JPH0694642A - Method and equipment for inspecting surface defect - Google Patents

Abstract

PURPOSE:To make definite the conditions of setting of a laser light to be projected, in accordance with the roughness of the surface of a material to be inspected, in the case when a surface defect of the material of which the roughness of the surface is large is detected by the laser light. CONSTITUTION:An infrared laser beam 2 of a wavelength lambda=3.39mum emitted from an infrared He-Ne laser 1 is passed through a collimator lens 3 and made to scan continuously in the direction of the plate width of a hot-rolled steel plate 5 by a polygon mirror 4. A reflected light is led to a photosensor 7 by a condenser lens 6, an electric signal of the sensor is processed on a real time basis by a signal processing device 8 and the result of detection is outputted from an output device 9. When the incidence is executed by setting an incident angle theta so that a condition of (sigmaXcostheta/lambda)<0.35 be set in accordance with a preparatory deviation sigma in the direction of the height of a surface profile of the hot-rolled steel plate 5, the peak intensity of a diffracted scattered light of an infrared wavelength of the reflected light can be detected with ease and thus the sensitivity of detection of a defect is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for detecting surface defects on a material to be inspected which moves at high speed such as a hot rolled steel sheet. The present invention relates to detection technology.

[0002]

2. Description of the Related Art A method and apparatus for projecting a laser beam on the surface of a material to be inspected and detecting a defect from the intensity and distribution change of the diffracted and scattered light have already been put to practical use in steel, aluminum, papermaking lines and the like. There is. This device is, for example, as shown in FIG.
The laser light 12 output from the laser light source 11 is condensed through the collimator lens 13 to a beam diameter optimum for defect inspection, and then projected by the polygon mirror 14 so as to scan in the width direction of the inspected material 15 to be inspected. The light reflected on the surface is measured by the photodetector 17 through the condenser lens 16, and the defect is detected from the intensity thereof. Here, in order to improve the detection sensitivity of a defect and the ability to identify a detected defect, a technique using a transmission filter 18 for reflected light called a spatial mask or arranging a photodetector at some specific reflection angle is used. The device for extracting the component necessary for defect detection, that is, the reflected light resulting from the defect, is made from the reflected light distribution.

[0003]

However, in the prior art, the conditions for the light source wavelength and the projection angle of the laser beam for accurately detecting the reflected light caused by the defect have not been clarified. In some cases, it was not possible to distinguish the diffracted and scattered light from the normal part of the above and the reflected light due to the defect. In particular, in the material to be inspected having a large surface roughness such as a hot rolled steel sheet, the spread of the diffracted and scattered light from the normal portion is large, and the result is that the reflected light due to the defect is buried in this. The S / N ratio deteriorates. Therefore, if a He-Ne laser with a wavelength of 633 nm, which is often found in the examples of conventional apparatuses, is used, it cannot be applied to the detection of surface defects of a material to be inspected such as a hot rolled steel sheet.

In order to solve the above problems, the present invention provides
The purpose is to clarify the setting conditions of the laser light source for projection according to the roughness of the surface of the material to be inspected.

[0005]

According to the present invention, when a light beam is projected on the surface of a material to be inspected and the diffracted and scattered light is detected to detect a surface defect of the material to be inspected, the surface profile of the material to be inspected is detected. Is a standard deviation σ in the height direction, and the wavelength λ of the projected light beam and the projection angle θ of the light beam onto the surface of the material to be inspected are set and measured according to the following equations.

(Σ × cos θ / λ) <0.35 This method can be preferably carried out when the surface of the material to be inspected is a hot rolled steel sheet, and the light beam is preferably an infrared laser. . The apparatus of the present invention is preferably used for carrying out the above method, and includes a laser light source that emits a light flux of wavelength λ, and a polygon mirror that projects this laser light onto the surface of a material to be inspected that moves at high speed. , A light receiver for receiving reflected light from the material to be inspected, and the spatial positional relationship between the polygon mirror and the surface of the material to be inspected is determined according to the standard deviation σ in the height direction of the surface profile of the material to be inspected. The surface defect inspection apparatus is characterized in that the laser beam projection angle θ according to the equation is set according to the following equation.

