JPWO2014171361A1 - Metal defect detection method - Google Patents

Abstract

In this metal defect detection method, the inspection surface of the metal sample is faced upward, the etching solution is held on the upper surface of the inspection surface so that the entire surface of the inspection surface is covered with the etching solution, and the defect appears. The thickness of the etching solution above the inspection surface is set to be greater than 0 mm and equal to or less than 10 mm, and the inspection surface is imaged by the imaging device in this state.

Description

The present invention relates to a metal defect detection method for detecting defects by etching an inspection surface of a metal sample to reveal defects and imaging the inspection surface with an imaging device.
This application claims priority on April 17, 2013 based on Japanese Patent Application No. 2013-086496 for which it applied to Japan, and uses the content here.

  Detection and evaluation of defects such as cracks and segregation in metal materials, particularly steel slabs and slabs, are indispensable for quality control of slabs and optimization and operation management of slab manufacturing processes. Conventionally, a sulfur printing method or an etch printing method has been used as a quality evaluation method inside a metal material. In any case, a sample is cut out from the metal material, the cross section of the metal material is polished as an inspection surface, and evaluation is performed.

  In the sulfur printing method, a sulfuric acid aqueous solution is immersed in a transfer printing paper containing silver bromide and attached to the inspection surface, and the state of segregation of sulfur in the sample appears on the printing paper. According to this sulfur printing method, it is possible to evaluate the center segregation or internal crack of a slab by utilizing the property that sulfur segregates at the center segregation part or crack part of the slab. The sulfur printing method is used for a material having a high sulfur concentration in a metal material, but cannot be applied to an extremely low sulfur steel having a low sulfur concentration.

  The etch print method uses the property that elements are segregated at the center segregation and cracks on the inspection surface. The inspection surface is etched to reveal the element segregation, and the sample is coated with petroleum jelly and then re-polished. This is a technique for visualizing segregation and cracking. This etch printing method can also be used for materials having a low sulfur concentration.

  With the recent improvement in quality and purity of metal materials, there has been a demand for higher accuracy in detecting defects in metal materials. Regarding the sulfur printing method, there is a problem in the stable supply of photographic paper to be used, and the sulfur printing method cannot be applied to extremely low-sulfur steel in the first place. The etch printing method requires several hours for the entire operation, and speeding up of defect detection is required.

  It is preferable if the inspection surface of the metal material is etched and then the inspection surface can be directly picked up by an imaging device such as a camera and recorded as an image because defects can be detected quickly unlike the etch printing method. In this case, since the defect portion is deeply dug due to the progress of etching, it is necessary to detect the defect using the deeply dug defect portion as a dark portion and the other normal portion (non-defect portion) as a bright portion. However, when the inspection surface is etched (macro corrosion) and the inspection surface is imaged with an imaging device, the lightness of the defective portion and the normal portion is low because the lightness of the normal portion is low. There is a problem that the contrast is small.

  On the other hand, in patent document 1, the test | inspection surface of a metal sample is grind | polished to one direction, the said metal sample is etched, and the defect in a metal sample appears. Then, the inspection surface is irradiated from a predetermined arrangement position using a linearly continuous light source or a light source (linear light source) in which a plurality of point light sources are linearly arranged, and the inspection surface is imaged by the imaging device. Thereby, the brightness of the normal part in the inspection surface captured image is made uniform and bright, and the defective part can be detected clearly.

Japanese Unexamined Patent Publication No. 2011-203201

  The method described in Patent Document 1 is intended to increase the contrast between the defective part and the normal part by increasing the lightness of the normal part, but not to reduce the lightness of the defective part. Therefore, the increase in contrast is naturally limited. In addition, the type of light source is limited to a linear light source, and further, the installation method of the light source, specifically, the relationship between the linear direction of the linear light source and the polishing direction, and the irradiation angle with respect to the polishing direction are limited. There was a need to improve the efficiency of defect detection work.

  The present invention relates to a metal defect detection method in which a defect is detected by etching an inspection surface of a metal sample to reveal a defect, and the defect is detected by imaging the inspection surface with an imaging device, thereby reducing the brightness of the defect portion. An object of the present invention is to provide a metal defect detection method capable of clearly detecting a defect portion without increasing the contrast between the portion and the normal portion and without limiting the light source and the polishing direction of the inspection surface.

