Eddy current flaw detection method and device

Abstract

To provide an eddy current flaw detection device capable of detecting flaw with accuracy, without correcting sensitivity of flaw detection for each thickness.SOLUTION: An eddy current flaw detection device 100 comprises: a pair of detection coils 2a, 2b; and a flaw detection tool 3. The flaw detection tool stores, a relationship between an induction voltage generated on one detection coil 2b, and a magnitude of a defect signal being a differential signal corresponding to an artificial defect, the relationship being acquired in advance using multiple correction electroseamed steel pipes. The flaw detection tool has a pre-detected induction voltage detected using one correction electroseamed steel pipe, and has flaw detection sensitivity which is corrected in advance. The flaw detection tool executes: a flaw detection sensitivity correction step for correcting the set flaw detection sensitivity, on the basis of the relationship, the induction voltage detected for the electroseamed steel pipe P, and an induction voltage stored about the correction electroseamed steel pipe; and a defect detection step for detecting defect which exists on the electroseamed steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an eddy current inspection method and an eddy current inspection device for detecting defects present in welds of electric resistance welded steel pipes during a pipe-making process for manufacturing electric resistance welded steel pipes. In particular, the present invention relates to an eddy current inspection method and an eddy current inspection device capable of performing eddy current inspection with high accuracy, even if the wall thickness of an electric resistance welded steel pipe changes within a predetermined thickness range during the pipe-making process, without calibrating the inspection sensitivity for each different wall thickness. The present invention also relates to an eddy current inspection method and an eddy current inspection device for detecting defects present in a steel pipe that has been subjected to a cold drawing process. In particular, the present invention relates to an eddy current inspection method and an eddy current inspection device capable of performing eddy current inspection with high accuracy, even if the outer diameter and wall thickness of a steel pipe that has been subjected to a cold drawing process change within a predetermined outer diameter range and a predetermined wall thickness range, respectively, without calibrating the inspection sensitivity for each different outer diameter and wall thickness. [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 steel pipes with various dimensions (outer diameter, wall thickness) using the steel pipe after the pipe-making process as a raw material, and has the advantage that the 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, in the pipe-making process, if scale invades between the ends of the butted plate materials, 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 forming of the plate materials. If a welding defect occurring in the pipe-making process remains until the next secondary processing process such as a cold drawing process, stress concentration may occur in the welding defect in the secondary processing process, which may cause cracks in the steel pipe. If a crack occurs in the steel pipe, not only will the yield of the steel pipe decrease, but there is also a risk that a problem will occur in the secondary processing process and a large amount of man-hours will be required to repair it. For this reason, it is desirable to be able to accurately detect welding defects in the pipe-making process for quality assurance.

[0005] Therefore, in the conventional pipe-making process, eddy current testing is generally performed to detect defects in electric-welded steel pipes based on a differential signal obtained by applying an alternating magnetic field to the electric-welded steel pipe to detect eddy currents generated in the electric-welded steel pipe using a pair of detection coils through which the electric-welded steel pipe passes.

[0006] However, in the pipe-making process, the rear end of one hoop material (leading hoop material) that is formed into a tubular shape first is generally joined to the front end of another hoop material (following hoop material) that is formed into a tubular shape next by welding (this is called intermediate welding), and electric resistance welded steel pipes are generally manufactured continuously. And, if the outer diameter of the electric resistance welded steel pipes to be manufactured is the same, even if the thickness of the leading hoop material and the thickness of the following hoop material are different, it is possible to manufacture electric resistance welded steel pipes continuously by intermediate welding them. For this reason, in the pipe-making process, electric-resistance welded steel pipes having a wall thickness over a predetermined range (electric-resistance welded steel pipes having different wall thicknesses within a predetermined thickness range) may be continuously manufactured by joint-welding hoop materials of different thicknesses, rather than having a constant wall thickness, as described in, for example, Patent Documents 1 and 2. Patent Document 1 discloses a method for continuously manufacturing electric-resistance welded steel pipes having a wall thickness range of 4.0 mm to 6.0 mm (e.g., claim 8 of Patent Document 1), and Patent Document 2 discloses a method for continuously manufacturing electric-resistance welded steel pipes having a wall thickness range of 2.5 mm to 5 mm (Figure 6 of Patent Document 2).

[0007] As described above, when eddy current testing is performed on electric resistance welded steel pipes having a thickness ranging over a certain range, if the same testing sensitivity (amplification of the differential signal) is used, there is a risk that changes in thickness will affect the testing accuracy of the eddy current testing. For this reason, for example, it is conceivable to adopt a method in which the pipe-making process is interrupted every time the thickness changes, and the testing sensitivity of the eddy current testing is calibrated using a calibration electric resistance welded steel pipe with an artificial defect. However, such a method requires the pipe-making process to be interrupted, thereby reducing the efficiency of the pipe-making process, and also requires the preparation of electric-resistance welded steel pipes for calibration (for example, by creating an artificial defect in part of a finished electric-resistance welded steel pipe to prepare an electric-resistance welded steel pipe for calibration), which has the problem of reducing the yield of electric-resistance welded steel pipes. As mentioned above, Patent Documents 1 and 2 describe the production of electric welded steel pipes having a wall thickness over a predetermined range, but do not disclose or suggest a method for eddy current inspection of electric welded steel pipes having a wall thickness over such a predetermined range.

[0008] Similarly, for quality assurance purposes, eddy current testing is performed on steel pipes after the cold drawing process in which an AC magnetic field is applied to the steel pipe to generate eddy currents, which are detected using a pair of detection coils through which the steel pipe passes. This produces a differential signal, and defects present in the steel pipe are detected based on the resulting signal. However, in the cold drawing process, steel pipes having various outer diameters and wall thicknesses are manufactured by changing the dies, plugs, etc. used, so if the same flaw detection sensitivity is used, the change in outer diameter or wall thickness may affect the flaw detection accuracy of eddy current flaw detection. For this reason, as in the case of the above-mentioned pipe-making process, it is possible to adopt a method of interrupting the cold drawing process every time the outer diameter or wall thickness changes, and calibrating the flaw detection sensitivity of eddy current flaw detection using a calibration steel pipe with an artificial defect. However, such a method reduces the efficiency of the cold drawing process, and requires the preparation of a calibration steel pipe (for example, an artificial defect is provided in a part of a steel pipe as a product to prepare the calibration steel pipe), which leads to a problem of reducing the yield of the steel pipe. In particular, when a small number of various types of steel pipes are manufactured in the cold drawing process, the above problem becomes significant. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 7081718 [Patent Document 2] Japanese Patent Application Publication No. 4-13486 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been made to solve the problems of the conventional techniques as described above, and has as its first object to provide an eddy current inspection method and an eddy current inspection device capable of performing eddy current inspection with high accuracy, even if the wall thickness of an electric resistance welded steel pipe changes within a predetermined thickness range in the pipe making process, without having to calibrate the inspection sensitivity for each different wall thickness. Also, the present invention has as its second object to provide an eddy current inspection method and an eddy current inspection device capable of performing eddy current inspection with high accuracy, even if the outer diameter and wall thickness of a steel pipe that has been subjected to a cold drawing process change within a predetermined outer diameter range and a predetermined wall thickness range, respectively. [Means for solving the problem]

[0011] In order to solve the first problem, the present inventors conducted extensive research and found that, depending on the type of steel of the electric-resistance welded steel pipe (for example, when the type of steel of the electric-resistance welded steel pipe is stainless steel), there is a good correlation between the wall thickness of the electric-resistance welded steel pipe and the magnitude of a defect signal, which is a differential signal corresponding to defects of the same size (having the same cross-sectional dimension in the direction perpendicular to the wall thickness direction of the electric-resistance welded steel pipe). Specifically, it was found that the larger the wall thickness of the electric-resistance welded steel pipe, the larger the magnitude of the defect signal. In addition, it was found that there is a good correlation between the wall thickness of the electric-resistance welded steel pipe and the induced voltage induced in one of the pair of detection coils (the induced voltage induced in a state where a defect-free portion of the electric-resistance welded steel pipe magnetized to a magnetic saturation level penetrates the pair of detection coils). Specifically, it was found that the larger the wall thickness of the electric-resistance welded steel pipe, the smaller the induced voltage induced in one of the pair of detection coils. Therefore, it has been found that there is a good correlation between the induced voltage induced in one of the detection coils and the magnitude of the defect signal, and as the induced voltage increases, the magnitude of the defect signal decreases. Therefore, it was conceived that by using calibration electric-welded steel pipes having different thicknesses and artificial defects to obtain in advance the relationship between the induced voltage induced in one of the detection coils and the magnitude of the defect signal, which is a differential signal corresponding to the artificial defect, the inspection sensitivity calibrated using one calibration electric-welded steel pipe can be corrected to an appropriate inspection sensitivity according to the wall thickness of the electric-welded steel pipe to be subjected to eddy current inspection based on the induced voltage detected in the electric-welded steel pipe whose quality is assured in the pipe making process and the induced voltage detected in advance in one calibration electric-welded steel pipe, and the previously obtained relationship, thereby enabling eddy current inspection to be performed with high accuracy.

