JPH10123107A - Method and device for measuring decarburized layer depth of steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure a decarburized layer depth without cutting a steel material. SOLUTION: An ultrasonic probe 2 generates an ultrasonic wave due to a signal that is transmitted from a transmitter 3 via an amplifier 4 and the signal of a reflected ultrasonic wave from a steel material 1 to be measured is received by a receiver 5. An envelope detector 6 detects the envelopes (each reception intensity at each time) of reception signals from the receiver 5 and sends them to an operator 7. The operator 7 has a memory that stores the envelope data of a number of known decarburized layer depths, transmitted envelope data are compared with the known envelope being stored in a memory, and a corresponding decarburized layer depth is set to the decarburized layer depth of the steel material 1 to be measured from the constant (α) of the closest envelope out of the envelope of the memory.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring the depth of a decarburized layer of a steel material for measuring the depth of a decarburized layer formed on the surface of the steel material.

[0002]

2. Description of the Related Art When a steel material is subjected to a high-temperature heat treatment in a furnace for a long time, the gas in the furnace reacts with the surface of the steel material to cause oxidation or a reduction in carbon diffusion (decarburization) in the surface layer of the steel material. Is too soft to obtain the required mechanical strength. Therefore, by measuring the depth of the oxidized layer and the degree of decarburization (decarburized layer depth), this is fed back to the heat treatment conditions to minimize the occurrence of oxidation and decarburization.
Or, they are checked to see if they are within acceptable limits. By the way, the scale (oxide) due to the oxidation can be confirmed from the appearance inspection, but the depth of the decarburized layer cannot be determined from the appearance, so that the steel material was cut and the structure was observed by microscopic observation.

[0003]

However, there has been a problem that cutting the steel material and observing with a microscope require a lot of trouble and time. In addition, for example, Japanese Patent Application Laid-Open No. 60-35253 proposes a means for non-destructively measuring the diameter of particles constituting a steel material by radiating ultrasonic waves into the steel material.
This means obtains the average of the diameters of the particles in the ultrasonic wave propagation direction, and cannot measure the depth of the decarburized layer composed of particles having different diameters from the particles in the non-decarburized portion.

An object of the present invention is to solve the above-mentioned problems in the prior art and to provide a method and an apparatus for measuring the depth of a decarburized layer of a steel material, which can measure the depth of the decarburized layer without cutting the steel material. .

[0005]

In order to achieve the above-mentioned object, according to the first aspect of the present invention, an ultrasonic wave is radiated from the surface of a steel material to be measured into the steel material, and the envelope of the reception intensity of the reflected wave signal is emitted. And comparing this envelope with the envelope of the reception intensity of the ultrasonic reflected wave signal for a plurality of known decarburized layer depths determined in advance, and corresponding to the closest envelope of these envelopes. The depth of the decarburized layer to be measured is defined as the decarburized layer depth of the steel material to be measured.

According to a second aspect of the present invention, an ultrasonic wave is radiated from the surface of the steel material to be measured into the steel material, and a frequency of a peak value among frequency components of the reflected wave signal at predetermined time intervals is obtained. The frequency of the steel material to be measured is based on a time corresponding to a frequency corresponding to a frequency of a peak value among frequency components at the predetermined time intervals previously determined for the steel material on which the decarburized layer is not formed. It is characterized by obtaining the depth of the decarburized layer.

According to a third aspect of the present invention, a surface acoustic wave is generated by radiating ultrasonic waves having different frequencies to the surface of the steel material to be measured, and the relative sensitivity of the reflected wave signal for each frequency is defined as:
The depth of the decarburized layer of the steel material to be measured is obtained based on a frequency corresponding to a predetermined relative sensitivity.

