JPH07198306A - Method for measuring deviation of metal pipe thickness - Google Patents

Abstract

PURPOSE:To provide a method for measuring thickness deviation of a metal pipe wherein the thickness deviation of the metal pipe can be measured even under condition in which many condition changes exist in an elongation extraction process. CONSTITUTION:Detection coils 5a, 5b and 6a, 6b opposedly disposed in the direction of a diameter of a metal pipe 4 are differentially connected and detect a difference of thickness in the direction of the diameter, thus a thickness deviation is detected. In addition, since the thickness deviation of the pipe formed by elongation extraction using a die and a plug, the sum of the thickness in the direction of the diameter of the pipe is the same in any direction, so the thickness deviation in the diameter of a plurality of orthogonal directions is detected and a quantity of the thickness deviation can be found by calculating the maximum thickness deviation. Furthermore, since a plurality of pairs of the opposedly disposed detection coils 5a, 5b, 6a, 6b, are arranged on a similar circumference, no influence of temperature change is effected, because the generation of heat of the metal pipe due to plastic deformation of the elongation extraction is constant on the same circumference and furthermore measurement of a thickness difference due to differential output is conducted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an uneven thickness measuring method for measuring an uneven state of wall thickness distribution of a metal pipe, and more specifically, a condition change during drawing process for producing a metal pipe and a condition having many measurement restrictions. The present invention also relates to a method for measuring the wall thickness unevenness of a metal tube, which enables measurement of the wall thickness distribution of the metal tube.

[0002]

2. Description of the Related Art An electromagnetic induction method is known as a method for measuring the wall thickness of a metal tube, which utilizes the fact that the impedance of a coil arranged close to the metal tube changes depending on the wall thickness. The impedance of the coil placed close to the metal pipe changes depending on the wall thickness of the pipe and the separation distance between the coil and the pipe, if the measurement conditions such as the pipe material and the excitation frequency are constant.
The wall thickness of the metal pipe can be measured by arranging at least a pair of inspection coils symmetrically at predetermined positions on both sides of the pipe and detecting a change in impedance thereof. The block diagram shown in FIG. 14 shows a configuration for carrying out the method for measuring the wall thickness of a metal pipe disclosed in Japanese Patent Laid-Open No. 1-209302, in which the wall thickness of a steel pipe is measured by the electromagnetic induction method. . The wall thickness of the steel pipe 31 can be measured by detecting the impedance changes of the inspection coils 32 and 33 arranged on both sides of the steel pipe 31 to be measured. In the above conventional configuration, the steel pipe 31 is rotated by the rotating device, and the impedance of each inspection coil 32, 33 is measured by the timing signal output from the pulse generator 34 according to the rotation angle of the steel pipe 31. Measured by The wall thickness distribution of the steel pipe 31 can be detected by phase-analyzing the impedance of each inspection coil 32, 33 for each rotation angle of the steel pipe 31.

[0003]

However, it is not possible to measure the wall thickness of the metal pipe during the drawing process by the above wall thickness measuring method. That is, in the pipe during the drawing process, the temperature of the pipe changes rapidly due to the temperature rise due to plastic deformation.
The electric resistance of the metal tube changes and the output of the inspection coil changes, so that an accurate measured value cannot be obtained. Also, during the drawing process, not only the temperature change but also the metal tube is moving at the drawing speed, and it is impossible to measure the wall thickness distribution on one circumference by rotating it as in the conventional method. is there. Therefore, an object of the present invention is to provide a method for measuring the uneven thickness of a metal pipe, which is capable of measuring the wall thickness distribution of the metal pipe, that is, the uneven thickness, under the condition that the state changes a lot during the drawing process. It is in.

[0004]

