JP6950811B2 - Metal plate surface defect detection method and equipment, and galvanized steel sheet manufacturing method - Google Patents

Description

The present invention relates to a detection method for optically detecting unevenness defects on the surface of a metal plate, and is particularly suitable for detecting sputter defects existing on the surface of a cold-rolled steel sheet or a galvanized steel sheet. The present invention relates to a detection method and an apparatus, and a method for manufacturing a plated steel sheet.

The required level for the surface quality of thin steel sheets such as hot-dip galvanized steel sheets has become stricter in recent years, and even defects having a size of about 0.2 mm have become a problem. Therefore, it is important to ensure quality so as not to ship products with such defects. In addition, since some of these defects occur randomly on the surface of the steel sheet, it is necessary to inspect the entire length of the steel sheet, and an automatic inspection device for that purpose (particularly, an inspection device using an optical method). Is under development.

In Patent Document 1, the illumination means and the image pickup means are arranged on the rear side with respect to the traveling direction of the steel sheet, and the incident angle of the illumination light by the illumination means on the steel sheet surface and the light receiving angle of the image pickup means are set in a predetermined angle range. Defect inspection equipment is shown. According to this device, it is possible to suppress the influence of a background that is not a defect and to image a minute defect.
Further, Patent Document 2 discloses a surface inspection apparatus including an image reflecting means having a reflecting surface for reflecting an image to be inspected in addition to an illuminating means and an imaging means, and displays defective images from a plurality of angles. To analyze. This makes it possible to distinguish between harmful defects and harmless defects such as dirt, and to identify the type of defect.

Further, in Patent Document 3, the surface of the metal plate is illuminated, the illuminated portion on the metal plate is imaged, and the obtained image information is subjected to threshold processing to extract surface defects of the metal plate. Defect inspection methods are shown. In Patent Document 3, among the obtained image information, the image information s1 in which the image brightness is less than the dark part threshold value a1 and the image information s2 in which the image brightness exceeds the bright part threshold value a2 are both image information. If there is image information s1 and s2 in which the distance between the parts p1 and p2 of s1 and s2 is equal to or less than the distance threshold value (x), one defect that combines the parts p1 and p2 of these image information s1 and s2. A method for detecting surface defects of a metal plate from which a portion is extracted is shown. Further, Patent Document 3 states that it is possible to suppress over-detection and extract minute harmful defects by classifying them according to the aspect ratio in addition to the lightness and darkness of the defective portion.

Japanese Unexamined Patent Publication No. 2012-103017 Japanese Unexamined Patent Publication No. 2011-53228 Japanese Unexamined Patent Publication No. 2016-188768

In the apparatus of Patent Document 1, defect identification is possible by the difference in brightness between the defective portion and the background portion. However, the processing of determining the type of defect and determining the severity of the defect is not mentioned, and therefore it is considered difficult to determine them.

Further, in the apparatus of Patent Document 2, the equipment is complicated due to the presence of the image reflecting means, the defect detection ability is affected if the reflector is not properly maintained, and the image processing is complicated, so that the inspection is performed. Problems include the need to secure sufficient communication speed and calculation speed when the target moves at high speed.

Further, in the method of Patent Document 3, defects in which the steel sheet is deformed in parallel with the width direction of the steel sheet can be classified and detected. However, it is difficult to classify and detect defects in which the steel sheet is deformed in parallel with the traveling direction of the steel sheet, for example, spatter defects existing on the surface of cold-rolled steel sheet or galvanized steel sheet. Cannot be applied. Spatter defects and dross defects and pickpocket defects are the same micro-defects, but the degree of influence on the press die, etc. when processing steel sheets is different (generally, spatter defects are harder and therefore more likely to have an adverse effect). It is necessary to detect it separately from dross defects and scratches. In addition, since the causes of spatter defects and dross defects and scratches are completely different, if they cannot be detected separately, it is not possible to identify the cause appropriately and take measures to prevent defects.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to detect surface defects of a metal plate, such as spatter defects existing on the surface of a cold-rolled steel sheet or a galvanized steel sheet with a simple equipment configuration. It is an object of the present invention to provide a surface defect detecting method and an apparatus capable of accurately detecting harmful defects in which deformation of a metal plate occurs in parallel with the traveling direction of the metal plate as described above, and a method for manufacturing a plated steel sheet.

The gist of the present invention for solving the above problems is as follows.
[1] In a surface defect inspection method in which an illuminated metal plate surface is imaged and the obtained image information is threshold-processed to extract surface defects of the plated metal plate.
In the obtained image information, the distance between the dark part (p1) where the image brightness is less than the dark part threshold value (a1) and the bright part (p2) where the image brightness exceeds the bright part threshold value (a2) is the distance. Detects dark areas (p1) and bright areas (p2) that are below the threshold value (x).
The detected dark part (p1) and bright part (p2) are combined and extracted as one defective part.
Among the extracted defective parts, a metal that satisfies at least [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≥ ratio threshold value (y) as a harmful defect. A method for inspecting surface defects on a plate.
[2] The method for detecting a surface defect of a metal plate according to [1], wherein the distance threshold value (x) is set in the range of 0.1 to 5.0 mm.