According to a fourth aspect of the present invention, there is provided an ultrasonic probe for radiating an ultrasonic wave to a steel material to be measured, a receiver for receiving a reflected wave of the ultrasonic wave radiated from the ultrasonic probe, and a plurality of the steel materials. A first memory storing, for each depth of the decarburized layer, data of the envelope of the received intensity comprising the reception time and the received intensity of the ultrasonic reflected wave signal with respect to the known decarburized layer depth, and a constant of the envelope. And a second memory for storing a relationship between the depth and the depth of the decarburized layer, the reception intensity of the reflected wave received by the receiver at each reception time, and the corresponding reception for each depth of the decarburized layer in the first memory. First calculating means for calculating the difference from the reception intensity of time, second calculating means for calculating the sum of the differences calculated by the first calculating means, and calculation by the second calculating means. A first search method for searching the envelope of each decarburized layer depth having the smallest sum of differences And a second searching means for searching the data of the second memory for a decarburized layer depth corresponding to the constant of the envelope searched by the first searching means, and It is characterized by comprising.

According to a fifth aspect of the present invention, there is provided an ultrasonic probe for emitting ultrasonic waves to a steel material to be measured, a receiver for receiving a reflected wave of the ultrasonic waves emitted from the ultrasonic probe, and a decarburized layer. A memory for storing a peak frequency of the received signal of the ultrasonic wave for the steel material to be measured which is not performed for each predetermined time interval, and a time interval corresponding to each time of the predetermined time interval in the received signal received by the receiver. A frequency analysis unit for extracting a peak frequency; a search unit for searching for a time when the peak frequency extracted by the frequency analysis unit matches the peak frequency stored in the memory; And a calculating means for calculating the depth of the decarburized layer by means of a decarburized layer depth measuring device for steel material.

Further, according to a sixth aspect of the present invention, there is provided an ultrasonic probe which emits an ultrasonic wave to the surface of a steel material to be measured to generate a surface acoustic wave and receives a reflected wave signal thereof, and radiates from the ultrasonic probe. Frequency changing means for continuously changing the frequency of the ultrasonic wave, comparing means for comparing the relative sensitivity of the reflected wave signal received by the ultrasonic probe with a predetermined relative sensitivity, and the comparing means When the relative sensitivity coincides with the predetermined relative sensitivity, the calculating means for calculating the decarburized layer depth based on the frequency corresponding to the relative sensitivity constitutes a steel material decarburized layer depth measuring device.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the illustrated embodiments. FIG. 1 is a block diagram of an apparatus for performing a decarburized layer depth measuring method according to a first embodiment of the present invention. In this figure, 1 is a steel material to be measured and 2 is an ultrasonic probe. A delay member (not shown) is provided at the tip of the ultrasonic probe 2 so that a transmitted wave of the ultrasonic wave and a reflected wave from the steel material 1 to be measured can be completely separated. Usually, a medium such as water is interposed between the ultrasonic probe 2 and the steel material 1 to be measured. 3 is a transmitter for exciting the ultrasonic probe 2 to generate an ultrasonic wave, 4 is an amplifier for amplifying the signal of the transmitter 3, 5 is a receiver for receiving a reflected wave from the ultrasonic probe 2, and 6 is an envelope. The detector 7 is an arithmetic unit. The functions of the envelope detector 6 and the calculator 7 will be described later.

FIG. 2 is a sectional view of the surface layer of the steel material 1 to be measured. 1a shows the surface of the steel material 1 to be measured. 1b indicates a decarburized layer (so-called ferrite layer), and 1c indicates a ferrite, pearlite layer, or martensite layer. The decarburized layer 1b is located near the surface 1a, and the diameter of the crystal grains constituting the decarburized layer 1b having a low carbon content is larger than the diameter of the crystal grains constituting the ferrite, pearlite layer, or martensite layer 1c thereunder. . As described above, the decarburized layer 1b has a large crystal grain diameter as compared with the ferrite, pearlite layer, or martensite layer 1c, so that the scattering of the incident ultrasonic waves is small. Or, the attenuation of the ultrasonic wave incident on the grain is larger than that of the martensite layer 1c.

