JP3534579B2 - Decarburized layer depth measurement method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for measuring the depth of a decarburized layer appearing on the surface of a steel material or the like.

[0002]

2. Description of the Related Art Steel materials (steel materials) such as special steels used for members of automobiles and industrial machines are exposed to oxygen in the atmosphere during the manufacturing process such as rolling and heat treatment. Oxygen inside reacts and is released from the steel as CO and CO ₂ . Therefore, the carbon content of the surface of the steel material and the vicinity of the surface are different from the carbon content of the steel material.
This is called decarburization, and the part where the carbon is low is called the decarburized layer. Since this decarburized layer has low strength, it may cause a problem of insufficient strength when the steel material is processed into a product.

Therefore, when a product is shipped from a steel material manufacturer to a user, it is necessary to inspect the depth of the decarburized layer after a predetermined treatment to ensure that it is within an allowable range.

As a method of inspecting the depth of the decarburized layer, a sample in which a part of a steel material is cut out and the periphery is hardened with a resin is prepared, and the cut surface of this sample is enlarged and inspected by a microscope or the like. When corroded with a corrosive liquid and examined under a microscope, the decarburized part has a color difference compared to the undecarburized part, so a method has been used to take a photograph and check the depth of the steel material sample. .

In any case, since the depth of the decarburized layer has been measured by visual observation, there are variations due to people,
Moreover, there is a problem that the inspection requires skill.

[0006] As a method for solving this, Japanese Patent Laid-Open No. 4-1
In Japanese Patent No. 74357 and Japanese Patent Application Laid-Open No. 4-174358, a method of detecting the depth of a decarburized layer by imaging a steel material cross section by an imaging means and detecting a change in the density of the image data from the surface of the steel material to the inside thereof is detected. Is disclosed.
The method is shown in FIGS. 6 and 7.

In FIG. 6, 100 is a steel material sample, 1 is
01 is a steel material piece, and 102 is a resin. First, the steel piece 10
The image of the screen F1 in the peripheral portion of 1 is captured. 1 screen is 2
It is divided into 55 sectors, and the density data of each sector is added and stored in the RAM. Next, the steel piece 10
The XY stage on which No. 1 is placed moves, and the image on the screen F2 is captured and processed in the same manner. Below, screens F3 and F
This operation is sequentially performed for 4, ...

FIG. 7 is obtained by making a graph of the added value of the density data written in the RAM for each screen and the distance from the center of the steel material piece 101 to each screen and smoothing it. In FIG. 7, the distance from the edge A of the steel material piece 101 to the point B where the surface concentration of the steel material piece 101 is not decarburized is determined to be the decarburization depth DT.

[0009]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 4
The methods disclosed in Japanese Patent Application Laid-Open No. 174357/1974 and Japanese Patent Application Laid-Open No. 4-174358 can automatically measure the depth of the decarburized layer, but when the inventors actually performed the test, it was found that they were not It was found that the depth of the decarburized layer cannot be accurately obtained under the conditions of. Although the reason for this is not clear, it is considered that the concentration in the cross section of the steel material has a complicated distribution, which causes noise and deteriorates the measurement accuracy.

The present invention has been made to solve the above problems, and provides a method for automatically and accurately measuring the depth of a decarburized layer by using an image processing technique. With the goal.

[0011]

Means for Solving the Problems The above problem is that a cross section near the surface of an object to be inspected in which a decarburized layer exists from the surface to the inner surface is photographed by an image pickup means, and the imaged image is decomposed into pixels, After each pixel is subjected to the maximization process a plurality of times and the minimization process equal to or less than the maximization process number, each image is binarized, and the high-luminance pixel is binarized among the binarized pixels. Depth of the decarburized layer characterized by determining the depth of the decarburized layer by measuring at each position the length of a pixel having a value corresponding to the degree in the direction perpendicular to the surface of the object to be inspected. This is solved by the measuring method (claim 1).

By performing such image processing, the distinction between the decarburized layer and the part that is not the decarburized layer is emphasized, and the decarburized layer can be accurately recognized, so that accurate depth measurement can be performed.

Here, the "maximization process" is a process in which the maximum value of the values of a certain pixel and its surrounding pixels is set as the value of the corresponding "certain pixel", and the "minimization process" is performed.
Is a process in which the minimum value of the values of a certain pixel and its peripheral pixels is set as the value of the corresponding “certain pixel”.

