JPH01110257A - Detection of cracking defect inside continuous cast piece - Google Patents

Abstract

PURPOSE:To detect a non-planar cracking defect at a high accuracy, by vibrating an end of a piece to be inspected to judge the arrival of a longitudinal wave extracting a high frequency vibration component of vibration at an end face as opposed to a vibrating surface. CONSTITUTION:An impulse impact signal containing an even frequency component is applied with a hammer 2 to a vibrating surface 1a of a piece 1 to be inspected such as stainless slab. A sensor 4 is fixed on a surface 1b to be detected as opposed to the vibrating surface 1a of the piece 1 being inspected to detect vibration caused in the piece 1 being inspected. After a high frequency component alone is extracted with a high pass filter, for example, exceeding 40kHz, a vibration detection output is inputted into a time difference detection circuit which calculates a time length to the detection of vibration from the time of applying a vibration. When the results coincide with the arrival time of a longitudinal wave, the absence of a cracking is determined and when it does with the arrival time of a lateral wave, a cracking is determined as present.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for detecting non-planar crack defects that have reached the inside and surface of a ferritic stainless steel slab obtained by continuous casting.

[Conventional technology]

When stainless steel slabs are manufactured by continuous casting, due to the physical properties unique to stainless steel, cracking defects occur inside the slabs, which are different from ordinary steel slabs.

FIG. 5 shows a schematic diagram of the main types of crack defects that occur inside stainless steel slabs obtained by continuous casting (hereinafter simply referred to as stainless steel slabs).

These are plane cracks (A) that occur parallel to the front and back surfaces of the stainless steel slab, and non-plane cracks (B) that occur perpendicularly to the front and back surfaces of the stainless steel slab.
) and (C).

Therefore, the crack is
When observed on the -L' cross-section (lengthwise direction of the slab) and the Z-2' projection plane (thickness direction of the slab), each cross-section has a different form.

Planar cracks (A) are as shown in (a), ra), and (C) in Figure 6, and non-plane cracks (B) are as shown in (a), ra), and (C) in Figure 7.
Similarly, non-planar cracks (C) are shown in Figure 8 (a).
), (6), and (C), respectively.

Furthermore, the definition of cracking at this time is shown in Table 1.

Table 1 Here, for ordinary steel slabs, (A) and (B)
is the main one, and (C) almost never occurs.

On the other hand, in the case of a stainless steel slab, the solidified structure is rough and embrittlement progresses as cooling progresses, so type (C) cracks (hereinafter simply referred to as cracks) are likely to occur.

One of the measures to prevent this occurrence of cracking is to maintain the slab at high temperatures. However, in the case of stainless steel, the scale-off in the rolling heating furnace is small, so the surface texture of the slab directly affects the finished product. Therefore, it is necessary to take care of the slab surface.

Such slab maintenance can be broadly divided into solvent maintenance and grinder maintenance.

Among these, solvent treatment is generally adopted for ordinary steels other than those with high crack susceptibility such as high carbon steel.

However, in the case of stainless steel, it is impossible to clean it with a solvent using the iron powder method, which utilizes melting through oxidative combustion of iron. Therefore, a grinder is used to clean stainless steel. Cleaning using a grinder is mainly performed in cold or warm conditions, and hot grinding requires appropriate equipment measures. In particular, in the case of stainless steel slabs, minute defects can be a problem, so care and inspection are strict enough to require sensory inspection by humans, and inspection in the state of high-temperature slabs is difficult.

In addition, stainless steel generally has a coarse cast structure, and intermetallic compounds and carbides precipitate at grain boundaries, causing segregation and
Defects such as microcracks are likely to occur. As a result, the toughness is inherently low and the core of the slab is particularly susceptible to cracking. In addition, the Charpy impact energy is 200
The transition temperature is around ℃.

These tendencies and physical properties of stainless steel, combined with the thermal stress generated in the slab, cause the above-mentioned placement cracks to occur. That is, in general, the temperature distribution of a slab is highest at the center of thickness and center of width, and lower at the cutting edge. When a stainless steel slab with this temperature distribution is uniformly cooled and its temperature drops, thermal stress is generated in accordance with the temperature difference. Then, in the process of cooling down, the material passes through a region with high crack susceptibility, so cracks caused by thermal stress, that is, place cracks occur. If these cracks are present, they propagate to the surface in the heating furnace before rolling and during rough rolling, causing troubles such as plate breakage, which requires a long time to deal with.

