JP2001242089A - Surface flaw detecting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface flaw detecting method stably detecting a surface flaw with good detectability even in a hot rolling process. SOLUTION: When light is radiated from a light projector 5 to a steel plate 1 positioned in the closest position to a final stand roll 2 of a finishing rolling machine, its reflected light is received by means of a line sensor camera 6 for image processing, and the surface flaw is detected. When a scale is formed on the steel plate 1, the scale becomes noise or covers the surface of the surface flaw and lowers flaw detection ability. Therefore, surface flaw detection is carried out while the scale is scarcely formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for optically detecting defects generated on the surface of a product in a process of manufacturing a metal material such as a steel plate, a bar or a steel pipe by hot rolling. It is.

[0002]

2. Description of the Related Art In the rolling process of steel products, various defects appear on the product surface due to segregation of impurities in the material itself and flaws generated during rolling. Heretofore, many methods for detecting defects in a process close to room temperature, such as a cold rolling process and a galvanizing process, have been proposed, and a plurality of methods have been widely put to practical use. On the other hand, as for surface inspection during the hot rolling process, although some forms using a cold inspection method have been proposed, they have not been widely used. One of the reasons for this is that in the hot process, the temperature of the steel sheet to be inspected is as high as 400 ° C to 1000 ° C or higher, and a large amount of water and water vapor exists in the inspection atmosphere. Has formed.

[0003] However, such a problem can be avoided by taking appropriate measures, and is not an essential problem.
For example, FIG. 5 (JP-A-6-102195) shows that a light source 41 irradiates a steel plate 42, which is an object to be inspected, and the reflected light is detected by a line sensor camera 43, and image processing is performed to detect a surface defect. This is a surface inspection device that simply attaches a filter 44 for removing self-luminous light from the steel plate 42 to a surface defect detection device used in the cold process. By installing an air jet etc. to remove water and water vapor,
It is possible to observe the steel sheet surface during the hot rolling process.

On the other hand, Japanese Patent Application Laid-Open No. Sho 53-12378 proposes that an imaging system be installed after a cooling section in a hot rolling process. This technique is characterized in that, when the scale is sufficiently grown, unevenness is hardly seen, and a lower temperature place is selected as the inspection position.

[0005]

The iron oxide called scale is formed on the surface of the steel sheet during the hot process.
Its thickness is constantly increasing as the steel sheet passes through the process. Since the scale is iron oxide, it has different optical properties such as spectral refractive index, spectral reflectance, and spectral absorptivity from iron which is the background of the steel sheet. Therefore, at the point where the scale of the steel sheet surface is very thin, upstream of the hot process, the steel sheet surface exhibits optical properties close to the iron surface, and as the scale grows through the process, the optical properties of iron oxide gradually increase. Will be presented.

That is, in the hot process, the optical properties of the steel sheet surface differ depending on the place of the process.
The optical properties mentioned here include not only optical properties but also changes in micro properties such as roughness and fine structure of the surface. The optical properties including these elements are
As the thickness of the scale increases, the variety increases, and the relative appearance of the defect appearing on the surface of the steel sheet largely depends.

This point is overlooked in the conventional proposal as described in Japanese Patent Application Laid-Open No. Hei 6-102195, and the change in the optical properties of the steel sheet surface depending on the location during the hot process can be performed in a cold state. A defect inspection was attempted by installing a device almost similar to the inspection device. That is, JP-A-6-10
In Japanese Patent No. 2195, although the temperature and emissivity of the scale are discussed in order to determine the filter wavelength for eliminating the effect of self-emission, the steel sheet surface depends on the thickness of the scale and the thickness unevenness. No attention is paid to the reflectance of the light, the appearance of the defect, and the limitation on the installation position of the apparatus.

On the other hand, in Japanese Patent Application Laid-Open No. 53-12378, attention is paid to this point to some extent, and a low-temperature place where the scale is sufficiently grown and unevenness is hardly seen is selected as the inspection position. However, in a typical hot rolling process, the steel sheet has a temperature of about 400 ° C. to 600 ° C. even before the winding device, and the scale is still growing. Since the degree of scale growth changes depending on the operating conditions of the process, such as the material of the steel sheet and the degree of cooling after the finish rolling mill, the relative appearance of the defective part with respect to the normal part (the brightness of the defective part with respect to the normal part) is different for each operating condition. And its contrast) change.

