Ultrasonic flaw detection method and ultrasonic flaw detection device

Abstract

To provide an ultrasonic flaw detection method and others, capable of accurately and efficiently calibrating a defect detection threshold value or flaw detection sensitivity.SOLUTION: The present invention prepares a plurality of calibration pipes P' having different external diameters, including as a first artificial defect AF1 a circumferential direction notch formed on an outer surface or an inner surface, and as a second artificial defect AF2 simulating a subepidermal defect F, a bottom surface of a cylindrical hole H having an open end surface and extending in an axial direction, uses an ultrasonic flaw probe 1 arranged so as to face an outer surface of each calibration pipe to calculate flaw detection sensitivity for the first artificial defect and the second artificial defect, and calculates a relation between parameters Ps obtained by dividing an area S of the second artificial defect by the product of a depth D of the first artificial defect and an effective beam width W of the ultrasonic flaw probe 1 and a difference Sd in flaw detection sensitivity between the first artificial defect and the second artificial defect. A difference Sd'in flaw detection sensitivity between the first artificial defect and the subepidermal defect is estimated on the basis of the area Sa of the subepidermal defect to be detected and the above relation, to correct a defect detection threshold value and others calibrated using the first artificial defect.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an ultrasonic inspection method and an ultrasonic inspection device for detecting subsurface defects that exist under the surface of the outer surface of a pipe such as an electric resistance welded steel pipe and extend in the circumferential direction of the pipe. In particular, the present invention relates to an ultrasonic inspection method and an ultrasonic inspection device that can accurately and efficiently calibrate a defect detection threshold value or inspection sensitivity. [Background technology]

[0002] As is well known, electric resistance welded steel pipes (also called electric resistance welded steel pipes, ERW steel pipes) are manufactured in a pipe-making line by forming a thin plate unwound from a coil into a tubular shape with rolls and then electrically resistance welding the ends of the tubular thin plate together. This electric resistance welding is a method in which an induction coil to which high-frequency power is applied is used to generate eddy currents at the ends of a thin plate, and the ends of the thin plate heated by this eddy current (induction heating) are pressure-welded with rolls.

[0003] Electric resistance welded steel pipes, particularly those made of high strength materials, may develop a defect called a delayed crack that extends in the circumferential direction of the steel pipe several days after welding in order to release the residual stress in the weld caused by the welding. This delayed crack is highly likely to remain below the surface of the outer surface (for example, about 1 mm from the outer surface of the steel pipe) without opening on the inner or outer surface of the steel pipe. If a subsurface defect extending in the circumferential direction, such as a delayed crack, remains, for example, when a welded steel pipe is induction heated and bent, the flow of eddy currents may concentrate around the subsurface defect, causing local overheating, which may lead to problems such as the breakage of the steel pipe. For this reason, it is important to detect subsurface defects extending in the circumferential direction and perform quality assurance of steel pipes.

[0004] Ultrasonic inspection methods are known as a method for non-destructively detecting defects present inside (inside the wall thickness) of steel pipes, such as subsurface defects. In particular, to detect defects extending in the circumferential direction of a steel pipe, axial angle bevel inspection is used among so-called ultrasonic angle inspections. Axial angle bevel inspection is a method for detecting defects by transmitting and receiving ultrasonic waves toward the end face of the steel pipe along the axial direction of the steel pipe using an ultrasonic probe arranged opposite to the outer surface of the steel pipe. Specifically, axial angle bevel inspection is a method in which ultrasonic waves transmitted from an ultrasonic probe are made to enter the steel pipe at a predetermined angle (oblique incidence) with respect to the normal direction of the outer surface of the steel pipe, and when the incident ultrasonic waves propagate toward the end face of the steel pipe along the axial direction of the steel pipe, if a defect exists, the ultrasonic waves are reflected, and the reflected ultrasonic waves (echoes) are received by the ultrasonic probe to detect defects.

[0005] Generally, a circumferential notch formed on the outer or inner surface of a steel pipe is used for calibration of axial angle beam testing (calibration of defect detection threshold and testing sensitivity). A circumferential notch formed on the outer surface (external circumferential notch) is a notch-shaped artificial defect that opens onto the outer surface of the steel pipe and extends in the circumferential direction of the steel pipe, and has a depth that is a certain percentage of the wall thickness, such as 10% of the wall thickness of the steel pipe. Similarly, a circumferential notch formed on the inner surface (internal circumferential notch) is a notch-shaped artificial defect that opens onto the inner surface of the steel pipe and extends in the circumferential direction of the steel pipe, and has a depth that is a certain percentage of the wall thickness, such as 10% of the wall thickness of the steel pipe. Axial angle beam testing is calibrated by adjusting the testing sensitivity (amplification of the echo signal) so that the strength of the echo signal obtained by receiving the echo reflected by the circumferential notch is a predetermined magnitude. Axial angle beam testing is also calibrated by adjusting the defect detection threshold so that the echo signal obtained by receiving the echo reflected by the circumferential notch can be detected.

[0006] However, when detecting a subsurface defect extending in the circumferential direction by axial angle beam testing, in order to improve the calibration accuracy, it is desirable to use an artificial defect that does not open on the outer or inner surface so as to simulate a subsurface defect, rather than an artificial defect that opens on the outer or inner surface such as a circumferential notch. As such an artificial defect, it is possible to use the bottom surface of a cylindrical hole that opens on the end face of the steel pipe and extends in the axial direction.

[0007] If the bottom surface of the cylindrical hole as described above is used as an artificial defect to calibrate axial angle beam inspection, it is expected that the calibration accuracy will be improved, and ultimately the detection accuracy of subsurface defects will be improved. However, in order to distinguish between the echo reflected at the bottom of the cylindrical hole used for calibration and the echo reflected at the end of the steel pipe, which becomes noise, the cylindrical hole extending in the axial direction of the steel pipe needs to be at least a certain depth (for example, at least three times the thickness of the steel pipe). It is difficult to precisely machine such a cylindrical hole at a position about 1 mm from the outer surface of the steel pipe. In addition, since the position of the bottom of the cylindrical hole cannot be visually observed from the outer surface of the steel pipe, it is also difficult to align the ultrasonic probe with the bottom of the cylindrical hole during calibration. Therefore, although it is possible to calibrate axial angle beam testing using the bottom surface of a cylindrical hole as an artificial defect, this is extremely time-consuming, and it is extremely inefficient to routinely use such an artificial defect for calibration.

[0008] For example, Patent Documents 1 to 3 propose ultrasonic flaw detection methods for detecting subsurface defects using ultrasonic waves, but do not solve the above problems.

[0009] In the above explanation, an electric welded steel pipe has been used as an example, but the present invention is not limited to this. This problem is common to ultrasonic flaw detection methods that use an ultrasonic probe arranged facing the outer surface of the pipe to transmit and receive ultrasonic waves toward the end face of the pipe along the axial direction of the pipe, thereby detecting subsurface defects that exist below the surface of the outer surface of the pipe and extend in the circumferential direction of the pipe. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Special Publication No. 64-11144 [Patent Document 2] Japanese Unexamined Patent Publication No. 58-144742 [Patent Document 3] Japanese Patent Application Publication No. 9-15215 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made to solve the problems of the conventional technology as described above, and has an object to provide an ultrasonic flaw detection method and ultrasonic flaw detection device that detect subsurface defects that exist beneath the surface of the outer surface of a pipe, such as an electric-resistance steel pipe, and extend in the circumferential direction of the pipe, and that are capable of accurately and efficiently calibrating the defect detection threshold value or flaw detection sensitivity. [Means for solving the problem]

[0012] In order to solve the above problem, the present inventors conducted intensive research. Specifically, a plurality of calibration tubes with different outer diameters were prepared, in which a circumferential notch was formed on the outer or inner surface as the first artificial defect, and a cylindrical hole with an opening at the end and an axially extending bottom surface was formed as the second artificial defect simulating a subsurface defect, and the inspection sensitivity of the first artificial defect and the inspection sensitivity of the second artificial defect were calculated by subjecting each calibration tube to axial oblique inspection. As a result of intensive research into the relationship between the inspection sensitivity of the first artificial defect and the inspection sensitivity of the second artificial defect, it was found that a good correlation was obtained between a parameter Ps (i.e., Ps=S / (D·W)) obtained by dividing the area S of the second artificial defect (i.e., the area of ​​the bottom surface of the cylindrical hole) by the product of the depth D of the first artificial defect and the effective beam width W of the ultrasonic probe, and a difference Sd between the inspection sensitivity of the first artificial defect and the inspection sensitivity of the second artificial defect.