[3] The method for detecting a surface defect of a metal plate according to [1] or [2], wherein the ratio threshold value (y) is set in the range of 1.0 to 5.0.
[4] Among the extracted defective parts, [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≥ the ratio threshold value (y) is satisfied, and the following (1) to The method for detecting a surface defect of a metal plate according to any one of [1] to [3], wherein a defective portion satisfying one or more of (3) is determined to be a harmful defect or a serious harmful defect.
(1) The total area of the bright part (p2) in the defective part is equal to or larger than the area threshold value (w).
(2) The total length of the bright portion (p2) in the defective portion in the longitudinal direction of the metal plate is equal to or greater than the length threshold value (z).
(3) The maximum brightness value of the bright part (p2) in the defective part exceeds the bright part threshold value (a20) set to be higher brightness than the bright part threshold value (a2).

[5] Among the extracted defective parts, [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≧ ratio threshold value (y) is satisfied, and the above (1) to (1) to The method for detecting a surface defect of a metal plate according to [4], wherein a defect portion satisfying two or more of (3) is determined to be a harmful defect or a serious harmful defect.

[6] Among the extracted defective parts, [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≧ ratio threshold value (y) is satisfied, and the above (1) to (1) to The method for detecting a surface defect of a metal plate according to [4], wherein a defective portion satisfying all of (3) is determined to be a harmful defect or a serious harmful defect.

[7] The method for detecting a surface defect of a metal plate according to any one of [4] to [6], wherein the area threshold value (w) is ^{set in the range of 0.06 to 34.00 mm 2.}
[8] The method for detecting a surface defect of a metal plate according to any one of [4] to [7], wherein the length threshold value (z) is set in the range of 0.6 to 10.0 mm.
[9] The illuminating means for illuminating the surface of the metal plate and the imaging means for imaging the illuminated portion on the metal plate by the illuminating means are either on the front side or the rear side of the metal plate normal line in the direction of travel of the metal plate. The method for detecting surface defects of a metal plate according to any one of [1] to [8] arranged in.

[11] When a spatter defect is determined by the method for detecting a surface defect of a metal plate according to [10], it is determined that there is a harmful defect, which is different from the case where it is determined to be a scratch defect and / or a dross defect. A method for manufacturing a plated steel sheet, which is carried out on a plated steel sheet.
[12] A surface defect inspection apparatus for a metal plate, comprising an image processing apparatus and a defect determination apparatus for detecting surface defects by the surface defect detecting method according to any one of [1] to [11].

According to the present invention, harmful defects such as spatter defects existing on the surface of a cold-rolled steel sheet or a galvanized steel sheet, which are deformed in parallel with the traveling direction of the metal plate, can be accurately detected with a simple equipment configuration. be able to.

Explanatory drawing which shows an example of the surface defect inspection apparatus used for carrying out this invention. A drawing showing an image of a spatter defect captured by an imager (left figure) and an image after image processing (right figure). Conceptual diagram showing an example of the threshold value processing performed on the image information of FIG. 2 in the present invention. A drawing schematically showing image information for extracting defects and determining harmful defects in one embodiment of the present invention. A drawing schematically showing image information for extracting defects and determining harmful defects in another embodiment of the present invention. A drawing showing each image of dross defects, scratches, and spatters captured by an imager.

The present invention is a method for detecting surface defects of various metal plates including a steel plate, but since it is a particularly suitable method for detecting spatter defects existing on the surface of a steel plate, the following mainly describes the method. The case of detecting surface defects of steel sheets such as cold-rolled steel sheets and galvanized steel sheets will be described as an example.
The minute defects include harmless defects such as stains, minute foreign substances, and minor scratches that are not harmful in actual use, and harmful defects that cause problems during processing and painting. For example, harmless defects include stains due to scattering of a very small amount of oil droplets, and harmful defects include sputter defects in cold-rolled steel sheets and galvanized steel sheets. This spatter defect is a spatter generated when welding the ends of steel sheets in a continuous line (metal droplets melted in welding scatter and become a small spherical solid after cooling) on the metal plate. It is a defect that occurs when it is pushed in. Sputter defects are surface defects consisting of spatter adhering to the surface of a steel sheet (oxidized sputter) and a deformed portion of the steel sheet generated from this spatter. With conventional methods, it is difficult to distinguish between the above-mentioned harmful defects and harmless defects. The reason is that if there is no difference in surface roughness or optical characteristics between the harmful defect and the harmless defect, the difference cannot be found by the optical method.

Therefore, in the present invention, the defect is determined in consideration of not only the defect portion but also the influence of the steel plate portion around the defect portion. For example, the spatter adhering to the steel sheet by welding before the pickling process is pushed into the steel sheet while forming cracks in the steel sheet during the subsequent cold rolling process or passing through the process roll. Further, since this crack is pushed in between runs, it has a shape extending in the running direction (longitudinal direction of the steel plate). It is known that the degree of this deformation has a strong relationship with whether or not a steel sheet becomes a harmful defect when press-molded, and also correlates with the high severity of the harmful defect. If the severity is high, the press die may be damaged in the press molding process, and the press die with such a defect needs to be maintained or replaced. Therefore, the quality and production efficiency of the press molding process are greatly adversely affected. Therefore, it is an extremely important technique to accurately detect spatter defects in steel sheets and appropriately determine their severity.

For example, defect detection is performed by an inspection device having a lighting means and an imaging means on the rear side with respect to the traveling direction of the steel sheet. Considering the characteristics of spatter defects in cold-rolled steel sheets, the surface of spatter itself is oxidized and the surface is black, resulting in a dark image with respect to the background. Further, even in the case of a galvanized steel sheet in which the upper layer of sputtering is plated, the surface of the sputter has a metallic luster, and the image becomes dark with respect to the background. On the other hand, with respect to the crack portion (steel plate deformed portion) that enters in the longitudinal direction before and after the sputtering, since the incident angle of the illumination is different from the background, there is a bright portion with respect to the background.

