Eddy current flaw detection method and device

Abstract

To provide an eddy current flaw detection device or the like, capable of accurately detecting flaw, without correcting sensitivity of flaw detection for each outer diameter and 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, a parameter PηL expressed by a formula (1) and magnitude of a defect signal being a differential signal corresponding to an artificial defect, the relationship acquired in advance, using multiple correction steel pipes. The flaw detection tool has flaw detection sensitivity which is corrected in advance using one correction steel pipe, the flaw detection tool executes: a flaw detection sensitivity correction step for, correcting the set flaw detection sensitivity on the basis of the relationship, a value of the parameter PηL calculated to the steel pipe P, the value of the parameter PηL calculated for the correction steel pipe used for correcting the flaw detection sensitivity; and a defect detection step for using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step, for detecting defect which is present on the steel pipe. The formula 1 is: PηL=a fill factor of the detection coil η×average peripheral length of the steel pipe L.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 a steel pipe. 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 without calibrating the inspection sensitivity for each different outer diameter and wall thickness, 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, respectively, due to the execution of a cold drawing process or the like. [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, conventionally, as described in Patent Document 1, for example, in the pipe making process, eddy current testing is generally performed to detect defects present in an electric-resistance welded steel pipe based on a differential signal obtained by applying an alternating magnetic field to the electric-resistance welded steel pipe to detect eddy currents generated in the electric-resistance welded steel pipe using a pair of detection coils through which the electric-resistance welded steel pipe passes. Furthermore, as described in Patent Document 1, for example, eddy current testing is also performed for quality assurance on steel pipes after the cold drawing process, in which defects present in the steel pipe are detected based on a differential signal obtained by applying an AC magnetic field to the steel pipe and detecting eddy currents generated in the steel pipe using a pair of detection coils through which the steel pipe passes.

[0006] 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 (amplification of the differential signal) is used, the change in the outer diameter or wall thickness may affect the flaw detection accuracy of the eddy current flaw detection. For this reason, for example, a method may be adopted in which the cold drawing process is interrupted every time the outer diameter or wall thickness changes, and the flaw detection sensitivity of the eddy current flaw detection is calibrated 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]

[0007] [Patent Document 1] Patent Publication No. 2023-000265 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made to solve the problems of the conventional technology as described above, and has an object to provide an eddy current flaw detection method and an eddy current flaw detection device that are capable of performing eddy current flaw detection with high accuracy, without having to calibrate the flaw detection sensitivity for each different outer diameter and wall thickness, 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, respectively, due to the cold drawing process or the like. [Means for solving the problem]

[0009] In order to solve the above problems, the inventors conducted extensive research and found that, for example, in steel pipes having various outer diameters and wall thicknesses after a cold drawing process, the parameter PηL, which is expressed by PηL = filling factor η of the detection coil × average circumference L of the steel pipe, and which is determined by the inner diameter of the detection coil and the outer diameter and wall thickness of the steel pipe, has a good correlation with the magnitude of a defect signal, which is a differential signal corresponding to a defect of the same dimension (having the same cross-sectional dimension in a direction perpendicular to the wall thickness direction of the steel pipe), and that the magnitude of the defect signal also increases as the value of the parameter PηL increases. Therefore, it was conceived that if the relationship between the parameter PηL and the magnitude of the defect signal, which is a differential signal corresponding to the artificial defect, is obtained in advance using calibration steel pipes having different outer diameters and different wall thicknesses and having artificial defects, then based on the value of the parameter PηL calculated for the steel pipe to be quality assured, the value of the parameter PηL calculated for one calibration steel pipe, and the previously obtained relationship, the inspection sensitivity calibrated using the one calibration steel pipe can be corrected to an appropriate inspection sensitivity according to the outer diameter and wall thickness of the steel pipe to be subjected to eddy current inspection, thereby enabling eddy current inspection to be performed with high accuracy.

