JPS58204355A - Method for detecting flaw on surface of metallic object - Google Patents

Abstract

PURPOSE:To enable the detection of the defect on a surface with high S/N by setting the angle between the incident direction of the incident light having a prescribed plane of polarization from a laternal side and the normal on the surface of a specimen at 15 deg.-55 deg. and detecting the prescribed polarization component of the specular reflected light. CONSTITUTION:A laser light source 22 disposed in the diagonal direction intersecting orthogonally with the traveling direction of a continuous casting slab 20 on the lateral side of the traveling line of said slab irradiates the external light having a linear polarization characteristic in such a way that the angle theta1 between the normal on the surface of the slab 20 and the incident direction of the irradiated light attains 15 deg.-55 deg.. The laser light 22a is expanded to a belt shape up to the necessary visual field on the slab 20 by a cylindrical lens 26 through a rotator 24 for the plane of polarization. On the other hand, a photodetection camera 30 which is disposed in the position on the opposite side of the slab traveling line equal to the angle theta1 between the normal on the surface of the slab 20 and the detection direction of the specular reflected light and has a polarization filter 28 detects the prescribed polarization component of the specular reflected light, and a signal processing circuit 32 outputs a defect signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for detecting defects on the surface of a metal object, and in particular, a method for detecting defects on the surface of a metal object during running, which is suitable for online detection of surface defects in running high-temperature steel materials such as continuous casting slabs. The present invention relates to an improvement in a method for detecting defects on the surface of a metal object in which surface defects on the object are detected by projecting light from the outside onto the surface of the object and receiving light reflected by the surface of the object.

An optical surface flaw detection method is known in which light is emitted from the outside onto the surface of a test object while it is running on a conveyance line, and the reflected light from the surface of the test object is received to detect surface defects on the test object. There is. This optical surface flaw detection method, for example, as shown in FIG. The object 10 is irradiated with external light and received by the light receiver 14, which is also placed directly above the object on the object travel line. This is used to detect surface defects. For example, when a laser light source is used as the light projector 12, a spot-shaped light spot is scanned in the width direction of the subject 10 using a flying spot scanning method, and the reflected light from the subject 10 is reflected by a photoelectron intensifier 8 tube. A light receiver 14 consisting of an optical cell or the like receives the light, and the position of the defective part in the ring direction is detected from the change in light intensity at each point. In addition, when a bar-shaped light source of white light is used as the light projector 12, the reflected light from the subject 10 is transmitted to the -dimensional image sensor or the light receiver 14 ("flying").
The light is received at a single point (-A) by the scanning method.

According to such an optical surface flaw detection method, surface defects of the moving object 10 can be measured online in a non-contact manner C. Feature 4! :Does it have?Conventional 1. &, Noise signals are treated as defective signals, and false detections become a real problem, creating a practical obstacle. Further, if the test object 10 is used as it is for surface flaw detection of a material, there are problems in terms of heat resistance and the like. Furthermore, it has a complicated mechanism such as a part of the rotating mirror, and heat resistance measures and adjustments for the entire device are extremely complicated.

The present invention was made in order to eliminate the drawbacks of the conventional technology as described in 1.
It is an object of the present invention to provide a method for detecting flaws on the surface of a metal object, which can detect defects with an N ratio of 11 degrees, and also allows easy heat resistance measures for a projector and a light receiver.

The present invention provides a method for detecting surface flaws on the surface of a metal object in which surface defects on the object are detected by irradiating light from the outside onto the surface of a moving object and receiving reflected light from the surface of the object. A light projector placed on the side of the travel line in an oblique direction orthogonal to the travel direction of the test object is directed onto the test object surface so that the angle between the normal line of the test object surface and the incident direction of the irradiated light is 15 degrees to 55 m. A change in the specularly reflected light or its predetermined polarization component that is irradiated with external light that has a predetermined plane of polarization or external light that does not have polarization characteristics, and is received by a receiver placed on the opposite side of the projector to the object travel line. The above object has been achieved by detecting surface defects on the object.

