JPH09145637A - Method for judging grade of roughness defect - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the method for judging the grades of the roughness defects, which can quantitatively judge the grades of the roughness defects of a material under inspection. SOLUTION: The surface of a cold rolled steel plate, which is ground with a grindstone, is photographed with a microscope (step S1). Then, the mark of the grindstone on the microscope photograph is manually colored (step S2). The colored part is extracted by imaging using a computer (step S3). Then, the area ratio or the colored part is computed (step S4). Thus, the grade of the defect is judged based on the area ratio of the grindstone mark obtained in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for determining the grade of uneven defects that determines the grade of uneven defects that occur on the surface of a steel plate, an aluminum plate or the like.

[0002]

2. Description of the Related Art In order to promote quality control or quality assurance in the manufacturing process of steel sheets, aluminum sheets, etc., it is important to detect surface defects occurring on these surfaces. In the manufacturing process of, the product surface is checked for defects by an automatic surface inspection device or visually inspected by an operator during the process until it is shipped in the form of cold rolled coil after various continuous steps. Is strictly quality controlled.

One of the surface defects of the cold-rolled steel sheet is an irregularity defect caused by a slight irregularity defect on the surface of the steel sheet, and many of these types of defects cannot be detected by visual inspection by humans. Therefore, it is usual to detect by a so-called grindstone mounting inspection, in which a part of the steel plate is ground by using a grindstone and a defect is exposed and then a visual inspection is performed.

The reason why the unevenness defect, which is difficult to detect visually, is easily detected by the grinding process is that the unevenness defect is a concave defect or a convex defect. This is because the surface shapes of the grindstone traces) are different from each other, and the gloss of the grindstone traces of these defective portions is different from the gloss of the sound portion.

Such unevenness defects are caused by the deposition of minute foreign matter on the roll surface or the slight deformation of the unevenness of the roll itself in the manufacturing process, resulting in the deformation of the foreign matter and the roll itself. Are often transferred to the surface of the steel sheet. Therefore, defects are generated periodically or continuously at a pitch corresponding to the circumference of the roll, and the entire product often becomes defective. Further, when a defect occurs in a continuous process, manufacturing of defective products continues until a defect is found and an action is taken, which may cause a large loss.

[0006] When such an unevenness defect occurs, for example, when press-forming a cold-rolled steel sheet for an automobile outer panel, the defective portion is rubbed or stretched with a stronger force than a sound portion having no defect. Therefore, the surface roughness of the molded product after press molding causes a difference between the defective part and the sound part,
The difference may remain on the surface even after painting. Therefore,
For cold-rolled steel sheets with strict requirements for surface properties, strict quality control is performed in the manufacturing process so that even minor surface defects are not overlooked, and strict inspections are carried out when shipping products to prevent defective products from being shipped. Is normal.

On the other hand, on the other hand, even in the case of cold-rolled steel sheet, depending on the application, slight defects may not be a problem. Therefore,
It is necessary to accurately judge the grade of the uneven defects and select defective products according to the appropriate inspection standard according to the application of the product. Conventionally, the grade determination of such irregularity defects has been performed by visual inspection by an inspector who is a skilled whetstone inspection.

[0008]

However, since the grade determination of uneven defects by such a skilled person depends on the human sense, there is a difference in grade due to individual differences among inspectors, and Even the same inspector may have different judgments due to various factors. For this reason, the range of determination of the grade of the defect to be detected is widened, and more strict determination than necessary is made in view of safety.

Under these circumstances, the development of an apparatus for automatically determining the grade of uneven defects has been eagerly advanced.
In developing such an automatic determination device, since an index that quantitatively expresses the degree of unevenness of the defect portion and the sound portion of the inspected material could not be found, the device that automatically determines the grade of the unevenness defect is still available. It has not been put to practical use. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for determining the grade of an irregularity defect capable of quantitatively determining the grade of an irregularity defect of an inspected material.

