JPS61277052A - Eddy current flaw detection test for steel plate - Google Patents

Abstract

PURPOSE:To enable the eddy current flaw detection of a steel plate at a high S/N ratio, by setting the diameter, test frequency, lift-off and phase difference of a flaw detection probe to specified values. CONSTITUTION:In the diameter of a flaw detection probe, a flaw detection probe with a diameter of 5mm has the highest detection capacity to a hot rolled steel plate S in order to detect the minute flaw of the hot rolled steel plate S and, in this case, said diameter is set to a range of 2.5-6mm. Sensitivity is well as lift-off, that is, the distance between the flaw detection probe 42 and the hot rolled steel plate S becomes small and, in order to stably detect a flaw, lift-off (l) is set to a range of 0.4-1.2mm. In usual, it is desirable that test frequency, that is, the AC frequency imparted to the flaw detection probe 42 is high in order to perform the flaw detection of a minute flaw but, contrarily, noise is also easy to enter said probe 42. Detection capacity is best at frequency of 256kHz with respect to the minute flaw of the hot rolled steel plate and frequency is set to a range of 100-300kHz. The phase difference between the AC applied to the flaw detection probe 42 and the flaw signal from the flaw detection probe 42 is set to 100-160 deg..

Description

[Detailed description of the invention] (Industrial application field) This invention relates to an eddy current testing method for steel plates.

(Prior art) In general, surface flaw inspection of hot-rolled steel sheets involves sampling one out of ten hot coils immediately after rolling and winding, unwinding a portion of the coil using unwinding equipment, and visually inspecting it. This method is common.

However, since scale is generated on the surface of the hot coil, it is impossible to detect microscopic paper (chill hege flaws, scale flaws, etc.) hidden inside the hot coil by human visual inspection.

Therefore, when the surface scale is removed in a post-process pickling line or the like, flaws under the scale are discovered, causing process confusion.

For this reason, there is a need for a surface flaw detection device that can detect minute paper defects in place of human visual inspection.

Eddy current detectors are excellent for detecting flaws on the surface and surface layer of conductors and are used in a wide variety of applications. When detecting flaws using this eddy current detector, the size of the flaw detection probe (diameter), -
The distance between the flaw detection probe and the object to be inspected, that is, the steel plate, the test frequency, and the phase difference between the human input signal and the output signal are important issues, and optimizing these is important for stable detection.

However, in eddy current testing of hot rolled steel sheets, the hot rolled steel sheet is transported longitudinally and detected using a mutual induction self-comparison probe, but noise is generated due to the vibration of the steel sheet due to transportation, the surface condition of the steel sheet, the form of flaws, etc. .

Therefore, prior to flaw detection work, a comparison test piece is used, and the SN
Set the test frequency and phase angle for the best ratio. Note that detecting flaws while feeding a metal plate is known, for example, in Japanese Patent Application Laid-Open No. 55-103459.

(Problems to be Solved by the Invention) Conventionally, as a method for eddy current flaw detection of hot rolled steel plates, there was no effective method for maintaining a constant distance between the flaw detection probe and the object to be inspected, that is, the steel plate, making it difficult to implement. .

Therefore, in the conventional method, flaw detection test conditions other than the above-mentioned test frequency and phase angle were set without particular consideration.

For this reason, depending on the flaw detection test conditions, the signal-to-noise ratio varies, and flaws may be overlooked, making it impossible to fully guarantee the quality of the product.

Therefore, the present invention aims to provide an eddy current flaw detection test method that can detect surface flaws on steel plates with the best signal-to-noise ratio.

(Means for Solving the Problems) The eddy current flaw detection testing method of the present invention performs an eddy current flaw detection test while feeding a steel plate using a mutual induction self-comparison 5 deep flaw probe. Then, the diameter should be 2.5~2.5 to give the best S/N ratio
Using a Gl1 layer flaw detection probe, the test frequency was set to 1011.
1-300kHz, lift-off 0.4-1.2 am
and the phase difference is set to 100 to 160 degrees. Here, lift-off indicates the distance between the lower surface of the test coil and the surface of the steel plate, and phase difference indicates the phase difference between the input signal (reference voltage or oscillator output voltage) and the output signal (signal voltage).

(Operation) Eddy current flaw detection of steel plates is performed with a high signal-to-noise ratio by using specified values of the flaw detection probe diameter, test frequency, lift-off, and phase difference. Therefore, even minute scratches such as Chirhege scratches can be detected.