[0012] The first eddy current flaw detection method according to the present invention has been completed based on the findings of the inventors described above. That is, in order to solve the first problem, the present invention provides a first eddy current inspection method, which is an eddy current inspection method for detecting defects in an electric welded steel pipe based on a differential signal obtained by applying an AC magnetic field to the electric welded steel pipe to generate eddy currents in the electric welded steel pipe during a pipe making process in which ends of plate material formed into a tubular shape are butted together and welded to produce an electric welded steel pipe having a thickness ranging over a predetermined thickness range, using a pair of detection coils through which the electric welded steel pipe passes to detect the eddy currents, the method using a plurality of calibration electric welded steel pipes each having different thicknesses within the thickness range of the electric welded steel pipe and each having artificial defects of the same dimensions, and detecting a defect in the electric welded steel pipe based on a differential signal obtained by detecting a defect in the electric welded steel pipe using one of the pair of detection coils. The present invention provides an eddy current flaw detection method, comprising: a relationship acquisition step of acquiring a relationship between an induced voltage and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect; a flaw detection sensitivity calibration step of calibrating flaw detection sensitivity using one of the calibration electric-resistance welded steel pipes; a flaw detection sensitivity correction step of correcting the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the induced voltage detected in the electric-resistance welded steel pipe, the induced voltage detected in the calibration electric-resistance welded steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step; and a defect detection step of detecting defects present in the electric-resistance welded steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. In the first eddy current inspection method according to the present invention, "artificial defects of the same dimensions" refer to artificial defects having the same cross-sectional dimensions in a direction perpendicular to the wall thickness direction of the electric resistance welded steel pipe for calibration. The same applies to the second eddy current inspection method described below. In the case of eddy current inspection of electric resistance welded steel pipes, through holes are often used as artificial defects. In the first eddy current flaw detection method according to the present invention, the term "flaw detection sensitivity" refers to the amplification degree of a differential signal. The same applies to the second eddy current flaw detection method described below. Furthermore, in the first eddy current inspection method according to the present invention, the induced voltage induced in one of the detection coils forming the relationship obtained using a plurality of electric resistance welded steel pipes for calibration means the induced voltage induced in a state where a portion of the electric resistance welded steel pipes for calibration magnetized to a magnetic saturation level and having no artificial defects passes through a pair of detection coils. The same applies to the second eddy current inspection method described below. In the first eddy current inspection method according to the present invention, some of the calibration electric resistance welded steel pipes used in the relationship acquisition step may include calibration electric resistance welded steel pipes having the same wall thickness (it is sufficient that at least a plurality of calibration electric resistance welded steel pipes having different wall thicknesses are included). The same applies to the second eddy current inspection method described below.

[0013] According to the first eddy current flaw detection method of the present invention, in the relationship acquisition step, the relationship between the induced voltage and the magnitude of the defect signal is acquired, and in the flaw detection sensitivity calibration step, the flaw detection sensitivity is calibrated using one calibration electric resistance welded steel pipe (using a calibration electric resistance welded steel pipe having one wall thickness within the wall thickness range of the electric resistance welded steel pipe to be quality guaranteed). Note that the relationship acquisition step and the flaw detection sensitivity calibration step are not limited to being performed in this order, and either one may be performed first. Next, in the flaw detection sensitivity correction step, the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step is corrected based on the induced voltage detected in the electric resistance welded steel pipe (the electric resistance welded steel pipe to be quality guaranteed), the induced voltage detected in one calibration electric resistance welded steel pipe in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step. As a correction of the flaw detection sensitivity, for example, if the magnitude of the defect signal obtained by the induced voltage detected in the electric resistance welded steel pipe and the relationship is A, and the magnitude of the defect signal obtained by the induced voltage detected in one calibration electric resistance welded steel pipe and the relationship is B, it is possible to correct the calibrated flaw detection sensitivity by a factor of B / A. Note that the flaw detection sensitivity correction step is preferably performed immediately after the thickness of the electric resistance welded steel pipe changes (immediately after the portion corresponding to the tip of the following hoop material, which has a different thickness from the preceding hoop material, reaches the pair of detection coils). The reason why it is preferable to perform the flaw detection sensitivity correction step at this timing is that the joint made by welding between the leading hoop material and the trailing hoop material (called the joint portion) and the specified length portion of the electric welded steel pipe based on this joint portion are generally treated as defective products (therefore, eddy current testing is not required for the joint portion and the specified length portion), so even if the flaw detection sensitivity correction step is performed immediately after the portion corresponding to the tip of the trailing hoop material reaches a pair of detection coils, no undetected areas will be created in the portions of the electric welded steel pipe that are not treated as defective products. Finally, in the defect detection step, defects present in the electric resistance welded steel pipe are detected using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. According to the first eddy current inspection method of the present invention, as described above, by using the inspection sensitivity corrected by B / A times the calibrated inspection sensitivity, if a defect of the same size as an artificial defect in one calibration electric-resistance welded steel pipe exists in the electric-resistance welded steel pipe, even if the wall thickness of one calibration electric-resistance welded steel pipe is different from that of the electric-resistance welded steel pipe, a defect signal of the same size as the artificial defect in the calibration electric-resistance welded steel pipe is obtained, so that eddy current inspection can be performed with high accuracy. In addition, if the relationship between the induced voltage and the magnitude of the defect signal is obtained using multiple calibration electric-resistance welded steel pipes and the induced voltage is detected for one calibration electric-resistance welded steel pipe, even if the wall thickness of the electric-resistance welded steel pipe changes within a predetermined thickness range, there is no need to calibrate the inspection sensitivity for each different wall thickness (it is only necessary to correct the calibrated inspection sensitivity), so there is no risk of a decrease in the efficiency of the pipe-making process or a decrease in the yield of the electric-resistance welded steel pipe.

[0014] Furthermore, the inventors conducted extensive research and found that, depending on the type of electric welded steel pipe (for example, when the steel type of the electric welded steel pipe is carbon steel), even if the wall thickness of the electric welded steel pipe changes, there is little change in the magnitude of the defect signal, which is a differential signal corresponding to a defect of the same dimension (having the same cross-sectional dimension in the direction perpendicular to the wall thickness direction of the electric welded steel pipe). Furthermore, they found that even if the wall thickness of the electric welded steel pipe changes, there is also little change in the induced voltage induced in one of the pair of detection coils (the induced voltage induced when a defect-free portion of the electric welded steel pipe passes through the pair of detection coils). For this reason, by using calibration electric-welded steel pipes having different thicknesses and artificial defects to obtain in advance the relationship between the induced voltage induced in one of the detection coils and the magnitude of the defect signal, which is a differential signal corresponding to the artificial defect, and by specifying in advance the range of induced voltages within which the magnitude of the defect signal falls within a predetermined range, it was conceived that, if the induced voltage detected in an electric-welded steel pipe whose quality is assured in the pipe-making process falls within the specified range of induced voltages, then accurate eddy current testing can be performed by using the inspection sensitivity calibrated using one calibration electric-welded steel pipe as is (without correcting the inspection sensitivity according to the thickness of the electric-welded steel pipe).

[0015] The second eddy current flaw detection method according to the present invention has been completed based on the findings of the inventors described above. That is, in order to solve the first problem, the present invention provides a second eddy current inspection method, which is an eddy current inspection method for detecting defects in an electric welded steel pipe based on a differential signal obtained by detecting eddy currents generated in the electric welded steel pipe by applying an AC magnetic field to the electric welded steel pipe during a pipe making process in which ends of plate material formed into a tubular shape are butted together and welded to produce an electric welded steel pipe having a thickness ranging over a predetermined thickness range, using a pair of detection coils through which the electric welded steel pipe passes, and using a plurality of calibration electric welded steel pipes each having different thicknesses within the thickness range of the electric welded steel pipe and each having artificial defects of the same dimensions, and detecting an induced voltage induced in one of the pair of detection coils and the differential signal corresponding to the artificial defects. and determining a range of the induced voltage within a predetermined range in which the magnitude of the defect signal falls; a flaw detection sensitivity calibration step of calibrating the flaw detection sensitivity using one of the calibration electric-resistance welded steel pipes; a determination step of determining whether the induced voltage detected in the electric-resistance welded steel pipe is within the range of the induced voltage determined in the induced voltage range determination step; and a defect detection step of detecting defects present in the electric-resistance welded steel pipe using the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step if it is determined in the determination step that the induced voltage detected in the electric-resistance welded steel pipe is within the range of the induced voltage determined in the induced voltage range determination step.

[0016] According to the second eddy current flaw detection method of the present invention, in the induced voltage range specifying step, a range of induced voltage in which the magnitude of the defect signal is within a predetermined range is specified, and in the flaw detection sensitivity calibrating step, the flaw detection sensitivity is calibrated using one calibration electric resistance welded steel pipe (using a calibration electric resistance welded steel pipe having one wall thickness within the wall thickness range of the electric resistance welded steel pipe to be quality assured). Note that the induced voltage range specifying step and the flaw detection sensitivity calibrating step are not limited to being performed in this order, and either one may be performed first. Next, in a judgment step, it is judged whether the induced voltage detected in the electric-resistance welded steel pipe (the electric-resistance welded steel pipe to be quality-guaranteed) is within the specified induced voltage range. The judgment step is preferably performed immediately after the thickness of the electric-resistance welded steel pipe changes (immediately after the portion corresponding to the tip of the following hoop material, which has a different thickness from the preceding hoop material, reaches the pair of detection coils). In order to strengthen quality assurance, it is also effective from the viewpoint of full-length guarantee to continuously perform the judgment step regardless of the presence or absence of a change in thickness. Finally, if it is determined that the induced voltage detected in the electric welded steel pipe is within the specified induced voltage range, in a defect detection step, defects present in the electric welded steel pipe are detected using the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step. According to the second eddy current inspection method of the present invention, if the induced voltage detected in the electric resistance welded steel pipe is determined to be within the range of the specified induced voltage, if a defect of the same size as the artificial defect in one calibration electric resistance welded steel pipe exists in the electric resistance welded steel pipe, even if the wall thickness of one calibration electric resistance welded steel pipe is different from that of the electric resistance welded steel pipe, the calibrated inspection sensitivity is used as it is to obtain a defect signal of the same magnitude as the artificial defect in the calibration electric resistance welded steel pipe, so that eddy current inspection can be performed with high accuracy. Furthermore, if the relationship between the induced voltage and the magnitude of the defect signal is obtained using multiple calibration electric resistance welded steel pipes and the range of induced voltages in which the magnitude of the defect signal is within a predetermined range is specified, even if the wall thickness of the electric resistance welded steel pipe changes within a predetermined thickness range, there is no need to calibrate the inspection sensitivity for each different wall thickness (it is sufficient to simply use the inspection sensitivity calibrated using one calibration electric resistance welded steel pipe as it is), so there is no risk of a decrease in the efficiency of the pipe making process or a decrease in the yield of the electric resistance welded steel pipe. If it is determined in the determination step that the induced voltage detected in the electric resistance welded steel pipe is not within the specified range of induced voltages, it is considered that some abnormality has occurred, and therefore, for example, a warning signal may be output. This makes it possible to take measures such as interrupting the pipe-making process and checking whether there is an abnormality in the detection coil or recalibrating the flaw detection sensitivity using a calibration electric resistance welded steel pipe, as necessary.