That is, a defect may appear that is brighter than a normal part under a certain operating condition, but becomes less visible under a different operating condition, or may be darker than a normal part. . Then, there are cases where similar defects can be detected depending on operating conditions and cases where similar defects cannot be detected. Therefore, even if the method described in JP-A-53-12378 is used, it is impossible to reliably detect a surface defect.

Another problem specific to surface inspection in the hot rolling process is that there is a case where a defect image obtained during the process cannot be matched with an actual defect that is visible after the product has cooled. At a thickly grown portion, there is a problem that it is difficult to discriminate between the defect and the unevenness of the scale, and a sufficient inspection cannot be performed.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a method for detecting a surface defect that can detect a surface defect more stably and with good detectability even in a hot rolling process. Make it an issue.

[0012]

A first means for solving the above-mentioned problem is to remove defects generated on the surface of a product in a process of producing a metal material such as a steel plate, a strip or a steel pipe by hot rolling. A method for detecting a surface defect by an optical method, wherein a surface defect is detected at a place where an oxide on an inspection target surface is extremely thin (claim 1).

[0013] A second means for solving the above-mentioned problems is as follows.
In the process of manufacturing a steel sheet by hot rolling, a method of detecting defects occurring on the surface of a product by an optical method, and detecting surface defects downstream of the final roll of the finishing mill and before the cooling equipment. A method for detecting a surface defect (claim 2).

A third means for solving the above-mentioned problem is as follows.
In the process of manufacturing a steel sheet by hot rolling, a method for detecting defects generated on the surface of a product by an optical method, wherein the inspected portion of the steel sheet is within 5 seconds after leaving the final roll of the finishing mill. And a surface defect detection method (claim 3).

The term "extremely thin" in the first means (claim 1) refers to a thickness in the range of 0 to 10 μm.

FIG. 2 shows the outline of the hot rolling process of the steel sheet and the installation position of the surface defect inspection apparatus. 2, 21 is a rough rolling mill, 22 is a finishing mill, 23 is a steel plate, 24 is a steel plate cooling device, 25 is a winding machine, 26 is a light source, and 27 is a line sensor camera.

A steel sheet 23 rolled to a product size by a rough rolling mill 21 and a finish rolling mill 22 is water-cooled by a steel sheet cooling device 24 and wound by a winder 25 to form a coil. On the entry side of the finish rolling mill 22, the scale of the steel sheet surface generated so far is once peeled off by a process called descaling by mechanical impact, high-pressure water injection, or the like. The scale gradually grows while being rolled by the rolls in the finish rolling mill 22, and gradually grows before reaching the winder 25. However, within the range where the inspection apparatus can be actually installed, it is obvious that the area immediately after the final stage of the roll is the thinnest point.

The present inventors actually set the position immediately after the finish rolling mill shown in FIG.
An optical system composed of a light source 26 and a line sensor camera 27 is simultaneously installed at a position indicated by B, which is the exit side (entrance side of the winder 25) of the steel sheet 4, and an image of the steel sheet surface being processed at each location is obtained. Observed. A position is 5 from the center of the final roll.
m downstream, the B position is further 50 m downstream from the A position.
Here, the camera lens is equipped with a heat ray cut filter,
In addition, a blue filter that cuts off the light emission of the steel plate was installed. The light projection and the light reception substantially form a regular reflection optical system, but the light reception angle is slightly out of the regular reflection position. This is because the defective portion looks better empirically.

One example of this observation is shown in FIGS. 3A and 3B. In both figures, the horizontal direction is about 8m in the direction in which the steel sheet flows,
About 2 m is shown in the vertical direction in the width direction of the steel plate. (A) is an image of the defect taken at the position A in FIG. 2 immediately after the finish rolling mill, while (B) is a defect image taken at the position B in FIG. 2 immediately before the winding machine. Both images are of the same steel plate. (A) The part that looks particularly white in the image is a defect caused by an abnormality in the finishing roll, and is periodically transferred to the steel sheet. Finish roll circumference is about 3
m, defects appear every 3 m.

When the images of (A) and (B) are compared, the defect observed by the camera at the position A immediately after the finish rolling mill is clearly seen. As a result of continued observation of many defects, it was found that in the image at the position (A), the defect caused by the wrinkle accompanied by the fold of the steel plate always looks brighter than the normal part. It was also found that the defects caused by foreign matter falling on the steel plate and denting the roll always seemed dark. That is, at the position A, the brightness is determined for each defect type.

On the other hand, in (B), the same defect as in (A) appears darker than the normal part, and the appearance at the position (A) and the brightness are reversed. Also, the outline of the defect is difficult to understand.
The normal portion also has many patterns as a whole, and is an image in which it is difficult to identify a defective portion. As a result of observation of many defects, it was found that at position (B), the brightness of each defect type was indefinite, and the appearance was unstable.