[0010] The eddy current flaw detection method according to the present invention has been completed based on the above-mentioned findings of the present inventors. That is, in order to solve the above-mentioned problems, the present invention provides an eddy current inspection method for detecting defects present in a steel pipe, based on a differential signal obtained by 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, to detect eddy currents generated in the steel pipe using a pair of detection coils through which the steel pipe passes, and 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 parameter PηL represented by the following formula (1) and a defect differential signal corresponding to the artificial defects are detected. The present invention provides an eddy current flaw detection method, comprising: a relationship acquisition step of acquiring a relationship with a signal magnitude; 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 value of the parameter PηL calculated for the steel pipe, the value of the parameter PηL calculated for 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. PηL = filling rate η of the detection coil × average circumference L of the steel pipe (1)

[0011] In the 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 include other steel pipes such as seamless steel pipes. In the eddy current inspection method according to the present invention, the term "artificial defects of the same size" means artificial defects having the same cross-sectional size in a direction perpendicular to the wall thickness direction of the calibration steel pipe. In the case of eddy current inspection of steel pipes, through holes are often used as artificial defects. In addition, in the eddy current flaw detection method according to the present invention, the "filling factor η of the detector coil" that defines the parameter PηL is a value expressed by the following formula (2), where OD is the outer diameter of the steel pipe and ID is the inner diameter of the detector coil. η = (OD) 2 / (ID) 2 (2) Generally, a through-type detection coil includes a cylindrical bobbin and a winding wound around the outer surface of the bobbin, and the inner diameter ID of the detection coil in the above formula (2) means the inner diameter of the bobbin. However, the "filling rate η of the detector coil" is not limited to the above definition using the inner diameter ID of the detector coil. It is also possible to define the filling rate η of the detector coil as the value expressed by the following formula (2)' using the average diameter d of the detector coil (d = (outer diameter of the windings constituting the detector coil + inner diameter of the windings constituting the detector coil) / 2). η = (OD) 2 / d 2 ...(2)' In addition, in the eddy current inspection method according to the present invention, the "average circumference L of the steel pipe" defining the parameter PηL is a value expressed by the following formula (3), where OD is the outer diameter of the steel pipe, WT is the wall thickness of the steel pipe, and π is the circular constant. L = π (OD-WT) (3) Furthermore, in the eddy current flaw detection method of the present invention, the "relationship between the parameter PηL and the magnitude of the defect signal" acquired in the relationship acquisition step does not only mean the relationship between the parameter PηL itself and the magnitude of the defect signal, but is a concept that includes the relationship between the parameter PηL multiplied by a predetermined coefficient (e.g., 1 / π) and the magnitude of the defect signal. When the relationship acquired in the relationship acquisition step is the relationship between the parameter PηL itself and the magnitude of the defect signal, the flaw detection sensitivity correction step "corrects the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the value of the parameter PηL itself calculated for the steel pipe, the value of the parameter PηL itself calculated for the calibration steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step." When the relationship acquired in the relationship acquisition step is the relationship between the parameter PηL multiplied by a predetermined coefficient and the magnitude of the defect signal, the flaw detection sensitivity correction step "corrects the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the value of the parameter PηL calculated for the steel pipe multiplied by a predetermined coefficient, the value of the parameter PηL calculated for the calibration steel pipe used in the flaw detection sensitivity calibration step multiplied by the coefficient, and the relationship acquired in the relationship acquisition step." Furthermore, in the eddy current flaw detection method according to the present invention, the "flaw detection sensitivity" means the amplification degree of the differential signal. In addition, in the eddy current testing method of 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).