[0017] In order to solve the first problem, the present invention provides a first eddy current inspection device for performing eddy current inspection on an electric welded steel pipe in a pipe making process in which electric welded steel pipes having a wall thickness ranging over a predetermined wall thickness range are manufactured by butting together ends of plate material formed into a tubular shape and welding the ends together, the eddy current inspection device comprising: a pair of detection coils through which the electric welded steel pipe passes and which detects eddy currents generated in the electric welded steel pipe by applying an AC magnetic field to the electric welded steel pipe; and a flaw detector which detects defects present in the electric welded steel pipe based on a differential signal obtained by detecting the eddy currents with the pair of detection coils, the flaw detector detecting defects present in the electric welded steel pipe based on a differential signal obtained by detecting the eddy currents with the pair of detection coils, the differential signal being obtained by the detection of ... The present invention is also provided as an eddy current flaw detection device, characterized in that a relationship between an induced voltage induced in one of the pair of detection coils and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect, is stored in the flaw detector, the induced voltage detected in advance using one of the calibration electric-welded steel pipes is stored in the flaw detector, and a pre-calibrated flaw detection sensitivity is set, and the flaw detector executes a flaw detection sensitivity correction step of correcting the set flaw detection sensitivity based on the induced voltage detected in the electric-welded steel pipe, the induced voltage stored in the calibration electric-welded steel pipe, and the relationship, and a defect detection step of detecting defects present in the electric-welded steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step.

[0018] In order to solve the first problem, the present invention provides a second eddy current inspection device for performing eddy current inspection on an electric welded steel pipe in a pipe making process in which electric welded steel pipes having a wall thickness ranging over a predetermined wall thickness range are manufactured by butting together ends of plate material formed into a tubular shape and welding the ends together, the eddy current inspection device comprising: a pair of detection coils through which the electric welded steel pipe passes and which detects eddy currents generated in the electric welded steel pipe by applying an AC magnetic field to the electric welded steel pipe; and a flaw detector which detects defects present in the electric welded steel pipe based on a differential signal obtained by detecting the eddy currents with the pair of detection coils, the flaw detector including: a pair of detection coils which are previously obtained using a plurality of calibration electric welded steel pipes each having different wall thicknesses within the wall thickness range of the electric welded steel pipe and each having artificial defects of the same dimensions; The present invention is also provided as an eddy current flaw detection device, characterized in that a range of induced voltages within which the magnitude of the defect signal falls within a predetermined range is stored, the range being determined based on the relationship between the induced voltage induced in one of the detection coils and the magnitude of the defect signal, which is the differential signal corresponding to the artificial defect, and a flaw detection sensitivity that has been pre-calibrated using one of the calibration electric-welded steel pipes is set in the flaw detector, and the flaw detector executes a determination step of determining whether the induced voltage detected in the electric-welded steel pipe is within the range of the stored induced voltages, and a defect detection step of detecting defects present in the electric-welded steel pipe using the set flaw detection sensitivity when it is determined in the determination step that the induced voltage detected in the electric-welded steel pipe is within the range of the stored induced voltages.

[0019] Furthermore, in order to solve the second problem, the present inventors conducted extensive research and found that the first eddy current inspection method is also effective for steel pipes after a cold drawing process. That is, the inventors found that, for steel pipes having various outer diameters and wall thicknesses after a cold drawing process, there is a good correlation between the induced voltage induced in one of the pair of detector coils and the magnitude of the defect signal, and that the larger the induced voltage, the smaller the magnitude of the defect signal. Therefore, it was conceived that by using calibration steel tubes having different outer diameters and different wall thicknesses and having artificial defects, and obtaining in advance the relationship between the induced voltage induced in one of the detection coils and the magnitude of the defect signal, which is a differential signal corresponding to the artificial defect, the inspection sensitivity calibrated using one calibration steel tube can be corrected to an appropriate inspection sensitivity according to the outer diameter and wall thickness of the steel tube to be eddy current inspected based on the induced voltage detected in the steel tube to be quality assured after the cold drawing process, the induced voltage detected in advance in one calibration steel tube, and the previously obtained relationship, thereby enabling eddy current inspection to be performed with high accuracy.

[0020] The third eddy current flaw detection method according to the present invention has been completed based on the findings of the inventors described above. That is, in order to solve the second problem, the present invention provides a third eddy current inspection method for detecting defects in a steel pipe, the steel pipe having an outer diameter in a predetermined outer diameter range and a wall thickness in a predetermined wall thickness range, which has been subjected to a cold drawing process using a die and a plug or mandrel, by applying an AC magnetic field to the steel pipe to detect eddy currents generated in the steel pipe using a pair of detection coils through which the steel pipe passes, based on a differential signal obtained by detecting the eddy currents, the differential signal being obtained by detecting the eddy currents using one of the pair of detection coils using a plurality of calibration steel pipes each having different outer diameters within the outer diameter range of the steel pipe and different wall thicknesses within the wall thickness range, and each having artificial defects of the same dimensions. The present invention provides an eddy current flaw detection method, comprising: a relationship acquisition step of acquiring a relationship between an induced voltage induced in an output coil and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect; a flaw detection sensitivity calibration step of calibrating flaw detection sensitivity using one of the calibration steel pipes; a flaw detection sensitivity correction step of correcting the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the induced voltage detected in the steel pipe, the induced voltage detected in the calibration steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step; and a defect detection step of detecting defects present in the steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. In the third eddy current inspection method according to the present invention, the terms "steel pipe" and "calibration steel pipe" are not limited to electric resistance welded steel pipes, but are concepts which also include other steel pipes such as seamless steel pipes. In addition, in the third eddy current flaw detection method according to the present invention, "artificial flaws of the same dimensions" means artificial flaws having the same cross-sectional dimensions in a direction perpendicular to the wall thickness direction of the calibration steel pipe. In the third eddy current flaw detection method according to the present invention, the term "flaw detection sensitivity" refers to the amplification degree of a differential signal. Furthermore, in the third eddy current testing method according to the present invention, the induced voltage induced in one of the detection coils forming the relationship obtained using multiple calibration steel pipes means the induced voltage induced in a state in which a portion of the calibration steel pipe, which has been magnetized to the magnetic saturation level and has no artificial defects, passes through a pair of detection coils. In addition, in the third eddy current testing method according to the present invention, some of the calibration steel pipes used in the relationship acquisition step may include calibration steel pipes having the same outer diameter or calibration steel pipes having the same wall thickness (it is sufficient that at least a plurality of calibration steel pipes having both different outer diameters and wall thicknesses are included).

[0021] According to the third eddy current flaw detection method of the present invention, by using the flaw detection sensitivity corrected from the calibrated flaw detection sensitivity, if a flaw of the same size as an artificial flaw in one calibration steel pipe exists in a steel pipe (steel pipe to be quality-guaranteed), even if the outer diameter and wall thickness of one calibration steel pipe are different from those of the steel pipe, a flaw signal of the same size as the artificial flaw in the calibration steel pipe can be obtained, so that eddy current flaw detection can be performed with high accuracy. In addition, if the relationship between the induced voltage and the magnitude of the flaw signal is obtained using multiple calibration steel pipes and the induced voltage is detected for one calibration steel pipe, even if the outer diameter and wall thickness of the steel pipe change within a predetermined outer diameter range and a predetermined wall thickness range, there is no need to calibrate the flaw detection sensitivity for each different outer diameter and wall thickness (it is only necessary to correct the calibrated flaw detection sensitivity), so there is no risk of the efficiency of the cold drawing process decreasing or the yield of the steel pipe decreasing.

[0022] Furthermore, in order to solve the second problem, the present invention provides a third eddy current inspection apparatus for performing eddy current inspection on a steel pipe having an outer diameter in a predetermined outer diameter range and a wall thickness in a predetermined wall thickness range, which has been subjected to a cold drawing process using a die and a plug or mandrel, the eddy current inspection apparatus comprising: a pair of detection coils through which the steel pipe passes and which detects eddy currents generated in the steel pipe by applying an AC magnetic field to the steel pipe; and a flaw detector which detects defects present in the steel pipe based on a differential signal obtained by detecting the eddy currents with the pair of detection coils, the flaw detector being configured to perform eddy current inspection in advance using a plurality of calibration steel pipes each having different outer diameters within the outer diameter range of the steel pipe and different wall thicknesses within the wall thickness range, and each having artificial defects of the same dimensions. The present invention is also provided as an eddy current flaw detection device, characterized in that the relationship between the acquired induced voltage induced in one of the pair of detection coils and the magnitude of the defect signal, which is the differential signal corresponding to the artificial defect, is stored, the flaw detector stores the induced voltage detected in advance using one of the calibration steel pipes and has a pre-calibrated flaw detection sensitivity set, and the flaw detector executes a flaw detection sensitivity correction step of correcting the set flaw detection sensitivity based on the induced voltage detected in the steel pipe, the induced voltage stored in the calibration steel pipe, and the relationship, and a defect detection step of detecting defects present in the steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. Effect of the Invention