[0010]

According to the method for determining the grade of irregularity defects of the present invention, which achieves the above object, a method of forming an uneven defect is formed in a predetermined inspection area on a surface of an inspection material by a grinding process using a grindstone. Measure the area of the grindstone trace, divide the area of the grindstone trace by the area of the area to be inspected to obtain the grindstone trace area ratio, and judge the grade of the irregularity defect of the material to be inspected based on the grindstone trace area ratio. Is characterized by.

Here, in the above-described method for determining the degree of irregularity defects according to the present invention, in measuring the area of a grindstone trace formed in a predetermined area to be inspected on the surface of the material to be inspected by a grinding process using a grindstone. , The area to be inspected, the area of the grindstone trace is measured for each of a plurality of divided areas divided into areas according to the size of the defect to be detected, and the area of the grindstone trace for each divided area is the area of each divided area Calculate the grindstone trace area ratio for each divided area by dividing by, obtain an index indicating the average value of the grindstone trace area ratio and the degree of variation from the grindstone trace area ratio for each divided area, for each of the above divided areas Obtain the normalized whetstone trace area ratio by normalizing the whetstone trace area ratio with an index representing the average value and the degree of variation of the whetstone trace area ratio, instead of the above whetstone trace area ratio, the normalized whetstone trace area ratio Judging the grade of irregularity defect of the material to be inspected based on Rukoto is preferable.

Further, the above-mentioned normalized grindstone trace area ratio is subjected to threshold value processing to extract a segmented region having a defect, and among a plurality of segmented regions extracted, a plurality of these segmented regions adjacent to each other are extracted. The divided area is set as one block, and for the independent divided areas in which adjacent divided areas do not exist, one of the divided areas is set as one block, and the defect evaluation index D represented by the following formula is obtained for each block, It is also a preferable embodiment to judge the grade of the unevenness defect of the material to be inspected based on the defect evaluation index D.

D = w ₂ · Σ {w ₁ · k (i, j)} Here, let k (i, j) be the grindstone trace area ratio for each of the extracted divided regions forming the block. Mark area ratio k
The weight determined according to (i, j) is w _1, and the weight determined according to the size of the defect to be detected is w ₂ .

[0014]

Embodiments of the present invention will be described below. The inventors of the present invention, in developing a method for determining the grade of uneven defects, as a result of a detailed investigation of the surface properties of the steel sheet after being subjected to a whetstone, the defect grade by visual judgment by an expert, and a whetstone mark. It has been found that there is a high correlation with the area ratio, that is, the area of the grindstone trace in a predetermined inspection area divided by the area of the inspection area.

Further, in the case of a convex defect, a defect having a larger grindstone trace area ratio is observed more clearly because the defect is more clearly observed, and a defect having a smaller grindstone trace area ratio is observed as being blurred and thus a light defect. It was found that, as for the concave defect, the smaller the grindstone trace area ratio, the larger the defect, and the smaller the grindstone trace area ratio, the lighter defect.

Based on these findings, the results of various studies,
The present invention has led to the development of a method for determining the grade of uneven defects shown in the following embodiment. In this embodiment, the grade determination of the uneven defect called an indentation mark generated in the cold rolled steel sheet will be described. FIG. 1 is a flow chart showing a procedure for measuring a grindstone trace area ratio.

As shown in FIG. 1, the surface of the cold-rolled steel sheet ground by grinding with a grindstone is photographed with a microscope (step S1), and then the grindstone trace portion of the micrograph is manually colored (step S2). Then, the colored portion is extracted by image processing using a computer (step S3), and then the area ratio of the colored portion is calculated (step S4). The area ratio is 3.3 mm in length x 2.5 mm in width near the center of the defect.
Measure the area of size.

The grade of the irregularity defect is judged based on the grindstone trace area ratio thus obtained. Since the colored portion and the non-colored portion can be easily identified by image processing, the area ratio calculation can be performed at high speed and with high accuracy. In addition,
In this embodiment, image processing is performed after a micrograph is taken. However, a camera is attached to the microscope, the image signal captured by the camera is directly input to the computer, and the grindstone trace portion and the non-magnet are input in the image processing process. You may make it identify a trace part.