[0013] Therefore, once the area Sa of the subcutaneous defect to be detected is determined, the difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subcutaneous defect to be detected can be estimated by using this area Sa and the relationship between the above-mentioned parameter Ps and the flaw detection sensitivity difference Sd. Specifically, the difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subcutaneous defect to be detected can be calculated (estimated) by using the parameter Ps' calculated by substituting the area Sa of the subcutaneous defect to be detected instead of the area S of the second artificial defect that defines the parameter Ps, and the above-mentioned relationship. By using the above estimated difference Sd', it is possible to correct the defect detection threshold or flaw detection sensitivity calibrated using the first artificial defect. In other words, once the relationship between the parameter Ps and the flaw detection sensitivity difference Sd is calculated, there is no need to calibrate using the second artificial defect, which is difficult to process and align, every time calibration is performed, and it is sufficient to calibrate using the first artificial defect, which is commonly used, so that calibration can be performed efficiently. In addition, not only are the defect detection threshold and flaw detection sensitivity calibrated using the first artificial defect as in general calibration, but the calibrated defect detection threshold or flaw detection sensitivity is corrected using the estimated difference Sd', so that calibration can be performed with high accuracy.

[0014] The present invention has been completed based on the above-mentioned findings of the present inventors. That is, in order to solve the above-mentioned problems, the present invention provides an ultrasonic inspection method for detecting subsurface defects that exist under the surface of the outer surface of the pipe to be inspected and extend in the circumferential direction of the pipe to be inspected by transmitting and receiving ultrasonic waves toward an end face of the pipe to be inspected along the axial direction of the pipe to be inspected using an ultrasonic probe arranged opposite the outer surface of the pipe to be inspected, and is characterized in that it includes the following first to fifth steps. (1) First step: Prepare a plurality of calibration tubes having different outer diameters. The first artificial defect is a circumferential notch formed on the outer or inner surface, and the second artificial defect simulating the subsurface defect is a cylindrical hole having an opening at its end and a bottom extending in the axial direction. (2) Second step: Using the ultrasonic probe arranged opposite the outer surface of each of the plurality of calibration tubes, ultrasonic waves are transmitted and received toward the end face of each calibration tube along the axial direction of each calibration tube, thereby calculating the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect. (3) Third step: Calculate the relationship between a parameter Ps obtained by dividing the area S of the second artificial defect by the product of the depth D of the first artificial defect and the effective beam width W of the ultrasonic probe, and a difference Sd between the inspection sensitivity of the first artificial defect and the inspection sensitivity of the second artificial defect. (4) Fourth step: Based on the area Sa of the subsurface defect to be detected and the relationship calculated in the third step, a difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subsurface defect to be detected is estimated, and the defect detection threshold value or the flaw detection sensitivity calibrated using the first artificial defect is corrected using the estimated difference Sd'. (5) Fifth step: Using the ultrasonic probe, ultrasonic waves are transmitted and received toward the end face of the test pipe along the axial direction of the test pipe, and the subsurface defects are detected using the defect detection threshold value or test sensitivity corrected in the fourth step.

[0015] The "calibration tube" in the present invention is not necessarily limited to this, but it is preferable to use a tube made of the same material as the "test tube" whose quality is assured by applying the ultrasonic testing method. Furthermore, "multiple calibration tubes having different outer diameters" is not limited to cases where only the outer diameters are different, but also includes cases where the wall thicknesses are different in accordance with the different outer diameters. Furthermore, the first artificial defect and the second artificial defect do not necessarily have to be formed simultaneously in the same calibration tube, and it is also possible to use a calibration tube in which only the first artificial defect is formed and a calibration tube in which only the second artificial defect is formed separately. In addition, "transmitting and receiving ultrasonic waves toward the end face" is not limited to the ultrasonic waves reaching the end face, but means transmitting and receiving ultrasonic waves in the direction of the end face. Furthermore, the "effective beam width of an ultrasonic probe" means the length of the range in which the intensity of the echo signal is equal to or greater than a predetermined intensity (for example, -3 dB or greater when the maximum intensity is 0 dB) in the intensity profile of the echo signal obtained by performing axial oblique angle inspection of a small diameter (for example, 3 mm in diameter) through hole formed to penetrate the radial direction of the pipe while moving the ultrasonic probe and the pipe relatively in the circumferential direction of the pipe, and receiving the echo reflected from the through hole with the ultrasonic probe.

[0016] According to the present invention, in the first step, a plurality of calibration tubes are prepared in which a first artificial defect (a circumferential notch formed on the outer surface or the inner surface) and a second artificial defect (a bottom surface of a cylindrical hole that opens at an end surface and extends in the axial direction) are formed, and in the second step, each calibration tube is subjected to axial oblique angle inspection (ultrasonic waves are transmitted and received toward the end surface of each calibration tube along the axial direction of each calibration tube), thereby calculating the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect. As a result, in the third step, the relationship between the parameter Ps (Ps=S / (D·W)) obtained by dividing the area S of the second artificial defect by the product of the depth D of the first artificial defect and the effective beam width W of the ultrasonic probe, and the difference Sd between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect can be calculated. Specifically, for example, a relational expression such as that shown in the following formula (1) can be calculated, in which the flaw detection sensitivity difference Sd is approximated by a quadratic function of the parameter Ps. Sd=a·Ps 2 +b·Ps+c··(1)

[0017] Furthermore, according to the present invention, in the fourth step, a difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subcutaneous defect to be detected can be estimated based on the area Sa of the subcutaneous defect to be detected and the relationship calculated in the third step. Specifically, a parameter Ps' (Ps'=Sa / (D·W)) calculated by substituting the area Sa of the subcutaneous defect to be detected instead of the area S of the second artificial defect that defines the parameter Ps, and the relationship calculated in the third step can be used, that is, for example, the value on the left side obtained by substituting this parameter Ps' instead of Ps on the right side of the above formula (1) can be calculated (estimated) as the flaw detection sensitivity difference Sd'.

