JPS6411144B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an ultrasonic flaw detection method for round steel bars, and detects subcutaneous defects such as minute subcutaneous nonmetallic inclusions in round steel bars by ultrasonic flaw detection. Regarding how to.

(Conventional technology) In the production line of round steel bars, flaw detection is generally carried out in the billet state at the production stage, and usually flaws are detected within a range of approximately 10% of the material diameter from the surface to ensure quality. are doing. Therefore, in the final inspection, it is necessary to detect minute nonmetallic inclusions present within this depth range. However, almost no technology has been proposed to date that employs ultrasonic flaw detection for subcutaneous inspection of this type of round steel bar.

A technology relatively similar to this is in the United States.
ASTM standard E588 stipulates a method for conducting ultrasonic flaw detection on bearing steel and determining the quality of the steel material based on the frequency of the obtained echoes. This is 5MHz
Alternatively, vertical flaw detection is performed using a 10MHz focused probe, the number of echoes exceeding the standard is counted and integrated, and the amount of inclusions is estimated from the integrated value to determine the quality level.

In addition, some flaw detection methods for round steel bars using a two-piece probe have been proposed.

(Problems to be Solved by the Invention) Vertical flaw detection has the disadvantage that echoes reflected from the material surface become noise and become an obstruction, making it impossible to detect nonmetallic inclusions directly beneath the surface.

For those using a two-piece probe, 5mm below the surface.
The disadvantage is that the shallower area is a dead zone and cannot be detected.

In view of the above, an object of the present invention is to provide an ultrasonic flaw detection method that can detect defects in a round steel bar whose subcutaneous depth is approximately 10% or less of the material diameter without being disturbed by noise from the material surface. shall be.

(Means for Solving the Problems) A feature of the present invention is that ultrasonic waves are incident on a round steel bar from the outer surface at a refraction angle of 45±8°, and the ultrasonic waves that have entered the material are The method is to converge the ultrasonic waves at a first reflection position where they are reflected on the material surface, and to detect subepidermal defects by capturing reflected signals from areas before and after the first reflection position.

(Example) Hereinafter, an example of the present invention will be described based on the drawings.

FIG. 1 shows an ultrasonic flaw detection apparatus for carrying out the method of the present invention, in which 1 is a water tank, 2 is a round steel bar that passes through the water in the water tank 1, and is connected to a pair of ski rolls 3, 3 in front and back of the water tank 1. They are held between the upper and lower sides by the rolls 4, respectively.

The skie rolls 3, 3 are arranged so as to be inclined at a certain angle with respect to the round steel bar 2 in a horizontal plane as shown in FIG.
6 to be rotatably supported around the axis, and each is driven in the direction of the arrow by a motor (not shown) via a winding transmission mechanism 7.

The pinch roll 4 is a pair of ski rolls 3,
In the upper position between 3 and 3, they skie roll 3,3
It is pivotally supported at the lower end of the support member 8 so as to be freely rotatable around an axis mirroring the axis of the round steel bar. As a result, the round bar 2 is moved to the ski roll 3,
3, and is conveyed in the longitudinal direction while rotating around the axis.

Reference numeral 9 denotes a point focus type probe using a bifocal type lens 10, which is located at the center between the pair of front and rear pinch rolls 4, and is configured to input ultrasonic waves into the round steel bar 2 from below. is set in tank 1. The probe 9 is supported by a probe holding and adjusting device 11 so that its position can be adjusted vertically and laterally. In addition, 12 is a flaw detector, 13 is a recorder,
14 is a monitoring oscilloscope.

When performing ultrasonic flaw detection on the round steel bar 2, first, the probe holding and adjusting device 11 is operated to adjust the vertical and lateral positions of the probe 9 relative to the round steel bar 2.
By adjusting the position of the probe 9 in the vertical direction, the third
The water distance H from the probe 9 to the ultrasonic incidence point of the round steel bar 2 shown in the figure is adjusted. Further, by adjusting the position of the probe 9 in the lateral direction, an offset amount W, which is the amount of shift between the center axis of the probe 9 and the center axis of the round steel bar 2 parallel to this center axis, is adjusted.