Therefore, by performing image processing as one defect location when the dark image and the bright image are in close proximity, it is possible to distinguish it from harmless defects such as scratches that do not involve deformation of the steel sheet due to spatter pressing. It can be recognized as a serious defect accompanied by deformation of the steel sheet. Therefore, in the present invention, in order to extract as one defect portion when the dark image and the bright image are close to each other and to distinguish from harmless defects such as scratches, the longitudinal direction of the steel plate of this defect portion. And the dimensional ratio (aspect ratio) in the width direction is used as an index. This makes it possible to accurately discriminate harmful defects for inspection purposes.
Here, FIG. 6 shows images of dross defects, scratches, and spatter defects captured by the imager.

The present invention is a surface defect inspection method in which the surface of a steel sheet is illuminated, an illuminated portion on the steel sheet is imaged, and the obtained image information is subjected to threshold processing to extract surface defects of the steel sheet. FIG. 1 shows an example of a surface defect inspection apparatus used for carrying out the present invention. The surface defect inspection device includes a floodlight 1, an imager 2, an image processing device 3, a defect determination device 4, and the like.
The floodlight 1 projects light onto the surface of a steel plate 5 (for example, a hot-dip galvanized steel plate). The type of illumination is not particularly limited, and for example, halogen illumination, metal halide illumination, fluorescent lamp, LED illumination, xenon strobe illumination and the like used in conventional surface defect inspection can be used.
The imager 2 captures the light reflected from the surface of the steel plate 5, and for example, a CCD area sensor camera, a CCD line sensor camera, or the like can be used. The spatial resolution of the imager 2 is appropriately set to 0.2 mm or less in order to detect minute defects.

In the example of FIG. 1, the floodlight 1 and the imager 2 are arranged on the rear side (downstream side) of the steel plate normal in the steel plate traveling direction. Generally, the incident angle α of the light from the floodlight 1 is set to, for example, about 50 ° to 80 ° with respect to the steel plate normal, and the light receiving angle β of the imager 3 is set to, for example, about 0 ° to 40 ° with respect to the steel plate normal. Is set to. The floodlight 1 and the imager 2 may be arranged on the front side (upstream side) of the steel plate normal in the steel plate traveling direction.
The image processing device 3 performs threshold processing on the image obtained by the imager 2 to obtain image information necessary for defect determination. The defect determination device 4 extracts defective portions and determines harmful defects based on the image information obtained by the image processing device 3. The incident angle α is preferably set in the range of 55 to 70 °, and the light receiving angle is preferably set in the range of 20 to 40 °.
The image pickup by the imager 2 is performed at a spot having a predetermined equipment resolution (preferably 0.2 mm or less) with respect to the overall width and overall length of the continuously conveyed steel sheet.

In the present invention, for example, in the surface defect inspection device as described above, the image captured by the imager 2 is image-processed by the image processing device 3, and the defect determination device 4 is defective from the image information (image signal). The location is detected. In the defect determination device 4, the dark part p1 of the image information whose image brightness is less than the dark part threshold value a1 and the bright part p2 of the image information whose image brightness exceeds the bright part threshold value a2 are detected. When the distance between the dark part p1 and the bright part p2 is equal to or less than the distance threshold value (x), the part corresponding to the dark part p1 and the part corresponding to the bright part p2 (however, the part corresponding to the dark part p1 and the bright part p2 If there is another part between the part corresponding to and the part corresponding to (including this part), one defect part is extracted. Here, the meaning of the parentheses is that if the distance between the dark part p1 and the bright part p2 is equal to or less than the distance threshold value (x), another part is between the part corresponding to the dark part p1 and the part corresponding to the bright part p2. That is, even if there is a portion of image information whose image brightness is equal to or greater than the dark portion threshold value a1 and equal to or less than the bright portion threshold value a2, this is also extracted as one defective portion.

Here, the dark portion p1 whose image brightness is less than the dark portion threshold value a1 is usually image information of deposits on the steel plate. Taking a spatter defect as an example, the spatter itself adhering to the steel sheet surface is oxidized on the surface and exhibits a black color, resulting in a dark image with respect to the background. Therefore, the image information of the spatter itself is the dark portion p1 described above. On the other hand, the bright portion p2 whose image brightness exceeds the bright portion threshold value a2 is usually image information of a deformed portion of the steel sheet around the deposit. Taking a spatter defect as an example, a crack (steel deformed part) that enters parallel to the longitudinal direction of the steel sheet before and after the spatter pushed into the steel sheet is a bright image with respect to the background because the incident angle of illumination is different from the background. It becomes. Therefore, the image information of the deformed portion of the steel plate is the bright portion p2 described above.