[0018] Then, in the fourth step, the estimated difference Sd' is used to correct the defect detection threshold value or the flaw detection sensitivity calibrated using the first artificial defect. For example, if the estimated difference Sd'>0, it is considered that the flaw detection sensitivity of the first artificial defect is greater than the flaw detection sensitivity of the sub-surface defect to be detected. In other words, when ultrasonic flaw detection is performed with the same flaw detection sensitivity, it is considered that the intensity of the echo signal of the first artificial defect is smaller than the intensity of the echo signal of the sub-surface defect to be detected. For this reason, when the flaw detection sensitivity calibrated using the first artificial defect is used as is, it is expected that the sub-surface defect can be detected even if the flaw detection threshold calibrated using the first artificial defect is used as is (without correction), or even if the flaw detection threshold calibrated using the first artificial defect is corrected to a larger value according to the flaw detection sensitivity difference Sd'. Also, when the flaw detection threshold calibrated using the first artificial defect is used as is, it is expected that the sub-surface defect can be detected even if the flaw detection sensitivity calibrated using the first artificial defect is used as is (without correction), or even if the flaw detection sensitivity calibrated using the first artificial defect is corrected to a smaller value according to the flaw detection sensitivity difference Sd'. Furthermore, for example, if the estimated difference Sd' = 0, it is considered that the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subsurface defect to be detected are the same. Therefore, the defect detection threshold value and the flaw detection sensitivity calibrated using the first artificial defect can be used as they are without correction. Furthermore, for example, if the estimated difference Sd'<0, it is considered that the flaw detection sensitivity of the first artificial defect is smaller than the flaw detection sensitivity of the sub-surface defect to be detected. In other words, when ultrasonic flaw detection is performed with the same flaw detection sensitivity, it is considered that the intensity of the echo signal of the first artificial defect is greater than the intensity of the echo signal of the sub-surface defect to be detected. For this reason, when the flaw detection sensitivity calibrated using the first artificial defect is used as is, if the flaw detection threshold value calibrated using the first artificial defect is also used as is, there is a risk that the sub-surface defect cannot be detected. For this reason, the flaw detection threshold value calibrated using the first artificial defect may be corrected to a smaller value according to the flaw detection sensitivity difference Sd'. This is expected to enable the detection of the sub-surface defect. Also, when the flaw detection threshold value calibrated using the first artificial defect is used as is, if the flaw detection sensitivity calibrated using the first artificial defect is also used as is, there is a risk that the sub-surface defect cannot be detected. For this reason, the flaw detection sensitivity calibrated using the first artificial defect may be corrected to a larger value according to the flaw detection sensitivity difference Sd'.

[0019] As described above, according to the present invention, in the fourth step, the difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subsurface defect to be detected is estimated, and the estimated difference Sd' is used to correct the defect detection threshold or flaw detection sensitivity calibrated using the first artificial defect. In other words, once the relationship between the parameter Ps and the flaw detection sensitivity difference Sd is calculated, it is not necessary to calibrate using the second artificial defect, which is difficult to process and align, every time calibration is performed, and it is only necessary to calibrate using the first artificial defect, which is commonly used, so that calibration can be performed efficiently. In addition, not only are the defect detection threshold and flaw detection sensitivity calibrated using the first artificial defect as in general calibration, but the calibrated defect detection threshold or flaw detection sensitivity is corrected using the estimated difference Sd', so that calibration can be performed with high accuracy. According to the present invention, in the fifth step, the test pipe is subjected to axial oblique angle testing (ultrasonic waves are transmitted and received toward the end face of the test pipe along the axial direction of the test pipe), and the defect detection threshold value or test sensitivity corrected in the fourth step is used, making it possible to detect subsurface defects accurately and efficiently.

[0020] The present invention is suitably used in the case where the pipe to be inspected is an electric resistance welded steel pipe and the subsurface defect is present in a weld of the electric resistance welded steel pipe.

[0021] Here, when performing axial angle inspection, the dead zone (range in which defects cannot be detected) at the end of a pipe is generally smaller on the end face side where ultrasonic waves are transmitted and received than on the opposite side of the end face where ultrasonic waves are transmitted and received. The dead zone on the end face side where ultrasonic waves are transmitted and received depends on the wavelength and wave number of the echo (end face echo) reflected at the end face of the pipe that becomes noise, and is usually about three times the pipe thickness (when the wall thickness is the commonly used electric resistance welded steel pipe and the inspection frequency is about 5 MHz), but the dead zone on the opposite side of the end face where ultrasonic waves are transmitted and received is determined by the range where ultrasonic waves do not propagate, and the latter is usually larger than the former.

[0022] Therefore, when the pipe to be inspected is an electric welded steel pipe and the subsurface defect is present in the weld of the electric welded steel pipe, in the fifth step, it is preferable to move the ultrasonic probe relatively along the weld of the pipe to be inspected from the center of the pipe to be inspected toward the end face that transmits and receives ultrasonic waves. According to the above-mentioned preferred method, by relatively moving the ultrasonic probe from the center of the test tube toward the end face that transmits and receives ultrasonic waves, it is possible to perform ultrasonic testing of the entire length of the test tube (the entire length excluding the dead zone) while reducing the dead zone. In addition, to ultrasonically inspect the entire length of the test pipe using the above-mentioned preferred method, for example, the ultrasonic probe is moved relatively from the center of the test pipe toward the end face for transmitting and receiving ultrasonic waves, and approximately half of the test pipe is ultrasonically inspected, and then the direction of the ultrasonic probe is reversed (the direction of the ultrasonic probe is changed so that ultrasonic waves are transmitted and received toward the end face opposite to the end face), and the ultrasonic probe is moved relatively from the center of the test pipe toward the opposite end face to ultrasonically inspect the remaining half of the test pipe. In addition, for example, two ultrasonic probes may be used, and one ultrasonic probe is moved relatively from the center of the test pipe toward one end face while transmitting and receiving ultrasonic waves toward the one end face, and the other ultrasonic probe is moved relatively from the center of the test pipe toward the other end face while transmitting and receiving ultrasonic waves toward the other end face, thereby ultrasonically inspecting the entire length of the test pipe.

[0023] Here, if the pipe to be inspected is an electric resistance welded steel pipe, and the subsurface defect is a defect such as a delayed crack that occurs in the welded part several days after welding, it cannot be detected by the ultrasonic inspection device installed in the pipe manufacturing line of the pipe to be inspected. In addition, the ultrasonic inspection device installed in the pipe manufacturing line generally aims to detect defects extending in the axial direction of the electric resistance welded steel pipe and welding defects, so circumferential angle beam inspection is used. Circumferential angle beam inspection is a method of detecting defects by transmitting and receiving ultrasonic waves along the circumferential direction of the pipe using ultrasonic probes arranged opposite to the outer surface of the pipe, so in principle it is not possible to detect subsurface defects extending in the circumferential direction of the pipe. For this reason, to detect subsurface defects that occur after pipe making, such as delayed cracks, ultrasonic testing (axial angle beam testing) is required offline, away from the pipe making line. When ultrasonic testing is performed offline, the pipe may rotate circumferentially during transportation to the offline line, so the position of the weld cannot be identified for each pipe. For this reason, when ultrasonic testing electric resistance welded steel pipes offline, a rotating probe ultrasonic testing device (an ultrasonic testing device that performs ultrasonic testing while the pipe moves relatively in the axial direction and the ultrasonic probe rotates in the circumferential direction of the pipe) is generally used, which is capable of ultrasonic testing the entire surface and entire length of the pipe. Although it is possible to perform axial angle beam testing using this rotating probe ultrasonic testing device, there is a problem that the dead zone at the end of the pipe becomes large due to disturbance of the test water (coupling medium) when the pipe is transported into and out of the rotating probe ultrasonic testing device. Furthermore, in order to keep the decrease in echo signal intensity that occurs depending on which part of the mountain-shaped sound pressure distribution of the ultrasonic probe a circumferentially extending defect passes through below a certain value and ensure the reproducibility of the echo signal intensity, it is necessary to reduce the relative movement speed in the axial direction of the pipe, which poses the problem of poor flaw detection efficiency.

