JPH08160006A - Defect detecting method for steel plate - Google Patents

Abstract

PURPOSE: To discriminate a surface defect and an inclusion defect by counting the number of detected defects at the same position in the lateral direction of a steel plate with a reference length preset in a traveling direction, and judging whether the counted value satisfies a predetermined equation or not. CONSTITUTION: A defect physical property discriminator 151 has a threshold value circuit 22 which inputs a defect signal V from a calculator 141 to generate a pulse when the signal V exceeds a preset threshold value, a counter 23 for counting the pulses from a pulse generator 104, a counter 24 for counting the pulses generated from the circuit 22, a setter 25 for previously setting the mean number N0 =L/r of the defects sequentially generated at an interval (r) within the length L, and a comparator 26 for comparing the counted value N of the counter 24 with the set value N0 of setter 25. The output pulses of the circuit 22 is supplied to the counters 23, 24. When the value N=the number N0 is satisfied, the surface defect is decided and when the number N≠ the number N0 is satisfied, the inclusion defect is judged to be detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet defect detection method for automatically detecting defects such as surface flaws and non-metallic inclusions on a steel sheet online in a threaded state.

[0002]

2. Description of the Related Art The leakage magnetic flux flaw detection test known as one of the nondestructive inspection methods for steel sheets is described in "Nondestructive Inspection Handbook" (edited by the Japan Nondestructive Inspection Society, published by Nikkan Kogyo Shimbun), pages 517 to 534. As described in JP-A-57-10, it is one of the effective methods for detecting surface defects such as blowholes in metal materials and non-metallic inclusions.
As disclosed in Japanese Patent No. 8656, it is also used for online flaw detection at the time of manufacturing a thin steel sheet.

FIG. 8 shows a circuit configuration of a thin steel sheet defect detecting apparatus which has been widely used in the past. In this conventional defect detection device, a large number (number 1
Magnetic sensors 81 ₁ , 81 ₂ , ...
.., 81 _n are arranged, the magnetic flux of the thin steel plate is detected by the magnetic sensor of each (channel), and the output of each magnetic sensor, that is, the channel output, is output to the corresponding amplifier 82 ₁ , 82 ₂ ,. After amplification by 82 _n , band pass filters (BPF) 83 ₁ , 83 ₂ , ..., 8
The noise component other than the frequency band of the defective portion signal is removed by supplying it to 3 _n .

Bandpass filters 83 ₁ , 83 ₂ , ...
, 83 _n are usually composed of analog circuits in which capacitors, resistors, inductors, and operational amplifiers are combined, and these bandpass filters 83 ₁ , 83 ₂ ,
The output of 83 _n is converted into a signal with only a positive sign by the corresponding rectifying circuits 84 ₁ , 84 ₂ , ..., 84 _n as necessary and is supplied to the arithmetic circuit 85. The outputs of the channels are collected for each predetermined block (this process is not necessary because the number of channels in the subsequent stage is thinned out) and supplied to the threshold circuit 86.
6, the output of the arithmetic circuit 85 is compared with a preset threshold value, and when the output of the arithmetic circuit 85 exceeds the threshold value, a defect detection signal is output and an alarm is issued.

[0005]

However, in the above-mentioned conventional defect detecting apparatus, even if the steel sheet has surface defects such as roll marks, all of them are detected as inclusion defects, so that the product yield is high. In addition to the problem of significantly lowering the production efficiency, erroneous countermeasures may be taken when the countermeasures are taken retrospectively from the generation state of inclusions to the manufacturing process, and this may impair the stability of operation.

An object of the present invention is to solve the above-mentioned conventional problems and to provide a steel sheet defect detection method capable of discriminating between surface defects such as roll marks and inclusion defects and detecting them.

[0007]

In order to achieve the above object, in the present invention, a magnetic flux generated from a running steel plate is detected by a magnetic sensor, and a defect of the steel plate is detected based on an output signal from the magnetic sensor. When the number of detected defects at the same position in the width direction of the steel plate is counted within the reference length L preset in the direction, and the count value N is the preset interval of the surface defects as r , N = L / r, the detected defect is discriminated as a periodic surface defect, and N ≠ L / r
In this case, the defect is distinguished from the inclusion defect and detected.

