JP2970198B2 - Surface defect detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting a surface defect of a steel material by using ultrasonic waves.

[0002]

2. Description of the Related Art As a surface defect detection method for inspecting a surface defect of a steel material, a method of detecting a surface defect using ultrasonic waves is known. This detection method is disclosed in
There is a method as disclosed in JP-A-61-266907. In this method, ultrasonic waves are perpendicularly incident on the surface of a steel material to be inspected, the reflected echo is detected, and a surface defect of the steel material is detected based on the level of the detected reflected echo. If there is a surface defect (irregularity) in the steel material, the level of the reflected echo will be lower than the level of the reflected echo in a healthy part of the smooth surface due to the scattering of the ultrasonic wave incident on that part. Utilizing such a technique, a portion of a steel material from which a reflection echo of a level lower than a predetermined level is obtained is detected as a surface defect.

[0003]

However, in the conventional surface defect detection method as described above, the relationship between the minimum surface defect to be detected and the frequency of the ultrasonic wave is not established, and the minimum surface defect is changed. In such a case, there is a problem that it is difficult to respond quickly while maintaining a predetermined detection accuracy.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a surface defect detection method capable of selecting an appropriate threshold value and ultrasonic frequency in accordance with the detection accuracy of the minimum surface defect to be detected. The purpose is to provide.

[0005]

According to the present invention, there is provided a method for detecting a surface defect, comprising the steps of: focusing an ultrasonic wave output from a probe on a surface of a material to be inspected and vertically entering the surface; In the surface defect detection method for receiving and detecting a surface defect of the inspection target material based on the level of the received reflection echo, a predetermined ratio to a level of the reflection echo from the surface of the sound portion of the inspection target material is determined by the surface defect detection method. A threshold value for determining whether or not the reflected echo is from the surface, the level of the reflected echo from the smallest surface defect to be detected is lower than the threshold value, and the reflection from the surface of the sound portion of the inspection material is The ultrasonic frequency is selected based on the reduction rate of the level of the reflected echo from the minimum surface defect with respect to the level of the echo, and an ultrasonic wave composed of a sine wave having a frequency higher than the determined frequency is output from the probe. The light is focused so as to be substantially equal to the width of the minimum surface defect to be detected and made incident on the surface of the material to be inspected, and the level of the reflected echo is compared with the threshold value. Is characterized by detecting a surface defect.

[0006]

When the ultrasonic wave is focused on the surface of the material to be inspected and is incident vertically, the level of the reflected echo decreases as the position of the surface defect moves away (or approaches) from the ultrasonic wave incident point. For this reason, a threshold value for clearly detecting surface defects is set for the level of the reflected echo,
By comparing this threshold with the level of the reflected echo,
It is possible to determine that a reflected echo having a level lower than the threshold value is an echo representing a surface defect. In addition, the level change of the reflected echo accompanying the change in the depth (height) of the surface defect has a characteristic that the higher the frequency of the ultrasonic wave, the larger the change. The higher the frequency of the ultrasonic wave, the higher the level of the reflected echo. , The detection accuracy of surface defects can be increased. Therefore, a higher frequency may be selected based on each of the above characteristics. Further, since the ultrasonic wave emitted from the probe is an ultrasonic wave composed of a sine wave, the frequency distribution of the ultrasonic wave becomes narrower, so that the frequency of the ultrasonic wave can be easily changed.

[0007]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a block diagram showing a configuration of a surface defect detection device used for implementing a surface defect detection method according to the present invention.

In the drawing, reference numeral 1 denotes a steel pipe as a material to be inspected. A probe 2 for transmitting and receiving ultrasonic waves is provided on a surface of the steel pipe 1 with water 8 interposed between the probe 2 and the surface of the steel pipe 1. Is set perpendicular to. The output of the oscillator 3 that excites the probe 2 is input to the probe 2, and the probe 2 generates a sine wave (sin wave) having a predetermined frequency determined as described later by the excitation.
The ultrasonic wave composed of the wave (wave) is vertically incident on the surface of the steel pipe 1. The ultrasonic wave generated from the probe 2 by the excitation of the oscillator 3 is an ultrasonic wave composed of a sine wave having a predetermined frequency as described above, and the frequency is arbitrarily changed by adjusting the excitation frequency of the oscillator 3. It is possible to do. The probe 2 is provided with an acoustic lens 20. The acoustic lens 20 focuses the ultrasonic wave on the surface of the sound portion of the steel tube 1 at the focal position, and focuses the ultrasonic wave at the focal position to the minimum beam diameter. And enters the surface of the steel pipe 1.