[0024] Therefore, when the pipe to be inspected is an electric welded steel pipe and the subsurface defect is present in a weld of the electric welded steel pipe, it is preferable that the fifth step includes the steps of detecting the position of the weld, rotating the pipe to be inspected relatively in the circumferential direction so that the position of the detected weld is positioned opposite the ultrasonic probe, and transmitting and receiving ultrasonic waves toward the end face of the pipe to be inspected along the axial direction of the pipe to be inspected when the position of the weld is positioned opposite the ultrasonic probe. According to the above-mentioned preferred method, even in situations where it is not possible to identify the position of the weld for each pipe, as in the case of offline ultrasonic testing, the position of the weld is detected, the pipe to be tested is rotated relatively circumferentially so that the position of the detected weld is positioned opposite the ultrasonic probe, and then axial angle bevel testing is performed. This eliminates the need to perform ultrasonic testing while rotating the ultrasonic probe circumferentially of the pipe, as is the case with a rotating probe ultrasonic testing device, and has the advantage of not causing problems such as a large dead zone or poor testing efficiency. The position of the welded portion can be detected by using a known detection method, for example, in which the outer surface of the pipe is imaged by an imaging means disposed opposite the outer surface of the pipe, and the image is subjected to image processing.

[0025] In order to solve the above-mentioned problems, the present invention provides an ultrasonic inspection device that includes an ultrasonic probe disposed opposite to an outer surface of a test pipe, and detects subsurface defects that exist under the surface of the outer surface of the test pipe and extend in a circumferential direction of the test pipe by transmitting and receiving ultrasonic waves from the ultrasonic probe toward an end face of the test pipe along an axial direction of the test pipe, the ultrasonic inspection device comprising: a first step of preparing a plurality of calibration tubes having different outer diameters from each other, the calibration tubes having a circumferential notch formed on an outer surface or an inner surface as a first artificial defect, and a cylindrical hole having a bottom surface that opens at an end face and extends in an axial direction as a second artificial defect simulating the subsurface defect; a second step of calculating the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect by transmitting and receiving ultrasonic waves from the ultrasonic probe disposed opposite to the outer surface of each of the plurality of calibration tubes toward an end face of each of the calibration tubes along an axial direction of each of the calibration tubes; and the effective beam width W of the ultrasonic probe, and a relationship between the parameter Ps, which is obtained by dividing the parameter Ps by the product of the parameter Ps and the effective beam width W of the ultrasonic probe, and a difference Sd between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect; and a fourth step of estimating a difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subcutaneous defect to be detected based on the area Sa of the subcutaneous defect to be detected and the relationship calculated in the third step, and correcting the defect detection threshold or flaw detection sensitivity calibrated using the first artificial defect using the estimated difference. The ultrasonic flaw detection device is also provided as an ultrasonic flaw detection device, characterized in that the ultrasonic probe transmits and receives ultrasonic waves toward an end face of the pipe to be detected along the axial direction of the pipe to be detected, and detects the subcutaneous defect using the set corrected defect detection threshold or flaw detection sensitivity. Effect of the Invention

[0026] According to the present invention, in an ultrasonic inspection method and ultrasonic inspection device for detecting subsurface defects that exist beneath the surface of the outer surface of a pipe, such as an electric-welded steel pipe, and extend circumferentially of the pipe, it is possible to accurately and efficiently calibrate the defect detection threshold or inspection sensitivity, and thus to accurately and efficiently detect subsurface defects. [Brief description of the drawings]

[0027] [Figure 1] 1 is a diagram showing a schematic configuration of an ultrasonic flaw detection device according to an embodiment of the present invention; [Diagram 2] FIG. 4 is a diagram illustrating a dead zone at the end of a tube to be inspected. [Diagram 3] 1 is a flow chart showing a schematic procedure of an ultrasonic flaw detection method according to an embodiment of the present invention. [Figure 4] FIG. 4 is a diagram for explaining steps ST1 and ST2 shown in FIG. [Diagram 5] 4 shows an example of the flaw detection sensitivities of the first artificial defect AF1 and the second artificial defect AF2 calculated in step ST2 shown in FIG. [Figure 6] An example of the relationship between the parameter Ps calculated in step ST3 shown in FIG. 3 and the flaw detection sensitivity difference Sd is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, with reference to the attached drawings as appropriate, an ultrasonic flaw detection device according to one embodiment of the present invention and an ultrasonic flaw detection method using the same will be described, taking as an example a case in which the pipe to be detected is an electric welded steel pipe and a subsurface defect is present in the weld of the electric welded steel pipe.

[0029] FIG. 1 is a diagram showing a schematic configuration of an ultrasonic inspection device according to an embodiment of the present invention. FIG. 1(a) is a front view seen from the axial direction of the pipe P to be inspected. FIG. 1(b) is a perspective view seen from a horizontal direction substantially perpendicular to the axial direction of the pipe P to be inspected at the ultrasonic inspection position shown in FIG. 1(a). FIG. 1(c) is a perspective view explaining the movement trajectory of the ultrasonic probe at the ultrasonic inspection position shown in FIG. 1(a). In FIG. 1(b), the illustration of the transverse conveyor 5 shown in FIG. 1(a) is omitted. In addition, in FIG. 1(c), the illustration of the flaw detector 2 and the transverse conveyor 5 shown in FIG. 1(a) is omitted. Note that the dimensions, scales, and shapes of the components shown in FIG. 1 may differ from the actual ones. The same applies to other figures. As shown in Figure 1, the ultrasonic flaw detection device 100 of this embodiment is equipped with an ultrasonic probe 1, a flaw detector 2, an imaging means 3, an image processing means 4, a cross-feed conveyor 5, and a rotating roller 6, and is an apparatus for detecting subsurface defects F that exist under the surface of the outer surface of the welded portion PW of the pipe P to be detected and extend in the circumferential direction of the pipe P to be detected.

[0030] 1(a), the test pipe P is carried to a carry-in position on a transverse conveyor 5 by a transport means such as a skid, and is then fed laterally to a weld detection position by the transverse conveyor 5. That is, the test pipe P is transported in a horizontal direction perpendicular to the axial direction of the test pipe P to the weld detection position.

[0031] At the weld detection position, an imaging means 3 such as a CCD camera or a CMOS camera is arranged above and a pair of rotating rollers 6 is arranged below so as to face the outer surface of the pipe P to be detected. The rotating rollers 6 are configured to be able to rise and fall, and wait at a lowered position where they do not interfere with the pipe P to be detected until the pipe P to be detected reaches the weld detection position, and rise until they come into contact with the lower surface of the pipe P to be detected after the pipe P to be detected reaches the weld detection position. The rotation of the raised rotating rollers 6 causes the pipe P to rotate in the circumferential direction. The image processing means 4 is connected to the imaging means 3, and detects the position of the weld PW of the pipe P to be detected by performing known image processing on the captured image of the outer surface (upper surface) of the pipe P to be detected acquired by the imaging means 3. While rotating the rotating roller 6 (while rotating the test pipe P), the imaging means 3 sequentially acquires captured images, and the image processing means 4 sequentially detects the position of the welded portion PW, thereby making it possible to detect the time when the welded portion PW reaches the top of the test pipe P (the position facing the ultrasonic probe 1). The rotating roller 6 stops rotating when the welded portion PW reaches the top of the test pipe P. This brings the welded portion PW into a state where it is located at the top of the test pipe P. In the example shown in FIG. 1(a), when the welded portion PW is in the position shown by the dashed line, the rotating roller 6 is rotating (the test pipe P is rotating), but when the welded portion PW is in the position shown by the solid line, the rotating roller 6 is stopped (the test pipe P is stopped).

