JP2012103017A - Dross defect inspection device and dross defect inspection method of molten metal plating steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inspection device which can stably inspect a dross defect of a molten metal plating steel plate by being separated from formation noise and a slight defect.SOLUTION: A dross defect inspection device has defect determination means (defect determination device 18) for classifying extracted defects into a dross defect and other defects, in which an incident angle of light to be emitted from illumination means (floodlight 12) to the surface of a steel plate 1 is set to an angle between 50° and 80° to the normal direction of the steel plate 1, imaging means (imaging unit 14) is arranged on the same side as that of the illumination means (12) to a normal of the steel plate 1, and a light receiving angle of light to be received by the imaging means (14) is set to an angle between 0° and 40° to the normal direction of the steel plate 1. It is preferable that the defect determination means (18) determines the extracted defects into the dross defect and other defects based on at least one of area of the defects, image luminance of the defects, and forms of the defects.

Description

  The present invention relates to a dross defect inspection apparatus and a dross defect inspection method for a molten metal plated steel sheet, for example, a melting that can be suitably used as an apparatus for optically inspecting a dross defect generated on the surface of a hot dip galvanized steel sheet. The present invention relates to a dross defect inspection apparatus and a dross defect inspection method for a metal-plated steel sheet.

  One of the most common processes for hot metal plating of steel strip is continuous hot dip galvanization of steel strip.

  One of the defects generated on the surface of the hot dip galvanized steel sheet during the continuous hot dip galvanizing process is a dross defect. The dross defect is a convex defect that is generated when the bottom dross adheres to the surface of the steel sheet in the zinc pot in the manufacturing process of the hot dip galvanized steel sheet. It is pushed in and becomes latent. For this reason, on the outgoing side of the hot dip galvanizing process, in particular, minute dross defects are often not visually recognized.

  On the other hand, hot-dip galvanized steel sheets are processed into door materials and panel materials for automobiles by press molding. Since the dross defect containing zinc as the main component is harder than the steel plate which is the base plate, it becomes a convex defect on the surface opposite to the surface where the defect is generated by press working, and the appearance of the steel plate surface is impaired. For this reason, the dross defect is regarded as one of the most harmful defects among various surface defects.

  As an inspection device for surface defects such as dross defects in hot-dip galvanized steel sheets, Patent Document 1 discloses a plurality of chucking devices that restrain the longitudinal ends of the steel sheets and a pair of thrust tensions that are movable in the longitudinal direction of the steel sheets. Disclosed is an apparatus composed of a carriage, a steel plate placing section that can be moved up and down, a chucking device that restrains a sheet width direction end of the steel sheet, and a plurality of side tension carriages that can move in the longitudinal direction and width direction of the steel sheet. ing. This apparatus is an apparatus for inspecting a surface flaw of a steel sheet by applying tensile strain to the steel sheet.

  Patent Document 2 discloses that the surface of a galvanized steel sheet is irradiated with visible light, the surface of the steel sheet is photographed with a high-resolution CCD camera having a size of 1 mm × 1 mm or less from the vicinity of the regular reflection direction, and the obtained image is displayed with a predetermined threshold. An optical surface inspection apparatus that cuts out values and detects surface defects is shown. When detecting surface defects, it is preferable to set the threshold value to a concentration excluding the high-intensity minute spots peculiar to galvanized steel sheets, and to switch the threshold value according to roughness information before plating. Yes.

  In Patent Document 3, the surface of the steel plate is irradiated with illumination light, and the surface of the steel plate is photographed with a high resolution CCD camera of 0.2 mm or less from the backward diffuse reflection direction. An optical surface inspection apparatus that cuts out and detects surface defects is shown. When detecting surface defects, the camera is arranged on the same side as the light source with respect to the normal of the surface to be inspected, and the incident angle α of the illumination light to the surface to be inspected (the angle formed with the normal of the surface to be inspected) ) Is an angle between 60 ° and 80 °, and the light receiving angle of the camera (the angle formed with the normal of the surface to be inspected) is preferably set to an angle between 20 ° and α.

JP-A-5-223728 JP-A-9-113465 JP 2008-275424 A

  On the other hand, the required level for the surface quality of the hot dip galvanized steel sheet has been getting stricter in recent years, and the dross defect has become a problem even with a minute one having a dimension of about 0.2 to 0.5 mm, for example. . Therefore, it is extremely important to inspect the entire surface of the steel sheet for dross defects including such minute ones to assure the surface quality of the steel sheet.

