JP2005300356A - Method and apparatus for measuring diameter distribution of crystal grain - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus, capable of high precision measurement of the diameter distribution of crystal grains, even if the measurable frequency band of ultrasonic waves is restricted. <P>SOLUTION: Ultrasonic waves are generated in an object to be measured made of a polycrystalline material, and attenuation factor of the ultrasonic waves is measured, to acquire observed attenuation factor. The attenuation factor of ultrasonic waves is theoretically computed, on the basis of an assumed diameter distribution of crystal grains to acquire the theoretical attenuation factor. The observed attenuation factor is compared with the theoretical attenuation factor, and the diameter distribution of crystal grains of the object to be measured is determined based on the comparison results. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  By refining the steel structure, characteristics of high strength and steel toughness can be obtained. Therefore, if the crystal grain size can be measured online in the steel production process, there is a great merit in steel production. For example, by measuring the crystal grain size online and monitoring the current crystal grain size, it is useful for reducing variation in material with high accuracy.

  The measurement of the average crystal grain size of steel is conventionally known as disclosed in Non-Patent Document 1, for example. Non-Patent Document 2 discloses a method for estimating the crystal grain size distribution from information on the ultrasonic attenuation rate.

“Direct measurements of grain size in low-cvarbon steels using the laser ultrasonic technique”, Transaction of Metallurgical and Materials, A, Vol. 33A, March 2002, p687-691 Denise Nicoletti and Aran Anderson, “Determination of grain-size distribution from ultrasonic attenuation: Transformation and inversion”, Journal of Acoustic Society of America 101 (2), February 1997, p686-691

  However, the method disclosed in Non-Patent Document 1 can only measure the average crystal grain size in the path through which the ultrasonic wave propagates. For example, when there is a crystal grain size distribution in the thickness direction of the steel material, The distribution information cannot be obtained.

  In addition, the method disclosed in Non-Patent Document 2 requires information on the ultrasonic attenuation rate in an infinitely wide frequency range, and if the frequency band that can be measured practically is limited, it can be obtained by calculation. The accuracy of the crystal grain size distribution is a problem.

  The present invention has a technical problem to solve such problems of the prior art, and provides a method and apparatus capable of accurately measuring the crystal grain size distribution even when the frequency band of ultrasonic waves that can be measured is limited. The purpose is that.

  According to a first aspect of the present invention, there is provided a method for measuring a crystal grain size distribution of a polycrystalline material, the step of generating an ultrasonic wave in a measurement object made of the polycrystalline material, and measuring the attenuation rate of the ultrasonic wave. Obtaining a measured attenuation rate; calculating a theoretical ultrasonic attenuation rate based on an assumed crystal grain size distribution to obtain a theoretical attenuation rate; and comparing the measured attenuation rate with a theoretical attenuation rate; The gist of the present invention is a crystal grain size distribution measuring method for determining a crystal grain size distribution of the measurement object based on a comparison result.

  According to another aspect of the present invention, in the crystal grain size distribution measuring apparatus for a polycrystalline material, means for generating an ultrasonic wave in a measurement object made of the polycrystalline material, and measuring the attenuation rate of the ultrasonic wave Means for obtaining the actual attenuation rate, means for theoretically calculating the ultrasonic attenuation rate based on the assumed crystal grain size distribution to obtain the theoretical attenuation rate, and means for comparing the actual attenuation rate with the theoretical attenuation rate And a crystal grain size distribution measuring apparatus for determining the crystal grain size distribution of the measurement object based on the comparison result.

  According to the present invention, the measured attenuation rate is compared with the theoretical attenuation rate, and the crystal grain size distribution of the measurement object is determined based on the comparison result. Therefore, the frequency band of ultrasonic waves that can be measured is limited. Even if this is done, the crystal grain size distribution can be measured with high accuracy.

