JP2003025017A - Hot-rolled steel sheet to which information of flaw detection result is appended and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot-rolled steel sheet using a magnetic leakage flux test by which flaws are detected with high accuracy even when the flaws are very small and unwanted magnetic fluxes, that is, noises are large. SOLUTION: The manufacturing method of the hot-rolled steel sheet having a stage where hot rolling is performed, a stage where flaw detection is performed by the magnetic leakage flux test and a stage where the position on the steel sheet where the detected flaw is present is specified is provided. The stage where the flaw detection is performed consists of a stage for successively magnetizing a ferromagnetic body to plural different intensities of magnetization to the steel sheet after rolling, a stage for detecting the magnetic flux which leaks from the same position of the ferromagnetic body which is magnetized to the each intensity of magnetization with a magnetic sensor and a stage for performing arithmetic processing of the output signal of the magnetic sensor corresponding to the intensity of each magnetization so as to emphasize a signal due to the flaw inside the ferromagnetic body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a leakage magnetic flux flaw detection method for detecting defects such as inclusions therein by applying a magnetic field to a ferromagnetic substance and detecting the magnetic flux leaking from the ferromagnetic substance. The present invention relates to a method for manufacturing a hot rolled steel sheet or a descaled steel sheet.

[0002]

2. Description of the Related Art Leakage magnetic flux flaw detection is widely used as a method for detecting defects such as inclusions existing in a ferromagnetic material such as a steel plate.

As an example thereof, FIG. 1 shows the configuration of a magnetic flaw detector using a magnetic sensor incorporated in a steel plate inspection line. A magnetic flaw detector 4 is arranged along the conveyance path of the steel sheet 1 (ferromagnetic material) conveyed on the product inspection line by the conveyance rollers 2 and 3 at a substantially constant speed V. This magnetic flaw detector 4 has a magnetizer 5 for magnetizing the steel plate 1 in a running state,
It is composed of a magnetic sensor 6 installed at a position facing the magnetizer 5 with the steel plate 1 interposed therebetween, and a signal processing device 7 that arithmetically processes an output signal of the magnetic sensor 6.

Now, when the steel plate 1 is magnetized by the magnetizer 5, if a defect 8 exists inside the steel plate 1, the magnetic flux passing through the inside of the steel plate 1 is disturbed by this defect 8 and a part of it is outside the steel plate 1. Leak to. If this magnetic flux leakage is detected by the magnetic sensor 6 and the output signal thereof is processed by the signal processing device 7, the defect 8 will be detected. Further, since the intensity of the leakage magnetic flux depends on the size of the defect 8, the size of the defect 8 can be evaluated by the output signal level of the magnetic sensor 6.

On the other hand, in addition to the leakage flux due to defects, the leakage flux detected by the magnetic sensor has non-uniform local magnetic characteristics in the steel sheet (uneven thickness of oxide scale and oxide scale / base steel sheet interface). (Such as unevenness) and disturbance of leakage magnetic flux due to surface roughness. Such disturbance of the magnetic flux becomes unnecessary magnetic flux, that is, noise from the viewpoint of detecting defects.

In order to eliminate the influence of this noise, different frequency characteristics of an output signal due to a defect (hereinafter simply referred to as a defect signal) and an output signal due to noise (hereinafter simply referred to as a noise signal) are used. The following method may be used.

FIG. 2 shows an example of frequency characteristics of a defect signal and a noise signal measured by running a steel sheet at a constant speed. In general, as shown in the figure, the defective signal has a higher frequency distribution than the noise signal. Therefore, by incorporating a high-pass filter having a cutoff frequency f in the signal processing device, the defective signal is relatively emphasized and extracted. It becomes possible to do.
In addition, the method of using a filter with an appropriate constant in order to improve the defect detection capability in the leakage flux flaw detection method is described in
It is also disclosed in Japanese Patent Publication No. 61-119760.

However, as shown in FIG. 2, since the frequency characteristics of the defect signal and the noise signal overlap with each other, even if the defect is very small or the noise is large, even if a high-pass filter is provided, the defect signal can be frequency filtered. Even if the discrimination is performed, it becomes difficult to remove the influence of noise to the extent that the defect can be detected accurately.

The object of the present invention is to provide a small number of defects,
It is an object of the present invention to provide a method for manufacturing a hot-rolled steel sheet using a leakage magnetic flux flaw detection method capable of detecting defects with high accuracy even if unnecessary magnetic flux, that is, noise is large.

[0010]

In order to achieve the above object, firstly, the present invention comprises a step of hot rolling, a step of detecting a defect by a leakage magnetic flux flaw detection method, and the detected defect. And a step of specifying the position on the steel sheet at which is present. The step of performing the defect detection includes a step of sequentially magnetizing a ferromagnetic body to a plurality of different magnetization strengths with respect to a rolled steel sheet, and the same position of the ferromagnetic body magnetized to each of the magnetization strengths. Magnetic flux leaking from the magnetic sensor is detected, and an output signal of the magnetic sensor corresponding to the strength of each magnetization is processed so that a signal caused by a defect in the ferromagnetic body is emphasized. And the process.

Secondly, according to the present invention, a step of hot rolling, a step of performing descaling treatment on the rolled steel sheet, and a step of performing leakage flux flaw detection on the steel sheet after the descaling treatment are performed. A method for manufacturing a hot-rolled steel sheet, comprising: a step of performing detection; and a step of identifying a position on the steel sheet where the detected defect exists. The step of performing the defect detection includes sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths, and magnetic flux leaking from the same position of the ferromagnetic material magnetized to each magnetization strength It comprises a step of detecting with a sensor and a step of arithmetically processing an output signal of the magnetic sensor corresponding to the strength of each magnetization so that a signal due to a defect in the ferromagnetic body is emphasized.

Thirdly, according to the present invention, a step of unwinding a hot-rolled coil (including a descale hot-rolled coil) obtained by winding a steel sheet into a coil shape, and a magnetic flux leakage flaw detection method for the unwound steel sheet. A hot-rolling coil having a step of performing defect detection, a step of identifying a position on the steel sheet where the detected defect is present, and a step of rewinding the unwound steel sheet ( Including) manufacturing method. The step of performing the defect detection includes sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths, and magnetic flux leaking from the same position of the ferromagnetic material magnetized to each magnetization strength It comprises a step of detecting with a sensor and a step of arithmetically processing an output signal of the magnetic sensor corresponding to the strength of each magnetization so that a signal due to a defect in the ferromagnetic body is emphasized.