In order to achieve the above object, the method adopted by the present invention is such that a detection coil is closely arranged on the surface of a metal tube and excited by a predetermined frequency power, and the impedance of the detection coil is metallic. Uneven thickness measurement of metal pipe wall thickness by detecting the wall thickness of the metal pipe by detecting the amount of change caused by the change in the wall thickness of the pipe, and measuring the uneven thickness from the wall thickness distribution at multiple positions on the circumference of the pipe In the method, a plurality of pairs of the detection coils are arranged on the same circumference so as to be opposed to each other on both sides of the circumference of the pipe separated from the central axis of the metal pipe by a predetermined distance, and the detection coil pairs facing each other are connected differentially From the differential output change component between the facing coils, the difference in wall thickness in the facing direction is measured from the magnitude of the direction component orthogonal to the facing direction, and the uneven thickness state of the metal pipe is calculated from the measured values by the multiple pairs of detection coils. By measuring the uneven thickness of the metal pipe That. In the above measuring method, the detection coil is arranged such that the long side direction of the rectangular coil formed with a winding diameter length ratio of 3 or more is aligned with the axial direction of the metal tube. In the above measuring method, the measured value can be corrected by the relationship data between the outer diameter of the metal tube and the wall thickness difference. In the above measuring method, the measured value can be corrected by the drawing speed data of the metal tube. In the above measuring method, the detection coil can be excited by electric power in which a high frequency is superimposed on a low frequency.

[0005]

When the detection coil excited by the power of the predetermined frequency is arranged close to the metal tube, the magnetic field due to the eddy current generated in the metal tube acts on the detection coil to change the impedance of the detection coil. The impedance change of the detection coil changes in proportion to the distance (lift-off) between the detection coil and the metal pipe and the wall thickness of the metal pipe at the position where the detection coil is close. The relationship between the change in impedance due to the lift-off and the wall thickness is that the impedance change in the direction of change in wall thickness changes in the direction orthogonal to the change in impedance in the direction of lift-off variation. The wall thickness of the tube can be measured.
In the present invention, the difference in the diametrical wall thickness is detected by differentially connecting the detection coils that are arranged to face each other in the diametrical direction of the metal tube and detecting the difference in the measurement output. The uneven thickness of a pipe formed by drawing using a die and a plug is the same in all directions in terms of the sum of thicknesses in the diameter direction of the pipe. The amount of uneven thickness can be obtained by calculating the maximum uneven thickness. Furthermore,
Since a plurality of pairs of detection coils arranged to face each other are arranged on the same circumference, the heat generation of the metal tube due to the plastic deformation of drawing is constant on the same circumference, and the difference in wall thickness due to the differential output is Since it is measured, it is not affected by temperature changes.

In the above detection coil, when the long sides of rectangular coils differing in length and width of the winding diameter are aligned with the axial direction of the metal tube, the metal coil is eccentric in the direction orthogonal to the facing direction of the detection coil. It is possible to prevent deterioration of measurement accuracy. When the length ratio of the winding diameter is 3 or more, the effect of eccentricity can be effectively prevented. Claim 2 corresponds to this. In the above measuring method, when the outer diameter of the metal tube becomes small, the average value of the wall thickness in a relatively large angle range is measured due to the spread of the eddy current, and the measured value is smaller than the actual wall thickness difference. However, it can be solved by obtaining the relationship between the outer diameter and the wall thickness difference in advance and correcting the measured value with this data. Claim 3 corresponds to this. Also, when measuring during the drawing process, when the drawing speed is fast, the magnetic flux is dragged in the drawing direction and the overcurrent changes from the static state to the static value. This can be solved by obtaining the relationship with the difference in advance and correcting the measured value with this data. Claim 4 corresponds to this. Furthermore, when measuring in a state where the lift-off of a metal tube is large, when the detection coil is excited by superimposing a high frequency on a low frequency, the measurement result of the thickness difference becomes non-linear due to the large lift-off. The error can be corrected. The non-linearity is corrected by using the difference between the detected value of low frequency and the detected value of high frequency as the measured value. Claim 5 corresponds to this.

[0007]

Embodiments of the present invention will be described below with reference to the accompanying drawings for the understanding of the present invention. The following embodiments are examples of embodying the present invention and do not limit the technical scope of the present invention. Here, FIG. 1 is a schematic view showing the configuration of a first embodiment of a thickness deviation measuring device to which the measuring method of the present invention is applied, and FIG. 2 is a front view (a) and a side view showing the arrangement of detection coils according to the embodiment. (B), FIG. 3 is a schematic diagram for explaining the principle of measuring the wall thickness, FIG. 4 is an explanatory diagram for explaining the principle of impedance change detection by the differential coil, and FIG. 5 is the angle of the metal tube detected by the differential coil. FIG. 6 is a graph showing an example of actually measuring the wall thickness distribution of the metal pipe, FIG. 7 is a graph showing the maximum amount of uneven thickness measured by the differential coil, and FIG. 8 is a measured uneven thickness ratio. Fig. 9 is a graph comparing the amount of eccentricity measured with a differential coil. Fig. 9 is a graph showing the change in eccentricity depending on the length ratio of the winding diameter of the detection coil.
FIG. 10 is a graph showing changes in measured values due to changes in the outer diameter of the metal pipe, FIG. 11 is a schematic diagram showing the configuration of the second embodiment of the thickness deviation measuring device to which the measuring method of the present invention is applied, and FIG. FIG. 13 is a graph showing the relationship between the drawing speed and the measured value, and FIG. 13 is a schematic diagram showing the configuration of the third embodiment of the thickness deviation measuring apparatus to which the measuring method of the present invention is applied.

