Metal strip surface inspection device, surface inspection method, and manufacturing method

Abstract

To provide a surface inspection device and a surface inspection method for metal strip capable of detecting and discriminating various types of surface defects with a simple device configuration.SOLUTION: A surface inspection device for a metal strip includes: two or more light source parts that selectively emit light of two or more wavelengths onto a surface of a metal strip; an imaging part that images reflected light of the metal strip from the light emitted from the light source parts; and a control part that controls the operation of the light source parts and the imaging part. The control part synchronizes with a scan signal that controls imaging timing of the imaging part and outputs an output light control signal that controls the light source parts so that wavelengths of the light emitted by the light source parts are different from each other.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a surface inspection device, a surface inspection method, and a manufacturing method for a metal strip. [Background technology]

[0002] Metal strips, such as thin steel sheets, are turned into products through various manufacturing processes. For example, a slab cast in a steelmaking process is turned into a hot-rolled steel strip through a hot-rolling process, and after the oxides on the surface are removed in a pickling process, the strip is turned into a product through multiple manufacturing processes, such as a cold rolling process in which the strip is cold-rolled to a predetermined thickness, an annealing process in which the metal strip, which has become hardened by processing strain, is softened, and a plating process and a temper rolling process are subsequently performed. In this way, while the metal strip is turned into a product through multiple manufacturing processes, it is appropriately inspected whether or not it has predetermined characteristics and quality. This is not only for the purpose of quality assurance of the final product, but also as a means of production management, in order to determine the suitability of each metal strip at an intermediate stage, such as changing the subsequent manufacturing process or stopping the production of the metal strip in the middle if a defect is detected in the intermediate process. For example, if a surface defect is detected in a metal strip during a pickling process, if the defect is minor, the subsequent cold rolling process is performed, and if the defect is major, the manufacturing process is stopped and the strip is scrapped. In addition, when surface defects are detected only at the leading or trailing end of the metal strip, the portion where the defect is detected may be cut off and only the portion of the metal strip without defects may be sent to the downstream manufacturing process. Surface inspection of metal strip may be performed at multiple steps in the manufacturing process of metal strip (e.g., annealing step, plating step, etc.).

[0003] There are various types of surface defects in metal strips, and the necessity of additional treatment processes, etc. is determined depending on the application of the metal strip to be produced. For example, for metal strips used in applications that require strict quality control for surface defects, strict management standards for surface defects are set, and in some cases, the parts containing surface defects are removed in the subsequent treatment process, or the destination of the product is changed. In addition, when there are only a few minor surface defects, the areas containing surface defects may not be removed from the metal strip, but the parts containing the detected surface defects may be marked and a notice of the presence of the defects may be given at the time of shipment. In this way, in the manufacturing process of metal strips, a policy for dealing with the treatment process, etc. is determined depending on the type and severity (grade) of the surface defect, so it is important to use a surface inspection device to determine the type and severity of the surface defects present on the surface of the metal strip and to classify them online.

[0004] In response to such problems, methods for detecting and distinguishing surface defects have been proposed. For example, Patent Document 1 describes a method for improving the detection sensitivity of defects having unevenness based on the characteristic that the ratio of the regular reflection component and the diffuse reflection component changes due to the unevenness of the metal strip surface by installing a light source that irradiates light toward the metal strip surface, a photoelectric converter that detects the regular reflection component of the light reflected from the metal strip surface, and a photoelectric converter that detects the diffuse reflection component. In addition, Patent Document 2 describes a method for irradiating an inspection object with monochromatic light of different wavelengths from different directions, dispersing the reflected light from the inspection object into three primary colors of red, green, and blue, and discriminating surface defects of a metal strip from the light receiving intensity level of each of the dispersed wavelength components. In addition, Patent Document 2 describes that this method can detect not only localized uneven shapes such as surface defects, but also color unevenness.

[0005] Furthermore, Patent Document 3 describes a surface defect inspection device that irradiates a metal strip with laser light corresponding to the three primary colors of light from different angles and captures the reflected light of each color with an imaging camera corresponding to each color. Patent Document 3 also describes a device that uses a composite light obtained by combining general light with a laser light in order to obtain an image by diffuse reflection light. Meanwhile, Patent Document 4 describes a surface inspection device that is configured to transmit or block different wavelength regions among the wavelength regions divided for each RGB color, by providing a filter device for multiple white light sources in an optical path from the white light source to a light receiving unit, and transmits or blocks different wavelength regions among the wavelength regions divided for each RGB color. According to this device, even when light is irradiated from multiple light sources to the same position on the metal strip surface, the light irradiated from each light source can be captured without interfering with each other, so that multiple reflected images from multiple light sources can be obtained. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-188010 [Patent Document 2] Japanese Patent Application Publication No. 4-113260 [Patent Document 3] JP 2003-121371 A [Patent Document 4] JP 2018-105631 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, the above-mentioned conventional techniques have the following problems: First, the method described in Patent Document 1 improves the detection sensitivity of uneven defects on the surface of a metal strip based on the ratio of the regular reflection component and the diffuse reflection component of light reflected on the metal strip surface, but the surface defects to be detected are limited to uneven defects, making it difficult to detect many types of defects.

[0008] In addition, the method described in Patent Document 2 utilizes the property that the diffuse reflection intensity varies depending on the surface defect, and detects surface defects by irradiating monochromatic light of different wavelengths from different directions. However, since the arrangement of the light source unit and the wavelength of light that can be selected are limited, the types of surface defects that can be detected are also limited.