The present invention will be explained in detail below.

The present invention, as shown in FIG. , in a surface flaw detection method for detecting surface defects of a test object 10, the inventors have variously changed the direction of incidence of the irradiated light by the light projector 12 and the direction of reception of reflected light by the light receiver 14 to determine the optimal positional relationship. This was done based on the results of experiments.

That is, the projector 12 is placed on the side of the subject running line in an oblique direction orthogonal to the subject running direction, and the angle θ1 between the normal to the subject surface and the irradiation light incident direction is changed,
The light is specularly reflected by the light receiver 14 placed at the specularly reflected light receiving position on the opposite side of the subject traveling line from the projector 12, where the angle between the normal to the subject's surface and the direction of receiving the reflected light is equal to the angle θ1. When the light was received and surface defects of the object 10 to be inspected were detected, the S/S of defect (mainly vertical scraping) signals were detected.
The N ratio was as shown by solid line A in FIG. As is clear from the figure, when the angle θ1 is within the range of 15 degrees to 55 degrees, the S/N ratio of the defect signal is 2.0, which is a level that can practically discriminate the defect signal. As described above, highly accurate defect detection is possible.

Furthermore, when the incident direction of the light irradiated by the projector 12 is an oblique direction toward the line side, as shown in FIG.
In this manner, the method of placing the projector 12 diagonally above the side of the fin is particularly effective for detecting vertical cracks parallel to the running direction. FIG. 3(B) is a reduced version of the received light waveform when the incident angle is 45 degrees, and beak B is a defect signal.

Furthermore, even when the vertical distance from the subject 10 is small, heat resistance measures can be easily taken. Furthermore, there is no need to provide a large distance between the projector and the reflection point, and sufficient light reaches the reflection point regardless of the influence of dust that is abundant at the site.

Incidentally, if the angle θ1 is made too small, the perspective of the projected field of view becomes too strong, which may cause problems in addressing during focusing and signal processing.

On the other hand, as the angle θ1 becomes larger, it becomes less susceptible to the vertical movement of the subject 10, but it is closer to the F plane of the subject 10, which is disadvantageous especially in terms of heat resistance of the hot material. or,
When inspecting the lower surface of the object 10 without inverting it, it is also necessary to increase the depth of the bit for arranging the light projector 12. Therefore, as mentioned above, the angle is preferably within the range of 15 degrees to 55 degrees, and in particular, the range of 20 degrees to 50 degrees is more effective in practice.

Furthermore, when the polarization conditions were set appropriately in the above angle range, the S/N ratio of the defect signal was further improved. That is, the irradiation light irradiated by the light projection 1112 is linearly polarized, and the plane of polarization is adjusted to illuminate the subject 10.
The external light has a polarization plane parallel to the short side of the rod-shaped (band-shaped) field of view on the surface, that is, parallel to the running direction of the subject 10, and when receiving the reflected light from the subject 10, the polarization plane is When only the light of
! ! As shown by the solid line C in Fig. 1 (and as shown in Fig. 183 (C)), the polarization condition was not limited to the above example. A similar effect was obtained even when only light was received.

The present invention was made based on the above findings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a continuous casting slab surface flaw detection apparatus employing the metal object surface flaw detection method according to the present invention will be described in detail below with reference to the drawings.