FIG. 3 is a waveform diagram of the reception intensity of the ultrasonic reflected wave received by the receiver 5 shown in FIG. FIG. 3A shows a waveform diagram when the decarburized layer is thick, and FIG. 3B shows a waveform diagram when the decarburized layer hardly exists. In the figure, the horizontal axis indicates time, and the vertical axis indicates reception intensity. I ₀ is a transmitted wave, R ₀ is a surface reflected wave of the steel material 1 to be measured, and R is a group of reflected waves after the surface reflected wave R ₀ . As described above, the decarburized layer 1b
Since there is little scattering of ultrasonic waves and large attenuation, when the decarburized layer exists as shown in FIG. 3A, the reception intensity of the reflected wave group R has a large attenuation, and conversely, When the decarburized layer 1b is scarcely present, the attenuation of the reception intensity of the reflected wave group R is extremely small as shown in FIG.

The present embodiment utilizes the above phenomenon, and a measuring method thereof will be described with reference to FIGS. FIG. 4 is the same waveform diagram as the waveform diagram shown in FIG.
(A) is a waveform diagram when the decarburized layer is thick, and (b) in FIG. 4 is a waveform diagram when the decarburized layer hardly exists. FIG.
The same reference numerals are given to the waveforms of the same portions as the waveforms shown in FIG. E ₁ is the envelope of the waveform shown in (a) of FIG. 4, E
₂ is an envelope of the waveform shown in FIG. By the way, in general, the envelope E can be represented by an exponential function f (t) = e− ^αt, where t is time and α is a constant.
Then, ultrasonic waves are radiated to steel materials having various decarburized layer depths to obtain the respective envelopes E of the reflected waveforms, and if the constant α is associated with the decarburized layer depth, the ultrasonic wave of the steel material 1 to be measured can be obtained. The depth of the decarburized layer of the steel material 1 to be measured can be known from the reflected wave.

FIG. 5 is a characteristic diagram of the constant α with respect to the depth of the decarburized layer. In the figure, the horizontal axis represents the decarburization depth and the vertical axis represents the constant α.
There is. As is apparent from the figure, the depth of the decarburized layer is substantially proportional to the constant α. A line indicating the relationship between the two is shown as a calibration curve. Now, if the constant α of the envelope obtained from the ultrasonic reflected wave is α ₁ as shown, the decarburized layer depth obtained from the calibration curve is d ₁ as shown.

In order to realize the above method, the apparatus shown in FIG. 1 is provided with an envelope detector 6 and a calculator 7 each constituted by a microcomputer. The arithmetic unit 7 has a memory 7 for storing data relating to a known envelope.
M (shown in FIG. 8) is provided. The contents of the memory 7M will be described with reference to FIGS. FIG. 6 is a diagram showing an envelope with respect to the reception intensity of the ultrasonic reflection of the steel material to be measured having a certain decarburized layer depth, where the horizontal axis represents time and the vertical axis represents reception intensity. Assuming that the reception intensity is A, the time is t, and the constant is α, this envelope E is A =
It is represented by e- ^αt . The envelope E of the constant α is expressed by the time t
And reception intensity A. This will be described with reference to FIG.

FIG. 7 is a diagram for explaining how to define the envelope E shown in FIG. 6. The envelope E in this figure is the same as the envelope shown in FIG. The envelope E of such a constant α
Indicates that the reception intensity at time t ₁ is A, as shown in FIG.
_1, time t reception intensity A ₂ when the _first predetermined time from Δt has elapsed time t _2, the reception strength A ₃ at time t ₃ when the time t ₂ a predetermined time Δt has elapsed, ... ……,
Reception intensity A ₉ when the reception intensity is a time t _9, which has passed a predetermined time Δt from A _8, time t ₈ at time t ₈ to time t ₇ predetermined time Δt has elapsed, ............ Is defined.

FIG. 8 is an explanatory diagram of the contents of the memory 7M. The memory 7M, as shown in FIG. 8, the data of a number of known constant alpha ₁ envelope of decarburized layer depth, as the reception intensity A ₁₁ to A _1n at each time t ₁ ~t _n The data of the envelope of the constant α ₂ is stored at each time t ₁ to
are stored as the respective reception intensities A _{21 to} A _{2n at} t _n ,
The data of the constant alpha ₃ of the envelope is stored as the reception intensity A ₃₁ to A _3n at each time t ₁ ~t _n, similarly, the data of the constant alpha ₄ or less of the envelope is stored.