Observation of more than 1000 steel plates shows that, with almost no exception, the camera at the position A can clearly observe the defect, and the defect at which the light and dark can be observed stably can be seen with the camera at the position B immediately before the winding machine. It became difficult, the light and darkness was reversed, or it was invisible.

This phenomenon will be described with reference to FIG. In FIG. 4, 31 is the steel sheet surface, 32 is the defect, 33 is the irradiation light,
34 is a reflected light and 35 is a scale. (A) is a conceptual diagram of the surface of the steel sheet immediately after leaving the finishing rolling mill, in which a wrinkle accompanied by a fold of the steel sheet causes a flaw in the rolling roll, which is transferred to form a diffusely reflective defective portion 32. Is modeled. The surface of the steel plate surface 31 is not a perfect mirror surface,
Although it has a certain degree of diffuse reflection, the defect portion 32 has a more complicated property, and the diffuse reflection is further enhanced optically as compared with the normal steel sheet surface 31 as shown in FIG. I have.

Therefore, when viewed from an optical system slightly deviated from the regular reflection position, the defective portion 32 looks brighter than the normal portion. Even if the steel plate has loose irregularities, it is buried in the irregular reflection of light by the fine structure, and the irregularities are not visible. (A)
Although some scales (not shown) have already been generated in this case, the defects are not considered in the model because they are formed on the surface.

Here, when the scale 35 further grows, the fine structure is buried as shown in FIG. 3B, and the contrast of the defective portion 32 is reduced in the image. At this time,
Since macroscopic irregularities remain, a portion where the incident light is reflected at an angle greatly deviated from the camera occurs at the defect portion, and the defect portion appears dark.

The above is the reason why the appearance of the defect changes in the image of FIG. 3 depending on the inspection place. In the case where the scale has grown when there is no macro unevenness in the defect part, it is considered that the defect part is covered with the thick scale like the normal part and cannot be distinguished from the normal part in (B).

The properties of the surface of the steel sheet immediately after the final roll of the finishing mill are almost constant since they have just been rolled, so that it can be easily inferred from observation that the appearance of the defective portion will be stable.

On the other hand, it is expected that the degree of scale formation greatly fluctuates depending on operating conditions such as plate thickness, rolling temperature, and rolling speed. Then, since the state of the surface in (B) depends on the operating conditions, it is easily presumed that the appearance of the defect also changes greatly.

From the results of many other image observations, the inventors of the present invention conclude that in addition to the fact that the defect itself is difficult to see at the position B in FIG. 2, the image at the position B in FIG. In many cases, luminance unevenness that is considered to be caused by unevenness appears, which is an obstacle, and it is difficult to discriminate from the background except for a large defect. On the other hand, in the detection at the position A in FIG. It was found that the brightness of the image was uniform and stable, and it was easy to detect only the defect. This fact supports the problem described in the section of "Problems to be solved by the invention".

Therefore, as a method of obtaining a more stable defect signal and obtaining a practical hot surface inspection apparatus, it was concluded that the inspection apparatus should be provided at a place where the scale of the steel sheet surface is as thin as possible. . In the hot rolling process for a steel sheet, the most stable surface inspection is possible immediately after the finish rolling mill as described above, and it is desirable to install the inspection device as close as possible to the final roll of the finishing rolling mill.

The scale of the surface of the steel sheet produced in the hot rolling process is several μm to 100 μm. In the process used in the experiment of the present invention, as a result of measurement by tomography after the product has cooled, when the steel sheet speed immediately after passing through the finishing mill is 800 ° C., the thickness of the scale is 20 to 30 μm.
m. Immediately before entering the finishing mill, the scale of the steel sheet is once peeled off by descaling, but regrows while passing through the finishing mill. When calculated from the growth rate of a general scale, the thickness of the scale at a position where a good inspection result is obtained is estimated to be 0 to 10 μm.

The place where the most stable surface inspection can be performed is immediately after the finish rolling mill as described above, and it is desirable to install the inspection apparatus as close as possible to the final roll of the finishing rolling mill. The same applies to hot rolling of other metals that generate scale on the surface.

On the other hand, when various peripheral devices are already installed, the guide of the installation location of the inspection device is as follows: (1) If there is a cooling facility using water or the like downstream of the finishing mill, immediately after the final roll, Until just before the cooling equipment. (2) If there is no cooling equipment, the part to be inspected of the steel sheet can be observed within 5 seconds after exiting the final roll. By doing so, generally good results can be obtained. Regarding (2), for example, the slowest rolling speed in the final roll is 4
m per second, the inspection equipment is 20
m (= 4 m per second × 5 seconds).

