JPH09189683A - Method and device for ultrasonic flaw detection in ductile grain steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect only reflections at a defect without detecting tree-like echoes at a grain boundary, so as to provide a high defect-detecting ability, by causing ultrasonic waves to enter from a direction tilted from a line perpendicular to a direction for rolling a ductile grain steel plate. SOLUTION: A transmitted signal with a preset frequency is input from a transmitter to a transmitter-receiver probe 2, and an ultrasonic wave is made to enter a steel plate 1 at an inclination angle θ. If there is a defect (d), the probe 2 detects a reflected echo γ, converts it into an electric signal, and imparts the signal to a receiver. A probe 3 used exclusively for reception constantly receives ultrasonic waves from the probe 2. If the defect (d) moves along with the steel 1 and enters an area connecting the probes 2, 3, part of the ultrasonic wave is reflected because of the presence of the defect, and the levels of the signals received by the probe 3 and the receiver are lowered. Therefore, the length of the defect (d) can be calculated by detecting the duration of sound pressure lowering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic flaw detection method and apparatus for wrought steel sheets, and more particularly, to a probe tilted at a constant angle from a line orthogonal to the rolling direction of the wrought steel sheets. The present invention relates to a method and an apparatus for accurately detecting defects existing in an expanded grain steel sheet by arranging them.

[0002]

2. Description of the Related Art When inspecting for defects existing inside or on the surface of a thin steel sheet, ultrasonic waves are made incident by using an ultrasonic probe.
An ultrasonic flaw detection method for detecting a surface defect and an internal defect by receiving the reflected echo is well known in the art. However, in such an ultrasonic flaw detection method, in the case of a material to be inspected with coarse crystal grains, the ultrasonic waves are scattered at the crystal grain boundaries, so the amount of attenuation is large and it is difficult to detect minute defects.

To explain this, in general, such attenuation of ultrasonic waves varies depending on the material and size of the material to be inspected, but can be expressed by equation (1).

[0004]

[Equation 1]

The sound pressure P at the propagation distance x decreases exponentially as the propagation distance x and the damping constant α increase, as shown in the equation (1). Where the damping constant α is
Since it is proportional to the fourth power of the frequency of the ultrasonic wave, it can be understood that the attenuation of the ultrasonic wave can be reduced by using the ultrasonic wave of low frequency when detecting the material to be inspected with coarse crystal grains. Therefore, conventionally, low frequency ultrasonic waves have been selected for flaw detection.

However, in ultrasonic flaw detection, the sound pressure P _r when a defect of the area S at the propagation distance x is detected is expressed by the following equation (2).

[0007]

[Equation 2]

From the equation (2), when the low frequency ultrasonic wave is used as described above, the sound pressure P _r becomes small because the wavelength λ is long, which causes a problem that the defect detectability is deteriorated. .

Further, in the expanded grain steel sheet in which crystals have a certain degree of directionality in the sheet passing direction, when ultrasonic waves are incident perpendicularly to the sheet passing direction and a reflected echo is detected, the reflected echo is reflected from the grain boundary. There is also a problem in that scattered echoes are overlapped and received to form forest-like echoes (shower echoes), and as a result, the S / N ratio of the received signal is greatly reduced.

As described above, when the crystal grains of the material to be inspected are coarse and directional, the detectability is lowered by the use of low-frequency ultrasonic waves, and the S / N ratio is greatly increased by the generation of forest echo. In order to deal with the decrease and the like, the following prior art examples have been proposed.

(1) In Japanese Patent Application Laid-Open No. 62-85859 (hereinafter referred to as "Prior Art 1"), a stainless clad tube is immersed in water and a wide band type frequency characteristic of 5 to 15 MHz is applied to the stainless clad tube. GS.No. is from 8.3 by narrowing the beam diameter and transmitting ultrasonic waves with a wavelength larger than the crystal grain size of the stainless clad tube from the oblique angle ultrasonic probe. Also disclosed is an ultrasonic flaw detection method that enables ultrasonic flaw detection of minute defects by suppressing the scattering and attenuation of ultrasonic waves even with a coarse-grained stainless clad tube.

(2) JP-A-2-186261 (hereinafter, referred to as
According to Japanese Patent Laid-Open Publication No. 2002-27242, the material to be inspected is subjected to ultrasonic flaw detection using high-frequency and wide-band ultrasonic waves.
An ultrasonic flaw detection method has been disclosed in which defect detection is good and S / N ratio is good by performing defect detection at a predetermined frequency in which a noise signal is small in the frequency range of reflected echo.