When a received signal is input from the receiver 5 to the envelope detector 6, the envelope detector 6 operates at each of the times t _{1 to} t.
Each of the received intensities of the received signal corresponding to _n is extracted, and these are output to the arithmetic unit 7. The arithmetic unit 7 calculates the respective reception intensities and the respective constants α ₁ and α ₁ stored in the memory 7M.
_2. Calculate the difference between the data at the same time in each of the envelopes and the sum of the differences. For example, when the reception intensity at each time t ₁ ~t _n of the received signal is the A ₀₁ to A _0n, for the constant alpha ₁ envelope, (A ₀₁ -
_{A 11), (A 02 -A} 12), ............, ( calculating the A _0n -A _{1n), then, [(A 01 -A 11)} + (A 02 -A 12) + ...
... + (A _0n −A _1n )]. Performs the same processing with respect to the constant alpha ₂ following the envelope, the total or envelope value of the standard deviation is the minimum difference and the envelope of the received signal.

In the above description, an example in which the reception intensity at each of the times t _{1 to} t _n is stored in the memory 7M for each of the constants α ₁ , α ₂ ,... , the constants α _1, α _2, ............ and may be stored only each time t ₁ ~t _n, when the reception strength of each time from the envelope detector 6 is inputted, the arithmetic unit 7 , A = e ^−αt , each reception value A is calculated by putting each numerical value into the constant α and the time t,
The above calculation may be performed on the obtained reception strength.

When the envelope is determined in this way, the arithmetic unit 7 stores the depth of the decarburized layer corresponding to the constant α of the envelope in another area of the memory 7M or a memory different from the memory 7M. The depth of the decarburized layer obtained from the relationship between the constant α and the decarburized layer depth (relation shown in FIG. 5) is defined as the decarburized layer depth of the steel material 1 to be measured.

As described above, in the present embodiment, the ultrasonic reflected wave from the steel material to be measured is received, and the envelope of the received intensity is defined as the envelope of the ultrasonic received intensity whose decarburization layer depth is known. Since the depth of the decarburized layer is determined based on the result of the comparison, the depth of the decarburized layer can be measured without cutting the steel material, and the measurement does not require any trouble or time.

FIGS. 9 and 10 are views for explaining a method for measuring the depth of the decarburized layer according to the second embodiment of the present invention. FIG. 9 is a waveform diagram of the reception intensity of the ultrasonic reflected wave, and FIG. FIG. 4 is a frequency characteristic diagram of a reception signal. FIG. 9A shows a waveform diagram when the decarburized layer is thick, and FIG. 9B shows a waveform diagram when the decarburized layer hardly exists. In FIG. 9, the same reference numerals are given to the waveforms of the same portions as the waveforms shown in FIG. t _{1 to} t _n indicate respective time divisions when the received signal R is divided at predetermined time intervals.

In general, the smaller the diameter of a crystal particle, the narrower the directivity of the high-frequency component of the reflected ultrasonic wave to the crystal particle, so that the frequency band of the received signal is narrower and the peak frequency is lower. In the present embodiment, the depth of the decarburized layer is measured using the peak frequency characteristic among the characteristics of the crystal particles with respect to the ultrasonic waves. This will be described with reference to the frequency characteristics of the received signal shown in FIG. In FIG. 10, the horizontal axis represents the time section shown in FIG. 9, and the vertical axis represents the peak frequency. In Figure, F ₀ is a characteristic line obtained by plotting the peak frequency in each time segment of the ultrasonic reception signal with respect to steel no decarburized layer, as is apparent from the characteristic line F _0, in the decarburized layer is not steel, time (E.g., deep portions of steel), the peak frequency of the ultrasonic reception signal is substantially constant.