That is, according to the present invention, when the magnetic flux generated from the running steel plate is detected by the magnetic sensor and the defect of the steel plate is detected based on the output signal thereof, within the reference length L preset in the running direction. , The number of defects detected at the same position in the width direction of the steel sheet is counted, and the count value N is a preset interval of surface defects r and an allowable value ΔN (however, ΔN
N is an integer of 0 or more), and when | N−L / r | ≦ ΔN is satisfied, the detected defect is discriminated as a periodic surface defect, and | N−L / r |> ΔN When satisfied, the defect is distinguished from the inclusion defect and detected.

Further, according to the present invention, the magnetic flux generated from the running steel plate is detected by the magnetic sensor, and the defect of the steel plate is detected based on the output signal thereof. The detected number of defects at the same position in the width direction of the steel sheet is counted, and when the count value in the sequential reference length is the same value different from zero, the detected defect is determined as a periodic surface defect. However, if they are different, the defect is distinguished from the inclusion defect and detected.

[0010]

When a roll mark is generated on the steel sheet, a large number of surface defects are arranged at equal intervals at the same width position of the steel sheet with a distance corresponding to the roll circumference. Is unlikely to occur periodically like a roll mark. Therefore, by examining the periodicity of the defect detection signal detected at the same width position of the steel plate, it becomes possible to determine whether or not the detected defect is a roll mark.

Here, the roll for generating the roll mark can be generally specified by following the manufacturing process thereof, and the interval r between the successive surface defects generated from the roll can be set in advance. Therefore, as in the first invention, the number of detected defects at the same position in the width direction of the steel plate is counted within the reference length L preset in the traveling direction, and the count value N and the reference length L are counted. It is possible to determine whether or not the detected defect is a roll mark by comparing it with a preset average number L / r of surface defects generated in the above.

Further, as in the second invention, the number of detected defects at the same position in the width direction of the steel plate is counted within the reference length L preset in the traveling direction, and the count value N and the reference value are counted. The absolute value of the difference between the preset average number L / r of surface defects generated within the length L is calculated, and the absolute value of the difference and the preset allowable value ΔN (where ΔN is a natural number) are calculated. By comparison, it becomes possible to determine and detect whether or not the detected defect is a roll mark.

Further, as in the third aspect, the number of detected defects at the same position in the width direction is counted for each reference length preset in the traveling direction, and the count value in the sequential reference length is calculated. By comparison, it becomes possible to determine and detect whether or not the detected defect is a roll mark.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an example of a steel sheet defect detecting apparatus embodying the present invention. In this embodiment, a surface defect of a steel plate 101 running on a line and a defect such as a non-metallic inclusion are automatically detected online in a threaded state. At a predetermined position of the traveling line of the steel plate 101, the magnetizing device 100 is arranged so as to face the steel plate 101, and a large number of magnetic sensors 11 ₁ , 11 ₂ , ... Are opposed to the magnetizing device 100 via the steel plate 101.・ ・ 、 11
Place _n .

The magnetizing device 100 is arranged in the width direction so as to magnetize the steel plate 101. This magnetizing device 100
At least the magnetic sensors 11 ₁ , 11 ₂ , ..., 11 _n
In order to magnetize the steel plate 101 in the detection portion where is arranged, for example, an electromagnet or other known magnetizing means is used. Further, the magnetic sensors 11 ₁ , 11 ₂ , ..., 1
1 _n are arranged in the width direction of the steel plate 101 so that the leakage magnetic flux of the steel plate 101 can be detected. These magnetic sensors 1
Each of the reference numerals 1 ₁ , 11 ₂ , ..., 11 _n is composed of a magnetic diode, a magnetoresistive element, a Hall element, a magnetic saturation sensor, a micro coil, or the like. It should be noted that, although it depends on the widthwise dimension of the steel sheet 101, each magnetic sensor is preferably about several mm so that flaws can be flawlessly detected at any position in the widthwise direction of the steel sheet 101, regardless of the widthwise dimension. A large number of sheets are arranged in the width direction of the steel plate 101.