Further, by setting the maximum value of the depth of the decarburized layer measured at each position as the total decarburized layer depth (claim 2), the total decarburized layer depth can be accurately measured.

When each image is binarized, among the binarized pixels, pixels having a value corresponding to low lightness, which are surrounded by pixels having a value corresponding to high lightness, remain in an island shape. There is. In such a case, if only the method according to claim 1 is used, the depth to the surface side of the pixel corresponding to the island-shaped low brightness will be determined as the depth of the decarburized layer, which is accurate. I can't measure easily. Therefore, when such a situation occurs, following binarization, the pixel is surrounded by pixels having a value corresponding to high brightness among the binarized pixels,
By inverting the value of the pixel having the value corresponding to the low brightness to obtain the value corresponding to the high brightness (filling in) (claim 3), the above-mentioned erroneous determination can be avoided.

When the object to be inspected is a low carbon content steel material (having a carbon content rate of less than 0.7% by weight near the eutectic point), the maximum value treatment is performed three times in succession, and then the minimum value is obtained. By performing the chemical treatment twice in succession (claim 4),
The distinction between the decarburized layer and the normal part can be emphasized.

On the other hand, the object to be inspected is a high carbon steel material (having a carbon content of 0.7% by weight or more near the eutectic point).
If it is, after performing the maximization process twice consecutively,
By performing the minimization process once (claim 5), it is possible to particularly emphasize the distinction between the decarburized layer and the normal portion.

By performing shading correction on each pixel before performing maximum value processing and minimum value processing (claim 6), it is possible to correct shading variation due to light source unevenness and the like, and a binarization level. Can be stabilized.

Further, after performing the maximum value processing and the minimum value processing, and before performing the binarization, the averaging processing is performed (claim 7) to remove noise components, and to remove the decarburized layer and the normal decarburization layer. The distinction between parts can be made clear.

In some cases, binarized pixels corresponding to high brightness may appear discontinuously from the surface of the object to be inspected in a direction perpendicular to the surface. In this case, if the distance between them is less than or equal to a predetermined value, they are treated as continuous, and if they are greater than the predetermined value, they are treated as discontinuous and continuous from the surface. By measuring the length of the pixel that is being used (claim 8), it is possible to remove the influence of noise and the like and to accurately measure the depth of the decarburized layer.

In the position of the surface of the object to be inspected, among the binarized pixels, the pixels corresponding to the high brightness are continuously arranged in a predetermined number (including one) from the surface side of the object to be inspected. By determining the position that appears as the position of the surface of the inspection object (claim 9), the position can be accurately determined.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of the configuration of an apparatus used for implementing the present invention. The microscope 1
An image pickup lens is attached to the eyepiece section, and an image pickup device 16 for picking up an image of the surface to be inspected through the image pickup lens is attached to the upper part. The stage 17 on which the object to be inspected is placed can be moved in the front-rear, left-right directions by driving a pulse motor provided in the plane drive mechanism 18 in response to a signal from the auto stage driver 10. Further, the vertical movement mechanism 19 is operated by the autofocus driver 11 to move the stage 17 in the vertical direction to focus the stage.

The A / D converter 2 converts the video signal from the image pickup device 16 into a digital signal, and the input buffer temporarily stores this data. The bus 4 transmits signals, the program memory 5 stores a program that defines the operation of the apparatus, and the CPU 6 controls the entire apparatus by this program.

The image processor 7 performs a gradation process, a maximum value process, a minimum value process, an average value process, a binarization process, an image analysis, etc. of the input image data. Gray image memory 8
Stores the grayscale image data with gradation, and the binary image memory 9 stores the binarized data.

The auto stage driver 10 includes a CPU 6
By controlling the plane moving mechanism 18 to move the stage 17 on which the object to be inspected is placed in the front-back and left-right directions,
The position and area of the object to be inspected are set. The autofocus driver 11 controls the vertical movement mechanism 19 according to an instruction from the CPU 6 to automatically focus.

The output buffer 12 temporarily stores the data to be output, and the D / A converter 13 converts this into a video signal and displays it on the CRT 14. Keyboard 15, mouse 1
The operator inputs instructions and data by 5 '.