[0009] Moreover, the device described in Patent Document 3 uses laser light corresponding to the three primary colors of light and a plurality of imaging devices that detect specularly reflected light corresponding to the wavelengths of each light. However, although laser light has excellent linearity and is advantageous for detecting specularly reflected light, it may be difficult to detect and distinguish defects based on information about diffusely reflected light. Patent Document 3 also describes a device that uses synthetic light obtained by synthesizing general light with laser light, but in that case, the device for generating the synthetic light becomes large, which increases the size of the surface inspection device and the cost of the device.

[0010] In addition, according to the device described in Patent Document 4, by dividing the wavelength range of light using white light, information on surface defects corresponding to each wavelength range can be obtained. However, strictly speaking, light of the same wavelength is not incident from different angles, even if it is within a specified wavelength range, so there is room for improvement in the detection and discrimination of multiple types of surface defects.

[0011] The present invention has been made to solve the above problems, and an object of the present invention is to provide a surface inspection device and a surface inspection method for a metal strip that are capable of detecting and distinguishing various types of surface defects with a simple device configuration. Another object of the present invention is to provide a manufacturing method for a metal strip that is capable of improving the manufacturing yield of the metal strip. [Means for solving the problem]

[0012] The metal strip surface inspection device of the present invention comprises two or more light source units that selectively emit light of two or more wavelengths onto the surface of the metal strip, an imaging unit that images the light reflected from the metal strip by the light emitted from the light source units, and a control unit that controls the operation of the light source units and the imaging units, and the control unit outputs an emitted light control signal that controls the light source units so that the wavelengths of light emitted by the light source units are different from each other, in synchronization with a scan signal that controls the imaging timing of the imaging unit.

[0013] The imaging unit may include a sensor unit capable of distinguishing between the light beams having the two or more wavelengths emitted from the light source unit.

[0014] The light source unit may include three light source units each emitting light of a wavelength corresponding to RGB.

[0015] It is preferable that each of the light source units is a line light source that emits a line-shaped light source, and that the light emitted from each of the light source units is irradiated onto the same location on the surface of the metal strip.

[0016] The metal strip may be provided with a surface defect discrimination unit that discriminates surface defects, and the surface defect discrimination unit may include an image data acquisition unit that acquires image data captured by the imaging unit, an emitted light cycle signal acquisition unit that acquires a signal output by the control unit for each emitted light cycle that completes a cycle of the wavelength of light emitted by the light source unit, and a defect discrimination unit that judges surface defects of the metal strip based on the image data acquired by the image data acquisition unit for each emitted light cycle in accordance with the signal acquired by the emitted light cycle signal acquisition unit.

[0017] A method for inspecting a metal strip surface according to the present invention includes a step of determining surface defects of the metal strip using the metal strip surface inspection device according to the present invention.

[0018] The method for manufacturing a metal strip according to the present invention includes a step of manufacturing a metal strip while determining surface defects of the metal strip using the method for inspecting the surface of the metal strip according to the present invention. Effect of the Invention

[0019] According to the metal strip surface inspection device and method of the present invention, it is possible to detect and distinguish various types of surface defects with a simple device configuration, and according to the metal strip manufacturing method of the present invention, it is possible to improve the manufacturing yield of the metal strip. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing the configuration of a surface inspection device according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a change in light emitted from a light source unit. [Diagram 3] FIG. 3 is a schematic diagram showing the time-series operations of the light source unit and the imaging unit. [Figure 4] FIG. 4 is a schematic diagram showing an optical system of a coaxial epi-illumination method. [Diagram 5] FIG. 5 is a diagram for explaining an example of defect information processing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, a metal strip surface inspection device, a surface inspection method, and a manufacturing method according to an embodiment of the present invention will be described with reference to the drawings.

[0022] [Test subjects] In the surface inspection device and the surface inspection method for a metal strip according to an embodiment of the present invention, a thin steel sheet in any process after the hot rolling process in the manufacturing process of the thin steel sheet is inspected for surface defects. Since the thin steel sheet is wound into a coil shape and then processed at least after the hot rolling process, the thin steel sheet is described as a "metal strip" in this embodiment. However, the present invention can be applied not only to metal strips such as thin steel sheets, but also to metal strips that are processed in the form of a sheet such as a thick steel plate. In addition, the metal strip is not limited to steel material, and metal strips containing aluminum, copper, etc. are also subject to the present invention. In addition, the inspection for surface defects can be performed at any position from the entrance side to the exit side of the equipment in each process of the hot rolling process, the pickling process, the cold rolling process, the annealing process, the plating process, and the temper rolling process. In addition, the inspection can be performed in the final inspection process for quality assurance of the metal strip.

[0023] The surface defects to be inspected by the surface inspection device of this embodiment can be roughly divided into two types called uneven defects and pattern defects. The uneven defects are defects consisting of concaves and convexities, or multiple concaves and convexities, and the uneven shape can be recognized to some extent by visual inspection. Specifically, the uneven defects are circular, elliptical, streak-like, or irregular defects having a diameter of 0.1 mm or more when viewed from the surface of the metal strip, and the depth of the concaves (or the height of the convexities) is about 5 to 1000 μm. Conventionally, the uneven defects are further divided into types, and are sometimes called dents, dull baldness, compression defects, scratches, scabs, etc. Dents are bite-like dent defects that are generated when a metal strip gets wrapped around some kind of hard foreign matter during line threading. Dulled baldness is a defect that occurs in a part of a metal strip during a rolling process (cold rolling process or temper rolling process) using a dulled roll, where the unevenness (dull grain) of the rolling roll is not transferred. Dulled baldness occurs due to wear of the rolling roll or adhesion of foreign matter to the rolling roll. Pressed defects are defects that occur when a part of the interlayer of a coil adheres to the metal strip when batch annealing is performed on the metal strip in a coiled state, and the adhered part is peeled off. Scratched defects are defects that are observed as sliding marks on the surface of the metal strip when the metal strip comes into contact with a foreign matter such as an equipment part. Scalded defects are defects that are formed in a wrap shape due to surface peeling caused by internal inclusions in the metal strip. Scratched and scalded defects are often observed as defects that extend continuously or intermittently in the longitudinal direction of the metal strip.