Fourthly, according to the present invention, a step of unwinding a hot rolled coil obtained by winding a steel sheet into a coil shape, a step of performing a descaling treatment on the unrolled hot rolled steel sheet, and a step of the descaling treatment For the hot-rolled steel sheet was performed, the step of performing defect detection by the leakage flux flaw detection method, the step of identifying the position on the descaled hot-rolled steel sheet in which the detected defects are present, the descaled hot-rolled steel sheet. A method for manufacturing a descaled hot-rolled coil having a step of rewinding. The step of performing the defect detection includes sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths, and magnetic flux leaking from the same position of the ferromagnetic material magnetized to each magnetization strength It comprises a step of detecting with a sensor and a step of arithmetically processing an output signal of the magnetic sensor corresponding to the strength of each magnetization so that a signal due to a defect in the ferromagnetic body is emphasized.

Fifth, the first to fourth methods preferably further include a step of marking defect information on the steel sheet.

Sixth, the first to fourth methods preferably include the step of attaching a tag, a sheet or an information recording medium indicating defect information.

Seventh, the first to fourth methods preferably further include the step of sending defect information to the user of the hot-rolled steel sheet.

Eighth, it is preferable that the first to fourth methods further include a step of sending a sheet or information recording medium showing defect information to a user of the hot rolled steel sheet.

Ninth, in the step of magnetizing the ferromagnetic material in any of the first to eighth methods, the ferromagnetic material is magnetically saturated to the maximum magnetization intensity when magnetizing the ferromagnetic material. It is preferable to set the magnetization.

Tenth, in the first to eighth methods, the leakage magnetic flux flaw detection method sequentially magnetizes the same position of a ferromagnetic material into two kinds of magnetization strengths, and corresponds to a large magnetization strength. It is desirable to weight and subtract the output signal of the magnetic sensor corresponding to the small magnetization intensity from the output signal of the magnetic sensor.

Eleventh, in the tenth method, it is desirable to set the high magnetization intensity to the magnetization at which the ferromagnetic material is magnetically saturated.

Twelfth, the tenth method calculates the ratio of the output of the magnetic sensor corresponding to a smaller magnetization intensity to the output of the magnetic sensor corresponding to a larger magnetization intensity, and calculates the ratio from the surface of the defect. It is desirable to have a step of calculating the depth of.

Thirteenth aspect of the present invention is the step of hot rolling, the step of detecting defects in the rolled steel sheet by the leakage flux flaw detection method, and the step of detecting the defects on the steel sheet. A method of manufacturing a hot-rolled steel sheet having a step of specifying a position. The step of performing the defect detection includes the step of magnetizing the ferromagnetic material with one or a plurality of magnetizers, and the step of moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. A step of sequentially detecting magnetic fluxes leaking from the same position of a magnetized ferromagnetic body by a plurality of magnetic sensors installed at positions having different magnetization intensities, and the output signal of the magnetic sensor after the detection is a defect in the ferromagnetic body. And a calculation process is performed so that the signal caused by

Fourteenth, the present invention comprises a step of hot rolling, a step of descaling the rolled steel sheet, and a defect of the descaling steel sheet by a leakage magnetic flux flaw detection method. A method for manufacturing a hot-rolled steel sheet, comprising: a step of performing detection; and a step of identifying a position on the steel sheet where the detected defect exists. The step of performing the defect detection includes the step of magnetizing the ferromagnetic material with one or a plurality of magnetizers, and the step of moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. A step of sequentially detecting magnetic fluxes leaking from the same position of a magnetized ferromagnetic body by a plurality of magnetic sensors installed at positions having different magnetization intensities, and the output signal of the magnetic sensor after the detection is a defect in the ferromagnetic body. And a calculation process is performed so that the signal caused by

The fifteenth aspect of the present invention is to unwind a hot-rolled coil (including a descale hot-rolled coil) obtained by winding a steel sheet in a coil shape, and a leakage magnetic flux flaw detection method for the unrolled steel sheet. A hot-rolling coil having a step of performing defect detection, a step of identifying a position on the steel sheet where the detected defect is present, and a step of rewinding the unwound steel sheet ( (Including) is provided. The step of performing the defect detection includes the step of magnetizing the ferromagnetic material with one or a plurality of magnetizers, and the step of moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. A step of sequentially detecting magnetic fluxes leaking from the same position of a magnetized ferromagnetic body by a plurality of magnetic sensors installed at positions having different magnetization intensities, and the output signal of the magnetic sensor after the detection is a defect in the ferromagnetic body. And a calculation process is performed so that the signal caused by

Sixteenth, according to the present invention, a step of unwinding a hot rolled coil obtained by winding a steel sheet into a coil shape, a step of performing a descaling treatment on the unrolled hot rolled steel sheet, and a step of the descaling treatment For the hot-rolled steel sheet is performed, a step of performing defect detection by a leakage flux flaw detection method, a step of specifying the position on the descaled hot-rolled steel sheet in which the detected defects are present,
A method for manufacturing a descaled hot rolled coil, comprising the step of rewinding the descaled hot rolled steel sheet. The step of performing the defect detection includes the step of magnetizing the ferromagnetic material with one or a plurality of magnetizers, and the step of moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. A step of sequentially detecting magnetic fluxes leaking from the same position of a magnetized ferromagnetic body by a plurality of magnetic sensors installed at positions having different magnetization intensities, and the output signal of the magnetic sensor after the detection is a defect in the ferromagnetic body. And a calculation process is performed so that the signal caused by

Seventeenth, it is preferable that the methods 13 to 16 further include a step of marking defect information on the steel sheet.

Eighteenth, it is preferable that the methods 13 to 16 further include a step of attaching a tag, a sheet or an information recording medium showing defect information.

Nineteenth, it is preferable that the methods 13 to 16 further include a step of sending defect information to the user of the hot-rolled steel sheet.

Twentieth, it is preferable that the methods 13 to 16 further include a step of sending a sheet or information recording medium showing defect information to a user of the hot rolled steel sheet.

Twenty-first, in the step of magnetizing the ferromagnet in the methods 13 to 16, the maximum magnetization intensity when magnetizing the ferromagnet is set to the magnetization at which the ferromagnet is magnetically saturated. It is preferable to set.