The gist of the present invention resides in that the imaging device is formed at a position where the oxide on the surface to be inspected is very thin (when the oxide thickness is 0).
In the case of a finish rolling mill, it is provided as close as possible to a finish rolling mill, and any imaging method may be used. The present inventors have adopted the method using a line sensor camera as described above, but the effect is the same in observation using a strobe and a normal television camera. Alternatively, a surface defect meter that scans with a laser beam, which is often used in a cold rolling process, may be used. In the case of the optical inspection method, the appearance of the defect differs depending on whether the light from the light source is received by specular reflection or at the scattering position,
The present invention is applicable to any optical system without depending on the light projecting / receiving angle.

[0035]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a hot rolling process for a steel sheet will be described below. FIG. 1 is a diagram showing an outline of a finishing mill in hot rolling of a steel sheet and a surface defect detector installed immediately after the finishing mill. In FIG. 1, 1 is a steel plate, 2 is a rolling roll of a finishing mill, 3 is a lamp, 4 is a bundle fiber, 5 is a floodlight, 6 is a line sensor camera, 7 is a filter, 8 is a rotary encoder, 9 is a camera controller, 10 is an image memory / image processing device,
11 is a computer and 12 is a display / output device.

The finishing mill has seven stages, and the first roll on the entry side is No. 1 and the last roll is No. 7 roll.
A rough-rolled steel sheet 1 at a temperature of about 1100 ° C. is inserted from the entry side, and is gradually rolled.
Rolled to a degree. The steel sheet moving speed here is the maximum
The width of the steel plate is about 1800 mm, for example, about 25 m per second. The scale layer on the steel sheet surface is generated and peeled even before No. 1 roll.
So once you remove the scale with a water jet etc.
Make it bite into the No.1 roll.

In this embodiment, the position of No. 7 roll is 5
The optical system of the surface defect detector was installed at a position m m downstream. In the surface defect detector, light from a metal halide lamp 3 which is a DC light source, which is a light source, is linearly formed by a bundle fiber 4, guided to a light projector 5, condensed by a cylindrical lens (not shown), and illuminates the surface of the steel plate 1. 250W lamp 6
The table irradiates a width of about 2000mm.

For light reception, two line sensor cameras 6 with a drive clock of 20 MHz are used. Since the steel plate is red hot, 600n
The visible light and heat rays of m or more are cut by the filter 7 so that only the reflected light from the light source is received. Since the number of pixels of the line sensor camera 6 is 1024, and the width per camera is 1000 mm, the pixel resolution in the horizontal direction is 1 mm. The camera controller 9 receives a signal from the rotary encoder 9 provided on the roll, and sends a luminance signal of 1024 pixels from the camera to the image memory 10 every time the steel plate advances by 1 mm. Therefore, even if the operating speed of the steel plate changes, one pixel always corresponds to an image of 1 mm in length and width on the actual steel plate in the image memory.

Thus, a two-dimensional image of the surface of the steel plate is developed in the image memory. The image of the steel plate surface is, for example, as shown in FIG. The microcomputer 11 also measures the advanced length of the steel plate based on a signal from the rotary encoder 8, and controls the image processing device 10 to capture an image at a specified distance from a specified time.

The image processing apparatus 10 performs processing, such as shading correction, binarization, and filtering, on the collected image as necessary, which is generally performed even in the detection of defects in cold rolling, to automatically detect defects. Perform detection. In the present embodiment, shading correction and luminance normalization are performed, and binarization is performed using an empirically obtained threshold value to extract a defect candidate. Then, it calculates the geometric features and the presence or absence of periodicity to determine whether or not it is a defect, and what kind and how much defect is located at which position. Then, the result is displayed on the display device, and the occurrence of the defect is notified to the operator by the alarm device. The operator checks the image of the defect or the defect candidate on the monitor device 12, and in some cases, after actually visually inspecting the coil of the steel plate separately,
Inspect defective products and inspect rolling mills.

Table 1 summarizes the effects of the present invention. When the inspection apparatus is installed at the position described in the embodiment, and when the inspection is performed at a place below the scale transformation temperature before the winding machine, And the rate of finding defects in the three methods as a result of a separate visual inspection after winding. However, image processing by a computer was not performed, and it was investigated whether a human could look at the collected image and see if a defective portion could be confirmed. We surveyed images of more than 1000 steel plates, and set the method that found the most defective parts to 100%, and expressed the detection rate by other methods.

