JP6179482B2 - Method and apparatus for detecting minute irregular surface defects - Google Patents

Description

  The present invention relates to a detection method and a detection apparatus for minute uneven surface defects formed on a steel plate.

In the steel sheet manufacturing process, the foreign matter attached to the roll installed in the production line and the irregularities formed on the roll surface by the foreign matter being bitten into the roll are transferred to the steel plate, thereby causing roll defects. May occur. Among these roll defects, the surface roughness Ra of a thin steel plate having a thickness of 0.8 to 1.6 mm exists in a rough surface having a thickness of 0.5 to 2 μm, and is gentle. a contour having a (radius of curvature R ≧ 10 mm), irregularity 5μm or less, there is an area 10 mm ² or more fine uneven surface defects. The area of this micro uneven surface defect is about 10 to several tens of mm ² , but the amount of unevenness is 5 μm or less, and the smallest is around 1 μm, which is the same order as the surface roughness Ra of the thin steel plate. It is a small one.

  Most roll defects having a relatively large unevenness are visible, so that they can be easily found on the production line. However, the surface irregularities of minute irregularities are in the same order as the surface roughness Ra of the thin steel sheet, and since the optical difference is small, it is difficult to find them even if they are observed as they are. It ’s difficult to discover. However, the micro uneven surface defect becomes clear when the surface roughness Ra of the thin steel plate becomes smooth when the thin steel plate is painted and buried in the paint, which is a serious problem in appearance. Therefore, it is an important problem in quality control to find minute surface irregularities before painting thin steel sheets.

  As the form of the micro uneven surface defect, there are a dot-like wrinkle such as a roll wrinkle and a wrinkle that continues in the longitudinal direction of the thin steel plate such as a linear mark or a drawing mark. The micro uneven surface defect occurs when the unevenness formed on the roll is transferred to the thin steel plate, and once generated, it continuously occurs until the roll is replaced or the manufacturing process is improved. For this reason, it is extremely important from the standpoint of yield improvement to detect microscopic surface irregularities at an early stage and take countermeasures.

  Conventionally, in order to detect minute irregular surface defects, in each inspection line of the thin steel sheet manufacturing process, the traveling of the thin steel sheet is stopped during operation for all coils, and the inspector grinds the thin steel sheet. Visual inspection is performed after the inspection. When grinding a thin steel plate, the convex part hits the grindstone more than the concave part, the reflectance of the convex part becomes high, and the difference between the reflectance of the concave part and the reflectance of the convex part becomes large. It becomes easy to visually confirm minute surface irregularities. Generally, this operation is called grinding wheel inspection.

  However, when the grinding wheel inspection is performed, the traveling of the thin steel plate must be stopped, and since a lot of time is required for the visual inspection, the manufacturing efficiency of the thin steel plate is lowered. Against this background, a method for automatically detecting minute uneven surface defects has been proposed in recent years. Specifically, Patent Document 1 describes a technique for automatically detecting a minute uneven surface defect using a leakage magnetic flux flaw detection method. Specifically, the inventors who proposed the method described in Patent Document 1 perform X-ray diffraction measurement on a plurality of defects in order to detect minute uneven surface defects generated by a rolling roll during the production of a cold-rolled steel sheet. The physical properties were analyzed. As a result of the analysis, the inventors have a distortion that is thought to have occurred when the wrinkle is transferred from the roll to the micro uneven surface defect, and the micro uneven surface defect cannot be sufficiently detected only by the shape-derived signal. However, it is proposed that a micro uneven surface defect can be detected by detecting a signal due to distortion.

  And in the method of patent document 1, as a magnetization condition of the thin steel plate for improving the S / N ratio of the defect signal, the magnetic flux density of the thin steel plate to which the magnetic flux is applied is 75% or more and less than 95% of the saturation magnetic flux density. Or a combination of this condition and a condition in which the magnetic flux density of the thin steel sheet to which the magnetic flux is applied is 95% or more, or the strength of the magnetic field applied to the thin steel sheet is 4000 to 25000 A / m. Conditions are listed. Further, it is assumed that a Hall element is used as a magnetic flux detection element for detecting a defect signal.