Cos θ <0.35 × (λ / σ) where θ: projection angle (angle of inclination from the vertical line of the surface of the material to be inspected)
(Degree) λ: wavelength (μm) σ: standard deviation (μm) in the height direction of the surface profile of the material to be inspected

[0008]

[Function] The method of measuring the surface roughness using the light diffraction phenomenon has been confirmed theoretically and experimentally that the diffracted and scattered light contains information on the fine shape of the surface to be measured. Therefore, many technologies have already been published. The present inventors have tried to apply this basic technique of surface roughness measurement to the detection of surface defects, particularly to the in-process defect detection of hot-rolled steel sheet. Specifically, the conditions under which the diffracted and scattered light from the normal portion and the diffracted and scattered light from the defective portion of the inspection material have a sufficient S / N ratio were examined.

The standard deviation σ in the height direction of the surface profile is used as a value indicating the degree of roughness of the surface of the material to be inspected. As shown in FIG. 2, when the laser light 2 having the wavelength λ is projected on the surface of the material 15 to be inspected having the roughness σ at the incident angle θ, the diffracted scattered light 21 is generated due to the minute irregularities on the surface. FIG. 3 shows the measurement results of the diffracted and scattered light with respect to steel sheet samples having different standard deviations σ. As is clear from FIG. 3, in a steel sheet having a large standard deviation σ, the scattered diffracted light is large, and it is no longer possible to recognize the peak intensity. The present inventors have examined conditions under which the peak intensity of diffracted and scattered light can be clearly detected. Here, in order to determine whether or not the peak intensity can be clearly detected, the kurtosis γ using the kurtosis of the scattered light distribution as an evaluation parameter is

[0010]

[Equation 1]

Is a value calculated by
In the case of, the distribution curve follows a normal distribution, the larger the distribution curve has a sharper peak, and the smaller the distribution curve is, the smoother the distribution becomes. FIG. 4 shows the kurtosis when the laser wavelength λ of the light source is changed under the condition that the incident angle θ is constant with respect to the steel plate samples having different standard deviations σ in the height direction of the material to be inspected, and FIG. 5 is the laser wavelength of the light source. These are the results of experimentally determining the kurtosis when the incident angle θ was changed under the condition that λ was constant. As is clear from the figure,
At a constant projection angle, the longer the laser wavelength λ,
Under a constant laser wavelength, the larger the incident angle θ, the larger the kurtosis γ, and the wider the range in which the peak intensity can be clearly observed. Therefore, in order to quantitatively evaluate such a phenomenon, the parameter α = σ × cos θ / λ 2 was introduced. This parameter is a parameter that includes the surface roughness of the material to be inspected, the laser wavelength, and the incident angle, and the smaller this value is, the more clearly the peak intensity can be detected. Using the experimental data shown in FIGS. 4 and 5,
FIG. 6 shows the result of examining the relationship between the parameter α and the kurtosis γ. As is clear from FIG. 6, it can be seen that an inverse proportional relationship is established between the parameter α and the kurtosis γ. The present inventors obtained the following empirical formula as a description of this relationship.

Γ = (1 / 1.5α) -1.9 (2) As a condition of peak observation necessary for defect detection, kurtosis γ = 0,
That is, if it is selected that the scattered light distribution follows a normal distribution, α <0.35 is required from the second equation. Therefore, (σ × c
If the optical conditions are set so as to satisfy osθ / λ) <0.35, it is possible to clearly detect the peak intensity of the diffracted and scattered light.

Further, the most problematic defect in the hot-rolled steel sheet production line is that irregularities are generated on the surface, such as blow-in flaws caused by foreign matter adhering to the rolling roll. The flaw has a distinctly larger shape than the roughness order. Therefore, the reflected light caused by such a defect is determined by the geometrical shape of the defect, rather than the scattering intensity and distribution thereof being determined by the diffraction phenomenon of light, and the scattering is very large. Become. Therefore, if the diffracted and scattered light from the normal portion of the hot-rolled steel sheet spreads too much, it becomes impossible to detect the change in the reflected light due to the defect, and the defect cannot be detected.

In the case of hot-rolled steel sheets, the standard deviation σ of the surface profile in the height direction often exceeds 1 μm. Therefore, according to the conditions of the previous term, it is necessary to detect defects using an infrared laser.

[0015]

EXAMPLES The results of applying examples according to the present invention to hot-rolled steel sheets will be described. FIG. 1 is an example of an in-process surface defect inspection apparatus for hot-rolled steel sheet according to the present invention, which is a defect detection apparatus for hot-rolled steel sheet using an infrared He—Ne laser having a wavelength of 3.39 μm as a light source.