On the other hand, F ₁ is a curve in which the peak frequency in each time section of the ultrasonic reception signal for the steel material having the decarburized layer is plotted. The diameter of the crystal particles in the decarburized layer is larger than the diameter of the crystal particles without decarburization as described above. Therefore, the peak frequency of the ultrasonic wave reception signal from the decarburized layer exhibits a higher frequency as shown by the curve F _1. However, as the position becomes deeper from the surface of the steel material, crystal grains having smaller diameters are mixed and the peak frequency shifts to a lower side. At a position beyond the boundary of the decarburized layer, the peak frequency coincides with the case where there is no decarburized layer. The matching time segment is indicated by t _d in FIG. Here, assuming that the propagation speed of the ultrasonic wave in the steel material is c (known), the depth d of the decarburized layer can be obtained by calculating d = c · t _d / 2.

In order to carry out the measuring method according to the present embodiment, a frequency analysis unit is provided instead of the envelope detector 6 shown in FIG.
A memory for storing peak frequencies for _{1 to} t _n is provided.
_A frequency analysis is performed for each of _{1 to} t _n , and a peak frequency for each time section is output to the arithmetic unit 7, and the arithmetic unit 7 compares each input peak frequency with the peak frequency stored in the memory, removed time interval t _d that both peak frequencies coincide, calculates the decarburized layer depth by performing the above-described operation.

As described above, in this embodiment, the ultrasonic reflected wave from the steel material to be measured is received, the received signal is divided at predetermined time intervals, and the peak frequency of the received signal is obtained for each time division. Since this peak frequency was compared with the peak frequency of each time section of the steel material without the decarburized layer, the depth of the decarburized layer was calculated based on the time section in which both coincided.
The depth of the decarburized layer can be measured without cutting the steel material, and the measurement does not require any trouble or time.

FIG. 11 is a block diagram of an apparatus for performing a decarburized layer depth measuring method according to a third embodiment of the present invention. In this figure, 1 is a steel material to be measured and 10 is an ultrasonic probe. The ultrasonic probe 10 includes a transmitting transducer 11, a focusing lens 12, a receiving lens 13, and a receiving transducer 14. w indicates a medium such as water. Fifteen
Is a voltage control oscillator, 16 is a data analysis / control device composed of a microcomputer, 17 is a D / A converter, 18 is a receiver, 19 is an A / D converter, and 20 is a peak hold circuit.

In the present embodiment, the depth of the decarburized layer is measured using an ultrasonic surface acoustic wave propagating on the surface of the steel material 1 to be measured. The surface acoustic wave propagating on the surface of the steel material 1 to be measured attenuates under the influence of the lower layer having a depth of 1/2 to 1 wavelength from the surface. That is, the attenuation of the surface acoustic wave is related to the wavelength of the surface acoustic wave and the depth of the decarburized layer. In the present embodiment, the depth of the decarburized layer is measured by changing the wavelength (frequency) of the ultrasonic wave.

FIG. 12 is a characteristic diagram of the relative sensitivity of the received signal of the surface acoustic wave to the frequency. In this figure, the horizontal axis represents the frequency, and the vertical axis represents the relative sensitivity of the received signal. R _S0
Is a line showing characteristics of a steel material without a decarburized layer, and since there is no decarburized layer, the attenuation is constant and does not change at any frequency.
On the other hand, R _S1 , R _S2 , and R _S3 indicate characteristic curves of a steel material having a decarburized layer. The curve R _S1 is a curve when the decarburized layer is relatively shallow, the curve R _S3 is a curve when the decarburized layer is deep, and the curve R _S2 is a curve when the depth of the decarburized layer is intermediate between the two. For example, considering the curve R _S1 , when the frequency is low (the wavelength is long), the influence of the base material having a smaller crystal particle diameter is larger than that of the decarburized layer having a larger crystal particle diameter, so that the attenuation is small, and the relative sensitivity decreases. However, when the frequency is increased (when the wavelength is shortened), the influence of the decarburized layer having a large crystal particle diameter is greatly affected, the attenuation is increased, and the relative sensitivity is rapidly reduced. Such characteristics are the same for each curve, and as shown by the curve R _S3 , when the decarburized layer is deep, even at a low frequency (even if the wavelength is long), it is attenuated to lower the relative sensitivity.

