Eddy Current Testing Method for Electric Resistance Welded Steel Tubes

Abstract

To provide eddy current flaw detection equipment or the like which can detect a fine defect in a tube manufacturing process, in a manufacturing process of an electric resistance welded tube having the tube making process and a secondary working process.SOLUTION: Eddy current flaw detection equipment 100 related to the present invention comprises a first eddy current flaw detection device 10 detecting an eddy current flaw of an electric resistance welded tube P in a tube making process, and a second eddy current flaw detection device 20 detecting an eddy current flaw of the electric resistance welded tube after a cold stretching process. A first defect volume ratio which is decided by a dimension of a defect for use in a calibration of flaw detection sensitivity or the like in the first eddy current flaw detection device, and a dimension of a detection coil 2 of the first eddy current flaw detection device, and a second defect volume ratio which is decided by a dimension of a defect being a detection object in the second eddy current flaw detection device and a dimension of a detection coil of the second eddy current flaw detection device are calculated in advance. Corrected flaw detection sensitivity which is obtained by correcting the calibrated flaw detection sensitivity on the basis of a ratio of the second defect volume ratio with respect to the calculated first defect volume ratio is set at the first eddy current flaw detection device, and the first eddy current flaw detection device detects the eddy current flaw of the electric resistance welded tube by using the corrected flaw detection sensitivity or the like.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an eddy current inspection method and eddy current inspection equipment for detecting defects present in welds, etc., of electric resistance welded steel pipes. In particular, the present invention relates to an eddy current inspection method and eddy current inspection equipment capable of detecting minute defects in electric resistance welded steel pipes during a pipe making process in a manufacturing process of electric resistance welded steel pipes, which includes a pipe making process for manufacturing electric resistance welded steel pipes and a secondary processing process for subjecting the electric resistance welded steel pipes to secondary processing (cold drawing, bending, pipe expansion / diameter reduction, etc.) after the pipe making process. [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 process by forming a plate material (called a hoop material) unwound from a coil into a tubular shape with a roll, and then butting the ends of the tubular plate material together and electrically resistance welding them. 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 the plate material, and the ends of the plate material heated by this eddy current (induction heating) are pressure-welded with a roll. The molten steel extruded onto the inner and outer surfaces of the steel pipe by electric resistance welding cools and remains on the steel pipe as a bead, and this bead is cut off with a cutting tool immediately after welding.

[0003] The electric resistance welded steel pipe obtained in the above pipe-making process is generally subjected to a cold drawing process as a secondary processing process. The cold drawing process is a process in which a steel pipe is drawn through a die with a plug or mandrel inserted in the steel pipe. This cold drawing process is suitable for manufacturing electric resistance welded steel pipes of various dimensions using the steel pipe after the pipe-making process as a raw material, and has the advantage that the electric resistance welded steel pipe after the cold drawing process has uniform outer diameter and wall thickness dimensions and improved surface roughness compared to the steel pipe after the pipe-making process, which has bead cutting marks on its inner and outer surfaces.

[0004] Here, conventionally, in the pipe-making process, eddy current inspection of electric resistance welded steel pipes is generally performed using a penetration type detection coil. Similarly, eddy current inspection using a penetration type detection coil is sometimes performed on electric resistance welded steel pipes after a cold drawing process. In eddy current testing of electric resistance welded steel pipes in the pipe-making process, it is necessary to pass unwelded plate material formed into a tubular shape through the detection coil for process reasons, so the gap between the outer diameter of the welded steel pipe and the inner diameter of the detection coil usually needs to be 5 mm or more. In other words, it is necessary to perform eddy current testing with a relatively small filling factor of the detection coil. In addition, electric resistance welded steel pipes manufactured by forming plate material into a tubular shape have dimensional variations in the outer diameter, which are a cause of noise in eddy current testing. On the other hand, in eddy current testing of electric resistance welded steel pipes after the cold drawing process, the difference between the outer diameter of the steel pipe and the inner diameter of the detection coil can be reduced to about 2 mm, since there is little variation in the outer diameter and wall thickness of the steel pipe. In other words, it is possible to perform eddy current testing with a relatively large filling factor of the detection coil. In general, the higher the filling factor of the detection coil, the higher the defect detection ability of eddy current testing (hereinafter referred to as "S / N ratio" where appropriate).

[0005] For the reasons mentioned above, eddy current inspection of electric resistance welded steel pipes after the cold drawing process generally has a higher S / N ratio than eddy current inspection of electric resistance welded steel pipes in the pipe making process. For this reason, as for defects (artificial defects) used for calibration of the inspection sensitivity (amplification of the inspection signal) and the defect detection threshold (the inspection signal exceeding this threshold is detected as the inspection signal corresponding to the defect) in eddy current inspection in the pipe making process, the size of the defect is determined by standards such as JIS, and for example, in the case of small diameter steel pipes, a drill hole (through hole) with a diameter (inner diameter) of 1.2 mm that penetrates the steel pipe in the wall thickness direction is used. On the other hand, for eddy current inspection after the cold drawing process, a drill hole with a diameter of, for example, 0.5 mm that penetrates the steel pipe in the wall thickness direction is used, which is smaller than the defects used in eddy current inspection in the pipe making process. For eddy current inspection after the cold drawing process, the dimensions of the defects used for calibration are not specifically specified in the standards, and the dimensions of the defects to be detected, which are determined based on the requirements of users of electric-welded steel pipes, are regarded as the dimensions of the defects used for calibration.

[0006] Here, in the pipe-making process, if scale invades between the ends of the butted plate materials, fine defects (welding defects) may occur in the welded portion. The invading scale may be contained in the cooling water used in the pipe-making process, or may be floating in the atmosphere during the formation of the plate materials. Fine defects such as welding defects occurring in the pipe-making process are difficult to detect by eddy current inspection in the pipe-making process calibrated as in the past, but it is expected that they can be detected by eddy current inspection after the cold drawing process. However, if a fine defect occurring in the pipe-making process remains until the cold drawing process, stress concentration occurs in the fine defect in the cold drawing process, which may cause cracks in the steel pipe. If a crack occurs in the steel pipe, the yield of the steel pipe will decrease, and there is a risk that a trouble will occur in the cold drawing process and a large amount of man-hours will be required to repair it. For this reason, it is desired to be able to detect fine defects such as welding defects in the pipe-making process to the same degree as after the cold drawing process. Similarly, when a bending process or a pipe expansion / reduction process is performed on an electric resistance welded steel pipe after a pipe making process (an electric resistance welded steel pipe that has not been subjected to a cold drawing process after a pipe making process) or an electric resistance welded steel pipe after a cold drawing process (an electric resistance welded steel pipe that has been subjected to a cold drawing process after a pipe making process) as a secondary processing process, if minute defects that occur in the pipe making process remain until these secondary processing processes, stress concentration occurs in the minute defects in the secondary processing process, which may cause cracks in the steel pipe. For this reason, it is desirable to be able to detect minute defects such as welding defects even in the pipe making process.

[0007] For example, Patent Documents 1 to 3 propose eddy current flaw detection methods for steel pipes, but do not solve the above problems. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Publication number 2-8210 [Patent Document 2] JP 2003-4708 A [Patent Document 3] JP 2007-121193 A Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made to solve the problems of the conventional technology as described above, and has an object to provide an eddy current inspection method and eddy current inspection equipment capable of detecting minute defects in electric welded steel pipes during the pipe making process, which includes a pipe making process for producing electric welded steel pipes and a secondary processing process for subjecting the electric welded steel pipes to secondary processing (cold drawing, bending, expanding / reducing diameter, etc.) after the pipe making process. [Means for solving the problem]

[0010] In order to solve the above problems, the present inventors have conducted extensive research. First, the present inventors investigated the relationship between the diameter of a defect (artificial defect) used in the calibration of eddy current inspection and the magnitude (amplitude) of the inspection signal obtained for this defect, and confirmed that there is a good positive correlation between the two, and further, that there is a good positive correlation between the volume of the defect and the magnitude of the inspection signal obtained for this defect. The present inventors then found that even when the outer diameter and wall thickness dimensions of the electric resistance welded steel pipe to be subjected to eddy current inspection are different, there is also a good positive correlation between the defect volume ratio (= volume of defect / (cross-sectional area of ​​electric resistance welded steel pipe×coil width of detection coil) and the magnitude of the inspection signal obtained for this defect. For this reason, we calculated the defect volume ratio of a defect (e.g., a drill hole with a diameter of 1.2 mm) conventionally used for calibrating eddy current testing of electric welded steel pipes in the pipe-making process, and the defect volume ratio of a defect (e.g., a drill hole with a diameter of 0.5 mm) to be detected (used for calibration) in eddy current testing of electric welded steel pipes after the cold drawing process, and found that by correcting the inspection sensitivity and / or defect detection threshold after calibration of eddy current testing in the pipe-making process based on the ratio of the latter defect volume ratio to the former defect volume ratio, it is possible to detect fine defects in the pipe-making process as well as after the cold drawing process.