JP 2009-52903 A

  By the way, an electromagnet is usually used to magnetize a thin steel plate. However, when a large magnetic field such as the magnetization condition described in Patent Document 1 is applied, the electromagnet becomes large and a large current needs to flow. Further, when the electromagnet is brought online, it is necessary to examine in detail the fixing method of the electromagnet, and the cost for manufacturing and operating the electromagnet increases.

  In addition, when a Hall element is used as the magnetic flux detection element, the defect signal level decreases and the S / N ratio of the defect signal decreases as the influence of thermal noise becomes significant as the applied magnetization level decreases. . In order to solve such a problem, it is conceivable to use a sensor having a magnetic sensitivity equivalent to that of a Hall element such as a SQUID (Superconducting Quantum Interference Device) or MI (Magneto Impedance) element as a magnetic flux detection element.

  However, when the SQUID is used, the SQUID needs to be cooled by a cryogen such as liquid helium, so that the apparatus becomes very large and the cost increases. Further, since the MI element is sensitive to temperature, it is necessary to take measures such as keeping the temperature around the MI element uniform, which is also disadvantageous in terms of cost.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and its object is to detect a micro uneven surface defect with a simple configuration without high cost and to detect a micro uneven surface defect with high accuracy, and to detect it. To provide an apparatus.

  A method for detecting a micro uneven surface defect according to the present invention is a method for detecting a micro uneven surface defect on a steel plate, wherein the micro uneven surface defect is detected by applying a magnetic field to the steel plate and using a tunnel magnetoresistive element. The method includes a step of detecting a minute uneven surface defect by detecting a magnetic signal caused by distortion of the steel sheet due to the uneven surface defect.

  The method for detecting a micro uneven surface defect according to the present invention is characterized in that, in the above invention, the magnitude of the magnetic field is less than 75% of the saturation magnetic flux density of the steel sheet.

  The method for detecting a micro uneven surface defect according to the present invention is characterized in that, in the above invention, the tunnel magnetoresistive element is arranged so that a magnetic field sensitivity direction of the tunnel magnetoresistive element substantially coincides with a surface vertical direction of a steel sheet. To do.

  A micro uneven surface defect detecting device according to the present invention is a micro uneven surface defect detecting device for detecting micro uneven surface defects on a steel plate, and includes a magnetic field applying means for applying a magnetic field to the steel plate, and a micro uneven surface defect. A tunnel magnetoresistive element that detects a magnetic signal caused by distortion of a steel plate, and a detection unit that detects a minute uneven surface defect based on the magnetic signal detected by the tunnel magnetoresistive element.

  According to the detection method and detection apparatus for minute uneven surface defects according to the present invention, it is possible to detect minute uneven surface defects with high accuracy with a simple configuration without requiring much cost.

FIG. 1 is a schematic diagram showing a configuration of a tunnel magnetoresistive element used in a minute uneven surface defect detection apparatus according to an embodiment of the present invention. FIG. 2 is a circuit diagram showing a configuration of a minute uneven surface defect detection apparatus according to an embodiment of the present invention. FIG. 3 is a schematic diagram for explaining a method of detecting a micro uneven surface defect according to an embodiment of the present invention. FIG. 4 is a diagram illustrating an example of a magnetic signal derived from a defect caused by transfer of foreign matter attached on a roll to a thin steel plate. FIG. 5 is a diagram showing the level change of the defect signal and the noise signal when the magnitude of the magnetic flux density applied to the thin steel plate is changed. FIG. 6 is a diagram showing the S / N ratio of this embodiment and the conventional defect signal.

  The inventors of the present invention have expanded the range of magnetization conditions of thin steel sheets in the method described in Patent Document 1 so that minute uneven surface defects can be detected with high accuracy with a simple configuration without requiring much cost. We investigated whether it was possible to detect surface irregularities with a small magnetic field. As a result, the inventors of the present invention have come up with the technical idea that a micro uneven surface defect can be detected with a smaller magnetic field by using a tunnel magnetoresistive element which is a very sensitive magnetic sensor. . The tunnel magnetoresistive element is very sensitive to a small magnetic field and has a feature that temperature drift is small compared to other magnetic sensors. Hereinafter, with reference to the drawings, a configuration of a minute uneven surface defect detection apparatus according to an embodiment of the present invention will be described.