Magnetic sensor 11₁, 11₂・ ・ ・ ・ ・ ・ 11
_nEach output signal (channel output) of the corresponding amplifier
12₁, 12₂・ ・ ・ ・ ・ ・ 12_nThe appropriate voltage range.
Width and signal processing circuit 13₁, 13₂, ・ ・ ・, 13_nTo
The noise signal outside the frequency band of the defective signal is supplied.
After removing the minute, the arithmetic circuit 14₁, 14₂・ ・ ・ ・ ・ ・ 1
4_nTo detect the defective signal. These arithmetic circuits 1
4₁, 14₂・ ・ ・ ・ ・ ・ 14_nEach output of
Falling property determination circuit 15₁, 15₂・ ・ ・ ・ ・ ・ 15 _nSupply to
Then, the running of the steel plate 101 from the pulse generator 104 is performed here.
Detected defect signal in synchronization with pulse proportional to line speed
Discriminates the property of the, and displays the result on the display device 103.
It

Amplifiers 12 ₁ , 12 ₂ , ..., 12
_n is composed of, for example, a bridge circuit or an operational amplifier according to the magnetic-electric conversion characteristics of the magnetic sensor used.
Further, the signal processing circuits 13 ₁ , 13 ₂ , ..., 13
Generally, _n is configured by combining a bandpass filter and a rectifier circuit, but it may be a simple one using a resistor and a capacitor, one using an inverting amplifier circuit, or it may be omitted. Furthermore, the pulse generator 1
Although 04 can be configured using, for example, a rotary encoder that rotates in contact with the steel plate 101, if the traveling speed of the steel plate 101 is constant, a commercially available pulse generator can be used alone.

FIG. 2 shows a defect property discriminating circuit 15 shown in FIG.
₁ shows an example of the configuration of 1. The other defect property determination circuits 15 ₂ to 15 _n are also configured in the same manner as in FIG. The defect property determination circuit 15 ₁ receives a defect signal V from the arithmetic circuit 14 ₁ and outputs a pulse when the defect signal V exceeds a preset threshold value, and a pulse generation circuit 104. Counter 23 for counting the pulses of
A counter 24 for counting the pulses generated from the threshold circuit 22; a setting circuit 25 for presetting the average number N ₀ = L / r of surface defects successively generated at intervals r within the reference length L; And a comparison circuit 26 for comparing the count value N of 24 and the set value N ₀ of the setting circuit 25.

The output pulse of the threshold circuit 22 is the counter 2
3 and 24 respectively. The counter 23 counts the pulses from the pulse generator 104 by using the pulse from the threshold circuit 22 as a trigger signal, and the pulses from the threshold circuit 22 and the pulse generator 104.
Each of the gates of the counter 24 and the comparison circuit 26 is controlled based on the count value M of the pulse from. That is,
When the pulse is input from the threshold circuit 22, the counter 2
4 and the gate of the comparison circuit 26 are opened, and the counter 24
At, the pulse from the threshold circuit 22 is counted. Further, the count value M of the pulses of the counter 23 is, at the time exceeds the number of pulses M _L corresponding to the reference length L of the steel plate 101 which is set in advance, the gate of the counter 24 and comparator circuit 26 is closed, the counter 24 Count value N at setting circuit 2
The set value N ₀ from 5 is compared in the comparison circuit 26,
The comparison result is supplied to the display device 103 to be displayed, and the counter 23 is reset to repeat the same operation. The threshold circuit 22 and the counter 2
Commercially available arithmetic elements can be used for 3, 24, the comparison circuit 26, and the setting circuit 25.

The operation of this embodiment will be described below with reference to the flow chart shown in FIG. In this embodiment, the steel plate 101 moving along the traveling line is magnetized by the magnetizing device 100 at a predetermined position as described above, and the leakage magnetic flux generated from the defective portion of the steel plate 101 is thereby detected by the magnetic sensor 11 _1. , 11 ₂ , ..., 11 _n are detected. These magnetic sensors 11 ₁ , 11 ₂ , ...
The output of each channel of 11 _n is the amplifier 12 ₁ , 12 ₂ ,
..., after being amplified by 12 _n , the signal processing circuit 13 ₁ ,
13 _2, ..., noise components 13 _n is removed, the arithmetic circuit 14 _1, 14 _2, ..., defect through 14 _n property determination circuit 15 _1, 15 _2, ..., to 15 _n Supplied.

