size distribution N (D) is determined for each value of the parameter? It calculates from Formula (7). An example of the calculation result is shown in FIG. As an example, Fig. 6 shows grain size distribution N (D) in six cases where the values of? Are 1.1, 1.3, 1.5, 1.7, 1.9, and 2.1.

When the grain size distribution N (D) is obtained for each value of the parameter sigma, the change in the ultrasonic attenuation rate with respect to the frequency change for each value of the parameter sigma is calculated from Equation (6) as the theoretical attenuation rate. An example of the calculation result is shown by the solid line of FIGS. For reference, FIGS. 7 to 9 show a case where the average crystal grain size is 27 µm and the parameters σ relating to the distribution width are 2.3 (FIG. 7), 1.1 (FIG. 8), and 3.0 (FIG. 9). In the present invention, the measured attenuation ratio and the theoretical attenuation ratio are compared in a specific frequency range, for example, a frequency band between 0 and 20 MHz, and a crystal grain size distribution giving a theoretical attenuation ratio showing the best degree of agreement based on the comparison result is obtained. It is to select as a grain size distribution. In Fig. 7 to Fig. 9, the frequency dependence curve of the theoretical attenuation rate is superimposed on the measured attenuation rate α _exp and the relationship curve of the frequency (Fig. 4). In this example, the case of Fig. 7 shows the best agreement degree. Therefore, N (D) when sigma = 2.3 is adopted as the grain size distribution.

According to the present invention, by comparing the measured attenuation rate with the theoretical attenuation rate, and determining the grain size distribution of the measurement object based on the comparison result, the crystal grain size distribution can be measured with excellent accuracy even if the frequency band of the measurable ultrasonic wave is limited. Will be.

Claims (6)

In the method for measuring the grain size distribution of a polycrystalline material, Generating ultrasonic waves in a measurement object made of a polycrystalline material; Measuring the attenuation rate of the ultrasonic wave to obtain a measured attenuation rate; Theoretically calculating the ultrasonic attenuation rate based on the assumed grain size distribution, and obtaining the theoretical attenuation rate; Comparing the measured attenuation rate with a theoretical attenuation rate; A crystal grain size distribution measuring method for determining a grain size distribution of the measurement object based on the comparison result. The method of claim 1, The assumed grain size distribution is a normal distribution, Obtaining the theoretical decay rate, Obtaining an average grain size of the measurement object based on the measured attenuation rate, and obtaining a grain size distribution for each of the average grain size and the width value assuming a plurality of different values with respect to the width of the normal distribution; Calculating a decay rate for each of the obtained grain size distributions to obtain a plurality of theoretical decay rates, The comparing step, Comparing the measured decay rate with each of the plurality of theoretical decay rates, The method includes the step of selecting a grain size distribution giving a theoretical reduction ratio showing the best agreement as a result of the comparison, as a grain size distribution of the measurement object. The method of claim 2, The method assumes that the normal distribution is a lognormal distribution. In the crystal grain size distribution measuring device of a polycrystalline material, Means for generating ultrasonic waves in a measurement object made of a polycrystalline material; Means for measuring the attenuation rate of the ultrasonic wave to obtain a measured attenuation rate; Means for theoretically calculating the ultrasonic attenuation rate based on the assumed grain size distribution and obtaining the theoretical attenuation rate; Means for comparing the measured attenuation rate with a theoretical attenuation rate, And a grain size distribution measuring device for determining a grain size distribution of the measurement object based on the comparison result. The method of claim 4, wherein The means for generating the ultrasonic wave includes an ultrasonic measuring laser beam source for irradiating the ultrasonic measuring laser beam in a pulse shape toward the measurement object, The means for measuring the attenuation rate of the ultrasonic wave to obtain the measured attenuation rate includes: a measuring laser beam source for continuously irradiating a measuring laser beam toward the measurement object; and a reflection laser beam of the measuring laser beam, and receiving light of a reflected ray beam. And a means for converting a change in intensity into an electrical signal, and means for calculating the attenuation rate for each frequency by receiving the electrical signal as waveform data and performing fast Fourier transform of the waveform data. The method of claim 5, Means for obtaining the theoretical decay rate, Obtaining the average grain size of the measurement object based on the measured attenuation ratio, assuming a normal distribution with respect to the grain size distribution, and a plurality of different values with respect to the width of the normal distribution, each of the average grain size and the width value Obtain a plurality of theoretical reduction ratios by obtaining a grain size distribution with respect to each other, and calculating attenuation ratios for each of the obtained crystal grain distributions. The means for comparing, Comparing the measured attenuation rate with each of the plurality of theoretical attenuation rates, And the apparatus further comprises means for selecting, as a grain size distribution of the measurement object, a grain size distribution giving a theoretical attenuation ratio showing the best agreement as a result of the comparison.

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