FIG. 2 shows an image of a spatter defect captured by an imager (left figure) and an image of the spatter defect after image processing (right figure), and the numerical value shown in the right figure is the image brightness. .. Further, FIG. 3 is a conceptual diagram showing an example of threshold processing performed on the image information of FIG. In FIG. 3, the dark portion p1 of the image information whose image brightness is less than the dark portion threshold value a1 and the bright portion of the image information whose image brightness exceeds the bright portion threshold value a2 on both sides of the dark portion p1 (both sides in the longitudinal direction of the steel plate). There is p2 (bright part p2 _A , bright part p2 _B ). In this case, for example, if the distance x _{0A between the dark part p1 and the bright part p2 A} and the distance x _0B between the dark part p1 and the bright part p2 _B are both equal to or less than the distance threshold value (x), it corresponds to the dark part p1. The part to be removed and the parts corresponding to the bright areas p2 _A and p2 _B are combined and extracted as one defective part. On the other hand, when the distance x _0B between the dark part p1 and the bright part p2 _B is equal to or less than the distance threshold value (x), but the distance x _{0A between the dark part p1 and the bright part p2 A} exceeds the distance threshold value (x). The part corresponding to the bright part p2 _A is excluded from the defective part, and one defective part including only the part corresponding to the dark part p1 and the part corresponding to the bright part p2 _{B is extracted.}

In the present invention, if the distance threshold value (x) regarding the distance between the dark part p1 and the bright part p2 is too small, it cannot be extracted as one defect portion when the dark image and the bright image are separated. On the other hand, if the distance threshold value (x) is too large, two images that are different defects are actually combined, and the type and severity of the defects cannot be correctly determined. Therefore, the distance threshold value (x) is preferably set in the range of 0.1 to 5.0 mm (for example, the distance threshold value (x) = 0.30 mm). Further, the distance threshold value (x) is more preferably set in the range of 0.1 to 1.0 mm.

Further, if the dark area threshold value a1 of the image brightness is too low, harmful defects may not be detected. On the other hand, if the dark area threshold value a1 is too high, harmless color tone unevenness or the like may be detected and cause over-detection. Therefore, it is preferable to set the dark area threshold value a1 in the range of 70 to 110 (for example, the dark area threshold value a1 = 90). Further, it is more preferable to set the dark area threshold value a1 in the range of 75 to 105.

Further, if the bright part threshold value a2 of the image brightness is too low, harmless color tone unevenness or the like may be detected and cause over-detection. On the other hand, if the bright part threshold value a2 is too high, harmful defects may not be detected. Therefore, it is preferable to set the bright part threshold value a2 in the range of 140 to 190 (for example, the bright part threshold value a2 = 150). Further, it is more preferable to set the bright part threshold value a2 in the range of 145 to 170.

An object of the present invention is to detect harmful defects such as spatter defects in which deformation of a steel sheet occurs in parallel with the traveling direction of the steel sheet. In the present invention, by extracting the defective portion as described above, most of the defects that are not harmful in the actual usage situation, such as dirt, minute foreign matter (harmless defect), and scratches, which are not accompanied by deformation of the steel sheet, are removed. .. However, the extracted defect locations may include defects other than the specific harmful defect of interest (for example, dross defect). Therefore, in the present invention, among the defect locations extracted as described above, defects satisfying at least [length in the longitudinal direction of the steel sheet] / [length in the width direction of the steel sheet] ≥ ratio threshold value (y). Judge the location as a harmful defect. As a result, defects such as spatter defects that cause deformation of the steel sheet in parallel with the running direction of the steel sheet (longitudinal direction of the steel sheet) are detected (determined) as harmful defects by distinguishing them from other defects (for example, scratches and dross defects). can do. Therefore, it is possible to identify the cause of each defect and take appropriate measures to prevent the defect.

In the present invention, if the ratio threshold value (y) relating to [length in the longitudinal direction of the steel sheet] / [length in the width direction of the steel sheet] is too small, it is formed parallel to the traveling direction of the steel sheet such as spatter flaws. Harmful defects cannot be detected well. On the other hand, if the ratio threshold value (y) is too large, a long linear scratch or the like occurs when an erroneous detection is made due to captured image noise or the like while the dark part and the bright part are combined and extracted as one defect. May be indistinguishable from defects in. Further, in the case of spatter defects, in general, [length in the longitudinal direction of the steel sheet] / [length in the width direction of the steel sheet] does not exceed 5.0. Therefore, the ratio threshold value (y) is preferably set in the range of 1.0 to 5.0 (for example, the ratio threshold value (y) = 3.0). Further, the ratio threshold value (y) is more preferably larger than 2.0. This is because if the ratio threshold value (y) is 2.0 or less, it may not be possible to distinguish it from a dross defect that is short in the longitudinal direction of the steel sheet.

FIG. 4 schematically shows image information for extracting a defective portion in the present invention.
In FIGS. 4A to 4C, the left figure is image information of the surface of the steel plate after image processing, and one square shows one pixel (0.11 mm in the width direction of the steel plate x 0.16 mm in the longitudinal direction of the steel plate). .. The figure on the right shows the defective part extracted from the image. Here, it is assumed that the distance threshold value (x) is set to 0.30 mm and the ratio threshold value (y) is set to 2.0.

In FIG. 4A, a dark portion p1 (a dark portion consisting of 4 pixels) whose image brightness is less than the dark portion threshold value a1 and one pixel on both sides in the longitudinal direction of the plate are separated so that the image brightness is the bright portion threshold value a2. There are bright areas p2 _A and p2 _B (bright areas consisting of 4 pixels each) that exceed. The distance x ₀ between the dark part p1 and the bright part p2 _{A and the} dark part p1 and the bright part p2 _B is 0.16 mm, and both are equal to or less than the distance threshold value (x). It is extracted as d (defect portion). Further, since the defect portion d is [length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] = 3.0 ≧ ratio threshold value (y), it is a “harmful defect” for inspection purposes. Is determined. That is, when the purpose of the inspection is to detect spatter defects, it is determined that the spatter defects are present.