Twenty-second, in the thirteenth to twentieth methods, in the magnetic flux leakage flaw detection method, the ferromagnetic material is moved while moving the ferromagnetic material along two magnetic sensors installed at positions having different magnetization intensities. It is preferable that the output signal of the magnetic sensor corresponding to the strength of small magnetization is weighted and subtracted from the output signal of the magnetic sensor corresponding to the strength of large magnetization at the same position.

Twenty-third, in the twenty-second method, it is desirable to set the high magnetization intensity to the magnetization at which the ferromagnetic material is magnetically saturated.

Twenty-fourth, in the twenty-second method, the ratio of the output of the magnetic sensor corresponding to the smaller magnetization intensity to the output of the magnetic sensor corresponding to the greater magnetization intensity is calculated to calculate from the surface of the defect. It is desirable to have a step of calculating the depth of.

Twenty-fifth, in the first to twenty-fourth methods, it is preferable that the defect information is defect position information.

Twenty-sixth, in the first to twenty-fourth methods, it is preferable that the defect information is defect density information.

Twenty-seventh, in the first to twenty-fourth methods, it is preferable that the defect information is a planar position of the defect.

Twenty-eighthly, in the first to twenty-fourth methods, it is preferable that the defect information is at least one of defect depth, size, and shape information in addition to the planar position of the defect.

Twenty-ninth, the present invention relates to a step of sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths, and a magnetic flux leaking from the same position of the ferromagnetic material magnetized to each of the magnetization strengths. Is detected by a magnetic sensor, and an output signal of the magnetic sensor corresponding to the strength of each magnetization is arithmetically processed so that a signal caused by a defect in the ferromagnetic body is emphasized. Provided is a hot-rolled steel sheet on which defect information detected by a leakage magnetic flux flaw detection method is marked.

The thirtieth aspect of the present invention is a step of sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths, and a magnetic flux leaking from the same position of the ferromagnetic material magnetized to each of the magnetization strengths. Is detected by a magnetic sensor, and an output signal of the magnetic sensor corresponding to the strength of each magnetization is arithmetically processed so that a signal caused by a defect in the ferromagnetic body is emphasized. Provided is a hot-rolled steel sheet to which a tag, a sheet, or an information recording medium indicating defect information detected by a magnetic flux leakage flaw detection method is attached.

Thirty-first, in the twenty-ninth or thirtieth hot-rolled steel sheet, the strongest magnetization intensity when magnetizing a ferromagnetic material by the leakage flux flaw detection method is set to a magnetization at which the ferromagnetic material is magnetically saturated. It is desirable to set.

Thirty-second, in the twenty-ninth or thirtieth hot-rolled steel sheet, the leakage magnetic flux flaw detection method is applied to the same position of the ferromagnetic material in two positions.
It is preferable that the types of magnetization are sequentially magnetized, and the output signal of the magnetic sensor corresponding to the small magnetization strength is weighted and subtracted from the output signal of the magnetic sensor corresponding to the large magnetization strength.

Thirty-third, in the thirty-second hot-rolled steel sheet, it is preferable that the magnetic flux leakage flaw detection method sets a large magnetization intensity to a magnetization at which a ferromagnetic material is magnetically saturated.

Thirty-fourth, in the thirty-second hot rolled steel sheet, the leakage magnetic flux flaw detection method uses the magnetic sensor output corresponding to a smaller magnetization strength and the magnetic sensor output corresponding to a larger magnetization strength. It is preferable to calculate the ratio and calculate the depth from the surface of the defect.

Fifty-fifthly, the present invention relates to the step of magnetizing a ferromagnetic material by one or a plurality of magnetizers, and moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. However, the magnetic flux leaking from the same position of the magnetized ferromagnetic material is sequentially detected by a plurality of magnetic sensors installed at the positions having different magnetization intensities, and the output signal of the magnetic sensor after the detection is the ferromagnetic value. Provided is a hot-rolled steel sheet on which defect information detected by a leakage magnetic flux flaw detection method is marked, which has a step of performing arithmetic processing so that a signal caused by a defect in the body is emphasized.

Thirty-sixth, the present invention relates to a step of magnetizing a ferromagnetic material by one or a plurality of magnetizers, and moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. However, the magnetic flux leaking from the same position of the magnetized ferromagnetic material is sequentially detected by a plurality of magnetic sensors installed at the positions having different magnetization intensities, and the output signal of the magnetic sensor after the detection is the ferromagnetic value. Provided is a hot-rolled steel sheet having a tag, a sheet, or an information recording medium indicating defect information detected by the leakage magnetic flux flaw detection method, which has a step of performing arithmetic processing so that a signal caused by a defect in the body is emphasized.

Thirty-seventh, in the thirty-fifth or thirty-sixth hot-rolled steel sheet, the leakage flux flaw detection method sets the strength of the magnetizer such that a large magnetization strength is a magnetization at which the ferromagnetic material is magnetically saturated. Is preferred.

Thirty-eighthly, in the thirty-fifth or thirty-sixth hot-rolled steel sheet, the leakage magnetic flux flaw detection method moves the ferromagnetic material along two magnetic sensors installed at positions having different magnetization intensities, It is desirable to weight and subtract the output signal of the magnetic sensor corresponding to the small magnetization intensity from the output signal of the magnetic sensor corresponding to the large magnetization intensity at the same position of the ferromagnetic material.

Ninety-ninth, in the thirty-eighth hot-rolled steel sheet, in the leakage flux flaw detection method, the strength of the magnetizer is set so that a large magnetization strength is a magnetization at which the ferromagnetic material is magnetically saturated. desirable.

Forty-fourth, in the thirty-eighth hot-rolled steel sheet, the leakage flux flaw detection method uses the magnetic sensor output corresponding to a smaller magnetization intensity and the magnetic sensor output corresponding to a larger magnetization intensity. It is preferable to calculate the ratio of the above to calculate the depth from the surface of the defect.

Forty-first, in the twenty-ninth to fortieth hot-rolled steel sheets, it is desirable that the defect information is defect position information.

Forty-second, in the twenty-ninth to fortieth hot-rolled steel sheets, it is desirable that the defect information is defect density information.

