JPH0423744B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for detecting surface flaws in a hot slab, and more particularly to an improvement in an optical flaw detection method using a laser beam. [Prior Art] In recent years, it is well known that direct rolling, in which slabs produced by continuous casting are directly sent to a rolling process in a hot state, is being adopted from the viewpoint of energy saving, labor saving, etc. In this way, when hot slabs are sent directly to the rolling process to produce finished products, the size and location of flaws that have occurred in the slabs are detected and quickly removed through hot maintenance. Depending on the state of occurrence of flaws, it is essential to quickly determine whether the product has turned into scraps or the like to prevent the production of defective products after rolling. However, in order to reliably detect defects in hot slabs, it is necessary to remove the solidified matter such as scale formed on the surface of the hot slab, or the deposits such as oil, dust, ash, etc. before using the laser beam. If it is not scanned and detected, abnormal noise will occur and detection accuracy will be extremely reduced. Therefore, in order to eliminate the drawbacks of the conventional optical flaw detection method described above, for example,
As disclosed in Japanese Patent Laid-Open No. 60228, a method of descaling a hot slab using high pressure water in a descaler and then scanning it (hereinafter simply referred to as the high pressure desqueping method), or as disclosed in Japanese Patent Application Laid-open No. 117264/1983, For example, a method is used in which a hot slab is melt-cut to a surface layer of about 1 mm using a hot scarf, and then the slab is scanned without bending (hereinafter simply referred to as the hot scarf method). However, these conventional methods cannot be said to be sufficient as methods for detecting flaws in hot slabs for the reasons described below. First, the high-pressure desqueination method does not sufficiently remove scale and other adhered substances from the surface of the hot slab, which tends to cause defects.Also, impurities in the injection water condense again, and the equipment cost is quite high. There are other problems. On the other hand, the hot scarf method also has problems such as a decrease in slab yield or the occurrence of unevenness on the hot slab surface due to hot scarf spots. [Object of the Invention] The present invention is directed to removing spots such as scale, recondensation of impurities in the sprayed water, etc. by processing for surface flaw detection.
It is an object of the present invention to provide a method for detecting flaws in a hot slab that does not cause unevenness on the surface of the hot slab. [Structure and operation of the invention] In the present invention, surface flaws in a hot slab obtained by continuous casting are detected by scanning with a laser beam. Scan the laser beam to detect surface flaws within 0.5-6.0 minutes. The present inventors have discovered that when a laser beam is scanned over the slab surface as a method for detecting defects in hot slabs, the defects caused by the laser beam are detected simply by scale adhering to the slab surface or by irregularities on the slab surface. The output (S)/normal output (N) in a certain case, the so-called signal S/noise N (S/N ratio), does not change, but the phosphates contained in the slab cooling water absorb the laser light. It was found that brush polishing has the same S/N ratio as in the case of cast slab defects, and that brush polishing is effective in removing phosphates together with scale and other deposits in hot slab conditions. As a result, we have proposed a method for detecting hot surface flaws on slabs that solves the problems of the conventional method mentioned above and makes it possible to detect micro flaws with extremely high accuracy. . However, subsequent research by the present inventors revealed that flaws can be detected in brush-polished hot slabs even if the elapsed time from polishing to flaw detection is longer than in conventional methods. I found out. Hereinafter, the flaw detection method according to the present invention will be explained in detail based on an embodiment shown in the drawings. FIG. 1 shows an embodiment of the method for detecting hot flaws in a cast slab according to the present invention, and FIG. 2 shows the surface condition of the hot slab before polishing treatment. First, the continuous casting slab 15 is cast into a ladle 2 and a tundish 3 placed on a turret 1, as is known.
After solidifying the molten steel poured into the mold and cooling support device (not shown), the molten steel is supplied to, for example, No. 1 strand line 4 and No. 2 strand line 5, respectively. ，
6', the surface is sufficiently polished by brush polishing 7, 7', and the cut surface is deburred by device 8,
After removing the burr at step 8', marking device 9,
After marking at step 9', the flaw detection device 10 scans. At this time, the slab 15 as the material to be inspected has poor surface properties and produces considerable noise harmful to flaw detection. As shown in Fig. 2, the causes include scale irregularities 11, water droplets, and oil droplets 11, which can be visually determined.
2. There are gas spatters 13 and the like during cutting, and although an improvement can be seen to some extent by removing these, it is not perfect for cutting out harmful noise in the laser photodetector. The reason is that phosphates are generally used as rust preventive agents in industrial water, and when this phosphate 14 dries and remains on the red-hot slab 15,