[0013]

However, the above-mentioned respective prior art examples have the following problems. <Problem of Prior Art Example 1> In this ultrasonic flaw detection method, 5 to 1
Even if it is effective to use a flaw detector and a probe having a broadband frequency of 5 MHz, when a wide range flaw detection is required, such as ultrasonic flaw detection of a thin steel plate targeted by the present invention, Ultrasonic waves above 5MHz cannot be adopted. Furthermore, when the inspection frequency is set so that the wavelength of the ultrasonic waves is larger than the crystal grain size, although the attenuation due to scattering at the crystal grain boundaries and the occurrence of forest echo are reduced, the ultrasonic wave Making the wavelength larger than the crystal grain size means that the wavelength of this ultrasonic wave may be larger than the defect diameter, and as the meaning of the above-mentioned formula (2), the wavelength is larger than the defect area S. Since the wavelength λ of the sound wave is increased, the detected sound pressure P _r is reduced and the defect detectability is reduced. Therefore, if the frequency is selected so that the wavelength is larger than the crystal grain size, there arises a problem that the defect cannot be detected.

<Problem of Prior Art 2> In the ultrasonic flaw detection method shown in Prior Art 2, a defect signal of only an arbitrary frequency band set based on preliminary flaw detection is detected from the reflected echo. However, when the crystal grains are extremely coarse or when the flaw detection range is wide as in the case of a thin steel plate, it is not suitable for the same reason as in the first example.

Such a problem is based on the limit to be solved only by selecting the frequency of the ultrasonic wave.

Therefore, the main object of the present invention is to detect only the defect reflection echo without directly detecting the forest-like echo due to the scattering echo at the crystal grain boundary, while using the ultrasonic wave of the frequency conventionally used. It is possible to provide an ultrasonic flaw detection method for an expanded grained steel sheet and a device therefor capable of achieving high defect detection ability.

[0017]

According to the invention of claim 1 which has achieved the above-mentioned problems, an ultrasonic wave is incident on the wrought steel sheet and a reflection echo at the defect portion is detected to detect the defect. An ultrasonic flaw detection method for detecting, wherein the steel plate is moved from a direction inclined from a line orthogonal to a rolling direction of the wrought steel sheet, more preferably from a position within an inclination angle range of 2.5 degrees to 45 degrees. It is an ultrasonic flaw detection method for wrought grained steel sheets, characterized by applying ultrasonic waves.

Since grain boundaries of wrought grained steel sheets such as ferritic stainless steel sheets have anisotropy in the rolling direction and many defects extend in the rolling direction, they are orthogonal to the rolling direction. When ultrasonic waves are transmitted and received from the direction, not only the defect reflection echo but also the direct scattering echo at the grain boundary is received. Further, when ultrasonic waves are transmitted and received from the rolling direction, the scattered echoes are not directly received and the noise is reduced, but since the target defect extends in the rolling direction, the reflected echo from the defect cannot be detected either.

Therefore, according to the present invention, a direction inclined from a line orthogonal to the rolling direction of the wrought steel sheet, or preferably 2.
When ultrasonic waves are transmitted and received from a position within the range of the inclination angle θ of 5 degrees to 45 degrees (see FIG. 1), although the ultrasonic waves themselves are scattered from the grain boundaries, the wrought grained steel sheet is rolled in the rolling direction. Due to the anisotropy direct reception of scattered echoes can be avoided and reflected echoes from defects can be detected.

FIG. 4 is an explanatory view of a defect drawn based on a microphotograph of a defect of an expanded grain steel sheet detected by an ultrasonic C-scan device. The defect extends in the rolling direction and its tip is rounded. It can be seen that it is oval-shaped. This is in agreement with the shape shown in the micrograph of the actual structure shown in FIG. However, when the ultrasonic waves are transmitted and received from the position within the tilt angle range according to the present invention, the ultrasonic waves are reflected at the rounded portion of the defect, so that the defect can be satisfactorily detected. Thus,
According to the ultrasonic flaw detection method of the present invention, the presence of a defect can be detected, but the length of the defect cannot be measured.

Therefore, according to the third and fourth aspects of the invention, the length of the defect is measured by receiving the incident ultrasonic wave and measuring the duration of the decrease in the sound pressure. is there.

That is, the invention described in claim 3 is to receive the incident ultrasonic wave and measure the length of the defect based on the duration time of the state of the reduced sound pressure.

