JP6547726B2 - Leakage flux detection apparatus and detection method for thin steel strip - Google Patents

Description

  In the present invention, a DC magnetic field is applied to a thin steel strip which is a ferromagnetic material, and the leakage fluxes caused by defects existing in or on the surface of the thin steel strip are opposed in an array in the width direction of the thin steel strip. The present invention relates to a leakage flux flaw detection apparatus and flaw detection method for a thin steel strip, which are flawed by detecting by a plurality of arranged magnetic sensors.

  BACKGROUND A magnetic flaw detection apparatus that detects a defect present inside or on a surface of a test object such as a steel strip by detecting a leakage magnetic flux is conventionally known. Specifically, in Patent Document 1, the leakage magnetic flux is detected by using an E-type magnetic sensor composed of a search core and an E-shaped ferromagnetic core arranged in the traveling direction of the test object. A magnetic flaw detector for detecting defects is disclosed.

  FIG. 7 is a diagram showing the operation of an E-type magnetic sensor according to the prior art. According to the magnetic flaw detector described in Patent Document 1, as shown in FIG. 7, the E-type magnetic sensor 13 is a thin steel in which the row of the three legs 1a to 1c of the ferromagnetic core 1 is a test object It is arranged along the traveling direction of the band 5.

  When the defect 6 is in the position shown in FIG. 7A, the defect leakage flux 7 passes from the leg 1 a of the ferromagnetic core 1 through the leg 1 c and passes downward through the search coil 2. When the thin steel strip 5 travels and the defect 6 is at the position shown in FIG. 7 (b), the defect leakage flux 7 passes from the leg 1c to the leg 1b and passes the search coil 2 upward.

  Therefore, the magnetic flux passing through the search coil 2 changes as the defect 6 passes, and a voltage proportional to the amount of change of the magnetic flux is generated in the search coil 2. The output signal of this E-type magnetic sensor 13 is input to the high pass filter, and the cut-off frequency of the high pass filter is set according to the traveling speed V of the thin steel strip 5 to detect defects with high S / N ratio. It is possible.

  Subsequently, in Patent Document 2, a plurality of the E-type magnetic sensors are arranged in a zigzag form in the width direction of a thin steel strip to be inspected, and after amplifying the output signal of each E-type magnetic sensor, a band pass filter is formed. The input signal to the full-wave rectifier, the output signal of a pair of full-wave rectifiers corresponding to a pair of magnetic sensors adjacent in the width direction of thin steel strip is subtracted by a subtractor, and the defect is determined based on the output signal of this subtractor A magnetic flaw detector to detect is disclosed.

  According to the magnetic flaw detector described in Patent Document 2, the sensitivity of the full-wave rectifier corresponding to each E-type magnetic sensor is previously adjusted to be the same among the full-wave rectifiers due to a reference sample defect or the like. Therefore, the sensitivity of the full-wave rectifier to vibration noise is also the same between the full-wave rectifiers as long as it is between adjacent channels, and vibration noise can be removed by subtraction processing, so defects can be detected with a high S / N ratio. There is. In addition, only the vibration noise can be removed without attenuating the defect signal by setting the distance between the E-shaped magnetic sensors in the traveling direction of the thin steel strip so as not to cancel each other by the defect detection signals. It is said that it is possible.

  In the following [Mode for Carrying Out the Invention], the contents proposed in Japanese Patent Application No. 2015-223668 (unpublished application 1) filed by the present inventor are also described.

Unexamined-Japanese-Patent No. 09-145679 JP, 2013-148449, A

  However, the magnetic flaw detection devices described in the above-mentioned prior art documents have the following problems.

  In the magnetic flaw detection apparatus configuration described in Patent Document 1, as described in Patent Document 2, when a minute defect is to be detected, when the thin steel strip as the inspection object moves in a relatively low speed region The noise caused by the minute vibration of E-type magnetic sensor or thin steel strip can not be removed because its frequency component overlaps with the defect signal, and when this is large, it is φ 0.1 mm (diameter 0.1 mm, specification below) In the description and figures, the unit of “φXX” and mm may be omitted.) The output level is higher than the equivalent level of the drill hole defect, so the defect judgment threshold can not be lowered and detection of minute defects is difficult. It is.

