JPH09257759A - C-scanning ultrasonic flaw detecting method and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic flaw detecting method suitably used in the detection of the internal flaw of about 10-100μm of a sample cut from a rolled metal panel. SOLUTION: A first process superposing the linearly converged ultrasonic beam from an ultrasonic transmitter 21 and the linearly converged receiving beam of an ultrasonic receiver 22 one upon another to detect a flaw and a second process allowing the linearly converged ultrasonic beam from the ultrasonic transmitter 21 to cross the linearly converged receiving beam of the ultrasonic receiver 22 at a right angle to detect a flaw are combined. By this constitution, the internal flaw of a panel 10 to be inspected can be detected inclusive of the form of the flaw without generating an insensitive zone in the vicinity of the surface thereof.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a C-scan ultrasonic flaw detection method and apparatus, and more particularly, to a C-scan ultra-sonic detector suitable for use in detecting internal defects of about 10 to 100 μm in a cut sample of a rolled metal plate. The present invention relates to an ultrasonic flaw detection method and apparatus.

[0002]

2. Description of the Related Art In recent years, thin steel sheets, which are materials for automobiles and cans, have become thinner due to demands for weight reduction and material cost reduction. In processing such as pressing and drawing, strong processing accompanied by remarkable deformation of the material has been performed. When subjecting a steel sheet to strong processing, cracks will occur if internal defects such as non-metallic inclusions are present in the part that is significantly deformed.
The smaller the thickness of the steel sheet, the more remarkable the occurrence of cracks due to internal defects, and the size of the internal defects that cause the cracks becomes smaller. There is also a relationship between the form of defects and the occurrence of cracks, and there are spherical simple substances, simple substances extending in one direction, and aggregates of minute spherical defects as defect forms. You can see the difference. In addition, products with severe operating conditions, such as thick steel plates used for sour gas line pipes, have increased, and from the size of 10 μm or more, they are considered to be harmful as they cause hydrogen-induced cracking, and hydrogen-induced cracking also occurs depending on the defect form. Ease is different. Therefore, in the above-mentioned steel sheet, it is required to reduce internal defects as much as possible, and to make the defect morphology less likely to cause cracks. It is necessary to evaluate.

As a means for detecting such internal defects of a steel sheet and evaluating the form thereof, a part of a product is cut out as a sample, and the internal defect in this sample is measured by using a device called a C-scan ultrasonic flaw detector. Testing has been widely used. FIG. 6 shows a flaw detection method using a conventional C-scan ultrasonic flaw detector. That is, the point-focusing ultrasonic wave transceiver 102 above the plate 101 to be inspected immersed in the solvent liquid.
Is a scanning device that is moved by a signal from the controller 114
The electric pulse scanned by 104 and transmitted from the electric pulse generator 116 at constant time intervals is converted into ultrasonic waves, and the ultrasonic beam 103 is transmitted substantially vertically toward the inspection plate 101, and at the same time, the inspection plate is inspected. It receives the internal defects of 101, reflected waves from the front and back, and converts them into electrical signals. The received signal is amplified by the reception amplifier 111, and the gate circuit 11
At 2, the reflected wave from the defect is extracted. The extracted signal is sent to the peak value detection circuit 113, where the amplitude of the defect reflected wave is detected and sent to the controller 114. The controller 114 controls the amplitude of the defect reflection wave and the scanning device 104.
And the position signal of 1) are output to the display unit 115, and the display unit 115 displays a two-dimensional distribution map of the internal defects and detects the internal defects in this way.

In such a method of detecting ultrasonic waves by transmitting ultrasonic waves substantially perpendicularly to the plate 101 to be inspected by one point-focusing ultrasonic wave transceiver 102 and receiving reflected waves from the plate to be inspected,
When an ultrasonic beam is incident on the surface, a surface reflection wave with a large amplitude and reverberation lasts for a while is generated, so the defect reflection wave near the surface overlaps with the surface echo or its reverberation, and its existence is identified. There is a problem in that it becomes impossible to detect defects near the surface.