[0023] According to the present invention, even if the wall thickness of an electric resistance welded steel pipe varies within a predetermined thickness range in the pipe making process, accurate eddy current inspection is possible without calibrating the inspection sensitivity for each different wall thickness. Also, according to the present invention, even if the outer diameter and wall thickness of a steel pipe that has been subjected to a cold drawing process vary within a predetermined outer diameter range and a predetermined wall thickness range, respectively, accurate eddy current inspection is possible without calibrating the inspection sensitivity for each different outer diameter and wall thickness. [Brief description of the drawings]

[0024] [Figure 1]1 is a diagram showing a schematic configuration of an eddy current flaw detector 100 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] FIG. 1 is a diagram showing an example of the relationship between the wall thickness, induced voltage, and the magnitude of a defect signal of a calibration electric resistance welded steel pipe made of stainless steel. [Figure 4] This figure shows an example of test results for a calibration electric-welded steel pipe made of ferritic stainless steel with an outer diameter of 48.6 mm and a wall thickness of 0.8 mm, and which has an artificial defect that is a through hole with a diameter of 1.5 mm extending in the wall thickness direction, determining the relationship between the magnetic saturation current and the magnitude of the defect signal. [Diagram 5] 10 is an explanatory diagram for explaining that the magnitude of a defect signal decreases as the induced voltage induced in a detection coil increases. FIG. [Figure 6] FIG. 2 is a diagram illustrating a schematic configuration of an eddy current flaw detector 100A according to a second embodiment of the present invention. [Figure 7] 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. [Figure 8] FIG. 1 is a diagram showing an example of the relationship between the wall thickness, induced voltage, and the magnitude of a defect signal of a calibration electric resistance welded steel pipe made of carbon steel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] Hereinafter, an eddy current flaw detection device according to embodiments (first to third embodiments) of the present invention and an eddy current flaw detection method using the same will be described with appropriate reference to the attached drawings. In addition, the majority of electric resistance welded steel pipes manufactured in the pipe-making process are made of carbon steel, but it is also possible to manufacture stainless steel.

[0026] First Embodiment [Eddy current flaw detection equipment] Fig. 1 is a diagram showing a schematic configuration of an eddy current flaw detector 100 according to a first embodiment of the present invention. 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. The eddy current inspection device 100 according to the first embodiment is an apparatus for eddy current inspection of an electric-resistance welded steel pipe P conveyed in the longitudinal direction of the thick arrow shown in FIG. 1 in a pipe-making process in which electric-resistance welded steel pipes (quality-guaranteed electric-resistance welded steel pipes) P having a wall thickness ranging over a predetermined wall thickness range are manufactured by butting together the ends of plate materials (hoop materials) formed into a tubular shape and welding them together. A preferred steel type of the electric-resistance welded steel pipe P to be eddy current inspected by the eddy current inspection device 100 according to the first embodiment can be stainless steel. The electric-resistance welded steel pipe P to be eddy current inspected by the eddy current inspection device 100 is manufactured continuously by joining the rear end of the preceding hoop material and the front end of the following hoop material by intermediate welding. By intermediate welding hoop materials having different thicknesses, electric-resistance welded steel pipes P having a wall thickness ranging over a predetermined wall thickness range are manufactured continuously. The eddy current inspection device 100 is arranged, for example, on the outlet side of a welding device installed in a pipe-making line that performs a pipe-making process.

[0027] As shown in FIG. 1, the eddy current flaw detector 100 includes a through-type coil (1, 2) through which an electric resistance welded steel pipe P passes, and a flaw detector 3 connected to the through-type coil (1, 2). The through-type coil (1, 2) 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) is a self-comparison type coil that includes a pair of detection coils 2a and 2b as the detection coil 2 and outputs a differential signal of each of the detection coils 2a and 2b. In the example shown in FIG. 1, the winding direction of the detection coil 2a and the winding direction of the detection coil 2b are the same, and the winding end of the detection coil 2a and the winding end of the detection coil 2b are connected in series to output an anti-phase sum signal (i.e., a differential signal). Although not shown in Fig. 1, the eddy current inspection device 100 is equipped with a through-type magnetic saturation coil commonly used in eddy current inspection, which is arranged to surround the electric resistance welded steel pipe P and the through-type coils (1, 2). By supplying a direct magnetic saturation current to the magnetic saturation coil, the electric resistance welded steel pipe P is magnetized to a magnetic saturation level during eddy current inspection.

[0028] As shown in FIG. 1, the flaw detector 3 includes an excitation circuit 31, a receiving circuit 32, a synchronous detection circuit 33, an amplifier 34, a defect determination unit 35, a voltage detection circuit 36, and a flaw detection sensitivity setting unit 37. The excitation circuit 31 supplies an AC excitation current to the excitation coil 1. As a result, an AC magnetic field extending in the longitudinal direction of the electric-resistance welded steel pipe P acts on the electric-resistance welded steel pipe P, generating an eddy current in the electric-resistance welded steel pipe P. The pair of detection coils 2a, 2b detects eddy currents generated in the electric resistance welded steel pipe P and outputs a differential signal obtained by the detection to the receiving circuit 32. The receiving circuit 32 adjusts the received differential 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 differential 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 differential signal after synchronous detection to the amplifier 34. The synchronous detection circuit 33 has a phase adjustment function and adjusts the differential signal (defect signal) corresponding to a defect to a predetermined phase. The flaw detection sensitivity (amplification degree of the differential signal) is set in the amplifier 34 by a flaw detection sensitivity setting unit 37. The amplifier 34 amplifies the differential signal output from the synchronous detection circuit 33 according to the set flaw detection sensitivity, and outputs the amplified signal to the defect determination unit 35. A defect detection threshold is set in the defect determination unit 35. The defect determination unit 35 A / D converts the amplified differential signal output from the amplifier 34, and then compares the A / D converted differential signal with the defect detection threshold, and detects a differential signal that exceeds the defect detection threshold as a defect signal.

[0029] 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 is equipped with a voltage detection circuit 36, and the voltage detection circuit 36 ​​detects the induced voltage induced in one of the pair of detection coils 2a, 2b (detection coil 2b in the example shown in Figure 1). In addition, the flaw detector 3 of the first embodiment is characterized in that the flaw detection sensitivity setting unit 37 corrects the pre-calibrated flaw detection sensitivity using the induced voltage detected by the voltage detection circuit 36, and sets the corrected flaw detection sensitivity in the amplifier 34. The specific operations of the voltage detection circuit 36 ​​and the flaw detection sensitivity setting unit 37 will be described later in the description of the eddy current flaw detection method.

[0030] [Eddy current testing method] Hereinafter, an eddy current flaw detection method using the eddy current flaw detection device 100 having the above 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 flaw detection method according to the first embodiment includes a relationship obtaining step ST11, a flaw detection sensitivity calibrating step ST12, a flaw detection sensitivity correcting step ST13, and a defect detecting step ST14. Each of steps ST11 to ST14 will now be described in detail.

[0031] (Relationship acquisition step ST11) In the relationship acquisition step ST11, a number of calibration electric-welded steel pipes are prepared, each having different wall thicknesses within the wall thickness range of the electric-welded steel pipe P (for example, 0.8 mm to 2.2 mm for stainless steel) and each having artificial defects of the same dimensions (artificial defects having the same cross-sectional dimensions in a direction perpendicular to the wall thickness direction of the calibration electric-welded steel pipe), and each calibration electric-welded steel pipe is placed in sequence in place of the electric-welded steel pipe P in Figure 1. Then, each calibration electric-resistance welded steel pipe is transported in the longitudinal direction, and the induced voltage induced in the detection coil 2b (the induced voltage induced when a portion of the calibration electric-resistance welded steel pipe where no artificial defect is provided passes through the pair of detection coils 2a, 2b) is detected by the voltage detection circuit 36. Also, the magnitude of the defect signal, which is a differential signal (differential signal output from the amplifier 34) corresponding to the artificial defect, is detected by the defect determination unit 35. Then, the relationship between the induced voltage and the magnitude of the defect signal is obtained.

[0032] Fig. 3 is a diagram showing an example of the relationship between the wall thickness of a calibration electric-resistance welded steel pipe made of stainless steel, the induced voltage, and the magnitude of a defect signal. Fig. 3(a) is a diagram showing an example of the relationship between the wall thickness of a calibration electric-resistance welded steel pipe and the magnitude of a defect signal. Fig. 3(b) is a diagram showing an example of the relationship between the wall thickness of a calibration electric-resistance welded steel pipe and the induced voltage. Fig. 3(c) is a diagram showing an example of the relationship between the induced voltage of a calibration electric-resistance welded steel pipe and the magnitude of a defect signal. The relationship shown in Figure 3 is the result obtained under the following conditions: all of the multiple calibration electric-resistance welded steel pipes had an outer diameter of 48.6 mm, all of the artificial defects were through holes with a diameter of 1.5 mm extending in the thickness direction, a DC magnetic saturation current of 0.5 A was supplied to the aforementioned magnetic saturation coil, and the amplifier 34 was set to a calibrated flaw detection sensitivity so that the magnitude of the defect signal of a calibration electric-resistance welded steel pipe with a wall thickness of 0.8 mm was 4 V.

[0033] As shown in Fig. 3(a), there is a good correlation between the wall thickness of the calibration electric resistance welded steel pipe and the magnitude of the defect signal. Specifically, the larger the wall thickness of the calibration electric resistance welded steel pipe, the larger the magnitude of the defect signal. In addition, as shown in Fig. 3(b), there is a good correlation between the wall thickness of the electric resistance welded steel pipe for calibration and the induced voltage. Specifically, the greater the wall thickness of the electric resistance welded steel pipe for calibration, the smaller the induced voltage. Therefore, as shown in FIG. 3(c), there is a good correlation between the induced voltage and the magnitude of the defect signal, and as the induced voltage increases, the magnitude of the defect signal decreases. In the above explanation, the relationship between the three parameters of wall thickness, induced voltage, and magnitude of the defect signal has been described, but in the relationship acquisition step ST11, it is sufficient to acquire at least the relationship between the induced voltage and the magnitude of the defect signal as shown in FIG. 3(c). This relationship may be expressed, for example, by an approximation such as a quadratic function, or may be expressed in a table format. Then, in the relationship acquisition step ST11, the acquired relationship is stored in the flaw detection sensitivity setting unit 37. At this time, the induced voltage detected for one calibration electric-resistance welded steel pipe to be used in the flaw detection sensitivity calibration step ST12 described later, among the multiple calibration electric-resistance welded steel pipes used in the relationship acquisition step ST11, is also stored in the flaw detection sensitivity setting unit 37.

