JPS649582B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for continuously detecting non-planar flaws in a slab during continuous casting using an ultrasonic flaw detection method. [Prior Art] First, FIG. 1 shows a schematic diagram of the main types of internal defects that occur in continuously cast slabs (hereinafter simply referred to as slabs). These can be broadly classified into planar defects A, which occur parallel to the front and back surfaces of the slab, and non-planar defects B, which occur perpendicularly to the front and back surfaces of the slab. If we look at these flaws on the C-C' cross section (in the width direction of the slab), the L-L' cross section (in the length direction of the slab), and the Z-Z' projected cross section (in the thickness direction of the slab), we can see that Flaw A is shown in a, b, and c in Fig. 2, and non-planar flaw B is shown in a, b, and c in Fig. 3.
It appears like this. The definition of such defects is shown in Table 1 below.
Shown below.

[Problem to be solved by the invention]

The sulfur print method is a method for determining prints by corroding the polished surface of the cross section with a reagent, so processing is required and online determination is impossible, so offline determination has to be performed. This has the disadvantage that it takes a long time to find out. In particular, modern continuous casting machines are prone to non-planar defects that occur instantaneously due to machine troubles such as roll breakage, bearing damage, and clogged spray nozzles in the secondary cooling zone due to the trend towards high efficiency and high production. The amount of debris generated due to internal cracks (for example, internal cracks) increases dramatically. Therefore, there is an increasing need for early online detection of non-planar defects during high-speed casting. An object of the present invention is to detect non-planar flaws online using ultrasonic flaw detection. [Means for Solving the Problems] In the present invention, when detecting internal non-planar flaws in a slab during continuous casting of molten steel using an ultrasonic flaw detection method, the above-mentioned slab and an ultrasonic flaw detector are used. Ultrasonic waves with a frequency of 0.5 to 2.0 MHz are repeatedly projected onto the slab while at least one of them is moving relative to the other, and the attenuated waves of the ultrasonic waves returning from the back of the slab are , the return attenuation wave whose attenuation amount is more than a predetermined value and the generation density of this return attenuation wave in the direction of slab movement are detected, and this generation density is 1/(2 to 16).
Ultrasonic flaws with a diameter of 1/mm or more are detected as areas corresponding to non-planar flaws inside the slab. [Operation] As shown in the left column of Fig. 4, when an ultrasonic wave Fi is projected from an ultrasonic transmitter T toward a non-planar flaw in a slab, the reflected wave from the non-planar flaw is transmitted to an ultrasonic receiver R. The reception level is extremely low, making detection difficult.
The received signal at this time is shown in the left column of FIG. Since the projected ultrasonic wave Fi is scattered by the non-planar flaw, the reception level of the reflected wave _B1 from the back surface of the slab is also low. That is,
The amount of attenuation of the reflected wave (return attenuated wave) is large. For reference, the left column of FIG. 5 shows an ultrasonic flaw detection mode when there is a plane flaw, and the right column of FIG. 5 shows received waves. In the case of a plane flaw, the level of the reflected wave due to the plane flaw is high, and since the return attenuated wave is also transmitted through the plane flaw and received, the amount of attenuation is small. Comparing FIG. 4 and FIG. 5, it is possible to detect a planar flaw based on the amount of attenuation caused by the flaw, but a non-planar flaw cannot be detected depending on the reflected wave from the flaw. However, it seems possible to detect flaws based on the amount of attenuation of the returned attenuated wave. Here, as a first feature, the selection of the optimum frequency of the slab necessary for detecting each of the flaws will be explained with reference to FIG. 6. In Figure 6, (A) Straight line A shows the S/N (ratio of flaw signal level to noise) required for online flaw detection, (B) Curve B shows the correlation between the frequency used and S/N. (C) Curve C shows the ultrasonic attenuation characteristics depending on the casting composition. From FIG. 6, considering the range of high S/N and low attenuation, 0.5 to 2.0 MHz is the usable frequency range for online ultrasonic flaw detection of slabs. Therefore, in the present invention, as described above, ultrasonic waves with a frequency of 0.5 to 2.0 MHz are projected onto the slab, and non-planar flaws are detected based on the return attenuated waves of the ultrasonic waves from the back surface of the slab. Next, the second feature will be explained. Slabs that were demoted to a low-quality grade because they could not achieve the required quality, and slabs that became scrap that could not be demoted (i.e., the Sulfur Aprint rating, which is the current management index, is 2.0)