In this embodiment, the relative sensitivity β is set in advance,
The frequency when the relative sensitivity of the received signal indicates β is extracted, and for example, half of the wavelength corresponding to this is estimated as the decarburized layer depth. That is, if the wavelength is λ, the propagation velocity is c, and the frequency is f, λ = c / f, so the decarburized layer depth d is d =
c / (2f).

The implementation of the measuring method according to the present embodiment is performed by the apparatus shown in FIG. The data analysis / control device 16 outputs a digital signal that continuously changes the frequency of the output signal of the voltage control oscillator 15. This signal is converted into an analog value by the D / A converter 17 and applied to the voltage control oscillator 15. As a result, a transmission signal whose frequency continuously changes is output from the voltage control oscillator 15, and the transmission signal is transmitted to the transmission vibrator 1.
1, ultrasonic waves having different frequencies are sequentially emitted from the transmitting transducer 11. These ultrasonic waves are focused by the focusing lens and radiated at a predetermined angle to the surface of the steel material 1 to be measured via the medium w, and the ultrasonic waves propagate as surface acoustic waves on the surface.

During this propagation, the surface acoustic wave is attenuated according to its frequency and the depth of the decarburized layer as described above. Of the reflected waves of the surface acoustic wave propagated on the surface, the focusing lens 11
The reflected wave having the same angle as the incident angle from the
To excite the receiving vibrator 14 to generate a received signal. This received signal is received by the receiver 18 and the A / D
The data is converted into a digital value by the converter 19, and only the peak value is held by the peak hold circuit 20, and this peak value is output to the data analysis / control device 16. Such processing is performed every time the frequency is changed, and eventually, the peak value of each frequency is output to the data analysis / control device 16. The data analysis / control device 16 calculates the relative sensitivities between the input peak values of the respective frequencies, and is equal to a predetermined value β (for example, a predetermined ratio with respect to the maximum relative sensitivity) of the relative sensitivities, The frequency of the closest relative sensitivity is taken out, and the depth of the decarburized layer is calculated as described above.

As described above, in the present embodiment, the surface acoustic wave is generated on the steel material to be measured by the ultrasonic wave whose frequency continuously changes, the peak value of the reflected wave is taken out, and the relative value of the peak value of each frequency is obtained. Of the sensitivities, the frequency of the relative sensitivity equal to or closest to the predetermined value was extracted and the depth of the decarburized layer was estimated based on this frequency, so the depth of the decarburized layer was measured without cutting the steel material And the measurement does not require any trouble or time.

[0035]

As described above, according to the present invention, ultrasonic waves are radiated to the steel material to be measured, and the envelope of the received reflected wave, the peak frequency of the time division of the received reflected wave, or a different frequency is used. Since the depth of the decarburized layer was measured based on the relative sensitivity of the peak value of the received reflected wave of the surface acoustic wave of the ultrasonic wave, the depth of the decarburized layer could be measured without cutting the steel material. No effort or time is required for the measurement.

[Brief description of the drawings]

FIG. 1 is a block diagram of an apparatus for performing a decarburized layer depth measuring method according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a surface layer portion of the steel material 1 to be measured.

FIG. 3 is a waveform diagram of the reception intensity of an ultrasonic reflected wave received by the receiver shown in FIG. 1;

FIG. 4 is a waveform diagram of the reception intensity of the ultrasonic reflected wave received by the receiver shown in FIG.

FIG. 5 is a characteristic diagram of a constant α with respect to a depth of a decarburized layer.

FIG. 6 is a diagram showing an envelope with respect to a reception intensity of ultrasonic reflection of a steel material to be measured having a decarburized layer depth.

FIG. 7 is a diagram for explaining the definition of an envelope E shown in FIG. 6;

FIG. 8 is an explanatory diagram of the contents of a memory.

FIG. 9 is a waveform diagram of reception intensity of an ultrasonic reflected wave.