FIG. 2 shows an example of a schematic diagram sketching the micrograph used for obtaining the grindstone trace area ratio.
In FIG. 2, the part with vertical lines is the part of the grindstone trace 21, and the white part is the non-grindstone part 22. Such, the area of the grindstone trace portion in the target measurement region of the pattern consisting of the grindstone trace portion and the non-grindstone trace portion is measured, and the area is divided by the area of the measurement region Is required.

With respect to defects of the same material to be inspected for which the grindstone trace area ratio is obtained in this manner, a visual judgment is made by a skilled person, and from convex defects to light defects to heavy defects, grades A to F are evaluated. FIG. 3 shows the results of the judgment by the 6-level evaluation, and the concave defect from the light defect to the heavy defect by the 5-level evaluation from the grade A ′ to the grade E ′. FIG. 3 is a graph showing the relationship between the grindstone trace area ratio according to the present embodiment and the conventional grade determination result by visual determination.

As a result of the judgment, as shown in FIG.
5 grade convex defects from grade E to grade E and 3 grade concave defects from grade A'to grade C'are detected, and between the grade judgment result by conventional visual judgment and the grindstone trace area ratio. It is recognized that there is a strong correlation. Therefore, it is possible to perform quantitative grade determination instead of the conventional qualitative grade determination based on the grindstone trace area ratio. In the grade judgment based on the grindstone area ratio, in the case of a convex defect, the higher the grindstone area ratio than the sound part, the more it is judged as a heavy defect, and in the case of the concave defect, the grindstone area ratio is the sound part. The lower the value, the more severe the defect is determined.

Next, a grade judging method based on the normalized area ratio of the traces of the whetstone will be described. The surface of the inspection area to be judged,
It is divided into divided areas having an area smaller than the size of the defect to be detected, and the position of each divided area on the two-dimensional coordinate system is the address i,
It is represented by using j. The area of the grindstone trace is measured for each of these divided regions, and the grindstone trace area ratio x (i, j) is obtained.

Next, the average M and the standard deviation σ of the grindstone trace area ratios in the sound portion are obtained, and the grindstone trace area ratio x (i, j) is normalized according to the equation (1) to obtain the normalized grindstone trace area ratio. k (i, j) = {x (i, j) −M} / σ (1) Since the expression (1) is based on the sound portion, the grindstone trace area ratio x
The difference between (i, j) and the average M is obtained, and the difference is divided by the standard deviation σ to obtain the grindstone trace area ratio x (i,
It is a calculation formula for normalizing j).

Here, if the size of the divided area is equal to the size of the minimum defect, a grindstone trace area ratio of an unexpectedly abnormal value may be calculated depending on the grindstone mounting state. It is difficult to distinguish whether it is due to a defect or a defect. Therefore, in order to prevent the occurrence of such an abnormal value, it is desirable that the size of the divided area be smaller than 1/2 of the area of the smallest defect to be detected.

Among the normalized whetstone trace area ratios k (i, j) obtained by the equation (1), those having a predetermined first threshold value or more are judged to be convex defects, and predetermined second defects are determined. Those below the threshold value are judged to be concave defects. If the grade of the defect is determined using the normalized grindstone trace area ratio, even if the grindstone applying conditions such as the surface roughness of the inspected material, the grain size of the grindstone, the pressing force of the grindstone, the number of times the grindstone is applied, etc. The grade can be judged correctly.

FIG. 7 illustrates the second invention of the present application for correcting the influence of such a grindstone mounting condition.
The area ratio of the grindstone marks is measured in a divided area divided into a certain size, and the frequency distribution of the measured values is shown. As shown in the figure, for example, the grindstone under condition A has a stronger grindstone than the grindstone under condition B, so that the average area ratio of the grindstone marks is, for example, 40%, and
It is larger than 0%.

Further, since the degree of hanging is also varied, the standard deviation of the area ratio of the grindstone is, for example, 5%, which is larger than 2% of the condition B. And In such a state where the grindstone is applied, for example, even in the case of convex defects of the same grade B, the average area ratio of the grindstone traces of the defective portion is higher than that of the sound portion by the condition A.
Then, if it becomes 10% larger, then under condition B, for example, 4
However, the amount of increase is smaller than that of the condition A. The reason why the amount of increase is small is that the method of applying the grindstone is weak. For this reason, if the states of the grindstones are different, even if the defects have the same grade, it may not be possible to determine the grade by the difference in the grindstone trace area ratio from the grindstone trace area ratio of the sound portion.