Defect property determining circuits 15 ₁ , 15 ₂ , ...
.., 15 _n , the corresponding arithmetic circuits 14 ₁ , 14 ₂ , ...
.. When the defect signal V from 14 _n exceeds the threshold value in the threshold circuit 22, the pulse generator 1 in the counter 23
The counting of the pulses from 04 is started, and at the same time, the counting of the pulses from the threshold circuit 22 is started in the counter 24. The counting operation of the counter 23 is omitted in FIG. Here, when the count value M of the counter 23 exceeds the number of pulses M _L corresponding to the preset reference length L of the steel plate 101, the count value N of the counter 24 at that time.
And the set value N ₀ from the setting circuit 25 are compared in a comparison circuit 26. When N = N ₀ , a signal corresponding to a surface defect is obtained, and in other cases, a signal corresponding to an inclusion defect is obtained. The signal is supplied to the display device 103 and displayed.

Therefore, according to this embodiment, surface defects such as roll marks and inclusion defects can be discriminated and detected, so that flaw detection can be performed with high accuracy and the yield of products can be greatly improved. You can Further, since the surface defect and the inclusion defect can be discriminated and detected, the occurrence frequency thereof can be managed separately, which is effective in improving the operation.

In the above embodiment, when N = N ₀ , it is detected as a surface defect, and when N ≠ N ₀ , it is detected as an inclusion defect. In the embodiment, in the first embodiment, the allowable value ΔN of | N−N ₀ | is set in the comparison circuit 26 (where ΔN is an integer of 0 or more), and | N− is set as shown in FIG. N ₀ | <ΔN
When it is, it is determined as a surface defect, and in other cases, it is determined and detected as an inclusion defect.

As described above, by setting the allowable value ΔN of | N−N ₀ |, even if there is a slight erroneous detection signal, the surface defect and the inclusion defect can be discriminated and detected, so that the accuracy is higher. Can perform flaw detection.

FIG. 4 shows the essential parts of a third embodiment of the present invention. In this embodiment, the arithmetic circuits 14 ₁ , 1
The outputs of 4 ₂ , ..., 14 _n are added to the adders 16 ₁ 16 ₂ ,
..., 16 _{n / L} , for each L channel, addition is performed, and the addition output thereof is corresponding to the defect property determination circuit 15 ₁ , 1
5 _2, ..., is supplied to the 15 n / _L, but were designed to detect and discriminate between surface defects and inclusions defects, other configurations are the same as the first embodiment. According to this structure, the number of parts can be reduced, which is advantageous in that the cost can be reduced. Not only for each L channel, the outputs of the arithmetic circuits of all the channels are added, and the output is supplied to one defect property determination circuit so that the surface defect and the inclusion defect can be discriminated and detected. You can also

FIG. 5 shows the construction of the essential parts of the fourth embodiment of the present invention. This embodiment has a sequential reference length L.
Based on the comparison of the detected number of defects in
The defect defect determination circuits 15 ₁ , 15 ₂ , ..., 15 _n are different from the first embodiment shown in FIG. 1 in that the inclusion defect and no defect are discriminated and detected. Other configurations are the same as those in FIG. Thus, FIG. 5 shows a configuration of a defect nature discriminating circuit 15 _1, also other defects property determination circuit 15 ₂ to 15 _n, are configured similarly to FIG.

The defect property determining circuit 15 ₁ receives the defect signal V from the arithmetic circuit 14 ₁ and generates a pulse when the defect signal V exceeds a preset threshold value, and the steel plate 101. A counter 23 that repeatedly counts the number of pulses M _L from the pulse generation circuit 104 corresponding to the reference length L of, and a p-bit (for example, p = 4) counter 24 that counts the pulses generated from the threshold circuit 22. The q-stage (q ≧ 2) shift registers 27 ₁ , 27 ₂ , ...
27 _p , comparison circuits 28 ₁ , 28 ₂ , ..., 28 _p for comparing outputs of q stages of each shift register, an AND gate 29 having inputs of outputs of these comparison circuits, a shift register 27 ₁ , 27 ₂ , ..., 27 _p , an OR gate 30 that receives the output of any one of the same stages, and an AND gate 2
9 and decision circuit 3 which receives the output of OR gate 30
1 and.