[0011] The first eddy current inspection method of the present invention, which is applicable when the secondary processing step is a cold drawing step, 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 a first eddy current inspection method, which includes a pipe making process for manufacturing an electric welded steel pipe by butting ends of a plate material formed into a tube shape together and welding them, and a cold drawing process for cold drawing the electric welded steel pipe after the pipe making process, and includes a first eddy current inspection step for performing eddy current inspection on the electric welded steel pipe in the pipe making process, and a second eddy current inspection step for performing eddy current inspection on the electric welded steel pipe after the cold drawing process, and further includes a defect volume ratio calculation step for calculating a first defect volume ratio, which is a defect volume ratio determined by the size of a defect used for calibrating the inspection sensitivity and / or defect detection threshold value in the first eddy current inspection step and the size of a penetrating detection coil used in the first eddy current inspection step, and a second defect volume ratio, which is a defect volume ratio determined by the size of a defect to be detected in the second eddy current inspection step and the size of a penetrating detection coil used in the second eddy current inspection step, In the first eddy current testing step, The defect used for the calibration is pre-calibrated so that the magnitude of the detection signal obtained by eddy current testing with the detection coil becomes a predetermined value. Calibrated sensitivity and / or A calibrated test coil is pre-calibrated so that the test signal obtained by performing eddy current testing on the defect used for the calibration can be detected by the detection coil. The present invention provides an eddy current inspection method for electric-resistance welded steel pipes, characterized in that a defect detection threshold is corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step, and the electric-resistance welded steel pipes are subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold. The "defect volume ratio" means the volume of the defect / (cross-sectional area of ​​the electric resistance welded steel pipe x coil width of the detection coil).

[0012] According to the first eddy current inspection method of the present invention, in the defect volume ratio calculation step, a first defect volume ratio is calculated, which is a defect volume ratio determined by the size of a defect used for calibrating the inspection sensitivity and / or the defect detection threshold in the first eddy current inspection step (a step of performing eddy current inspection on an electric resistance welded steel pipe in a pipe making process) and the size of a penetrating detection coil used in the first eddy current inspection step. Also, a second defect volume ratio is calculated, which is a defect volume ratio determined by the size of a defect to be detected in the second eddy current inspection step (a step of performing eddy current inspection on an electric resistance welded steel pipe after a cold drawing process) and the size of a penetrating detection coil used in the second eddy current inspection step. As mentioned above, there is a good positive correlation between the defect volume ratios (first defect volume ratio, second defect volume ratio) and the magnitude of the inspection signal obtained for the defects, so in the first eddy current inspection step, the calibrated inspection sensitivity and / or defect detection threshold is corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step, and the corrected inspection sensitivity and / or defect detection threshold is used to perform eddy current inspection of the electric welded steel pipe, making it possible to detect fine defects of the same level as those after the cold drawing process even in the pipe making process.

[0013] In the first eddy current inspection method of the present invention, the defect volume ratio calculation step preferably includes the steps of preparing an electric welded steel pipe after the cold drawing process, in which a plurality of defects of different dimensions are formed, as a calibration steel pipe; calculating the relationship between defect dimensions and defect detectability by performing eddy current inspection on each of the plurality of defects in the calibration steel pipe with the detection coil used in the second eddy current inspection step; determining a minimum defect dimension at which a desired defect detectability can be obtained based on the relationship; and calculating the second defect volume ratio using the determined minimum dimension as the dimension of the defect to be detected.

[0014] According to the above-mentioned preferred method, the minimum defect dimension at which the desired defect detectability (S / N ratio) is obtained is used as the dimension of the defect to be detected in the second eddy current inspection step to calculate the second defect volume ratio, so that it is expected that even in the pipe making process, it will be possible to detect fine defects with the desired defect detectability.

[0015] Preferably, in the first eddy current inspection step, both eddy current inspection of the electric welded steel pipe using the corrected inspection sensitivity and / or defect detection threshold, and eddy current inspection of the electric welded steel pipe using the calibrated and uncorrected inspection sensitivity and / or defect detection threshold are performed.

[0016] According to the above-mentioned preferred method, in the first eddy current inspection step, eddy current inspection of the electric resistance welded steel pipe is performed using the corrected inspection sensitivity and / or defect detection threshold, so that even in the pipe making process, it is possible to detect fine defects to the same extent as after the cold drawing process, according to the request of the user of the electric resistance welded steel pipe. In addition, in the first eddy current inspection step, eddy current inspection of the electric resistance welded steel pipe is performed using the inspection sensitivity and / or defect detection threshold after calibration and before correction, so that the defects used for calibration are in accordance with standards such as JIS, thereby enabling eddy current inspection in accordance with the standards. In other words, it is possible to perform both eddy current inspection targeting fine defects according to the request of the user of the electric resistance welded steel pipe and eddy current inspection in accordance with standards such as JIS in parallel.

[0017] In the first eddy current inspection method according to the present invention, preferably, in the first eddy current inspection step, a first step is performed in which a time axis of the inspection signal obtained by performing eddy current inspection on the electric resistance welded steel pipe with the corrected inspection sensitivity is divided into a plurality of first sections having a predetermined time width, and a maximum value of the inspection signal in each of the first sections is detected as a noise value; and after the first step, a moving average value of the noise value in a second section consisting of L (L≧2) consecutive first sections out of the plurality of first sections is calculated, and the noise value of the first section subsequent to the second section and the calculated moving average value are compared. The method includes a second step of replacing the noise value of the subsequent first section with the noise value of the last first section that constitutes the second section if the difference with the moving average value within the second section exceeds a predetermined threshold value; a third step of calculating, after the second step, the moving average value of the noise value within a third section that is composed of M (M>L) consecutive first sections among the plurality of first sections as a noise level; and a fourth step of determining the defect detection threshold for each first section based on the noise level calculated in the third step, and performing eddy current inspection of the electric welded steel pipe using the determined defect detection threshold.

[0018] According to the above-mentioned preferred method, in the first step, the time axis of the flaw detection signal is divided into a plurality of first sections having a predetermined time width, and the maximum value of the signal in each first section is detected as a noise value. This noise value may include the magnitude of the flaw detection signal corresponding to a defect, not noise. For this reason, in the above-mentioned preferred method, in the second step, after the first step, a second section consisting of L (L≧2) consecutive first sections is set from among the plurality of first sections, and a moving average value of the noise value in this second section is calculated. Then, it is determined whether or not the difference between the noise value of the first section following the second section and the calculated moving average value in the second section exceeds a predetermined threshold value. If the difference between the two exceeds the predetermined threshold value, the noise value of the first section following the second section may correspond to the magnitude of the flaw detection signal corresponding to a defect. For this reason, in the second step, if the difference between the two exceeds the predetermined threshold value, the noise value of the following first section is replaced with the noise value of the last first section constituting the second section. As a result, it is highly likely that all the noise values ​​of the first section after the replacement correspond to the magnitude of the flaw detection signal corresponding to noise. In the above-mentioned preferred method, in the third step, after the second step, a moving average value of the noise value within a third section consisting of M (M>L) consecutive first sections among the multiple first sections is calculated as a noise level, and in the fourth step, a defect detection threshold is determined for each first section based on the calculated noise level, and eddy current inspection is performed on the electric welded steel pipe using the determined defect detection threshold. In other words, according to the above-mentioned preferred method, in the first eddy current inspection step, the electric welded steel pipe is not always inspected using a constant defect detection threshold, but rather, the electric welded steel pipe is inspected using a defect detection threshold determined for each first section based on the calculated noise level (in other words, a defect detection threshold that follows fluctuations in the noise level).Therefore, even if the noise level fluctuates during the pipe making process, overdetection and non-detection are suppressed, and it is possible to perform eddy current inspection of the electric welded steel pipe with high accuracy.

[0019] In order to solve the above-mentioned problems, the present invention provides a first eddy current inspection equipment for performing eddy current inspection on an electric welded steel pipe in a manufacturing process of the electric welded steel pipe, the manufacturing process including a pipe making process in which an electric welded steel pipe is manufactured by butting together ends of a plate material formed into a tubular shape and welding the ends of the plate material, and a cold drawing process in which the electric welded steel pipe is cold drawn after the pipe making process, the first eddy current inspection equipment includes a first eddy current inspection device for performing eddy current inspection on the electric welded steel pipe in the pipe making process, and a second eddy current inspection device for performing eddy current inspection on the electric welded steel pipe after the cold drawing process, and a calibration method for the defect detection sensitivity and / or defect detection threshold value in the first eddy current inspection device is performed based on the dimensions of a defect used for calibrating the defect detection sensitivity and / or defect detection threshold value in the first eddy current inspection device and the dimensions of a penetrating detection coil provided in the first eddy current inspection device. The present invention is also provided as an eddy current flaw detection equipment for electric welded steel pipes, characterized in that a first flaw volume ratio, which is a flaw volume ratio determined by the dimensions of the flaw to be detected by the second eddy current flaw detection device and the dimensions of a penetrating detection coil equipped in the second eddy current flaw detection device, are calculated in advance, the first eddy current flaw detection device is set with a corrected flaw detection sensitivity and / or defect detection threshold obtained by correcting the calibrated flaw detection sensitivity and / or defect detection threshold based on the ratio of the second flaw volume ratio to the calculated first flaw volume ratio, and the first eddy current flaw detection device performs eddy current flaw detection on the electric welded steel pipe using the corrected flaw detection sensitivity and / or defect detection threshold. The "defect volume ratio" means the volume of the defect / (cross-sectional area of ​​the electric resistance welded steel pipe x coil width of the detection coil).