[0034] Before explaining the flaw detection sensitivity calibration step ST12, the inventors will explain their findings regarding why the magnitude of the defect signal increases as the thickness of the calibration electric-welded steel pipe increases, as shown in Figure 3(a) (and therefore why the magnitude of the defect signal increases as the thickness of the electric-welded steel pipe P increases), and why the magnitude of the defect signal decreases as the induced voltage increases, as shown in Figure 3(c).

[0035] It is known that the properties of magnetic materials such as electric-resistance welded steel pipe P are generally represented by a BH curve, with the horizontal axis representing the magnetic field strength H and the vertical axis representing the magnetic flux density B. When performing eddy current testing on magnetic materials such as electric-resistance welded steel pipe P, in order to suppress partial variations in the magnetic properties, it is common to magnetize the magnetic material to the magnetic saturation level by supplying a DC magnetic saturation current to the magnetic saturation coil, as described above. The inventors inferred that, for example, when the inspection sensitivity is calibrated using a calibration electric-welded steel pipe having the minimum value (e.g., 0.8 mm) of the wall thickness range of the electric-welded steel pipe P, if the wall thickness of the electric-welded steel pipe P to be inspected with eddy current after calibration becomes greater than the wall thickness of the calibration electric-welded steel pipe, the magnetic flux density in the calibration electric-welded steel pipe will decrease to the magnetic unsaturation level on the BH curve, causing the magnetic permeability of the electric-welded steel pipe P to increase, and as a result, the defect signal will become larger.

[0036] Specifically, the inventors used a calibration electric-resistance welded steel pipe and conducted tests to change the value of the magnetic saturation current supplied to the magnetic saturation coil in order to change the magnetic flux density within the calibration electric-resistance welded steel pipe, and to investigate the effect of the magnetic saturation current on the magnitude of the defect signal. FIG. 4 is a diagram showing an example of a test result of determining the relationship between the magnetic saturation current and the magnitude of the defect signal for a calibration electric resistance welded steel pipe made of ferritic stainless steel with an outer diameter of 48.6 mm and a thickness of 0.8 mm, and having an artificial defect that is a through hole with a diameter of 1.5 mm extending in the thickness direction. As shown in FIG. 4, it was found that the magnitude of the defect signal increases when the magnetic saturation current decreases. Therefore, it was inferred that if a magnetic saturation current of 0.5 A is supplied during calibration, when the thickness of the electric resistance welded steel pipe P to be subjected to eddy current inspection becomes larger than the thickness of the calibration electric resistance welded steel pipe, the magnetic flux density in the electric resistance welded steel pipe P decreases, as in the case of reducing the magnetic saturation current shown in FIG. 4 below 0.5 A, and the magnitude of the defect signal increases. Based on this inference, the inventors actually changed the thickness of the calibration electric resistance welded steel pipe to investigate the effect on the magnitude of the defect signal, and obtained the results shown in FIG. 3(a) described above.

[0037] FIG. 5 is an explanatory diagram for explaining that as the induced voltage induced in the detection coil increases, the magnitude of the defect signal decreases. As shown in Figure 5, if the induced voltage induced in one of the pair of detection coils 2a, 2b (detection coil 2b in the example shown in Figure 5) is Vcoil, the excitation voltage excited by the excitation coil 1 is Vt, and the induced voltage induced by the eddy current is Ve, it is considered that the following equation (1) holds. Vcoil = Vt - Ve (1) Here, when the excitation voltage Vt is constant, if the magnitude of the defect signal decreases, the induced voltage Ve also decreases, and therefore, according to the above formula (1), it is considered that the induced voltage Vcoil increases. Based on the above speculation, the inventors actually changed the wall thickness of the calibration electric-resistance welded steel pipe (changing the magnitude of the induced voltage Vcoil) and investigated the effect on the magnitude of the defect signal, and obtained the results shown in Figure 3(c) above.

[0038] (Flaw detection sensitivity calibration step ST12) In the flaw detection sensitivity calibration step ST12, the flaw detection sensitivity is calibrated using one calibration electric resistance welded steel pipe. Specifically, for example, the flaw detection sensitivity is calibrated so that the magnitude of a defect signal of a calibration electric resistance welded steel pipe with a wall thickness of 0.8 mm becomes 4 V. Then, in the flaw detection sensitivity calibration step ST12, the calibrated flaw detection sensitivity is stored in the flaw detection sensitivity setting unit 37 and set in the amplifier 34.

[0039] In the eddy current testing method of the first embodiment, the relationship acquisition step ST11 and the testing sensitivity calibration step ST12 described above are performed as preparatory steps using a calibration electric-welded steel pipe, and then the testing sensitivity correction step ST13 and the defect detection step ST14 are performed to perform eddy current testing on the electric-welded steel pipe P whose quality is actually guaranteed.

[0040] (Flaw detection sensitivity correction step ST13) The flaw detection sensitivity correction step ST13 is executed, for example, immediately after the thickness of the electric welded steel pipe P (referred to as the "electric welded steel pipe to be flaw detected" in Figure 2) changes (immediately after the portion corresponding to the tip of the following hoop material, which has a different thickness from the preceding hoop material, reaches the pair of detection coils 2a, 2b). In the flaw detection sensitivity correction step ST13, the induced voltage detected by the voltage detection circuit 36 ​​for the electric resistance welded steel pipe P is input to the flaw detection sensitivity setting unit 37. Then, the flaw detection sensitivity setting unit 37 corrects the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step ST12 based on the input induced voltage of the electric resistance welded steel pipe P, the induced voltage detected and stored for the calibration electric resistance welded steel pipe used in the flaw detection sensitivity calibration step ST12, and the relationship acquired and stored in the relationship acquisition step ST11.

[0041] For example, consider a case where the relationship (approximation formula) shown in Fig. 3(c) is stored in the flaw detection sensitivity setting unit 37, and an induced voltage of 19.5V in a calibration electric resistance welded steel pipe with a wall thickness of 0.8 mm is stored as the induced voltage in the calibration electric resistance welded steel pipe. Then, consider a case where an induced voltage of 18.7V in an electric resistance welded steel pipe P with a wall thickness of 2 mm is detected by the voltage detection circuit 36 ​​and input to the flaw detection sensitivity setting unit 37, the flaw detection sensitivity setting unit 37 calculates the magnitude of the defect signal corresponding to the induced voltage of 18.7V in the electric resistance welded steel pipe P as 8.4V from the relationship shown in Fig. 3(c). Also, the flaw detection sensitivity setting unit 37 calculates the magnitude of the defect signal corresponding to the induced voltage of 19.5V in the calibration electric resistance welded steel pipe as 4.3V from the relationship shown in Fig. 3(c). Then, the flaw detection sensitivity setting unit 37 corrects the flaw detection sensitivity calibrated and stored in the flaw detection sensitivity calibration step ST12 by 4.3 / 8.4=0.51 times. In other words, the flaw detection sensitivity setting unit 37 performs a correction to reduce the calibrated flaw detection sensitivity by 5.8 dB. Then, the flaw detection sensitivity setting unit 37 sets the corrected flaw detection sensitivity in the amplifier 34.

[0042] (Defect detection step ST14) In the defect detection step ST14, defects present in the electric resistance welded steel pipe P are detected using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step ST13. Specifically, the amplifier 34 amplifies the differential signal output from the synchronous detection circuit 33 in accordance with the set corrected flaw detection sensitivity, and outputs the amplified signal to the defect determination unit 35. Then, the defect determination unit 35 A / D converts the amplified differential signal output from the amplifier 34, and compares the A / D converted differential signal with a defect detection threshold, and detects a differential signal exceeding the defect detection threshold as a defect signal.

[0043] According to the eddy current inspection device 100 according to the first embodiment and the eddy current inspection method using the same described above, the inspection sensitivity calibrated in the inspection sensitivity calibration step ST12 is corrected based on the induced voltage detected in the electric resistance welded steel pipe P in the inspection sensitivity correction step ST13, the induced voltage detected in one calibration electric resistance welded steel pipe used in the inspection sensitivity calibration step ST12, and the relationship acquired in the relationship acquisition step ST11. By using this corrected inspection sensitivity, if a defect of the same size as an artificial defect in one calibration electric resistance welded steel pipe exists in the electric resistance welded steel pipe P, even if the wall thickness of one calibration electric resistance welded steel pipe and the wall thickness of the electric resistance welded steel pipe P are different, a defect signal of the same size as the artificial defect in the calibration electric resistance welded steel pipe is obtained, so that eddy current inspection can be performed with high accuracy. In addition, since there is no need to calibrate the flaw detection sensitivity for each different thickness of the electric welded steel pipe P (it is only necessary to correct the calibrated flaw detection sensitivity), there is no risk of a decrease in the efficiency of the pipe making process or a decrease in the yield of the electric welded steel pipe P.