FIG. 10 is a frequency characteristic diagram of a received signal.

FIG. 11 is a block diagram of an apparatus for performing a decarburized layer depth measuring method according to a third embodiment of the present invention.

FIG. 12 is a characteristic diagram of a relative sensitivity of a received signal of a surface acoustic wave to a frequency.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel to be measured 2 Ultrasonic probe 3 Transmitter 4 Amplifier 5 Receiver 6 Envelope detector 7 Computing unit

Claims (6)

[Claims] An ultrasonic wave is radiated from the surface of a steel material to be measured into the steel material, an envelope of a reception intensity of a reflected wave signal is obtained, and the envelope is determined by a plurality of known decarburization layers which are obtained in advance. Compared with the envelope of the reception intensity of the reflected wave signal of the ultrasonic wave with respect to the depth, the decarburized layer depth corresponding to the closest envelope of these envelopes is defined as the decarburized layer depth of the steel material to be measured. The method for measuring the depth of the decarburized layer of steel. 2. The ultrasonic wave is radiated from the surface of the steel material to be measured into the steel material, and the frequency of the peak value of the frequency components of the reflected wave signal at predetermined time intervals is determined. Obtaining the decarburized layer depth of the steel material to be measured based on the time corresponding to the frequency corresponding to the frequency of the peak value of the frequency component for each of the predetermined time intervals previously determined for the unformed steel material A method for measuring the depth of a decarburized layer of steel. 3. A surface acoustic wave is generated by radiating ultrasonic waves having different frequencies on the surface of a steel material to be measured, and the reflected wave signal corresponds to a predetermined relative sensitivity among the relative sensitivities at each frequency. A decarburized layer depth of the steel material to be measured based on a frequency to be measured. 4. An ultrasonic probe for radiating ultrasonic waves to a steel material to be measured, a receiver for receiving a reflected wave of the ultrasonic wave radiated from the ultrasonic probe, and a plurality of known decarburized layer depths of the steel material A first memory storing, for each of the decarburized layer depths, the data of the envelope of the received intensity composed of the reception time and the received intensity of the reflected wave signal of the ultrasonic wave, and the relationship between the constant of the envelope and the decarburized layer depth A second memory for storing the reception intensity of the reflected wave received by the receiver at each reception time and the first
A first calculating means for calculating a difference between the reception intensity at each corresponding reception time for each depth of the decarburized layer of the memory, and a second calculating means for calculating the sum of the differences calculated by the first calculation means. Calculating means; first searching means for searching an envelope of each decarburized layer depth having a minimum sum of differences calculated by the second calculating means; and And a second searching means for searching for a decarburized layer depth corresponding to the constant of the envelope searched by the searching means. 5. An ultrasonic probe for radiating ultrasonic waves to a steel material to be measured, a receiver for receiving a reflected wave of the ultrasonic wave radiated from the ultrasonic probe, and the steel material to be measured without a decarburized layer A memory for storing a peak frequency of a received signal of an ultrasonic wave for each predetermined time interval, and a frequency analysis unit for extracting a peak frequency of each time corresponding to each time of the predetermined time interval in the received signal received by the receiver Search means for searching for a time at which the peak frequency extracted by the frequency analysis section matches the peak frequency stored in the memory; and calculating the depth of the decarburized layer based on the time searched by the search means. An apparatus for measuring the depth of a decarburized layer of a steel material, comprising: 6. An ultrasonic probe which emits an ultrasonic wave to the surface of a steel material to be measured to generate a surface acoustic wave and receives a reflected wave signal thereof, and continuously adjusts the frequency of the ultrasonic wave radiated from the ultrasonic probe. Frequency changing means for changing the relative sensitivity of the reflected wave signal received by the ultrasonic probe to a predetermined relative sensitivity; and the comparing means makes the relative sensitivity of the reflected wave signal equal to the predetermined relative sensitivity. Calculating means for calculating a decarburized layer depth based on a frequency corresponding to the relative sensitivity when the sensitivity is matched with the sensitivity.