The gist of the present invention is as follows.
(1) In the first aspect of the present invention, the inspection surface of the metal sample faces upward, and the etching solution is held on the inspection surface so that the entire surface of the inspection surface is covered with the etching solution. In this method, the thickness of the etching solution above the inspection surface is set to more than 0 mm and not more than 10 mm, and the inspection surface is imaged by the imaging device in this state.
(2) In the metal defect detection method according to (1) above, the metal sample may be immersed in the etching solution.
(3) In the metal defect detection method according to the above (1) or (2), the metal sample may be steel having a carbon concentration of 0.5% by mass or less.

  In a metal defect detection method in which a defect is detected by etching an inspection surface of a metal sample and the inspection surface is imaged by an imaging device, at least the entire inspection surface is covered with an etching solution on the inspection surface. The etching solution is held as shown, and the inspection surface is imaged by the imaging device before the etching solution is removed. As a result, since the elution element or the compound of the elution element having a low brightness remains in the segregation part, the contrast becomes stronger compared to the surrounding normal part, so that the segregation area is narrow or the concentration of the elution element in the segregation part is reduced. Even if it is not high, the segregation part can be detected by imaging with the imaging device without limiting the light source and the polishing direction of the inspection surface.

It is a fragmentary sectional view of the inspection surface vicinity of a metal material, and is a conceptual diagram which shows the location of a segregation part. It is a fragmentary sectional view near the inspection surface of a metal material, and is a conceptual diagram at the end of etching. It is a fragmentary sectional view near the inspection surface of a metal material, and is a conceptual diagram showing a situation where an etching solution is removed after etching. It is a fragmentary sectional view near the test | inspection surface of a metal material, and is a conceptual diagram which shows the condition after etching liquid removal. It is a perspective conceptual diagram showing one embodiment of the present invention. It is sectional drawing which shows one Embodiment of this invention. It is a perspective conceptual diagram showing one embodiment of the present invention. It is a perspective conceptual diagram showing one embodiment of the present invention including an imaging device and a light source.

  In the segregation part in the metal material, the eluted element is segregated positively. In the metal sample 1 shown in FIG. 1A, the segregated portion 3a and the segregated portion 3b are positively segregated from the eluted element. When the etching solution 5 is brought into contact with the inspection surface of such a metal sample 1, the etching rate is slow for the normal part 4 where the normal metal material is exposed, and the segregation part 3 (defect part) where the eluted element segregates. ) Has a high etching rate, a difference occurs in the etching depth, and as shown in FIG. 1B, the defect portions such as the segregation portion 3a and the segregation portion 3b are deeply etched to form the recesses 9a and 9b.

  In addition to the difference in the etching rate as described above, it has been found that the defect portion has characteristics in the lightness of the eluted element or the compound of the eluted element generated by etching. The eluting element or compound of the eluting element generated in the segregation part 3 has a lightness that is generally dark enough to be black. Therefore, as shown in FIG. 1B, the elution element or the compound (8a, 8b) of the elution element exists in the depression (9a, 9b) formed in the defect portion (segregation portion 3a, 3b) such as the segregation portion. If these eluting elements or compounds of the eluting elements can remain in the defective part as they are, the lightness of the defective part will be dark, so the brightness difference (contrast) from the bright normal part will be intense, and the inspection surface was imaged by the imaging device. Sometimes the defect can be made even more pronounced. However, after the etching has progressed, the etching surface is conventionally removed and the inspection surface is cleaned, so as shown in FIG. 1C, the dark-colored elution elements or elution element compounds (8c, 8d) are also washed away. Therefore, it could not be used for defect detection. As shown in FIG. 1D, only the dents (9a, 9b) remain on the inspection surface after cleaning, and the normal part and the defective part have the same lightness. Therefore, only the defective part deeply recessed like the dent 9a is present. It was supposed to be able to differentiate from the normal part by the brightness becoming darker. As for the segregation part 3b, since the segregation region is originally narrow or the concentration of the eluted element in the segregation part is not high, the depth of the recess 9b becomes shallow and the brightness difference from the normal part is small, so that it is difficult to detect as a defect. Become.