Therefore, in the past, in order to minimize the occurrence of plate breakage during rolling, slabs with the same charge as the slab that caused the trouble were tested in advance by blooming, and then refinished. Then, it was decided that the final rolling would be carried out.

In order to avoid this extra step, methods have been developed to detect cracks inside the slab, and ultrasonic flaw detection is generally used. For example, Japanese Unexamined Patent Publication No. 59-52750 discloses that a vertical probe is disposed at the end of a steel plate to propagate longitudinal waves in the width direction of the plate to detect defects.

[Problem that the invention seeks to solve]

However, in the case of stainless steel, the ultrasonic waves are attenuated rapidly due to the generally brittle cast structure, and a large amount of noise is mixed into the detection signal, complicating the electrical processing. Moreover, in the case of ultrasonic waves, it is easily affected by the surface properties of the slab (such as unevenness caused by mold vibration, etc.), and even after cleaning and finishing, unevenness remains especially due to partial cleaning, making it difficult to use this method. .

Therefore, an object of the present invention is to appropriately detect crack defects inside a cast slab, which are unique to stainless steel.

[Means for solving problems]

In order to achieve this objective, the internal crack detection method of the present invention applies an impact force of a predetermined strength to the end of the slab, and directs the vibration of the slab excited by this impact force to a direction opposite to the excitation surface. extracting only the high frequency vibration component from this detection output, monitoring the time from the time when the impact force is applied to the time when the vibration of the high frequency vibration component rises,
When the time corresponds to the arrival time of longitudinal waves, it is determined that there is no cracking defect, and when the time corresponds to the arrival time of transverse waves, it is determined that there is a cracking defect.

[Effect]

When vibration is applied to a rigid body, longitudinal waves are generated due to the density of particles in the rigid body in the same direction as the direction of vibration.

In addition, transverse waves are also generated due to the displacement of particles in the direction perpendicular to the direction of vibration. Therefore, vibrations transmitted through a rigid body consist of longitudinal waves and transverse waves. The propagation speed of longitudinal waves in the slab is 5900 m/s, and that of transverse waves is 3200 m/s.
/second.

As a method for detecting cracks inside a rigid body, highly directional high-frequency components are extracted from the vibration detection signal of the surface facing the excited surface. If a space such as a crack exists between the straight line connecting the excitation point and the detection point, no movement of particles due to density will occur, and no longitudinal waves will be transmitted. On the other hand, transverse waves propagate regardless of the strength of the directivity, because even if there is space between straight lines, if there is a continuous part, the particles will be distorted. Therefore, when a highly directional high frequency component is extracted, if a crack exists, longitudinal waves will not be transmitted and will not be extracted, and only transverse waves will be extracted.

As mentioned above, the propagation speed of longitudinal waves is different from the propagation speed of transverse waves, and longitudinal waves propagate in about 1.8 times longer than transverse waves.

Therefore, the presence or absence of longitudinal waves can be determined based on the time from excitation to the detection of vibration, and thereby the presence or absence of cracks in the rigid body can be determined.

〔Example〕

Hereinafter, features of the present invention will be specifically explained based on embodiments shown in the drawings.

Figure 1 is a diagram showing an example of cracks existing inside a slab.
FIG. 2 is a cross-sectional view of a portion of the slab shown in FIG. 1 including cracks, divided in the X direction.

As shown in FIGS. 1 and 2, one end surface of the stainless steel slab 1 is an excitation surface 1a, and the other end surface is a detection surface 1b.
shall be. An impulse shock signal containing a -like frequency component is applied from the excitation surface 1a by a normalizer 2. Hammer 2 has
A load cell sensor 3 is attached that detects an excitation force, detects an electric signal, and generates an output. Excitation surface 1a
A sensor 4, such as a piezoelectric sensor or an electrodynamic sensor, is fixed on the detection surface 1b facing the stainless steel slab 1, which detects vibrations generated in the excited stainless steel slab 1 and generates a detection output as an electric signal. That's what I do.