  Subsequently, the problems of the magnetic flaw detection apparatus described in Patent Document 2 will be described. FIG. 8 is a diagram showing the configuration of an off-line test machine for experimentally investigating the reduction effect of vibration noise according to the prior art. Based on the embodiment described in Patent Document 2, the signals of two adjacent E-type magnetic sensors 13 arranged in a staggered manner in two rows in the width direction of the thin steel strip are respectively amplified and band pass filtered, This figure shows the configuration of an off-line test machine used to experimentally investigate how much vibration noise can be reduced by subtracting both full-wave rectifier output signals connected to full-wave rectifiers.

  In the experiment, as shown in FIG. 8, the thin steel strip sample 10 is wound around the nonmagnetic hollow roll 9 and rotated, and the thin steel strip sample 10 is made longitudinal by a DC magnetizer (not shown) fixed inside. The magnetic sensor housed in the sensor holder 11 fixed to the sensor holder support mechanism 12 on the upper side is DC-magnetized to simulate the flaw detection of the thin steel strip online. In the sensor holder 11, as described in the embodiment of Patent Document 2, the E-type magnetic sensors 13 are spaced 3 mm apart (not shown) in the width direction of the thin steel strip sample 10 and 5 mm apart in the traveling direction. Two channels are stored.

  The thin steel strip sample 10 has a thickness of 0.16 mm, and on the same line in the width direction, φ 0.05 mm, φ 0.1 mm, φ 0.2 mm drill hole defects are machined at constant intervals in the longitudinal direction, and this is lifted off 0.5 mm, The flaw is detected at a traveling speed of 50 m / min. At this time, each output after full-wave rectification of 2-channel E-type magnetic sensor 13 in sensor holder 11 has a voltage of 3 V when a φ 0.1 mm drill hole defect passes directly under each E-type magnetic sensor 13 Each gain was adjusted so that

  In this state, no. Generation of vibration noise in on-line flaw detection by striking the sensor holder support mechanism 12 with a hammer while rotating the nonmagnetic hollow roll 9 so that the drill hole defect passes immediately below the E channel magnetic sensor 13 of one channel. Simulated. Vibration noise, while monitoring the output waveform after full-wave rectifier of E-type magnetic sensor 13, strikes with force addition and subtraction such that this vibration noise level is about 3 V (equivalent to φ0.1 mm drill hole output) The output waveform after full wave rectifier of No. 1 and 2 channel E type magnetic sensor 13 in a timing, and subtraction output (No. 1 channel-No. 2 channel) waveforms of both were sampled.

  FIG. 9 is a view showing an example of an output waveform of vibration noise in the off-line test. 9 (a), (b), and (c), respectively, when vibration was given, No. No. 1 channel full-wave rectifier output waveform Two-channel full-wave rectifier output waveform, waveform after half-wave rectifying both subtraction signals (No. 1 channel-No. 2 channel) and No. 1 channel when no vibration is given to (d) The drill hole flaw detection waveform is shown. The tip of the upward arrow line in each waveform of FIG. 9 is a drill hole defect detection signal, and the tip of the downward arrow is a vibration noise output.

  Here, according to FIG. The drill hole defect passing immediately below the E channel magnetic sensor 13 of one channel is No. 1; Although the level is small but output is also made to the E type magnetic sensor 13 of 2 channels, since a gap of 5 mm in the running direction of the thin steel strip sample 10 between the both sensors causes a shift in the output timing, There is no defect signal drop due to subtraction.

  As shown in FIGS. 9A to 9C, even if the sensitivity adjustment is performed so that the defect has the same sensitivity to the defect, the vibration noise is no. The vibration levels detected are not completely identical because the E-type magnetic sensor 13 of 1 and 2 channels is disposed differently, and although it is possible to reduce by subtraction, the output after subtraction is It does not become zero.

  Therefore, as shown in FIG. 9 (d), when there is no vibration noise, it has a level at least twice as high as that of the material noise caused by the magnetic permeability unevenness in the thin steel strip sample shown by the dotted line. The vibration noise level shown by the dotted line in FIG. 9C is close to the φ0.05 drill hole defect output, so it is difficult to stably detect this.

  From the above, the vibration noise reduction method according to Patent Document 2 is effective when the level of vibration noise is relatively small or when the defect to be detected is large, but the disturbance noise due to the vibration equivalent to φ0.1 mm drill hole defect or more The effect is insufficient when detecting a defect of a size equal to or less than a φ 0.05 mm drill hole defect under an environment where

  The present invention has been made in view of such conventional problems, and a thin steel strip leakage flux flaw detection apparatus which can be removed to an extremely low level even when disturbance noise due to vibration is relatively large, and It aims to provide a flaw detection method.