By rotationally driving each of the skie rolls 3, 3, the round steel bar 2 is rotated around its axis while being pinched from above and below by the skie rolls 3, 3 and the pinch rolls 4 at the front and back of the water tank 1. It is conveyed at a constant speed in the longitudinal direction. Then, flaw detection is performed by transmitting ultrasonic waves from the probe 9.

Using the above ultrasonic flaw detection device, ultrasonic waves are incident on the round steel bar 2 from the outer surface so that the refraction angle α is 45± ⁸⁰ , and the ultrasonic waves that have entered the material of the round steel bar 2 are detected. The first reflection position reflected by the surface [hereinafter 1B
(abbreviated as point)]. In addition, flaw detector 12
Now, apply a gate to the range G before and after point 1B,
The reflected signal generated from the range G is captured to detect nonmetallic inclusions within a range of about 10% below the surface of the round steel bar 2.

In addition, in the above flaw detection, in order to detect fine inclusions in the round steel bar 2, it is necessary to reduce noise from grain boundaries, and it is desirable to use an ultrasonic beam with a small beam diameter. Generally, when an ultrasonic wave is converged by a lens 10 or the like, the spread angle θ (6 dB down spread angle) generated in the ultrasonic beam is expressed by the following formula.

θ=2sin ^-1 (0.51λ/D) λ: Wavelength D: Diameter of probe Also, beam diameter d of the focused ultrasound beam
is determined by the distance L to the focal point and the spread angle θ expressed by the following equation.

d=2L tanθ/2 However, d cannot be less than the wavelength λ.

Therefore, the larger the probe 9, the shorter the wavelength, and the shorter the focal length, the narrower the ultrasound beam can be focused, and the sound pressure at that point will also increase, improving detection sensitivity. It's good to be able to do it.

Furthermore, when the round steel bar 2 is submerged in water and ultrasonic waves are applied from its outer surface, the beam spreads inside the material due to the lens effect caused by the curvature of the surface of the round steel bar 2. Therefore, the ultrasonic beam is as shown in Figure 4.
In order to have a focal point at point 1B, it is necessary to consider the spread of the ultrasonic beam and enter it into the material. In this case, if the lens 10 is kept constant, increasing the water distance H will move the focal point toward you, and decreasing it will move the focal point in the opposite direction. This can also be seen from FIGS. 5A to 5C, which show the results of geometrical optical drawing while changing the water distance H. Note that, as is clear from this plotting result, if the water distance H is increased, there is a condition for refocusing beyond point 1B.

Further, in order to adjust the refraction angle α, the offset amount W may be adjusted. This is because the relationship between the refraction angle α and the offset amount W is expressed by the following equation.

sin α=Vs/Vw×W/R Vs: Transverse sound velocity in the material, Vw: Sound velocity in water, R: Radius in the material In other words, by introducing the concept of offset amount W, the parallel movement of the probe 9 The refraction angle α can be controlled simply by using the same method, and the flaw detection work can be simplified.

Figures 6 to 8 show the results of actually measuring the echo heights of artificial horizontal holes with a diameter of 1 mm made at different depths in the round steel bar 2. In addition, round steel bar 2
The outer diameter of the lens is 40 mm, and the focal length of the bifocal lens is 69 in the circumferential direction.
mm, axial direction 110 mm, frequency 5 MHz, and probe diameter 19 mm. In addition, the signals at each point ABC in the round steel bar 2 shown in FIG. 3 are shown as 0.0 in FIG. 6 and FIG. 7, respectively. Indicated by △ and × marks.

Figures 6A to 6D show the relationship between water distance H (μsec) and sensitivity, that is, water distance H and echo height (V) at a refraction angle of 45°, for artificial defects of different depths. .

FIGS. 7A to 7D show the relationship between offset change (mm) and sensitivity, that is, change in refraction angle (α) and echo height (V), for artificial defects of different depths.

FIG. 8 shows the relationship between the offset change (mm), that is, the change in the refraction angle (α), and the noise echo height (V) (noise echo from point 1B).