Further, as the prior art relating to the C-scan ultrasonic flaw detection method or apparatus, there are JP-A-59-17153 and JP-A-5-333000 which use high-frequency ultrasonic waves. The former is 30-100MHz, the latter is 15-50MH
In each of z, high-resolution ultrasonic waves are used to reduce the beam diameter to improve resolution and detectability of internal defects. The latter is the ultrasonic frequency,
By optimizing the focal length and the relationship between the plate to be inspected and the focal position, it is possible to ensure the detection of minute defects existing near the surface and to quantitatively evaluate the flaw detection result.

[0006]

[Patent Document 1] Japanese Patent Application Laid-Open No. 5-33
Although the dead zone just below the surface is reduced in the 3000 publication, it cannot be said that there is no dead zone, and the problem remains that microscopic defects existing in the vicinity of the surface cannot be detected in C scan ultrasonic flaw detection by the vertical flaw detection method. ing. Further, in C-scan ultrasonic flaw detection using a point-focusing type transceiver, there is a problem in that flaw detection takes time because the ultrasonic transducer is two-dimensionally scanned to detect internal defects.

By the way, the inventors of the present invention have already disclosed in JP-A-7-
In 253414, a line focus ultrasonic transmitter and a one-dimensional array ultrasonic receiver having a plurality of ultrasonic transducers arranged in the width direction of the ultrasonic beam are arranged to face each other with a plate to be inspected, A band-shaped ultrasonic beam is transmitted from the transmitter almost vertically to the plate to be inspected, and the reflected wave from the internal defect caused by the ultrasonic waves incident on the plate to be inspected is received by the one-dimensional array type ultrasonic receiver. Then, it proposes an ultrasonic flaw detection method and device characterized by detecting the presence or absence of a reflected wave that has reached a predetermined amplitude after amplifying the received ultrasonic wave and extracting only the reflected wave. It became possible to detect a linear area of a constant width at a high speed at a time over the entire area in the plate thickness direction without a dead zone directly below.

However, according to this method, although the presence or absence of even minute defects can be clearly seen, the ultrasonic waves to be transmitted and received are not two-dimensionally focused, so that the resolution of the unfocused ultrasonic beam in the width direction is low. However, there is a problem that the form of the defect cannot be determined. The present invention has been made to solve the above-described problems of the prior art, and in C-scan ultrasonic flaw detection, there is no dead zone near the surface of the plate to be inspected, and flaw detection of the entire cross section in the plate thickness direction is performed at high speed. An object of the present invention is to provide a C-scan ultrasonic flaw detection method and apparatus capable of detecting even the form of a fine internal defect.

[0009]

According to the present invention, a line focus type ultrasonic transmitter and a line focus type ultrasonic receiver are arranged to face each other with a plate to be inspected immersed in a liquid sandwiched therebetween for scanning. At the same time, the ultrasonic beam linearly focused from the ultrasonic transmitter is made to enter the plate to be inspected substantially vertically, and the transmitted wave of the ultrasonic beam and the reflection from the internal defect caused by the ultrasonic beam are reflected. A method for detecting an internal defect of a plate to be inspected on the basis of a received signal by receiving a wave and the ultrasonic wave receiver, wherein the ultrasonic beam linearly focused from the ultrasonic wave transmitter and the A first step of flaw detection such that the linearly focused reception beam of the ultrasonic receiver overlaps, and the linearly focused ultrasonic beam from the ultrasonic transmitter and the linear shape of the ultrasonic receiver So that the received beam focused on A second step of flaw detection by, it is C-scan ultrasonic flaw detection method comprising consisting.

Further, according to the present invention, a line focus type ultrasonic transmitter for transmitting ultrasonic waves substantially perpendicularly to the surface of the plate to be inspected and a position facing the ultrasonic transmitter with the plate to be inspected interposed therebetween. Then, a line focus type ultrasonic wave receiver for receiving the reflected wave of the ultrasonic wave and the reflected wave from the internal defect caused by the ultrasonic wave, and the ultrasonic beam linearly focused from the ultrasonic wave transmitter. And from the position where the reception beam converged linearly of the ultrasonic receiver overlaps the position orthogonal to
A rotating mechanism for rotating the ultrasonic transmitter and / or the ultrasonic receiver, a support arm for supporting the ultrasonic transmitter and the ultrasonic receiver with a plate to be inspected, and scanning the support arm. A scanning device, an electric pulse generator that generates electric pulses for transmitting a pulsed ultrasonic beam from the ultrasonic receiver, a reception amplifier that amplifies a reception signal from the ultrasonic receiver, and an amplifier And a gate means for extracting a reflected wave from the internal defect from the generated signal, and a C-scan ultrasonic flaw detector.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram including a partial perspective view showing the structure of the present invention and the beam state of the first step, and FIG. 2 is a perspective view showing the beam state of the second step of the present invention. is there.