This is because even on a smooth surface, uneven reflections occur due to absorption of laser light. In addition, as is well known, hot slabs (surface temperature 600
℃~1250℃) is chemically active and always covered with iron oxide (FexOy) or scale in air. This scale can be forcibly removed using a scale breaker using high-pressure water, but as long as the hot slab is in an air atmosphere, oxidation of the steel surface will begin immediately even if the scale is removed. Then, new scale is generated, floats up on the casting surface, spreads in spots, and causes a drop in the temperature of the scale itself, making it impossible to perform good flaw detection using the optical flaw detection method. Therefore, for example, as a measure against regeneration scale,
There is a method of detecting flaws within a time range of 3 seconds to 20 seconds after passing through a descaler, as disclosed in Japanese Patent Application Laid-open No. 54-60228, but as shown in Fig. 2, it is necessary to install equipment after the continuous casting oxygen gas cutter 6. In many cases, there are restrictions regarding this, and it is nearly impossible to detect flaws within 20 seconds. For example, the time required from the brush polishing 7 to the flaw detection device 10 is approximately 50 minutes on the No. 1 strand line 4 side.
It takes approximately 3 minutes on the No. 2 strand line 5 side, and it would require a large amount of equipment cost to perform flaw detection by connecting it to the slab cutting device without interfering with the inherent high productivity of the continuous casting device. In addition, it has become impossible to perform highly accurate detection. Therefore, when brushing a hot slab to remove oxidized scale, phosphates, oil, etc.
An extremely thin base scale 16 is left on the base steel, and the unevenness, thick scale, phosphates, and oil of the scale are removed to ensure a surface quality that allows sufficient flaw detection with a small diameter laser beam. Moreover, even with the base scale 16 left on the hot slab surface even in an air atmosphere, by suppressing scale formation due to oxidation of the slab surface, the allowable time from polishing to flaw detection is greatly extended. If the base scale 16 is brush-polished to a thickness of 0.8 mm or less, preferably 0.5 mm or less, desirable detection accuracy can be obtained and the allowable time can be extended. Here, in the hot flaw detection method according to the present invention, the allowable time from the surface brush polishing of the hot slab to the flaw inspection is preferably 0.5 to 6.0 minutes. Erroneous detection occurs due to linear marks (surface irregularities) on the surface of the hot slab. Furthermore, if the inspection exceeds 6 minutes, reoxidized scale is rapidly formed in addition to the base scale, leading to scale spots, and as described above, the detection accuracy becomes extremely poor. Therefore, for the reasons mentioned above, the allowable time from brush polishing to flaw inspection is preferably within 0.5 to 6 minutes.
Figure 3 also shows the slab surface temperature F and descaling temperature F ₁ by high-pressure water in brush polishing according to the present method as surface treatment of hot slabs, and the scale generation rate G in high-pressure water and descaling in the case of brush polishing. The test results of the scale generation rate G′ during scaling and the optimal allowable flaw detection time associated with this are shown. The effect of suppressing reoxidation generated scale by base scale 16 is extremely large, and the extension of the allowable flaw detection time (H) As a result, it has become possible to detect minute defects without impeding improved detection accuracy, simplification of the detection device, and high productivity. Furthermore, as shown in Fig. 4, the conditions for polishing with a brush include, for example, the amount M value of polishing (brushing) with a brush at one point on a piece of slab is 200 to 200.
200,000H is preferable; if it is less than 200, unevenness will occur in the brushing area, if it exceeds 200,000, problems such as the life of the brush will occur, and if it is more than 300,000, the surface of the slab will become mirror-like due to excessive brushing, and the laser beam will be totally reflected. Since diffused reflection disappears, flaws can be detected. Here, the M value will be defined. As shown in FIG. 5, first, when the slab 14 is conveyed in the E direction and attention is paid only to the brush roll 6, the number of brushes passing through the point O per unit time is m=V'×ρ. Next, the contact time between the brush roll and the slab to reach points O to O' is given by t=l/V. Therefore, the total number of brushes that point O encounters while passing through the contact area between the brush roll and slab can be defined as M=m·t=(πDN/60)×ρ×(l/V). However, m: Number of brushes passing point O in unit time (pieces/
sec) V': Peripheral speed of brush roll (mm/sec) ρ: Linear density of brushes (pieces/mm) t: Time to reach point O to point O' (sec) l: Contact length of brush roll O ~O' (mm) V: Conveying speed of the slab 14 (mm/sec) N: Number of rotations of the brush roll (rpm) D: Outer diameter of the brush roll (mm) M: Amount of polishing by the brush at a certain point (brush number).