In FIG. 1, the thickness deviation measuring device 1 includes two pairs of detection coils 5a, 5b and 6a, 6b arranged around a metal tube 4 to be measured, and the detection coils 5a, 5b.
And 6a, 6b, the eddy current measuring device 7 for measuring the impedance change due to the eddy current, the signal processing device 8 for measuring the uneven thickness from the measurement output of the eddy current measuring device 7, and the measurement condition provided in the signal processing device 8. And an outer diameter correction table 9 for correcting fluctuations in measurement results due to changes in the outer diameter of the metal tube 4 based on the outer diameter data of the metal tube 4 input as The detection coils 5a, 5b and 6a, 6b
As shown in FIG. 2, the metal pipe 4 is orthogonal to the central axis 10 in the drawing direction which is drawn out from the drawing process.
The detection coils 5a and 5b and the detection coils 6a and 6b are opposed to each other at an equal distance from 0, and
The pair of 5b (detection coil pair 5) and the pair of detection coils 6a and 6b (detection coil pair 6) are arranged with a predetermined angle therebetween. In the present embodiment, the detection coil pair 5 and the detection coil pair 6 are arranged at an orthogonal angle. Further, each of the detection coils 5a, 5b, 6a, 6b is formed as a rectangular coil having different winding diameters in the vertical and horizontal directions. In the case of this embodiment, the diameter of the short side of the rectangular coil is 3 mm, which is about 25% of the diameter of the metal tube 4, and the diameter of the long side is 10 mm, which is three times or more the diameter of the short side. The direction of the long side is arranged in parallel with the direction of the central axis 10 of the metal tube 4.

The respective detection coils 5a, 5b, 6a, 6b are
Each of them is excited by a low frequency power of 2 kHz, and the detection coils 5a and 5b and the detection coils 6a and 6b are differentially connected. This differentially connected detection coil pair 5
The outputs of the detection coil pair 6 and the detection coil pair 6 are input to the eddy current measuring device 7, respectively. The eddy current measuring device 7 performs synchronous detection with the excitation voltage as a reference, and detects the in-phase component (real number component) and the orthogonal component (imaginary number component) of the excitation current. The principle and method of measuring the thickness deviation by the thickness deviation measuring device 1 having the above-described configuration will be described below. As shown in the principle of measuring the wall thickness shown in Fig. 3, the metal pipe 1
A pair of detection coils 15 and 16 are arranged on both sides of 7 so as to be opposed to each other while maintaining a constant distance between them, and both detection coils 15 and 16 are differentially connected to each other.
16 synchronous detection outputs are measured. Now, when the metal tube 17 is moved (lifted off) in the direction opposite to the detection coils 15 and 16 in FIG. 3, the synchronous detection output changes, and the direction of change on the impedance plane is eccentric as shown in FIG. Draw a linear trajectory that is almost proportional to the direction and amount. When the metal tube 17 is rotated with the axial center kept at a constant position with the direction of this locus as the X axis and the direction orthogonal to the X axis as the Y axis, the impedance change of the metal tube 17 with uneven thickness is shown in FIG. Draw a Lissajous figure with the Y component as shown in. When the magnitude of the Y component is measured while changing the rotation angle of the metal tube 17 in steps, the result shown in FIG. 5 is obtained. Comparing this with the measurement graph of the wall thickness distribution in the circumferential direction of the metal pipe having the same uneven thickness as shown in FIG. 6, the measurement result shown in FIG. 5 is a signal proportional to the difference in wall thickness. I understand. In this way, the output obtained by differentially connecting the pair of detection coils according to this embodiment can detect the difference in wall thickness in the measurement direction.