In these figures, 21 is a line focus type ultrasonic transmitter having a transducer width of W (hereinafter, simply referred to as an ultrasonic transmitter), and 22 is a line focus having a transducer width of W. Type ultrasonic receivers (hereinafter, simply referred to as ultrasonic receivers), which are arranged to face each other with the thin steel plate 10 to be inspected. Reference numeral 23 is a U-shaped support arm that supports the ultrasonic transmitter 21 and the ultrasonic receiver 22. 24 is an ultrasonic receiver
The ultrasonic receiver 22 is interposed between the support arm 23 and the support arm 23.
It is a rotating mechanism for rotating 90 °. 25 is a support arm
It is a scanning device for scanning 23. Water, which is preferably used as an ultrasonic propagation medium, is interposed between the ultrasonic transmitter 21 and the ultrasonic receiver 22 and the plate 10 to be inspected.

Reference numeral 31 is an electric pulse generator for transmitting electric pulses from a built-in clock circuit (not shown) to a piezoelectric vibrator (not shown) built in the ultrasonic transmitter 21 at regular time intervals. 32 is a reception amplifier that receives the signal from the ultrasonic receiver 22, 33 is a gate circuit, 34 is a peak value detection circuit,
Reference numeral 35 is a controller, 36 is a display, and 37 is a memory device.

The ultrasonic transmitter 21 converts an electric pulse transmitted from the electric pulse generator 31 at a constant time interval into an ultrasonic wave, and transmits a transmission line focus beam (substantially perpendicular to the plate 10 to be inspected through the water). Hereinafter, it will be simply referred to as a transmission beam) 40 is transmitted. The ultrasonic receiver 22 receives a reflected wave from an internal defect caused by the ultrasonic wave incident on the plate 10 to be inspected through water by a reception line focus beam (hereinafter, simply referred to as a reception beam) 50. The reception signal amplified by the reception amplifier 32 is sent to the gate circuit 33, and the gate circuit 33 extracts the reflected wave from the internal defect from the reception signal. The extracted signal is transmitted to the peak value detection circuit 34, and the peak value detection circuit 34 detects the amplitude of the reflected wave and outputs it to the controller 35 as an analog amount or a digital amount.
Then, the support arm 23 is scanned by the scanning device 25 driven by a signal from the controller 35, and thereby the ultrasonic transmitter 21 and the ultrasonic receiver 22 arranged to face each other are square-scanned on the surface of the plate 10 to be inspected. , To detect its internal defects.

Therefore, in the first step of flaw detection, as shown in FIG. 1, the reception beam 50 of the ultrasonic receiver 22 is made parallel to the transmission beam 40 transmitted from the ultrasonic transmitter 21. Two-dimensional scanning is performed with the width W of the line focus beam as the index feed amount. At this time, the controller 35 outputs the amplitude of the defect reflected wave and the position signal of the scanning device 25 to the display 36 to create a two-dimensional distribution map of the internal defects, and at the same time, to reflect the defects reflected with an amplitude of a certain threshold or more. The position where the wave is detected is stored in the memory device 37.

Next, in the second step of flaw detection, as shown in FIG. 2, the ultrasonic receiver 22 is rotated by 90 ° by the rotating mechanism 24 interposed between the support arm 23 and the ultrasonic receiver 22. Rotate so that the reception beam 50 of the ultrasonic receiver 22 is orthogonal to the transmission beam 40 transmitted from the ultrasonic transmitter 21, and a minute measurement pitch near the position of the internal defect stored in the memory device 37, For example, two-dimensional scanning is performed with 0.1 mm or less (it varies depending on the size of the internal defect to be detected). At this time, the controller 35 outputs the amplitude of the reflected wave of the defect and the position signal of the scanning device 25 to the display 36 to create a two-dimensional distribution map showing the form of the internal defect. This second step is carried out for all the defects detected in the first step of flaw detection, so that it becomes possible to detect internal defects including their forms.