[0012] According to the eddy current flaw detection method of the present invention, in the relationship acquisition step, the relationship between the parameter PηL 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 steel pipe (using a calibration steel pipe having one outer diameter and wall thickness within the outer diameter and wall thickness range of the 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 a flaw detection sensitivity correction step, the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step is corrected based on the value of the parameter PηL calculated for the steel pipe (steel pipe to be quality-guaranteed), the value of the parameter PηL calculated for one calibration steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step. As a correction of this flaw detection sensitivity, for example, if the magnitude of the defect signal obtained by the value of the parameter PηL calculated for the steel pipe and the relationship is A, and the magnitude of the defect signal obtained by the value of the parameter PηL calculated for one calibration steel pipe and the relationship is B, it is considered to correct the calibrated flaw detection sensitivity by a factor of B / A. Finally, in the defect detection step, defects present in the steel pipe are detected using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step. According to the eddy current flaw detection method of the present invention, as described above, by using the flaw detection sensitivity corrected by B / A times 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 is obtained, so that eddy current flaw detection can be performed with high accuracy. In addition, if the relationship between the parameter PηL and the magnitude of the flaw signal is obtained using multiple calibration steel pipes and the value of the parameter PηL is calculated 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 a decrease in the yield of the steel pipe.

[0013] Furthermore, the inventors conducted extensive research and found that, although the correlation is somewhat lower than that of the aforementioned parameter PηL, the three parameters of the cross-sectional area S of the steel pipe, the filling factor η of the detection coil, and the filling factor η of the detection coil × the cross-sectional area S of the steel pipe also have a good correlation with the magnitude of the defect signal, and that as the values ​​of these parameters increase, the magnitude of the defect signal also increases. Therefore, in the relationship acquisition step, instead of the parameter PηL, a relationship between an alternative parameter, which is one of the cross-sectional area S of the steel pipe, the filling factor η of the detection coil, and the filling factor η of the detection coil x the cross-sectional area S of the steel pipe, and the magnitude of the defect signal is acquired, and in the flaw detection sensitivity correction step, it is also possible to correct the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step based on the value of the alternative parameter calculated for the steel pipe, the value of the alternative parameter calculated for the calibration steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step. According to the eddy current flaw detection method using the above alternative parameter, even if the outer diameter and wall thickness of the steel pipe vary within a specified outer diameter range and a specified wall thickness range, respectively, eddy current flaw detection can be performed with the same accuracy as the eddy current flaw detection method using the parameter PηL without having to calibrate the flaw detection sensitivity for each different outer diameter and wall thickness.

[0014] In the eddy current flaw detection method according to the present invention, the outer diameter of the steel pipe in which defects are detected in the defect detection step is preferably an outer diameter in which a filling factor η of the detection coil is 0.5 or more. If the filling factor η of the detection coil is large, such as 0.5 or more, the magnetic field strength of the AC magnetic field acting on the steel pipe becomes large, which is preferable in that the defect detection ability of eddy current testing becomes high.

[0015] The eddy current inspection method according to the present invention is suitably used in the case where the steel pipe in which defects are detected in the defect detection step is a steel pipe that has been subjected to a cold drawing process using a die and a plug or a mandrel. By applying the eddy current flaw detection method of the present invention to a steel pipe that has undergone a cold drawing process, 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, respectively, due to the cold drawing process, there is no need to calibrate the flaw detection sensitivity for each different outer diameter and wall thickness (it is sufficient to correct the calibrated flaw detection sensitivity), which has the advantage that there is no risk of a decrease in the efficiency of the cold drawing process.

[0016] Further, in order to solve the above-mentioned problems, the present invention provides an eddy current flaw detection device for performing eddy current flaw detection on a steel pipe having an outer diameter in a predetermined outer diameter range and a wall thickness in a predetermined wall thickness range, the device 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 including: a detection signal represented by the following formula (1) which is obtained 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 flaws of the same dimensions: The present invention is also provided as an eddy current flaw detection device, characterized in that a relationship between the parameter PηL calculated for the steel pipe and the magnitude of the defect signal, which is the differential signal corresponding to the artificial defect, is stored in the flaw detector, a flaw detection sensitivity that has been pre-calibrated using one of the calibration steel pipes is set in the flaw detector, and the flaw detector executes a flaw detection sensitivity correction step of correcting the set flaw detection sensitivity based on the value of the parameter PηL calculated for the steel pipe, the value of the parameter PηL calculated for the calibration steel pipe used to calibrate the flaw detection sensitivity, 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. PηL = filling rate η of the detection coil × average circumference L of the steel pipe (1) Effect of the Invention