FIG. 2 is a flow chart showing an example of the embodiment of the present invention. In FIG. 2, a block surrounded by a solid line corresponds to claim 1, and a block surrounded by a broken line is added in another claim and is adopted as necessary.

First, in step S1, an image of a cross section near the surface of the object to be inspected is picked up by using the apparatus shown in FIG. 1, and the image pickup screen is divided into 512 × 512 pixels.
Then, the lightness is stored in the grayscale image memory 8 as a grayscale image of 8 bits, that is, 256 levels.

Next, in step S2, shading correction is performed. The shading correction is widely performed in order to remove the grayscale noise of the image caused by the unevenness of the illumination, and a known one can be selected and used. After performing maximization processing on a wide neighborhood of, the same minimization processing image is subtracted from the original image to obtain an image with fine unevenness, and a bias with a density close to the average value is added to this. is doing. As a result, a gradual and non-uniform background due to uneven illumination is corrected.

As a maximum value widening process, 41 ×
41 operators should be used. 3x3 operators (8 neighborhood operators) instead of 41x41 operators
By repeating 20 times, a similar effect can be obtained in a short time. Similarly, in the minimization process, the 8-neighbor operator process is repeated 20 times.

As an example of a 3 × 3, that is, 8-neighboring operator, among the 512 × 512 pixels, the gray scale of the pixel existing in the i-th row and the j-th column is indicated by x _ij , and the gray-scale of the i-th row and the j-th column after the processing is indicated. If the gradation of existing pixels is x ′ _ij , then x ′ _ij = MAXx _mn
The processing is (i−1 ≦ m ≦ i + 1, i−1 ≦ n ≦ i + 1). In the minimization process, if the gradation of the pixel existing in the i-th row and the j-th column is indicated by x _ij , and the gradation of the pixel existing in the i-th row and the j-th column is x ′ _ij , x ′ _ij = MINx _mn ( i-1
≦ m ≦ i + 1, i−1 ≦ n ≦ i + 1).

The shading correction is not limited to the above method, but the output of the low-pass filter may be subtracted from the original image, or the output of the wide area average value filter may be subtracted from the original image. Using the above-described method of maximizing and minimizing the value has an advantage that the device described later can be used.

Then, in step S3, maximum value processing and minimum value processing are performed. The maximum value processing and the minimum value processing in this case are the same as those performed by the shading correction, but the pixels to be calculated are 24 neighboring pixels. That is, with respect to the pixel existing in the i-th row and the j-th column, the pixel existing in the m-th row and the n-th column within the range of i−2 ≦ m ≦ i + 2 and i−2 ≦ n ≦ i + 2 is the target of the calculation of MAX and MIN. .

The pixels to be calculated may be 8 neighboring pixels or 48 neighboring pixels.

In order to emphasize the difference between the decarburized layer and the non-carburized layer, the number of times of maximization processing may be equal to the number of times of minimization processing. A preferable result is obtained when the number of times is larger than the number of times.

According to experiments conducted by the inventors, when the object to be inspected is a low carbon content steel material, the maximization process is performed three times in succession and then the minimization process is performed twice in succession. , And the difference between the decarburized layer and the non-decarburized layer could be distinguished most clearly. On the other hand, when the inspected object is a high carbon content steel material, if the maximization process is performed twice and then the minimization process is performed once, the decarburized layer and the non-decarburized layer The differences could be most clearly distinguished.

Next, in step S4, an averaging process is performed. This is performed in order to reduce noise and is a well-known method. However, the average value of gradations of a certain pixel and its neighboring pixels is used as the average value of the corresponding “certain pixel”.

According to experiments by the inventors, the target of the averaging process is up to 8 neighboring pixels, and when the object to be inspected is a low carbon content steel material, the averaging process is performed three times in succession. When the object to be inspected is a high carbon content steel material, the noise was most efficiently removed by continuously performing the averaging process four times.

Next, in step S5, these images are binarized. The threshold value for binarization is determined by, for example, taking the average value of the gradation of the decarburized layer and the gradation of the non-decarburized layer.

3 to 4 show examples of image processing data.
FIG. 3 is an image obtained by immediately binarizing the captured original image, and FIG. 4 is an image binarized by the image processing from step 1 to step 5.

In FIG. 4, the white portion corresponds to high brightness.