The inventors of the present invention have found that in visual judgment by humans, the grade judgment is performed while taking into consideration such a difference in the grindstone mounting conditions, and the relative condition is determined based on the grindstone mounted state of the sound part. It was found that the grade of the defect can be judged by the normalized area ratio of the traces of the whetstone by obtaining the difference and further calculating the ratio with the standard deviation to correct the variation of the whetstone mounting state. According to the equation (1), the convex defect shown in FIG. 7 has a normalized grinding wheel trace area ratio of k (i, j) = 10/5 = 2 in the condition A grindstone and even in the condition B grindstone. , K (i, j)
= 4/2 = 2, which can be evaluated as defects of the same grade.

In FIG. 7, the convex defect is taken as an example, but in the case of the concave defect, the frequency distribution indicating the defect appears on the side smaller than the average value of the sound part, and the corrected area ratio k of the grindstone mark is k. (I, j) is calculated as a negative value. Next, the third invention will be described. In the case of a convex defect, when there is a group of segmented regions having a normalized grindstone trace area ratio k (i, j) of a predetermined first threshold value or more,
The block is regarded as one block, and the grade of the block is judged. In the case of a concave defect, the same processing may be performed on the segmented region having a predetermined second threshold value or less.

First, the case of a convex defect will be described in detail. First, a segmented region in which the above-described normalized grindstone trace area ratio k (i, j) is equal to or larger than a predetermined first threshold value, that is, a segmented region in which a convex defect exists is extracted from all the segmented regions. .
Next, among each of the extracted segmental areas, for a plurality of adjacent segmental areas that exist as a group,
One of these blocks is a block.

In order to find a segmented area that exists as a group, it is sufficient to check whether or not each segmented area adjacent to the extracted segmented area is the extracted segmented area. For example, adjacent up / down / left / right directions (near 4 connections), or adjacent up / down / left / right directions and diagonal directions (8
If any of the divided areas (in the vicinity of the connection) is equal to or larger than a predetermined first threshold value, it is regarded as the same cluster, and if it is less than the first threshold value, the same cluster is not considered to be the same cluster, and the extracted segment areas are sequentially extracted. The same processing may be continued for.

When the above processing is completed in this way, the sum of the normalized grindstone trace area ratios k (i, j) for the extracted segmental areas that make up each block is calculated to find the area ratio of the grindstone traces. It is possible to extract only a segmented area that has a difference compared with a sound portion and is a block as a convex defect. In the case of a concave defect, a segmented area equal to or smaller than a predetermined second threshold value is extracted, and then a block of the extracted segmented areas is regarded as one block. Similarly to the above, the sum of the normalized grindstone trace area ratios may be calculated for the extracted segmental areas that make up each block.

By the way, in the above total, the threshold value is different between the case of the convex defect and the case of the concave defect, such as the cause of the defect, the type of the defect, the type of the material to be inspected, the state of the grindstone treatment, etc. It is necessary to change the grade judgment method accordingly. Therefore, the following defect evaluation index is set so that the defect can be always evaluated with a constant reference regardless of the type of defect and various other conditions.

That is, after the above processing is finished, the grindstone trace area ratio for each of the extracted divided regions forming each block is k (i, j), and the grindstone trace area ratio k (i, j) Where w ₁ is the weight determined by the above and w ₂ is the weight determined according to the size of the defect to be detected, the equation D = w ₂ · Σ {w ₁ · k (i, j)} The defect evaluation index D represented by (2) is obtained, and the grade of the uneven defect of the inspected material is determined based on the defect evaluation index D.

The defect evaluation index D is obtained as follows.
Depending on the type of the defect, there is no direct proportional relationship between the value of the grindstone trace area ratio k (i, j) and the defect grade, and the acceleration increases as the value of the grindstone trace area ratio k (i, j) increases. In some cases, the defect grade increases, and depending on the defect, the defect size and the defect grade may not be proportional.