(Example) Fig. 1 and Fig. 2 show a part of an eddy current flaw detection test device that implements the present invention, Fig. 1 is a side view of the entire flaw detection head, and Fig. 2 shows details of the housing. It shows.

A head body 2 of the flaw detection head I is supported by a frame 3 directly above the hot rolled steel plate S. The head body 2 includes a cylindrical shaft 5 that rotates around a vertical axis, and within the cylindrical shaft 5 are a signal wiring conduit and a conduit (none of which are shown).
is installed. Further, a reduction gear 7 is connected to the cylinder shaft 5 via a fluid supply rotary joint 6, and a fluid supply iK (not shown) is connected via the fluid supply rotary joint 6 and a fixed pipe 9. The rotational drive AC motor 8 rotationally drives the cylindrical shaft 5 via the reducer 7 .

Further, the flaw detection head 1 includes a signal wiring section 11 and a non-contact rotating transformer section or slip ring 12, which transmit and receive signals to and from the cylinder shaft 5. The signal wiring 12 is connected to an input/output section (not shown) of the flaw detector.

A horizontally extending arm 15 is attached to the lower end of the cylinder shaft 5, and a housing 21 is fixed to each end of the arm 15. A fluid supply hole (not shown) is provided in the arm 15 .

The housing 21 has a cylindrical shape, with a closed top and an open bottom. A fluid supply pipe 27 extends concentrically with the housing 21 from the top of the housing 21 to near the bottom end. The fluid supply pipe 27 communicates with the supply hole. A plurality of guide holes 23 extending vertically are provided in the peripheral wall 22 of the housing 21, and a plurality of small holes 24 opening inward are provided near the lower end of the peripheral wall 22. The upper end of the guide hole 23 communicates with the supply hole, and the lower end communicates with the small hole 24.

The casing 31 has a double cylindrical shape, and the outer peripheral surface is connected to the lower part of the housing 21, and the fluid supply pipe 27 is connected to the through hole 32 in the center, and the gap between the connected parts is 1.
It is set to layer - degree. In this example, the casing 3
1 has an outer diameter of 52 mm and a height of 45 mm. Also,
The outer diameter of the fluid supply pipe 27 is 10 wm, and the diameter of the through hole 32 is 12 mm. Therefore, the casing 31 is movable up and down, left and right, and can be tilted within the range of this gap. In the through hole 32 of the casing 31, a portion below the lower end of the fluid supply pipe 27 is a fluid reservoir 33. A flaw detection probe 42 is built into an annular portion between an outer wall 35 and an inner wall 36 of the casing 31 . Hereinafter, the casing 31, the flaw detection probe 42, and the AGC detection probe 43 will be referred to as the fluid float section 30.

The flaw detection head l is attached so that the lower end surfaces of the casing 31, the flaw detection probe 42, and the AGO detection probe 43 are aligned when the fluid float portion 30 is at the highest position. Therefore, the length of the fluid supply pipe 27 is such that the lower end of the fluid supply pipe 27 is slightly higher than the lower end surface of the housing 21 (approximately 3
mm).

In order to minimize the effects of transport vibrations, changes in ambient temperature, etc., the flaw detection probe 42 uses a mutual induction self-comparison type flaw detection probe. The flaw detection probe 42 is connected to the signal wiring, and its lower end face faces the surface of the hot rolled steel plate S. A
The GO detection probe 43 is a distance correction detector, and the GO detection probe 43 is a distance correction detector.
If a change occurs in the distance between the flaw detection probe 42 and the flaw detection probe 42 within a short period of time, the detection signal is compensated.

In order to detect minute flaws in the hot rolled steel sheet S, the diameter of the flaw detection probe must not be too large or too small. Generally, the flaw detection probe 42 should be large enough to fit within the diameter of the flaw, and the flaw detection probe 42 should not fit inside the flaw.With these considerations in mind, flaw detection probes 42 of various diameters are manufactured. The test results are shown in Figure 3.As is clear from Figure 3, a flaw detection probe with a diameter of 5cm has the highest detection ability for the hot rolled steel plate S, and a flaw detection probe of 2.5 to 6cm is appropriate. This is a range.

The eddy current flaw detection test is performed on the hot rolled steel sheet S while it is running. When the cylinder shaft 5 is rotationally driven by the rotational drive AC motor 8, the arm 15 rotates. The flaw detection probes 42 at both ends of the arm 15 scan the surface of the hot rolled steel sheet S in the width direction while drawing a circular arc.