In this example, as shown in Figure 4, continuous casting slurry 120
Continuously cast slabs are continuously cast so that the angle θ1 between the light incident direction and the normal to the surface of the continuous casting slab 20, which is disposed in an oblique direction orthogonal to the slab running direction on the side of the running line, is between 15 degrees and 55 degrees. A laser light source 22 for irradiating the surface of the casting slab 20 with external light having linear polarization characteristics, and a polarization plane of the laser light emitted from the laser light source 22 is set to a predetermined polarization plane
For example, a polarization plane rotator 24 for making the laser beam 22a have a polarization plane parallel to the short side of the bar-shaped (band-shaped) field of view of the continuous casting slab 20 (that is, parallel to the running direction of the continuous casting slab 20), and A laser beam 22a having a predetermined polarization plane output from the surface rotator 24 is directed onto the continuous casting slab 20! The angle θ1 is the angle between the cylindrical lens 26 for widening the field of view in a strip shape, the normal to the surface of the continuous casting slab 20 on the opposite side of the slab running line from the laser light source 22, and the direction in which the reflected light is received. A light receiving camera 3o equipped with one or more polarizing filters 28 for receiving a predetermined polarized light component of the specularly reflected light is placed at a specularly reflected light receiving position equal to Signal processing circuit 3 that processes the reflected light signal and outputs a defect signal
It is composed of 2. Figure 41. :, 34 is a laser beam a* disposed in the light receiving part of the light receiving camera 30.
This is an interference filter for removing the shadow of the self-luminous energy of the continuous casting slab 2o and improving detection accuracy by passing only the usable wavelength range of the laser beam 22a irradiated from the continuous casting slab 2o.

As the laser light source 22, for example, an argon laser with an output of 5 W can be used. Generally, when a hot object is the object to be tested, the self-luminous energy of the object has a strong energy component on the long wavelength side of infrared and visible light, so when receiving reflected light to obtain a defect signal, It is advantageous to project light of a short wavelength with as few self-luminous components as possible. Argon laser has a wavelength of #1 high output component near 5001B-, which falls under the relatively weak wavelength range of the self-luminous component of high-temperature steel materials such as continuous casting slabs. A laser is effective as a light source for surface flaw detection because it produces continuous and relatively strong protrusions.

As the light-receiving camera 30, for example, one in which an electronic scanning image sensor using a charge-coupled f-vis is disposed on the focal plane can be used. The lens of the light-receiving 7J camera is selected to have an optimum aperture depending on the distance between the subject and the light-receiving camera, the width of the band-shaped light projection surface, etc. Now, when inspecting a field of view width of 1 m+ using a sensor with 2048 probes, its geometric resolution is about 0.5-1.

The light projecting device including the laser light source 22, the polarization plane rotator 24, the cylindrical lens 26, etc., and the light receiving device including the polarizing filter 28, the light receiving camera 3o, the interference filter 34, etc. are all connected to the continuous casting slab 20. They are arranged at a sufficiently large distance in the diagonal direction, and are heat-resistant with gas or liquid so that they can be used continuously for a long time.

The action will be explained below.

Surface a! The continuously cast slab 20, which has a temperature of 500° C. or higher and is F, runs along the maxillary line at a substantially constant speed in the direction of the arrow, and is in a state where at least the surface to be inspected can be considered flat. The laser beam 22a emitted from the laser light source 22 has its polarization plane set to a predetermined polarization plane by the polarization plane rotator 24, and then is projected onto the continuous casting slab 20 in a band shape by the cylindrical lens 26. The radar light reflected by the test surface of the continuous casting slab 20 enters the light receiving camera 30 via the interference filter 34 and the polarizing filter 28,
The image of the band-shaped light is manually printed with one-dimensional information on the focal plane of the light-receiving camera 30, converted into a defect signal by the signal processing circuit 32, and output.

In this embodiment, the projector and 5. Since the laser light source 22 is used, the distance between the pass line between the laser light source 22 and the cylindrical lens 26 and the continuous casting slab 20, which is a high-temperature material, and the continuous casting slab 20
It is possible to widen the distance between the path line and the light receiving camera 3o, which is more advantageous than heat-resistant steel.In other words, the laser beam has strong directionality, and the beam is very Since it is small and has a very high energy density, there is almost no decrease in distance, and even if the cylindrical lens 26 is used to spread the energy laterally, a sufficiently high energy density can be obtained.
As a characteristic of laser light, it has a single wavelength component, so by attaching an interference filter 34 suitable for the laser to be used in front of the lens of the light-receiving camera 30 as in this embodiment, only the laser light can be isolated. It is possible to selectively receive light, and it is easy to effectively remove the influence of self-luminous energy. Note that the type of projector is not limited to this, and for example, a mercury lamp that projects white light can also be used.