First, with reference to FIG. 10, the crystal grain size distribution measuring method according to the present invention will be described.
In the present invention, first, an ultrasonic wave is generated in a measurement object using a laser beam, and the average crystal grain size of the measurement object is measured by measuring the attenuation rate at a specific frequency of the ultrasonic wave. (Step S10). Then, based on the measured average crystal grain size, assuming that the crystal grain size distribution follows a specific distribution, in the embodiment described below, a lognormal distribution, the distribution width is changed to Curve data indicating a change in attenuation rate with respect to a change in sound wave frequency is created (step S12). Next, in a specific frequency region, for example, a region of 0-20 MHz, the change in attenuation rate with respect to the change in frequency is compared with the actually measured value and the calculation data obtained in step S12 (step S14). Next, as a result of the comparison in step S14, it is determined whether the degree of coincidence of the curves is the best, and the crystal grain size distribution is determined (step S16).

Hereinafter, a crystal grain size distribution measuring apparatus according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.
First, referring to FIG. 1, the crystal grain size distribution measuring apparatus according to the present embodiment includes an ultrasonic generation laser source 10, an ultrasonic detection laser source 20, a head unit 30, an interferometer 50, and light detection. And a computer (calculation means) 70. The crystal grain size distribution measuring apparatus is provided with optical fibers 91a, 91b, 91c, a condensing lens 92, and the like as optical components.

The ultrasonic wave generation laser source 10 irradiates the measurement object 2 with a laser for exciting ultrasonic waves. 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 emitted from the ultrasonic wave generation laser source 10 is guided to the head unit 30 via the optical fiber 91a.

  The ultrasonic detection laser source 20 is a laser for detecting ultrasonic waves generated in the measurement object 2 by irradiation of the laser beam from the ultrasonic generation laser source 10 and propagating through the measurement object 2. is there. A single frequency continuous laser beam is used as the ultrasonic detection laser. The laser beam emitted from the ultrasonic detection laser source 20 is guided to the head unit 30 through the optical fiber 91b.

As shown in FIG. 2, the head unit 30 is connected with optical fibers 91a and 91b, and measures an ultrasonic wave generation laser beam L ₁ and an ultrasonic detection laser L ₂ guided through the optical fibers 91a and 91b. An optical coupler 31 having an emission port 31 a that emits toward the surface 2 a of the object 2, and a laser beam L ₁ and L ₂ emitted from the optical coupler 31 are collected on the surface _{2 a} of the measurement object 2. a lens 32, a reflected laser beam L ₃ from the surface 2a of the object 2 and the half mirror 33 for reflecting the optical fiber 91c, parallel toward a reflected laser beam L ₃ from the half mirror 33 to the optical fiber 91c And a molded lens 34 as a light beam.

The interferometer 50 can comprise, for example, a Fabry-Perot interferometer. The Fabry-Perot interferometer 50 detects a change in frequency of the reflected laser beam L ₃ caused by ultrasonic vibration, and has two reflecting mirrors facing each other. The two reflecting mirrors constitute a resonator, and function as a band-pass filter by multiple reflection of the reflected laser beam L ₃ between the two reflecting mirrors. By adjusting the distance between the two reflecting mirrors, the frequency of light transmitted through the resonator can be adjusted.

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

Hereinafter, the operation of the present embodiment will be described.
First, a continuous laser beam is irradiated as an ultrasonic detection laser beam L ₂ from the ultrasonic detection laser source 20 and transmitted to the head unit 30 through the optical fiber 91b. In the head unit 30, the ultrasonic measurement laser beam L ₂ is emitted from the emission port 31 a of the photocoupler 31 toward the condenser lens 32, and is focused on the surface 2 a of the measurement object 2 by the condenser lens 32. It is condensed to. The ultrasonic measurement laser beam L ₂ continuously irradiated toward the surface 2 a of the measurement object ₂ is irregularly reflected on the surface 2 a, but a part thereof is reflected by the half mirror 33 as a reflected laser beam L ₃ , It becomes a parallel beam by the shaping lens 34 and is guided to the optical fiber 91c.