In FIG. 4B, the dark portion p1 (dark portion consisting of 8 pixels) whose image brightness is less than the dark portion threshold value a1 and the bright portion threshold value a2 whose image brightness is separated by one pixel on both sides in the longitudinal direction of the plate. There are bright areas p2 _A and p2 _B (bright areas consisting of 3 pixels each) that exceed. In this case as well, the distance x _{0 between the dark part p1 and the bright part p2 A and the} dark part p1 and the bright part p2 _B are both 0.16 mm, which are both equal to or less than the distance threshold value (x). It is extracted as one defect portion d (defect portion). However, since this defect location d is [length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] = 1.5 <ratio threshold value (y), what is a "harmful defect" for inspection purposes? Not judged. That is, when the purpose of the inspection is to detect spatter defects, it is not determined to be spatter defects.

In FIG. 4C, a dark portion p1 (a dark portion consisting of 4 pixels) in which the image brightness is less than the dark portion threshold value a1 and a bright portion in which the image brightness is separated by 2 pixels on one side in the longitudinal direction of the plate. There is a bright part p2 _A (a bright part consisting of 2 pixels) that exceeds the value a2, and a bright part p2 _B (a bright part consisting of 4 pixels) whose image brightness exceeds the bright part threshold value a2 with one pixel on the other side. ). In this case, the distance x _0B between the dark part p1 and the bright part p2 _B is 0.16 mm, which is equal to or less than the distance threshold value (x). On the other hand, the distance x _0A between the dark part p1 and the bright part p2 _A is 0.32 mm, which exceeds the distance threshold value (x). Therefore, the bright part p2 _A is not extracted as the defective part d, but the dark part p1 and the bright part p2 _B are combined and extracted as one defective part d (defect part). Further, since the defect portion d is [length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] = 2.5 ≧ ratio threshold value (y), it is a “harmful defect” for the purpose of inspection. Is determined. That is, when the purpose of the inspection is to detect spatter defects, it is determined that the spatter defects are present.

Further, in the present invention, in order to detect harmful defects with higher accuracy in the defect portion extracted as described above, the above-mentioned [length in the longitudinal direction of the steel sheet] / [length in the width direction of the steel sheet] described above ] ≥ In addition to the criterion of ratio threshold value (y), the determination can be made using the following criteria (1) to (3). In other words, in order to improve the detection accuracy, it is possible to determine whether or not the detection result of the harmful defect based on the distance threshold value (x) and the ratio threshold value (y) is correct. Alternatively, among the harmful defects detected based on the distance threshold value (x) and the ratio threshold value (y), whether or not the harmful defect is a serious harmful defect is determined by the criteria (1) to (3) below. May be used for determination.

Here, when the extracted defective portion satisfies one or more of the criteria (1) to (3) below, it may be determined that the defective portion is a harmful defect or a serious harmful defect. Alternatively, it may be determined that the defect is a harmful defect or a serious harmful defect when two or more or all of the following criteria (1) to (3) are satisfied. In addition to determining whether or not it is serious, the more satisfied the criteria (1) to (3) below, the more serious the adverse defect is. May be judged step by step.
(1) The area of the bright part p2 (however, when there are two or more bright parts p2, the total area of them) is equal to or larger than the area threshold value (w).
(2) The length of the bright portion p2 in the longitudinal direction of the steel sheet (however, if there are two or more bright portions p2, the total length thereof) is equal to or greater than the length threshold value (z).
(3) The maximum luminance point of the bright part p2 (however, when there are two or more bright parts p2, the maximum luminance point of at least one bright part p2) is set to be higher than the bright part threshold value a2. The brightness threshold value a20 is exceeded.

It can be said that the larger the area of the bright part p2 as in the standard of (1) above, the higher the possibility of a harmful defect, or the higher the possibility of a serious harmful defect. In particular, in the case of spatter defects, the area of the bright part p2, which is the cracked part of the base iron, is large. And from the area of the bright portion p2 of the sputtering defects, the area threshold (w) is set in the range of about 0.06~34.00mm ² (e.g., an area threshold (w) = 0.08mm ² ) Is preferable. Further, in the case of spatter defects, the area of the bright part p2 is usually larger than the area of the dark part p1 (often about twice as large), so the ratio of the area of the bright part p2 to the area of the dark part p1 p2. The determination condition may be that / p1 is equal to or greater than the area threshold value (w)'. Further, it is more preferable to set the area threshold value (w) in ^{the range of 0.6 to 8.0 mm 2.}

It can be said that the larger the length of the bright part p2 in the longitudinal direction of the steel sheet as in the standard (2) above, the higher the possibility of a harmful defect, or the higher the possibility of a serious harmful defect. In particular, in the case of spatter flaws, there is a feature that the length of the bright part p2, which is the cracked part of the base iron, is large. The length threshold value (z) is set in the range of about 0.6 to 10.0 mm from the length of the bright part p2 such as spatter defects (for example, the length threshold value (z) = 0. 8 mm) is preferable. Further, in the case of spatter defects, the length of the bright portion p2 in the longitudinal direction of the steel plate is usually larger than the length of the dark portion p1 in the longitudinal direction of the steel plate. The determination condition may be that the ratio p2 / p1 of the length of the dark portion p1 in the longitudinal direction of the steel sheet is equal to or greater than the length threshold (z)'. Further, it is more preferable to set the length threshold value (z) in the range of 0.6 to 5.0 mm.