Forty-third, in the twenty-ninth to fortieth hot rolled steel sheets, it is preferable that the defect information is a planar position of the defect.

Forty-fourth, in the twenty-ninth to fortieth hot rolled steel sheets, the defect information is at least one of defect depth, size and shape information in addition to the planar position of the defect. desirable.

The 29th to 44th hot-rolled steel sheets include hot-rolled coils, descaled hot-rolled steel sheets, and descaled hot-rolled coils.

[0055]

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 3 shows a result of measuring a noise signal by chemically scraping a steel plate having a thickness of 1 mm gradually from the surface so as to prevent distortion. The noise signal level in FIG. 3 is normalized by the signal level before being trimmed.

As the cutting allowance increases, the noise signal level gradually decreases, and when the cutting allowance is about 20 μm, the level becomes less than half of the level before the cutting and becomes stable. This result is considered to be based on the fact that grinding reduces the influence of surface roughness and non-uniformity of the magnetic properties of the surface layer portion caused by cooling from the surface during steel sheet production. Such a phenomenon has been confirmed in other than the above steel plate,
It can be said that the noise source is mainly in the surface layer of the ferromagnetic material. On the other hand, defects such as inclusions are generally located inside the surface layer.

We focused on the phenomenon that the magnetic shield effect of a ferromagnetic material depends on the strength of magnetization, and when a steel sheet is magnetized with different magnetic fields by a magnetizer, it exists between a magnetic sensor and a defect or noise source. The effect of the magnetic shield effect of the steel plate itself on the defect signal and noise signal was investigated.

FIG. 4 shows the relationship between the strength of the magnetic field and the defect signal level and noise signal level. The signal level in FIG. 4 is normalized by the signal level when the steel sheet is magnetically saturated with a magnetic field strength of 2500 AT or more.

When the strength of the magnetic field is lowered below 2500 AT, which is the strength of the magnetic field at which the steel sheet is magnetically saturated, both the defect signal level and the noise signal level decrease, but the extent of the decrease is greater in the defect signal. It turns out to be big. This phenomenon can be understood as follows.

When the steel sheet is magnetically saturated, the differential relative permeability is
The level difference between the defect signal and the noise signal depends only on the difference in the distance between the magnetic sensor and the defect and noise source because the magnetic shield effect becomes close to 1. On the other hand, when the strength of the magnetic field is reduced to make the steel sheet magnetically unsaturated, the differential relative permeability becomes greater than 1 and the magnetic shielding effect of the steel sheet occurs. At that time, the defect signal caused by the defect located farther from the magnetic sensor has a larger thickness of the steel plate between the magnetic sensor and the defect than the noise signal from the surface layer portion. It is strongly affected and the degree of signal level drop becomes large.

Therefore, a ferromagnetic material such as a steel plate is magnetized to have a plurality of different magnetization intensities, and the magnetic flux leaking from the same position of the magnetized ferromagnetic material is detected by a magnetic sensor to determine the strength of each magnetization. Even if the defect is very small or the noise is large, the output signal of the corresponding magnetic sensor is processed so that the noise signal of relatively high level is canceled and the defect signal is relatively emphasized. Therefore, the defect can be detected with high accuracy.

At this time, it is more effective to set the strongest magnetization strength of the plurality of different magnetization strengths to the magnetization strength at which the ferromagnetic material having a differential relative permeability of 1 is magnetically saturated. is there.

Further, the ferromagnetic material is magnetized to have two kinds of magnetization strengths, and the output signal of the magnetic sensor corresponding to the strong magnetization strength is weighted from the output signal of the magnetic sensor corresponding to the strong magnetization strength. If it is added and subtracted, the defect signal can be emphasized more simply and effectively. Also in this case, as described above, it is preferable to set the large magnetization intensity to the magnetization intensity at which the ferromagnetic substance is magnetically saturated. In addition, when calculating (output of magnetic sensor corresponding to small magnetization intensity) / (output of magnetic sensor corresponding to large magnetization intensity), a large value indicates defects close to the surface, and a small value indicates internal defects. It turns out that this is a defect. The depth from the surface of the defect can be calculated by using an appropriate function.

FIG. 5 shows an example of a magnetic flaw detector for carrying out the leakage magnetic flux flaw detection method of the present invention. In this device, the magnetizer 5 installed in the conventional device shown in FIG.
a, aside from the magnetic sensor 6a, another set of magnetizers 5b and a magnetic sensor are located at a position away from the magnetic sensor 6a by a distance d in the traveling direction of the steel plate 1.
6b is installed. Note that here, the magnetic sensors 6a and 6b
The distance from the steel plate 1, that is, the lift-off L is set to be the same.

Now, the magnetizer 5a magnetizes the steel plate 1 so as to magnetically saturate it, and the magnetizer 5b magnetizes the steel plate 1 so as to be magnetically unsaturated, so that the steel plate 1 is magnetized along the magnetizers 5a to 5b. Magnetic fluxes from the same part of the steel plate 1 are detected by the magnetic sensors 6a and 6b while being moved, and their output signals Va (t) and Vb
(t) by the signal processing device 7 so that A in the following equation (1) is close to 0 in the absence of the defect 8, that is, Va (t)
If Vb (t) is weighted with K ₂ and subtracted from it, the noise signal can be reduced and the S / N ratio of the defective signal can be improved.

A = K ₁ · (Va − K ₂ · Vb) (1) At this time, the output signal Va (t) of the magnetic sensor 6 a is controlled by the delay processing circuit 9 to be the distance between the magnetic sensors 6 a and 6 b, that is, It is relatively delayed with respect to the output signal Vb (t) of the magnetic sensor 6b by using the time difference Δt corresponding to the same steel plate position obtained by dividing the positional deviation amount d by the steel plate speed V which is sequentially measured. V
A (t-Δt) corresponds to Vb (t). In addition, the detection signals Va (t-Δt) and Vb (t) reduce the direct current component and low-frequency formation noise components, and cut electrical noise higher than the defect signal frequency. Can be filtered. The subtraction, delay processing, filtering, etc. of the output signal may be performed with an analog signal, but may be performed with a digital signal after analog-digital conversion with a sampling frequency of 20 kHz, for example.

The lift-off L is not always the magnetic sensor 6
It is not necessary for a and 6b to be the same. Further, in order to magnetize to different magnetizations, it is not always necessary to use a plurality of magnetizers and magnetic sensors, and even if the current of the magnetizers is changed by using a set of magnetizers and magnetic sensors, the strength of magnetization can be changed. Good.