[Configuration of tunnel magnetoresistive element]
First, a configuration of a tunnel magnetoresistive element used in a minute uneven surface defect detection apparatus according to an embodiment of the present invention will be described with reference to FIG.

  FIGS. 1A and 1B are schematic views showing the configuration of a tunnel magnetoresistive element used in a minute uneven surface defect detection apparatus according to an embodiment of the present invention. As shown in FIGS. 1A and 1B, a tunnel magnetoresistive element 1 used in a minute uneven surface defect detection apparatus according to an embodiment of the present invention includes a fixed layer 2a formed of a ferromagnetic thin film. And the free layer 2b and the insulating film 3 sandwiched between the fixed layer 2a and the free layer 2b, and the magnetization direction B1 of the fixed layer 2a when a magnetic field is applied to the tunnel magnetoresistive element 1 is fixed in advance. ing.

  In the tunnel magnetoresistive element 1 having such a configuration, when a magnetic flux enters the free layer 2b in a state where a voltage is applied from the free layer 2b side to the fixed layer 2a side as indicated by an arrow A in the figure, The magnetoresistance changes according to the magnetization direction or the magnetic flux density of the fixed layer 2a and the free layer 2b.

  That is, as shown in FIG. 1 (a), when the magnetization direction B1 of the fixed layer 2a and the magnetization direction B2 of the free layer 2b are the same direction, the magnetoresistance of the tunnel magnetoresistive element 1 decreases and current flows. It becomes easy. On the other hand, as shown in FIG. 1B, when the magnetization direction B1 of the fixed layer 2a and the magnetization direction B2 of the free layer 2b are different, the magnetoresistance of the tunnel magnetoresistive element 1 increases and the current hardly flows. .

[Configuration of the micro uneven surface defect detection device]
Next, with reference to FIG. 2 and FIG. 3, the configuration of a minute uneven surface defect detection device according to an embodiment of the present invention will be described. FIG. 2 is a circuit diagram showing a configuration of a minute uneven surface defect detection apparatus according to an embodiment of the present invention. FIG. 3 is a schematic diagram for explaining a method of detecting a micro uneven surface defect according to an embodiment of the present invention.

  As shown in FIG. 2, a minute uneven surface defect detection device (defect detection device) 10 according to an embodiment of the present invention includes a tunnel magnetoresistive element 1, a DC power source 11, a resistance element 12, and the like shown in FIG. 1. , And a DC voltage measuring unit 13.

  The DC power source 11 is connected in series to the tunnel magnetoresistive element 1 and applies a DC voltage to the tunnel magnetoresistive element 1.

  The resistance element 12 is a resistance element connected in series to the tunnel magnetoresistance element 1.

  The DC voltage measuring unit 13 measures a current flowing through a series circuit constituted by the tunnel magnetoresistive element 1, the DC power source 11, and the resistive element 12 by measuring a voltage applied to the resistive element 12. The current flowing through the series circuit changes depending on the magnetoresistance of the tunnel magnetoresistive element 1. Although not shown, the DC voltage measuring unit 13 may be provided with an amplifier that amplifies the voltage signal and a filter that removes a high-frequency noise signal included in the voltage signal.

  When detecting a micro uneven surface defect using the defect detection apparatus 10 having such a configuration, as shown in FIG. 3A, in a state separated from the surface of the thin steel sheet S by a predetermined lift-off amount. The tunnel magnetoresistive element 1 is arranged so that the magnetic field sensitivity direction (the direction of the bidirectional arrow B shown in FIG. 2) substantially coincides with the surface vertical direction of the thin steel sheet S. Then, a DC voltage is applied to the tunnel magnetoresistive element 1 using the DC power supply 11.

  In such a state, when the magnetization direction or magnetic flux density of the free layer 2b of the tunnel magnetoresistive element 1 changes due to minute uneven surface defects formed on the thin steel sheet S, the magnetoresistance of the tunnel magnetoresistive element 1 changes. A change in the magnetoresistance of the tunnel magnetoresistive element 1 can be detected by a change in the current value measured by the DC voltage measuring unit 13. Therefore, the minute uneven surface defect formed in the thin steel sheet S can be detected by detecting a change in the current value measured by the DC voltage measuring unit 13.