  However, the conventional inspection apparatus described above has the following problems.

  In the inspection apparatus of Patent Document 1, it is necessary to cut a steel plate into a predetermined size (length: 0.6 to 2.5 m, width: about 0.6 to 1.9 m) and inspect it offline. It is difficult to inspect a galvanized steel sheet covering m over the entire length.

  In the inspection apparatus of Patent Document 2, discrimination between a minute defect and a high-luminance minute point is determined based on the number of integrated pixels, but a minute dross defect with a diameter of about 0.2 to 0.5 mm is ground noise (defect It is difficult to detect the defect separately from the formation noise because the size is similar to that of the high-intensity minute dots that are not.

  In the inspection apparatus of Patent Document 3, since the imaging is performed from the backward diffuse reflection direction, it is possible to suppress the above-described formation noise, but it is difficult to separate the dross defect from the minor minute defect other than the dross defect. .

  The present invention has been made in view of such problems, and provides an inspection apparatus capable of stably inspecting dross defects of a molten metal plated steel sheet from ground noise and minor defects. For the purpose.

  A first aspect of a dross defect inspection apparatus for a molten metal plated steel sheet according to the present invention that solves the above problems includes an illumination means for illuminating the surface of the steel sheet, an imaging means for imaging an illumination portion on the steel sheet, and the imaging means. An image processing means for extracting a surface defect of a steel sheet by thresholding the obtained image signal, and a dross defect inspection apparatus for a molten metal plated steel sheet, wherein the extracted defect is a dross defect and other defects And the incident angle of the light emitted from the illumination unit with respect to the steel sheet surface is set to an angle between 50 ° and 80 ° with respect to the normal direction of the steel plate. And the imaging means is disposed on the same side as the illumination means with respect to the normal line of the steel plate, and the light receiving angle of light received by the imaging means is 0 ° with respect to the normal direction of the steel plate Between 40 ° and 40 ° It is those that have been set.

  In this first aspect, the defect determination means determines the extracted defect based on at least one of the area of the defect, the image brightness of the defect, and the form of the defect, It is preferable to determine a defect.

  A second aspect of the dross defect inspection apparatus for a molten metal plated steel sheet according to the present invention that solves the above-described problems is an illumination means for illuminating the surface of the steel sheet, an imaging means for imaging an illumination part on the steel sheet, and the imaging means. An image processing means for extracting a surface defect of a steel sheet by thresholding the obtained image signal, and a dross defect inspection apparatus for a molten metal plated steel sheet, wherein the extracted defect is a dross defect and other defects The defect determination means classifies the extracted defect based on at least one of the area of the defect, the image brightness of the defect, and the form of the defect. It is judged as a dross defect and other defects.

  In these first and second aspects, it is preferable that the spatial resolution of the imaging means is 0.2 mm or less.

  Moreover, it is preferable to install in the upstream position of a skin pass mill in a molten metal plating line.

  A method for inspecting a dross defect of a molten metal plated steel sheet according to the present invention that solves the above-mentioned problem illuminates the surface of the steel sheet, images an illuminated part on the steel sheet, thresholds the obtained image signal, and A method for inspecting a dross defect of a molten metal plated steel sheet for extracting a surface defect, wherein the extracted defect is based on at least one of the area of the defect, the image brightness of the defect, and the form of the defect. It is judged as a dross defect and other defects.

  The first aspect of the dross defect inspection apparatus for molten metal plated steel sheet according to the present invention has a defect determination means for classifying the extracted defect into a dross defect and other defects, and the light emitted from the illumination means The incident angle with respect to the surface of the steel sheet is set to an angle between 50 ° and 80 ° with respect to the normal direction of the steel sheet, and the imaging means is the illumination means with respect to the normal line of the steel plate. Since the light receiving angle of the light received by the imaging means is set to an angle between 0 ° and 40 ° with respect to the normal direction of the steel plate, it is suppressed on formation noise. And a dross defect having a convex shape can be detected with high sensitivity.

  The second aspect of the dross defect inspection apparatus for molten metal plated steel sheet according to the present invention has a defect determination means for classifying the extracted defect into a dross defect and other defects, and the defect determination means is an extraction The detected defect is determined as a dross defect and other defects based on at least one of the defect area, the image brightness of the defect, and the form of the defect. It becomes possible to inspect separately.