  In the present invention, after etching is performed by bringing the etching solution into contact with the inspection surface, the etching solution is not washed and removed, and further, the etching solution is not allowed to flow on the inspection surface, and the etching product is eluted. The inspection surface is imaged with the imaging device while the element or the compound of the eluted element is kept in the segregation part. Specifically, as shown in FIGS. 2A to 4, with the inspection surface 2 facing upward, the etching solution 5 is held on the inspection surface so that at least the entire inspection surface is covered with the etching solution 5. The inspection surface 2 is imaged by the imaging device 12. As shown in FIG. 1B, since the elution element having a low brightness or the compound 8 of the elution element remains in the segregation part 3, the contrast becomes stronger than that of the surrounding normal part 4, so that the segregation part in the captured image. Can be recognized as a dark part. As a result, as in the case of the segregation part 3b in FIG. 1A, even when the segregation region is originally narrow or the concentration of the eluted element in the segregation part is not high, and the depth of the recess 9b after etching is shallow, Since the eluting element having a darkness or the compound 8b of the eluting element remains in the depression 9b (FIG. 1B), the segregation part 3b can be detected by imaging with the imaging device. Furthermore, cracks accompanied by segregation can also be detected.

  After the etching solution is brought into contact with the inspection surface, the etching liquid is held for a predetermined time necessary for the etching to proceed. Even during this holding, it is necessary to continue the contact of the etching solution with the inspection surface and not cause the etching solution to flow.

  The necessary minimum amount of the etching solution that contacts the inspection surface may be present so that at least the entire inspection surface is covered with the etching solution at the time of imaging with the imaging device. Specifically, it is only necessary to maintain a state in which the entire inspection surface is wet with the etchant.

  On the other hand, if the thickness H of the etching solution covering the upper portion of the inspection surface is too thick (if the depth from the etching solution surface to the inspection surface is too deep), as shown in FIG. Is likely to occur. If the elution element or the compound 8 of the elution element flows out of the segregation part due to this flow, the brightness of the segregation part cannot be kept dark. For example, the flow 7 of the etching solution may be a flow caused by the convection of the etching solution due to a temperature difference between the inspection surface temperature and the etching solution temperature, or a flow when the etching solution is poured onto the inspection surface. Both become more significant as the thickness H of the etching solution on the inspection surface increases. In the present invention, by setting the thickness H of the etching solution on the inspection surface to 10 mm or less, the flow 7 of the etching solution can be effectively suppressed, and the eluted element or the compound 8 of the eluted element can be held in the segregation part 3. It becomes possible. The thickness H of the etching solution on the inspection surface is preferably 7 mm or less, more preferably 5 mm or less, and further preferably 3 mm or less. The lower limit of the thickness H of the etching solution on the inspection surface is more than 0 mm because the entire inspection surface only needs to be covered with the etching solution. However, it is good also as 0.3 mm or more or 0.5 mm or more in order to elute a segregation part suitably.

  As a method of holding the etching solution on the upper surface of the inspection surface, a method of immersing the metal sample 1 in the etching solution in the etching container 10 as shown in FIGS. 2A and 2B can be employed. In this case, either the method in which the metal sample 1 is put in the etching container 10 and then the etching solution 5 is poured or the method in which the etching solution 5 is put in the etching container 10 and the metal sample 1 is immersed after that is adopted. You may do it. Further, as shown in FIG. 3, an etchant outflow prevention plate 11 is installed around the inspection surface 2 of the metal sample 1, and the etchant 5 is placed in a frame formed by the inspection surface 2 and the etchant outflow prevention plate 11. You may employ | adopt the method of pouring.

  A method for confirming that the thickness H of the etching solution on the inspection surface is 10 mm or less will be described. If the method shown in FIGS. 2A and 2B is used, a length measurement scale is installed on the side surface of the etching container 10 and the water level of the etching solution is measured using this scale. If the thickness of the metal sample 1 is known in advance, the difference between the height at which the bottom of the metal sample 1 is in contact with the etching container and the water level of the etching solution is measured, and the thickness of the metal sample 1 is subtracted from this measured value. Thus, the thickness H of the etching solution can be calculated. If the method shown in FIG. 3 is used, the thickness H of the etchant can be measured by installing a scale on the inner side surface of the etchant outflow prevention plate 11.