[0024] On the other hand, a pattern defect is a defect that can be visually recognized as an uneven color tone on the surface of a metal strip. Even if it is a pattern defect, it may be formed from fine irregularities when observed microscopically. Here, a pattern defect refers to one with a height of irregularities of less than 5 μm. For example, when a plating process is performed on the surface of a metal strip, the deposition state of metal particles in the plating film may be different from other parts and recognized as a pattern defect. The fine irregularities caused by such precipitated particles scatter light of a specific wavelength or less (for example, 600 nm or less) and are observed as a brown-colored defect (brown pattern) depending on the viewing angle. There are many causes of pattern defects. For example, they may be called rust patterns, scale patterns, brown patterns, temper colors, stains, etc. A rust pattern is a pattern formed on the surface of a metal strip that is oxidized and cannot be distinguished from other parts of the metal strip in terms of the irregular shape during the rolling process or the sheet threading process, but has a rust-colored pattern (different in color from other areas) on the surface of the metal strip. The temper color is caused by a partial change in the thickness of the oxide film on the surface of the metal strip during annealing, and the interference color that occurs between the surface of the oxide film and the bare metal strip varies depending on the location on the surface of the metal strip. The color of the pattern defect may be caused by the interference color when light is reflected on the surface of the metal strip, and the color tone of the pattern defect varies depending on the type of defect. The above-mentioned classification of the types of defects into uneven defects and pattern defects is preset for each metal strip production line, and the characteristics of the defects are usually identified visually by an inspector and managed on each production line.

[0025] The above classification of uneven defects and pattern defects is based on the difference in how they are recognized by human vision. Therefore, even if there is a geometric unevenness, if it is large, it may be judged as an uneven defect, and if it is small, it may be judged as a pattern defect, and the difference between them can be said to be for convenience. In this specification, if the height (amplitude) of the unevenness existing on the surface of the metal strip is 5 μm or more, it is called an uneven defect, and if the height (amplitude) of the unevenness is less than 5 μm, or if it is not due to unevenness but is observed only by color tone or brightness difference due to light interference, etc., it is called a pattern defect.

[0026] 〔composition〕 The configuration of the surface inspection device of this embodiment is shown in FIG. 1. As shown in FIG. 1, the surface inspection device 1 of this embodiment includes a plurality of light source units 2a to 2c that emit light toward the metal strip S, an imaging unit 3 that captures the light reflected from the surface of the metal strip S, and a control unit 4 that controls the operation of the plurality of light source units 2a to 2c and the imaging unit 3. The plurality of light source units 2a to 2c selectively emit light in a plurality of wavelength bands and may be disposed at any angle toward the inspection position on the surface of the metal strip S. The imaging unit 3 receives a scan signal from the control unit 4 and acquires image data including the inspection position on the surface of the metal strip S. The control unit 4 outputs a scan signal that controls the timing at which the imaging unit 3 acquires the image data. In addition, the control unit 4 outputs an emission light control signal to each light source unit so as to switch the wavelength of the light emitted by the plurality of light source units 2a to 2c in synchronization with the scan signal. The emission light control signal is also a signal that specifies the wavelength of the light emitted by each light source unit, and controls the plurality of light source units 2a to 2c so that the wavelengths of the emitted light are different from each other.

[0027] In the surface inspection device 1 shown in FIG. 1, the light source units 2a to 2c are arranged so that the incident angles of the emitted light with respect to the surface of the metal strip S are different, such as α, β, and γ, and each light source unit emits light of a different wavelength toward an inspection position on the surface of the metal strip S. Each light source unit has a function of selectively emitting light of a wavelength selected from the RGB wavelength bands, and an emitted light control signal from the control unit 4 determines which wavelength band the light is to be emitted. In addition, the control unit 4 outputs an emitted light control signal so that the wavelengths of the light emitted from the light source units 2a to 2c are different from each other. In other words, at any timing, the light source units 2a to 2c are in a state of emitting light of different wavelengths selected from the RGB wavelength bands. In the configuration example shown in FIG. 1, the light source unit 2a emits light of the R wavelength band, the light source unit 2b emits light of the G wavelength band, and the light source unit 2c emits light of the B wavelength band.

[0028] The light source units 2a to 2c in this embodiment selectively emit light of two or more wavelengths, and are light sources capable of emitting a plurality of light beams of different wavelength bands. Different wavelength bands refer to light having different spectral distributions. However, the spectral distributions may overlap partially, as long as the peak wavelengths of the spectral distributions are different. Furthermore, since the imaging unit 3 separates a plurality of images corresponding to light of different wavelength bands from image data acquired by one imaging, it is preferable that the peak wavelengths of the spectral distributions of the light emitted by the light source units 2a to 2c are at least 50 nm apart. More preferably, they are 100 nm or more apart.

[0029] When visible light is selected as the light emitted by the light source units 2a to 2c, a wavelength selected from purple (light with a wavelength range of 380 to 430 nm), indigo blue (light with a wavelength range of 430 to 460 nm), blue (light with a wavelength range of 460 to 500 nm), green (light with a wavelength range of 500 to 570 nm), yellow (light with a wavelength range of 570 to 590 nm), orange (light with a wavelength range of 590 to 610 nm), red (light with a wavelength range of 610 to 780 nm), etc. may be used. A more preferred form of the light source units 2a to 2c is an LED lighting that can independently adjust the three wavelengths of RGB. For example, RGB high-brightness line lighting manufactured by ITEC System Co., Ltd. can be used. The light emitted from the light source units 2a to 2c is not necessarily limited to visible light. It may be a light source unit that selectively emits infrared rays (IR), ultraviolet rays (UV), etc. However, it is necessary that the imaging section 3 is capable of capturing images by receiving light in these wavelength bands.