  When the thin steel plate S is magnetized in the detection process of the micro uneven surface defect, the tunnel magnetoresistive element 1 is positioned at a position corresponding to the center position between the magnetic poles of the magnetizer 20, as shown in FIG. Is installed. Further, in this case, when the thin steel sheet S having no micro uneven surface defect is magnetized at a predetermined magnetization level, the current value measured by the DC voltage measuring unit 13 is the tunnel magnetoresistive element 1 when the magnetic field is 0 gauss. The position of the tunnel magnetoresistive element 1 is adjusted in advance so as to have a value corresponding to the magnetoresistance value.

  In addition, it is required to perform a wide area inspection simultaneously on the actual line. It is efficient to detect wrinkles by arranging a plurality of tunnel magnetoresistive elements 1 in a row in the width direction of the steel plate to be passed. In such a case, the elements are arranged so that the distance between the elements is not too large with respect to the size of the minute irregularity defect that needs to be detected. Considering the range in which the magnetic flux leaking due to the strain introduced into the thin steel sheet S is expanded, the element spacing is allowed to be in the range of about 2 to 10 times the size of the surface of the steel sheet to be detected. This is determined by the sensitivity and efficiency (economic efficiency) of the device. Each element is preferably arranged in a direction perpendicular to the traveling direction of the steel sheet S and in a direction parallel to the steel sheet S.

[Magnetic condition of thin steel sheet]
In the magnetization curve (BH curve) in a state in which a force is applied from the outside to the thin steel plate to cause distortion in the entire thin steel plate and the magnetization curve in a state in which no strain is applied, the magnetization level is smaller than the saturation magnetic flux region. A large difference occurs in the region (rotational magnetization region). For this reason, when detecting a micro uneven surface defect, it is desirable to magnetize a thin steel plate with a magnetization level smaller than the saturation magnetic flux density in order to detect with high sensitivity a signal caused by the strain due to the micro uneven surface defect. Therefore, the inventors of the present invention have repeatedly investigated the magnetization level that can detect a minute uneven surface defect with high sensitivity. The strength of the magnetic field applied to the thin steel sheet is not the sum of the magnetic fields generated from the magnetizer that magnetizes the thin steel sheet, but the magnetic field directly applied to the inspection area of the thin steel sheet, that is, the inspection area of the thin steel plate. This is an amount corresponding to the magnetic field strength H when the magnetization curve is drawn.

  As a result of gradually decreasing the magnetization level of the thin steel sheet, the inventors of the present invention have found that even when the thin steel sheet is not magnetized at all, it is possible to detect a magnetic signal due to distortion due to minute uneven surface defects. This means that the thin steel plate region to which the strain due to the micro uneven surface defect is given is magnetized by locally aligning the spins. However, since the S / N ratio of the defect signal is lowered as the magnetization level of the thin steel plate is lowered, it is desirable to magnetize the thin steel plate according to the magnetic signal level of the minimum defect to be detected.

  When the Hall element found in Patent Document 1 or the like is used as a magnetic flux detection element, the level of the defect signal decreases as the magnetization level decreases and the influence of thermal noise becomes significant. The ratio decreases. On the other hand, when the tunnel magnetoresistive element 1 is used as the magnetic flux detecting element, the influence of thermal noise is small, so that even if the level of the defect signal is reduced as the magnetization level is reduced, a good S / N is obtained. A ratio is obtained.

  Normally, the defect inspection apparatus is used for product inspection on the final line, and various processes exist from the time of defect occurrence to the inspection. In these processes, for example, a phenomenon in which the strain generated at the time of defect occurrence is removed by heat, the strain is released by other stresses such as tension during feeding, and the strain is released There is a possibility that a magnetic signal due to distortion cannot be detected in such a process. For this reason, it is the same as in the case of a normal leakage magnetic flux flaw detector that the location for detecting a minute uneven surface defect needs to be examined.