[0032] After the rotating rollers 6 stop, the test pipe P is fixed by a predetermined chuck (not shown) so as not to rotate in the circumferential direction when it is fed laterally to the ultrasonic testing position. The rotating rollers 6 are also lowered to a lowered position where they do not interfere with the test pipe P. Then, the test pipe P, which is fixed by the chuck, is fed laterally by the lateral feed conveyor 5 to the ultrasonic testing position.

[0033] At the ultrasonic inspection position, the ultrasonic probe 1 is disposed above so as to face the outer surface of the pipe P to be inspected. As described above, the weld PW is located at the top of the pipe P to be inspected at the weld detection position, and in this state, the pipe P to be inspected is fed laterally to the ultrasonic inspection position, so that the ultrasonic probe 1 faces the weld PW of the pipe P to be inspected. In this embodiment, two ultrasonic probes 1a and 1b are arranged as the ultrasonic probe 1 along the axial direction of the pipe P to be detected. Each ultrasonic probe 1 is attached to a traveling cart (not shown). Each traveling cart has a function of raising and lowering each ultrasonic probe 1 and moving along the axial direction of the pipe P to be detected. As each traveling cart moves along the axial direction of the pipe P to be detected, each ultrasonic probe 1 moves along the axial direction of the pipe P to be detected. 1(b), the ultrasonic probe 1 is configured to transmit and receive ultrasonic waves U toward an end face of the test pipe P along the axial direction of the test pipe P (along the welded portion PW) so that axial angle inspection can be performed. In this embodiment, one ultrasonic probe 1a transmits and receives ultrasonic waves U toward one end face PE1 of the test pipe P, and the other ultrasonic probe 1b transmits and receives ultrasonic waves U toward the other end face PE2 of the test pipe P.

[0034] As shown by the solid arrow in FIG. 1(c), the traveling cart (not shown) to which the ultrasonic probe 1a is attached lowers the ultrasonic probe 1a until the ultrasonic probe 1a comes into contact with the pipe P to be inspected (until it comes into contact with the welded portion PW) through the contact medium (not shown) at the center of the pipe P to be inspected. After that, the ultrasonic probe 1a starts transmitting and receiving ultrasonic waves U, and the traveling cart moves the ultrasonic probe 1a toward the end face PE1 along the welded portion PW of the pipe P to be inspected. By moving the ultrasonic probe 1a a predetermined distance (a distance corresponding to the dead zone of the end of the pipe P to be inspected) from the end face PE1, approximately half of the end face PE1 side of the pipe P to be inspected is ultrasonically inspected. After the ultrasonic inspection is completed, the traveling cart raises the ultrasonic probe 1a until it is separated from the pipe P to be inspected, and then moves the ultrasonic probe 1a toward the end face PE2 to its original position (the center of the pipe P to be inspected). Similarly, as shown by the moving trajectory in FIG. 1(c) with the dashed arrow, the traveling cart (not shown) on which the ultrasonic probe 1b is attached lowers the ultrasonic probe 1b until the ultrasonic probe 1b comes into contact with the pipe P to be inspected (until it comes into contact with the welded portion PW) through the contact medium (not shown) at the center of the pipe P to be inspected. After that, the ultrasonic probe 1b starts transmitting and receiving ultrasonic waves U, and the traveling cart moves the ultrasonic probe 1b along the welded portion PW of the pipe P to be inspected toward the end surface PE2. By moving the ultrasonic probe 1b to a predetermined distance (distance corresponding to the dead zone of the end of the pipe P to be inspected) from the end surface PE2, approximately half of the end surface PE2 side of the pipe P to be inspected is ultrasonically inspected. After the ultrasonic inspection is completed, the traveling cart raises the ultrasonic probe 1b until it is separated from the pipe P to be inspected, and then moves the ultrasonic probe 1b toward the end face PE1 to its original position (the center of the pipe P to be inspected). 1(c), each traveling carriage moves each ultrasonic probe 1 so that the movement trajectory of ultrasonic probe 1a and the movement trajectory of ultrasonic probe 1b overlap at the center of the test pipe P, so that no uninspected area is generated at the center of the test pipe P. The movement timing of each traveling carriage is controlled so that each traveling carriage and each ultrasonic probe 1 do not interfere with each other at the portion where their movement trajectories overlap. As described above, according to the ultrasonic inspection device 100 of this embodiment, the entire length of the pipe P to be inspected can be ultrasonically inspected.

[0035] FIG. 2 is a diagram for explaining the dead zone at the end of the test tube P. As shown in FIG. In the case of performing axial angle inspection, the dead zone (range in which defects cannot be detected) at the end of the test pipe P is generally smaller on the end face side where ultrasonic waves U are transmitted and received than on the opposite side of the end face where ultrasonic waves U are transmitted and received. In the case of the ultrasonic probe 1a shown in FIG. 2, ultrasonic waves U are transmitted and received toward the end face PE1 of the test pipe P, so the dead zone on the end face PE1 side shown in FIG. 2(a) is smaller than the dead zone on the end face PE2 side shown in FIG. 2(b). The dead zone on the end face PE1 side depends on the wavelength and wave number of the echo (end face echo) reflected by the end face PE1 that becomes noise, and is usually about three times the wall thickness of the test pipe P (when the wall thickness is the commonly used electric resistance welded steel pipe and the inspection frequency is about 5 MHz), but the dead zone on the end face PE2 side is determined by the range where ultrasonic waves U do not propagate, and the latter is usually larger than the former. Therefore, if ultrasonic testing is performed while moving the ultrasonic probe 1a in the direction of the arrow X1 in FIG. 2 over the entire length of the test pipe P, the dead zone on the end face P2 side becomes large, which is undesirable. For this reason, in this embodiment, as described above, the ultrasonic probe 1 is moved from the center of the test pipe P toward the end face that transmits and receives the ultrasonic wave U. Specifically, the ultrasonic probe 1a is moved from the center of the test pipe P toward the end face PE1 that transmits and receives the ultrasonic wave U (in the direction of the arrow X1 shown in FIG. 1(b)), and the ultrasonic probe 1b is moved from the center of the test pipe P toward the end face PE2 that transmits and receives the ultrasonic wave U (in the direction of the arrow X2 shown in FIG. 1(b)). This makes it possible to perform ultrasonic inspection of the entire length of the test pipe P (the entire length excluding the dead zone) while reducing the dead zone.

[0036] In this embodiment, the ultrasonic probe 1 is illustrated as being provided with two ultrasonic probes 1a and 1b, but the present invention is not limited thereto, and it is also possible to adopt a configuration in which a single ultrasonic probe 1 is provided. When a single ultrasonic probe 1 is provided, the ultrasonic probe 1 is moved from the center of the test pipe P toward the end face (e.g., end face PE1) that transmits and receives ultrasonic waves U, and approximately half of the test pipe P is ultrasonically inspected, and then the direction of the ultrasonic probe 1 is reversed (the direction of the ultrasonic probe 1 is changed so that ultrasonic waves U are transmitted and received toward the end face (e.g., end face PE2) opposite to the end face), and the ultrasonic probe 1 is moved from the center of the test pipe P toward the end face on the opposite side, and the remaining half of the test pipe P is ultrasonically inspected. This allows ultrasonic inspection of the entire length of the test pipe P while reducing the dead zone. In addition, in the present embodiment, a single ultrasonic probe 1 is arranged in the circumferential direction of the test pipe P, but the present invention is not limited to this, and it is also possible to adopt a configuration in which multiple ultrasonic probes 1 are arranged in close proximity to the circumferential direction of the test pipe P. Since the welded portion PW of the test pipe P may not be formed in a straight line parallel to the axial direction of the test pipe P but may meander, adopting a configuration in which multiple ultrasonic probes 1 are arranged in close proximity to the circumferential direction of the test pipe P can reduce the risk of an uninspected area being generated.