[0017] According to the present invention, 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, respectively, due to the cold drawing process being performed, eddy current testing is possible with high accuracy without having to calibrate the testing sensitivity for each different outer diameter and wall thickness. [Brief description of the drawings]

[0018] [Figure 1] 1 is a diagram showing a schematic configuration of an eddy current flaw detector 100 according to an embodiment of the present invention. [Diagram 2] 1 is a flow chart showing a schematic procedure of an eddy current inspection method according to an embodiment of the present invention. [Diagram 3] FIG. 13 is a diagram showing an example of the relationship between a parameter PηL of a steel pipe and the magnitude of a defect signal. [Figure 4] 1 is a diagram showing an example of the relationship between three parameters, namely, the cross-sectional area S of a steel pipe, the filling factor η of a detection coil 2, and the filling factor η of the detection coil 2 × the cross-sectional area S of the steel pipe, and the magnitude of a defect signal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Below, with appropriate reference to the attached drawings, an eddy current testing device according to one embodiment of the present invention and an eddy current testing method using the same will be described, taking as an example a case in which the steel pipe to be quality assured is an electric welded steel pipe that has undergone a cold drawing process.

[0020] [Eddy current flaw detection equipment] Fig. 1 is a diagram showing a schematic configuration of an eddy current flaw detector 100 according to an embodiment of the present invention. In Fig. 1, a 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 this embodiment is an apparatus for performing eddy current inspection on a steel pipe P that has an outer diameter ranging over a predetermined outer diameter range and a wall thickness ranging over a predetermined wall thickness range by performing a cold drawing process using a die (not shown) and a plug or mandrel (not shown) and is transported in the longitudinal direction indicated by the thick arrow in Fig. 1. The eddy current inspection device 100 is disposed, for example, on the outlet side of a welding device installed in a pipe making line that performs a pipe making process. It is also disposed on the outlet side of a straightening machine that straightens bends, etc., in the steel pipe P after it has been cold drawn in the cold drawing process.

[0021] As shown in FIG. 1, the eddy current flaw detector 100 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). 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 flaw detection device 100 includes a through-type magnetic saturation coil commonly used in eddy current flaw detection, which is arranged to surround the steel pipe P and the through-type coils (1, 2). By supplying a DC magnetic saturation current to the magnetic saturation coil, the steel pipe P is magnetized to a magnetic saturation level during eddy current flaw detection.

[0022] 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 , a defect determination section 35, and a flaw detection sensitivity setting section . 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 steel pipe P acts on the steel pipe P, generating an eddy current in the steel pipe P. The pair of detection coils 2a, 2b detects the eddy current generated in the 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 36. 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.

[0023] 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 this embodiment is characterized in that the flaw detection sensitivity setting unit 36 ​​corrects the pre-calibrated flaw detection sensitivity using the value of the parameter PηL described below, and sets the corrected flaw detection sensitivity in the amplifier 34. The specific operation of the flaw detection sensitivity setting unit 36 ​​will be described later in the description of the eddy current flaw detection method.

[0024] [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 diagram showing an outline of the procedure of the eddy current flaw detection method according to this embodiment. As shown in FIG. 2, the eddy current flaw detection method according to this embodiment includes a relationship obtaining step ST1, a flaw detection sensitivity calibrating step ST2, a flaw detection sensitivity correcting step ST3, and a defect detecting step ST4. Each of steps ST1 to ST4 will now be described in detail.