However, if the defect evaluation index D obtained by the above equation (2) is used, in the case of a convex defect, a defect that is clearly observed even if it is small has a significantly larger grindstone trace area ratio than a sound part. The defect evaluation index D becomes large, and even if the defect evaluation is large, the defect evaluation index D becomes small because the defect rate of the grindstone is slightly larger than that of the sound portion. That is, the defect evaluation index D becomes larger as the defect is visually judged to be a heavy defect, and the defect evaluation index D becomes smaller as the defect is visually judged to be the light defect.

On the other hand, in the case of a concave defect, the defect evaluation index D
Is a negative value, and a smaller defect evaluation index D indicates a heavy defect, and a larger defect evaluation index D indicates a light defect.

[0038]

[Embodiment] The object to be measured is 6 from the grade A to the grade F, which is generated from cold rolled steel sheets, from light defects to heavy defects by visual judgment.
It is a convex defect determined by the graded evaluation, and the size of the divided area for measuring the grindstone trace area ratio is 0.25 mm × 0.2.
The area ratio x (i, j) of the grindstone was set to 5 mm and measured. Further, the grindstone trace area ratio was also measured under the same conditions for a sound portion where no defect occurred, and then the normalized grindstone trace area ratio k (i, j) was obtained according to the equation (1). A first threshold value is set to 1.0, and a segmented area in which a defect having a grindstone trace area ratio k (i, j) is equal to or larger than the first threshold value is extracted,
The defect evaluation index D was obtained according to the equation (2). The weight w ₂ when calculating the defect evaluation index D is determined according to the area of the defect.
For defects of φ2 mm or more, w ₂ = 1 and for defects of less than φ2 mm, w ₂ = 1.4. Also, the weight w ₁ is
In this embodiment, all are set to w ₁ = 1.

For example, FIG. 4 shows a normalized grindstone trace area ratio in a defect portion which is judged to be a convex defect of grade C by visual judgment,
It is a figure which respectively shows the normalized grindstone trace area ratio in a sound part. FIG. 4A shows the normalized grindstone trace area ratio k (i, j) for each divided area in the inspected area 40 that includes the convex defect 41 that is visually determined to have the defect grade C. ing. The value of the normalized grindstone trace area ratio for each divided area is indicated by the height of the square pillar, and the square pillar with diagonal lines on the upper surface of the square pillar is a divided area below the first threshold value. The square prism having the white upper surface of the square prism indicates that it is a segmented region of the first threshold value or more.

In FIG. 4A, a portion 4 surrounded by a one-dot chain line
Since 1 is detected, it can be determined as a convex defect. On the other hand, FIG. 4B shows the normalized grindstone trace area ratio for each of the divided areas in the sound part, but the portion surrounded by the two-dot chain line is unevenness 42 due to unevenness of the grindstone mounting. However, since the threshold value is not appropriate, it may be erroneously determined as a defect. However, by obtaining the defect evaluation index D,
For the convex defect 41 of FIG. 4A, D = 92.2 is obtained, and it is determined to be a defect of grade C according to FIG. 5 described later. In addition, since D = 13.4 is obtained for the convex defect 42 of FIG. 4B, the D value of the light defect A is 5 as shown in FIG.
Since it is smaller than 0, it is not erroneously determined as a convex defect.

FIG. 5 is a graph showing the relationship between the defect evaluation index D obtained for convex defects and the defect grade by visual judgment. Each data shows the size of the defect is 2 mm or more in diameter,
Although it is plotted as stratified to less than 2 mm, the defect evaluation index D and the defect grade by visual judgment show a good correlation regardless of the size of the defect. Thus, by using the defect evaluation index D, , It is possible to quantitatively judge the defect grade regardless of the condition of the grindstone and the size of the defect.

Regarding the concave type defect, the second threshold value is given to the defect which is generated in the cold-rolled steel sheet and is judged by the five-level evaluation from the light defect to the heavy defect by the grade A'to the grade E'by visual judgment. Is set to -1.4, and the whetstone trace area ratio K
A segmented area in which a defect having (i, j) is equal to or less than the second threshold value is extracted, and the defect evaluation index D is obtained according to the equation (2).