  In this example, a DC voltage of 5 V was applied to the tunnel magnetoresistive element 1, and a 1 kΩ resistive element was used as the resistive element 12. The DC voltage measurement unit 13 uses a low-pass filter that amplifies the voltage signal with a 40 db amplifier and can change the filter constant according to the flaw detection speed. The tunnel magnetoresistive element 1 had a width of 0.9 mm, a thickness of 0.3 mm, and a length of 0.8 mm. The lift-off amount from the thin steel plate was set to 1 mm, which is optimal for leakage magnetic flux inspection. The magnetic field sensitivity direction of the tunnel magnetoresistive element 1 was perpendicular to the length direction of the element and parallel to the width direction of the element.

  FIG. 4 is a diagram illustrating an example of a magnetic signal derived from a defect caused by transfer of foreign matter attached on a roll to a thin steel plate. In this example, the shape of the defect was a circular shape having a diameter of about 1 mm, and the cross-sectional shape of the defect was a convex shape. In this example, the thin steel plate was not magnetized. From the S / N ratio of the defect signal measured under the same magnetization conditions as those described in Patent Document 1, it was confirmed that a distortion of about 0.00037 occurred. Moreover, the level of the defect signal increased as the thin steel plate was gradually magnetized. At this time, the noise signal gradually increased, but the increase rate of the noise signal was much smaller than the increase rate of the defect signal, and the S / N ratio increased.

  FIG. 5 shows the level change of the defect signal and the noise signal when the magnetic flux density (saturation magnetic flux density) when the thin steel plate is magnetically saturated is 100% and the magnitude of the magnetic flux density applied to the thin steel plate is changed. FIG. FIG. 6 is a diagram showing the S / N ratio of the defect signal of the present embodiment and the S / N ratio of the conventional defect signal calculated from FIG. As shown in FIG. 6, according to the present example, even when the thin steel sheet is not magnetized, the defect signal S / N ratio is about 8 and the defect according to the method described in Patent Document 1, which is a conventional example. It was confirmed that it was sufficiently larger than the S / N ratio of the signal. Further, it was confirmed that a sufficient S / N ratio was obtained over the entire range where the ratio of the magnetic flux density to the measured saturation magnetic flux density was less than 75%.

  In addition, the value of the magnetic flux density in the present embodiment is obtained from the magnetic field strength H based on a previously measured magnetization curve. Regarding the magnetic field strength H, the value of the space near the inspection position of the thin steel plate may be measured and used. In addition, in this embodiment, a micro uneven surface defect was detected using a DC leakage magnetic flux. However, if a signal due to distortion can be detected, a tunnel magnetoresistive element can be used even in the AC leakage magnetic flux method and the eddy current flaw detection method. Thus, it is possible to detect a minute uneven surface defect.

  The embodiment to which the invention made by the present inventors is applied has been described above, but the present invention is not limited by the description and the drawings that constitute a part of the disclosure of the present invention. That is, other embodiments, examples, operational techniques, and the like made by those skilled in the art based on this embodiment are all included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Tunnel magnetoresistive element 2a Fixed layer 2b Free layer 3 Insulating film 10 Defect detection apparatus 11 DC power supply 12 Resistance element 13 DC voltage measurement part

Claims (3)

A method for detecting micro uneven surface defects that detect micro uneven surface defects on a steel sheet,
The magnetic field is applied to the steel plate, it viewed including the steps of detecting a minute uneven surface defect by sensing a magnetic signal due to distortion of the steel sheet due to the minute uneven surface defect by utilizing a tunneling magneto-resistance element, the magnitude of the magnetic field Is a size of less than 75% of the saturation magnetic flux density of the steel sheet, and a method for detecting minute uneven surface defects. Detection method for a micro uneven surface defects according to claim 1, characterized in that the magnetic field sensitivity direction of the tunnel magneto-resistance element is disposed the tunnel magneto-resistance element such that substantially coincides with the direction perpendicular to the surface of the steel sheet. A micro uneven surface defect detection device for detecting micro uneven surface defects on a steel plate,
Magnetic field applying means for applying a magnetic field to the steel sheet;
A tunnel magnetoresistive element that detects a magnetic signal caused by a distortion of a steel sheet due to a micro uneven surface defect;
Detecting means for detecting a micro uneven surface defect based on a magnetic signal detected by the tunnel magnetoresistive element ,
The apparatus for detecting minute uneven surface defects, wherein the magnitude of the magnetic field is less than 75% of the saturation magnetic flux density of the steel sheet .

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