Here, the internal defect is near the surface of the inspection plate 10 as shown in FIG. 3, that is, the distance d from the surface 11 of the inspection plate 10 is t / 2 (t; the thickness of the inspection plate 10). ) A specific description will be given of the detection method when it exists below. That is, the ultrasonic transmission beam 40 transmitted from the ultrasonic transmitter 21 propagates in water, reaches the surface 11 of the inspection plate 10, enters the inspection plate 10, and propagates inside. At this time, the ultrasonic waves propagated inside the inspection plate 10 have internal defects 13
Is present in the ultrasonic propagation path, the internal defect 13
Reflected once on the surface 11, reflected on the surface 11, propagated once within the thickness t of the plate 10 to be inspected (0.5 round trips or one way), propagated from the back surface 12 into the water and received ultrasonic waves. The reflected wave 52 received by the child 22 propagates once (0.5 round trips or one way) in the thickness t of the plate 10 to be inspected, is reflected by the back surface 12, goes to the internal defect 13, and then the internal defect 13 After reflecting once, back side 12
A reflected wave 53 is generated which is propagated from and received in the water. The reflected waves 52 and 53 described above are further reflected once or more on the back surface 12 and make one or more round trips within the thickness t of the plate 10 to be inspected to generate reflected waves received by the ultrasonic receiver 22. However, the illustration is omitted.

As shown in FIG. 3 described above, the internal defect 13 is near the surface 11 of the inspection plate 10, that is, the distance d from the surface.
Is less than t / 2, the reflected wave 53 reaches the ultrasonic receiver 22 later than the reflected wave 52. Further, the temporal movement of the signal received by the ultrasonic receiver 22 will be described with reference to FIG. In FIG. 4, reference numeral 51 is a signal of a direct transmitted wave propagating once in the thickness t of the plate 10 to be inspected (0.5 round trip or one way), and reference numeral 54 is 1 in the thickness t of the plate 10 to be inspected. This is a secondary transmitted wave signal that propagates once (0.5 round trips or one way) and then makes one round trip on the plate 10 to be inspected. Further, τ ₀ is the time when the direct transmitted wave 51 propagated once (0.5 round trips or one way) in the thickness t of the inspection plate 10 reaches the ultrasonic receiver 22, and τ ₁ is the ultrasonic wave. Is the time required to propagate the thickness t.

Reference numerals 52 and 53 are reflected waves from the internal defect 13 as described above. Since the propagation distance of the reflected wave 53 is larger than that of the reflected wave 52, the reflected wave 53 is received with a delay. From this figure, the reflected wave 53 due to the internal defect 13 is
From the time τ ₀ when the direct transmitted wave 51 propagated once (0.5 round trips or one way) within the thickness t of 10 reaches the ultrasonic receiver 22, ultrasonic waves propagate through the thickness t of the plate 10 to be inspected. Time required τ
_{After one} lapse of time, the direct transmitted wave 51 and the reverberation appear outside the dead zone region, and before (2 × τ ₁ ) from the time τ ₀ , and the secondary transmitted wave 54
Appears earlier than. Further, the closer the internal defect is to the surface, τ ₂ (τ ₂ ;
However, it appears earlier than the secondary transmitted wave 54. Therefore, internal defects 13
According to the present invention, the internal defects 13
Since the reflected wave 53 due to can be clearly identified and extracted, the internal defect can be surely detected.

The case where the internal defect 13 exists near the front surface has been described above. Next, the case where the internal defect 13 exists near the rear surface 12, that is, the distance d from the front surface 11 is t / 2 or more will be described. In this case, unlike FIG. 4, the propagation distance of the reflected wave 52 due to the internal defect becomes larger than the propagation distance of the reflected wave 53, so the reflected wave 52 is received with a delay. As described above, the reflected wave 52 appears at a position outside the dead zone due to the direct transmitted wave 51, and appears earlier than the secondary transmitted wave 54. Therefore, even if the internal defect 13 exists immediately below the back surface 12, the reflected wave 52 due to the internal defect 13 can be clearly identified and extracted according to the present invention, so that the internal defect 13 can be detected. From this, even if the internal defect 13 is located at any position including immediately below the front surface and immediately below the back surface, according to the present invention, there is no dead zone on the surface, and the reflected wave from the internal defect is clearly identified and extracted. Therefore, the internal defect can be reliably detected.