The invention according to claim 4 is an ultrasonic flaw detector for detecting a defect by injecting ultrasonic waves into a wrought grained steel sheet and detecting a reflection echo at the defect portion,
A transmission / reception probe which is arranged at a position inclined from a line orthogonal to the rolling direction of the wrought steel sheet and receives ultrasonic waves and reflected echoes, and a reception which is arranged at a position facing the transmission / reception probe The presence or absence of a defect based on the reflection echo level received by the dedicated probe and the transmission / reception probe, and the length of the defect based on the duration of the state in which the reception sound pressure is reduced in the reception-only probe. And a signal processing system for detecting each of the above.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION An ultrasonic flaw detector for a wrought grained steel sheet for carrying out the present invention has a directivity above the wrought grained steel sheet 1, as shown in FIGS. 1 and 2, for example. The transmitting and receiving probe 2 having the above is arranged in contact with the expanded grained steel plate 1 through the contact medium so as to form an inclination angle θ from the direction line 5 orthogonal to the rolling direction, and the receiving-only probe 3 is transmitted and received. The probe 2 and the probe 2 are arranged so as to face each other with the flaw detection site interposed therebetween. Further, the two probes 2 and 3 are held by the probe turning jigs 4a and 4b, and the displacement angle (tilt angle) θ of the ultrasonic wave transmission / reception direction with respect to the orthogonal line 5 is arbitrarily set. You can do it.

In such a device, first, an oscillation signal of a preset frequency is input from the oscillator 7 to the transmission / reception probe 2, and the transmission / reception probe 2 spreads the ultrasonic wave of the frequency at the inclination angle θ. It enters the grain-rolled steel sheet 1 (t in the figure). In the transmission / reception probe 2, when there is a defect d, the reflection echo r
Is detected, converted into an electric signal, and given to the receiver 6. The signal received by the receiver 6 is given to the calculator 10.

On the other hand, the reception-only probe 3 always receives the ultrasonic waves from the transmission / reception probe 2, and the receiver 8 receives the signal. The timing of this reception is synchronized with the oscillation timing from the oscillator 7.

The defect d moves as the steel sheet 1 moves, and when it enters the area connecting the transmitting / receiving probe 2 and the reception-only probe 3, a part of the ultrasonic wave is generated due to the defect d. Reflection occurs, and the (sound pressure) level of the reception signals at the reception-only probe 3 and the receiver 8 decreases. This reduction in the sound pressure level continues until the defect d comes out of the area. Therefore, by detecting the duration of the decrease in the sound pressure level, the defect d
The length of can be detected. The reception signal of the receiver 8 is taken into the calculator 10 through the filter 9.

On the other hand, the signal processing of the defect judgment by the signal processing system consisting of the receivers 6, 8, the oscillator 7, the filter 9, and the calculator 10 will be described in more detail with reference to the explanatory view of FIG. The signals received by the transmission / reception probe 2 and the receiver 6 are:
A change is shown as the defect reflection echo intensity shown in FIG. 7. In this case, since noise may be determined as a defect, the arithmetic unit 10 appropriately sets a threshold value in advance, and only when the threshold value is exceeded, a defect is determined.

On the other hand, when determining the length of the defect d, the defect d
The input signal from the reception-dedicated probe 3 to the receiver 8 due to the presence of is reduced by only a few percent as compared with the input signal in the normal part.

However, the decrease of the input signal of about several percent also changes due to the following factors. -Instability of the amount of the contact medium used for the incidence of ultrasonic waves, which is mainly due to the change in the steel sheet passing speed. -Variation in the amount of attenuation of the received signal due to changes in the steel sheet structure due to variations in manufacturing conditions (rolling reduction rate, cooling pattern, annealing temperature, etc.). However, the fluctuation cycle due to the above factors is a low cycle, whereas the fluctuation cycle such as normal part-defective part-normal part is a high cycle. Thus, as shown in FIG. 7, by removing low-cycle received signal fluctuations, fluctuations due to the above factors and fluctuations due to the presence or absence of defective portions can be clearly discriminated, and the length of defects can be accurately measured.

Even if the defect is detected only by the reception-only probe 3, the position of the defect cannot be specified even if the presence / absence of the defect can be grasped. Therefore, it is not suitable for the defect detection. The probe 2 is used.

<Experimental example> Next, the result of the flaw detection experiment of the ferritic stainless steel sheet in the process of reaching the completion of the ultrasonic flaw detection method for the wrought grain steel sheet according to the present invention will be described.

As shown in FIG. 5, after the through hole 11 is machined into this ferritic stainless steel plate as an artificial defect, the transmission / reception probe 2 is placed at 300 from the center of the through hole 11.
mm apart, and the tilt angle θ is 0 degree, 2.5
The flaw detection was performed by setting the angle to 5 degrees, 10 degrees, 22.5 degrees, and 45 degrees. The results of this flaw detection are shown in FIGS.