FIG. 6 is a graph showing the relationship between the defect evaluation index D obtained for concave defects according to this embodiment and the defect grade by visual judgment. As shown in FIG. 6, also for the concave defect, a good correlation is found between the defect evaluation index D and the defect grade by visual judgment. By thus obtaining the defect evaluation index D, it is possible to more accurately determine the defect grade of the uneven defect.

[0044]

As described above, according to the method for determining the degree of unevenness defects of the present invention, the defect grade of unevenness defects, which has hitherto been qualitatively evaluated, is quantified by the grindstone trace area ratio. Since it is possible to make a decision manually, it is possible to prevent a mistaken decision made by an inspector.

Further, since it is possible to clearly determine whether the defect is a harmful defect or a harmless defect by the grindstone trace area ratio, which has been unclear in the past, it is possible to prevent a stricter defect inspection than necessary. , The product yield can be improved. Further, it is possible to automate the grade determination work of uneven defects, which conventionally depends on an expert.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a procedure for measuring a grindstone trace area ratio.

FIG. 2 is an example of a schematic diagram sketching the microphotograph used for obtaining a grindstone trace area ratio.

FIG. 3 is a graph showing a relationship between a grindstone trace area ratio according to the present embodiment and a conventional grade determination result by visual determination.

FIG. 4 is a diagram showing a normalized grindstone trace area ratio in a defective portion which is determined to be a convex defect of grade C by visual judgment, and a normalized grindstone trace area ratio in a sound portion.

FIG. 5 is a graph showing a relationship between a defect evaluation index D obtained for a convex defect and a defect grade by visual judgment.

FIG. 6 is a graph showing the relationship between the defect evaluation index D obtained for concave defects and the defect grade by visual judgment.

FIG. 7 is a second view of the present application for correcting the influence of the grindstone mounting condition.
FIG. 3 is a diagram for explaining the invention of FIG.

[Explanation of symbols]

 21 grindstone trace 22 non-grindstone trace 40 inspected area 42 unevenness

Claims (3)

[Claims] 1. An area of a grindstone trace formed in a predetermined inspected region on a surface of an inspected material by a grinding process using a grindstone is measured, and the area of the grindstone trace is divided by the area of the inspected region. Then, the grindstone trace area ratio is obtained, and the grade of the unevenness defect of the material to be inspected is determined based on the grindstone trace area ratio. 2. The size of a defect to be detected in the inspected area when measuring the area of a grindstone trace formed in a predetermined inspected area on the surface of the inspected material by a grinding process using a grindstone. The area of the grindstone trace is measured for each of the plurality of divided areas divided according to the area, and the area of the grindstone trace for each of the divided areas is divided by the area of each of the divided areas to obtain the grindstone of each of the divided areas. The scratch area ratio is obtained, and the index indicating the average value of the grindstone area ratio and the degree of variation is obtained from the grindstone area ratio for each of the divided areas, and the grindstone area ratio for each of the divided areas is defined as the grindstone area. Obtain the normalized grindstone trace area ratio by normalizing with an index representing the average value of the rate and the degree of variation, instead of the grindstone trace area ratio, the unevenness of the material to be inspected based on the normalized grindstone trace area ratio The unevenness according to claim 1, wherein the grade of the property defect is determined. Grade determination method of the defect. 3. A segmented region having a defect is extracted by thresholding the normalized grindstone trace area ratio, and a plurality of adjacent segmented regions are extracted from the extracted segmented regions. Is defined as one block, and for independent segmented areas where adjacent segmented areas do not exist, one segmented area is defined as one block, and a defect evaluation index D represented by the following formula is obtained for each block. 3. The method for determining the grade of irregularity defects according to claim 2, wherein the grade of irregularity defects of the material to be inspected is determined based on the defect evaluation index D. D = w ₂ · Σ {w ₁ · k (i, j)} Here, the address of each of the extracted divided regions forming the block is i, j, and the grindstone trace area ratio of each of the divided regions is k.
(I, j), the weight determined according to each grindstone trace area ratio k (i, j) was w _1, and the weight determined according to the size of the defect to be detected was w ₂ .