Instead of magnetizing the ferromagnetic material to a plurality of different magnetization strengths, the ferromagnetic material is magnetized by one or a plurality of magnetizers under a constant magnetizing condition, and the ferromagnetic material Even if the magnetic fluxes leaking from the same position are detected by a plurality of magnetic sensors installed at different positions along the magnetization direction, the same effect can be obtained as described below.

FIG. 6 shows the flow of magnetic flux when the steel sheet is magnetized by the magnetizer.

A part of the magnetic flux emitted from the N pole of the magnetizer 5 is a steel plate.
Pass the inside of 1 and magnetize the steel plate 1. At this time, the amount of magnetic flux supplied from the magnetizer 5 to the steel plate 1 increases as it approaches the magnetic pole center of the magnetizer 5 and reaches its maximum at the magnetic pole center. Therefore, the strength of magnetization of the steel plate 1 also reaches its maximum at the magnetic pole center. The lower it gets, the lower it gets. Therefore, if a ferromagnetic material is magnetized under a certain magnetizing condition by a magnetizer and a leakage magnetic flux is detected by a plurality of magnetic sensors installed at positions having different magnetization directions, the ferromagnetic material described above has a plurality of different magnetization strengths. The same effect as when magnetized is obtained.

In FIG. 7, the magnetic sensor 6 shown in FIG. 1 is used to fix the position of the magnetic sensor 6 at the magnetic pole center with respect to two kinds of defects and noises existing in the steel plate 1 having a thickness of 1 mm. Magnetizer 5
The output signal level obtained by changing the strength M of the applied magnetic field due to M when the output signal level obtained by
The results of the relationship between S and S are shown below. At this time, the steel sheet 1 was conveyed at a lift-off of 1 mm and a speed of 300 m / min, and magnetized by the magnetizer 5 having a magnetic pole interval of 12 mm located at a position of 4 mm from the steel sheet 1.
Also, in the test in which the magnetic sensor 6 is displaced from the magnetic pole center,
The strength M of the applied magnetic field was fixed at 3000 AT. Further, for example, in defect 8 in FIG. 7, the output signal level when the magnetic sensor 6 is installed on the magnetic pole center and the magnetic field of 2500 AT is applied, and the magnetic sensor 6 is installed at the position of S = 5 mm and the magnetic field of 3000 AT is set. It can be seen that the output level when applying is the same.

As shown in FIG. 7, the data of the two types of defects and the noise almost overlap, and the effectiveness of measuring the magnetic sensor 6 by shifting it from the magnetic pole center of the magnetizer 5 in the magnetization direction can be confirmed.

In such a method in which a ferromagnetic material is magnetized by a magnetizer under a constant magnetizing condition, and the magnetic flux leaking from the magnetized ferromagnetic material is detected by a plurality of magnetic sensors installed at positions having different magnetization directions. Since it is possible to measure with one magnetizer at a time, the device configuration is simple compared to the case where magnetizers are provided for each magnetizing condition.Because the magnetizing condition can be changed with one magnetizer, it is possible to measure multiple times. High-speed measurement is also possible.

As shown in FIGS. 13 to 15, the above-described magnetic flux leakage flaw detection method of the present invention is applied to the hot-rolled steel sheet with a scale or hot-rolled coil, and the hot-rolled steel sheet with scale, which has been more difficult to detect defects with higher accuracy than before. Or descaling of hot rolled coil
(Pickling, shot blasting, etc.) is applied to detect defects, and the positions of the detected defects on the steel sheet are specified, and the density of the detected defects is calculated to determine the position of the defects. Or a tag showing the density information, a sheet or an information recording medium is attached, or if the information of the defect can be marked on the steel sheet to provide the hot rolled steel sheet or hot rolled coil or the descaled steel sheet or the descaled coil, Since the accurate position and density information are known in advance, the steel plate user in the post-process or customer can take advance measures in consideration of defects.

[0075]

EXAMPLES The present invention will be specifically described below based on examples.

Example 1 A magnetic flaw detector as shown in FIG. 5 was placed on a steel plate inspection line, and a steel plate 1 having a thickness of 1 mm was set to 100 m.
At a speed of / min, the magnetizer 5a applies a magnetic field 2500AT at which the steel plate 1 is magnetically saturated (strong magnetization condition), and the magnetizer 5b applies a magnetic field 1000AT at which the magnetization of the steel plate 1 is unsaturated ( Weak magnetization condition), the magnetic sensor 6a measured the output signal Va under the strong magnetization condition, and the magnetic sensor 6b measured the output signal Vb under the weak magnetization condition by the above-described method.

At this time, a magnetic field of 2500 AT was applied as the strong magnetization condition and a magnetic field of 1000 AT was applied as the weak magnetization condition, because the noise signal common to both conditions was measured and the change in the defect signal level was caused by the noise signal level. This is because it is larger than the change of. The lift-off L of the magnetic sensors 6a and 6b was set to 0.7 mm. Then, the subtraction difference processing was performed by multiplying the output signal Va by 2.5 times the output signal Vb, that is, with K ₂ = 2.5 in the above equation (1).

As shown in FIG. 9, the S / N ratio of the defect signal obtained under the strong magnetization condition has a large noise and is as low as 1.3, but the defect signal is obtained by performing the differential calculation process on the output signal obtained under the weak magnetization condition. It can be seen that the signal S / N has improved to 3.5.

In the case of simultaneously measuring the plate width by the method of the present invention, it is necessary to install the magnetic sensors 6a and 6b also in the plate width direction of the steel plate 1 at a constant pitch.
If the steel plate is installed at a pitch of 5 mm, 200 sets of 400 magnetic sensors 6a and 6b are required.

(Embodiment 2) FIG. 8 shows another example of a magnetic flaw detector for carrying out the leakage flux flaw detection method of the present invention. In this device, in addition to the magnetic sensor 6a installed opposite to the magnetic pole center of the magnetizer 5 of the conventional device shown in FIG. 1, another magnetic sensor 6b is provided at a position 4 mm away from the magnetic sensor 6a. is set up. In addition, with the magnetic sensor 6a
Liftoff L of 6b is set to 0.7mm.