By the way, as shown in FIG. 4, black portions may be scattered like islands in the white portion. Such a point causes an error when the depth of the decarburized layer is measured later, and thus needs to be removed. Therefore, in step S6, filling processing is performed. That is, among the binarized pixels, the value of a pixel having a value corresponding to high brightness (hereinafter, referred to as “white pixel”, and having a value corresponding to low brightness (hereinafter referred to as “black pixel”)) Is inverted to a value corresponding to high brightness to eliminate the island-shaped black pixel portion.Of course, when it is known that the island-shaped black pixel portion does not exist, it is not necessary to perform hole filling processing.

The depth of the decarburized layer is measured by measuring the depth to which the white pixels are continuous from the surface of the object to be inspected after the hole filling process. That is, a measurement line is provided in step S8, and the length of white pixels on the measurement line that are continuous from the surface of the material to be inspected is set as the depth of the decarburized layer at that location. The measurement line is a line perpendicular to the surface of the object to be inspected, and a predetermined number of measurement lines are provided.

However, in some cases, white pixels are scattered and appear intermittently on one measurement line. This state is schematically shown in FIG. In FIG. 5, 21 is the surface of the object to be inspected, 22 to 26 hatched are white pixel groups, and l _{1 to} l ₃ are measurement lines. In the case like Fig. 5,
It is unclear how far the depth of the decarburized layer is.

Therefore, the continuity determination process is performed in step S7. In the continuity determination process, when white pixels appear discontinuously in the direction perpendicular to the surface from the surface of the object to be inspected, if the interval between them is less than a predetermined value, they are treated as being continuous. If the distance between them exceeds a predetermined value, they are treated as discontinuous. That is, the threshold value T is determined in advance, and if the interval between the white pixels on the measurement line is equal to or smaller than the threshold value T, it is treated as continuous, and if it exceeds the threshold value T, it is treated as separated.

In FIG. 5, t ₁ and t ₃ are less than or equal to T,
If t ₂ and t ₄ are larger than T, the white pixel group 22, the white pixel group 23, and the white pixel group 25 are treated as being continuous, and the white pixel group 22, the white pixel group 24, and the white pixel group 25. And the white pixel group 26 are treated as being separated.

Therefore, in measuring the depth of the decarburized layer, the measurement line l
_At the position ₁ , the depth of the decarburized layer is from the surface 21 of the object to be inspected to the lower end of the white pixel group 23, and at the position of the measurement line l ₂ , the depth of the decarburized layer is white from the surface 21 of the object to be inspected. Up to the lower end of the pixel group 22 at the position of the measurement line l ₃ ,
The depth of the decarburized layer is from the surface 21 of the inspection object to the white pixel group 25.
Is determined to the lower end of. As for the measurement line, a plurality of measurement lines are drawn at regular intervals to measure, and the decarburization depth for each measurement line is obtained.

When obtaining the total decarburization depth of the material to be measured, the maximum value among these measurement data is selected and used as the total decarburization depth. When obtaining the average decarburization depth, the average value of each measurement data is used as the average decarburization depth. However, in any case, noise may generate data that exceeds the common sense of decarburization depth, so a threshold is set and data that exceeds that is not adopted.

The position of the surface of the object to be inspected is detected as follows. That is, on the measurement line, the position where the white pixels first consecutively appear in the predetermined number from the surface side of the inspection object is determined as the surface position. here,
The expression "predetermined number of consecutive appearances" includes not only a plurality of consecutive appearances but also a case of one appearance in some cases.

[0050]

EXAMPLE The depth of the decarburized layer was measured using the method described in the above-described embodiment of the present invention and compared with the visual data obtained by humans. In this embodiment, shading correction, maximum value processing, minimum value processing, average value processing,
As the specific methods for the binarization processing, the hole filling processing, the continuous determination processing, and the decarburized layer depth determination processing, the methods described in the embodiments of the invention were used.

As a result, for the low carbon content steel material, the number of samples was 10, the average error was 0.01 mm, and the standard deviation was 0.015 mm. For the high carbon content steel material, the number of samples was 10, the average error was 0.01 mm, and the standard deviation was 0.02 mm. there were. This is a very satisfactory value as an automatic measurement result.