(1) In case of 0 degree (FIG. 6 (a)) Since the scattered echo at the crystal grain boundary is directly received, it becomes a forest echo as a whole and discrimination of the defect reflection echo shown by the symbol F Is impossible.

(2) Case of 2.5 degrees (FIG. 6 (b)) Compared with the case of 0 degrees, the scattered echoes at the grain boundaries are not directly received, so the forest echoes are reduced. However, since the reflection echo of the edge of the steel sheet indicated by the symbol B is largely received, it cannot be said that discrimination of the defect reflection echo F is optimal. (3) Case of 5 degrees (FIG. 6C) Compared with the case of 2.5 degrees, the number of forest echoes is further reduced, and the defect reflection echo F can be easily discriminated. However, although the reflected echo B of the edge is still received, the level is small and there is no problem in defect discrimination. (4) In case of 10 degrees, 22.5 degrees, 45 degrees (respectively,
6 (d), (e), (f)) Compared to the case of 5 degrees, the forest echo is small and the reflected echo B at the edge is not received, and the discrimination of the defective reflected echo F is excellent.

On the other hand, FIG. 8 shows the change in the S / N ratio due to the difference in the tilt angle. From these series of results, the inclination angle is preferably 2.5 to 45 degrees, particularly preferably 5 to 45 degrees. However, even if the angle of inclination exceeds 45 degrees, flaw detection is possible, but the flaw detection range in the plate width direction becomes narrower, and it is necessary to provide a plurality of flaw detection devices including the probe in the plate width direction. However, it is not a good idea in terms of cost.

Further, the ultrasonic flaw detection method for wrought grain steel sheet of the present invention is preferably used mainly for detecting inclusions present inside.

The flaw detection frequency in the present invention is 1 MHz to 5 MHz.
It can be MHz. The transmission / reception probe can transmit ultrasonic waves while selecting an optimal angle while changing at various angles in addition to a right angle to the steel plate surface.

[0039]

As described above, according to the present invention, the defect reflection echo can be directly detected without directly detecting the forest-like echo due to the scattering echo at the crystal grain boundary, while using the ultrasonic wave of the frequency conventionally used. Only the defects can be detected, and thus high defect detectability is exhibited.

Further, the length of the defect can be detected.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an ultrasonic flaw detection method according to the present invention.

FIG. 2 is a block diagram showing a configuration of an ultrasonic flaw detector according to the present invention.

FIG. 3 is a micrograph of a metal structure of a ferritic stainless steel plate.

FIG. 4 is an explanatory diagram of a detected defect in an expanded grain steel sheet.

FIG. 5 is an explanatory view of an inclination angle selection experiment for ultrasonic flaw detection.

FIG. 6 is a result of ultrasonic flaw detection at each inclination angle of a ferritic stainless steel plate.

FIG. 7 is an explanatory diagram of signal processing.

FIG. 8 is a graph showing changes in S / N ratio.

[Explanation of symbols]

1 ... steel plate, 2 ... transmission / reception probe, 3 ... reception-only probe, 5
... Orthogonal line, 6 ... Receiver, 7 ... Oscillator, 8 ... Receiver, 9 ...
Filter, 10 ... calculator.

Claims (4)

[Claims] 1. An ultrasonic flaw detection method for detecting defects by injecting ultrasonic waves to a wrought grained steel sheet and detecting reflection echoes at the defect portion, the method comprising: An ultrasonic flaw detection method for an expanded grain steel sheet, characterized in that ultrasonic waves are incident on the steel sheet from a direction inclined from an orthogonal line. 2. The ultrasonic flaw detection method for a wrought grained steel sheet according to claim 1, wherein the direction is within a tilt angle range of 2.5 to 45 degrees from a line orthogonal to the rolling direction of the wrought grained steel sheet. 3. The length of the defect is measured based on the duration of the state in which the sound pressure is reduced, by receiving the incident ultrasonic wave.
Ultrasonic flaw detection method for the wrought expanded steel sheet described. 4. An ultrasonic flaw detector for detecting a defect by injecting an ultrasonic wave into the expanded grained steel sheet and detecting a reflection echo at the defect portion, the ultrasonic flaw detection device comprising: A transmission / reception probe that is arranged at a position inclined from the orthogonal line and that receives ultrasonic waves and reflected echoes, and a reception-only probe that is arranged at a position facing the transmission / reception probe, and the transmission / reception A signal processing system for detecting the presence or absence of a defect on the basis of the reflected echo level received by the probe, and a signal processing system for detecting the length of the defect on the basis of the duration of the reduced state of the received sound pressure in the reception-only probe. An ultrasonic flaw detector for expanded grained steel sheets, which is characterized by being provided.