[0037] As shown in FIG. 1, the ultrasonic probe 1 is connected to the flaw detector 2. The flaw detector 2 has the same configuration as a flaw detector included in a conventionally known general ultrasonic flaw detector. Specifically, the flaw detector 2 has a standard equipment configuration for controlling the transmission and reception of ultrasonic waves and detecting defects, such as a pulser (not shown) that supplies a pulse signal for transmitting an ultrasonic wave U (see FIG. 1(b)) from the ultrasonic probe 1, a receiver (not shown) that amplifies an echo signal output from the ultrasonic probe 1 that receives an echo at a set flaw detection sensitivity, a gate unit (not shown) that extracts an echo signal in a predetermined time region where a defect may exist from the echo signal amplified by the receiver, an A / D converter (not shown) that A / D converts the echo signal extracted by the gate unit, and a detection unit that compares the A / D converted echo signal with a set defect detection threshold value to detect defects.

[0038] The ultrasonic inspection device 100 is equipped with a mechanism (not shown) that distinguishes and transports the pipe to be inspected P depending on whether or not a subsurface defect F is detected, such that if a subsurface defect F is detected in the pipe to be inspected P at the ultrasonic inspection position, the pipe to be inspected P is transported and stored in a defective rack, and if no subsurface defect F is detected in the pipe to be inspected P, the pipe to be inspected P is transported and stored in a good rack.

[0039] Hereinafter, an ultrasonic flaw detection method (axial angle beam flaw detection) using the ultrasonic flaw detection device 100 having the above configuration will be described. FIG. 3 is a flow diagram showing an outline of the procedure of an ultrasonic flaw detection method according to one embodiment of the present invention. 3, the ultrasonic inspection method according to this embodiment includes steps ST1 to ST8. Step ST1 corresponds to the first step of the present invention, step ST2 corresponds to the second step of the present invention, step ST3 corresponds to the third step of the present invention, step ST6 corresponds to the fourth step of the present invention, and step ST7 corresponds to the fifth step of the present invention. Each of steps ST1 to ST8 will be described in order below.

[0040] <Step ST1> Fig. 4 is a diagram for explaining steps ST1 and ST2. Fig. 4(a) is a front view of the calibration tube P' as seen from the axial direction. Fig. 4(b) and Fig. 4(c) are cross-sectional views of the calibration tube P' as seen from a horizontal direction perpendicular to the axial direction. As shown in FIG. 4, in step ST1, a calibration tube P' is prepared in which a circumferential notch is formed on the outer surface or inner surface (outer surface in the example shown in FIG. 4) as the first artificial defect AF1, and a bottom surface of a cylindrical hole H that opens at the end surface and extends in the axial direction is formed as the second artificial defect AF2 simulating a subsurface defect F (see FIG. 1(b)). A plurality of calibration tubes P' with different outer diameters are prepared. It is preferable to use a tube made of the same material as the test tube P as the calibration tube P'. In the example shown in FIG. 4(a), the first artificial defect AF1 and the second artificial defect AF2 are simultaneously formed in the same calibration tube P', but this is not necessarily limited to this, and it is also possible to separately use a calibration tube P' in which only the first artificial defect AF1 is formed and a calibration tube P' in which only the second artificial defect AF2 is formed.

[0041] <Step ST2> In step ST2, each of the multiple calibration tubes P' is transported sequentially to an ultrasonic inspection position (see Figure 1(a)), and an ultrasonic probe 1 (either ultrasonic probe 1a or ultrasonic probe 1b) arranged opposite the outer surface of each calibration tube P' is used to perform axial angle inspection of each calibration tube P' (transmitting and receiving ultrasonic waves U toward the end face of each calibration tube P' along the axial direction of each calibration tube P'), and the inspection sensitivity of the first artificial defect AF1 and the inspection sensitivity of the second artificial defect AF2 are calculated. Specifically, as shown in Figure 4(b), the circumferential position of the calibration tube P' is adjusted so that the first artificial defect AF1 is located at the top of the calibration tube P', and then the ultrasonic probe 1 is brought into contact with the outer surface of the calibration tube P' via a contact medium, and the position of the ultrasonic probe 1 is adjusted along the axial direction of the calibration tube P' so that the intensity of the echo signal of the first artificial defect AF1 is maximized. Then, the flaw detection sensitivity (amplification of the echo signal) at which the intensity of the echo signal of the first artificial defect AF1 becomes a predetermined magnitude is calculated. In addition, after calculating the inspection sensitivity of the first artificial defect AF1 as described above, the circumferential position of the calibration tube P' is adjusted so that the second artificial defect AF2 is located at the top of the calibration tube P', as shown in Figure 4(c), and then the ultrasonic probe 1 is brought into contact with the outer surface of the calibration tube P' via a contact medium, and the position of the ultrasonic probe 1 is adjusted along the axial direction of the calibration tube P' so that the intensity of the echo signal of the second artificial defect AF2 is maximized, and the inspection sensitivity is calculated so that the intensity of the echo signal of the second artificial defect AF2 becomes a predetermined magnitude. In this embodiment, an example has been given of calculating the inspection sensitivity of the first artificial defect AF1 and the second artificial defect AF2 using an ultrasonic probe 1 placed at an ultrasonic inspection position, as in the case of ultrasonically inspecting the test pipe P. However, this is not limited to this, and it is also possible, for example, to remove the ultrasonic probe 1 from the traveling cart and manually adjust the position of this ultrasonic probe 1 relative to the calibration pipe P' to calculate the inspection sensitivity of the first artificial defect AF1 and the second artificial defect AF2.

[0042] FIG. 5 shows an example of the flaw detection sensitivity of the first artificial defect AF1 and the second artificial defect AF2 calculated in step ST2. FIG. 5(a) shows the relationship between the outer diameter of the calibration tube P' and the flaw detection sensitivity, and FIG. 5(b) shows the relationship between the outer diameter of the calibration tube P' and the flaw detection sensitivity difference Sd (the flaw detection sensitivity of the first artificial defect AF1 - the flaw detection sensitivity of the second artificial defect AF2). The results shown in FIG. 5 were obtained when using a calibration tube P' in which an outer surface circumferential notch having a depth of 10% of the wall thickness of the calibration tube P' is formed as the first artificial defect AF1, and a bottom surface of a hole with a diameter of 2 mm that opens at the end face and extends in the axial direction is formed as the second artificial defect AF2. As can be seen from the results shown in Fig. 5(b), there is no clear correlation between the outer diameter of the calibration tube P' and the flaw detection sensitivity difference Sd. Therefore, the inventors have conducted extensive research and found that a good correlation can be obtained between the parameter Ps used in step ST3 described below and the flaw detection sensitivity difference Sd.