[0052]

As described above, according to the present invention, the cross section near the surface of the cross section of the object to be inspected in which the decarburized layer exists from the surface to the inner surface is photographed by the image pickup means,
The captured image is decomposed into pixels, and each pixel is binarized after the maximization process is performed a plurality of times and the minimization process is performed a number of times equal to or less than the number of maximization processes. ,
Of the binarized pixels, the pixel corresponding to the high brightness is measured at each position for the length from the surface of the object to be inspected continuous in the direction perpendicular to the surface, and the maximum value of the measured length is used to remove the value. Since the depth is determined as the depth of the coal seam, the depth of the decarburized seam can be automatically and accurately measured.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the configuration of an apparatus for carrying out the present invention.

FIG. 2 is a flowchart showing an example of an embodiment of the present invention.

FIG. 3 is a drawing-substituting photograph showing a cross-sectional image of an image of an object to be inspected.

FIG. 4 is a drawing-substituting photograph showing a state where the image shown in FIG. 3 is binarized after image processing.

FIG. 5 is a diagram showing an example of a distribution state of white pixels.

FIG. 6 is a diagram showing a method for measuring a decarburized layer in a conventional example.

FIG. 7 is a diagram showing a method for measuring a decarburized layer in a conventional example.

[Explanation of symbols]

1 microscope 2 A / D converter 3 input buffer 4 bus 5 Program memory 6 CPU 7 Image processor 8 gray image memory 9 Binary image memory 10 Auto stage driver 11 Autofocus driver 12 output buffers 13 D / A converter 14 CRT 15 keyboard 15 'mouse 16 Imaging device 17 stages 18 Plane movement mechanism 19 Vertical movement mechanism 21 Surface of object to be inspected 22-26 White pixel group

Continuation of the front page (56) Reference JP-A-4-174359 (JP, A) JP-A-6-83323 (JP, A) JP-A-61-160094 (JP, A) JP-A-2-24542 (JP , A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 33/20 G01B 11/22 G06T 7/00

Claims (9)

(57) [Claims] 1. A cross section near the surface of an object to be inspected in which a decarburized layer exists from the surface to the inner surface is photographed by an image pickup means, the imaged image is decomposed into pixels, and a plurality of times are taken for each pixel. After performing the maximization process and the minimization process that is equal to or less than the maximization process number, each image is binarized, and a pixel having a value corresponding to high brightness among the binarized pixels is binarized. A method for measuring the depth of a decarburized layer, which comprises measuring a continuous length from a surface of an object to be inspected in a direction perpendicular to the surface at each position and determining the depth as the depth of the decarburized layer of the measurement target material. 2. The method for measuring the depth of a decarburized layer according to claim 1, wherein the maximum value of the depth of the decarburized layer measured at each position is used as the total decarburized layer depth. 3. After binarizing each image, the value of a pixel having a value corresponding to a low brightness surrounded by pixels having a value corresponding to a high brightness of the binarized value is subsequently inverted. 2. The value corresponding to high brightness is set.
Alternatively, the method for measuring the depth of the decarburized layer according to claim 2. 4. The low carbon content steel material to be inspected, wherein the maximization process is performed three times in succession and then the minimization process is performed twice in succession. The method for measuring the depth of a decarburized layer according to any one of claims 1 to 3. 5. The method according to claim 1, wherein the object to be inspected is a high carbon content steel material, and the maximization process is performed twice and then the minimization process is performed once. Item 5. A method for measuring the depth of a decarburized layer according to any one of Items 3. 6. The method according to claim 1, wherein shading correction is performed on each pixel before performing maximum value processing and minimum value processing. Coal bed depth measurement method. 7. The averaging process is performed after performing the maximum value conversion process and the minimum value conversion process and before performing the binarization process. The method for measuring the depth of a decarburized layer according to the item. 8. A binarized pixel corresponding to high brightness,
When the object to be inspected appears discontinuously in the direction perpendicular to the surface, and if the distance between them is less than a predetermined value, they are treated as continuous and the distance exceeds the predetermined value. Treat them as discontinuous in some cases,
The depth measurement method of the decarburized layer according to any one of claims 1 to 7, wherein the length of pixels continuous from the surface is measured. 9. The position of the surface of the object to be inspected is determined by the position where a predetermined number of pixels corresponding to high brightness among the binarized pixels first appear consecutively from the surface of the object to be inspected. The method for measuring the depth of a decarburized layer according to any one of claims 1 to 8.