[0044] <Second embodiment> [Eddy current flaw detection equipment] 6 is a diagram showing a schematic configuration of an eddy current flaw detector 100A according to a second embodiment of the present invention, in which an electric resistance welded steel pipe P and a through-type coil (excitation coil 1, detection coil 2) are shown in cross section. The eddy current inspection device 100A according to the second embodiment, like the eddy current inspection device 100 according to the first embodiment, is an apparatus for performing eddy current inspection on the electric-resistance welded steel pipe P transported in the longitudinal direction of the thick arrow shown in FIG. 6 in a pipe-making process in which electric-resistance welded steel pipe P (electric-resistance welded steel pipe with quality assurance) having a thickness ranging over a predetermined thickness range is manufactured by butting together the ends of plate material (hoop material) formed into a tubular shape and welding them together. A preferred steel type of the electric-resistance welded steel pipe P to be eddy-current inspected by the eddy current inspection device 100A according to the second embodiment is carbon steel. The electric-resistance welded steel pipe P to be eddy-current inspected by the eddy current inspection device 100A is manufactured continuously by joining the rear end of the preceding hoop material and the front end of the following hoop material by intermediate welding, like the electric-resistance welded steel pipe P to be eddy-current inspected by the eddy current inspection device 100 according to the first embodiment. By jointly welding hoop materials of different thicknesses, electric resistance welded steel pipes P having a thickness over a predetermined range rather than a constant thickness are continuously manufactured. As with the eddy current flaw detector 100 according to the first embodiment, the eddy current flaw detector 100A is disposed, for example, on the outlet side of a welding device installed in a pipe making line that carries out a pipe making process. Below, the eddy current flaw detection apparatus 100A of the second embodiment will be described, focusing mainly on the components that differ from the eddy current flaw detection apparatus 100 of the first embodiment. Components that are similar to those of the eddy current flaw detection apparatus 100 of the first embodiment will be given the same symbols, and duplicate explanations will be omitted as appropriate.

[0045] As shown in Fig. 6, the eddy current flaw detector 100A according to the second embodiment, like the eddy current flaw detector 100 according to the first embodiment, is provided with a through-type coil (1, 2) through which the electric-resistance welded steel pipe P passes, and a flaw detector 3A connected to the through-type coil (1, 2). Although not shown in Fig. 6, the eddy current flaw detector 100A is provided with a through-type magnetic saturation coil commonly used in eddy current flaw detector, which is arranged to surround the electric-resistance welded steel pipe P and the through-type coil (1, 2). The configuration of the through-type coil (1, 2) provided in the eddy current flaw detector 100A is the same as that in the first embodiment, but the configuration of the flaw detector 3A is different from that in the first embodiment.

[0046] As shown in Fig. 6, the flaw detector 3A includes an excitation circuit 31, a receiving circuit 32, a synchronous detection circuit 33, an amplifier 34, a defect determination unit 35, a voltage detection circuit 36, and a voltage determination unit 38. That is, the flaw detector 3A of the second embodiment is similar to the flaw detector 3 of the first embodiment in that it includes the excitation circuit 31, the receiving circuit 32, the synchronous detection circuit 33, the amplifier 34, the defect determination unit 35, and the voltage detection circuit 36. On the other hand, the flaw detector 3A of the second embodiment differs from the flaw detector 3 of the first embodiment in that it includes a voltage determination unit 38 instead of the flaw detection sensitivity setting unit 37 included in the flaw detector 3 of the first embodiment. The basic configuration of the flaw detector 3A is similar to that of a known flaw detector that is typically used for eddy current flaw detection. However, the flaw detector 3A of the second embodiment is characterized in that it includes a voltage detection circuit 36, which detects an induced voltage induced in one of the pair of detection coils 2a, 2b (detection coil 2b in the example shown in FIG. 6). This is similar to the voltage detection circuit 36 ​​included in the flaw detector 3 of the first embodiment. Furthermore, the flaw detector 3A of the second embodiment is characterized in that the voltage determination unit 38 determines whether or not the induced voltage detected by the voltage detection circuit 36 ​​is within the range of the stored induced voltages. The specific operation of the voltage determination unit 38 will be described later in the description of the eddy current flaw detection method.

[0047] [Eddy current testing method] Hereinafter, an eddy current flaw detection method using the eddy current flaw detection apparatus 100A having the above configuration will be described. FIG. 7 is a flowchart showing an outline of the procedure of an eddy current flaw detection method according to the second embodiment of the present invention. As shown in FIG. 7, the eddy current flaw detection method according to the second embodiment includes an induced voltage range identifying step ST21, a flaw detection sensitivity calibrating step ST22, a determination step ST23, a defect detecting step ST24, and an alarm signal output step ST25. Each of steps ST21 to ST25 will now be described in detail.

[0048] (Induced voltage range determination step ST21) In the induced voltage range identification step ST21, multiple electric-welded steel pipes for calibration are prepared, each having different wall thicknesses within the wall thickness range of the electric-welded steel pipe P (for example, 1 mm to 6 mm for carbon steel) and each having artificial defects of the same dimensions (artificial defects having the same cross-sectional dimensions in a direction perpendicular to the wall thickness direction of the electric-welded steel pipe for calibration), and each of the electric-welded steel pipes for calibration is placed in sequence in place of the electric-welded steel pipe P in Figure 6. Then, each calibration electric-resistance welded steel pipe is transported in the longitudinal direction, and the induced voltage induced in the detection coil 2b (the induced voltage induced when a portion of the calibration electric-resistance welded steel pipe without an artificial defect penetrates the pair of detection coils 2a, 2b) is detected by the voltage detection circuit 36. Furthermore, the magnitude of the defect signal, which is a differential signal (differential signal output from the amplifier 34) corresponding to the artificial defect, is detected by the defect determination unit 35. Then, the relationship between the induced voltage and the magnitude of the defect signal is obtained. Furthermore, the range of induced voltage within which the magnitude of the defect signal is within a predetermined range is identified.

[0049] Fig. 8 is a diagram showing an example of the relationship between the wall thickness of a calibration electric-resistance welded steel pipe made of carbon steel, the induced voltage, and the magnitude of a defect signal. Fig. 8(a) is a diagram showing an example of the relationship between the wall thickness of a calibration electric-resistance welded steel pipe and the magnitude of a defect signal. Fig. 8(b) is a diagram showing an example of the relationship between the wall thickness of a calibration electric-resistance welded steel pipe and the induced voltage. Fig. 8(c) is a diagram showing an example of the relationship between the induced voltage of a calibration electric-resistance welded steel pipe and the magnitude of a defect signal. The relationship shown in Figure 8 is the result obtained under the following conditions: the outer diameters of the multiple calibration electric-resistance welded steel pipes were either 31.8 mm, 38.1 mm, 42.7 mm, or 48.6 mm, all of the artificial defects were through holes with a diameter of 1.5 mm extending in the thickness direction, a DC magnetic saturation current of 1.0 A was supplied to the above-mentioned magnetic saturation coil, and the amplifier 34 was set to a calibrated flaw detection sensitivity so that the magnitude of the defect signal of the calibration electric-resistance welded steel pipe with a wall thickness of 1.2 mm was 4 V.

[0050] As shown in FIG. 8(a), there is a low correlation between the wall thickness of the calibration electric resistance welded steel pipe and the magnitude of the defect signal, and even if the wall thickness changes, there is little change in the magnitude of the defect signal. Furthermore, as shown in FIG. 8(b), the correlation between the wall thickness of the calibration electric resistance welded steel pipe and the induced voltage is low, and even if the wall thickness changes, there is only a small change in the induced voltage. Therefore, as shown in Fig. 8(c), the correlation between the induced voltage of the calibration electric resistance welded steel pipe and the magnitude of the defect signal is low, and even if the induced voltage changes, the magnitude of the defect signal changes little. Therefore, in the induced voltage range identification step ST21, the relationship between the induced voltage of the calibration electric resistance welded steel pipe and the magnitude of the defect signal is obtained, and the range of induced voltage in which the magnitude of the defect signal is within a predetermined range is identified. In the example shown in Fig. 8(c), the range of induced voltage is identified as 14.5V to 16.1V. In the above explanation, the relationship between the three parameters of the thickness, the induced voltage, and the magnitude of the defect signal has been described, but in the induced voltage range specifying step ST21, it is only necessary to obtain at least the relationship between the induced voltage and the magnitude of the defect signal as shown in Fig. 8(c) and specify the range of the induced voltage in which the magnitude of the defect signal is within a predetermined range. Then, in the induced voltage range specifying step ST21, the specified range of the induced voltage is stored in the voltage judgment unit 38.

[0051] (Flaw detection sensitivity calibration step ST22) In the flaw detection sensitivity calibrating step ST22, the flaw detection sensitivity is calibrated using one calibration electric resistance welded steel pipe, as in the flaw detection sensitivity calibrating step ST12 in the first embodiment. Specifically, the flaw detection sensitivity is calibrated so that the magnitude of a defect signal of a calibration electric resistance welded steel pipe with a wall thickness of 1.2 mm becomes 4 V, for example. Then, in the flaw detection sensitivity calibrating step ST22, the calibrated flaw detection sensitivity is set in the amplifier 34. Note that in the second embodiment, unlike the first embodiment, the flaw detection sensitivity set in the amplifier 34 is not corrected even if the wall thickness of the electric resistance welded steel pipe P changes (the calibrated flaw detection sensitivity is used as is).

[0052] In the eddy current inspection method of the second embodiment, the induced voltage range identification step ST21 and the inspection sensitivity calibration step ST22 described above are performed as preparatory steps using a calibration electric-welded steel pipe, and then a judgment step ST23, a defect detection step ST24 and an alarm signal output step ST25 are performed to perform eddy current inspection on the electric-welded steel pipe P whose quality is actually guaranteed.

[0053] (Decision step ST23) Judgment step ST23 is executed, for example, immediately after the thickness of the electric welded steel pipe P (referred to as the "electric welded steel pipe to be inspected" in Figure 7) changes (immediately after the portion corresponding to the tip of the following hoop material, which has a different thickness from the preceding hoop material, reaches a pair of detection coils 2a, 2b). In judgment step ST23, the induced voltage detected by the voltage detection circuit 36 ​​for the electric-resistance welded steel pipe P is input to the voltage judgment unit 38. Then, the voltage judgment unit 38 judges whether the input induced voltage of the electric-resistance welded steel pipe P is within the range of the stored induced voltages. For example, if the induced voltage range of 14.5 V to 16.1 V shown in Figure 8 (c) is stored in the voltage judgment unit 38, and an induced voltage of 15.9 V is detected by the voltage detection circuit 36 ​​for an electric welded steel pipe P with a thickness of 2.3 mm and input to the voltage judgment unit 38, the voltage judgment unit 38 will determine that the input induced voltage of the electric welded steel pipe P is within the stored range of induced voltages.