FIG. 3 is a block diagram for determining the presence or absence of internal cracks in the stainless steel slab 1 based on the detection outputs s and S2 of the sensors 3.4. The output s of the sensor 3.4 mentioned above,
S2 is amplified by amplifiers 5 and 6, respectively. The vibration detection output S2 of the stainless steel slab 1 is, for example, 40 k.
After extracting only high frequency components using a high-pass filter 7 of Hz or higher, the components are manually inputted to a time difference detection circuit 8 that calculates the time from the time of vibration until the vibration is detected.

On the other hand, the length of the stainless steel slab 1 to be inspected is determined by the judge 9
The arrival time of the longitudinal wave and the arrival time of the transverse wave from the excitation point to the vibration detection point of the stainless steel slab to be inspected 1 are calculated manually, and compared with the output of the time difference detection circuit 8 described above, the arrival time of the longitudinal wave is calculated. If it matches or approximates, it is determined that there is no crack, and if it matches or approximates the arrival time of the transverse wave, it is determined that there is a crack.

Figure 4 shows the excitation force detection signal (a-1), (a-
2), Vibration detection signals (b-1), (b-2), and signals (c-1) and (c-2), which are obtained by extracting only high frequency components from the vibration detection signal using a high-pass filter 7, respectively. And (a-1).

(b-1), (C-1) are output examples of slabs without cracks, (a-2).

(b-2) and (c-2) are output examples of slabs with cracks.

The slab length of one of the stainless steel slabs used in this experiment was L = 5415 m, and the theoretical longitudinal wave propagation time was T
p = 0.918 ms, and the transverse wave propagation time is T s −
It is 1,692ms. Looking at the signal waveform (C-1), the rise time of the signal waveform with only high frequency components extracted is 0.9570 m5 from the excitation time, which corresponds to the propagation time of the longitudinal wave, so there is no cracking. It can be determined that it is a stainless steel slab. The slab length of the other stainless steel slab is L = 5350 m,
Similarly, the theoretical longitudinal wave propagation time is T, =0.907 m
s, the transverse wave propagation time is T5=1.672m5. Looking at the signal waveform (C-2>), the rise time of the signal waveform is 1.5586 m5 from the excitation time, and this is the arrival time of the transverse wave and not the longitudinal wave, so this slab It can be determined that there is a crack inside. By extracting the high frequency component of the excited vibration in this way, it is possible to determine whether the propagated vibration is a longitudinal wave or a transverse wave, and it is possible to determine whether there is a crack or not. can be clearly detected.

In addition, compared to ultrasonic flaw detection, the method of the present invention enables detection at a warm temperature, and is capable of detecting not only slabs but also rigid bodies with internal cracks. is also free.

〔Effect of the invention〕

As explained above, in the present invention, the end of a continuously cast slab is vibrated with a predetermined strength, and an excitation signal is detected at the end face opposite to this excitation surface, and the excitation signal is detected inside the slab. The presence or absence of a non-planar crack is determined based on the difference in arrival time between longitudinal waves and transverse waves caused by the presence or absence of a non-planar crack. This solves the problems of sensor installation and sound wave attenuation, and allows cracking defects inside the slab to be detected with high precision.

[Brief explanation of the drawing]

Fig. 1 is a perspective view for explaining the present invention in detail, Fig. 2 is a side view thereof, Fig. 3 is a block diagram showing an example of an electric circuit for processing the output of the sensor, and Fig. 4 is a A graph showing the difference in output signals when there is no defect and when there is a defect. Figure 5 is a perspective view showing the types of defects that occur in stainless steel slabs. Figures 6 to 8 are explanatory diagrams showing the state of each defect. be. Patent applicant Nippon Steel Corporation Representative
Masu Kobori (and 2 others) Figure 1 Figure 2 Figure 4: Sensor Figure 3

Claims (1)

[Claims] 1. Apply an impact force of a predetermined strength to the end of the slab, detect the vibration of the slab excited by this impact force at the end face facing the excitation surface, and select high-frequency vibration from this detection output. Only the component is extracted, and the time from the time when the above-mentioned equilibrium force is applied to the time when the vibration of the high-frequency vibration component rises is monitored, and when the time corresponds to the arrival time of the longitudinal wave, it is determined that there is no crack defect. . A method for detecting a crack defect inside a continuously cast slab, characterized in that when the time corresponds to the arrival time of a transverse wave, it is determined that a crack defect exists.