  Further, when the spatial resolution of the imaging means is 0.2 mm or less, it becomes possible to detect a minute dross defect.

  In addition, when this device is installed on the upstream side of the skin pass mill of the molten metal plating line, it is possible to inspect the dross defect before latentizing with the skin pass mill. It becomes possible to detect.

The apparatus block diagram which shows one Embodiment of the dross defect inspection apparatus of the molten metal plating steel plate by this invention. Illustration of the definition of the incident angle of the projector and the receiving angle of the imager Characteristic diagram showing changes in dross defect detection sensitivity (defect image brightness) depending on the incident angle of the projector and the acceptance angle of the imager Scatter plot plotting data of harmful dross and harmless defects using defect image brightness and defect area as parameters Chart showing an example of defect determination logic by the defect determination device

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is an apparatus configuration diagram showing an embodiment of a dross defect inspection apparatus for a molten metal plated steel sheet according to the present invention.

  As shown in FIG. 1, the dross defect inspection apparatus 10 includes a projector 12, an imager 14, an image processing device 16, and a defect determination device 18.

  The projector 12 projects light onto the surface of a molten metal-plated steel plate 1 (hereinafter sometimes simply referred to as the steel plate 1). The type of illumination is not particularly limited, and for example, halogen illumination, metal halide illumination, fluorescent lamp, LED illumination, xenon strobe illumination, etc. used in conventional surface defect inspection can be used.

  The imaging device 14 images light reflected from the surface of the steel plate 1, and for example, a CCD area sensor camera or a CCD line sensor camera can be used. The spatial resolution of the imager is suitably 0.2 mm or less in order to detect minute dross defects of φ0.5 mm or less.

  The image processing device 16 performs threshold processing on the image signal transmitted from the imager 14 to extract surface defects, and the same image processing device as a conventional surface defect inspection device may be used. it can.

  The defect determination device 18 classifies the defects extracted by the image processing device 16 into dross defects, others, and defects. The defect determination device 18 can also be configured by a computer that stores the determination logic in advance.

  Next, the arrangement of the projector 12 and the imager 14 will be described. As shown in FIG. 1, in this embodiment, the imager 14 is arranged on the same side as the projector with respect to the normal line of the steel plate 1, and the incident angle α of the projector 12 is set with respect to the normal direction of the steel plate 1. The angle between 50 ° and 80 ° is set, and the light receiving angle β of the imager 14 is set at an angle between 0 ° and 40 ° with respect to the normal direction of the steel plate 1.

  The basis for setting this angle will be described below with reference to FIGS.

  The inventors of the present invention have an optical arrangement for detecting a minute convex defect such as a dross defect with a high S / N ratio, that is, an incident angle α of a projector and a light receiving angle β of an imager (see FIG. 2. A careful study was conducted with respect to (note that the positive and negative directions of β are opposite to those in FIG. 1).

  As a result, it was found that when the incident angle α of the illumination is small, the detection sensitivity of the convex defect such as the Dross defect is lowered.

  Moreover, the following knowledge was acquired about the dross defect on the steel plate before rolling down with a skin pass mill.

  1) When the dross defect is observed in the regular reflection direction, it is detected as a bright spot having a higher luminance than the formation portion. Here, the regular reflection direction is a direction in which light emitted from the projector 12 and having an angle α with respect to the normal direction of the steel plate 1 is reflected at an angle α on the opposite side with respect to the normal line of the steel plate 1. (In FIG. 2, an arrangement where β = α).

  2) When observed from an angle of about 30 ° from the regular reflection direction to the normal direction of the steel plate 1, there is no difference in strength between the dross defect and the formation portion, and the dross defect is difficult to detect (in FIG. 2, | β −α | ≈30 °).

  3) When observed from an angle separated from the regular reflection direction by 30 ° or more in the normal direction of the steel plate 1, it is detected as a dark spot having a lower luminance than the formation portion (in FIG. 2, | β−α |> 30 ° Arrangement).

  Furthermore, the formation noise observed from the surface of the molten metal (here, zinc) plated steel plate 1 before the skin pass pressure is detected as a bright spot with high brightness, and the imager 14 is connected to the normal of the steel plate 1. It has been found that if it is arranged on the side opposite to the projector 12 (corresponding to β> 0 in FIG. 2), the strength of the ground noise signal increases and the SN ratio decreases.

  The present embodiment devises an apparatus for inspecting dross defects generated in a hot dip galvanized steel sheet with a high SN ratio based on the above knowledge.