[0020] As mentioned above, the secondary processing of electric resistance welded steel pipes is not limited to the cold drawing process, but also includes bending, pipe expansion and diameter reduction, etc. Unlike after the cold drawing process, it is difficult to perform eddy current inspection of electric resistance welded steel pipes after these secondary processing processes. On the other hand, if minute defects that occur in the pipe making process remain until these secondary processing processes, stress concentration occurs at these minute defects in the secondary processing process, which may cause cracks in the steel pipe. If a crack occurs, it is possible to actually measure the size of the defect. For this reason, in the defect volume ratio calculation step of the above-mentioned first eddy current inspection method, the second defect volume ratio was calculated based on the dimensions of the defect to be detected in the second eddy current inspection step and the dimensions of the penetrating detection coil used in the second eddy current inspection step, but instead, it is also possible to actually measure the dimensions of a defect that occurs in the weld of an electric-welded steel pipe during the secondary processing process of the electric-welded steel pipe, and calculate the second defect volume ratio as the defect volume ratio determined by the measured dimensions of the defect and the dimensions of the penetrating detection coil used in the first eddy current inspection step.

[0021] That is, in order to solve the above-mentioned problems, the present invention provides a second eddy current inspection method, in which an electric-resistance welded steel pipe manufacturing process includes a pipe-making process in which an electric-resistance welded steel pipe is manufactured by butting ends of a plate material formed into a tubular shape together and welding them, and a secondary processing process in which the electric-resistance welded steel pipe is subjected to secondary processing after the pipe-making process, the method further includes a first eddy current inspection step of performing eddy current inspection on the electric-resistance welded steel pipe in the pipe-making process, and a defect volume ratio calculation step of actually measuring the dimensions of defects generated in the welded portion of the electric-resistance welded steel pipe in the secondary processing process and calculating a second defect volume ratio which is a defect volume ratio determined by the dimensions of the defects used for calibrating the inspection sensitivity and / or defect detection threshold value in the first eddy current inspection step and the dimensions of a penetrating detection coil used in the first eddy current inspection step, and a defect volume ratio calculation step of actually measuring the dimensions of defects generated in the welded portion of the electric-resistance welded steel pipe in the secondary processing step and calculating the measured defect dimensions and the dimensions of the detection coil used in the first eddy current inspection step, The defect used for the calibration is pre-calibrated so that the magnitude of the detection signal obtained by eddy current testing with the detection coil becomes a predetermined value. Calibrated sensitivity and / or A calibrated test coil is pre-calibrated so that the test signal obtained by performing eddy current testing on the defect used for the calibration can be detected by the detection coil. The present invention provides an eddy current inspection method for electric-resistance welded steel pipes, characterized in that a defect detection threshold is corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step, and the electric-resistance welded steel pipes are subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold. The "defect volume ratio" means the volume of the defect / (cross-sectional area of ​​the electric resistance welded steel pipe x coil width of the detection coil). Specifically, the second defect volume ratio calculated by the second eddy current inspection method is expressed as the volume of the defect actually generated in the secondary processing step / (cross-sectional area of ​​the electric resistance welded steel pipe after the secondary processing step x coil width of the detection coil used in the first eddy current inspection step).

[0022] Also in the second eddy current inspection method of the present invention, there is a good positive correlation between the defect volume ratios (first defect volume ratio, second defect volume ratio) and the magnitude of the inspection signal obtained for the defects, so that in the first eddy current inspection step, the calibrated inspection sensitivity and / or defect detection threshold is corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step, and the corrected inspection sensitivity and / or defect detection threshold is used to perform eddy current inspection of the electric welded steel pipe, making it possible to detect fine defects of the same order of magnitude as defects caused by the secondary processing process, even in the pipe making process. The "secondary processing step" in the second eddy current inspection method according to the present invention includes (1) secondary processing steps such as bending and expanding / reducing performed on electric resistance welded steel pipes after a pipe making process, (2) secondary processing steps such as bending and expanding / reducing performed on electric resistance welded steel pipes after a pipe making process and a subsequent cold drawing process, and (3) cold drawing performed on electric resistance welded steel pipes after a pipe making process. That is, the second eddy current inspection method according to the present invention is preferably used when the secondary processing step is a bending process or an expanding / reducing process (cases (1) and (2) above) in which it is difficult to perform eddy current inspection after the secondary processing step (i.e., it is difficult to perform the second eddy current inspection step of the first eddy current inspection method), but is not necessarily limited thereto, and can also be used when the secondary processing step is a cold drawing process (case (3) above).

[0023] Furthermore, in order to solve the above-mentioned problem, the present invention provides a second eddy current inspection equipment for performing eddy current inspection on an electric welded steel pipe in a manufacturing process of the electric welded steel pipe, the manufacturing process including a pipe making process in which an electric welded steel pipe is manufactured by butting together and welding the ends of a plate material formed into a tubular shape, and a secondary processing process in which the electric welded steel pipe is subjected to secondary processing after the pipe making process, the second eddy current inspection equipment comprising a first eddy current inspection device for performing eddy current inspection on the electric welded steel pipe in the pipe making process, and a first defect volume ratio, which is determined by the size of a defect used for calibrating the defect inspection sensitivity and / or defect detection threshold value in the first eddy current inspection device and the size of a penetrating detection coil provided in the first eddy current inspection device. and a second defect volume ratio which is a defect volume ratio determined by the measured dimensions of the defects generated in the welded portion of the electric welded steel pipe in the secondary processing step and the dimensions of the detection coil used in the first eddy current inspection step, the first eddy current inspection device is set with a corrected defect inspection sensitivity and / or defect detection threshold obtained by correcting the calibrated defect inspection sensitivity and / or defect detection threshold based on the ratio of the second defect volume ratio to the calculated first defect volume ratio, and the first eddy current inspection device performs eddy current inspection of the electric welded steel pipe using the corrected defect inspection sensitivity and / or defect detection threshold. The "defect volume ratio" means the volume of the defect / (cross-sectional area of ​​the electric resistance welded steel pipe x coil width of the detection coil). Effect of the Invention

[0024] According to the present invention, when eddy current inspection is performed on both electric resistance welded steel pipes in the pipe making process and electric resistance welded steel pipes after the cold drawing process, minute defects can be detected in the pipe making process. [Brief description of the drawings]

[0025] [Figure 1] 1 is a diagram showing a schematic configuration of a first eddy-current testing device and a second eddy-current testing device provided in an eddy-current testing facility according to a first embodiment of the present invention; [Diagram 2] 1 is a flowchart showing a schematic procedure of an eddy current flaw detection method according to a first embodiment of the present invention. [Diagram 3]An example of the results of eddy current testing of an electric resistance welded steel pipe P having multiple defects with different diameters formed therein using the first eddy current testing device 10 shown in Figure 1 and evaluating the magnitude of the testing signal obtained for each defect is shown below. [Figure 4] Using the first eddy current testing device 10 shown in Figure 1, multiple defects with different diameters are formed in multiple electric welded steel pipes P with different outer diameters and wall thicknesses, and these electric welded steel pipes P are subjected to eddy current testing.An example of the results of evaluating the S / N ratio calculated from the magnitude of the testing signal obtained for each defect is shown below. [Diagram 5] This is a flow chart showing an outline of the procedure of the defect volume ratio calculation step ST1 (step ST12) shown in Figure 2, in which the dimensions of the defect to be detected are determined taking into account the S / N ratio, and the second defect volume ratio is calculated using these defect dimensions. [Figure 6] 6 is a diagram showing an example of the relationship between the defect size and the S / N ratio calculated in step ST122 shown in FIG. 5. [Figure 7] 3 is an explanatory diagram for explaining a procedure for determining a defect detection threshold value by a first determination unit 35a of the first eddy current flaw detector 10 shown in FIG. [Figure 8] FIG. 6 is a flowchart showing a schematic procedure of an eddy current flaw detection method according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Hereinafter, with reference to the attached drawings as appropriate, an eddy current inspection equipment for electric resistance welded steel pipes and an eddy current inspection method for electric resistance welded steel pipes using the same according to an embodiment of the present invention will be described.

[0027] First Embodiment [Eddy current flaw detection equipment] FIG. 1 is a diagram showing a schematic configuration of a first eddy current inspection device 10 and a second eddy current inspection device 20 provided in an eddy current inspection equipment 100 according to a first embodiment of the present invention. FIG. 1(a) shows a schematic configuration of the first eddy current inspection device 10, and FIG. 1(b) shows a schematic configuration of the second eddy current inspection device 20. In FIG. 1, an electric-resistance welded steel pipe P and a through-type coil (excitation coil 1, detection coil 2) are shown in cross section. In FIG. 1, similar components provided in the first eddy current inspection device 10 and the second eddy current inspection device 20 are given the same reference numerals, and duplicated explanations are omitted as appropriate. The eddy current inspection equipment 100 according to the first embodiment is equipment for performing eddy current inspection of an electric-resistance welded steel pipe P in a manufacturing process of the electric-resistance welded steel pipe P, which includes a pipe-making process in which the electric-resistance welded steel pipe P is manufactured by butting together and welding the ends of a plate material formed into a tubular shape, and a cold drawing process in which the electric-resistance welded steel pipe P is cold-drawn after the pipe-making process. As shown in Fig. 1, the eddy current inspection equipment 100 includes a first eddy current inspection device 10 for performing eddy current inspection of the electric-resistance welded steel pipe P in the pipe-making process, and a second eddy current inspection device 20 for performing eddy current inspection of the electric-resistance welded steel pipe P after the cold drawing process. The first eddy current inspection device 10 is disposed, for example, at the exit side of a welding device installed in a pipe manufacturing line that performs a pipe manufacturing process. The second eddy current inspection device 20 is disposed, for example, at the exit side of a straightening machine installed immediately after the discharge table of a cold drawing line that performs a cold drawing process, or offline where the electric resistance welded steel pipe P after the cold drawing process is carried in.