While the ultrasonic generation laser beam L ₂ is continuously irradiated toward the surface 2 a of the measurement object 2, a high energy laser beam is generated as an ultrasonic generation laser beam L ₁ from the ultrasonic generation laser source 10. Irradiated in a pulse shape and transmitted to the head unit 30 via the optical fiber 91a. In the head unit 30, the ultrasonic generation laser beam L ₁ is mixed with the ultrasonic measurement laser beam L _{2 in the} photocoupler 31, emitted from the emission port 31 a of the photocoupler 31 toward the condenser lens 32, and collected. Light is condensed by the optical lens 32 so as to focus on the surface 2a of the measurement object 2. When the surface 2a of the object 2 ultrasonic generation laser beam L ₁ is irradiated, part of the surface 2a of the object 2 that receive a radiation (hereinafter referred to as measuring point) is heated locally Thermal expansion. Thereby, an elastic wave or an ultrasonic wave is generated on the surface 2 a of the measurement object 2, and this is transmitted into the measurement object 2. This elastic wave or ultrasonic wave is transmitted from the measurement point to the inside of the measurement object 2 in various directions. In this embodiment, in particular, the thickness direction of the measurement object 2 (vertical direction in FIG. 2), that is, A longitudinal wave propagating in a direction parallel to the incident direction of the laser beams L ₁ and L ₂ is measured.

When the elastic wave or ultrasonic wave reaches the bottom surface 2b from the surface 2a of the measurement object 2, it reflects on the bottom surface 2b and propagates in the measurement object 2 toward the surface 2a. The elastic wave or ultrasonic wave is repeated while being attenuated. The acoustic wave or ultrasound, the surface 2a reaches the surface 2a of the measured object 2 is locally displaced, this displacement, the interference as a Doppler effect of the reflected laser beam L ₃ of the ultrasonic measurement laser beam L ₂ It is detected by the meter 50, converted into an electrical signal by the photodetector 60, and sent to the computer 70. Recorded as waveform data by the computer 70.

An example of the waveform data thus obtained is shown in FIG. FIG. 3 shows a change in the amplitude (A) of the waveform with respect to time (t). Elastic waves or ultrasonic waves generated on the surface 2a of the measurement object 2 are generated on the surface 2a and the bottom surface 2b of the measurement object 2. It is shown that it is periodically observed on the surface 2a of the measurement object 2 due to multiple reflection between the two.
The computer 70 performs fast Fourier transform on the acquired waveform data.

The elastic wave or ultrasonic wave transmitted in the measurement object 2 is attenuated in the measurement object 2 and gradually decreases according to the following equation. In this equation, d is the thickness of the object to be measured, α is the ultrasonic attenuation factor, f is the frequency, and i is a number indicating the number of the reflected signals among the multiple reflected signals measured as shown in FIG. It is.
A (f) _i = A (f) _i-1 × exp (-2dα)
As described above, the computer 70 performs fast Fourier transform on the waveform data, and calculates the actually measured value α _exp of the ultrasonic attenuation rate for each frequency (FIG. 4).

On the other hand, as described above, it has been experimentally found that there is a certain correlation between the ultrasonic attenuation rate α and the average crystal grain size. For example, referring to FIG. 5, there is shown an experimental result showing the relationship between the average crystal grain size and the attenuation rate when the frequency of the ultrasonic wave propagating through the measurement object made of austenitic steel is 4.7 MHz. Therefore, the relationship between the ultrasonic attenuation rate α and the average crystal grain size for various materials or steel materials is experimentally obtained for each of a plurality of specific frequencies, and is stored in the computer 70 in the form of a table or an empirical formula. Based on the actually measured ultrasonic attenuation rate α _exp , the average crystal grain size μ of the measurement object 2 is obtained by referring to the table or substituting it into the empirical formula.