[0054] (Defect detection step ST24) In the defect detection step ST24, if it is determined in the determination step ST23 that the induced voltage detected in the electric resistance welded steel pipe P is within the range of the induced voltage identified in the induced voltage range identification step ST21 (if the determination step ST23 in FIG. 7 is "Yes"), a defect existing in the electric resistance welded steel pipe P is detected using the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step ST22. Specifically, the amplifier 34 amplifies the differential signal output from the synchronous detection circuit 33 according to the calibrated and set flaw detection sensitivity, and outputs the amplified signal to the defect determination unit 35. Then, the defect determination unit 35 A / D converts the amplified differential signal output from the amplifier 34, and then compares the A / D converted differential signal with a defect detection threshold, and detects a differential signal exceeding the defect detection threshold as a defect signal.

[0055] (Alarm signal output step ST25) In the warning signal output step ST25, if it is determined in the determination step ST23 that the induced voltage detected in the electric resistance welded steel pipe P is not within the induced voltage range identified in the induced voltage range identification step ST21 (if the determination step ST23 in FIG. 7 is "No"), the voltage determination unit 38 outputs a predetermined warning signal such as an alarm sound. If the induced voltage detected in the electric resistance welded steel pipe P is not within the identified induced voltage range, it is considered that some abnormality has occurred, and therefore, if a warning signal is output, it is possible to take measures such as interrupting the pipe making process and inspecting the detection coil 2 for abnormalities or recalibrating the flaw detection sensitivity using a calibration electric resistance welded steel pipe, as necessary.

[0056] According to the eddy current inspection device 100A according to the second embodiment and the eddy current inspection method using the same described above, it is determined whether or not the induced voltage detected in the electric current welded steel pipe P is within the range of induced voltages identified in the induced voltage range identification step ST21. If it is determined that the induced voltage detected in the electric current welded steel pipe P is within the range of the identified induced voltages, even if a defect of the same size as the artificial defect in one electric current welded steel pipe for calibration is present in the electric current welded steel pipe P and the wall thickness of one electric current welded steel pipe for calibration is different from the wall thickness of the electric current welded steel pipe P, a defect signal of the same size as the artificial defect in the electric current welded steel pipe for calibration can be obtained by using the calibrated inspection sensitivity as it is, so that eddy current inspection can be performed with high accuracy. Furthermore, if the induced voltage detected for the electric welded steel pipe P is determined to be within the specified induced voltage range, there is no need to calibrate the flaw detection sensitivity for each different thickness of the electric welded steel pipe P (it is sufficient to simply use the calibrated flaw detection sensitivity as is), so there is no risk of a decrease in the efficiency of the pipe making process or a decrease in the yield of the electric welded steel pipe P.

[0057] In the above description, the eddy current inspection device 100 according to the first embodiment and the eddy current inspection device 100A according to the second embodiment are described as separate eddy current inspection devices, but the present invention is not limited to this. It is also possible to adopt a single eddy current inspection device having components of both the eddy current inspection devices 100 and 100A, and to use this eddy current inspection device to perform the eddy current inspection method. Specifically, for example, when the steel type of the electric welded steel pipe P is stainless steel, the components of the eddy current flaw detection apparatus 100 (excitation coil 1, detection coil 2, excitation circuit 31, receiving circuit 32, synchronous detection circuit 33, amplifier 34, defect judgment unit 35, voltage detection circuit 36, flaw detection sensitivity setting unit 37) are used to execute the above-mentioned eddy current flaw detection method (steps ST11 to ST14), and when the steel type of the electric welded steel pipe P is carbon steel, the components of the eddy current flaw detection apparatus 100A (excitation coil 1, detection coil 2, excitation circuit 31, receiving circuit 32, synchronous detection circuit 33, amplifier 34, defect judgment unit 35, voltage detection circuit 36, voltage judgment unit 38) are used to execute the above-mentioned eddy current flaw detection method (steps ST21 to ST25). In this way, it is also possible to adopt an embodiment in which both components are used depending on the steel type of the electric welded steel pipe P.

[0058] In the above description, the eddy current inspection is performed on an electric welded steel pipe P whose steel type is stainless steel using the eddy current inspection device 100 according to the first embodiment, and the eddy current inspection is performed on an electric welded steel pipe P whose steel type is carbon steel using the eddy current inspection device 100A according to the second embodiment. However, the present invention is not limited to this. It is also possible to select an eddy current inspection device to be used according to the relationship between the induced voltage and the magnitude of the defect signal for each steel type, for example, by investigating the relationship between the induced voltage induced in the detection coil and the magnitude of the defect signal in advance, and performing eddy current inspection using the eddy current inspection device 100 according to the first embodiment for an electric welded steel pipe P whose steel type has a good correlation between the two, and performing eddy current inspection using the eddy current inspection device 100A according to the second embodiment for an electric welded steel pipe P whose steel type has a tendency to have little change in the induced voltage and the magnitude of the defect signal even when the wall thickness changes.

[0059] <Third embodiment> The eddy current inspection apparatus and eddy current inspection method of the third embodiment differ from the eddy current inspection apparatus 100 and eddy current inspection method of the first embodiment in that the object for which eddy current inspection is performed is not an electric welded steel pipe P in a pipe making process, but steel pipes having various outer diameters and wall thicknesses after a cold drawing process; the other configurations are basically the same as those of the first embodiment. Hereinafter, the eddy current inspection device and eddy current inspection method according to the third embodiment will be described mainly in terms of differences from the eddy current inspection device and eddy current inspection method according to the first embodiment, and explanations of similarities will be omitted as appropriate. In addition, for the eddy current inspection device and eddy current inspection method according to the third embodiment, Fig. 1 and Fig. 2 referred to in the explanation of the first embodiment will be used, and components similar to those of the eddy current inspection device 100 according to the first embodiment and steps similar to those of the eddy current inspection method according to the first embodiment will be described using the same reference numerals.

[0060] [Eddy current flaw detection equipment] Like the eddy current flaw detector 100 according to the first embodiment, the eddy current flaw detector according to the third embodiment also includes a through-type coil (1, 2) through which a steel pipe P passes, and a flaw detector 3 connected to the through-type coil (1, 2) (see FIG. 1). In addition, the eddy current flaw detector includes a through-type magnetic saturation coil commonly used in eddy current flaw detectors, which is disposed so as to surround the steel pipe P and the through-type coil (1, 2). The flaw detector 3 of the third embodiment, like the first embodiment, includes an excitation circuit 31, a receiving circuit 32, a synchronous detection circuit 33, an amplifier 34, a defect determination unit 35, a voltage detection circuit 36, and a flaw detection sensitivity setting unit 37 (see FIG. 1). Like the first embodiment, the flaw detection sensitivity setting unit 37 of the third embodiment also uses an induced voltage detected by the voltage detection circuit 36 ​​to correct the flaw detection sensitivity calibrated in advance, and sets the corrected flaw detection sensitivity in the amplifier 34. However, unlike the eddy current flaw detection apparatus 100 of the first embodiment, the eddy current flaw detection apparatus of the third embodiment is arranged, for example, at the outlet side of a straightening machine that straightens bends, etc., of a steel pipe (e.g., an electric resistance welded steel pipe) P after it has been cold drawn in a cold drawing process.

[0061] [Eddy current testing method] The eddy current inspection method of the third embodiment, like the eddy current inspection method of the first embodiment, also includes a relationship acquisition step ST11, a flaw detection sensitivity calibration step ST12, a flaw detection sensitivity correction step ST13, and a defect detection step ST14 (see Figure 2). However, in the relationship acquisition step ST11 of the third embodiment, unlike the first embodiment, a plurality of calibration steel pipes are prepared, each having different outer diameters within the outer diameter range of steel pipe P (e.g., 31.8 mm to 48.6 mm) and different wall thicknesses within the wall thickness range (e.g., 1.2 mm to 2.3 mm), and each having artificial defects of the same dimensions (artificial defects having the same cross-sectional dimensions in a direction perpendicular to the wall thickness direction of the calibration steel pipe), and each calibration steel pipe is placed in sequence in place of steel pipe P at the position of steel pipe P in Figure 1. Then, each calibration steel pipe is transported in the longitudinal direction, and the induced voltage induced in the detection coil 2b (the induced voltage induced when a portion of the calibration steel pipe where no artificial defect is provided penetrates the pair of detection coils 2a, 2b) is detected by the voltage detection circuit 36. Also, the magnitude of the defect signal, which is a differential signal corresponding to the artificial defect (differential signal output from the amplifier 34), is detected by the defect determination unit 35. Then, the relationship between the induced voltage and the magnitude of the defect signal is obtained.

[0062] Although details are omitted, as in the first embodiment, there is a good correlation between the induced voltage and the magnitude of the defect signal, and the larger the induced voltage, the smaller the magnitude of the defect signal. This relationship may be expressed, for example, by an approximation such as a quadratic function, or in table format. Then, in the relationship acquisition step ST11, the acquired relationship is stored in the flaw detection sensitivity setting unit 37. At this time, the induced voltage detected for one calibration electric-resistance welded steel pipe used in the flaw detection sensitivity calibration step ST12, among the multiple calibration electric-resistance welded steel pipes used in the relationship acquisition step ST11, is also stored in the flaw detection sensitivity setting unit 37.

[0063] The flaw detection sensitivity calibration step ST12, the flaw detection sensitivity correction step ST13, and the defect detection step ST14 in the third embodiment are similar to those in the first embodiment, except that the target is the steel pipe P after the cold drawing process.