Regarding the above defective slabs, ultrasonic waves with a frequency of 0.5 to 2.0 MHz are repeatedly projected onto the slab while at least one of the slab and the ultrasonic flaw detector is moving relative to the other, FIG. 8 shows measurement data when a return attenuated wave having an attenuation amount of a predetermined value or more is detected among the return attenuated waves from the back surface of the slab due to the ultrasonic waves. The horizontal axis shows the width (mm) of non-planar flaws in the direction of slab movement, and the vertical axis shows the density of returned attenuated waves with an attenuation amount of a predetermined value or more (pieces/m; distance is in the direction of slab movement). . In this type of slab, the level of the returned attenuated wave shows a distribution as shown in FIG. 13 at non-planar flaw sites.
In FIG. 13, the horizontal axis represents the amount of movement of the slab relative to the ultrasonic flaw detector, and the vertical axis represents the reception level of the returned attenuated wave. B ₀ is the level of the returned attenuated wave at a portion without a non-planar flaw, and B ₁ to B ₆ are the levels of the returned attenuated wave at the non-planar flaw. Note that the vertical axis in FIG. 8 indicates the density of the returned attenuated waves B ₁ to _{B 6} at a level of 25% or less of B ₀ . Figures 13 and 8 show that in slabs that are demoted or scrapped, in the flaw detection method that detects non-plane flaws based on returned attenuated waves, if an ultrasonic flaw detector is placed on the non-plane flaws, is 2 to 16 mm against the flaw detector.
During the movement, the return attenuated wave B ₀ of the part without flaws
This means that return attenuated waves B ₁ to B ₆ that are attenuated to 25% or less appear. In other words, FIGS. 8 and 13 show that in a non-planar flawed part, the attenuated waves B ₁ to B ₆ at a level that is 25% or less of the level B ₀ of the attenuated return wave in a part without a flaw are ultrasonic waves. This means that one or more defects appear for every 2 to 16 mm of movement of the slab relative to the flaw detector. Therefore, in the present invention, as described above, a return attenuation signal wave whose attenuation amount is more than a predetermined value and the generation density of this return attenuation signal wave in the slab movement direction are detected, and this generation density is 1/(2 to 16 ) Detection of ultrasonic flaws of 1/mm or more as areas corresponding to non-planar flaws inside the slab. We will now consider this in more detail. Figure 7 shows the ratio (%) of non-planar defects detected by the above-mentioned flaw detection method of the present invention among the non-plane flaws detected by the sulfur print method, and the S/N ratio in the flaw detection method of the present invention. It is shown in a modified form. In the method of the present invention,
The detection rate is low at low S/N. However, in general, in online ultrasonic flaw detection such as the one targeted by the present invention, the higher the S/N, the better, and S/N≧3 is required due to the measurement environment and measurement equipment, and usually S/N≧3. It is. Therefore, when S/N≧3, as shown in FIG. 7, it can be seen that the flaw detection method of the present invention achieves detection accuracy that is almost equivalent to the detection accuracy of non-surface flaws by the sulfur print method. At S/N≧3 or higher, which is required in normal ultrasonic flaw detection, the flaw detection method of the present invention has the same detection accuracy as the sulfa print method, as shown in Figure 7.
The method for detecting non-planar flaws using the flaw detection method of the present invention can also perform an evaluation equivalent to the screening evaluation using the sulfur print method. In addition, we usually sulfur print the L-L' cross section of the slab, thereby detecting the presence or absence of flaws.
Based on the density and size of defects, the degree of defects in slabs is ranked from 0 (no defects) to 3.0 (with serious defects), and this is called an index that expresses the degree of presence or absence of defects. The index range of 0 to 3.0 is divided into 5 to 6 levels, from good to poor. An index of 2.0 means that there is a (normal) defect with a high probability of occurrence in slab manufacturing. Detecting non-planar flaws by the flaw detection method of the present invention,