[0030] The imaging unit 3 is disposed so as to capture an image of an irradiation position (inspection position) where the light emitted from the light source units 2a to 2c is irradiated on the surface of the metal strip S. The imaging unit 3 may be a two-dimensional camera or a line camera. It is sufficient if it can capture the light emitted from the light source units 2a to 2c and reflected at the inspection position on the surface of the metal strip S. It is preferable to apply a color camera using a solid-state imaging element (CCD, CMOS) as the imaging unit 3. The solid-state imaging element has a sensor unit capable of distinguishing light of two or more wavelengths, and can decompose the captured image into three-channel images of RGB. The color separation method by the solid-state imaging element may be a multi-plate method or a single-plate method. It is sufficient if the captured image is color-separated by the solid-state imaging element and each image is converted into an electric signal and acquired. The imaging unit 3 may be configured to have one or more RGB color sensors using Si photodiodes as light receiving units for receiving RGB light. For example, an RGB color sensor manufactured by Hamamatsu Photonics KK may be used. However, the imaging unit 3 is not limited to a color camera, and multiple monochrome cameras may be installed at the same position (the position of the light receiving angle θ in the example shown in FIG. 1), and each camera may be provided with a filter unit that selectively transmits the wavelengths of light emitted from the light source units 2a to 2c. This allows the imaging unit 3 to obtain images corresponding to the multiple wavelengths of light emitted from the light source units 2a to 2c using the multiple cameras.

[0031] It is preferable to use an imaging unit 3 in which the peak wavelengths of the light emitted by the light source units 2a to 2c match the transmission wavelengths of the sensor units of the imaging unit 3. It is also preferable that the sensors that receive light of each wavelength constituting the sensor unit of the imaging unit 3 do not overlap with each other in terms of transmission wavelengths. However, even if there is a partial overlapping area in the transmission wavelengths, it is acceptable as long as the overlap is 10% or less with respect to the amount of transmitted light.

[0032] With the light source units 2a to 2c and the imaging unit 3 having the above configuration, the imaging unit 3 acquires a reflection image of light in a wavelength band R at an incidence angle α by the light source unit 2a, a reflection image of light in a wavelength band G at an incidence angle β by the light source unit 2b, and a reflection image of light in a wavelength band B at an incidence angle γ by the light source unit 2c. The imaging unit 3 also acquires an image captured by the solid-state imaging element by decomposing it into three channel images of RGB. However, when a monochrome camera is used as the imaging unit 3, a filter unit that transmits only light in one of the wavelength bands of RGB is provided for each of the three monochrome cameras, thereby acquiring a reflection image of each of the wavelength bands of RGB. This allows the imaging unit 3 to acquire a total of three reflection images corresponding to light in different wavelength bands from three different incidence angles. In this way, the imaging unit 3 acquires a plurality of reflection images of light of multiple wavelengths incident from multiple incidence angles and reflected on the surface of the metal strip S, making it possible to detect and distinguish a plurality of types of surface defects.

[0033] [Switching of light emitted from the light source unit] The control unit 4 outputs an emission light control signal to the light source units 2a to 2c in synchronization with the timing of outputting a scan signal to the imaging unit 3, for controlling the wavelengths of light emitted from each of the light source units to be different from each other. The imaging unit 3 captures an image of the reflected light from the metal strip S in accordance with the scan signal output from the control unit 4. The light source units 2a to 2c emit light of a predetermined wavelength in accordance with the emission light control signal output from the control unit 4. In this case, if the control unit 4 is programmed to update the wavelengths of light emitted by the light source units 2a to 2c in a preset order, the emission light control signal does not need to be a signal that specifies the wavelength of light emitted from each light source unit, but may be a signal for switching the wavelength of light emitted from the light source units 2a to 2c in a preset order.

[0034] 2(a) to 2(c) are schematic diagrams showing changes in the light emitted by the light source units 2a to 2c. Scan1 shown in FIG. 2(a) shows a state in which the light source unit 2a emits light with an R wavelength, the light source unit 2b emits light with a G wavelength, and the light source unit 2c emits light with a B wavelength. At the timing of Scan1, the imaging unit 3 acquires image data of the light reflected on the surface of the metal strip S. Next, the state transitions to Scan2 shown in FIG. 2(b) by a scan signal generated by the control unit 4. Scan2 is a state in which an emitted light control signal is generated according to the scan signal of the control unit 4, and the wavelengths of the emitted light of the light source units 2a to 2c are switched in a preset order. In the example shown in FIG. 2(b), the emitted light of the light source unit 2a is switched to light with a B wavelength, the emitted light of the light source unit 2b is switched to light with an R wavelength, and the emitted light of the light source unit 2c is switched to light with a G wavelength. Then, at the timing of Scan2, the imaging unit 3 acquires image data of the light reflected on the surface of the metal strip S again. Furthermore, the next scan signal generated by the control unit 4 causes a transition to the Scan3 state shown in Fig. 2(c). In the Scan3 state, the light emitted by the light source unit 2a is switched to light with a G wavelength, the light emitted by the light source unit 2b is switched to light with a B wavelength, and the light emitted by the light source unit 2c is switched to light with an R wavelength. In the Scan3 state, the imaging unit 3 also acquires image data of the light reflected from the surface of the metal strip S in the same manner as described above.