Is the counter 23 a pulse generation circuit 104?
Counter 24 in synchronization with the start of counting the pulses from
Open the gate of the
The pulses generated from the value circuit 22 are counted. Also,
The count value M in the unter 23 corresponds to the reference length L in advance.
Set pulse number M_LWhen it exceeds the
The gate is closed, and each bit output representing the count value N is
For example, the least significant bit is the shift register 27₁The best
Shift bits to shift register 27_pShift in accordance with
Register 27₁, 27₂・ ・ ・ ・ ・ ・ 27_pMemorized in the first stage of
And output each bit output of the past counting result to the next stage.
To shift. In this way, the counter 23
Number M_LIs set every time, that is, the steel plate 101 is preset.
Each time the vehicle travels the specified reference length L, within the reference length L
The generated defect signal is counted by the counter 24 and sequentially
Each bit output of the count value at the reference length L of
Shift register, that is, the least significant bit of the sequential count value.
Shift register 27 ₁Shift the most significant bit to
Register 27_pSequentially shift to and store.

The shift registers 27 ₁ , 27 ₂ , ...
In each of the 27 _p , the bit outputs N ₀ and N of the q stage
₁ , ..., N _q are corresponding comparator circuits 28 ₁ , 2
8 _2,..., Compared with 28 _p, it provides a logic "1" when the bit output of the q stages are all equal, the AND gate 29 a logical "0" otherwise. Further, the output of an arbitrary same stage of each shift register 27 ₁ , 27 ₂ , ..., 27 _p , the output of the first stage in FIG. 5, is supplied to the OR gate 30. These AND gate 29 and OR gate 30
Is supplied to the determination circuit 31, where it is determined whether there is an inclusion defect, a surface defect, or no defect based on the outputs of the AND gate 29 and the OR gate 30.

FIG. 6 shows an example of the configuration of the determination circuit 31. This determination circuit 31 includes an inverter 32 ₁ ,
32 ₂ and AND gates 33 ₁ and 33 ₂ . The output of the AND gate 29 is supplied to the inverter 32 ₁ and also to one input terminals of the AND gates 33 ₁ and 33 ₂ . The output of the OR gate 30 is
It is supplied to the other input terminal of the AND gate 33 ₁ and is also supplied to the other input terminal of the AND gate 33 ₂ via the inverter 32 ₂ , whereby the inverter 32 ₁ is supplied.
When the output of is a logic "1", it is determined to be an inclusion defect, and when the output of the AND gate 33 ₁ is a logic "1", it is determined to be a surface defect, and the output of the AND gate 33 ₂ is a logic "1". , It is determined that there is no defect and detected. Each circuit element shown in FIGS. 5 and 6 can be configured by a commercially available arithmetic element or a known combination thereof.

The operation of the main part of this embodiment will be described below with reference to FIG.
This will be described with reference to the flowchart shown. The figure
7, the pulse generator 104 by the counter 23
The pulse counting operation is omitted. Defect property determination times
Road 15₁, 15₂・ ・ ・ ・ ・ ・ 15_nAt the counter
23 starts counting pulses from the pulse generator 104
Then, the gate of the corresponding counter 24 is opened. This
Thereby, in the threshold circuit 22, the corresponding arithmetic circuit 1
4₁, 14₂・ ・ ・ ・ ・ ・ 14_nThe defect signal V from the threshold
The pulse generated when it exceeds the
To be counted. The count value M of the counter 23 is set in advance.
Number of pulses M corresponding to the reference length L of the steel plate 101_LBeyond
Then, at that point, the gate of the counter 24 is closed and
Shift register corresponding to each bit output representing the count value N of
27₁, 27₂・ ・ ・ ・ ・ ・ 27 _pMemorized in the first stage of
In addition, each bit output of the past counting result is
Is shifted to.

Each shift register 27 ₁ , 27 ₂ , ...
.., 27 _{p p-} bit output N ₀ , N ₁ , ..., N
_q is the corresponding comparison circuit 28 ₁ , 28 ₂ , ..., 28
_p is compared, the comparison results are supplied to the AND gate 29, and each shift register 27 ₁ , 27 ₂ ,
The output of the same stage of 27 _p is the OR gate 30
Is supplied to. Here, the AND gate 29 is configured such that, in each of the shift registers 27 ₁ , 27 ₂ , ..., 27 _p , when the bit outputs of the q stages are all equal, that is, the past q
When the count results of the number of times of defects (maximum) are equal, a logic "1" is output, and in other cases, a logic "0" is output. Further, the OR gate 30 outputs a logic "1" when the counter 24 counts the defect signal V within a certain reference length L, that is, when there is a defect, and outputs a logic "1" when the defect signal V is not counted. "0" will be output.