ここで、
ｆ（ｘ）：求めるべきある関数
α：超音波減衰率
Ｄ：結晶粒径
λ：超音波パルスの波長
である。 Next, a method of creating curve data indicating the relationship between the attenuation rate and the frequency from the actually measured average crystal grain size μ will be described.
Non-Patent Document 2 described above describes the attenuation rate of ultrasonic waves propagating through a polycrystalline material. According to Non-Patent Document 2, the crystal grain size distribution in the polycrystalline material and the attenuation rate of the ultrasonic wave propagating through the polycrystalline material are expressed by the following equation (1).

here,
f (x): a function to be obtained α: ultrasonic attenuation factor D: crystal grain size λ: wavelength of ultrasonic pulse

ここで、
Ｎ（Ｄ）：粒径Ｄの結晶の数
である。 Assuming that equation (1) is true for all wavelengths, the attenuation factor can be rewritten into the following equation (2) to integrate the effects of all crystal grains.

here,
N (D): Number of crystals having a particle size D.

Here, by replacing N (D) = x (lnD) and f (λ / D) = y (ln (λ / D)), Equation (2) is transformed into Equation (3) below. be able to.

Here, equation (3) can be transformed into the following equation (4) by performing variable transformation with t = ln (λ) and τ = ln (D).

Furthermore, the following equation (5) is finally obtained by performing variable conversion to z (t) = α (e ^t ).

ここで、FFTおよびFFT^-1は高速フーリエ変換および高速逆フーリエ変換である。 From the equation (5), the particle size distribution x (t) is expressed by the following equation (6).

Here, FFT and FFT ⁻¹ are fast Fourier transform and fast inverse Fourier transform.

ここで、
μ：平均結晶粒径
σ：分布幅を決定するパラメータ
である。 Furthermore, Non-Patent Document 2 shows a particle size distribution model according to a lognormal distribution according to the following equation (7).

here,
μ: Average crystal grain size σ: A parameter that determines the distribution width.

  The average grain size μ actually measured as described above is fixed, and the parameter σ relating to the distribution width in the equation (7) is changed to several values. (D) is calculated from equation (7). An example of the calculation result is shown in FIG. As an example, FIG. 6 shows the crystal grain size distribution N (D) in the case of six values of σ of 1.1, 1.3, 1.5, 1.7, 1.9, and 2.1. Has been.

After obtaining the crystal grain size distribution N (D) for each value of the parameter σ, based on this, the change of the ultrasonic attenuation rate with respect to the change of the frequency is calculated from the equation (6) for each value of the parameter σ, and the theoretical attenuation rate. And An example of the calculation result is shown by a solid line in FIGS. 7 to 9 show cases where the average crystal grain size is 27 μm and the parameter σ relating to the distribution width is 2.3 (FIG. 7), 1.1 (FIG. 8), and 3.0 (FIG. 9). ing. In the present invention, the measured attenuation rate and the theoretical attenuation rate are compared in a specific frequency region, for example, a frequency band between 0-20 MHz, and the theoretical attenuation rate indicating the best match based on the comparison result is obtained. The crystal grain size distribution to be given is selected as the crystal grain size distribution of the measurement object. In FIGS. 7 to 9, the relationship between the actually measured attenuation rate α _exp and the frequency (FIG. 4) is superimposed on the frequency dependence curve of the theoretical attenuation rate. In this example, Since it is considered that the case of FIG. 7 shows the best degree of coincidence, N (D) when σ = 2.3 is adopted as the crystal grain size distribution.