[0028] As shown in FIG. 1(a), the first eddy current flaw detector 10 includes a through-type coil (1, 2) into which an electric resistance welded steel pipe P is inserted, and a flaw detector 3 connected to the through-type coil (1, 2). The through-type coil (1, 2) of the first embodiment is a mutual induction type coil having a through-type excitation coil 1 and a through-type detection coil 2 arranged in the excitation coil 1. The through-type coil (1, 2) of the first embodiment is a self-comparison type coil having a pair of detection coils 2a and 2b as the detection coil 2, and a differential signal of each of the detection coils 2a and 2b is output. In the first eddy current flaw detector 10, the gap amount G, which is the difference between the outer diameter of the electric resistance welded steel pipe P and the inner diameter of the detection coil 2, is set to, for example, 5 mm or more, and the filling rate of the detection coil 2 is relatively small. In the first embodiment, a mutual induction type coil in which the excitation coil 1 and the detection coil 2 are separate is used as the through coil (1, 2), but the present invention is not necessarily limited to this, and it is also possible to use a self-induction type coil in which the same coil functions as both the excitation coil and the detection coil. However, according to the knowledge of the inventors, by using a mutual induction type coil as in the first embodiment, even if the filling factor of the detection coil 2 is relatively small, the noise level is reduced compared to the case of using a self-induction type coil, which is preferable in that it can increase the S / N ratio.

[0029] The flaw detector 3 includes an excitation circuit 31 , a receiving circuit 32 , a synchronous detection circuit 33 , an amplifier 34 , and a determination unit 35 . The excitation circuit 31 supplies an AC excitation current to the excitation coil 1. As a result, an AC magnetic field acts on the electric resistance welded steel pipe P, and an eddy current is induced in the electric resistance welded steel pipe P. The detection coil 2 outputs a flaw detection signal (a differential signal in the first embodiment) obtained by detecting an eddy current induced in the electric resistance welded steel pipe P to the receiving circuit 32. The receiving circuit 32 adjusts the received flaw detection signal to a size required for subsequent processing, and outputs the signal to the synchronous detection circuit 33. The synchronous detection circuit 33 synchronously detects the flaw detection signal output from the receiving circuit 32 based on a reference signal (a reference signal having the same frequency as the excitation current supplied to the excitation coil 1) output from the excitation circuit 31, and outputs the flaw detection signal after synchronous detection to an amplifier 34. The synchronous detection circuit 33 has a phase adjustment function and adjusts the flaw detection signal corresponding to a defect to a predetermined phase. A flaw detection sensitivity (amplification degree of the flaw detection signal) is set in the amplifier 34. The amplifier 34 amplifies the flaw detection signal output from the synchronous detection circuit 33 according to the set flaw detection sensitivity, and outputs the amplified signal to the determination unit 35. A defect detection threshold is set in the judgment unit 35. After A / D converting the flaw detection signal output from the amplifier 34, the judgment unit 35 compares the flaw detection signal after A / D conversion with the defect detection threshold, and detects a flaw detection signal that exceeds the defect detection threshold as a flaw detection signal corresponding to a defect.

[0030] The basic configuration of the flaw detector 3 described above is the same as that of a known flaw detector that is typically used for eddy current flaw detection. However, the flaw detector 3 of the first embodiment is characterized in that it includes a plurality of amplifiers 34 (first amplifier 34a and second amplifier 34b) as the amplifier 34. The first amplifier 34a and the second amplifier 34b are connected in parallel to the synchronous detection circuit 33, and the same flaw detection signal is input to the first amplifier 34a and the second amplifier 34b from the synchronous detection circuit 33. The first amplifier 34a is set to a flaw detection sensitivity after correction as described below (however, in the case where only the defect detection threshold is corrected, the flaw detection sensitivity after calibration that is not corrected). Moreover, the second amplifier 34b is set to a flaw detection sensitivity after calibration and before correction as described below. The flaw detector 3 of the first embodiment is characterized in that it includes a plurality of the determination units 35 (first determination unit 35a and second determination unit 35b) as the determination unit 35. The first determination unit 35a receives an amplified flaw detection signal from the first amplifier 34a, and the second determination unit 35b receives an amplified flaw detection signal from the second amplifier 34b. The first determination unit 35a is set with a defect detection threshold value after correction as described below (however, when only the flaw detection sensitivity is corrected, an uncorrected defect detection threshold value is set). The second determination unit 35b is set with a defect detection threshold value after calibration and before correction as described below. In the first embodiment, the amplifier 34 and judgment unit 35 of the flaw detector 3 of the first eddy current flaw detection device 10 that performs eddy current flaw detection on the electric welded steel pipe P in the pipe making process have the above-mentioned configuration, so that it is possible to perform both eddy current flaw detection (eddy current flaw detection using the first amplifier 34a and the first judgment unit 35a) that detects minute defects in accordance with the requirements of the user of the electric welded steel pipe, and eddy current flaw detection that complies with standards such as JIS (eddy current flaw detection using the second amplifier 34b and the second judgment unit 35b) in parallel.

[0031] As shown in Fig. 1(b), the second eddy current flaw detector 20 also includes a through-type coil (1, 2) into which an electric-resistance welded steel pipe P (a calibration pipe P' in step ST12 described below) is inserted, and a flaw detector 3A connected to the through-type coil (1, 2). Like the through-type coil (1, 2) included in the first eddy current flaw detector 10, the through-type coil (1, 2) included in the second eddy current flaw detector 20 is a mutual induction type self-comparison coil having a through-type excitation coil 1 and a through-type detection coil 2 (detection coils 2a, 2b) arranged in the excitation coil 1. However, the through-type coil (1, 2) provided in the second eddy current flaw detection apparatus 20 differs from the through-type coil (1, 2) provided in the first eddy current flaw detection apparatus 10 in that the gap amount G, which is the difference between the outer diameter of the electric welded steel pipe P and the inner diameter of the detection coil 2, is set to a size of, for example, about 2 mm, and the filling rate of the detection coil 2 is relatively large.

[0032] The flaw detector 3A provided in the second eddy current flaw detector 20, like the flaw detector 30 provided in the first eddy current flaw detector 10, also includes an excitation circuit 31, a receiving circuit 32, a synchronous detection circuit 33, an amplifier 34, and a judgment unit 35. However, the flaw detector 3A provided in the second eddy current flaw detector 20 differs from the flaw detector 3 provided in the first eddy current flaw detector 10 in that the flaw detector 3A provided in the second eddy current flaw detector 20 is equipped with a single amplifier 34 and a single judgment unit 35, similar to conventionally known flaw detectors.

[0033] [Eddy current testing method] Hereinafter, a method for eddy current inspection of an electric resistance welded steel pipe P using the eddy current inspection equipment 100 having the above-mentioned configuration will be described. FIG. 2 is a flow chart showing an outline of the procedure of the eddy current flaw detection method according to the first embodiment of the present invention. As shown in Fig. 2, the eddy current inspection method according to the first embodiment includes a defect volume ratio calculation step ST1, a first eddy current inspection step ST2, and a second eddy current inspection step ST3. The defect volume ratio calculation step ST1 is a step of calculating a first defect volume ratio and a second defect volume ratio as described below. The first eddy current inspection step ST2 is a step in which a first eddy current inspection device 10 performs eddy current inspection on an electric resistance welded steel pipe P in a pipe making process. The second eddy current inspection step ST3 is a step in which a second eddy current inspection device 20 performs eddy current inspection on an electric resistance welded steel pipe P after a cold drawing process. The steps ST1 to ST3 will be described in detail below.

[0034] (Defect volume ratio calculation step ST1) In the defect volume ratio calculation step ST1, a first defect volume ratio is calculated, which is a defect volume ratio determined by the dimensions of the defect used to calibrate the inspection sensitivity and / or defect detection threshold in the first eddy current inspection device 10 shown in Figure 1(a) (in other words, in the first eddy current inspection step ST2) and the dimensions of the penetrating detection coil 2 provided in the first eddy current inspection device 10 (in other words, used in the first eddy current inspection step ST2) (step ST11 in Figure 2). Similarly, in the defect volume ratio calculation step ST1, a second defect volume ratio is calculated (step ST12 in FIG. 2), which is a defect volume ratio determined by the dimensions of the defect to be detected by the second eddy current testing device 20 shown in FIG. 1(b) (in other words, in the second eddy current testing step ST3) and the dimensions of the penetrating detection coil 2 provided in the second eddy current testing device 20 (in other words, used in the second eddy current testing step ST3). The defect volume ratio means defect volume / (cross-sectional area of ​​electric resistance welded steel pipe P×coil width W of detector coil 2). In the first embodiment, detector coil 2 is composed of a pair of detector coils 2a and 2b, and the coil width W1 of detector coil 2a is equal to the coil width W2 of detector coil 2b. The coil width W of detector coil 2 is expressed by the coil width W1 of detector coil 2a or the coil width W2 of detector coil 2b, but in this embodiment, W=W1=W2.