[0035] In this embodiment, the wavelength of the light emitted by each light source unit is updated in sequence, and the cycle is called an "emitted light cycle." When one emitted light cycle is completed, image data of the reflected light on the metal strip surface corresponding to the number of light source units (positions of the light source units) x the number of wavelengths of the emitted light is acquired. For example, when Scan1 to Scan3 shown in Fig. 2(a) to (c) are taken as one emitted light cycle and the emitted light cycle is completed, nine types of images are acquired (number of light source units 3 x number of wavelengths of the emitted light 3). As a result, as shown in Table 1 below, image data of the reflected light on the metal strip surface is acquired for all combinations of the positions of the light source units and the wavelengths of the emitted light, making it possible to detect many types of surface defects. Note that the number of light source units and the number of wavelengths of the light emitted from the light source units do not need to be the same, and the number of wavelengths of the light emitted from the light source units may be greater than the number of light source units.

[0036] [Table 1]

[0037] [Scan signal] The scan signal output by the control unit 4 may be a pulse signal output at a preset time interval. The time interval of the pulse signal may be set arbitrarily, and the set value is stored in the control unit 4 in advance. In addition, it is preferable to set the time interval at which the scan signal is output so as to match the scan rate of the imaging unit 3. This is because a lot of image data can be acquired within a certain time by switching the emitted light from the light source units 2a to 2c in synchronization with the scan rate of the imaging unit 3. On the other hand, the scan signal output by the control unit 4 does not necessarily have to be at a constant time interval during the surface inspection of the metal strip S. For example, the pulse signal may be output at a time interval inversely proportional to the conveying speed of the metal strip S, in synchronization with the conveying speed of the metal strip S. In this case, when the conveying speed of the metal strip S is fast, the imaging unit 3 acquires image data at a long time interval, and when the conveying speed is slow, the imaging unit 3 acquires image data at a short time interval. This makes it possible to acquire image data at equal intervals in the longitudinal direction of the metal strip S. Specifically, a conveying speed measuring means installed in the conveying equipment for the metal strip S may output a PLG signal to the control unit 4 every certain distance traveled by the metal strip S, and the control unit 4 may output the acquired PLG signal as a scan signal to the light source units 2a to 2c and the imaging unit 3.

[0038] In this embodiment, various surface defects are detected by acquiring a large number of images based on a combination of the incident angle from the light source unit and the wavelength of light used, so the more the number of light source units and the number of wavelengths of light emitted from the light source units, the more detailed information on the surface defects can be acquired. However, in this case, the number of scans performed by the imaging unit 3 until one emitted light cycle is completed also increases, and it takes a certain amount of time to acquire multiple images of the surface defects. As a result, when the metal strip S to be inspected is transported at high speed, the position of the image acquired for each scan by the imaging unit 3 may be shifted slightly. Therefore, in order to acquire detailed image information for the same location of the metal strip S, it is preferable to shorten the time interval of the scan signal of the imaging unit 3 and to limit the number of light source units and the number of wavelengths of light emitted from the light source units to a certain extent.

[0039] From this viewpoint, when a line scan camera is used as the imaging unit 3, it is preferable to use one with a scan rate of 10 MHz or more, and the frequency of the scan signal may be determined according to the scan rate of the line scan camera. When a line scan camera is used, the image acquired by the imaging unit 3 per scan is linear information in the conveying direction of the metal strip S, but when the scan rate is high, multiple images acquired during one emitted light cycle may be considered to be image information of approximately the same location. Specifically, when three scans are performed using three light source units and three wavelengths of light as shown in Figs. 2(a) to (c), when a surface inspection is performed at a conveying speed of the metal strip S of 50 to 2000 m / min, if the scan rate of the line scan camera is 10 to 640 MHz, nine types of images acquired during one emitted light cycle may be considered to be image information of the same location. This is because the scan rate is sufficiently high compared to the line speed.

[0040] On the other hand, when a two-dimensional camera is used as the imaging unit 3, the scan rate is preferably about 50 to 60 Hz. However, when the line speed (movement speed of the object to be inspected) of the metal strip S on the inspection line exceeds 150 m / min, it is preferable to use a high-speed camera as the imaging unit 3. In this case, the scan rate is preferably about 50 to 2000 kHz. When a two-dimensional camera is used, an image of a certain range can be acquired in the traveling direction of the metal strip S, so that even if the metal strip S to be inspected moves during the transition of the scan timing, an image including the same part can be acquired during one emission light cycle. Therefore, when a two-dimensional camera is used as the imaging unit 3, it is not necessary to use a camera with a scan rate as high as that of a line scan camera. It is preferable that the position of the multiple image data acquired during one emission light cycle is corrected in the surface defect discrimination unit 5 described later so that the image data is image data of the same position of the metal strip S.

[0041] 3(a) to (f) are schematic diagrams showing the time-series operation of the light source unit and the imaging unit. The scan signal generated by the control unit 4 is set according to the scan rate of the imaging unit 3. In this example, the light in the RGB wavelength band is selectively emitted from the three light source units, and the sensor units capable of identifying the three wavelengths of light provided in the imaging unit 3 are respectively designated as sensor R, sensor G, and sensor B. At this time, the control unit 4 generates a scan signal at the set scan rate, outputs the scan signal to the imaging unit 3, and simultaneously outputs an emitted light control signal to the light source units 2a to 2c. The emitted light control signal is set so that the light source units 2a to 2c emit light of different wavelengths from the RGB wavelengths in a preset order. Since the light source units 2a to 2c require a certain switching response time (50 to 100 ns in this case) to switch the emitted light, the imaging unit 3 is set to have an exposure timing at which the sensor unit acquires image data with a preset time delay after acquiring the scan signal. Then, when the emission light cycle in which the light emitted from the light source units 2a to 2c undergoes a full cycle of RGB switching, the imaging unit 3 acquires nine image data corresponding to the three light source units 2a to 2c and the three wavelengths of light. By repeating this emission light cycle, continuous surface inspection of the metal strip S in the longitudinal direction becomes possible.