If the maximum brightness point of the bright part p2 exceeds the bright part threshold value a20 set to be higher than the bright part threshold value a2 as in the reference of (3) above, it may be a harmful defect. It can be said that there is a high possibility that the sex is higher or that the harmful defect is serious. In particular, in the case of spatter defects, the brightness of the bright part p2, which is the cracked part of the base iron, is high. From the viewpoint of the brightness of the bright part p2 such as spatter defects, the bright part threshold value a20 (luminance) is preferably set in the range of 145 to 205 (for example, the bright part threshold value a20 = 180). Further, it is more preferable to set the bright part threshold value a20 in the range of 145 to 190 mm.

FIG. 5 schematically shows the image information to be determined in the case where the defect portion is extracted and the determination criteria (1) to (3) are added to perform the determination in the present invention. .. Here, in addition to the criteria of the distance threshold value (x) and the ratio threshold value (y) described above, when any one of the criteria (1) to (3) above is satisfied, the inspection purpose “ A case where it is determined to be a "harmful defect" or a "serious harmful defect" among the harmful defects will be described. If two or more or all of the criteria (1) to (3) above are satisfied, it is judged to be a "harmful defect" for inspection purposes, or it is considered to be a "serious harmful defect" among the harmful defects. You may judge.

In FIGS. 5A to 5C, the left figure is an image of the surface of the steel plate after image processing, and one square shows one pixel (0.11 mm in the width direction of the steel plate x 0.16 mm in the longitudinal direction of the steel plate). The figure on the right shows the defective part extracted from the image. Here, the distance threshold value (x) is 0.30 mm, the ratio threshold value (y) is 2.0, the area threshold value (w) is 0.10 mm ² , and the length threshold value (z) is 0. It is assumed that .40 mm and the bright part threshold value a20 are set to 180, respectively.

In FIG. 5A, a dark portion p1 (a dark portion consisting of 4 pixels) whose image brightness is less than the dark portion threshold value a1 and one pixel on both sides in the longitudinal direction of the plate are separated so that the image brightness is the bright portion threshold value a2. There are bright areas p2 _A and p2 _B (bright areas consisting of 4 pixels each) that exceed the above, and the distance x _{0 between the dark areas p1 and bright areas p2 A and the} dark areas p1 and bright areas p2 _B is 0.16 mm. Since it is equal to or less than the distance threshold value (x), one defect portion d (defect portion) in which these are combined is extracted. Further, the defective portion d is [length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] = 3.0 ≧ ratio threshold value (y), and the highest luminance point (portion) of the bright portion s2. ) Exceeds the bright part threshold value a20. Therefore, the defective portion is determined to be a "harmful defect" for inspection purposes, or is determined to be a "serious harmful defect" among the harmful defects. When the purpose of the inspection is to detect spatter defects, it is determined to be spatter defects (harmful defects) or serious spatter defects (harmful defects). On the other hand, unlike FIG. 5A, the maximum luminance point (part) of the bright portion s2 is equal to or less than the bright portion threshold value a20, and the total length of the bright portion p2 in the longitudinal direction of the steel plate is the length. If it is less than the threshold value (x), it is determined that the defect portion d is not a "harmful defect" for inspection purposes, or it is a "harmful defect" but is not serious.

In FIG. 5B, a dark portion p1 (a dark portion consisting of 4 pixels) whose image brightness is less than the dark portion threshold value a1 and a bright portion whose image brightness exceeds the bright portion threshold value a2 in contact with both sides in the longitudinal direction of the plate. Since there are p2 _A and p2 _B (bright areas consisting of two pixels each), the _{distances between the dark areas p1 and the bright areas p2 A and the} dark areas p1 and the bright areas p2 _B are all equal to or less than the distance threshold value (x). One defect portion d (defect portion) in which these are combined is extracted. The defect portion d is [length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] = 2.0 ≧ ratio threshold value (y). _{However, the total length of the bright portions p2 A} and p2 _{B in} the longitudinal direction of the steel sheet is 0.32 mm, which is less than the length threshold value (z) (0.40 mm). Therefore, it is determined that it is not a "harmful defect" for inspection purposes, or it is determined that it is a "harmful defect" but is not serious. When the purpose of the inspection is to detect spatter defects, it is determined that the inspection is not a spatter defect, or that the spatter defect is not serious. On the other hand, unlike FIG. 5B, if _{the total length of the bright portions p2 A} and p2 _{B in} the longitudinal direction of the steel sheet is the length threshold value (z) (0.40 mm) or more, the defective portion. d is determined to be a "harmful defect", or is determined to be a "serious harmful defect" among the harmful defects.

In FIG. 5C, the dark portion p1 (dark portion consisting of two pixels) whose image brightness is less than the dark portion threshold value a1 and the bright portion threshold value a2 whose image brightness is in contact with one side in the longitudinal direction of the plate. There is a bright part p2 _A (bright part consisting of 2 pixels) that exceeds, and there is a bright part p2 _B (bright part consisting of 1 pixel) whose image brightness exceeds the bright part threshold value a2 in contact with the other side, and there is a dark part p1. Since the distances between the bright part p2 _{A and the} dark part p1 and the bright part p2 _B are all equal to or less than the distance threshold value (x), they are collectively extracted as one defective part d (defect part). This defect location d is [length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] = 5.0 ≧ ratio threshold value (y), but the total area of _{bright areas p2 A} and p2 _B. Is 0.09 mm ^2, which is less than the area threshold value (w) (0.10 mm ^2). Further, if the total length of the bright part p2 in the longitudinal direction of the steel sheet is less than the length threshold value (x), it is determined that it is not a "harmful defect" for inspection purposes, or it is a "harmful defect". It is judged that it is not serious. That is, when the purpose of the inspection is to detect spatter defects, it is determined that the inspection is not a spatter defect, or that the spatter defect is not serious. On the other hand, unlike FIG. 5C, if _{the total area of bright areas p2 A} and p2 _B is equal to or greater than the area threshold value (w) (0.10 ^{mm 2} ), the defect location d is a “harmful defect”. Or, among the harmful defects, it is judged to be a "serious harmful defect".