[0025] (Relationship acquisition step ST1) In the relationship acquisition step ST1, a number 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 turn in place of steel pipe P in the position of steel pipe P in Figure 1. Then, each calibration steel pipe is transported in the longitudinal direction, and the magnitude of the defect signal, which is a differential signal corresponding to the artificial defect (differential signal output from amplifier 34), is detected by defect determination unit 35. Then, the relationship between the parameter PηL and the magnitude of the defect signal, which is expressed by the following formula (1), is obtained. PηL = filling rate η of detection coil 2 × average circumference of steel pipe L (1) In the above formula (1), the filling rate η of the detection coil 2 is a value expressed by the following formula (2), where OD is the outer diameter of the steel pipe and ID is the inner diameter of the detection coil 2 (the inner diameter of the bobbin that constitutes the detection coil 2). η = (OD) 2 / (ID) 2 (2) In addition, in the above formula (1), the average circumference L of the steel pipe is a value expressed by the following formula (3), where OD is the outer diameter of the steel pipe, WT is the wall thickness of the steel pipe, and π is the circular constant. L = π (OD-WT) (3) Therefore, the parameter PηL is a parameter that is determined by the inner diameter ID of the detection coil 2, the outer diameter OD and the wall thickness WT of the steel pipe.

[0026] Fig. 3 is a diagram showing an example of the relationship between the parameter PηL of a steel pipe and the magnitude of a defect signal. Fig. 3(a) is a diagram showing an example of the relationship between the parameter PηL of a calibration steel pipe and the magnitude of a defect signal. Fig. 3(b) is a diagram showing an example of the relationship between the parameter PηL of a steel pipe other than the calibration steel pipe shown in Fig. 3(a) and the magnitude of a defect signal. The relationship shown in Figure 3(a) was obtained by using five calibration steel tubes with combinations of outer diameter and wall thickness of (outer diameter, wall thickness) = (48.6 mm, 2.3 mm), (42.7 mm, 1.2 mm), (38.1 mm, 1.2 mm), (38.1 mm, 1.8 mm), and (31.8 mm, 2.3 mm), and by detecting the magnitude of the defect signal using two types of detection coils 2 with different inner diameters ID (ID = 53.6 mm, 47.7 mm) under eight conditions, No. 1 to No. 8, shown in Table 1 below, while supplying a DC magnetic saturation current of 1.0 A to the magnetic saturation coil described above. In addition, when obtaining the results shown in Figure 3(a), the amplifier 34 was set to a calibrated flaw detection sensitivity so that the magnitude of the defect signal of the calibration steel pipe having an outer diameter OD = 38.1 mm and a wall thickness WT = 1.2 mm, and the magnitude of the defect signal of the calibration steel pipe having an outer diameter OD = 31.8 mm and a wall thickness WT = 2.3 mm were each 4 V, as shown in No. 3 and No. 8 in Table 1. [Table 1]

[0027] As shown in Fig. 3(a), the parameter PηL and the magnitude of the defect signal have a good correlation (the coefficient of determination R 2=0.94). Specifically, the larger the value of the parameter PηL, the larger the magnitude of the defect signal. The relationship as shown in FIG. 3(a) may be expressed, for example, by an approximation such as a quadratic function, or in table format. Then, in the relationship acquisition step ST1, the acquired relationship is stored in the flaw detection sensitivity setting unit 36. At this time, the value of the parameter PηL for one calibration steel pipe to be used in the flaw detection sensitivity calibration step ST2 described later, among the multiple calibration steel pipes used in the relationship acquisition step ST1, is also stored in the flaw detection sensitivity setting unit 36.

[0028] (Flaw detection sensitivity calibration step ST2) In the flaw detection sensitivity calibration step ST2, the flaw detection sensitivity is calibrated using one calibration steel pipe. Specifically, for example, the flaw detection sensitivity is calibrated so that the magnitude of the defect signal of the calibration steel pipe having an outer diameter OD=31.8 mm and a wall thickness WT=2.3 mm shown in No. 8 of Table 1 above becomes 4 V. Then, in the flaw detection sensitivity calibration step ST2, the calibrated flaw detection sensitivity is stored in the flaw detection sensitivity setting unit 36 ​​and set in the amplifier 34.