[0042] [Linear light source] The light source units 2a to 2c are preferably linear light sources that irradiate the surface of the metal strip S with light in a line shape. In addition, when the linear light sources from the light source units 2a to 2c irradiate the surface of the metal strip S, it is preferable to arrange them so that the irradiating unit that irradiates linearly is perpendicular to the conveying direction of the metal strip S. When detecting surface defects while conveying the metal strip S, it is possible to detect surface defects at any position in the width direction of the metal strip S by irradiating linear light in the width direction of the metal strip S and detecting the reflected light with the imaging unit 3. The linear light source is not limited to a linear light source, but refers to a light source that irradiates light to a rectangular range on the metal strip S, and the irradiation range in the direction of one side is 10 times or more longer than the irradiation range of the other side. In this embodiment, it is preferable to irradiate light such that the direction of the long side of the rectangular irradiation range is at least longer than the width direction of the metal strip S. In addition, it is preferable that the light emitted from the multiple light source units 2a to 2c is irradiated to the same place on the surface of the metal strip S. However, it is not necessary that the irradiation areas of the emitted light from the multiple light source units 2a to 2c completely coincide on the surface of the metal strip S. There may be areas where the irradiation areas of the light source units do not overlap, because in areas where the irradiation lights overlap, surface defect inspection in the above-mentioned manner is possible.

[0043] [Arrangement of light source and imaging unit] The positional relationship between the light source units 2a to 2c and the imaging unit 3 can be arbitrarily selected as long as the imaging unit 3 is located in a position where the light from the light source units 2a to 2c reflected by the surface of the metal strip S can be received by the imaging unit 3. For example, the imaging unit 3 can be located in a position where it receives specularly reflected light from one of the light source units, and may be located in a position where it receives diffusely reflected light from the other light source units. This is because specularly reflected light from the metal strip surface makes it easy to detect uneven defects on the surface, and diffusely reflected light makes it easy to detect pattern defects. In this case, the light source units emit red light, and the image data captured by the imaging unit 3 as specularly reflected light from the metal strip S makes it easy to identify uneven defects. In addition, blue light is emitted from the light source unit located in a position where the imaging unit 3 receives diffusely reflected light, and the image data acquired by the imaging unit 3 that receives the diffusely reflected light from the metal strip S makes it easy to detect pattern defects such as rust patterns. In this embodiment, in addition to the positional relationship between the light source units 2a to 2c and the imaging unit 3, reflected images of various wavelengths of light are acquired, improving detection sensitivity for surface defects of different color tones as pattern defects.

[0044] A preferred arrangement in the case where three light source units 2a to 2c shown in FIG. 1 are provided will be described. The imaging unit 3 is arranged so that the incident angle α=θ is such that the light source unit 3 receives the specular reflected light emitted from one light source unit selected from the light source units 2a to 2c (light source unit 2a in FIG. 1). As the other light source unit, the light source unit 2b emits specular light from a position in the same quadrant as the light source unit 2a at an angle different from that of the light source unit 2a, and the imaging unit 3 receives the diffuse reflected light, so that β≠θ is satisfied (forward scattering arrangement). The light source unit 2c is arranged in the same quadrant as the imaging unit 3 at a position where it receives the diffuse reflected light (backward scattering arrangement). In this way, by capturing an image of the reflected light of light of different wavelengths emitted from each light source unit at different angles, many types of surface defects can be detected.

[0045] It is also possible to arrange one of the light source units as perpendicular to the surface of the metal strip S as possible, and to arrange the imaging unit 3 as perpendicular to the surface of the metal strip S as possible. In this case, as shown in FIG. 4, a coaxial epi-illumination method can be selected in which a light source unit 2a is provided to emit light from a direction perpendicular to the surface of the metal strip S. The coaxial epi-illumination method is a method in which the direction of emitted light and the imaging direction are made to coincide with each other via a half mirror 10, and is effective for inspecting glossy surfaces. Furthermore, the other light source units 2b and 2c may be arranged symmetrically with respect to the vertical plane of the metal strip S, and each may emit light of a different wavelength. According to the above-mentioned configuration of the surface inspection device, which is provided with one imaging unit 3 and multiple light source units 2a to 2c that irradiate light to the same position on the surface of the metal strip S, and enables surface inspection compatible with light of multiple wavelengths, there is no need to arrange multiple sets of light source units and imaging units, and various surface defects can be inspected with a compact equipment configuration.

[0046] [Surface defect detection unit] Returning to FIG. 1, as shown in FIG. 1, the surface inspection device 1 of this embodiment includes a surface defect discrimination unit 5 that discriminates defects detected on the surface of the metal strip S from a plurality of image data acquired by a combination of a plurality of light source units and wavelengths of emitted light as described above. The surface defect discrimination unit 5 is configured by a general-purpose computer such as a workstation or a personal computer. The surface defect discrimination unit 5 includes an emitted light cycle signal acquisition unit 5a that acquires an emitted light cycle signal from the control unit 4, an image data acquisition unit 5b that acquires image data captured by the imaging unit 3, a defect judgment unit 5c that judges surface defects of the metal strip S based on the image data acquired by the image data acquisition unit 5b for each emitted light cycle, and a storage unit 5d. The defect judgment unit 5c is, for example, a CPU or the like, and executes a predetermined calculation to realize a function of judging the type and grade (severity) of a surface defect using a computer program and data stored in the storage unit 5d.