  As an etching solution used in the etching treatment of the present invention, an etching solution that is a dark color (for example, black) with a lightness of an eluted element or a compound of the eluted element generated in a segregation part is easily selected from etching liquids that are usually used. Can be adopted. For example, an aqueous solution of ammonium persulfate or an aqueous solution of picric acid is preferably used. In addition, any liquid may be used as long as it is an etching liquid that produces a segregated part or a cracked part with a segregated part to produce a dark color (for example, black) eluted element or a compound of the eluted element. Moreover, what is necessary is just to set it as the holding time normally used about the etching liquid to be used about the holding time after an inspection surface is immersed in etching liquid until it images.

  In the present invention, a normal part (non-defect part) has a bright brightness, and a defect part such as a segregation part has a dark brightness, and a defect is detected by a brightness difference (contrast difference) between the normal part and the defect part. Therefore, it is necessary to make the normal part bright. In the present invention, since the lightness of the defective portion can be a dark dark color, sufficient lightness can be ensured by making the normal portion a normal polished surface. Specifically, polishing may be performed using polishing means having a roughness of 1000 or less and 100 or more as defined in JIS R6252 (2006). Thereby, the roughness of an inspection surface can be made into suitable roughness. The direction of polishing need not be particularly limited. As the polishing means, any of polishing paper, polishing cloth, and polishing material may be used.

  There is no particular limitation on the illumination that illuminates the polished surface for imaging. It is only necessary to ensure the brightness necessary for imaging with the imaging device. When the inspection surface is observed from the imaging device and it is assumed that the inspection surface is a mirror surface, it is preferable to avoid a position where the illumination light source is reflected on the inspection surface to be observed. This is because even if the inspection surface is an irregular reflection surface, the etching liquid surface on the inspection surface is a regular reflection surface, and the image of the light source is reflected on the liquid surface and reflected in the captured image.

  As the imaging device 12, any film camera or digital camera can be used as long as the imaging device can image the inspection surface. It is preferable to use an imaging device using a semiconductor imaging device or the like because captured image data can be taken into an image processing device and image analysis of a defective portion can be performed. As shown in FIG. 4, it is preferable to arrange the imaging device 12 vertically above the geometric center C of the inspection surface 2. In this case, under the condition that the height of the light source is the same as the height of the imaging device, an area 14 having a width and length twice that of the inspection surface 2 is assumed as the arrangement position of the light source 13. If the light source 13 is arranged outside the region 14 with the geometric center of the image pickup device 12, the light directly reflected by the light source is not reflected on the inspection surface portion in the picked-up image.

  After the imaging is completed, the etching liquid is removed from the inspection surface 2 by washing the metal sample 1.

  When the amount of impurities contained in a metal sample, for example, the amount of carbon contained in the sample increases, even if it is a normal part other than the segregation part, the element normally contained in the metal during the etching behaves as an elution element. An elemental compound is generated and acts to darken the brightness of the normal part. If the brightness of the normal part becomes too dark, it becomes difficult to recognize a brightness difference from a defect part such as a segregation part. When the metal sample is carbon steel, when the carbon concentration in the steel is 0.5% by mass or less, the brightness of the normal part is not so dark, and the present invention can be suitably used.

  The present invention was applied to three types of carbon steel having the components shown in Table 1 as three levels of carbon steel having different carbon concentrations. Continuous cast slabs (width: 1200 mm, thickness: 250 mm, length: 5000 mm) of each steel type are cut perpendicularly in the length direction (casting direction) to cut out a metal sample having a length direction of 50 mm and further divided into two in the width direction. Then, a metal sample 1 of 600 mm × 250 mm × 50 mm was cut out, and a vertical cross section with respect to the length direction of the continuous cast slab was set as an inspection surface 2 (size: 600 mm × 250 mm). The inspection surface 2 was polished using No. 300 polishing paper defined in JIS R6252. Polishing was performed in a random direction without being aware of the polishing direction.