Note that the echo heights in FIGS. 6 and 7 are data when the sensitivity of the flaw detector is set to 40 dB, and the noise echo height in FIG. 8 is data when the sensitivity of the flaw detector is set to 80 dB. Furthermore, since the offset change and the refraction angle change uniquely correspond as mentioned above, the seventh
The offset amount and the refraction angle are also shown on the horizontal axes of the figures and FIG. 8. Furthermore, since the noise echo height varies depending on the subtle unevenness of the material surface, Fig. 8 shows the vertical width between the maximum and minimum values, and plots their average value with a circle.

From Figures 6 and 7, when detected at point C after point 1B, it does not depend much on the water distance H;
It can be seen that detection is possible with high sensitivity.

This corresponds to the fact that the actually measured shape of the ultrasonic beam incident on the round steel bar 2 during the detection of the artificial defect is narrowly converged from point 1B onwards, as shown in FIG. Therefore, it can be seen that if the ultrasonic beam is focused at point 1B, highly sensitive flaw detection can be performed in the area before and after point 1B.

Furthermore, the range G to which the gate is applied in the flaw detector 12 is as follows, if the reflection of ultrasonic waves at the defect is isotropic.
Only the area in front of point 1B or only the area behind point 1B is sufficient. However, since the reflection of ultrasonic waves from an actual defect is anisotropic, the area before and after point 1B is set so that the defect can be observed from two different directions by rotating the round steel bar 2 relative to the probe 9 around its axis. A gate shall be applied.

In addition, in Fig. 3, the gate range G is 1/2L to 1B, where L is the length from the ultrasound incident position to point 1B.
The gate is applied to the range of 3/2L, that is, the range of 1/2L before and after the 1B point, but it is not limited to this, and any range before and after the 1B point is sufficient, and the range depends on the needs. It is determined by

Next, from Fig. 8, we can see that by changing the refraction angle α, S/
It can be seen that N changes significantly. That is, when the refraction angle α becomes smaller than 35°, the noise echo increases rapidly. This is because the ultrasonic beam hits the point 1B at the same angle as the refraction angle α, and as the refraction angle α becomes smaller, the noise echo caused by minute irregularities on the surface of the round steel bar 2 becomes larger. Therefore, the refraction angle α needs to be 35° or more.

Next, FIG. 10 shows the relationship between the refraction angle α and the percentage of the detectable depth with respect to the diameter of the material. this is,
As shown in FIG. 14, when the diameter of the round steel bar 2 is D and the detectable depth (the area through which the central axis of the ultrasonic wave passes as shown) is d, it is determined by the following formula.

d/D×100=(1/2D-1/2D・sinα)10
0/D = 50 (1-sin α) As a result, it can be seen that if the detection range is set to 10% of the material diameter, the upper limit of the refraction angle α is 53°.

In addition, from FIG. 14, the gate range G is 1/2
It can be seen that if L to 3/2L is used, the entire depth range that can be detected can be covered.

From the above, if the refraction angle of the ultrasonic wave is set to 45° ± 8° and the reflected signals from the front and rear regions of point 1B are captured,
It has been confirmed that defects shallower than approximately 10% of the material diameter can be detected without being disturbed by noise from the material surface.

Note that, in general, the diameter of the round steel bar 2 may vary due to manufacturing errors between products. In order to estimate the change in the refraction angle based on the variation in the offset amount W caused by this error, when the nominal diameter of the target round steel bar 2 is inspected using the offset amount when ultrasonic waves are incident at a refraction angle of 45°, , the result of calculating how many times the refraction angle changes due to the outer diameter error is shown in the 12th
As shown in the figure. As a result, the outer diameter variation of the actual material is ±1.8%
It was found that the resulting variation in the refraction angle was about ±1°. Therefore, if there is variation in the diameter of the flaw detection material, it is necessary to take this error into account.

Furthermore, when actually performing flaw detection, it is necessary to take into account variations in the amount of offset caused by bending of the material. Therefore, the relationship between flaw detection accuracy and flaw detection equipment will be explained by focusing on changes in the refraction angle due to material bending.