[0035] Figure 3 shows an example of the results of eddy current testing performed by the first eddy current testing device 10 on an electric welded steel pipe P having multiple defects (artificial defects, drill holes that penetrate the thickness direction of the electric welded steel pipe P) with different diameters (inner diameters), and evaluating the magnitude (amplitude) of the testing signal obtained for each defect (specifically, the output signal of the first amplifier 34a). As shown in Fig. 3, there is a good positive correlation between the diameter of a defect and the magnitude of the inspection signal obtained for that defect. Therefore, there is also a good positive correlation between the volume of a defect and the magnitude of the inspection signal obtained for that defect. Furthermore, there is also a good positive correlation between the defect volume ratio and the magnitude of the inspection signal obtained for that defect (and therefore the S / N ratio of the defect).

[0036] Figure 4 shows an example of the results of evaluating the S / N ratio calculated from the magnitude (amplitude) of the inspection signal (specifically, the output signal of the first amplifier 34a) obtained for each defect by forming multiple defects (artificial defects) with different diameters (inner diameters) in multiple electric welded steel pipes P having different outer diameters and wall thicknesses using the first eddy current inspection device 10. The results shown in Fig. 4 were obtained for each electric resistance welded steel pipe P in which the defects shown in Table 1 below were formed. The inspection frequency (frequency of the excitation current) was 8 kHz, the coil width W of the detection coil 2 was 2 mm, the distance between the detection coils 2a and 2b was 2 mm, and the gap amount, which is the difference between the outer diameter of the electric resistance welded steel pipe P and the inner diameter of the detection coil 2, was 5 mm. [Table 1] As shown in FIG. 4, as long as the diameter of the defect is equal to or less than the coil width W of the detection coil 2, it is found that there is a good positive correlation between the defect volume ratio and the defect S / N ratio. Therefore, in the eddy current flaw detection method of the first embodiment, the defect volume ratios (first defect volume ratio, second defect volume ratio) are calculated in the defect volume ratio calculation step ST1, and used to correct the flaw detection sensitivity and / or defect detection threshold in the first eddy current flaw detection step ST2, as described below.

[0037] In step ST11 of the defect volume ratio calculation step ST1, for example, it is assumed that an electric welded steel pipe P having an outer diameter of 21.7 mm and a wall thickness of 1.4 mm is subjected to eddy current inspection by the first eddy current inspection device 10, and a drill hole having an outer diameter of 1.2 mm is considered to be the defect to be used for calibrating the first eddy current inspection device 10. If the inner diameter of the detection coil 2 of the first eddy current flaw detector 10 is 27 mm, the simplified packing factor of the detection coil 2 in this case is (21.7) 2 / (27) 2 × 100 ≒ 65%. Strictly speaking, the filling rate of the detection coil 2 is determined by the outer diameter of the test material (in the first embodiment, the electric resistance welded steel pipe P). 2 / (average diameter of detector coil 2) 2 In the above calculation, the filling rate is calculated using the inner diameter of the detection coil 2 instead of the average diameter, so it is expressed as the "simplified filling rate" to distinguish it from the strict filling rate. The same applies to the "simplified filling rate" described later. The volume of the defect is (1.2 / 2) 2 ×3.14×1.4 ≒ 1.583 mm 3 It becomes. In addition, if the coil width W of the detection coil 2 of the first eddy current flaw detector 10 is 2.0 mm, the cross-sectional area of ​​the electric resistance welded steel pipe P×the coil width W of the detection coil 2 is (21.7−1.4)×1.4×3.14×2.0≒178.48 mm 3 It becomes. Therefore, in the above example, the first defect volume ratio is 1.583 / 178.48×100≈0.887%.

[0038] In step ST12 of the defect volume ratio calculation step ST1, for example, it is assumed that an electric welded steel pipe P after a cold drawing process, having an outer diameter of 18.0 mm and a wall thickness of 1.0 mm, is subjected to eddy current inspection by the second eddy current inspection device 20, and a drill hole with a diameter of 0.5 mm is determined to be the defect to be detected by the second eddy current inspection device 20. If the inner diameter of the detection coil 2 of the second eddy current flaw detector 20 is 20 mm, the simplified packing factor of the detection coil 2 in this case is (18.0). 2 / (20) 2 ×100=81%. The volume of the defect is (0.5 / 2) 2×3.14×1.0 ≒ 0.1963 mm 3 It becomes. In addition, if the coil width W of the detection coil 2 of the second eddy current flaw detector 20 is 2.0 mm, the cross-sectional area of ​​the electric resistance welded steel pipe P×the coil width W of the detection coil 2 is (18.0−1.0)×1.0×3.14×2.0=106.76 mm 3 It becomes. Therefore, in the above example, the second defect volume ratio is 0.1963 / 106.76×100≈0.184%.

[0039] (First eddy current testing step ST2) In the first eddy current inspection step ST2, the first eddy current inspection device 10 performs eddy current inspection on the electric resistance welded steel pipe P during the pipe making process. At this time, for example, as described above, a flaw detection sensitivity calibrated using a drill hole having a diameter of 1.2 mm is set in advance in the second amplifier 34b of the first eddy current flaw detector 10. Specifically, the flaw detection sensitivity of the second amplifier 34b is set so that the magnitude of the flaw detection signal (output signal of the second amplifier 34b) obtained for this drill hole becomes a predetermined value. Furthermore, for example, as described above, a defect detection threshold calibrated using a drill hole having a diameter of 1.2 mm is set in advance in the second judgment unit 35b of the first eddy current flaw detector 10. Specifically, the defect detection threshold of the second judgment unit 35b is set so that the flaw detection signal (output signal of the second amplifier 34b) obtained for this drill hole is detectable (so that the magnitude of the flaw detection signal exceeds the defect detection threshold).

[0040] On the other hand, the first amplifier 34a of the first eddy current flaw detector 10 is preset with a corrected flaw detection sensitivity obtained by correcting the calibrated flaw detection sensitivity (the flaw detection sensitivity set in the second amplifier 34b) based on the ratio of the second flaw volume ratio to the first flaw volume ratio calculated in the flaw volume ratio calculation step ST1. Specifically, in the above example, the first flaw volume ratio is 0.887% and the second flaw volume ratio is 0.184%, so the ratio of the second flaw volume ratio to the first flaw volume ratio is 0.184 / 0.887≒0.207. This is converted to dB and is approximately -13.7 dB. Therefore, for example, when only the flaw detection sensitivity of the first amplifier 34a is corrected (when the flaw detection threshold of the first judgment unit 35a is not corrected), the flaw detection sensitivity after calibration is corrected to be increased by 13.7 dB, and this corrected flaw detection sensitivity is set in the first amplifier 34a. In addition, a corrected defect detection threshold value obtained by correcting the calibrated defect detection threshold value (the defect detection threshold value set in the second judgment unit 35b) based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step ST1 is set in advance in the first judgment unit 35a of the first eddy current flaw detector 10. Specifically, in the above example, the ratio of the second defect volume ratio to the first defect volume ratio is −13.7 dB when converted into dB, so that, for example, when only the defect detection threshold value of the first judgment unit 35a is corrected (when the flaw detection sensitivity of the first amplifier 34a is not corrected), the calibrated defect detection threshold value is corrected to be lowered by 13.7 dB, and this corrected defect detection threshold value is set in the first judgment unit 35a. In addition, when correcting both the flaw detection sensitivity of the first amplifier 34a of the first eddy current flaw detector 10 and the defect detection threshold of the first judgment unit 35a, in the above example, the flaw detection sensitivity of the first amplifier 34a and the defect detection threshold of the first judgment unit 35a are corrected so that the sum of both correction amounts is 13.7 dB, and the corrected values ​​are set in the first amplifier 34a and the first judgment unit 35a, respectively. In this case, for example, the flaw detection sensitivity after calibration is corrected to be increased by 7.7 dB, and this corrected flaw detection sensitivity is set in the first amplifier 34a, and the defect detection threshold after calibration is corrected to be decreased by 6 dB, and this corrected defect detection threshold is set in the first judgment unit 35a.

[0041] In the first eddy current inspection step ST2 of the first embodiment, the first eddy current inspection device 10 performs eddy current inspection using the first amplifier 34a in which the corrected inspection sensitivity is set and the first judgment unit 35a in which the corrected defect detection threshold is set. Therefore, in the pipe making process, it is possible to detect fine defects similar to those after the cold drawing process according to the request of the user of the electric resistance welded steel pipe P. In addition, the first eddy current inspection device 10 also performs eddy current inspection using the second amplifier 34b in which the calibration and pre-correction inspection sensitivity is set and the second judgment unit 35b in which the calibration and pre-correction defect detection threshold is set. Therefore, by making the defects used for calibration conform to standards such as JIS, eddy current inspection conforming to the standards is possible. In other words, both eddy current inspection for detecting fine defects according to the request of the user of the electric resistance welded steel pipe P and eddy current inspection conforming to standards such as JIS can be performed in parallel.