A magnetic flaw detector as shown in FIG. 8 is arranged in a steel plate inspection line, a steel plate 1 having a thickness of 1 mm is conveyed at a speed of 100 m / min, and a magnetic field of 3000 AT is applied by a magnetizer 5a to make a steel plate.
1 was magnetically saturated, and the output signal Va corresponding to the strong magnetization condition of Example 1 was measured by the magnetic sensor 6a, and the output signal Vb corresponding to the weak magnetization condition of Example 1 was measured by the magnetic sensor 6b. Then, as in the case of the first embodiment, a difference calculation process is performed that subtracts the output signal Vb from the output signal Va by 2.5. Here, a magnetic field of 3000 AT is applied by the magnetizer 5a, and the reason why the magnetic sensor 6b is placed at a position 4 mm away from the magnetic sensor 6a along the magnetization direction is that the strong magnetization condition and the weak magnetization condition are set as described above. This is because the common noise signal is measured and the change in the defect signal level becomes larger than the change in the noise signal level.

As a result, as in the case of FIG. 9 obtained in the first embodiment, the magnetic sensor 6a installed at the magnetic pole center of the magnetizer 5 is shown.
The S / N ratio of the defect signal obtained by (corresponding to strong magnetization condition) was large with noise as low as 1.3, but the magnetic sensor 6b installed at a position 4 mm away from the magnetic sensor 6a in the magnetization direction (weak magnetization condition). The S / N ratio of the defect signal is improved to 3.5 by performing the difference calculation processing on the output signal obtained by (1).

Although the case where the measurement is performed at two types of magnetization levels is shown here, the same is true when the measurement is performed at three or more types of magnetization levels by further installing a magnetic sensor. Also,
In this embodiment, the magnetic sensors 6a and 6b and the magnetizer 5 are arranged on opposite sides of each other with the steel plate 1 in between,
They may be on the same side, and the direction of shifting the position of the magnetic sensor may be the traveling direction of the steel sheet 1 or the opposite direction.

Note that the calculation of the measured values in the magnetic sensors 6a and 6b, delay processing, processing such as filtering may be performed with an analog signal, or after the analog signal is converted into a digital signal. Further, like the first embodiment, in the case of simultaneously measuring the plate width by the method of the present invention, it is necessary to install the magnetic sensors 6a and 6b in the plate width direction of the steel plate 1 at a constant pitch.

(Embodiment 3) A low-carbon steel hot-rolled coil having a plate thickness of 1.8 mm and a plate width of 1 m was manufactured by the manufacturing process of the hot-rolled coil incorporating the inclusion inspection process and the defect information addition process as shown in FIG. Manufacturing, defect inspection with the scale attached,
The defect location and density information was determined. At this time, in the inclusion inspection process, the magnetic sensor is placed in the width direction of the coil by 5 mm.
Defects were detected under the same magnetization conditions as in Example 1 by using the same magnetic flaw detector as that shown in FIG.

FIG. 11 shows the longitudinal direction at the portion where the hot rolled coil is present.
An example of marking the defect position over 1000 m is shown.
As described above, even in the case of the hot-rolled coil with scale, the marking can provide the planar position of the defect. In addition, at least one of depth, size, and shape information of the defect can be provided. Further, the more accurate position can be provided as a sheet as shown in Table 1 or stored in an information recording medium or provided with information using a transmission means. At least one of more detailed defect depth, size, and shape information
One can also be provided.

[0087]

[Table 1]

FIG. 12 shows an example of defect density information in the longitudinal direction of the hot-rolled coil. Also, density information of defects on the outer peripheral surface of the hot rolled coil may be provided.

In addition to the hot-rolled coil, it is possible to provide a hot-rolled coil that has been descaled (pickled, shot blasted, etc.) with marking of defect information and with defect information attached.

Since the defect rolling position information and density information as shown in FIGS. 11 and 12 can be provided by the method for manufacturing a hot rolling coil or a descaling hot rolling coil of the present invention, the hot rolling coil or the descaling hot rolling coil can be provided. For the user of the coil, it is possible to perform a preliminary examination such as avoiding a portion having many defects or changing the application, which is a great advantage.
Further, as shown in FIG. 10, if the defect information is fed back to the upper process, it is possible to provide effective data for quality control of the hot rolled coil.

[0091]

EFFECTS OF THE INVENTION The hot rolling coil and the descaling hot rolling coil manufacturing method according to the present invention can provide defect position information and density information. Preliminary examination such as avoiding a large number of parts or changing the usage can be performed, which is a great advantage. Further, by feeding back the defect information to the upper process, it is possible to provide effective data for quality control of the hot rolled coil.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a configuration of a conventional magnetic flaw detector.

FIG. 2 is a diagram showing an example of frequency characteristics of a defect signal and a noise signal.

FIG. 3 is a diagram showing a relationship between a cutting allowance and a normalized noise signal level.

FIG. 4 is a diagram showing a relationship between a magnetic field strength and a defect signal level and a noise signal level.

FIG. 5 is a diagram showing an example of a magnetic flaw detector for carrying out the leakage magnetic flux flaw detection method used in the method of the present invention.

FIG. 6 is a diagram showing a flow of magnetic flux when magnetizing a steel sheet by a magnetizer.

FIG. 7 is a diagram showing the relationship between the magnetic field strength M of the magnetizer and the magnetic sensor displacement amount S that can obtain the same output signal level.

FIG. 8 is a diagram showing another example of a magnetic flaw detection apparatus for performing the leakage magnetic flux flaw detection method used in the method of the present invention.

FIG. 9 is a diagram showing each magnetization condition and an output signal after a difference calculation process.

FIG. 10 is a view showing a manufacturing process of a hot-rolled steel sheet incorporating the magnetic flux leakage flaw detection method according to the present invention.

FIG. 11 is a diagram showing an example of a hot rolling coil on which defect position information is marked.

FIG. 12 is a diagram showing an example of defect density information on the outer peripheral portion of the hot-rolled coil.

FIG. 13 is a diagram showing another manufacturing process of the hot-rolled steel sheet incorporating the magnetic flux leakage flaw detection method according to the present invention.

FIG. 14 is a diagram showing a manufacturing process of a hot-rolled coil incorporating the leakage magnetic flux flaw detection method according to the present invention.

FIG. 15 is a diagram showing another manufacturing process of the hot-rolled coil incorporating the leakage magnetic flux flaw detection method according to the present invention.