[0029] In the eddy current flaw detection method of this embodiment, the relationship acquisition step ST1 and flaw detection sensitivity calibration step ST2 described above are performed as preparatory steps using a steel pipe for calibration, and then the flaw detection sensitivity correction step ST3 and defect detection step ST4 are performed to perform eddy current flaw detection on the steel pipe P whose quality is actually to be assured.

[0030] (Flaw detection sensitivity correction step ST3) The flaw detection sensitivity correction step ST3 is executed, for example, before the tip of the steel pipe P to be subjected to eddy current flaw detection (indicated as "tested steel pipe" in FIG. 2) reaches the pair of detection coils 2a, 2b. In the flaw detection sensitivity correction step ST3, the inner diameter ID of the detection coil 2 to be used in the upcoming eddy current flaw detection, and the outer diameter OD and wall thickness WT of the steel pipe P to be subjected to eddy current flaw detection are input to the flaw detection sensitivity setting unit 36 ​​(see FIG. 1). Based on the input inner diameter ID of the detection coil 2, the outer diameter OD and wall thickness WT of the steel pipe P, the flaw detection sensitivity setting unit 36 ​​calculates the value of the parameter PηL for the steel pipe P by the above-mentioned formulas (1) to (3). Then, the flaw detection sensitivity setting unit 36 ​​corrects the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step ST2 based on the value of the parameter PηL calculated for the steel pipe P, the value of the parameter PηL calculated and stored for the calibration steel pipe used in the flaw detection sensitivity calibration step ST2, and the relationship acquired and stored in the relationship acquisition step ST1.

[0031] For example, when the relationship (approximation) shown in FIG. 3(a) is set in the flaw detection sensitivity setting unit 36 ​​as Y=0.0097X 2 Consider a case where the parameter PηL for the calibration steel pipe is stored as a quadratic function represented by -1.0004X+30.407. Also consider a case where the parameter PηL for the calibration steel pipe, which is 41.2 mm, is the value of the parameter PηL for the calibration steel pipe having the inner diameter ID of the detection coil 2 of 47.7 mm, the outer diameter OD of 31.8 mm, and the wall thickness WT of 2.3 mm, as shown in No. 8 in Table 1 above, is stored in the flaw detection sensitivity setting unit 36. Then, when considering the case where the inner diameter ID of the detection coil 2 shown in No. 9 of Table 2 below, the outer diameter OD of the steel pipe P, and the wall thickness WT of the steel pipe P are input to the flaw detection sensitivity setting unit 36 ​​and the parameter PηL is calculated as 70.8 mm, the flaw detection sensitivity setting unit 36 ​​calculates the magnitude of the defect signal corresponding to the parameter PηL=70.8 mm for the steel pipe P as 8.20 V from the relationship (approximation formula) shown in FIG. 3(a). Also, the flaw detection sensitivity setting unit 36 ​​calculates the magnitude of the defect signal corresponding to the parameter PηL=41.2 mm for the calibration steel pipe as 5.66 V from the relationship (approximation formula) shown in FIG. Then, the flaw detection sensitivity setting unit 36 ​​corrects the flaw detection sensitivity calibrated and stored in the flaw detection sensitivity calibration step ST2 to 5.66 / 8.20=0.69 times. In other words, the flaw detection sensitivity setting unit 36 ​​performs a correction to reduce the calibrated flaw detection sensitivity by 3.2 dB. Then, the flaw detection sensitivity setting unit 36 ​​sets the corrected flaw detection sensitivity in the amplifier 34. In addition, for another steel pipe P, the inner diameter ID of the detection coil 2 shown in No. 12 of Table 2 below, the outer diameter OD of the steel pipe P shown in No. 12 of Table 2, and the wall thickness WT of the steel pipe P shown in No. 12 of Table 2 below are input to the flaw detection sensitivity setting unit 36, and the parameter PηL = 91.9 mm is calculated. In this case, the flaw detection sensitivity setting unit 36 ​​calculates the magnitude of the defect signal corresponding to the parameter PηL = 91.9 mm for the steel pipe P as 20.39 V from the relationship (approximation formula) shown in FIG. 3(a). Similarly to the above, the flaw detection sensitivity setting unit 36 ​​calculates the magnitude of the defect signal corresponding to the parameter PηL = 41.2 mm for the calibration steel pipe as 5.66 V from the relationship (approximation formula) shown in FIG. 3(a). Then, the flaw detection sensitivity setting unit 36 ​​corrects the flaw detection sensitivity calibrated and stored in the flaw detection sensitivity calibration step ST2 to 5.66 / 20.39 = 0.28 times. In other words, the flaw detection sensitivity setting unit 36 ​​performs a correction to reduce the calibrated flaw detection sensitivity by 11.1 dB. Then, the flaw detection sensitivity setting unit 36 ​​sets the corrected flaw detection sensitivity in the amplifier 34.