[0009] The reflected echo generated by the reflection of the ultrasonic wave on the surface of the steel pipe 1 is received by the probe 2, and the flaw detection signal is input to the amplifier 4. Amplifier 4
Is amplified and output to the detector 5.

The detector 5 shapes the flaw detection signal into a predetermined signal, and outputs the shaped flaw detection signal to the gate circuit 6. In addition to the flaw detection signal, the gate circuit 6 is provided with threshold value information regarding the level of the flaw detection signal for determining a surface defect. The gate circuit 6 performs preliminary flaw detection.
A gate is set at a position on the time axis at which an artificially provided surface defect can be detected, and a signal whose signal level is equal to or less than the threshold value among the reflected echoes is recognized as a defect signal representing the surface defect. Then, an alarm is output. Reference numeral 7 denotes a display for displaying the flaw detection signal input to the gate circuit 6 and the gate position in an image.

Next, the basic principle of a surface defect detection method implemented by the operation of the above-described surface defect detection device will be described. FIG. 2 is a graph showing the correlation between the distance from the probe 2 to the ultrasonic reflection position, the level of the reflected echo, and the frequency of the ultrasonic wave in the above-described apparatus. (The distance from the probe 2 to the ultrasonic wave reflection position (the acoustic lens 20 for focusing the ultrasonic wave is initially set so that the focal point coincides with the surface of the healthy part of the steel material, so the focal point position is centered) + On the farther side,
The near side is indicated by-. ). Also,
In the graph, the ultrasonic frequency is indicated by a solid line when the frequency is low, and is indicated by a broken line when the frequency is high.

As is apparent from the graph of FIG. 2, the level of the reflected echo from the focal position is the largest, and has a characteristic of decreasing in a curve as the distance becomes longer (or shorter). Also, it can be seen that the rate of decrease in the level of the reflected echo, which occurs as the distance becomes longer (or shorter), increases as the frequency of the ultrasonic wave increases.

For this reason, utilizing the characteristic that the level of the reflected echo decreases with the distance from the focal point, the reflected echo at the focal point position shown in FIG. Since the echo is reflected at a position distant (or close) by a predetermined distance from the focal position (the surface of the healthy part of the steel pipe 1), the reflected echo can be regarded as an echo reflected by a surface defect.

Specifically, among the surface defects to be detected in the steel pipe 1 to be inspected, the level of the reflected echo from the surface defect having the smallest height or depth (hereinafter referred to as the minimum surface defect) is: A predetermined threshold value S (as a value that can be clearly distinguished from the level of the reflected echo from the focal position on the surface of the healthy part (eg, 70% of the level of the reflected echo from the focal position on the surface of the healthy part)) The following frequency, for example, indicated by a dashed line in FIG. 2 is obtained in advance, and the probe 2 generates an ultrasonic wave having this frequency or a frequency higher than this frequency.

According to this configuration, the level of a reflected echo from a surface defect whose height or depth is greater than the height or depth of the minimum surface defect is smaller than the threshold value S. When a reflected echo equal to or less than the threshold value S is obtained, the reflected echo can be regarded as an echo reflected by a surface defect. If the probe 2 generates a frequency higher than the frequency obtained as described above, a surface defect having a smaller height or depth can be detected.

The ultrasonic waves are focused on the surface of the steel pipe 1 to a predetermined diameter by the acoustic lens 20, and the beam diameter of the ultrasonic waves is changed according to the width of the surface defect to be detected. However, it is necessary to make the diameter at the top and bottom of the surface defect having the minimum width substantially equal to the width of the defect.