Thus, in the first step of flaw detection, the ultrasonic transmitter is used only for the purpose of detecting the internal defect 13.
The flaw detection is performed by making the transmission beam 40 transmitted from 21 and the reception beam 50 of the ultrasonic receiver 22 overlap in parallel. Since the internal defect in the portion where the two beams overlap can be detected, flaw detection can be performed in the region where the two beams overlap with each other by one ultrasonic wave transmission / reception. For example, if a line focus type ultrasonic wave transmitter 21 and an ultrasonic wave receiver 22 having a frequency of 25 MHz and a transducer width of 6 mm are used, the flaw detection width is about 6 mm. By the way, in the C-scan flaw detection using a normal point-focusing ultrasonic wave transceiver, it is necessary to perform two-dimensional scanning in the y direction at a measurement pitch of 0.1 mm or less, and stroke scanning of 60 times or more is required. Therefore, in the first flaw detection process of the present invention, it is possible to detect defects at a high speed in a short time of 1/60 or less as compared with the conventional method.

However, in this first step only, the ultrasonic beams to be transmitted and received are not focused two-dimensionally,
Although the resolution in the non-focusing direction is low and the presence / absence of a defect can be clearly seen, it is not possible to detect even the form of the defect. Therefore, according to the present invention, in the second step of flaw detection, the ultrasonic beam 22 is rotated by 90 ° and the transmitted beam 40 transmitted from the ultrasonic wave transmitter 21 and the received beam of the ultrasonic wave receiver 22 are transmitted.
50 is orthogonal to each other, and the flaw detection is performed only in the vicinity of the portion where the defect is detected in the flaw detection in the first stage. In this case, since the same resolution as that when the ultrasonic beam is two-dimensionally focused can be obtained by the intersection of the line focus beams, it becomes possible to detect even the form of the defect with high resolution. In this second step, the intersection of the line focus beam is measured with respect to the plate 10 to be inspected with a measurement pitch of 0.1 m, for example.
It is necessary to perform two-dimensional scanning at m or less, but since this scanning needs to be performed only in the vicinity of the internal defect 10 detected in the first step of flaw detection, the area of the region requiring scanning is small and It is possible to detect flaws in a time much shorter than the time required for two-dimensional scanning of the entire surface.

In the above embodiment, the ultrasonic receiver 22 is attached to the support arm 23 via the rotating mechanism 24 to detect the first flaw.
Although the case where the second step is performed after rotating the ultrasonic receiver 22 by 90 ° after the above step is completed, the present invention is not limited to this, and the ultrasonic transmitter 21 is rotated by a rotating mechanism. The ultrasonic transmitter 21 is attached to the support arm 23 via 24.
May be rotated.

Further, the rotation mechanism 24 may rotate the ultrasonic transmitter 21 or the ultrasonic receiver 22 by a driving device (not shown). In addition, the rotation mechanism 24 includes a support arm 23 on which the ultrasonic transmitter 21 or the ultrasonic receiver 22 can be manually rotated by 90 ° so that the ultrasonic transmitter 21 or the ultrasonic receiver 22 can be rotated by 90 °.
It may be one with 22 attached. In the above example, the support arm 23 scans the ultrasonic transmitter 21 and the ultrasonic receiver 22 arranged to face the plate 10 to be inspected,
It goes without saying that the plate to be inspected 10 may be configured to scan.

[0025]

[Example] The present invention was applied to the flaw detection of a thin steel plate having a thickness of 4.0 mm. At this time, the ultrasonic transmitter 21 and the ultrasonic receiver 22 are line-focused and have an ultrasonic frequency of 25
MHz, a transducer width W of 6 mm, and a focal length F in water of 38 mm were used. Then, first, flaw detection was performed by making the transmission beam 40 transmitted from the ultrasonic transmitter 21 and the reception beam 50 of the ultrasonic receiver 22 overlap in parallel. The positional relationship between the ultrasonic transmitter 21 and the ultrasonic receiver 22 at this time is, as shown in FIG. 5, a distance L ₁ between the ultrasonic transmitter 21 and the thin steel plate which is the plate to be inspected 11 mm, or The distance L ₂ between the ultrasonic receiver 22 and the thin steel plate was set to 11 mm, and the transducer width of the transmission beam 40 and the transducer width of the reception beam 50 were set to match.