When fluid (compressed air) is supplied to the fluid reservoir 33 of the through hole 32 of the fluid float section 30 through the fluid supply pipe 27, the supplied fluid is temporarily stored in the fluid reservoir 33 and is ejected onto the surface of the hot rolled steel sheet S. As a result, the fluid float section 3
0 floats to a certain height due to buoyancy and moves up and down according to the waves or undulations of the hot-rolled steel sheet S.

The shorter the distance between the lift-off probe 42 and the hot-rolled steel sheet S, the better the sensitivity. However, if they are brought too close, there is a risk that the hot rolled steel plate S and the flaw detection probe 42 will collide.

FIG. 4 shows an example of the relationship between lift-off observation and S/N ratio. As is clear from this figure, in order to stably detect flaws, the lift-off height is preferably in the range of 0.4 to 1.2 mm. 33, and the larger the diameter d and depth h of this, the larger the lift-off sentence becomes. As mentioned above, lift-off or holding at an optimum value is important. Therefore, in this embodiment, the diameter d of the fluid reservoir 33 is made as large as possible to 48 mm, and the height h is set to 5 mm to maintain a constant lift-off angle of about 0.6 ffiffl regardless of the pressure of the supplied fluid. ing. Note that by appropriately selecting the dimensions of the fluid reservoir 33, the lift-off position can be maintained substantially constant regardless of the pressure of the supplied fluid, as shown in FIG. - The fluid ejected from the small hole 24 of the housing 21 forms a high-pressure fluid layer around the float part 30. As a result, the casing 31 is held substantially concentrically with the housing 21,
It does not come into contact with the housing 21 or the fluid supply pipe 27.

Generally, it is desirable that the test frequency, that is, the frequency of the alternating current applied to the flaw detection probe 42, be higher in order to detect minute flaws, but conversely, noise is more likely to enter. Taking these things into consideration, we conducted a frequency-based flaw inspection test and presented the results in the sixth section.
As shown in Figure 6, for fine defects on hot rolled steel sheets, 25
It can be seen that 8 kHz has the best detectability. From a practical point of view, the test frequency may be in the range of 100-300kHz.

In addition, for the flaw detection ratio, it is desirable to optimize the phase difference between the AC applied to the flaw detection probe 42 and the flaw signal from the flaw detection probe 42 depending on the type of flaw. FIG. 7 shows the relationship between the magnitudes of the signals. As is clear from FIG. 7, a temperature of 100 to 180 degrees is optimal for hot rolled steel sheets.

(Effects of the Invention) As described above, an example in which the invention is applied to a hot-rolled steel plate has been described; however, the present invention is not limited to only a hot-rolled steel plate;
It can be widely applied to surface-treated steel sheets, and steel sheets can always be subjected to eddy current testing with the best signal-to-noise ratio. therefore,
It is possible to detect even the smallest defects without overlooking them, and it is possible to provide products of excellent quality.

[Brief explanation of drawings]

Fig. 1 is a side view showing an example of a flaw detection head, Fig. 2 is a detailed view of the fluid float section, Fig. 3 is a diagram showing the relationship between flaw depth and S/N ratio using the flaw detection probe diameter as a parameter, and Fig. 4 is a diagram showing the relationship between lift-off and S/N ratio, FIG. 5 is a diagram showing the relationship between air pressure and lift-off of the fluid float section, and FIG. 6 is a diagram showing the relationship between test frequency and S/N ratio.
FIG. 7 is a diagram showing the relationship between the signal magnitudes of bald spots and noise with respect to the phase difference. DESCRIPTION OF SYMBOLS 1... Flaw detection head, 5... Cylindrical shaft, 6... Fluid supply rotary joint, 8... AC motor for rotational drive, 9... Fluid supply piping, 11... Signal wiring section, 1
3... Non-contact rotating transformer part, 21... Housing, 22... Peripheral wall, 23... Guide hole 24... Small hole
27...Fluid supply pipe, 30...Fluid float part, 3
DESCRIPTION OF SYMBOLS 1...Casing, 32...Through hole, 33...Fluid reservoir part, 42...Flaw detection probe, S...Hot rolled steel plate.

Claims (1)

[Claims] In the method of eddy current flaw detection while feeding a steel plate using a mutual induction self-comparison type flaw detection probe, the diameter of the flaw detection probe is 2.5 to 6 mm and the test frequency is 100 to 300 kHz.
z, a lift-off of 0.4 to 1.2 mm, and a phase difference between an input signal and an output signal of 100 to 160 degrees.