In the figure, an infrared laser beam 2 having a wavelength λ = 3.39 μm emitted from an infrared He-Ne laser 1 is shown.
After the beam diameter is adjusted to a predetermined value through the collimator lens 3, the hot rolled steel sheet 5 is formed by the polygon mirror 4.
Are continuously scanned in the plate width direction. The reflected light is guided to the light receiver 7 by the condenser lens 6, and its intensity and distribution are converted into an electric signal. The signal processing device 8 processes this electric signal in real time, and outputs the detection result from the output device 9. The diffracted and scattered light of the infrared wavelength reflected by the hot-rolled steel plate 5 has a distribution such that its peak intensity can be easily detected, and therefore, for example, when irregularities called roll marks pass through the scanning infrared light, they are scattered. Generate a very large reflection, which improves the sensitivity of defect detection.

FIG. 7 shows the results of measuring the reflected light distributions of the normal part and the defective part using a hot-rolled steel sheet with σ = 1.1 ± 1.0 μm as a sample. In the example of FIG. 7, the result is obtained by projecting an infrared He—Ne laser having a wavelength of 3.39 μm at an incident angle θ = 30 degrees, and the parameter α≈0.28 in the normal part.
As is clear from the figure, there is a significant difference in the reflected light between the normal portion and the defective portion, and surface defect inspection with high detection sensitivity can be performed.

[0018]

According to the present invention, since the optimum conditions of the surface roughness of the material to be inspected, the wavelength of the light beam to be projected, and the projection angle are clarified, the sensitivity of defect detection even when the surface roughness of the material to be inspected is large. Can be increased. In particular, by using infrared laser light on the hot-rolled steel sheet, it becomes possible to detect in-process irregularity defects that cause a large number of defective products.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the configuration of an embodiment according to the present invention.

FIG. 2 is an explanatory diagram of diffracted and scattered light of laser light.

FIG. 3 is a graph showing an example of measurement results of reflected light distribution from a steel plate.

FIG. 4 is a graph showing the kurtosis of the distribution of diffracted and scattered light when the laser wavelength λ of the light source is changed under the condition that the projection angle θ is constant.

FIG. 5 is a graph showing the kurtosis of the distribution of the diffracted and scattered light when the projection angle θ is changed under the condition that the laser wavelength λ of the light source is constant.

FIG. 6 is a graph showing the relationship between parameter α and kurtosis.

FIG. 7 is a graph showing measurement results of reflection angle and reflection intensity.

FIG. 8 is a schematic explanatory diagram of a conventional defect detection device.

[Explanation of symbols]

1 Infrared Laser 2 Infrared Laser Beam 3 Collimator Lens 4 Polygon Mirror 5 Hot Rolled Steel Plate 6 Condensing Lens 7 Light Receiver 8 Signal Processor 9 Output Device 11 Laser Light Source 12 Laser Light 13 Collimator Lens 14 Polygon Mirror 15 Inspected Material 16 Condensing lens 17 Photodetector 18 Transmission filter 19 Signal processing device 20 Output device

Front page continuation (72) Inventor Masakazu Yokoo 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Satoshi Maruyama 1 Kawasaki-cho, Chuo-ku Chiba City Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Susumu Moriya 1 Kawasaki-cho, Chuo-ku, Chiba City Kawasaki Steel Co., Ltd. Technical Research Division

Claims (4)

[Claims] 1. A standard deviation σ in the height direction of a surface profile of a material to be inspected when a light beam is projected onto the surface of the material to be inspected and the diffracted and scattered light is detected to detect a surface defect of the material to be inspected.
Is obtained, and the wavelength λ of the projected light beam and the projection angle θ of the light beam on the surface of the material to be inspected are set and measured according to the following equations. (Σ × cos θ / λ) <0.35 2. The surface defect inspection method according to claim 1, wherein the surface of the material to be inspected is a hot rolled steel sheet. 3. The surface defect inspection method according to claim 1, wherein the light flux is an infrared laser. 4. A laser light source which emits a light flux of wavelength λ,
Equipped with a polygon mirror that projects laser light onto the surface of the inspected material that moves at high speed, and a light receiver that receives the reflected light from the inspected material, depending on the standard deviation σ in the height direction of the surface profile of the inspected material. The surface defect inspection apparatus is characterized in that the spatial positional relationship between the polygon mirror and the surface of the material to be inspected is matched with the laser beam projection angle θ according to the following equation. cos θ <0.35 × (λ / σ)