[Explanation of symbols]

1 steel plate 2 Conveyor roller 3 Conveyor roller 4 Magnetic flaw detector 5 magnetizer 5a magnetizer 5b magnetizer 6 Magnetic sensor 6a Magnetic sensor 6b magnetic sensor 7 Signal processor 8 defects 9 Delay processing circuit

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Junichi Shitsuji             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F term (reference) 2G053 AA11 AB22 BA03 BA15 BB03                       BB08 BB12 CB12 CB22 DB02

Claims (31)

[Claims] 1. A step of performing hot rolling, a step of sequentially magnetizing a ferromagnetic material to a plurality of different magnetization intensities of a rolled steel sheet, and a step of magnetizing the ferromagnets to the respective magnetization intensities. A step of detecting a magnetic flux leaking from the same position of the magnetic body with a magnetic sensor, and an output signal of the magnetic sensor corresponding to the strength of each magnetization is emphasized with a signal caused by a defect in the ferromagnetic body. A method of manufacturing a hot-rolled steel sheet, comprising: a step of performing defect detection by a leakage magnetic flux flaw detection method having a step of performing a calculation process; and a step of identifying a position on the steel sheet where the detected defect exists. 2. A step of performing hot rolling, a step of performing descaling treatment on the rolled steel sheet, and a step of performing descaling treatment on the steel sheet after the descaling treatment using a ferromagnetic material and a plurality of different magnetization strengths. The step of sequentially magnetizing, the step of detecting a magnetic flux leaking from the same position of the ferromagnetic material magnetized to the strength of each magnetization with a magnetic sensor, and the step of magnetizing the magnetic sensor corresponding to the strength of each magnetization. A step of performing a defect detection by a leakage magnetic flux flaw detection method having a step of processing an output signal so that a signal caused by a defect in the ferromagnetic body is emphasized; and on the steel plate on which the detected defect exists And a step of identifying the position of the hot rolled steel sheet. 3. A step of unwinding a hot-rolled coil obtained by winding a steel sheet in a coil shape, a step of sequentially magnetizing a ferromagnetic material to the unwound steel sheet to a plurality of different magnetization strengths, A step of detecting a magnetic flux leaking from the same position of the ferromagnetic material magnetized to each magnetization intensity with a magnetic sensor, and an output signal of the magnetic sensor corresponding to each magnetization intensity A step of performing a defect detection by a leakage magnetic flux flaw detection method having a step of performing a calculation process so that a signal resulting from the defect is emphasized, and a step of specifying a position on the steel plate where the detected defect exists, And a step of rewinding the unwound steel plate. 4. A step of unwinding a hot rolled coil obtained by winding a steel sheet into a coil, a step of descaling the unrolled hot rolled steel sheet, and a step of descaling the hot rolled steel sheet. On the other hand, a step of sequentially magnetizing the ferromagnetic material to a plurality of different magnetization strengths, and a step of detecting a magnetic flux leaking from the same position of the ferromagnetic material magnetized to each of the magnetization strengths with a magnetic sensor. Defect detection is performed by a leakage magnetic flux flaw detection method including a step of performing arithmetic processing on an output signal of the magnetic sensor corresponding to the strength of each magnetization so that a signal caused by a defect in the ferromagnetic body is emphasized. A method for manufacturing a descaled hot rolled coil, comprising: a step, a step of identifying a position on the descaled hot rolled steel sheet where the detected defect exists, and a step of rewinding the descaled hot rolled steel sheet. 5. The method according to claim 1, further comprising the step of marking defect information on the steel sheet. 6. The method according to claim 1, further comprising the step of attaching a tag, a sheet or an information recording medium indicating defect information.
Method 3 or 4. 7. The method according to claim 1, further comprising the step of sending defect information to the user of the hot-rolled steel sheet. 8. The method according to claim 1, further comprising the step of sending a sheet or information recording medium showing defect information to a user of the hot rolled steel sheet. 9. The depth from the surface of the defect is calculated by calculating the ratio of the output of the magnetic sensor corresponding to a smaller magnetization intensity to the output of the magnetic sensor corresponding to a larger magnetization intensity in the leakage magnetic flux flaw detection method. 9. The method of claim 8 including calculating the height. 10. A step of performing hot rolling, a step of magnetizing a ferromagnetic material to a rolled steel sheet by one or a plurality of magnetizers, and a plurality of magnetic sensors installed at positions having different magnetization intensities. Magnetic flux leaking from the same position of the magnetized ferromagnetic material while moving the ferromagnetic material along the magnetic material is sequentially detected by a plurality of magnetic sensors installed at different positions of the magnetization intensity; A step of performing a calculation of an output signal of a magnetic sensor so that a signal caused by a defect in the ferromagnetic body is emphasized, a step of performing a defect detection by a leakage magnetic flux flaw detection method; and a step of detecting the detected defect. A method of manufacturing a hot-rolled steel sheet, comprising: a step of identifying a position on the steel sheet. 11. A step of performing hot rolling, a step of performing descaling treatment on the rolled steel sheet, and a ferromagnetic material for the steel sheet after descaling by one or more magnetizers. While magnetizing and moving the ferromagnetic body along a plurality of magnetic sensors installed at positions where the magnetization strength is different, the magnetic flux leaking from the same position of the magnetized ferromagnetic body is moved to a position where the magnetization strength is different. Magnetic flux flaw detection having a step of sequentially detecting with a plurality of magnetic sensors installed in the magnetic flux sensor, and a step of performing arithmetic processing on an output signal of the magnetic sensor after the detection so that a signal caused by a defect in the ferromagnetic body is emphasized. A method for manufacturing a hot-rolled steel sheet, comprising: a step of detecting defects by a method; and a step of identifying a position on the steel sheet where the detected defects exist. 12. A step of unwinding a hot-rolled coil obtained by winding a steel sheet into a coil shape, a step of magnetizing a ferromagnetic material with respect to the unwound steel sheet by one or a plurality of magnetizers, and While moving the ferromagnetic material along a plurality of magnetic sensors installed at different positions, the magnetic flux leaking from the same position of the magnetized ferromagnetic material is detected by the plurality of magnetic sensors installed at different magnetization strengths. A step of performing defect detection by a leakage magnetic flux flaw detection method having a step of sequentially detecting and a step of performing an arithmetic operation on the output signal of the magnetic sensor after the detection so that a signal resulting from the defect in the ferromagnetic body is emphasized. A method for manufacturing a hot-rolled coil, comprising: a step of identifying a position on the steel sheet where the detected defect exists; and a step of rewinding the unwound steel sheet. 