  As a method of holding the etching solution on the upper surface of the inspection surface, mainly using a method of immersing a metal sample in the etching solution in the etching container 10 as shown in FIGS. 2A and 2B, as shown in FIG. An etching solution outflow prevention plate 11 is installed around the inspection surface 2 of the metal sample 1 and only one example is adopted in which the etching solution 5 is poured into a frame formed by the inspection surface 2 and the etching solution outflow prevention plate 11. An etching solution was supplied after using a 10% aqueous solution of ammonium persulfate as the etching solution 5 and placing the metal sample 1 in the etching container 10. In the method shown in FIG. 2A and FIG. 2B, a scale is installed on the inner side surface of the etching container 10 to measure the thickness of the etching solution in a portion where the metal sample 1 is not present in the etching container, and the measurement is performed in advance. The thickness H of the etching solution at the upper part of the inspection surface was evaluated by subtracting the metal sample thickness. No measured value was obtained for the thickness H when the inspection surface was not wet.

  As an imaging device, a digital single-lens reflex camera (imaging screen size: 22.3 mm × 14.9 mm, effective pixels: about 15.1 million pixels, lens: focal length 28 mm), as shown in FIG. It was installed at a position 1.5 m above the geometric center C). Further, as a light source 13 for illumination, as shown in FIG. 7, a bulb with a rated power consumption of 500 W is formed at four corners of a square (1.5 m × 1.5 m) parallel to the inspection surface 2 with the position of the imaging device 12 as the center of gravity. Four were arranged. Since the region 14 which is not preferable as the arrangement position of the light source 13 is in a range of 1.2 m × 0.5 m, all the light sources 13 are arranged outside the range of the region 14, and the region of the inspection surface of the captured image is The reflected image of the light source is not reflected.

  Under various conditions, an etching process for 60 seconds and an image of the inspection surface 2 were taken. Table 2 shows processing conditions and evaluation results. In Table 2, Examples 1 to 6 and Comparative Examples 1 to 5 are cases where the method shown in FIGS. 2A and 2B is used, and Example 7 is a case where the method shown in FIG. 3 is used. Conditions that deviate from the scope of the present invention are underlined. The temperature of the etching solution and the metal sample was room temperature.

For each inspection level, the captured image captured by the imaging device was subjected to image analysis, the brightness threshold value was set, the image was binarized, and the black portion after binarization was determined to be a defect. The number of defects on the inspection surface 2 was counted. Next, the number of defects evaluated for Examples 1 to 4 and Comparative Examples 1 to 3 was normalized with the number of defects evaluated in Comparative Example 1, and the number of defects evaluated for Example 5 and Comparative Example 4 was compared with Comparative Example 4. The number of defects evaluated was normalized by the number of defects evaluated in Comparative Example 5 for Example 6 and Comparative Example 5, and the number of defects NNL in each condition was calculated. From this NNL, the inspection status was evaluated according to the following criteria and entered in Table 2.
1.6 <NNL ... pretty good 1.3 <NNL ≤ 1.6 ... good 1.0 <NNL ≤ 1.3 ... good but a little inferior 0.7 <NNL ≤ 1.0・ Inferior
NNL ≦ 0.7 ... considerably inferior

  In Example 1 and Comparative Example 1, the imaging timing of the inspection surface 2 is changed using Comparative Example 1 as a reference. Comparative Example 1 imaged after removing the etching solution had a result of “inferior”, but Example 1 imaged before removing the etching solution had a result of “pretty good”.

  Examples 1 to 4 and Comparative Examples 2 and 3 are based on Example 1 and change the wetness of the inspection surface and the etching solution and the thickness of the etching solution held on the inspection surface. If the inspection surface is wetted with the etching solution and the thickness H of the etching solution is 10 mm or less, a good evaluation result can be obtained. Also, good results could be obtained for Example 7 using the method shown in FIG.

  In Comparative Examples 1, 4, and 5, Comparative Example 1 is the same as the standard, and the steel type is changed. In Examples 1, 5, and 6, Example 1 is the same as the standard, and the steel type is changed. When the carbon content in the metal material exceeds 0.50%, the evaluation result decreases to “good but slightly inferior”.