[0042] However, the present invention is not necessarily limited to this, and it is also possible to adopt a configuration including only the first amplifier 34a and the first judgment unit 35a (a configuration not including the second amplifier 34b and the second judgment unit 35b) and perform only eddy current testing to detect minute defects in accordance with the requirements of the user of the electric welded steel pipe P, etc.

[0043] <Second eddy current testing step ST3> In the second eddy current inspection step ST3, the second eddy current inspection device 20 performs eddy current inspection on the electric resistance welded steel pipe P after the cold drawing process. At this time, as described above, for example, a drill hole with a diameter of 0.5 mm is set as the defect to be detected, and the flaw detection sensitivity calibrated using this defect is preset in the amplifier 34 of the second eddy current flaw detection device 20. Specifically, the flaw detection sensitivity of the amplifier 34 is set so that the magnitude of the flaw detection signal (output signal of the amplifier 34) obtained for this drill hole becomes a predetermined value. Furthermore, as described above, for example, a defect detection threshold calibrated using a drill hole having a diameter of 0.5 mm, which is the defect to be detected, is set in advance in the determination unit 35 of the second eddy current flaw detector 20. Specifically, the defect detection threshold of the determination unit 35 is set so that the flaw detection signal (output signal of the amplifier 34) obtained for this drill hole is detectable (so that the magnitude of the flaw detection signal exceeds the defect detection threshold).

[0044] In the second eddy current testing step ST3 of the first embodiment, the second eddy current testing device 20 performs eddy current testing using an amplifier 34 in which the calibrated testing sensitivity is set and a judgment unit 35 in which the calibrated defect detection threshold is set. Therefore, by setting the defects used for calibration as the defects to be detected, it is possible to detect minute defects in accordance with the requirements of the user of the electric welded steel pipe P, etc.

[0045] [Calculation of the second defect volume ratio taking into account the S / N ratio] In the first embodiment described above, when determining the dimensions (specifically, the diameter) of the defect to be detected by the second eddy current flaw detector 20 (a drill hole with a diameter of 0.5 mm in the above example), the S / N ratio of this defect is not particularly taken into consideration. However, if the size of the defect that provides the desired S / N ratio is set as the size of the defect to be detected by the second eddy current flaw detector 20 (if the second defect volume ratio is calculated using this defect size), it is expected that even in the pipe making process, minute defects can be detected with the desired S / N ratio by the first eddy current flaw detector 10, which is preferable. This preferable embodiment will be described below.

[0046] FIG. 5 is a flow diagram showing an outline of the procedure of defect volume ratio calculation step ST1 (step ST12) in the case where the dimensions of the defect to be detected are determined taking into account the S / N ratio, and the second defect volume ratio is calculated using the dimensions of this defect. 5, in the defect volume ratio calculation step ST1 (step ST12) when the S / N ratio is taken into consideration, first, in step ST121, an electric resistance welded steel pipe after a cold drawing process in which a plurality of defects of different dimensions are formed is prepared as a calibration steel pipe P'. Specifically, for example, a calibration steel pipe P' is prepared which has the same cross-sectional dimensions as the electric resistance welded steel pipe P to be actually subjected to eddy current inspection by the second eddy current inspection device 20 and in which a plurality of drill holes of different diameters are formed.

[0047] Next, in step ST122, the second eddy current flaw detector 20 performs eddy current flaw detection on each of the multiple flaws in the calibration steel pipe P' (see FIG. 1(b)), thereby calculating the relationship between the flaw size and the S / N ratio. FIG. 6 is a diagram showing an example of the relationship between the size of the defect calculated in step ST122 and the S / N ratio. Specifically, the horizontal axis of FIG. 6 is the diameter of the drill hole formed in the steel pipe P' for calibration, and the vertical axis of FIG. 6 is the magnitude (amplitude) of the flaw detection signal (the output signal of the amplifier 34 of the second eddy current flaw detection device). That is, FIG. 6 directly shows the relationship between the size of the defect and the magnitude of the flaw detection signal. Then, the relationship between the two is approximated by an approximation line by performing an approximation calculation such as the least square method using the magnitude of the flaw detection signal for each defect size (diameters 0.4 mm, 0.5 mm, and 0.6 mm in the example shown in FIG. 6). In the example shown in FIG. 6, the noise level NL of the flaw detection signal shown by the dashed line is 0.13 V, so the S / N ratio is calculated by dividing the magnitude of the flaw detection signal on the vertical axis by this noise level NL. Therefore, the approximation line in FIG. 6 essentially shows the relationship between the size of the defect and the S / N ratio.

[0048] Next, in step ST123, the minimum defect size that provides a desired S / N ratio is determined based on the relationship (approximate line) calculated in step ST122. For example, if the desired S / N ratio is 2, the magnitude of the flaw detection signal that gives this S / N ratio is 0.26 V in the example shown in Fig. 6. The minimum size of the flaw that gives this magnitude of the flaw detection signal is a diameter of 0.4 mm in the example shown in Fig. 6.

[0049] Finally, in step ST124, the second defect volume ratio is calculated using the minimum dimension determined in step ST123 as the dimension of the defect to be detected. For example, as in the above example, if the second eddy current inspection device 20 performs eddy current inspection on the electric resistance welded steel pipe P having an outer diameter of 18.0 mm and a wall thickness of 1.0 mm after the cold drawing process, and the drill hole having a diameter of 0.4 mm determined in step ST123 is determined to be the defect to be detected by the second eddy current inspection device 20, the volume of this defect will be calculated as follows: (0.4 / 2) 2 ×3.14×1.0 ≒ 0.1256 mm 3 Therefore, as in the previous example, the cross-sectional area of ​​the electric resistance welded steel pipe P × the coil width W of the detection coil 2 is 106.76 mm 3 If this is the case, the second defect volume ratio is 0.1256 / 106.76×100 ≒ 0.118%.

[0050] As in the previous example, if the first defect volume ratio is 0.887%, and the second defect volume ratio is 0.118%, the ratio of the second defect volume ratio to the first defect volume ratio is 0.118 / 0.887 ≒ 0.133. Converting this to dB, it is approximately -17.5 dB. As described above, when the second defect volume ratio is calculated taking the S / N ratio into consideration, for example, when only the flaw detection sensitivity of the first amplifier 34a is corrected (when the flaw detection threshold of the first judgment unit 35a is not corrected), the flaw detection sensitivity after calibration is corrected to be increased by 17.5 dB, and this corrected flaw detection sensitivity is set in the first amplifier 34a. Also, for example, when only the flaw detection threshold of the first judgment unit 35a is corrected (when the flaw detection sensitivity of the first amplifier 34a is not corrected), the defect detection threshold after calibration is corrected to be decreased by 17.5 dB, and this corrected defect detection threshold is set in the first judgment unit 35a. Furthermore, when correcting both the flaw detection sensitivity of the first amplifier 34a and the defect detection threshold of the first judgment unit 35a of the first eddy current flaw detection device 10, the flaw detection sensitivity of the first amplifier 34a and the defect detection threshold of the first judgment unit 35a are corrected so that the sum of the two correction amounts becomes 17.5 dB, and are set in the first amplifier 34a and the first judgment unit 35a, respectively.

[0051] [Determination of defect detection threshold following noise level fluctuations] In the first embodiment, it is also possible to always set the defect detection threshold value in the first judgment unit 35a of the first eddy current flaw detection device 10 to a constant value (for example, to set it to a constant value while one electric resistance welded steel pipe P is subjected to eddy current flaw detection). However, in order to increase the S / N ratio, it is preferable that the first judgment unit 35a determines a defect detection threshold value that follows fluctuations in the noise level (the magnitude of the flaw detection signal corresponding to the noise) and uses this to perform eddy current flaw detection on the electric resistance welded steel pipe P. This preferable aspect will be described below.

[0052] Fig. 7 is an explanatory diagram for explaining a procedure for determining a defect detection threshold (a defect detection threshold that follows a noise level) by the first judgment unit 35a of the first eddy current flaw detector 10. Fig. 7(a) is a diagram for explaining a procedure for calculating a noise level, and Fig. 7(b) is a diagram for explaining a procedure for determining a defect detection threshold. As shown in FIG. 7A, the first judgment unit 35a divides the time axis of the amplified flaw detection signal input from the first amplifier 34a into a plurality of first sections S each having a predetermined time width (e.g., 50 mm in length). 1 (From the left in Fig. 7(a), 11 , S 12 Then, the first determination unit 35a divides the first section S 1 Every 1st section S 1 In the example shown in FIG. 7(a), for example, the first section S 11 The maximum value of the inspection signal within the noise value N 1 It is detected as the first section S 12 The maximum value of the inspection signal within the noise value N 2 It is detected as the first section S 13 The maximum value of the inspection signal within the noise value N 3 It is detected as the first section S 14 The maximum value of the inspection signal within the noise value N 4 The first determination unit 35a detects this first section S 1 Detect noise values ​​in all first intervals S 1 This will be carried out regarding:

[0053] Next, the first determination unit 35a determines a plurality of first sections S 1 The first section S of the L consecutive (L ≥ 2) 1 The second section S consists of 2 (For example, in terms of length, 800 mm) is calculated. In FIG. 7(a), for convenience, the second section S 2 The first section S of 4 pieces (L=4) 1 The second section S on the left side of FIG. 21 is the first section S 11 ~S 14 followed by one first section S 1 (located 1 minute apart) 22 is the first section S 12 ~S 15 For example, the second section S 21 The moving average of the noise value in the first interval S 11 ~S 14 The second section S is calculated as the average value of each noise value. 22 The moving average of the noise value in the first interval S 12 ~S 15 The first judgment unit 35a calculates the average value of each noise value in the second section S 2 Calculate the moving average of the noise value in all second intervals S 2 This will be carried out regarding:

[0054] Next, the first determination unit 35a determines whether the second section S 2 The first section following S 1 The noise value of and the calculated second section S 2 In the example shown in FIG. 7(a), for example, in the second section S 21 The first section following S 15 The noise value of and the calculated second section S 21 It is determined whether the difference between the moving average value and the second interval S exceeds a predetermined threshold value. 22 The first section following S 16 The noise value of and the calculated second section S 22It is determined whether the difference between the moving average value and the average value exceeds a predetermined threshold value. If it is determined that the threshold value is exceeded, the first determination unit 35a performs the following first section S 1 The noise value of the second interval S 2 The last section S 1 In the example shown in FIG. 7(a), for example, the noise value in the second section S 21 The first section following S 15 The noise value of and the calculated second section S 21 When it is determined that the difference between the moving average value and the moving average value in the first section S exceeds a predetermined threshold value, the first determination unit 35a 15 The noise value of the second interval S 21 The last section S 14 In addition, the noise value in the second section S 22 The first section following S 16 The noise value of and the calculated second section S 22 When it is determined that the difference between the moving average value and the moving average value in the first section S exceeds a predetermined threshold value, the first determination unit 35a 16 The noise value of the second interval S 22 The last section S 15 The first decision unit 35a performs the above calculations for all the second sections S 2 And the following first section S 1 This will be carried out regarding: 2nd Section S 2 The first section following S 1 The noise value of and the calculated second section S 2 If the difference between the moving average value and the second interval S exceeds a certain threshold, 2 The first section following S 1 The noise value of the first section S may correspond to the magnitude of the flaw detection signal corresponding to the flaw. 1 The noise value of the second interval S 2 The last section S 1 After replacing the noise value with the noise value of the first section S 1 It is highly likely that each of the noise values ​​corresponds to the magnitude of the flaw detection signal corresponding to the noise.

[0055] Next, the first determination unit 35a determines a plurality of first sections S 1 The first interval S of M (M>L) consecutive 1 The third section S consists of 3 (For example, 6400 mm in terms of length) is calculated as the noise level NL (see FIG. 7(b)). In FIG. 7(a), for convenience, the third section S 3 The first section S of 7 (M=7) 1 The third section S on the left side of FIG. 31 is the first section S 11 ~S 17 followed by one first section S 1 (located 1 minute apart) 3rd section S 32 is the first section S 12 ~S 18 For example, the third section S 31 The moving average of the noise value in the first interval S 11 ~S 17 The third section S is calculated as the average value of each noise value. 32 The moving average of the noise value in the first interval S 12 ~S 18 The first judgment unit 35a calculates the average value of each noise value in the third section S 3 Calculation of the moving average noise value (i.e., calculation of the noise level NL) in all the third sections S 3 This will be carried out regarding:

[0056] Finally, as shown in FIG. 7B, the first judgment unit 35a sets the defect detection threshold value Th within the first section S based on the calculated noise level NL. 1 7(b), the defect detection threshold Th is determined to be twice the noise level NL (i.e., S / N ratio=2). The first judgment unit 35a then performs eddy current inspection of the electric resistance welded steel pipe P using the determined defect detection threshold Th.

[0057] As described above, by determining the defect detection threshold Th that tracks the fluctuations in the noise level NL, overdetection and non-detection are suppressed, and eddy current inspection of the electric resistance welded steel pipe P can be performed with high accuracy. For example, in the example shown in Fig. 7(b), a defect detection flag is set for the inspection signals corresponding to defects F1 and F2, indicating that the threshold Th has been exceeded. However, when considering the noise level of the entire inspection signal shown in Fig. 7(b), the S / N ratio is about 1.5, making detection difficult in general. However, it can be seen that detection is possible by determining the defect detection threshold Th that tracks the fluctuations in the noise level NL.

[0058] <Second embodiment> [Eddy current flaw detection equipment] The eddy current inspection equipment according to the second embodiment is an equipment for performing eddy current inspection on an electric-resistance welded steel pipe P in a manufacturing process of the electric-resistance welded steel pipe P, which includes a pipe-making process in which the electric-resistance welded steel pipe P is manufactured by butting together the ends of a plate material formed into a tubular shape and welding them, and a secondary processing process in which the electric-resistance welded steel pipe P is subjected to secondary processing (in the second embodiment, bending, expanding, reducing the diameter, etc.) after the pipe-making process. The eddy current inspection equipment according to the second embodiment is equipped with a first eddy current inspection device 10 for performing eddy current inspection on the electric-resistance welded steel pipe P in the pipe-making process as shown in FIG. 1(a), like the eddy current inspection equipment 100 according to the first embodiment, but unlike the eddy current inspection equipment 100 according to the first embodiment, it is not equipped with a second eddy current inspection device 20 as shown in FIG. 1(b). The specific configuration of the first eddy current testing device 10 provided in the eddy current testing equipment of the second embodiment is the same as that of the eddy current testing equipment 100 of the first embodiment, so Figure 1(a) will be reused and a detailed description thereof will be omitted.

[0059] [Eddy current testing method] Fig. 8 is a flow diagram showing a schematic procedure of an eddy current inspection method according to a second embodiment of the present invention using an eddy current inspection equipment having the above-mentioned configuration. For convenience of explanation, Fig. 8 also shows a secondary processing step ST3' performed after the eddy current inspection method according to the second embodiment is performed. As shown in FIG. 8, the eddy current flaw detection method according to the second embodiment includes a defect volume ratio calculation step ST1' and a first eddy current flaw detection step ST2. The defect volume ratio calculation step ST1' of the eddy current inspection method according to the second embodiment is different from the defect volume ratio calculation step ST1 of the eddy current inspection method according to the first embodiment, while the first eddy current inspection step ST2 of the eddy current inspection method according to the second embodiment is similar to the first eddy current inspection step ST2 of the eddy current inspection method according to the first embodiment. Therefore, hereinafter, the defect volume ratio calculation step ST1' different from the eddy current inspection method according to the first embodiment will be described, and a detailed description of the first eddy current inspection step ST2 will be omitted.

[0060] In the defect volume ratio calculation step ST1' of the eddy current flaw detection method according to the second embodiment, the method of calculating the first defect volume ratio is the same as that of the first embodiment. That is, in step ST11 of Fig. 8, a first defect volume ratio is calculated, which is a defect volume ratio determined by the size of a defect used for calibrating the flaw detection sensitivity and / or the defect detection threshold value in the first eddy current flaw detection device 10 shown in Fig. 1(a) (in other words, in the first eddy current flaw detection step ST2) and the size of the penetrating detection coil 2 included in the first eddy current flaw detection device 10 (in other words, used in the first eddy current flaw detection step ST2).

[0061] On the other hand, in the defect volume ratio calculation step ST1' of the eddy current inspection method according to the second embodiment, the calculation method of the second defect volume ratio is different from that of the first embodiment. Specifically, in step ST12' of FIG. 8, the dimensions of the defect generated in the welded part of the electric resistance welded steel pipe P in the secondary processing step ST3' are actually measured in advance. Among the dimensions of the defect, the cross-sectional area of ​​the defect can be measured, for example, when a crack occurs in the electric resistance welded steel pipe P in the secondary processing step ST3', by imaging the defect contained in the cracked part using a magnifying glass or a microscope, and subjecting the captured image of the defect to image processing or using a printed captured image. In addition, the depth of the defect can be measured, for example, by polishing the defect using a polishing tool, and the moving distance of the polishing tool from the polishing start point to the point where the defect disappears. Then, in step ST12', the second defect volume ratio, which is a defect volume ratio determined by the measured defect dimensions and the dimensions of the detection coil 2 included in the first eddy current inspection device 10 (in other words, used in the first eddy current inspection step ST2), is calculated. Specifically, the second defect volume ratio calculated in the second embodiment is expressed as the volume of the defect actually generated in the secondary processing process ST3' divided by (the cross-sectional area of ​​the electric welded steel pipe P after the secondary processing process ST3' x the coil width W of the detection coil 2 used in the first eddy current inspection step ST2).

[0062] In the first eddy current inspection step ST2 of the eddy current inspection method according to the second embodiment, the electric resistance welded steel pipe P is subjected to eddy current inspection in the pipe making process, but in this case, as in the first embodiment, the calibrated inspection sensitivity and / or defect detection threshold value are corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step ST1', and the corrected inspection sensitivity and / or defect detection threshold value are used. This makes it possible to detect minute defects of the same order of magnitude as defects caused by the secondary processing step ST3' in the pipe making process as well. Then, after the first eddy current flaw detection step ST2 is performed, the electric resistance welded steel pipe P is subjected to secondary processing in a secondary processing step ST3'.

[0063] As with the eddy current inspection method of the first embodiment, it is also possible for the eddy current inspection method of the second embodiment to adopt a configuration including only the first amplifier 34a and the first judgment unit 35a (a configuration not including the second amplifier 34b and the second judgment unit 35b) and perform only eddy current inspection to detect minute defects. Furthermore, with regard to the eddy current inspection method of the second embodiment as well, as with the eddy current inspection method of the first embodiment, it is preferable that the first judgment unit 35a determines a defect detection threshold that tracks fluctuations in the noise level (the magnitude of the inspection signal corresponding to the noise) and uses this to perform eddy current inspection of the electric welded steel pipe P, as with the eddy current inspection method of the first embodiment.