The defect to be inspected is a periodic defect that occurs when an abnormality on the roll surface is transferred to a steel plate, and is called a roll defect. Defects are rated on a five-point scale of A, B, C, D, and E, with the mildest being Class A and the most severe being Class E. This is a serious defect that is claimed by the process.

[0043]

[Table 1]

Method 3 in Table 1 is the inspection method by position limitation of the present invention, and the detection rate was the highest in any grade. The advantage of the present invention is remarkable as the grade is milder,
In the class A and B, even a defect that could not be confirmed by visual observation after the steel sheet was cooled could be detected. On the other hand, in the detection near the winder, the detection rate was lower than the visual inspection separately performed. This is because it is difficult to discriminate a normal part from an abnormal part only from an image due to the fact that the contrast of the defect is low at this position and the unevenness of the scale is large.

According to this embodiment, if the steel sheet surface is inspected by being installed immediately after the rolling mill in accordance with the gist of the present invention under the hot rolling process, the defect can be more reliably detected.
It was confirmed that it could be detected with higher sensitivity, and it was put to practical use.

As the effects of practical use, there is an improvement in the accuracy of finding a defect by visual observation of an image by a human, and an improvement in the reliability of the apparatus by making it possible to reliably perform automatic detection by a computer. In some cases, a point-like severe C-class defect exists among the mild A- and B-class defects. Nevertheless, production may be continued and loss may occur. According to the present invention, the occurrence of Class A and Class B defects can be detected with high accuracy, and it is possible to prevent such an accident beforehand by using it together with a visual inspection.

[0047]

As described above, according to the present invention, (1) in the hot rolling process, inspection can be performed at a place where the oxide on the surface is extremely thin, so that a defect can be reliably detected. . (2) Minor defects, which were difficult to detect by visual observation and conventional installation methods, can be stably detected, and the quality assurance level and yield are improved. The remarkable effect is obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration and an installation position of a surface defect detector used in an embodiment of the present invention.

FIG. 2 is a view showing an outline of a hot rolling process of a steel sheet and an installation position of a surface defect inspection device.

FIG. 3 is a diagram showing an example of a defect image observed at each position shown in FIG. 2;

FIG. 4 is a diagram illustrating a change in detectability of a defect.

FIG. 5 is a diagram illustrating an example of a configuration of a conventional surface defect detector.

[Explanation of symbols]

1 ... steel plate, 2 ... rolling roll of finishing mill, 3 ... lamp,
4: bundle fiber, 5: floodlight, 6: line sensor camera, 7: filter, 8: rotary encoder,
9 camera controller, 10 image memory / image processing device, 11 computer, 12 display / output device, 2
DESCRIPTION OF SYMBOLS 1 ... Rough rolling mill, 22 ... Finish rolling mill, 23 ... Steel plate, 24 ...
Steel plate cooling device, 25: winder, 26: light source, 27: line sensor camera, 31: steel plate surface, 32: defective part, 3
3: irradiation light, 34: reflected light, 35: scale

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yasuo Kushida 1-2-1 Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 2F065 AA49 AA61 BB05 BB08 BB11 BB12 BB13 BB15 CC00 CC06 EE00 FF42 FF67 GG02 GG08 GG16 HH12 JJ02 JJ08 JJ19 JJ25 LL03 LL21 LL26 MM02 MM22 PP16 QQ03 QQ04 QQ21 QQ24 QQ33 QQ42 RR05 RR06 SS02 SS04 SS09 SS13 2G051 AA37 AB02 CA03 CB01 CC07 DA06 EA11 EA23

Claims (3)

[Claims] In a process for producing a metal material such as a steel plate, a bar, or a steel pipe by hot rolling, a method for optically detecting a defect occurring on a surface of a product,
A method for detecting a surface defect, comprising detecting a surface defect at a place where an oxide on a surface to be inspected is very thin. 2. A method for detecting defects generated on the surface of a product by an optical method in a process of manufacturing a steel sheet by hot rolling, wherein the surface is provided downstream of a final roll of a finishing mill and before a cooling facility. A method for detecting a surface defect, comprising detecting a defect. 3. A method for detecting defects generated on the surface of a product by an optical method in a process of manufacturing a steel sheet by hot rolling, wherein a portion to be inspected of the steel sheet exits a final roll of a finishing mill. Detecting a surface defect within 5 seconds from the detection of the surface defect.