  The above problems can be solved by the following invention.

[1] A leakage flux flaw detector for a thin steel strip for detecting a defect of the thin steel strip by a magnetic sensor that detects a magnetic flux leaking to the outside from a DC magnetized thin steel strip,
A bar-like ferromagnetic core on which search coils with the same number of turns are wound respectively near and far from the thin steel strip and the bar-like ferromagnetic core are surrounded by a gap between the front and rear faces and the upper side A differential magnetic sensor having a U-shaped ferromagnetic core,
An amplifier for amplifying an output in which the search coils are differentially connected in series; a band pass filter set to a predetermined pass frequency band; and a full wave rectifier for full wave rectifying a waveform after the band pass filter A signal processing unit provided,
What is claimed is: 1. A leakage flux detection apparatus for thin steel strips, comprising:
[2] A leakage flux detection method for a thin steel strip for detecting a defect in the thin steel strip by a magnetic sensor that detects a magnetic flux leaking to the outside from a DC magnetized thin steel strip,
The magnetic sensor includes a rod-like ferromagnetic core on which search coils with the same number of turns are wound respectively near and far from the thin steel strip, and the rod-like ferromagnetic core with a gap interposed between back and forth A differential magnetic sensor comprising: a face and a U-shaped ferromagnetic core surrounding the top,
A method for detecting a leakage flux of a thin steel strip characterized in that an output obtained by connecting in series the search coils so as to be differential is processed by an amplifier, a band pass filter and a full wave rectifier.

  According to the present invention, in the magnetic sensor used, a rod-like ferromagnetic core is disposed with a gap inside the U-shaped ferromagnetic core, and wound with the same number of turns. Of the two search coils, the search coil wound closer to the thin steel strip detects a defect signal and a vibration noise component when vibration is applied to the sensor unit. On the other hand, in the search coil wound to the far side, the vibration noise component passing through the same rod-like ferromagnetic core, that is, the vibration of the same level as the search coil wound to the near side. While the noise component is detected, even if the defect signal is detected, the leakage flux due to the defect has a local spatial distribution compared to the vibration noise, so the signal level is significantly attenuated. As a result, since both are differentially connected in series, it is possible to cancel the vibration noise component and electrically amplify so that the predetermined artificial defect output has a predetermined value for the defect output reduction due to the difference. If it does, it becomes possible to inspect with the S / N equivalent to the E-type magnetic sensor when there is no vibration.

It is a figure which shows the apparatus structural example of the leakage flux flaw detection apparatus of the thin steel strip which concerns on this invention. It is a figure which shows a mode that two search coils are connected in series so that it may become differential. It is a figure which shows the flaw detection waveform of a drill hole defect thin steel strip sample by various magnetic sensors. It is a figure which shows the vibration noise output waveform by various magnetic sensors. It is a figure which shows typically the magnetic flux line path | route of vibration noise in an E-type difference sensor and the difference sensor by this invention. It is a figure which shows the output waveform of vibration noise by various magnetic sensors. It is a figure which shows operation | movement of the E-type magnetic sensor by a prior art. It is a figure which shows the structure of the off-line test machine for investigating the reduction effect of the vibration noise by a prior art experimentally. It is a figure which shows the output waveform example of the vibration noise by an off-line test.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a view showing an example of the apparatus configuration of a leakage flux flaw detection apparatus for thin steel strip according to the present invention. In the figure, 1 is a ferromagnetic core, 1a, 1b are U-shaped ferromagnetic core legs, 1c is a rod-like ferromagnetic core leg, 2 (2f, 2n) is a search coil, 3 is a search coil The differential magnetic sensor according to the present invention, 4 is a signal processor, 5 is a thin steel strip which is an inspection object, 6 is a defect in the thin steel strip 5, 7 is a defect leakage flux from the defect 6, 8 is a DC magnetizer And G represent gaps respectively.

  The ferromagnetic core 1 is formed of a U-shaped ferromagnetic core and a rod-shaped ferromagnetic core. The leg 1c of the rod-like ferromagnetic core is formed on the front center surface of the U-shaped ferromagnetic core on the front and rear surfaces (surfaces with respect to 1a and 1b) and the upper surface (U-shaped ferromagnetic core top surface) 1 and the direction G in FIG. 1), the U-shaped ferromagnetic core is surrounded by the U-shaped ferromagnetic core.