  Based on the above knowledge, the present inventor has found that the apparatus configuration in which the projector 12 is arranged at a large incident angle and the imager 14 is arranged on the same side as the illumination (projector) with respect to the normal line of the steel sheet is a dross defect inspection. It came to the judgment that it would be suitable for.

  With this configuration, the dross defect has a lower brightness than the formation part, whereas the formation noise becomes a bright spot with high brightness, so the polarity of the dross defect signal and the formation noise signal is different, Easy to separate.

  FIG. 3 is a graph showing an example of the result of examining the detection sensitivity of dross defects while changing α and β. The horizontal axis represents the light receiving angle β of the imager, and the vertical axis represents the image brightness of the defect (of the defect portion). Image luminance peak value). 3 that α is an angle between 50 ° and 80 °, and β is an angle between 0 ° and 40 °.

  FIG. 3 shows that the defect detection sensitivity improves as the incident angle α of the light emitted from the projector 12 to the surface of the steel plate 1 is increased. However, if α is increased too much, the steel plate being run by the inspection apparatus 10 is increased. In the case of inspecting 1, there is a demerit that the steel plate is easily affected by the flickering of the steel plate and the change in the plate thickness. Therefore, it can be determined that α should not be larger than necessary, and the reason why α> 80 ° is not described in FIG. 3 is due to this situation.

  Next, the function of the defect determination device 18 will be described.

  The surface defects extracted by the image processing apparatus include minor point defects and line defects with low toxicity, in addition to dross defects with high toxicity. Therefore, the present inventor recognizes that it is important to provide the defect determination device 18 with a function of separating dross defects from other surface defects, and seeks a method for classifying them into dross defects and other defects. As a result of intensive studies, it was found that dross defects have the following characteristics.

a) The area of the harmful dross defect is limited to the range of S1 to S2. Here, S1 and S2 are predetermined defect areas. Area S1 means that smaller defects than this can be considered harmless. Further, the dross defect cannot be extremely large due to its generation mechanism, and a defect having a predetermined area S2 or more can be determined as a defect other than the dross defect such as baldness and dirt. The specific values of S1 and S2 depend on the galvanizing line to which the present invention is applied and the strictness of dross defect inspection. For example, S1 = 0.05 mm ² and S2 = 2.0 mm ² are appropriate. is there.

  b) The harmful dross defect has an image brightness lower than a predetermined value. A dross defect is detected as a dark defect, that is, a defect having a lower luminance than the formation portion when inspected with the above-described optical arrangement (α = 50 ° to 80 °, β = 0 ° to 40 ° in FIG. 1). As a result of investigating the harmfulness of dross defects during press processing, as shown in Fig. 4 (scatter diagram plotting data of harmful dross defects and harmless defects using defect image luminance and defect area as parameters), image luminance and defects It has been found that there is a correlation between the harmfulness of the image and the image brightness of the dross defect is less than the predetermined image brightness. That is, it is possible to determine that the defect having the predetermined image luminance or higher is a defect other than the dross defect. Note that the image luminance of the defect in FIG. 4 represents the luminance peak value of the defective portion, that is, the minimum luminance value among the pixels forming the defective portion. As the image luminance of the defect, for example, an average luminance value of the defective portion may be used in addition to the luminance peak value.

  c) When the form of the dross defect was investigated, the dross defect has a shape close to a perfect circle, and a defect having a low roundness such as a vertically long defect or a horizontally long defect has other characteristics such as oysters, dirt, and scales. It turns out to be a defect. Accordingly, whether or not the defect is a dross defect is determined based on an image feature amount indicating the form of the defect, for example, circularity (circumference of a circle having the same area / perimeter of an actual figure), roundness (a circular shape is divided into two concentric shapes). When sandwiched between geometric circles, it can be determined using the difference in radius of two circles when the interval between concentric circles is minimum) and the aspect ratio (ratio of the length and width of the defect). When an aspect ratio is used as an image feature amount representing a defect form, a value having a value around 1.0 (for example, 0.8 to 1.2) is determined as a dross defect, and other defects are determined as other defects. Can be separated.

  From the above findings a) to c), the defect determination device 18 of the dross defect inspection apparatus 10 according to the present embodiment determines the defect based on at least one of the defect area, the defect image luminance, and the defect form. The defect determination device 18 can determine the dross defect separately from other minor defects.