[0043] <Step ST3> In step ST3, based on the findings of the present inventors described above, the area S of the second artificial defect AF2 (see FIG. 4(c). For example, when the second artificial defect AF2 is the bottom surface of a hole having a diameter of 2 mm, the area S is set to 3.14 mm. 2 The relationship between the parameter Ps (Ps=S / (D·W)) obtained by dividing the depth D of the first artificial defect (see FIG. 4(b)) by the product of the depth D of the first artificial defect (AF1) and the effective beam width W of the ultrasonic probe 1 and the difference Sd between the detection sensitivity of the first artificial defect AF1 and the detection sensitivity of the second artificial defect AF2 is calculated. FIG. 6 shows an example of the relationship between the parameter Ps calculated in step ST3 and the difference Sd in flaw detection sensitivity. Specifically, the data plotted with "◯" in FIG. 6 is data obtained by rearranging the difference Sd in flaw detection sensitivity shown in FIG. 5(b) by changing the horizontal axis to the parameter Ps. The effective beam width W that defines the parameter Ps shown in FIG. 6 is the length of the range in which the echo signal has an intensity of -3 dB or more when the maximum intensity of the echo signal is 0 dB in the profile of the intensity of the echo signal obtained by performing axial oblique flaw detection on a through hole having a diameter of 3 mm formed so as to penetrate the calibration tube P' in the radial direction while moving the ultrasonic probe 1 and the calibration tube P' relatively in the circumferential direction of the calibration tube P', and receiving the echo reflected by the through hole by the ultrasonic probe 1. As shown in Figure 6, a good correlation is obtained between the parameter Ps and the difference Sd in flaw detection sensitivity. By using an approximation calculation such as the least squares method, it is possible to calculate the relational equation shown in the following equation (1), which approximates the difference Sd in flaw detection sensitivity as a quadratic function of the parameter Ps, as shown by the dashed line in Figure 6. Sd=a·Ps 2 +b·Ps+c··(1) In the above formula (1), a, b, and c each represent a predetermined constant. However, the relational equation between the parameter Ps and the flaw detection sensitivity difference Sd is not necessarily limited to this, and various relational equations can be used as long as a high degree of approximation accuracy can be obtained, such as approximating the flaw detection sensitivity difference Sd with an exponential function of the parameter Ps.

[0044] <Step ST4> In step ST4, as described above, it is determined whether or not to calibrate the defect detection threshold set in the detection section of the flaw detector 2 of the ultrasonic flaw detection device 100, or the flaw detection sensitivity set in the receiver of the flaw detector 2. These calibrations do not need to be performed every time the test pipe P is ultrasonically inspected, except when the first test pipe P is ultrasonically inspected, and may be performed at a predetermined timing, for example, when the outer diameter or wall thickness of the test pipe P to be ultrasonically inspected changes. If calibration is performed (if "Yes" in step ST4), step ST5 described below is executed, and if calibration is not performed (if "No" in step ST4), step ST7 described below is executed. If calibration is not performed, the test pipe P will be ultrasonically inspected using the defect detection threshold value and inspection sensitivity set in the flaw detector 2 after having already been calibrated and corrected in step ST7 described below.

[0045] <Step ST5> In step ST5, the defect detection threshold value and the flaw detection sensitivity are calibrated using a first artificial defect AF1 that is commonly used. Specifically, for example, from among a plurality of calibration tubes P', a calibration tube P' having an outer diameter equivalent to that of the test tube P is selected, and the inspection sensitivity is adjusted so that the intensity of the echo signal of the first artificial defect AF1 formed in this calibration tube P' becomes a predetermined magnitude. Also, the defect detection threshold is adjusted so that the echo signal of the first artificial defect AF1 formed in this calibration tube P' can be detected.

[0046] <Step ST6> In step ST6, the area Sa of the subsurface defect F to be detected is determined, and the difference Sd' between the inspection sensitivity of the first artificial defect AF1 and the inspection sensitivity of the subsurface defect F to be detected is estimated based on this area Sa and the relationship expressed by equation (1) calculated in step ST3. Specifically, by substituting the area Sa of the subcutaneous defect to be detected for the area S of the second artificial defect AF2 that defines the parameter Ps, the parameter Ps’ (Ps’ = Sa / (D·W)) thus calculated is substituted for Ps on the right side of the above formula (1), and the value on the left side obtained is calculated (estimated) as the difference Sd’ from the flaw detection sensitivity. That is, based on the following formula (1)’, the difference Sd’ between the flaw detection sensitivity of the first artificial defect AF1 and the flaw detection sensitivity of the subcutaneous defect F to be detected is estimated. Sd’ = a·Ps’ 2 +b·Ps’+c ···(1)’

[0047] Then, in step ST6, using the estimated difference Sd’, the defect detection threshold or flaw detection sensitivity calibrated using the first artificial defect AF1 in step ST5 is corrected. The above-mentioned Figure 6 illustrates the case of correcting the defect detection threshold. For example, if the estimated difference Sd’ > 0, it is considered that the flaw detection sensitivity of the first artificial defect AF1 is greater than the flaw detection sensitivity of the subcutaneous defect F to be detected. In other words, when ultrasonic flaw detection is performed with the same flaw detection sensitivity, it is considered that the intensity of the echo signal of the first artificial defect AF1 is smaller than the intensity of the echo signal of the subcutaneous defect F to be detected. Therefore, when directly using the flaw detection sensitivity calibrated using the first artificial defect AF1, or when correcting the defect detection threshold Th calibrated using the first artificial defect AF1 to a larger value according to the difference Sd’ in flaw detection sensitivity, it is expected that the subcutaneous defect can be detected. In the example shown in Figure 6, when the parameter Ps satisfies 1.4 < Ps ≦ 1.6, since Sd’ > 0 is estimated, the case of directly using the defect detection threshold Th calibrated using the first artificial defect AF1 (the defect detection threshold Th1 after correction = Th) is illustrated. Although not shown in the figure, when the defect detection threshold Th calibrated using the first artificial defect AF1 is used as is, it is expected that the subsurface defect F can be detected even if the flaw detection sensitivity calibrated using the first artificial defect AF1 is used as is (corrected flaw detection sensitivity = flaw detection sensitivity calibrated using the first artificial defect AF1) or if the flaw detection sensitivity calibrated using the first artificial defect AF1 is corrected to a smaller value according to the flaw detection sensitivity difference Sd'.

[0048] Furthermore, for example, if the estimated difference Sd' = 0, it is considered that the flaw detection sensitivity of the first artificial defect F1 is the same as the flaw detection sensitivity of the subsurface defect F to be detected. Therefore, the defect detection threshold value and the flaw detection sensitivity calibrated using the first artificial defect AF1 can be used as is without correction.

[0049] Furthermore, for example, if the estimated difference Sd'<0, it is considered that the flaw detection sensitivity of the first artificial defect AF1 is smaller than that of the sub-surface defect F to be detected. In other words, when ultrasonic flaw detection is performed with the same flaw detection sensitivity, it is considered that the intensity of the echo signal of the first artificial defect AF1 is greater than that of the sub-surface defect F to be detected. For this reason, when the flaw detection sensitivity calibrated using the first artificial defect AF1 is used as is, if the defect detection threshold Th calibrated using the first artificial defect AF1 is also used as is, there is a risk that the sub-surface defect F cannot be detected. For this reason, the defect detection threshold Th calibrated using the first artificial defect AF1 may be corrected to a smaller value according to the flaw detection sensitivity difference Sd'. As a result, it is expected that the sub-surface defect F can be detected. In the example shown in FIG. 6, when the parameter Ps satisfies 1.2 < Ps ≦ 1.4, since Sd’ < 0 is estimated for most of the range, an example is illustrated where the flaw detection threshold Th calibrated using the first artificial flaw AF1 is corrected to a small value according to the difference Sd’ in flaw detection sensitivity. Specifically, an example is illustrated where the corrected flaw detection threshold Th2 is corrected to a value approximately 3 dB smaller than the flaw detection threshold Th. Also, when the parameter Ps satisfies 0.4 < Ps ≦ 1.2, since Sd’ < 0 is estimated for all of the range, an example is illustrated where the flaw detection threshold Th calibrated using the first artificial flaw AF1 is corrected to a small value according to the difference Sd’ in flaw detection sensitivity. Specifically, an example is illustrated where the corrected flaw detection threshold Th3 is corrected to a value approximately 6 dB or more smaller than the flaw detection threshold Th. Although not shown in the figure, if the flaw detection threshold Th calibrated using the first artificial flaw AF1 is used as it is, there is a possibility that the subsurface flaw F cannot be detected because the flaw detection sensitivity calibrated using the first artificial flaw AF1 is also used as it is. Therefore, the flaw detection sensitivity calibrated using the first artificial flaw AF1 may be corrected to a large value according to the difference Sd’ in flaw detection sensitivity.