It is the schematic of the crystal grain size distribution measuring apparatus by this invention. It is a schematic block diagram of the head part of the crystal grain size distribution measuring apparatus of FIG. It is a figure which shows an example of waveform data. It is a graph which shows measured attenuation factor. It is a graph which illustrates the relationship between an attenuation factor and an average crystal grain size. It is a graph which shows an example of crystal grain size distribution. It is a graph which shows the comparative example of the attenuation factor theoretically obtained and the measured attenuation factor. It is a graph which shows the comparative example of the attenuation factor theoretically obtained and the measured attenuation factor. It is a graph which shows the comparative example of the attenuation factor theoretically obtained and the measured attenuation factor. It is a flowchart which shows the outline of the method of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Laser source 20 for ultrasonic generation ... Laser source 30 for ultrasonic detection ... Head part 31 ... Optical coupler 32 ... Condensing lens 33 ... Half mirror 34 ... Molding lens 50 ... Interferometer 60 ... Photo detector 70 ... Computer ( Calculation means)
91a ... Optical fiber 91b ... Optical fiber 91c ... Optical fiber 92 ... Condensing lens

Claims (6)

In the method for measuring the crystal grain size distribution of a polycrystalline material,
Generating ultrasonic waves in a measurement object made of a polycrystalline material;
Measuring an attenuation rate of the ultrasonic wave to obtain an actual attenuation rate;
Calculating the ultrasonic attenuation rate theoretically based on the assumed crystal grain size distribution to obtain the theoretical attenuation rate;
Comparing the measured attenuation rate with a theoretical attenuation rate;
A crystal grain size distribution measuring method for determining a crystal grain size distribution of the measurement object based on a comparison result. The assumed crystal grain size distribution is a normal distribution or a log normal distribution,
The step of obtaining the theoretical attenuation rate includes obtaining an average crystal grain size of the measurement object based on the measured attenuation rate,
Assuming a plurality of different values for the spread width of the normal distribution and determining a crystal grain size distribution for each of the average crystal grain size and the spread width value;
Calculating a damping rate for each of the determined crystal grain size distributions to obtain a plurality of theoretical damping rates;
The step of comparing includes comparing the measured attenuation rate with each of the plurality of theoretical attenuation rates;
2. The method according to claim 1, wherein the method includes a step of selecting a crystal grain size distribution that gives a theoretical damping rate that shows a best match as a result of the comparison as a crystal grain size distribution of the measurement object.   The method according to claim 2, wherein the assumed crystal grain size distribution is an arbitrary distribution function having one peak value in the distribution. In the crystal grain size distribution measuring device for polycrystalline materials,
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 an actual attenuation rate;
Means for theoretically calculating the ultrasonic attenuation rate based on the assumed crystal grain size distribution to obtain the theoretical attenuation rate;
Means for comparing the measured attenuation rate with a theoretical attenuation rate,
A crystal grain size distribution measuring apparatus for determining a crystal grain size distribution of the measurement object based on a comparison result. The means for generating the ultrasonic wave comprises a laser beam source for ultrasonic measurement that irradiates the measurement target with a laser beam for ultrasonic measurement in a pulsed manner.
The means for measuring the attenuation rate of the ultrasonic wave to obtain the actual attenuation rate includes a measurement laser beam source for continuously irradiating the measurement object with the measurement laser beam, and a reflection laser of the measurement laser beam. A means for receiving the beam and converting the change in the light intensity of the reflected laser beam into an electrical signal, and taking the electrical signal as waveform data and performing a fast Fourier transform on the waveform data, thereby obtaining an attenuation factor for each frequency. 5. The apparatus according to claim 4, further comprising means for calculating. The means for obtaining the theoretical attenuation rate obtains the average crystal grain size of the measurement object based on the measured attenuation rate, assumes a distribution function for the crystal grain size distribution, and sets a plurality of different values for the spread width of the distribution function. Assuming that a crystal grain size distribution is obtained for each of the average crystal grain size and the spread width value, and a plurality of theoretical damping rates are obtained by calculating a damping rate for each of the obtained crystal grain size distributions. And
The means for comparing compares the measured attenuation rate with each of the plurality of theoretical attenuation rates,
6. The apparatus according to claim 5, further comprising means for selecting, as a result of the comparison, a crystal grain size distribution that gives a theoretical damping rate that shows the best degree of coincidence as a crystal grain size distribution of the measurement object. .

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