Therefore, in the decision circuit 31, as shown in the truth table in Table 1, when the output of the AND gate 29 is logic "0", only the output of the inverter 32 ₁ becomes logic "1". When the output of the AND gate 29 is logic "1" and the output of the OR gate 30 is logic "1", it is determined that it is an inclusion defect, and only the output of the AND gate 33 ₁ becomes logic "1", and If the output of the AND gate 29 is logic "1" and the output of the OR gate 30 is logic "0", the AND gate 33 ₂ is judged to be defective.
Is output as logic "1", it is determined that there is no defect, and each is displayed on the display device 103.

[Table 1]

As described above, according to this embodiment, surface defects such as roll marks, inclusion defects and no defects can be discriminated and detected, so that the yield of products can be significantly increased as in the above-mentioned embodiments. In addition to being able to improve, there is an effect that operation improvement can be facilitated.

In FIG. 5 and FIG. 6, all the circuits from the counting of the number of defects to the determination of the defect property are composed of logic elements and combinations thereof. It can also be realized by a computer or the like. Also in this embodiment, in the same way as described with reference to FIG. 4, in order to reduce the number of parts and reduce the cost, the output of the arithmetic circuit of each channel is added for each L channel, and several blocks are added. Alternatively, the surface defects, the inclusion defects, and the absence of defects can be discriminated and detected after they are combined into one.

[0036]

As described above, according to the present invention, the defects detected during flaw detection of the steel sheet can be detected by discriminating between defects corresponding to internal inclusions and surface defects such as roll marks. It is possible to prevent an error in which a defect is erroneously detected as an internal defect, and it is possible to perform flaw detection with high accuracy. In addition, since inclusion defects and surface defects can be detected separately, it is possible to control the frequency of occurrence of them separately, so it is possible to accurately provide feedback from product defects to the manufacturing process and improve operation. There is an advantage that you can easily.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of an example of a defect detection device embodying the present invention.

FIG. 2 is a block diagram showing a configuration of an example of the defect property determination circuit shown in FIG.

FIG. 3 is a flowchart for explaining the operation of the first and second embodiments of the present invention.

FIG. 4 is a block diagram for explaining a third embodiment of the present invention.

FIG. 5 is a block diagram for explaining a fourth embodiment of the present invention.

6 is a block diagram showing a configuration of an example of a determination circuit shown in FIG.

FIG. 7 is a flowchart for explaining the operation of the fourth embodiment.

FIG. 8 is a block diagram for explaining a conventional technique.

[Explanation of symbols]

11 _{1 to} 11 _n Magnetic sensor 12 _{1 to} 12 _n Amplifier 13 _{1 to} 13 _n Signal processing circuit 14 _{1 to} 14 _n Arithmetic circuit 15 ₁ to 15 _n Defect property determination circuit 22 Threshold circuit 23, 24 Counter 25 Setting circuit 26 Comparison circuit 26 27 _{1 to} 27 _p shift register 28 _{1 to} 28 _p comparison circuit 29 AND gate 30 OR gate 31 determination circuit 100 magnetizing device 101 steel plate 103 display device 104 pulse generator

Claims (2)

[Claims] 1. A magnetic sensor detects a leakage magnetic flux generated from a running steel plate, and detects a defect in the steel plate based on an output signal thereof, in a reference length L preset in a running direction. When the number of defects detected at the same position in the width direction is counted, and the count value N is a preset sequential surface defect interval is r and the allowable value is ΔN (where ΔN is an integer of 0 or more) , | N−L / r | ≦ ΔN, the detected defect is discriminated as a periodic surface defect, and when | N−L / r |> ΔN, the defect is an inclusion. A method for detecting defects in a steel sheet, which is characterized by detecting defects. 2. A magnetic sensor detects leakage magnetic flux generated from a running steel sheet, and detects a defect in the steel sheet based on an output signal thereof, the width of the steel sheet being set for each reference length preset in the traveling direction. Counting the number of defects detected at the same position in the direction, the count value in the sequential reference length is the same value different from zero, the detected defect is determined as a periodic surface defect,
If they are different, the defect detection method for a steel sheet is characterized by detecting the defect by discriminating it as an inclusion defect.