[0064] According to the eddy current flaw detection device according to the third embodiment and the eddy current flaw detection method using the same described above, the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step ST12 is corrected based on the induced voltage detected for the steel pipe P in the flaw detection sensitivity correction step ST13, the induced voltage detected for one of the calibration steel pipes used in the flaw detection sensitivity calibration step ST12, and the relationship acquired in the relationship acquisition step ST11. By using this corrected flaw detection sensitivity, if a defect of the same size as an artificial defect in one of the calibration steel pipes exists in the steel pipe P, even if the outer diameter and wall thickness of one of the calibration steel pipes are different from the outer diameter and wall thickness of the steel pipe P, a defect signal of the same size as the artificial defect in the calibration steel pipe is obtained, so that eddy current flaw detection can be performed with high accuracy. In addition, since it is not necessary to calibrate the flaw detection sensitivity for each different outer diameter and wall thickness of the steel pipe P (it is only necessary to correct the calibrated flaw detection sensitivity), there is no risk of the efficiency of the cold drawing process decreasing or the yield of the steel pipe P decreasing.

[0065] In the above explanation, the subject of eddy current inspection is a steel pipe P having various outer diameters and wall thicknesses after the cold drawing process, but this is not necessarily limited to this. It is also possible to manufacture steel pipes P having the same outer diameter and various wall thicknesses (steel pipes having a wall thickness ranging over a predetermined thickness range) by the cold drawing process, and to use this steel pipe P as the subject of eddy current inspection. In this case, in the relationship acquisition step ST11 of the third embodiment, the relationship between the induced voltage and the magnitude of the defect signal is acquired using a plurality of calibration steel pipes, each having different wall thicknesses within the wall thickness range of the steel pipe P and each having artificial defects of the same dimensions, and in the flaw detection sensitivity calibration step ST12, the flaw detection sensitivity is calibrated using one of the plurality of calibration steel pipes. [Explanation of symbols]

[0066] 1. Excitation coil 2, 2a, 2b...Detection coil 3, 3A...Flaw detector 31...excitation circuit 32 Receiving circuit 33. Synchronous detection circuit 34 Amplifier 35 Defect judgment section 36 Voltage detection circuit 37....Flaw detection sensitivity setting section 38 Voltage judgment unit 100, 100A...eddy current flaw detection device P...Erw steel pipe (steel pipe)

Claims

1. In a pipe-making process in which an electric-welded steel pipe having a thickness ranging over a predetermined thickness range is manufactured by butting together the ends of a plate material formed into a tubular shape and welding them, an eddy-current inspection method is provided for detecting defects in an electric-welded steel pipe based on a differential signal obtained by applying an AC magnetic field to the electric-welded steel pipe to detect eddy currents generated in the electric-welded steel pipe using a pair of detection coils through which the electric-welded steel pipe passes, the method comprising: a relationship acquisition step of acquiring a relationship between a voltage induced in one of the pair of detection coils and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect, using a plurality of calibration electric-resistance welded steel pipes each having different wall thicknesses within the wall thickness range of the electric-resistance welded steel pipe and each having artificial defects of the same dimensions; A flaw detection sensitivity calibration step of calibrating the flaw detection sensitivity using one of the calibration electric resistance welded steel pipes; a flaw detection sensitivity correction step of correcting the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the induced voltage detected in the electric resistance welded steel pipe, the induced voltage detected in the calibration electric resistance welded steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step; A defect detection step of detecting defects present in the electric resistance welded steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. The eddy current flaw detection method according to the present invention is characterized in that

2. In a pipe-making process in which an electric-welded steel pipe having a thickness ranging over a predetermined thickness range is manufactured by butting together the ends of a plate material formed into a tubular shape and welding them, an eddy-current inspection method is provided for detecting defects in an electric-welded steel pipe based on a differential signal obtained by applying an AC magnetic field to the electric-welded steel pipe to detect eddy currents generated in the electric-welded steel pipe using a pair of detection coils through which the electric-welded steel pipe passes, the method comprising: an induced voltage range specifying step of obtaining a relationship between an induced voltage induced in one of the pair of detection coils and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect, using a plurality of calibration electric-resistance welded steel pipes each having different wall thicknesses within the wall thickness range of the electric-resistance welded steel pipe and each having artificial defects of the same dimensions, and specifying a range of the induced voltage within which the magnitude of the defect signal is within a predetermined range; A flaw detection sensitivity calibration step of calibrating the flaw detection sensitivity using one of the calibration electric resistance welded steel pipes; a determination step of determining whether or not the induced voltage detected in the electric resistance welded steel pipe is within the range of the induced voltage specified in the induced voltage range specifying step; and a defect detection step of detecting defects present in the electric resistance welded steel pipe by using the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step when it is determined in the determination step that the induced voltage detected in the electric resistance welded steel pipe is within the range of the induced voltage specified in the induced voltage range specification step. The eddy current flaw detection method according to the present invention is characterized in that

3. In a pipe-making process in which electric-resistance welded steel pipes having a thickness ranging over a predetermined thickness range are manufactured by butting ends of tubular plate materials together and welding them, an eddy current inspection device is provided for inspecting the electric-resistance welded steel pipes, a pair of detection coils through which the electric resistance welded steel pipe passes and which detect eddy currents generated in the electric resistance welded steel pipe by applying an AC magnetic field to the electric resistance welded steel pipe; a flaw detector for detecting defects present in the electric resistance welded steel pipe based on a differential signal obtained by detecting an eddy current with the pair of detection coils, The flaw detector stores a relationship between a voltage induced in one of the pair of detector coils and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect, that is previously obtained using a plurality of calibration electric-resistance welded steel pipes each having different wall thicknesses within the wall thickness range of the electric-resistance welded steel pipe and each having artificial defects of the same dimensions, and The flaw detector stores the induced voltage detected in advance using one of the calibration electric resistance welded steel pipes, and a flaw detection sensitivity calibrated in advance is set in the flaw detector; The flaw detector includes: a flaw detection sensitivity correction step of correcting the set flaw detection sensitivity based on the induced voltage detected for the electric resistance welded steel pipe, the induced voltage stored for the calibration electric resistance welded steel pipe, and the relationship; A defect detection step of detecting defects present in the electric resistance welded steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. An eddy current flaw detection device characterized by:

4. In a pipe-making process in which electric-resistance welded steel pipes having a thickness ranging over a predetermined thickness range are manufactured by butting ends of tubular plate materials together and welding them, an eddy current inspection device is provided for inspecting the electric-resistance welded steel pipes, a pair of detection coils through which the electric resistance welded steel pipe passes and which detect eddy currents generated in the electric resistance welded steel pipe by applying an AC magnetic field to the electric resistance welded steel pipe; a flaw detector for detecting defects present in the electric resistance welded steel pipe based on a differential signal obtained by detecting an eddy current with the pair of detection coils, The flaw detector stores a range of induced voltages within a predetermined range, the range being determined based on a relationship between an induced voltage induced in one of the pair of detection coils, which is previously obtained using a plurality of calibration electric-resistance welded steel pipes each having different wall thicknesses within the wall thickness range of the electric-resistance welded steel pipe and each having artificial defects of the same dimensions, and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect; The flaw detector is set with a flaw detection sensitivity that is pre-calibrated using one of the calibration electric resistance welded steel pipes, The flaw detector includes: a determination step of determining whether or not the induced voltage detected in the electric resistance welded steel pipe is within a range of the induced voltage stored in memory; a defect detection step of detecting defects present in the electric resistance welded steel pipe using the set flaw detection sensitivity when it is determined in the determination step that the induced voltage detected in the electric resistance welded steel pipe is within a range of the stored induced voltages. An eddy current flaw detection device characterized by:

5. An eddy current inspection method for detecting defects present in a steel pipe, the method comprising: applying an AC magnetic field to a steel pipe having an outer diameter in a predetermined outer diameter range and a wall thickness in a predetermined wall thickness range, the steel pipe being subjected to a cold drawing process using a die and a plug or mandrel; detecting eddy currents generated in the steel pipe by using a pair of detection coils through which the steel pipe passes, the method comprising the steps of: a relationship acquisition step of acquiring a relationship between a voltage induced in one of the pair of detection coils and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect, using a plurality of calibration steel pipes each having different outer diameters within the outer diameter range of the steel pipe and different wall thicknesses within the wall thickness range, and each having artificial defects of the same dimensions; A flaw detection sensitivity calibration step of calibrating flaw detection sensitivity using one of the calibration steel pipes; A flaw detection sensitivity correction step of correcting the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the induced voltage detected in the steel pipe, the induced voltage detected in the calibration steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step; A defect detection step of detecting defects present in the steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. The eddy current flaw detection method according to the present invention is characterized in that

6. An eddy current inspection apparatus for performing eddy current inspection on a steel pipe having an outer diameter ranging over a predetermined outer diameter range and a wall thickness ranging over a predetermined wall thickness range, the apparatus comprising: a pair of detection coils through which the steel pipe passes and which detect eddy currents generated in the steel pipe by applying an AC magnetic field to the steel pipe; and a flaw detector for detecting defects present in the steel pipe based on a differential signal obtained by detecting an eddy current with the pair of detection coils, The flaw detector stores a relationship between a voltage induced in one of the pair of detector coils and a magnitude of a defect signal, which is the differential signal corresponding to the artificial defect, that is previously obtained using a plurality of calibration steel pipes each having different outer diameters within the outer diameter range of the steel pipe and different wall thicknesses within the wall thickness range, and each having artificial defects of the same dimensions; The flaw detector stores the induced voltage detected in advance using one of the calibration steel pipes, and a flaw detection sensitivity calibrated in advance is set in the flaw detector; The flaw detector includes: A flaw detection sensitivity correction step of correcting the set flaw detection sensitivity based on the induced voltage detected for the steel pipe, the induced voltage stored for the calibration steel pipe, and the relationship; A defect detection step of detecting defects present in the steel pipe using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. An eddy current flaw detection device characterized by:

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