[0047] In this case, the imaging unit 3 outputs a plurality of image data acquired at each scan timing to the surface defect discrimination unit 5. The control unit 4 outputs an output light cycle signal, which is information that an output light cycle is completed, to the surface defect discrimination unit 5 each time an output light cycle is completed. The defect determination unit 5c analyzes the image data acquired in each output light cycle, consolidates information on defect candidates obtained from the plurality of image data, and determines surface defects. In this case, image data corresponding to the number of light source units (positions of the light source units) x the number of wavelengths of output light is acquired for one output light cycle. Here, the plurality of image data acquired in one output light cycle is called inspection data. For example, when one output light cycle from Scan1 to Scan3 shown in Figs. 2(a) to (c) is completed, nine types of image data (3: the number of light source units x 3: the number of wavelengths of output light) are acquired, and this becomes the inspection data in the defect determination unit 5c.

[0048] Specifically, the defect determination unit 5c determines surface defects by the defect information processing shown in FIG. 5. FIG. 5 shows processing of arbitrary image data included in the inspection data as an example of the defect information processing of this embodiment. The example shown in FIG. 5 is an example in which a line light source is used as the light source unit, and the surface of the metal strip S is scanned in the width direction by a line scan camera to capture an image. When the imaging unit 3 is a line scan camera, the image data becomes received light intensity information as shown in FIG. 5. The received light intensity information is information on the light intensity of a specific wavelength band from a specific light source unit acquired by the imaging unit 3, and is information in which a correspondence relationship with the width direction position of the metal strip S to be inspected is specified. FIG. 5 shows the relationship between the width direction position of the metal strip and the received light intensity as a chart image.

[0049] In addition, when the received light intensity information has an unavoidable distribution in the width direction due to the configuration of the optical system (when the received light intensity information has a certain distribution in the width direction even in a state where there is no surface defect on the metal strip S), the defect determination unit 5c may perform shading correction on the received light signal as a pre-processing to reduce the error of the received light signal generated in the imaging unit 3 due to the characteristics of the optical system, and use the result as the received light intensity information. In the defect information processing shown in FIG. 5, the defect determination unit 5c compares the distribution of the received light intensity with upper and lower thresholds (tolerance range) set in advance within a measurement range corresponding to the width of the metal strip S. Then, when a part of the received light intensity falls outside the tolerance range, the defect determination unit 5c stores information about that part (the part indicated by the broken line in FIG. 5) in the storage unit 5d as information about a defect candidate. Here, the information about the defect candidate generated in this way is called defect candidate information. The defect candidate information is information in which the width direction position of the metal strip S where the received light intensity information exceeds the upper and lower thresholds is associated with the magnitude of the received light intensity at that position (for example, the deviation from the threshold value, etc.). The defect determination unit 5c then performs the same process on the other image data included in the inspection data. As a result, defect candidate information is stored in the storage unit 5d for each image data included in the inspection data. Note that the upper and lower thresholds in the above process may be set to different values ​​depending on the arrangement of the light source unit and the wavelength of the selected light, or may be the same value. They may also be set empirically depending on the results of past surface inspections.

[0050] Next, the defect determination unit 5c executes defect information processing on the inspection data, obtains the defect candidate information obtained from the storage unit 5d, and classifies the combination of the position of the light source unit and the wavelength of light that includes the defect candidate information in the image data included in the inspection data into the combination of the position of the light source unit and the wavelength of light that does not include the defect candidate information. However, when multiple pieces of defect candidate information are detected in the width direction of the metal strip S, the defect determination unit 5c aggregates the defect candidate information among the defect candidate information detected at the same width direction position based on the position information of the defect candidate information. For example, the following Tables 2 and 3 are examples of aggregated defect candidate information that are aggregated based on nine image data acquired using a surface inspection device configured with three light sources and three wavelengths (RGB).

[0051] [Table 2]

[0052] [Table 3]

[0053] In the table, "◯" indicates that defect candidate information was not detected, and "X" indicates that defect candidate information was detected. However, these are a summary of evaluation results at the same position in the width direction of the metal strip S. Table 2 is an example in which defect candidate information was detected only when red R light was emitted from the light source unit 2a, which is arranged so that the imaging unit 3 receives specularly reflected light. Table 3 is an example in which defect candidate information was detected only when blue B light was emitted from the light source unit 2c, which is arranged in a backscattering position relative to the imaging unit 3.

[0054] In this case, in the example shown in Table 2, since defect candidate information is detected for the image data of the regular reflection light, it is presumed that the detected surface defect is caused by strong light scattering due to the unevenness of the surface. Furthermore, since defect candidate information is obtained for the red light, it can be determined that the defect has a relatively large unevenness. However, since no defect candidate information is detected for the forward scattering and backward scattering arrangements, it can be seen that the uneven defect does not show a color change specific to a pattern-like defect. On the other hand, in the example shown in Table 3, since a defect candidate is detected by blue light, which has a shorter wavelength than red light, for the backward scattering arrangement, it can be determined that the defect is not classified as an uneven defect but as a rust-colored pattern-like defect. However, since no defect candidate information is obtained for the forward scattering arrangement, it can be determined that the surface of the metal strip S is highly smooth and that the defect is a rust-like surface defect with an extremely thin oxide film formed thereon.

[0055] In this manner, it is possible to determine whether a detected surface defect is a concave-convex defect or a pattern defect based on the determination of defect candidate information based on the combination of the arrangement of the multiple light source units and the wavelengths of the emitted light. In addition, since it is possible to distinguish between defects that are detected as defect candidate information and those that are not based on the wavelength of light, it is also possible to determine the color tone of the detected surface defect, and to determine surface defects in more detailed classifications within the classification of pattern defects. When determining the severity of a defect, for example, in the defect information processing shown in FIG. 5, the severity (grade) of the defect may be determined based on the degree of deviation of the received light intensity information from the allowable range.