  Next, the installation location of the dross defect inspection apparatus 10 according to the present embodiment will be described. As described above, the convex dross defects adhering to the surface of the steel plate are pushed into the steel plate by the skin pass mill downstream of the zinc pot and become latent. Therefore, it is preferable that the dross defect inspection apparatus 10 be installed at a position downstream of the molten zinc facility and upstream of the skin pass mill.

  The dross defect inspection apparatus of the molten metal plating steel plate which concerns on this invention is demonstrated concretely based on an Example. In this example, the dross defect inspection apparatus shown in FIG. 1 was installed upstream of the skin pass mill in the hot dip galvanizing production line.

  The projector used a line-shaped light guide for metal halide fiber illumination, and was further condensed by a cylindrical lens and irradiated onto the steel sheet at an incident angle of 65 °. The image pickup device was a CCD line sensor camera having a spatial resolution of 0.1 mm (plate width direction) × 0.1 mm (steel plate conveyance direction), and was installed at the light receiving angle β = 30 ° on the same side as the projector. The image processing apparatus cuts out a portion where the image signal (image brightness) of the camera (imaging device) falls below a predetermined threshold value. This is based on the knowledge that dross defects are detected as dark defects. The defect determination apparatus determined the surface defect extracted by the image processing apparatus using the determination logic shown in FIG. 5 using the three feature amounts of the defect area S, the defect image luminance A, and the defect circularity C.

  The inspection apparatus having the above configuration included in the present invention has improved the detection rate of dross defects to 99%, which was about 20% in the conventional surface inspection apparatus. In addition, the misdetection rate for erroneously recognizing defects other than dross defects as dross defects has been reduced from 40% to 8%.

  From the above results, it was confirmed that the dross defect inspection performance of the dross defect inspection apparatus according to the present invention is remarkably superior to the conventional surface defect inspection apparatus.

DESCRIPTION OF SYMBOLS 1 ... Molten metal plating steel plate 10 ... Dross defect inspection apparatus 12 ... Light projector 14 ... Imaging device 16 ... Image processing apparatus 18 ... Defect determination apparatus alpha ... Incident angle beta ... Light reception angle

Claims (6)

Illuminating means for illuminating the surface of the steel sheet, imaging means for imaging the illuminated portion on the steel sheet, and image processing means for extracting a surface defect of the steel sheet by thresholding the image signal obtained from the imaging means, A dross defect inspection device for a molten metal-plated steel sheet,
It has a defect determination means that classifies the extracted defects into dross defects and other defects,
The incident angle of the light emitted from the illumination means with respect to the steel plate surface is set to an angle between 50 ° and 80 ° with respect to the normal direction of the steel plate,
The imaging means is disposed on the same side as the illumination means with respect to the normal line of the steel plate, and the light receiving angle of light received by the imaging means is 0 ° to 40 ° with respect to the normal direction of the steel plate. An apparatus for inspecting a dross defect of a molten metal-plated steel sheet, characterized in that the angle is set between the two.   The defect determination means determines the extracted defect as a dross defect and other defects based on at least one of the defect area, the image brightness of the defect, and the form of the defect. The dross defect inspection apparatus of the molten metal plating steel plate of Claim 1. Illuminating means for illuminating the surface of the steel sheet, imaging means for imaging the illuminated portion on the steel sheet, and image processing means for extracting a surface defect of the steel sheet by thresholding the image signal obtained from the imaging means, A dross defect inspection device for a molten metal-plated steel sheet,
It has a defect determination means that classifies the extracted defects into dross defects and other defects,
The defect determination means determines the extracted defect as a dross defect and another defect based on at least one of the area of the defect, the image brightness of the defect, and the form of the defect. A dross inspection system for molten metal plated steel sheets.   4. The dross defect inspection apparatus for a hot-dip galvanized steel sheet according to claim 1, wherein a spatial resolution of the imaging means is 0.2 mm or less.   5. The dross defect inspection apparatus for a molten metal plated steel sheet according to claim 1, wherein the apparatus is installed at an upstream position of a skin pass mill in the molten metal plating line. A method for inspecting a dross defect of a molten metal plated steel sheet by illuminating a steel sheet surface, imaging an illumination part on the steel sheet, and thresholding the obtained image signal to extract a surface defect of the steel sheet,
A hot-dip galvanized steel sheet characterized in that an extracted defect is determined as a dross defect or another defect based on at least one of the area of the defect, the image brightness of the defect, and the form of the defect Dross inspection method.

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