In carrying out the present invention, it is desirable to set appropriate threshold values for parameters such as defect size and integrated concentration, which are also used in conventional inspection devices, to improve the discrimination accuracy.

Although there is no limitation on the surface defects to be detected in the present invention, the present invention is suitable for detecting spatter defects of plated steel sheets (for example, hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, etc.). A spatter defect is detected separately from a scratch defect and / or a dross defect, and is determined to be a harmful defect.

Here, although the detection of harmful defects (spatter defects) for inspection purposes has been described above, dross defects are also mentioned as one of the harmful defects. That is, in the manufacturing process of the plated steel sheet, the dross adhering to the surface of the plated steel sheet may be pushed in by winding or rolling on a roll. Then, the surface of the plated steel sheet is slightly deformed and becomes a dross defect. This dross defect is also a harmful defect that adversely affects painting and the like. Here, since the dross defect is also accompanied by the dross-attached portion and the deformed portion around it, it can be determined by using the distance threshold value (x) of the dark portion lang = EN-US> p1 and the bright portion p2. can. Therefore, based on the above-mentioned method for detecting a harmful defect (spatter defect) for inspection purposes, it is possible to distinguish between a dross defect and a spatter defect among the harmful defects.

For example, a distance threshold value (x) that can be extracted with both dross defects and spatter defects as defect locations is set, and among the extracted defect locations, the ratio threshold value (y) and further (1). A defect portion determined not to be a spatter defect may be determined to be a dross defect using the criteria of (3). Alternatively, the dross distance threshold value for dross defect detection is set separately from the range of, for example, 0.1 to 2.0 mm, and the dark part lang = EN-US> p1 and the bright part p2 are equal to or less than the dross distance threshold value. If, it may be determined that the defective portion is a dross defect. Further, the dross defect may be determined by utilizing the property of extending in the plate width direction and adding the condition that the length in the plate width direction is equal to or more than the set length among the defective portions. In this way, it is possible to distinguish between spatter defects and dross defects having different types of harmful defects.

A surface defect inspection device as shown in FIG. 1 was installed in the continuous hot-dip galvanizing line to detect surface defects in the plated steel sheet. In the continuous hot-dip galvanizing line, the plate passing speed of the steel strip is 80 to 140 mpm, and the dimensions of the steel strip are: steel strip width 820 to 1840 mm.
The inspection equipment resolution was 0.11 mm in the width direction × 0.16 mm in the plate-passing direction, and in the image processing, the brightness of each pixel was classified into 256 stages (0 to 255) and processed.
In the present invention, when extracting the defective portion d, the dark part threshold value a1 is set to the brightness 85 and the bright part threshold value a2 is set to the brightness 150, and the distance threshold value (x) regarding the distance between the dark part p1 and the bright part p2 is set. ) Was set to 0.48 mm. Further, as a criterion for determining whether the extracted defect portion d is a harmful defect, the ratio threshold value (y) regarding [length in the steel plate longitudinal direction] / [length in the steel plate width direction] is set to 2.0. Set the length threshold (z) for the length of the bright part p2 in the longitudinal direction of the steel sheet (however, if there are two or more bright parts p2, the total length of them) to 0.8 mm, respectively, and set [ The length in the longitudinal direction of the steel plate] / [length in the width direction of the steel plate] is equal to or greater than the ratio threshold value (y) (= 2.0), and the total length of the bright portion p2 in the longitudinal direction of the steel plate is the length. A defect portion d having a threshold value (z) (= 0.8 mm) or more was determined to be a harmful defect for inspection purposes.

A surface defect detection test was carried out by the present invention and the conventional method using a hot-dip galvanized steel sheet provided with a spatter defect sample (length 3 mm in the longitudinal direction of the steel sheet − length about 1 mm in the width direction of the steel sheet). As a conventional method, when the lowest value of the brightness of the region determined as a defect falls below the dark area threshold value a1, a method of determining the severity of the defect according to the brightness is used.
When the conventional method was applied, it was impossible for the inspection equipment to distinguish between harmless dirt / minute foreign matter and harmful spatter defects, and it was possible to detect only by visual inspection of the inspector with the steel plate running stopped. .. Therefore, it was impossible to confirm the total length of the product.

On the other hand, when the present invention is applied, it has been possible to distinguish a sputter-defect sample from harmless stains and minute foreign substances by an inspection facility. Specifically, 13 spatter defects were actually investigated and their detection status was evaluated. As a result, the exact match (judgment: harmful, actual defect: harmful) = 10 and undetected (judgment: harmless, actual defect: harmful) = 3, and the reliability rate (ratio of determining harmful actual defect as harmful). ) Was 77%, and it was confirmed that harmful surface defects could be detected accurately.
As described above, the total length of the product can be confirmed in the inspection equipment. For example, the detection location of spatter defects may be re-inspected and visually inspected by an inspector to make a final judgment.