Based on the detection results, the cast slabs were classified into ranges in the same manner as the range classification from good to poor in 5 to 6 stages based on sulfur print. Then, we detect flaws in the cast slab using the sulfa print method, and divide it into ranges in the five to six stages that we have conventionally used. Based on this, we calculated the match rate with the conventional range classification. FIG. 9 shows the above-mentioned matching rate for the conventional slab with the above-mentioned index of 2.0 based on the sulfur print method. The horizontal axis in FIG. 9 indicates the width of the non-planar flaw detected by the flaw detection method of the present invention in the direction of slab movement. As shown in Figure 9, by excluding the areas () and long areas () where the width of the non-planar flaws (in the direction of slab movement) is short, we can find non-flat defects with a width of 2 to 16 mm in the direction of slab movement. By specifying the flaw detection of the present invention to flaws, false detections and detection errors (failure to detect what was supposed to be detected) caused by the surface properties of the slab, such as oscillation marks, large vertical cracks, etc., can be avoided. , the match rate of detection judgment with sulfur print index of 2.0 or higher is maximized. [Example] Next, an ultrasonic flaw detection system used for implementing the present invention will be described. FIG. 10 is a block diagram of the apparatus system, and 1 is a power supply unit, which is a power supply for supplying DC power to the flaw detector unit and the data processing unit. 2 is a data processing unit that makes full use of a microcomputer to perform defect judgment output processing and interfaces with a process computer, recorder, and flaw detector. 3
is an FD unit, which is composed of parts that generate transmission pulses to the probe 4, amplify reflected echo signals, and A/D convert each analog signal of the reflected echo signals. A plurality of probes 4 are incorporated in the probe holding mechanism and have a sensor function for detecting reflected ultrasonic echoes. 5 is a CRT monitor that displays reflected echo signals, gate signals, and marker signals. 6 is the UST (ultrasonic flaw detection) panel control section, which is the control center and controls the UST mechanical device, UST water supply/air treatment device, operation monitoring panel,
It receives signals from process computers, electrical equipment, etc. and performs logical calculation processing. 7 is electrical equipment for the slab conveyance system, 8 is an operation monitoring panel that displays the status of the equipment,
Performs equipment abnormality alarms, displays signals from external equipment, and selects flaw detection mode. 9 is a front operation panel consisting of data input switches. 10
This is a recorder that records monitor data in analog and events, and uses a pen recorder. A process computer 11 provides instructions to the flaw detector and outputs digital values of data.
12 is a data output terminal of the process computer 11; Now, a case will be described in which the apparatus system configured as described above is applied to internal detection of a slab during casting.
FIG. 11 is an overall schematic diagram of the continuous casting apparatus, where A is a gas cutting device for slabs, B is an ultrasonic flaw detection device used for carrying out the present invention, and C is an offline quality check section. . FIG. 12 shows an embodiment of the present invention. Before use, the flaw detection device B performs automatic sensitivity calibration using the calibration sample 13, moves on the traversing carriage bridge 14, and waits on the conveyance roller table 15 in an automatic flaw detection mode. The slab S being transported on the transport roller table 15 in the direction of the arrow (from right to left) blocks the light from the laser master 16a, so that the process computer 11 controls the data processing unit 2 and the UST (ultrasonic flaw detection) panel. In part 6,
When the flaw detection instruction is transmitted and the tip of the slab S blocks the laser master 16b, the pulse generator 17 starts counting the pulses generated, and the spray device 18 is activated in order to cool the upper and lower surfaces of the slab and to purge scale. Then, after the flaw detection water is activated, the probe holding mechanism 19 activates the slab contact plate timing of the probe. The length of the flaw can also be recognized from this pulse counter value. Through the flaw detection control by the data processing unit 2 and the UST (ultrasonic flaw detection) panel control unit 6 working together, ultrasonic waves are repeatedly emitted from the probe 4 at predetermined short time intervals, and the returned attenuated waves are detected. Of the
Those whose attenuation amount is greater than a predetermined value are extracted, and their density is detected. That is, when a return attenuated wave having an amount of attenuation equal to or greater than a predetermined value is detected, the counter value of the pulse generated by the generator 17 at that time is read. After the flaw detection is completed, the magnetic sensor attached to the probe holding mechanism 19 detects the rear end of the slab S, thereby completing the flaw detection of the single slab S. When the flaw detection is completed, the difference between the read count values of adjacent ones in the reading order is calculated, and if the error is within the range of 2 to 16 mm, it is determined that there is a non-planar flaw. The position of the non-planar flaw is represented by the read count value. A plurality of probes are arranged at the center in the widthwise direction of the slab, and the above-mentioned processing is performed in parallel for each probe. Next, Table 2 shows a list of examples using this device.