The effect of the wall thickness difference detection by the pair of differentially connected detection coils is effectively exerted in the uneven thickness measurement in the drawing process of the metal tube, which is the main object of the present invention. One of the factors that make it difficult to measure uneven thickness during the drawing process of metal tubes
For example, the electrical conductivity may change due to the rise in the temperature of the pipe due to plastic deformation, and the detection output may change. The temperature rise of the pipe is determined by factors such as the amount of plastic deformation (workability), the deformation rate, and the thermal conductivity, but it can be considered that the temperature is constant in the circumferential direction of the pipe. Therefore, the differential output of the detection coils arranged on the same circumference is not affected by the temperature change of the tube. Further, in the conventional technique, it is necessary to measure the wall thickness of each of the detection coils, but the measuring method using the differential output according to the present invention also has the cost reduction effect that the structure of the wall thickness measurement can be halved. Using the principle of measuring the difference in wall thickness by the above-mentioned pair of detecting coils, two pairs of detecting coils 5 as in this embodiment are used.
The uneven thickness can be measured by making a, 5b and 6a, 6b orthogonal. Occurrence of uneven thickness in the drawing process is caused by eccentricity of the plug with respect to the die. Since the inner shape of the pipe is determined by the shape of the plug and the outer shape is determined by the shape of the die,
In the diametrical direction of the pipe, the difference in wall thickness is constant in all directions. The average wall thickness is determined by the shapes of the die and the plug, and the amount of uneven thickness can be determined by the following formula from the wall thickness difference in the two directions. Maximum thickness deviation = √ {(difference in wall thickness measured by the detection coils 5a and 5b) ² + (difference in wall thickness measured by the detection coils 6a and 6b) ² } / (die diameter-plug diameter) Fig. 7 shows the results obtained by selecting the measured values in two orthogonal directions from the measured wall thickness and calculating the maximum wall thickness deviation of the metal pipe having three types of wall thickness deviation using the above equation. It can be seen that the same amount of uneven thickness can be measured using any of the data in eight directions.
FIG. 8 shows a comparison between the results of measuring the uneven thickness of three types of metal pipes having different amounts of uneven thickness and the results of measuring the wall thickness by cutting the same pipe. It can be seen from the figure that uneven thickness can be measured by the configuration of this embodiment.

In the configuration of this embodiment, each detection coil 5
As described above, a, 5b, 6a and 6b are arranged such that the long side direction formed in the rectangular shape is parallel to the drawing direction of the metal tube 4. With this shape and arrangement, it is possible to prevent the measurement accuracy from deteriorating due to a change in output when the metal tube 4 is eccentric (moves in the direction orthogonal to the lift-off) in a direction orthogonal to the pair of opposed axes. FIG. 9 shows the results of measuring the effect on the eccentricity when the detection coil is circular and when it is rectangular. As shown in the figure, when the difference between the long side and the short side of the rectangular coil is 3 or more, the influence of eccentricity is reduced. In the configuration of the above embodiment, when the outer diameter of the metal tube to be measured is sufficiently large, the detected differential output can be detected in proportion to the thickness difference in the measuring direction. However, when the outer diameter becomes smaller, the average value of the wall thickness in a relatively large angle range is measured due to the spread of the eddy current, which is smaller than the actual wall thickness difference, and the difference between the wall thickness difference and It is no longer in direct proportion to the output. Therefore, the relationship between the outer diameter and the wall thickness difference is measured in advance, and the error due to the outer diameter change is corrected by this relational data. Since the relationship between the outer diameter and the differential output is as shown in FIG. 10, this relational data is stored in advance in the outer diameter correction table 9 provided in the signal processing device 8 and the signal processing device 8 stores it. By inputting, the measurement error due to the outer diameter can be corrected.