[0064] In the second embodiment, a method for eddy current inspection of an electric-resistance welded steel pipe P in a pipe-making process has been described in a manufacturing process of an electric-resistance welded steel pipe P, which includes a secondary processing step ST3' in which the electric-resistance welded steel pipe P is subjected to secondary processing (bending, pipe expansion / diameter reduction, etc.) without performing a cold drawing process after the pipe-making process, but the present invention is not limited to this. The eddy current inspection method according to the second embodiment is also applicable to a manufacturing process in which a secondary processing step ST3' such as a bending process or a pipe expansion / diameter reduction process is performed on the electric-resistance welded steel pipe P after the pipe-making process and the subsequent cold drawing process, and a manufacturing process in which a cold drawing process is performed as a secondary processing step on the electric-resistance welded steel pipe after the pipe-making process (without performing a bending process or a pipe expansion / diameter reduction process, etc.).

[0065] When the eddy current inspection method of the second embodiment is applied to the former manufacturing processes (pipe making process, cold drawing process, and secondary processing process ST3' such as bending process and pipe expansion / reduction process), it is possible to perform not only the first eddy current inspection step ST2 of performing eddy current inspection of the electric welded steel pipe P in the pipe making process, but also the second eddy current inspection step ST3 (see Figure 2) of performing eddy current inspection of the electric welded steel pipe P after the cold drawing process (before the secondary processing process ST3' such as bending process and pipe expansion / reduction process), as in the first embodiment. When the second eddy current inspection step ST3 is performed, as described above, in the first eddy current inspection step ST2, the calibrated inspection sensitivity and / or defect detection threshold are corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step ST1', and the electric resistance welded steel pipe P is subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold. In addition, in the second eddy current inspection step ST3, the electric resistance welded steel pipe P may be subjected to eddy current inspection using the inspection sensitivity and / or defect detection threshold after similar correction. In the second eddy current inspection step ST3, when the electric resistance welded steel pipe P is subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold, a third defect volume ratio, which is a defect volume ratio determined by the measured dimensions of the defects generated in the welded part of the electric resistance welded steel pipe P in the secondary processing step ST3' and the dimensions of the detection coil 2 (corresponding to the detection coil 2 in FIG. 1(b)) used in the second eddy current inspection step ST3, may be calculated. This third defect volume ratio is expressed as the volume of the defect actually generated in the secondary processing step ST3' / (cross-sectional area of ​​the electric resistance welded steel pipe P after the secondary processing step ST3'×coil width W of the detection coil 2 used in the second eddy current inspection step ST3). Then, in the second eddy current inspection step ST3, the calibrated inspection sensitivity and / or defect detection threshold value are corrected based on the ratio of the third defect volume ratio to the second defect volume ratio (defect volume ratio determined by the size of the defect to be detected in the second eddy current inspection step ST3 and the size of the detection coil 2 used in the second eddy current inspection step ST3) similar to the first embodiment, and the electric resistance welded steel pipe P is subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold value. This makes it possible to detect fine defects similar to those generated by the secondary processing step ST3' in the second eddy current inspection step ST3. [Explanation of symbols]

[0066] 1. Excitation coil 2, 2a, 2b...Detection coil 3, 3A...Flaw detector 10...1st eddy current flaw detection device 20...Second eddy current flaw detection device 31...excitation circuit 32 Receiving circuit 33. Synchronous detection circuit 34 Amplifier 34a First amplifier 34b Second amplifier 35... Judgment section 35a...1st judgment section 35b...Second judgment section 100...Eddy current testing equipment P...ERW steel pipe P'...Calibration tube

Claims

1. A manufacturing process for electric-resistance welded steel pipe, comprising a pipe-making process for manufacturing an electric-resistance welded steel pipe by butt-welding ends of a plate material formed into a tubular shape, and a cold drawing process for cold-drawing the electric-resistance welded steel pipe after the pipe-making process, A first eddy current inspection step of performing eddy current inspection on the electric resistance welded steel pipe in the pipe making process; A second eddy current inspection step of performing eddy current inspection on the electric resistance welded steel pipe after the cold drawing process, The method further includes a defect volume ratio calculation step of calculating a first defect volume ratio, which is a defect volume ratio determined by the size of a defect used for calibrating the inspection sensitivity and / or the defect detection threshold in the first eddy current inspection step and the size of a penetrating detection coil used in the first eddy current inspection step, and a second defect volume ratio, which is a defect volume ratio determined by the size of a defect to be detected in the second eddy current inspection step and the size of a penetrating detection coil used in the second eddy current inspection step, In the first eddy current inspection step, a calibrated inspection sensitivity, which has been pre-calibrated so that the magnitude of the inspection signal obtained by eddy current inspection of the defect used for the calibration with the detection coil becomes a predetermined value, and / or a calibrated defect detection threshold, which has been pre-calibrated so that the inspection signal obtained by eddy current inspection of the defect used for the calibration with the detection coil can be detected, are corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step, and the electric resistance welded steel pipe is subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold. The eddy current inspection method for electric resistance welded steel pipes is characterized by the above. The "defect volume ratio" means the volume of the defect / (cross-sectional area of ​​the electric resistance welded steel pipe×coil width of the detection coil).

2. The defect volume ratio calculation step includes: a step of preparing an electric resistance welded steel pipe after the cold drawing process, the electric resistance welded steel pipe having a plurality of defects having different sizes formed therein, as a steel pipe for calibration; A step of calculating a relationship between a defect size and a defect detectability by detecting each of the plurality of defects in the calibration steel pipe by the detection coil used in the second eddy current flaw detection step; determining a minimum defect size that provides a desired defect detectability based on the relationship; and calculating the second defect volume ratio by using the determined minimum dimension as a dimension of the defect to be detected. The eddy current inspection method for electric resistance welded steel pipes according to claim 1.

3. In the first eddy current inspection step, eddy current inspection of the electric welded steel pipe is performed using the corrected flaw detection sensitivity and / or defect detection threshold value, and eddy current inspection of the electric welded steel pipe is performed using the calibrated flaw detection sensitivity and / or defect detection threshold value before and after the correction.

3. The eddy current inspection method for electric resistance welded steel pipe according to claim 1 or 2.

4. In the first eddy current testing step, a first step of dividing a time axis of the inspection signal obtained by performing eddy current inspection on the electric resistance welded steel pipe with the corrected inspection sensitivity into a plurality of first sections having a predetermined time width, and detecting a maximum value of the inspection signal within each of the first sections as a noise value; a second step of calculating, after the first step, a moving average value of the noise value in a second section that is composed of L (L≧2) consecutive first sections among the plurality of first sections, and replacing the noise value of the subsequent first section with the noise value of a last first section that constitutes the second section when a difference between the noise value of the first section subsequent to the second section and the calculated moving average value in the second section exceeds a predetermined threshold value; a third step of calculating, after the second step, a moving average value of the noise value within a third section constituted by M (M>L) consecutive first sections among the plurality of first sections as a noise level; and a fourth step of determining the defect detection threshold for each of the first sections based on the noise level calculated in the third step, and performing eddy current inspection of the electric resistance welded steel pipe using the determined defect detection threshold.

4. The eddy current inspection method for electric resistance welded steel pipe according to claim 1,

5. In the manufacturing process of the electric-resistance welded steel pipe, the manufacturing process includes a pipe-making process in which ends of a plate material formed into a tubular shape are butted together and welded to manufacture the electric-resistance welded steel pipe, and a secondary processing process in which the electric-resistance welded steel pipe is subjected to secondary processing after the pipe-making process, A first eddy current inspection step of performing eddy current inspection on the electric resistance welded steel pipe in the pipe making process, The method further includes a defect volume ratio calculation step of measuring the size of a defect generated in the welded portion of the electric resistance welded steel pipe in the secondary processing step and calculating a first defect volume ratio, which is a defect volume ratio determined by the size of a defect used for calibrating the inspection sensitivity and / or the defect detection threshold value in the first eddy current inspection step and the size of a penetrating detection coil used in the first eddy current inspection step, and a second defect volume ratio, which is a defect volume ratio determined by the size of the defect measured and the size of the detection coil used in the first eddy current inspection step, In the first eddy current inspection step, a calibrated inspection sensitivity, which has been pre-calibrated so that the magnitude of the inspection signal obtained by eddy current inspection of the defect used for the calibration with the detection coil becomes a predetermined value, and / or a calibrated defect detection threshold, which has been pre-calibrated so that the inspection signal obtained by eddy current inspection of the defect used for the calibration with the detection coil can be detected, are corrected based on the ratio of the second defect volume ratio to the first defect volume ratio calculated in the defect volume ratio calculation step, and the electric resistance welded steel pipe is subjected to eddy current inspection using the corrected inspection sensitivity and / or defect detection threshold. The eddy current inspection method for electric resistance welded steel pipes is characterized by the above. The "defect volume ratio" means the volume of the defect / (cross-sectional area of ​​the electric resistance welded steel pipe×coil width of the detection coil).

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