After performing the flaw detection in the first step, the rotation mechanism
24 is operated to rotate the ultrasonic receiver 22 by 90 °, the transmission beam 40 and the reception beam 50 are made orthogonal to each other, and the vicinity of the defect detected in the first step is set to a measurement pitch of 0.002 mm and the flaw detection in the second step is performed. I went. As a result, it was possible to detect a disc-shaped ultra-fine defect of 10 μmφ including its form regardless of the position of the defect in the thickness direction.

[0027]

As described above, according to the present invention, it is possible to detect internal defects such as fine inclusions in a plate to be inspected such as a rolled metal plate in a short time without a dead zone near the surface. It is also possible to obtain the excellent effect that the form of the defect can be evaluated in detail.

[Brief description of drawings]

FIG. 1 is a block diagram including a partial perspective view showing a configuration of the present invention and a state of a beam in a first step.

FIG. 2 is a perspective view showing a state of a beam in a second step of the present invention.

FIG. 3 is a cross-sectional view showing a propagation path of a reflected wave from an internal defect.

FIG. 4 is an explanatory diagram showing a relationship between a received ultrasonic signal and time.

FIG. 5 is an explanatory diagram showing a positional relationship between an ultrasonic transmitter and an ultrasonic receiver according to the present invention.

FIG. 6 is a block diagram including a partial perspective view showing a conventional example.

[Explanation of symbols]

10 Inspected Plate 11 Front Surface 12 Back Surface 13 Internal Defect 21 Ultrasonic Transmitter (Line Focus Ultrasonic Transmitter) 22 Ultrasonic Receiver (Line Focus Ultrasonic Receiver) 23 Support Arm 24 Rotation Mechanism 25 Scanning Device 31 Electric pulse generator 32 Receive amplifier 33 Gate circuit 34 Peak value detection circuit 35 Controller 36 Display 37 Memory device 40 Transmit beam (transmit line focus beam) 50 Receive beam (receive line focus beam) 51 Direct transmission wave 52 From internal defect Reflected wave of the light 53 Reflected wave from the internal defect 54 Secondary transmitted wave

Claims (2)

[Claims] 1. A line focus type ultrasonic wave transmitter and a line focus type ultrasonic wave receiver are arranged so as to face each other with a plate to be inspected immersed in a liquid sandwiched therebetween, and the ultrasonic wave transmitter is also used. An ultrasonic beam that is linearly focused into the plate to be inspected substantially perpendicularly to the plate to be inspected, and the transmitted wave of the ultrasonic beam and the reflected wave from the internal defect caused by the ultrasonic beam are received by the ultrasonic receiver. In the method of detecting an internal defect of the plate to be inspected based on the received signal, the ultrasonic beam linearly focused from the ultrasonic transmitter and the ultrasonic receiver linearly. The first step of detecting flaws by making the focused reception beam overlap with each other, and the linearly focused ultrasonic beam from the ultrasonic transmitter and the linearly focused reception beam of the ultrasonic receiver are orthogonal to each other. Second step to detect flaws by doing , C scan ultrasonic testing method characterized by comprising the. 2. A line focus type ultrasonic transmitter for transmitting ultrasonic waves substantially perpendicularly to the surface of the plate to be inspected, and an ultrasonic wave which is arranged at a position facing the ultrasonic transmitter with the plate to be inspected therebetween. Line focus type ultrasonic receiver for receiving the reflected wave from the internal defect caused by the ultrasonic wave, and the ultrasonic beam linearly focused from the ultrasonic transmitter and the ultrasonic wave. A rotation mechanism that rotates the ultrasonic transmitter and / or the ultrasonic receiver from a position where the linearly focused reception beam of the receiver overlaps to a position where the reception beam is orthogonal to the ultrasonic transmitter, and the ultrasonic transmitter and the ultrasonic wave. A support arm for supporting the receiver with the plate to be inspected, a scanning device for scanning the support arm, and an electric pulse for generating an electric pulse for transmitting a pulsed ultrasonic beam from the ultrasonic transmitter. A generator, A C-scan ultrasonic flaw detector, comprising: a reception amplifier that amplifies a reception signal from the ultrasonic receiver, and a gate unit that extracts a reflected wave from an internal defect from the amplified signal.