[0056] Furthermore, the defect determination unit 5c may generate new image data by superimposing a plurality of image data from the inspection data aggregated for each emitted light cycle, and perform the above-mentioned defect information processing on this image data. For example, the defect determination unit 5c may perform defect information processing on image data generated by light of different wavelengths emitted from the same light source unit. This is because it becomes easier to determine a pattern defect that exhibits an intermediate color in the wavelength band of the light of the light source unit as a surface defect of the metal strip. In addition, when the light source unit 2b and the light source unit 2c are arranged symmetrically with respect to the surface perpendicular direction of the metal strip S as shown in FIG. 4, the defect determination unit 5c may calculate the difference between two image data when the light from the light source unit 2b and the light from the light source unit 2c have the same wavelength, and perform defect information processing on image data composed of the difference information. This emphasizes the shape of the uneven surface defect, making it easier to determine the uneven defect.

[0057] It is preferable to perform an offline inspection using the above-mentioned surface inspection device 1 on a sample of surface defects previously determined by an inspector, obtain a summary table of defect candidate information for each defect (lists shown in Tables 2 and 3), and store the information in the storage unit 5d as defect determination information. For example, for uneven defects classified as dents, dull baldness, and compression defects, a summary table of defect candidate information is created based on the inspection data. Similarly, for pattern defects, an offline surface inspection is performed on rust patterns, scale patterns, brown patterns, temper colors, stains, etc., and a summary table of defect candidate information is created based on the inspection data. These are indexes representing the feature amounts of surface defects identified offline, and are stored in the storage unit 5d as defect determination information.

[0058] The defect determination unit 5c performs defect information processing on the inspection data acquired online at any time, and generates an aggregate table of defect candidate information as described above. The defect determination unit 5c then identifies the type of the detected surface defect by comparing it with defect determination information previously stored in the storage unit 5d. Information on the surface defect identified by the defect determination unit 5c is output to the output unit of the surface inspection device 1, making it possible to appropriately select a processing step for the metal strip to be inspected. For example, a processing step may be added to the surface inspection step of the metal strip to remove the detected surface defect. This makes it possible to manufacture a metal strip with fewer surface defects.

[0059] The above defect information processing can also be applied when a two-dimensional camera is used as the imaging unit, or when a two-dimensional image is generated by combining images obtained for each emitted light cycle using a line scan camera in the traveling direction of the metal strip. In this case, the received light intensity information shown in Fig. 5 is specified by the two-dimensional coordinates of the width direction position and the length direction position of the metal strip, and by setting upper and lower thresholds, defect candidate information can be obtained and its position information can also be specified.

[0060] Although the embodiments of the present invention have been described above, the present invention is not limited by the descriptions and drawings that form part of the disclosure of the present invention according to the present embodiments. In other words, other embodiments, examples, and operation techniques, etc., made by those skilled in the art based on the present embodiments are all included in the scope of the present invention. [Explanation of symbols]

[0061] 1 Surface inspection equipment 2a~2c Light source section 3. Imaging unit 4. Control section 5. Surface defect detection section 5a Emitted light cycle signal acquisition unit 5b Image data acquisition unit 5c Defect Judgment Section 5d Storage area 10 Half Mirror S Metal Strip

Claims

1. Two or more light source units selectively emitting light of two or more wavelengths onto a surface of the metal strip; an imaging unit that images reflected light of the metal strip caused by light emitted from the light source unit; A control unit that controls operations of the light source unit and the imaging unit; a surface defect discrimination unit that discriminates surface defects of the metal strip, the control unit outputs an emission light control signal that controls the light source unit so that wavelengths of light emitted by the light source unit are different from each other in synchronization with a scan signal that controls an imaging timing of the imaging unit; The surface defect determination unit includes: an image data acquisition unit that acquires image data captured by the imaging unit; an emission light cycle signal acquisition unit that acquires a signal output by the control unit for each emission light cycle in which the wavelength of the light emitted by the light source unit goes around; a defect determination unit that determines a type of a surface defect of the metal strip based on image data acquired by the image data acquisition unit for each emitted light cycle in accordance with a signal acquired by the emitted light cycle signal acquisition unit, A surface inspection device for a metal strip, wherein the defect determination unit determines the type of surface defect based on information regarding a combination of a position of a light source unit and a wavelength of light when the image data includes defect candidate information and information regarding a combination of a position of a light source unit and a wavelength of light when the image data does not include defect candidate information.

2. The metal strip surface inspection device according to claim 1 , wherein the imaging unit includes a sensor unit capable of identifying the light of the two or more wavelengths emitted from the light source unit.

3. 3. The metal strip surface inspection device according to claim 1, wherein the light source unit includes three light source units each emitting light of a wavelength corresponding to RGB.

4. A metal strip surface inspection device as described in any one of claims 1 to 3, wherein the light source units are all line light sources that emit line-shaped light, and the light emitted from the light source units is all irradiated to the same location on the surface of the metal strip.

5. 5. The metal strip surface inspection device according to claim 1, wherein the surface defects are uneven defects and pattern defects.

6. A method for inspecting a surface of a metal strip, comprising a step of determining surface defects of the metal strip using the metal strip surface inspection device according to any one of claims 1 to 5.

7. A method for manufacturing a metal strip, comprising the step of manufacturing the metal strip while determining surface defects of the metal strip using the method for inspecting the surface of the metal strip according to claim 6.

Images