13. A step of unwinding a hot rolled coil obtained by winding a steel sheet into a coil shape, a step of descaling the unrolled hot rolled steel sheet, and a step of descaling the hot rolled steel sheet. On the other hand, the step of magnetizing the ferromagnetic material with one or a plurality of magnetizers and the step of magnetizing the ferromagnetic material while moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. A step of sequentially detecting magnetic flux leaking from the same position with a plurality of magnetic sensors installed at positions having different magnetization intensities, and an output signal of the magnetic sensor after the detection is emphasized by a signal caused by a defect in the ferromagnetic body. A step of performing defect detection by a leakage magnetic flux flaw detection method having a step of performing arithmetic processing as described above; a step of specifying a position on the descaled hot rolled steel sheet where the detected defect exists; Manufacturing method of descaling a hot rolled coil having a step of unwinding the hot rolled steel sheet, a. 14. The method according to claim 10, 11, 12 or 13, further comprising the step of marking defect information on the steel sheet. 15. The method according to claim 10, further comprising the step of attaching a tag, a sheet or an information recording medium indicating defect information.
11, 12 or 13 methods. 16. The method according to claim 10, 11, 12 or 13 further comprising the step of sending defect information to a user of the hot-rolled steel sheet.
the method of. 17. The method according to claim 10, 11, 12 or 13, further comprising the step of sending a sheet or information recording medium showing defect information to a user of the hot rolled steel sheet. 18. The defect information is at least one of depth, size and shape information of the defect in addition to the planar position of the defect. The method described in. 19. A magnetic sensor detects a step of sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths and a magnetic flux leaking from the same position of the ferromagnetic material magnetized to each of the magnetization strengths. And a step of arithmetically processing an output signal of the magnetic sensor corresponding to the strength of each magnetization so that a signal caused by a defect in the ferromagnetic body is emphasized. Hot rolled steel sheet with marked defect information. 20. A magnetic sensor detects a step of sequentially magnetizing a ferromagnetic material to a plurality of different magnetization strengths and a magnetic flux leaking from the same position of the ferromagnetic material magnetized to each of the magnetization strengths. And a step of arithmetically processing an output signal of the magnetic sensor corresponding to the strength of each magnetization so that a signal caused by a defect in the ferromagnetic body is emphasized. Hot-rolled steel sheet with attached tag, sheet or information recording medium showing the defect information. 21. The leakage magnetic flux flaw detection method sets the strongest magnetization intensity when magnetizing a ferromagnetic material to a magnetization at which the ferromagnetic material is magnetically saturated. Hot rolled steel sheet. 22. The leakage magnetic flux flaw detection method sequentially magnetizes the same position of a ferromagnetic material into two kinds of magnetization strengths, and outputs a small magnetization strength from a magnetic sensor output signal corresponding to a large magnetization strength. 21. The hot rolled steel sheet according to claim 19, wherein the output signal of the magnetic sensor corresponding to is weighted and subtracted. 23. The hot-rolled steel sheet according to claim 22, wherein the leakage magnetic flux flaw detection method sets a high magnetization intensity to a magnetization at which a ferromagnetic material is magnetically saturated. 24. The depth from the surface of the defect is calculated by calculating the ratio of the output of the magnetic sensor corresponding to a smaller magnetization intensity to the output of the magnetic sensor corresponding to a larger magnetization intensity in the leakage magnetic flux flaw detection method. 23. The hot-rolled steel sheet according to claim 22, wherein the hot-rolled steel sheet is calculated. 25. A step of magnetizing a ferromagnetic material by one or a plurality of magnetizers, and a step of magnetizing the ferromagnetic material while moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization strengths. A step of sequentially detecting magnetic flux leaking from the same position of the magnetic body by a plurality of magnetic sensors installed at positions having different magnetization intensities, and an output signal of the magnetic sensor after the detection is caused by a defect in the ferromagnetic body. A hot-rolled steel sheet on which defect information detected by a leakage magnetic flux flaw detection method having a step of performing arithmetic processing so that a signal is emphasized is marked. 26. A step of magnetizing a ferromagnetic material by one or a plurality of magnetizers, and a step of magnetizing the ferromagnetic material while moving the ferromagnetic material along a plurality of magnetic sensors installed at positions having different magnetization intensities. A step of sequentially detecting magnetic flux leaking from the same position of the magnetic body by a plurality of magnetic sensors installed at positions having different magnetization intensities, and an output signal of the magnetic sensor after the detection is caused by a defect in the ferromagnetic body. A hot-rolled steel sheet to which a tag, a sheet, or an information recording medium indicating defect information detected by a leakage magnetic flux flaw detection method having a step of performing arithmetic processing so that a signal is emphasized is attached. 27. The hot rolling according to claim 25 or 26, wherein in the leakage magnetic flux flaw detection method, the strength of the magnetizer is set so that a large magnetization strength is a magnetization at which the ferromagnetic material is magnetically saturated. steel sheet. 28. In the leakage magnetic flux flaw detection method, while moving a ferromagnetic substance along two magnetic sensors installed at positions having different magnetization intensities, a large magnetization intensity is obtained at the same position of the ferromagnetic substance. 27. The hot-rolled steel sheet according to claim 25 or 26, wherein an output signal of the magnetic sensor corresponding to a small magnetization intensity is weighted and subtracted from an output signal of the magnetic sensor corresponding to. 29. The hot-rolled steel sheet according to claim 28, wherein the leakage magnetic flux flaw detection method sets the strength of the magnetizer so that a large magnetization strength is a magnetization at which a ferromagnetic material is magnetically saturated. . 30. The leakage magnetic flux flaw detection method calculates the ratio of the output of the magnetic sensor corresponding to a smaller magnetization intensity to the output of the magnetic sensor corresponding to a larger magnetization intensity, and calculates the ratio from the surface of the defect. 3. The depth is calculated.
8 hot rolled steel sheet. 31. The defect information is at least one of depth, size and shape information of the defect in addition to the planar position of the defect. Hot-rolled steel sheet according to.