Further, the above-mentioned surface defect inspection may be applied to a method for manufacturing a plated steel sheet. As the manufacturing process of the plated steel sheet, for example, a step of annealing the steel sheet, a step of adhering a plating layer to the surface of the annealed steel sheet, a step of chemical treatment of the steel sheet to which the plating layer is attached, and a surface inspection of the chemicalized plated steel sheet. There is a step, and the above-mentioned surface defect inspection is carried out in this surface inspection step. Then, when it is determined by this surface inspection that the plated steel sheet has a harmful defect (sputter defect), that fact may be displayed on a display device installed on the plating production line. The displayed information includes, for example, the presence / absence of a harmful defect, the location information of the harmful defect, the severity, the length or area of the harmful defect, and the like.

Further, when it is determined that the plated steel sheet has a spatter defect, a countermeasure different from the case where it is determined to be a scratch and / or a dross defect may be taken for the plated steel sheet.

Different measures include the following. For example, when it is determined that the defective portion is a scratch and / or a dross defect, the mixing rate of the dross defect or the like in the steel plate coil in which the metal plate is wound is calculated. The mixing rate means (length (m) / total length (m) from which dross defects and the like are extracted). Then, when the mixing ratio is equal to or higher than the set threshold value, it is determined that the plated steel sheet coil has failed.

On the other hand, when it is determined that the defective portion is a spatter defect, the spatter defect may cause a serious defect such as damage to the press die, unlike a dross defect or the like. Therefore, the rejection is determined regardless of the number of the steel plate coils. Then, spatter defects are removed.

1 Floodlight 2 Imager 3 Image processing device 4 Defect judgment device 5 Steel plate

Claims (13)

In a surface defect detection method in which an illuminated metal plate surface is imaged and the obtained image information is threshold-processed to extract surface defects of the plated metal plate.
In the obtained image information, the distance between the dark part (p1) where the image brightness is less than the dark part threshold value (a1) and the bright part (p2) where the image brightness exceeds the bright part threshold value (a2) is the distance. Detects dark areas (p1) and bright areas (p2) that are below the threshold value (x).
The detected dark part (p1) and bright part (p2) are combined and extracted as one defective part.
Among the extracted defect locations, those that satisfy at least [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≥ ratio threshold value (y) are judged to be harmful defects. And
The metal plate is a plated steel plate, and spatters adhering to the surface of the base steel plate of the plated steel plate and spatter defects consisting of the deformed portion of the steel plate generated from this spatter are distinguished from dross defects and are judged to be harmful defects. Method for detecting surface defects on a plate. The method for detecting a surface defect of a metal plate according to claim 1, wherein the distance threshold value (x) is set in the range of 0.1 to 5.0 mm. The method for detecting a surface defect of a metal plate according to claim 1 or 2, wherein the ratio threshold value (y) is set in the range of 1.0 to 5.0. The method for detecting a surface defect of a metal plate according to claim 3, wherein the ratio threshold value (y) is set to be larger than 2.0. Claims 1 to 4 of the extracted defective parts, in which the total area of the bright part (p2) in the defective part is equal to or larger than the area threshold value (w) is determined as a harmful defect or a serious harmful defect. The method for detecting surface defects of a metal plate according to any one of. Among the extracted defective parts, [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≥ ratio threshold value (y) is satisfied, and the following (1) to (3) are satisfied. The method for detecting a surface defect of a metal plate according to any one of claims 1 to 4, wherein a defective portion satisfying one or more of the above is regarded as a harmful defect or a serious harmful defect.
(1) The total area of the bright part (p2) in the defective part is equal to or larger than the area threshold value (w).
(2) The total length of the bright portion (p2) in the defective portion in the longitudinal direction of the metal plate is equal to or greater than the length threshold value (z).
(3) The maximum brightness value of the bright part (p2) in the defective part exceeds the bright part threshold value (a20) set to be higher brightness than the bright part threshold value (a2). Among the extracted defective parts, [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≥ ratio threshold value (y) is satisfied, and the above (1) to (3) are satisfied. The method for detecting a surface defect of a metal plate according to claim 6, wherein a defective portion satisfying two or more of them is determined to be a harmful defect or a serious harmful defect. Among the extracted defective parts, [length in the longitudinal direction of the metal plate] / [length in the width direction of the metal plate] ≥ ratio threshold value (y) is satisfied, and the following (1) to (3) are satisfied. The method for detecting a surface defect of a metal plate according to claim 6, wherein the defective portion satisfying all of the above is determined to be a harmful defect or a serious harmful defect. The method for detecting a surface defect of a metal plate according to any one of claims 5 to 8, wherein the area threshold value (w) is ^{set in the range of 0.06 to 34.00 mm 2.} The method for detecting a surface defect of a metal plate according to any one of claims 6 to 9, wherein the length threshold value (z) is set in the range of 0.6 to 10.0 mm. An illuminating means for illuminating the surface of the metal plate and an imaging means for imaging the illuminated portion on the metal plate by the illuminating means are arranged on either the front side or the rear side of the metal plate normal line in the direction of travel of the metal plate. The method for detecting surface defects of a metal plate according to any one of claims 1 to 10. When a spatter defect is determined by the method for detecting a surface defect of a metal plate according to any one of claims 1 to 11, there is a harmful defect in a countermeasure different from that when it is determined to be a scratch defect and / or a dross defect. A method for manufacturing a plated steel sheet to be carried out on a plated steel sheet determined to be. A surface defect detection device for a metal plate including an image processing device and a defect determination device for detecting surface defects by the surface defect detection method according to any one of claims 1 to 11.

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