[0032] Table 2 below shows the results of detecting the magnitude of defect signals for four steel pipes P other than the calibration steel pipe shown in the above-mentioned Table 1. The results shown in Table 2, like the calibration steel pipe shown in the above-mentioned Table 1, all of the artificial defects were through holes with a diameter of 1.5 mm extending in the wall thickness direction, and were obtained by supplying a DC magnetic saturation current of 1.0 A to the above-mentioned magnetic saturation coil and detecting the magnitude of the defect signal using a detection coil 2 with an inner diameter ID = 47.7 mm. [Table 2] The data and approximate equation plotted with "◯" in Fig. 3(b) are the same as those shown in Fig. 3(a), and the data plotted with "●" in Fig. 3(b) correspond to the results shown in Table 2. As can be seen from Fig. 3(b), the magnitude of the defect signal actually detected for steel pipe P, which is different from the calibration steel pipe plotted with "●", matches the approximate equation relatively well. Therefore, in the flaw detection sensitivity correction step ST3, it is possible to appropriately correct the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step ST2.

[0033] (Defect detection step ST4) In the defect detection step ST4, defects present in the steel pipe P are detected using the flaw detection sensitivity corrected in the flaw detection sensitivity correction step ST3. 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 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.

[0034] According to the eddy current flaw detection device 100 according to the present 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 ST2 is corrected based on the value of the parameter PηL calculated for the steel pipe P in the flaw detection sensitivity correction step ST3, the value of the parameter PηL calculated for one calibration steel pipe used in the flaw detection sensitivity calibration step ST2, and the relationship acquired in the relationship acquisition step ST1. By using this corrected flaw detection sensitivity, if a defect of the same size as an artificial defect in one calibration steel pipe exists in the steel pipe P, even if the outer diameter and wall thickness of one calibration steel pipe 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.

[0035] In addition, the inventors also evaluated the correlation between the magnitude of the defect signal and the three parameters, namely, the cross-sectional area S of the steel pipe, the filling factor η of the detection coil 2, and the filling factor η of the detection coil 2 × the cross-sectional area S of the steel pipe. Fig. 4 is a diagram showing an example of the relationship between three parameters, namely, the cross-sectional area S of the steel pipe, the filling factor η of the detection coil 2, and the filling factor η of the detection coil 2 × the cross-sectional area S of the steel pipe, and the magnitude of the defect signal. Fig. 4(a) shows the relationship between the cross-sectional area S of the steel pipe and the size of the defect, Fig. 4(b) shows the relationship between the filling factor η of the detection coil 2, the cross-sectional area S of the steel pipe, and the size of the defect, and Fig. 4(c) shows the relationship between the filling factor η of the detection coil 2 × the cross-sectional area S of the steel pipe and the size of the defect. Note that Fig. 4 corresponds to a result in which the parameters on the horizontal axis are rewritten using the same data as the results shown in Fig. 3 described above, which were obtained using multiple calibration steel pipes. As shown in Figure 4, the correlation is slightly lower than that of the parameter PηL mentioned above (coefficient of determination R when approximated by a quadratic function). 2 However, it was found that, although the correlations between the parameters are small (all smaller than 0.94), the larger the values ​​of these parameters, the larger the defect signal magnitude.