  In Table 2, the reference Example 1 and Comparative Example 1 are duplicated for each example group for easy comparison.

  In Table 2, it was found that all of the examples of the present invention were in the range from “pretty good” to “good but slightly inferior”.

  The present invention is not limited to the embodiments specifically described in the above-described embodiments and examples, and can be modified within the scope defined in the claims. For example, The present invention is also applicable to the case of configuring a method for detecting a defective portion in a metal of the present invention, in particular, a steel piece, with high accuracy and speed by combining one or all of the above embodiments, examples and modifications. Is included in the scope of rights.

  ADVANTAGE OF THE INVENTION According to this invention, the defect detection method of the metal which can detect a defect part clearly can be provided.

DESCRIPTION OF SYMBOLS 1 Metal sample 2 Inspection surface 3 Segregation part 4 Normal part 5 Etching liquid 7 Flow of etching liquid 8 Elution element or compound of elution element 9 Depression 10 Etching container 11 Etching liquid outflow prevention plate 12 Imaging device 13 Light source 14 Area

The gist of the present invention is as follows.
(1) In the first aspect of the present invention, the inspection is performed so that the inspection surface of a metal sample made of steel having a carbon concentration of 0.5% by mass or less faces upward and the entire inspection surface is covered with an etching solution. Defect detection of metal in which the etching solution is held on the upper surface to reveal defects, the thickness of the etching solution on the upper surface of the inspection surface is more than 0 mm and 10 mm or less, and the inspection surface is imaged by the imaging device in this state Is the method.
(2) In the metal defect detection method according to (1) above, the metal sample may be immersed in the etching solution.

Under various conditions, an etching process for 60 seconds and an image of the inspection surface 2 were taken. Table 2 shows processing conditions and evaluation results. In Table 2, Examples 1 to 5, Reference Example 1 and Comparative Examples 1 to 5 are cases where the method shown in FIGS. 2A and 2B is used, and Example 7 is a case where the method shown in FIG. 3 is used. . Conditions that deviate from the scope of the present invention are underlined. The temperature of the etching solution and the metal sample was room temperature.

For each inspection level, the captured image captured by the imaging device was subjected to image analysis, the brightness threshold value was set, the image was binarized, and the black portion after binarization was determined to be a defect. The number of defects on the inspection surface 2 was counted. Next, the number of defects evaluated for Examples 1 to 4 and Comparative Examples 1 to 3 was normalized with the number of defects evaluated in Comparative Example 1, and the number of defects evaluated for Example 5 and Comparative Example 4 was compared with Comparative Example 4. The number of defects evaluated was normalized by the number of defects evaluated in Comparative Example 5 for Reference Example 1 and Comparative Example 5, and the number of defects NNL in each condition was calculated. From this NNL, the inspection status was evaluated according to the following criteria and entered in Table 2.
1.6 <NNL ... pretty good 1.3 <NNL ≤ 1.6 ... good 1.0 <NNL ≤ 1.3 ... good but a little inferior 0.7 <NNL ≤ 1.0・ Inferior
NNL ≦ 0.7 ... considerably inferior

In Comparative Examples 1, 4, and 5, Comparative Example 1 is the same as the standard, and the steel type is changed. Examples 1, 5 and Reference Example 1 are the same as those in Example 1 except that the steel type is changed. When the carbon content in the metal material exceeds 0.50%, the evaluation result decreases to “good but slightly inferior”.

Claims (3)

With the inspection surface of the metal sample facing upward, the etching solution is held on the upper surface of the inspection surface so that the entire surface of the inspection surface is covered with the etching solution, so that defects appear, and the etching solution above the inspection surface is exposed. A metal defect detection method, wherein the inspection surface is imaged by an imaging apparatus in this state, with a thickness of 0 mm to 10 mm. The metal defect detection method according to claim 1, wherein the metal sample is immersed in the etching solution. The metal defect detection method according to claim 1 or 2, wherein the metal sample is steel having a carbon concentration of 0.5 mass% or less.

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