Next, the measuring method of the second embodiment of the present invention will be described. The present embodiment is a measuring method in which uneven thickness measurement of a metal pipe moving at a drawing speed is added with a process of correcting an error generated when the drawing speed is high. In FIG. 11, the thickness deviation measuring device 2 to which the method of the second embodiment is applied takes out a drawing speed signal from a drawing machine control device 12 which controls the drawing device 11, and inputs the drawing speed signal to a signal processing device 8 to obtain measurement data. It is configured so that it can be corrected. The rest of the configuration is the same as the configuration of the first embodiment, so description thereof will be omitted. The detection coils 5a, 5 are attached to the metal tube 4 which moves at the drawing speed.
When the uneven thickness is measured by arranging b, 6a, and 6b, when the drawing speed becomes faster, the magnetic flux is dragged in the transport direction of the metal tube 4 due to the eddy current, and a deviation from the measured value in the stationary state occurs. The influence of the movement of the object due to the differential detection is less than the method of individually measuring with a single coil, but it is affected to a large extent by the drawing speed. As a result of measuring the drawing speed dependence of the differential output, as shown in FIG. 12, when the drawing speed becomes faster, the output becomes smaller even if the uneven thickness is constant. Therefore, by correcting the measurement data by the speed signal, ，
The occurrence of measurement error due to changes in the drawing speed is eliminated. Next, the measuring method of the third embodiment of the present invention will be described.
The present embodiment is a measuring method in which a process of correcting a measurement error that occurs when the lift-off of the metal tube 4 becomes large is added. In FIG. 11, the thickness unevenness measuring device 3 to which the method of the third embodiment is applied is the same as the structure of the first embodiment and the eddy current measuring device 7a.
Are configured differently. In this configuration,
2 kHz for exciting each detection coil 5a, 5b, 6a, 6b
The high frequency of 100 kHz is superimposed on the frequency of. A lift-off correction circuit 13 is incorporated in the eddy current measuring device 7a. The rest of the configuration is the same as the configuration of the first embodiment, so description thereof will be omitted.

While the lift-off of the metal tube 4 is small, the impedance change due to the lift-off changes in a straight line. As described above in the first embodiment, the thickness difference can be detected from the Y-direction component, but the lift-off does not occur. When it becomes large, the change in impedance becomes non-linear (X-axis component) and a measurement error occurs. In order to correct this measurement error, each detection coil 5a, 5b, 6a, 6b is excited by a low frequency (2 kHz) and a high frequency (100 kHz) to correct the non-linearity of the change component. Since the penetration depth into the metal tube 4 is small at a high frequency, the influence of the wall thickness does not appear, but only the effect of lift-off occurs and nonlinearity occurs. The non-linearity due to this lift-off also occurs at low frequencies, but since the penetration depth into the metal tube 4 is large at low frequencies, the Y-direction component due to the wall thickness is detected. Since the low-frequency nonlinearity and the high-frequency nonlinearity have the same shape, the difference between the two is taken out by the lift-off correction circuit 13 and the measurement data of the eddy current measuring device 7a is corrected to measure even when the lift-off becomes large. The result is not affected. In each of the embodiments described above, the example in which the detection coils are configured in two pairs has been described.
It is also possible to arrange the detection coil pairs in more than one pair. In this case, two unknowns of the eccentricity direction and the maximum eccentricity are calculated from the measured values of the individual detection coil pairs by the least square method.

[0014]

As described above, according to the present invention, the detection coils that are arranged to face each other in the diametrical direction of the metal tube are differentially connected to each other to detect the difference between the measurement outputs, so that the diametrical direction can be inexpensively obtained. The difference in wall thickness, that is, uneven thickness is detected. In addition, the uneven thickness of a pipe formed by drawing using a die and a plug is the same in all directions in terms of the sum of the thicknesses in the diameter direction of the pipe. The amount of uneven thickness can be obtained by detecting and calculating the maximum uneven thickness. Further, since the plurality of pairs of detection coils arranged to face each other are arranged on the same circumference, the heat generation of the metal tube due to the plastic deformation of the drawing is constant on the same circumference, so that it is not affected by the temperature change. In the above detection coil, when the long sides of the rectangular coils which are different in length and width of the winding diameter are arranged so as to coincide with the axial direction of the metal tube, the measurement accuracy of the metal tube is decentered in the direction orthogonal to the facing direction of the detection coil. Deterioration can be prevented. When the length ratio of the winding diameter is 3 or more, the effect of eccentricity can be effectively prevented. (Claim 2) In the above measuring method, when the outer diameter of the metal tube becomes small, the average value of the wall thickness in a relatively large angular range is measured due to the spread of the eddy current. Although the value is smaller than the wall thickness difference, it can be resolved by obtaining the relationship between the outer diameter and the wall thickness difference in advance and correcting the measured value with this data. (Claim 3) Also, when measuring during the drawing process, when the drawing speed is fast, the magnetic field of the eddy current is dragged in the drawing direction, which causes a change from the measured value in the stationary state. This can be solved by obtaining the relationship with the difference in advance and correcting the measured value with this data. (Claim 4) Furthermore, in the case where the measurement is performed in a state where the lift-off of the metal pipe is large, when the detection coil is excited by superimposing a high frequency on a low frequency, the measurement result of the thickness difference becomes non-linear due to the large lift-off. Therefore, it is possible to correct the measurement error that occurs. The non-linearity is corrected by using the difference between the detected value of low frequency and the detected value of high frequency as the measured value. (Claim 5)