In addition, the odor gate of this example is Rays Hikari i! The light from the i22 is projected onto the surface of the continuous casting slab 20 in a band shape, and the reflected light is received by an electronic scanning image sensor C to obtain an output signal. It can be much faster than conventional mechanical scanning. Therefore, even if the running speed of the subject is 1000-1/min or more, it is possible to respond. Also, compared to the case where the receiver is composed of a photomultiplier tube, silicon photocell, amplifier, etc.
The light receiver is small, making it easy to take shielding measures such as humidity, heat, and acid resistance. Furthermore, since there are no rotating parts in the flaw detection device as a whole, maintenance is easy.

In the above embodiment, the laser light source 22 was provided with the light plane rotator 24 and the polarizing filter 28 was provided in front of the light receiving camera 30.
However, in the case of a linearly polarized laser light source, the polarization plane rotator 24 can be omitted by carefully arranging its position.

Also, if the laser light source 22 is a randomly polarized laser light source or a white light source, it is also possible to add one or more polarizing filters to the front of the projector.

Further, the light irradiated by the light projector is external light having a polarization characteristic of 4, and a polarizing filter is disposed in front of the #2 light receiver so that only a predetermined light component of the scattered reflected light is received by the light receiver. It is also possible to configure...

in front! In Example C, the present invention was applied to the flaw detection of a continuously cast slab with high-temperature material, but the scope of application of the present invention is not limited to this. It is clear that the present invention can be similarly applied to online flaw detection of hot-rolled steel strips, online flaw detection of hot-rolled steel strips, online flaw detection of cold-rolled steel strips, steel plates, etc.

As explained above, according to the present invention, continuous casting souffle @
Significantly reduces surface defects on the object being inspected while driving.
It has the advantages of being able to detect ζ with high accuracy in terms of ratio, and also making it easy to take heat-resistant measures for the projector and receiver.

[Brief explanation of drawings]

Figure 11 is a perspective view showing the state in which the conventional surface flaw detection method is being performed, and Figure 2.1 shows the principle of the present invention. Figure 3 is a diagram comparing the relationship between the angle between the surface normal and the incident direction of the irradiation light and the S/N ratio of the defect signal detected from the change in the specularly reflected light (A> is the same as above). Figures 3 (B) and 3 (C) are cross-sectional views showing the state in which irradiation light is incident on the surface of a specimen with vertical cracks from an oblique direction. FIG. 4 is a diagram showing a comparison of changes in reflected light; FIG. 4 is a partial block diagram showing the configuration of an embodiment of the surface flaw detection equipment for a continuous casting slab in which the metal object surface flaw detection method according to the present invention is adopted. It is a perspective view including a diagram. 10... Subject, 10a... Vertical crack, 12
... Emitter, 14... Light receiver, 20...
Continuous casting slab, 22... Laser light source, 24... Polarization plane rotator, 26... Cylindrical lens, 28... Polarizing filter, 30... Light receiving camera, 3
2... Signal processing circuit, 34... Interference filter. Agent ^ Arrow Theory (1 other person) Figure 1 Figure 2 - Angle θ1

Claims (1)

[Claims] (1) In a metal object surface flaw detection method in which the surface of a moving object is irradiated with light from the outside and the reflected light from the surface of the object is received to detect surface defects on the object, the object is From a projector placed on the side of the line in an oblique direction perpendicular to the running direction of the subject, the light is projected so that the angle between the normal to the subject's surface and the direction of incidence of the irradiated light is 15 degrees to 55 degrees.
External light having a predetermined plane of polarization or external light having no polarization characteristics is irradiated onto the surface of the test object, and specularly reflected light or its predetermined value is received by a light receiver placed on the opposite side of the projector from the projector in the travel line of the test object. A method for detecting defects on the surface of a metal object, characterized by detecting surface defects on the object from changes in polarized light components.