This is for the following reason. FIG. 3 is a schematic diagram showing an incident state of ultrasonic waves generated from the probe 2 on the surface of the steel pipe 1. Steel pipe 1 as described above
When the ultrasonic wave U reflected on the surface of the steel tube 1 is reflected on a sound portion of the steel pipe 1, most of the energy is specularly reflected, so that a high-level reflected echo can be obtained. The beam diameter of the ultrasonic wave U is equal to the width W of the surface defect A.
When it becomes larger, the number of reflected echoes reflected at a sound portion around the surface defect A increases, and the number of reflected echoes from the surface defect A decreases, so that the surface defect A cannot be detected. It must be incident on defect A. To realize this, the beam diameter R of the ultrasonic wave U is set such that the beam diameter R is smaller at the top or bottom of the surface defect A whose width is minimum among the surface defects to be detected. Since it is necessary not to spread, the beam diameter R of the ultrasonic wave U is
It is necessary that the diameter at the top or bottom is substantially equal to the width W of the surface defect A.

On the basis of the principle as described above, the above-described surface defect detection device detects a surface defect of the steel pipe 1. That is, the oscillator 3 excites the probe 2 to generate an ultrasonic wave having a sine wave having the frequency determined as described above, and thereby the ultrasonic wave having the sine wave is incident on the surface of the steel pipe 1 from the probe 2. Then, the reflected echo is received by the probe 2, and a flaw detection signal representing the received reflected echo is given to the gate circuit 6 via the amplifier 4 and the detector 5, and the gate circuit 6
In the above, when a flaw detection signal of a reflected echo that is equal to or smaller than the threshold value S as described above is determined, an alarm indicating that a surface defect exists is output.

In the surface defect detection method as described above, the steel pipe 1
If there is a surface defect, the level of the reflected echo at the top or bottom of the surface defect due to the ultrasonic waves focused on the surface of the steel tube 1 by the acoustic lens 20 is higher than the level of the reflected echo at the surface of the sound portion of the steel tube 1 Since the surface defect is detected by utilizing the characteristic of lowering the surface defect, the surface defect can be detected irrespective of its shape.

[0020]

As described above in detail, in the surface defect detection method according to the present invention, the threshold value used as a criterion for judging whether or not the defect is a reflection echo, and the defect portion relative to the level of the reflection echo in the sound portion are determined. By selecting the frequency of the ultrasonic wave in consideration of the rate of decrease in the level of the reflected echo, the minimum surface defect can be appropriately set, and the detection accuracy of the surface defect is improved. Play.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a surface defect detection device used for implementing a surface defect detection method according to the present invention.

FIG. 2 is a graph showing a characteristic of a correlation between a distance from a probe to an ultrasonic reflection position, a level of a reflected echo, and an ultrasonic frequency in the surface defect detection device.

FIG. 3 is a schematic diagram showing an incident state of ultrasonic waves generated from a probe on a surface of a steel pipe.

[Explanation of symbols]

 Reference Signs List 1 steel pipe 2 probe 6 gate circuit 20 acoustic lens

Claims (1)

(57) [Claims] An ultrasonic wave output from a probe is focused on a surface of a material to be inspected and vertically incident thereon, a reflected echo on the surface is received, and the ultrasonic wave is received based on a level of the received reflected echo. In a surface defect detection method for detecting a surface defect of an inspection material, a predetermined ratio to a level of a reflection echo from a surface of a healthy part of the inspection material is set as a threshold value for determining whether or not the reflection echo is from a surface defect. The level of the reflected echo from the minimum surface defect with respect to the level of the reflected echo from the surface of the sound portion of the inspection material is set such that the level of the reflected echo from the minimum surface defect to be detected falls below the threshold value. The frequency of the ultrasonic wave is selected based on the drop rate, and the width of the minimum surface defect to be detected by outputting the ultrasonic wave composed of a sine wave having a frequency higher than the determined frequency from the probe is detected. A surface defect of the material to be inspected which is detected by comparing the level of the reflected echo with the threshold value and detecting the surface defect based on the comparison result. Detection method.