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a first embodiment of a thickness deviation measuring device to which a measuring method of the present invention is applied.

FIG. 2 is a front view (a) and a side view (b) showing an arrangement of detection coils according to an embodiment.

FIG. 3 is a schematic diagram illustrating the measurement principle of the thickness difference.

FIG. 4 is an explanatory diagram illustrating the principle of impedance change detection using a differential coil.

FIG. 5 is a graph showing an output change for each angle of the metal tube detected by the differential coil.

FIG. 6 is a graph showing an example of actually measuring the wall thickness distribution of a metal tube.

FIG. 7 is a graph of the maximum thickness deviation measured with a differential coil.

FIG. 8 is a graph comparing the measured thickness deviation rate with the thickness deviation measured with a differential coil.

FIG. 9 is a graph in which a change in eccentricity according to a length ratio of a winding diameter of a detection coil is measured.

FIG. 10 is a graph showing changes in measured values due to changes in the outer diameter of the metal tube.

FIG. 11 is a schematic diagram showing the configuration of a second embodiment of a thickness deviation measuring device to which the measuring method of the present invention is applied.

FIG. 12 is a graph showing the relationship between the drawing speed of a metal tube and the measured value.

FIG. 13 is a schematic diagram showing the configuration of a third embodiment of a thickness deviation measuring device to which the measuring method of the present invention is applied.

FIG. 14 is a schematic diagram showing a configuration of a wall thickness measuring device to which a metal tube wall thickness measuring method according to a conventional example is applied.

[Explanation of symbols]

 1, 2, 3 ... Uneven thickness measuring device 4 ... Metal tube 5a, 5b, 6a, 6b ... Detection coil 7 ... Eddy current measuring device 8 ... Signal processing device 9 ... Outer diameter correction table 10 ... Central axis (drawing direction) 11 ... Drawing device 12 ... Lift-off correction circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsumi Okubo 65 Hirazawa, Hadano City, Kanagawa Prefecture Kobe Steel Works, Ltd. Hadano Plant

Claims (5)

[Claims] 1. A wall thickness of the metal tube is obtained by arranging a detection coil close to the surface of the metal tube, exciting the coil with a predetermined frequency electric power, and detecting the impedance of the detection coil changed due to a variation in the wall thickness of the metal tube. In the method for measuring the uneven thickness of a metal pipe, in which the unevenness is measured from the wall thickness distribution at a plurality of positions on the circumference of the pipe, the detection coil is arranged on the circumference of the pipe at a predetermined distance from the central axis of the metal pipe. The detection coil pairs are arranged on both sides so as to face each other, and a plurality of the detection coil pairs are arranged on the same circumference at a predetermined angle, and the detection coil pairs are differentially connected to measure the difference in wall thickness in the facing direction. A method for measuring the wall thickness unevenness of a metal pipe, which calculates the uneven thickness state of the metal pipe from the difference in wall thickness measured by a plurality of pairs of detection coils. 2. The detection coil is a rectangular coil having a winding diameter length ratio of 3 or more, and the detection coil is arranged with its long side direction aligned with the axial direction of the metal tube. Measuring method for uneven wall thickness of metal tubes. 3. The method for measuring the uneven thickness of a metal pipe according to claim 1, wherein the measured value is corrected by preset data of the relationship between the outer diameter of the metal pipe and the thickness difference. 4. The method for measuring the uneven thickness of a metal tube according to claim 1, wherein the measured value is corrected by preset drawing speed data of the metal tube. 5. The method for measuring the wall thickness deviation of a metal tube according to claim 1, wherein the detection coil is excited by electric power in which a high frequency is superimposed on a low frequency.