[0050] As described above, by estimating the difference Sd’ between the flaw detection sensitivity of the first artificial flaw AF1 and the flaw detection sensitivity of the subsurface flaw F to be detected in step ST6 and using this estimated difference Sd’, it is possible to correct the flaw detection threshold Th or the flaw detection sensitivity calibrated using the first artificial flaw AF1. That is, once the relationship between the parameter Ps and the difference Sd in flaw detection sensitivity is calculated in step ST3, it is not necessary to calibrate using the second artificial flaw AF2, which is difficult to process and align each time, and it is only necessary to calibrate using the first artificial flaw AF1, which is generally commonly used. Therefore, calibration can be performed efficiently. Also, not only the flaw detection threshold and the flaw detection sensitivity are calibrated using the first artificial flaw AF1 in the same way as general calibration, but also the calibrated flaw detection threshold or the flaw detection sensitivity is corrected using the estimated difference Sd’, so that calibration can be performed with high accuracy. The defect detection threshold calibrated in step ST5 and corrected in step ST6 is set in the detection unit of the flaw detector 2. Similarly, the flaw detection sensitivity calibrated in step ST5 and corrected in step ST6 is set in the receiver of the flaw detector 2.

[0051] <Step ST7> In step ST7, at the aforementioned ultrasonic testing position (see FIG. 1(a)), the test pipe P is subjected to axial oblique angle testing (ultrasonic waves U are transmitted and received toward the end face of the test pipe P along the axial direction of the test pipe P) to detect subsurface defects F. At this time, by using the defect detection threshold value or test sensitivity corrected in step ST6, it is possible to detect subsurface defects F accurately and efficiently.

[0052] <Step ST8> In step ST8, it is determined whether or not there is a next pipe P to be ultrasonically inspected. If there is a next pipe P to be inspected (if "Yes" in step ST8), the above-mentioned operations are repeated from step ST4. If there is no next pipe P to be inspected (if "No" in step ST8), the ultrasonic inspection method according to this embodiment is terminated.

[0053] According to the ultrasonic inspection method of this embodiment described above, the defect detection threshold or inspection sensitivity can be calibrated accurately and efficiently, and thus, the subsurface defect F can be detected accurately and efficiently. [Explanation of symbols]

[0054] 1, 1a, 1b...Ultrasonic probe 2...Flaw detector 3. Imaging means 4. Image processing means 5. Horizontal feed conveyor 6. Rotating roller 100...Ultrasonic flaw detection equipment AF1: First artificial defect AF2: Second artificial defect F. Subepidermal defect H: Cylindrical hole P...Pipe to be tested for flaws P'...Calibration tube PE1, PE2...end face PW···Welded part

Claims

1. An ultrasonic inspection method for detecting subsurface defects that exist under the surface of the outer surface of the pipe to be inspected and extend in a circumferential direction of the pipe to be inspected by transmitting and receiving ultrasonic waves toward an end face of the pipe to be inspected along an axial direction of the pipe to be inspected using an ultrasonic probe disposed opposite the outer surface of the pipe to be inspected, a first step of preparing a plurality of calibration tubes having different outer diameters, the calibration tubes having a circumferential notch formed on an outer surface or an inner surface as a first artificial defect and a cylindrical hole having a bottom surface opening at an end surface and extending in an axial direction as a second artificial defect simulating the subsurface defect; a second step of calculating the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect by transmitting and receiving ultrasonic waves toward an end face of each of the calibration tubes along an axial direction of each of the calibration tubes using the ultrasonic probes arranged to face the outer surfaces of the respective calibration tubes; a third step of calculating a relationship between a parameter Ps obtained by dividing an area S of the second artificial defect by a product of a depth D of the first artificial defect and an effective beam width W of the ultrasonic probe, and a difference Sd between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect; a fourth step of estimating a difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subcutaneous defect to be detected based on an area Sa of the subcutaneous defect to be detected and the relationship calculated in the third step, and correcting the flaw detection threshold value or the flaw detection sensitivity calibrated using the first artificial defect using the estimated difference Sd'; and a fifth step of transmitting and receiving ultrasonic waves toward an end face of the test pipe along an axial direction of the test pipe using the ultrasonic probe, and detecting the subsurface defect using the defect detection threshold value or the test sensitivity corrected in the fourth step. The ultrasonic flaw detection method according to claim 1,

2. The test pipe is an electric resistance welded steel pipe, The subsurface defect is present in the weld of the electric resistance welded steel pipe.

2. The ultrasonic flaw detection method according to claim 1 .

3. In the fifth step, the ultrasonic probe is moved relatively along the welded portion of the test pipe from a center portion of the test pipe toward an end surface for transmitting and receiving ultrasonic waves.

3. The ultrasonic flaw detection method according to claim 2.

4. The fifth step is detecting a position of the weld; a step of relatively rotating the test pipe in a circumferential direction so that the position of the detected welded portion faces the ultrasonic probe; and transmitting ultrasonic waves to and from the end face of the test pipe along an axial direction of the test pipe while the welded portion is positioned opposite the ultrasonic probe.

4. The ultrasonic flaw detection method according to claim 2 or 3.

5. An ultrasonic inspection device comprising an ultrasonic probe disposed opposite an outer surface of a test pipe, the ultrasonic probe transmitting and receiving ultrasonic waves toward an end face of the test pipe along an axial direction of the test pipe, the ultrasonic inspection device detecting subsurface defects that exist under the surface of the outer surface of the test pipe and extend in a circumferential direction of the test pipe, a first step of preparing a plurality of calibration tubes having different outer diameters, the calibration tubes having a circumferential notch formed on an outer surface or an inner surface as a first artificial defect and a cylindrical hole having a bottom surface opening at an end surface and extending in an axial direction as a second artificial defect simulating the subsurface defect; a second step of calculating the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect by transmitting and receiving ultrasonic waves toward an end face of each of the calibration tubes along an axial direction of each of the calibration tubes using the ultrasonic probes arranged to face the outer surfaces of the respective calibration tubes; a third step of calculating a relationship between a parameter Ps obtained by dividing an area S of the second artificial defect by a product of a depth D of the first artificial defect and an effective beam width W of the ultrasonic probe, and a difference Sd between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the second artificial defect; a fourth step of estimating a difference Sd' between the flaw detection sensitivity of the first artificial defect and the flaw detection sensitivity of the subcutaneous defect to be detected based on an area Sa of the subcutaneous defect to be detected and the relationship calculated in the third step, and correcting the flaw detection threshold value or the flaw detection sensitivity calibrated using the first artificial defect using the estimated difference, The ultrasonic inspection device is set with a defect detection threshold value or inspection sensitivity after the correction in the fourth step, The ultrasonic inspection device detects the subsurface defect by using the ultrasonic probe to transmit and receive ultrasonic waves toward the end face of the test pipe along the axial direction of the test pipe, and using the set corrected defect detection threshold value or test sensitivity. An ultrasonic flaw detection device characterized by:

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