【table】

〔Effect of the invention〕

As described above, according to the present invention, non-planar flaws can be detected online.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the appearance of the slab. Fig. 2 is a cross-sectional view of the slab, a is a sectional view showing the C-C' section of the slab shown in Fig. 1, and b is a cross-sectional view showing the L-
A cross-sectional view showing the L' cross section, and c is a Z-Z' projection view.
Figure 3 is a diagram showing non-planar flaws in the slab, a
1 is a cross-sectional view taken along the line C-C' of the slab shown in FIG. 1, b is a cross-sectional view taken along the line L-L', and c is a projected view taken along the line Z-Z'. FIG. 4 shows a cross-sectional view showing the arrangement of a probe for detecting return attenuated waves from the back surface of the slab due to non-planar flaws inside the slab, and a plan view showing the received waves. FIG. 5 shows a cross-sectional view showing a probe arrangement for detecting a reflected signal from inside the slab with respect to a plane flaw inside the slab, and a plan view showing received waves. FIG. 6 is a graph showing a selection range of frequencies that allow detection of non-planar flaws inside the slab. FIG. 7 is a graph showing the detection accuracy of the ultrasonic method based on the S/N in the ultrasonic flaw detection method for non-planar flaws and the flaw correspondence of the sulfur print method. FIG. 8 is a graph showing the relationship between the length of non-planar flaws in the longitudinal direction (L-L' cross section) of the slab and the generation density of return attenuated waves having an attenuation amount of a predetermined value or more. Figure 9 shows the concordance rate of slab evaluation based on the detection of the present invention with evaluation based on sulfa print, as a function of the length of non-planar flaws for slabs with a sulfa print index of 2.0 or more. It is a graph. FIG. 10 is a block diagram of one ultrasonic flaw detection system used for implementing the present invention. FIG. 11 is an overall schematic side view of a continuous casting apparatus system for implementing the present invention, and FIG. 12 is a plan view. FIG. 13 is a graph showing detection data by the non-planar flaw detection method of the present invention. 1: power supply unit, 2: data processing unit,
3: FD unit, 4: Probe, 5: CRT monitor, 6: UST panel control section, 7: Electrical equipment, 8: Operation monitoring panel, 9: Front operation panel, 10: Recorder,
11: Process computer, 12: Data output terminal, A: Gas cutting device, B: Ultrasonic flaw detection device, C: Quality check section, S: Slab, 13: For calibration, 14: Crosslinking, 15: Transport Roller table, 16a, 16b: laser master, 17: pulse generator, 18: spray device, 19: probe holding mechanism.

Claims (1)

[Claims] 1. In continuous casting of molten steel, when detecting internal non-planar flaws in a slab during continuous casting using an ultrasonic flaw detection method, at least one of the slab and an ultrasonic flaw detector is used relative to the other. Ultrasonic waves with a frequency of 0.5 to 2.0 MHz are repeatedly projected onto the slab while it is moving, and among the return attenuated waves of the ultrasonic waves from the back surface of the slab, the return attenuation wave whose attenuation amount is more than a predetermined value is generated. Then, the generation density of this returned attenuated wave in the direction of slab movement is detected, and the ultrasonic flaw detection area where the generation density is 1/(2 to 16) parts/mm or more is identified as a part corresponding to a non-planar flaw inside the slab. A method for detecting ultrasonic flaws in slabs during continuous casting.