  Here, the ferromagnetic core 1 of FIG. 1 functions as an equivalent to the ferromagnetic core of the E-type magnetic sensor 13 in the prior art described above with reference to FIG. 7, and the central leg 1 c of FIG. The central leg 1c of the E-type magnetic sensor in FIG. 7 can be regarded as being separated from the outer ferromagnetic core by the gap G.

  Further, the two search coils 2n and 2f (one closer to the thin steel strip 5 and the other distant) have the same number of turns (100 turns) in the same direction from the lower surface of the rod-like ferromagnetic core 1c upward. And are connected in series so as to be differential (this will be described later).

  The differential magnetic sensor 3 includes the components described above. The differential magnetic sensor 3 proposed in the present invention is an E-type ferromagnetic core in the magnetic sensor proposed in the unpublished application 1 (E-type differential magnetic sensor 14 shown in FIG. 3C, etc. described later). The core in the middle can be regarded as a bar-shaped ferromagnetic core separated from the outer ferromagnetic core.

  Specifically, in order to provide a gap from the outer ferromagnetic core as a rod-like ferromagnetic core and divide it, for example, a hole is provided halfway through the thickness of the Bakelite plate (the remaining plate The rod-like ferromagnetic core is vertically fixed in this hole, the thickness is the gap), and these are incorporated into the outer ferromagnetic core, and the outer gap is surrounded by resin or the like. Not limited to this creation method.

  An output (between 2 fs and 2 ns) in which the search coils 2 f and 2 n are connected in series to be differential is input to the signal processing unit 4. The signal processing unit 4 includes an amplifier for amplifying the output of the differential magnetic sensor 3, a band pass filter set in a predetermined pass frequency band according to the moving speed of the thin steel strip 5, and a full wave waveform after the band pass filter. Consists of a full-wave rectifier to rectify.

  In FIG. 1, when the defect 6 is in the position shown in the figure, the defect leakage flux 7 passes from the lower surface of the leg 1a through the side surface, enters from the side surface of the leg 1c, and passes downward through the search coil 2. When the thin steel strip 5 travels and the defect 6 is positioned between the leg 1c and the leg 1b, the defect leakage flux 7 enters from the lower surface of the leg 1c and passes upwards through the search coil 2 and enters from the side of the leg 1b I get out of it. As described above, since the loop through which the defect leakage magnetic flux 7 passes by the legs 1a, 1c and 1b of the ferromagnetic core 1 is formed, the search coil can be efficiently and hence highly sensitive like the E-type magnetic sensor 13. 2 is detected.

  Here, since the spatial distribution of the defect leakage flux 7 due to the defect 6 is locally spread, the amount of defect leakage flux passing through the central leg 1 c is sharply attenuated as it goes in the height direction. As a result, the voltage induced in the search coil 2f due to the defect leakage flux change along with the movement of the thin steel strip 5 becomes smaller compared to the search coil 2n, so the difference detection output of the two makes the defect detection signal zero. There is nothing to do.

  FIG. 2 is a diagram showing how two search coils are connected in series so as to be differential. FIG. 2 (a) shows the same connection method as FIG. 1, and although two search coils 2n and 2f are wound in the same direction with the same number of turns, search coil 2n and search coil 2f and Are connected to the terminals 2 ne and 2 fe, and the signal to the signal processing unit 4 is input from the terminals 2 ns and 2 fs.

  On the other hand, in FIG. 2B, the winding directions of the coils of the two search coils are different. In FIG. 2 (b), the search coil 2f in the winding direction opposite to that of the search coil 2f in FIG. 2 (a) is disposed, and the search coil 2n and the search coil 2f are connected by the terminals 2ne and 2fs. Signals to the processing unit 4 are input from terminals 2 ns and 2 fe.

  As described above, in any of the connection methods, signals from two search coils connected in series to be differential are subjected to signal amplification, band pass filter processing, full wave rectification in the signal processing unit 4, Defect determination is performed by comparing with a defect determination threshold set in advance.

  The differential magnetic sensor 3 configured as described above separates the thin steel strip 5 magnetized by the magnetic pole 8 of the direct current electromagnet in the traveling direction of the thin steel strip 5 by a predetermined lift-off, and makes a predetermined width direction. A plurality of strips are arranged in an array over the maximum width of the thin steel strip 5 at pitch, and full width flaw detection of the thin steel strip 5 is performed.