[Examples and effects]

As an embodiment of the hot flaw detection method according to the present invention
Table 1 shows a comparison between the conventional method and the conventional method when used for hot slabs of a 300T continuous casting machine.The method of the present invention does not cause a drop in the temperature of the slab, and has very few false positives, so it is highly accurate. It can be seen that this method is an excellent method for detecting minute defects on paper. As described above, by using the method for detecting flaws in hot slabs according to the present invention, there is no removal of scale or other flaws, and there is no need to reduce the temperature of the slab due to descaling water and prevent steps due to slab melting (machine scarfing). This method suppresses retaining loss and obtains an extremely smooth surface of the cast slab.Moreover, since the allowable time range for flaw detection is large, the position of the detection device is not restricted, so high detection accuracy is obtained even with a low-cost device. This is an extremely excellent hot flaw detection method that can detect minute flaws with high precision, which was previously impossible, and can also efficiently achieve high efficiency in casting lines and direct hot feed in rolling lines.

[Brief explanation of drawings]

Fig. 1 is a plan view showing the configuration of an apparatus for carrying out one embodiment of the present invention, Fig. 2 is a vertical cross-sectional view of an untreated hot slab before detection of surface flaws, and Fig. 3 is a diagram showing the surface temperature of the slab and flaw detection tolerance. A graph showing the relationship between time periods, comparing the method of the present invention and high-pressure descaling. Figure 4 is a graph showing the relationship between the M value and S/N during brushing. Figure 5 is a graph showing the relationship between the brush and slab. It is a longitudinal cross-sectional view showing a contact state. 1: Turret, 2: Ladle, 3: Tundish, 6: Brush roll, 7: Polishing device, 8: Deburring device, 9: Marking device, 10: Flaw detection device, 11: Irregularity scale, 12: Water drop, oil drops, 1
3: Gas spatter, 14: Phosphate, 15: Slab, 16: Base scale, F: Slab temperature, G: Uneven scale reoccurrence time, H: Flaw detection allowable time range.

Claims (1)

[Claims] 1. A laser beam that projects and scans the surface of a hot slab obtained by continuous casting, receives the reflected laser beam from the surface of the slab, and detects surface flaws based on its intensity. In the hot flaw detection by: The surface of the hot slab is polished with a brush so that a base scale of 0.8 mm or less remains, and the hot flaw detection is performed within 0.5 to 6.0 minutes from this brush polishing. A method for detecting hot defects in slabs during continuous casting.