[0036] Therefore, in the relationship acquisition step ST1, instead of the parameter PηL, it is possible to acquire a relationship between the magnitude of the defect signal and an alternative parameter, which is any one of the cross-sectional area S of the steel pipe, the filling factor η of the detection coil 2, and the filling factor η of the detection coil 2 × the cross-sectional area of ​​the steel pipe. Then, in the flaw detection sensitivity correction step ST3, it is also possible to correct the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step ST2 based on the value of the alternative parameter calculated for the steel pipe P, the value of the alternative parameter calculated for the calibration steel pipe used in the flaw detection sensitivity calibration step ST2, and the relationship acquired in the relationship acquisition step ST1. According to the eddy current flaw detection method using the above alternative parameter, even if the outer diameter and wall thickness of the steel pipe vary within a specified outer diameter range and a specified wall thickness range, respectively, eddy current flaw detection can be performed with the same accuracy as the eddy current flaw detection method using the parameter PηL without having to calibrate the flaw detection sensitivity for each different outer diameter and wall thickness. [Explanation of symbols]

[0037] 1. Excitation coil 2, 2a, 2b...Detection coil 3...Flaw detector 31...excitation circuit 32 Receiving circuit 33. Synchronous detection circuit 34 Amplifier 35 Defect judgment section 36....Flaw detection sensitivity setting section 100...Eddy current flaw detection equipment P...Steel pipe (ERW steel pipe)

Claims

1. An eddy current inspection method for detecting defects 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; detecting eddy currents generated in the steel pipe using a pair of detection coils through which the steel pipe passes; and detecting the eddy currents based on a differential signal obtained by the detection of the eddy currents; a relationship acquisition step of acquiring a relationship between a parameter PηL expressed by the following formula (1) 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 value of the parameter PηL calculated for the steel pipe, the value of the parameter PηL calculated for 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 PηL = filling rate η of the detection coil × average circumference L of the steel pipe (1)

2. In the relationship acquisition step, instead of the parameter PηL, a relationship between an alternative parameter, which is any one of a cross-sectional area S of the steel pipe, a filling factor η of the detection coil, and a value obtained by multiplying the filling factor η of the detection coil by the cross-sectional area S of the steel pipe, is acquired, and In the flaw detection sensitivity correction step, the flaw detection sensitivity calibrated in the flaw detection sensitivity calibration step is corrected based on the value of the alternative parameter calculated for the steel pipe, the value of the alternative parameter calculated for the calibration steel pipe used in the flaw detection sensitivity calibration step, and the relationship acquired in the relationship acquisition step. The eddy current inspection method according to claim 1 .

3. The outer diameter of the steel pipe in which defects are detected in the defect detection step is an outer diameter in which the filling rate η of the detection coil is 0.5 or more.

3. The eddy current inspection method according to claim 1 or 2.

4. The steel pipe in which defects are detected in the defect detection step is a steel pipe that has been subjected to a cold drawing process using a die and a plug or a mandrel.

3. The eddy current inspection method according to claim 1 or 2.

5. An eddy current flaw detection apparatus for performing eddy current flaw detection on a steel pipe having an outer diameter in a predetermined outer diameter range and a wall thickness in a predetermined wall thickness range, 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 parameter PηL represented by the following formula (1), which 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 of the steel pipe, and each having artificial defects of the same dimensions, and a defect signal, which is the differential signal corresponding to the artificial defects, The flaw detector is set with a flaw detection sensitivity that is pre-calibrated using one of the calibration steel pipes, The flaw detector includes: A flaw detection sensitivity correction step of correcting the set flaw detection sensitivity based on the value of the parameter PηL calculated for the steel pipe, the value of the parameter PηL calculated for the calibration steel pipe used to calibrate the flaw detection sensitivity, 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: PηL = filling rate η of the detection coil × average circumference L of the steel pipe (1)

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