  In the following, according to the apparatus configuration example according to the present invention shown in FIG. 1, even if noise caused by vibration is generated, the E-type magnetic sensor 13 disclosed in Patent Document 1 is flawed in the absence of vibration noise. Explain that flaw detection is possible with the same S / N as when

  FIG. 3 is a figure which shows the flaw detection waveform of a drill hole defect thin steel strip sample by various magnetic sensors. Using the off-line test machine described in FIG. 8, the E-type magnetic sensor 13 (coil winding number 100 turns) of the prior art, differential magnetic sensor 3 (coil winding number 100 × 2 places) according to the present invention, for comparison and verification The E-type differential magnetic sensor 14 (the number of coil turns: 100 turns × 2 places, the magnetic sensor proposed in the undisclosed application 1) is a result of examining the detectability of defects (see FIGS. b) displayed as (c)).

  The thin steel strip samples used were the same as those described above, and for the various magnetic sensors (13, 3, 14), lift-off 0.5 mm, flaw detection speed 50 m / min conditions of each magnetic sensor It is an example of the result of sampling the waveform after passing the φ 0.05 mm, φ 0.1 mm, φ 0.2 mm drill hole defects right under it, amplifying each magnetic sensor output at this time, band pass filtering and full-wave rectification .

  The gain of each output waveform is adjusted so that the φ0.1 mm drill hole output is 3 V. It can be seen from FIG. 3 that these three magnetic sensors can be flawed at the same S / N when there is no noise due to vibration.

Subsequently, similarly, an off-line test machine was used to prepare the following two sensor holders 11 (1) and (2), and each was examined for vibration noise effects.
(1) An E-type magnetic sensor 13 according to the prior art and the differential-type magnetic sensor 3 according to the present invention, which are separated by a certain distance in the width direction and stored (2) E-type magnetic sensor 13 according to the prior art and E-type differential magnetic sensor In the case where the sensor 14 was stored at a certain distance in the width direction, the width direction position of each magnetic sensor was in the state where the thin steel strip sample 10 was wound around the nonmagnetic hollow roll 9 as shown in FIG. It was placed at a distance from immediately above the drill hole defect, and was performed at a lift-off 0.5 mm and a flaw detection speed of 50 mpm. At this time, the sensitivity of each magnetic sensor was gain-adjusted in advance so that the φ0.1 mm drill hole defect output on the same sample was 3 V.

  FIG. 4 is a figure which shows the vibration noise output waveform by various magnetic sensors.

  First, using the sensor holder 11 of (1), the sensor holder is supported so that the vibration noise output value after the full-wave rectifier of the E-type magnetic sensor 13 according to the prior art becomes about 3 V (equivalent to φ0.1 drill hole output) The mechanism 12 was hit with a hammer, and the output waveforms after full-wave rectification of the E-type magnetic sensor 13 and the differential magnetic sensor 3 according to the present invention at this time were sampled at the same timing. The results are shown in the upper part (a) and (b) of FIG.

  Subsequently, the same experiment as described above was performed using the sensor holder of (2). The results are shown in the lower part (c) and (d) of FIG.

  It can be seen from FIG. 4 that in the differential type magnetic sensor 3 according to the present invention, the vibration noise has completely disappeared (see FIGS. 4 (a) and 4 (b)). On the other hand, in the E-type differential magnetic sensor 14 having a configuration in which the gap G above the detection coil 2 is not provided in the differential magnetic sensor 3 according to the present invention added to the vibration noise influence investigation for comparison and verification Although the vibration noise is reduced to about 50% as compared with the magnetic sensor 13, the vibration noise reduction effect as high as that of the differential magnetic sensor 3 according to the present invention is not obtained (Fig. 4 (c), (d) See).

  This mechanism is described with reference to FIG. FIG. 5 is a view schematically showing magnetic flux line paths of vibration noise in the E-type differential sensor and the differential sensor according to the present invention. FIG. 5 (a) shows the path of disturbance noise flux lines 15 coming from the outside, such as vibration noise, in the ferromagnetic core 1 of the E-type differential magnetic sensor 14 shown for comparison and verification. b) shows the path of the disturbance noise flux lines 15 in the ferromagnetic core 1 of the differential magnetic sensor 3 according to the present invention.