As in the embodiment shown in FIG. 2 above, the skie rolls 3 and 3 and the pinch roll 4
When the round steel bar 2 is forcibly held by .

Δζ=XY Δζ: Amount of displacement at the center of the material (mm) As small as.

The allowable amount of bending to keep the change in refraction angle due to offset variation within the range of 45±8° is -2.1% to +1, taking into account the variation in Figure 12 due to diameter error. .8%. Therefore, if the stricter value of 1.8% is taken, the allowable amount of bending for the device of the above embodiment is as follows.

Xcr=0.018/YD D: Material diameter Xcr: Bending allowance A pair of ski rolls 1 as shown in Figure 13A
5, 15 while transporting the tube material 16, or as shown in FIG. If flaw detection is performed while rotating the rotary ring 19 by the motor 21 via the winding transmission mechanism 20, an offset variation equivalent to half the error in the diameter of the tube material 16 is inevitably added. Similarly, in order to suppress the refraction angle to 45±8°, it is necessary to suppress the change in the refraction angle due to bending to 0.9%. Therefore, in the case of the device shown in FIG. 13, the allowable amount of bending is expressed by the following equation.

Comparing the equations Xcr=0.019/YD and Equation, the bending tolerance in the device shown in FIG. 13 is about twice as small as in the device of the previous embodiment. For example, material diameter is 50mm. When the distance between the pinch rolls is 400mm, the bending tolerance according to the formula is 2.25mm/m, which is within the 2mm/m bending standard for normal round steel bar products. Therefore, the first
In the device shown in Fig. 3, it is necessary to strictly control the bending of the material for flaw detection, whereas according to the device of the above embodiment, there is no need for special bending control for flaw detection. It is better to use the one shown in Figure 2 than the one shown in Figure 13.
However, if there is no problem in strictly controlling bending of the material, flaw detection may be performed using the apparatus shown in FIG. 13.

In addition, when detecting defects that are long in the axial direction,
A line focus type probe may be used.

(Effects of the Invention) According to the present invention, ultrasonic waves are incident on a round steel bar at an angle of 45°±8°, converged at the first reflection position on the material surface, and defects are detected by the reflected signals from the areas before and after the first reflection position. , all defects shallower than approximately 10% of the material diameter can be detected without being disturbed by noise from the material surface.

Moreover, since the reflected signals are captured not only from the front or rear area of the first reflection position but also from the front and rear areas, the same defect can be detected twice from different directions as the material rotates relative to the probe. Even if objects have directionality, they can be detected reliably.

[Brief explanation of the drawing]

The drawings illustrate one embodiment of the present invention, in which Fig. 1 is a partially cutaway perspective view showing an outline of the flaw detection device, Fig. 2 is a plan view of the skie roll section, and Figs. An explanatory diagram showing the incident state of ultrasonic waves, FIG. 5 is a geometrical optical diagram showing the convergence state of ultrasound waves,
Figure 6 shows the relationship between water distance and sensitivity, Figure 7 shows the relationship between refraction angle and sensitivity, Figure 8 shows the relationship between refraction angle and noise, and Figure 9 shows the relationship between beam Figure 10 shows the relationship between the refraction angle and the detectable depth, Figure 11 shows the relationship between the offset amount and the refraction angle, and Figure 12 shows the relationship between the outer diameter error and the refraction angle. FIG. 13 is a diagram showing a different flaw detection apparatus, and FIG. 14 is an explanatory diagram of the relationship between refraction angle and flaw detection depth. 2... Round steel bar, 3... Skew roll, 4...
Pinch roll, 9... probe, 10... lens,
12...Flaw detector.

Claims (1)

[Claims] 1. Ultrasonic waves are incident on the round steel bar from the outer surface at a refraction angle of 45 ± 8°, and the ultrasonic waves that have entered the material are focused at the first reflection position where they are reflected on the material surface, An ultrasonic flaw detection method for round steel bars, characterized in that subcutaneous defects are detected by capturing reflected signals from areas before and after the first reflection position.