  As illustrated, in the E-type differential magnetic sensor 14, the disturbance noise flux lines 15 passing through the leg 1c of the ferromagnetic core 1 around which the search coil 2 is wound are the differential magnetic sensors according to the present invention In No. 3, it is considered that detouring to the legs 1a and 1b on both sides of which the magnetic resistance is small by the gap G is an effect due to the attenuation of the magnetic flux amount due to the disturbance noise which intersects the search coil 2.

  In order to verify the above effects, in the differential magnetic sensor 3 according to the present invention and the E-type differential sensor 14 for comparison and reference, of the two search coils 2 respectively provided, the one closer to the thin steel strip sample 10 Only the search coil 2n was used, that is, one in which the search coil was not differentiated. After adjusting the sensitivity so that the output voltage after full-wave rectifier of φ0.1 drill hole defect by each search coil 2 n is 3 V, the same vibration noise investigation experiment as described in the result of FIG. 4 was performed.

  The results are shown in FIG. FIG. 6 is a diagram showing output waveforms of vibration noise by various magnetic sensors. In FIG. 6, the upper row shows vibration noise waveforms when only the E-type magnetic sensor 13 according to the prior art and the search coil 2 n of the differential magnetic sensor 3 according to the present invention are used at the same timing (FIGS. Fig. 6 (b). The lower part similarly shows vibration noise waveforms when using only the search coil 2 n of the conventional E-type magnetic sensor 13 and E-type differential magnetic sensor 14 at the same timing (FIG. 6 (c) and FIG. 6 respectively). (D)).

  From the lower waveform in FIG. 6, when the gap G is not provided, disturbance magnetic flux is collected due to the increase of the ferromagnetic core length, and it is worse against vibration noise as compared with the E-type magnetic sensor 13 according to the prior art. I understand. On the other hand, according to the upper waveform in FIG. 6, the magnetic shield effect by providing the gap G causes the magnetic flux of the disturbance noise generated by the vibration to pass through the legs 1a and 1b on both sides and bypass the central leg 1c. It was confirmed that the noise output value was attenuated to 30% or less.

  From the above, even in the environment where relatively large vibration noise occurs, the leakage flux testing apparatus for thin steel strip and the leakage flux testing method for thin steel strip according to the present invention are the prior art in an environment free from vibration noise. It is possible to perform flaw detection with the same S / N as when using an E-type magnetic sensor. In the present invention, an apparatus and method capable of reducing noise due to vibration to an extremely low level as disturbance noise have been described, but it is equally effective to reduce electrical noise coming from the outside besides vibration noise. .

Reference Signs List 1 ferromagnetic core 2 search coil 3 differential magnetic sensor according to the present invention 4 signal processing unit 5 thin steel strip (inspection object)
6 Defects 7 Defective Leakage Flux 8 Magnetic Pole of DC Magnetizer 9 Non-Magnetic Hollow Roll G Gap 10 Thin Steel Strip Sample 11 Sensor Holder 12 Sensor Holder Support Mechanism 13 E-Type Magnetic Sensor 14 E-Type Differential Magnetic Sensor 15 Disturbance Noise Flux Line

Claims (2)

A thin steel strip leakage flux flaw detection apparatus for detecting a defect of the thin steel strip by a magnetic sensor that detects a magnetic flux leaking to the outside from a DC magnetized thin steel strip,
A bar-like ferromagnetic core on which search coils of the same number of turns are wound respectively closer to and farther from the thin steel strip, and a gap is enclosed on the front and rear faces and above the bar-like ferromagnetic core A differential magnetic sensor having a U-shaped ferromagnetic core,
An amplifier for amplifying an output in which the search coils are differentially connected in series; a band pass filter set to a predetermined pass frequency band; and a full wave rectifier for full wave rectifying a waveform after the band pass filter A signal processing unit provided,
What is claimed is: 1. A leakage flux detection apparatus for thin steel strips, comprising: A leakage flux detection method for a thin steel strip for detecting a defect in the thin steel strip by a magnetic sensor that detects a magnetic flux leaking to the outside from a DC magnetized thin steel strip,
The magnetic sensor comprises a rod-like ferromagnetic core on which search coils of the same number of turns are wound respectively in the direction closer to the thin steel strip and on the front and back surfaces and above the rod-like ferromagnetic core A differential magnetic sensor comprising a U-shaped ferromagnetic core surrounding a gap;
A method for detecting a leakage flux of a thin steel strip characterized in that an output obtained by connecting in series the search coils so as to be differential is processed by an amplifier, a band pass filter and a full wave rectifier.