JP4298444B2 - Ultrasonic flaw detection method - Google Patents

Description

  The present invention relates to a method for inspecting the presence or absence of internal flaws in a metal round bar using ultrasonic waves.

  Round steel bar made of special steel is obtained by rolling a steel ingot or steel slab. Usually, multi-stage rolling is performed by a rough row rolling mill, a middle row rolling mill, and a finishing row rolling mill arranged in tandem. By this rolling, the billet is gradually reduced in diameter and lengthened to obtain a round bar steel. Inclusions present in the steel ingot and the steel slab extend in the longitudinal direction of the round bar steel as the length increases. This inclusion has a cylindrical shape. When the inclusion is a high-hardness substance such as alumina, it may not be deformed even by rolling and may be granular. These inclusions are internal scissors. Depending on the extent of the internal defects, the internal defects may deteriorate the quality of the round bar steel and hinder its use.

  Steel manufacturers take various measures at the refining stage to reduce inclusions that cause internal defects. However, it is impossible to completely prevent internal defects. Steel manufacturers inspect the quality of round bar steel and ship only round bar steel that has not been found to have large defects. An ultrasonic flaw detection method is employed for the inspection. In the ultrasonic flaw detection method, an ultrasonic wave having a frequency of about 2 MHz to 5 MHz is used. A probe that can be used in an ultrasonic flaw detection method is disclosed in Japanese Patent Laid-Open No. 10-31823.

  FIG. 4 is an explanatory view showing a state in which the round steel bar S is inspected by a conventional ultrasonic flaw detection method. 4A shows a cross section of the round steel bar S, and FIG. 4B shows a vertical cross section of the round steel bar S. What is indicated by reference numeral 2 in FIG. 4 is a probe. The probe 2 includes a vibrator 4 and a concave lens 6. The lower surface of the concave lens 6 is recessed in FIG. 4A and is flat in FIG. 4B. An ultrasonic wave U is emitted from the vibrator 4. The ultrasonic wave U passes through the concave lens 6 and enters the round steel bar S. When the round bar steel S has an internal flaw, the ultrasonic wave U is reflected by the inner flaw. When this reflected wave is sensed by the probe 2, an internal defect is detected. As the probe 2 relatively rotates around the round steel bar S, the entire cross-sectional area of the round steel bar S is inspected. As the round steel bar S advances in the direction indicated by the arrow A in the figure, the inspection is performed over the entire region in the length direction of the round steel bar S.

  In the cross section of the round steel bar S, the ultrasonic wave U is focused at the focal point F. On the other hand, in the longitudinal section of the round steel bar S, the ultrasonic wave U is dispersed without being focused. In this flaw detection method, the sound pressure of the ultrasonic wave U in the vicinity of the focusing point F is small in the longitudinal section. Therefore, the energy of the reflected wave from the inner hook shorter than that of the vibrator 4 is small, and the detection accuracy of the inner hook is insufficient. In this ultrasonic flaw detection method, granular internal defects having a diameter of 0.15 mm or less are difficult to detect.

By using a two-dimensional concave lens, ultrasonic waves can be focused even in a longitudinal section. FIG. 5 is an explanatory view showing a state in which the round steel bar S is inspected by an ultrasonic flaw detection method using the two-dimensional concave lens 8. 5A shows a cross section of the round steel bar S, and FIG. 5B shows a vertical cross section of the round steel bar S. The probe 10 includes a vibrator 12 together with a two-dimensional concave lens 8. An ultrasonic wave U is emitted from the vibrator 12. When a metal material having a flat upper surface is inspected by the probe 10, the ultrasonic wave U is focused on one point. However, since the surface of the round steel bar S in the cross section is curved, according to Snell's law, the depth Da of the focusing point Fa shown in FIG. 5A and the focusing shown in FIG. 5B. The depth Db of the point Fb is different. Even in this ultrasonic flaw detection method, since the sound pressure of the ultrasonic wave U in the vicinity of the focusing points Fa and Fb is small, the energy of the reflected wave from the inner flaw shorter than the vibrator 12 is small. Even with this ultrasonic flaw detection method, the accuracy of detecting internal defects is insufficient.
Japanese Patent Laid-Open No. 10-31823

  The round steel bar S made of special steel is often used for important parts (for example, shafts of automobile steering). In this application, the round steel bar S is required to have high strength and toughness. In this application, even internal flaws that cannot be detected by conventional ultrasonic flaw detection methods may hinder use. Although microscopic internal wrinkles can be detected by microscopic observation, the efficiency of microscopic observation is poor. Microscopic observation is possible at the laboratory level, but lacks practicality in a factory that performs mass production. An object of the present invention is to provide an ultrasonic flaw detection method capable of efficiently selecting a round bar material S that satisfies the required quality.

  In the ultrasonic flaw detection method according to the present invention, ultrasonic waves emitted from the probe are incident on the metal round bar from the side surface. The ultrasonic waves are focused inside the round bar, and the presence of internal flaws is inspected by reflected waves. The focal point of the ultrasonic wave in the cross section of the round bar material and the focal point of the ultrasonic wave in the vertical cross section of the round bar material substantially coincide.

Preferably, when the effective area of the transducer included in the probe is Ae (mm ² ) and the distance between the transducer and the inner rod is xmm, the value of (Ae / x ² ) is 0.03 or more. It is. Preferably, the frequency of the ultrasonic wave is 6 MHz or more.

  The ultrasonic flaw detection method according to the present invention is suitable for detecting granular internal defects having a diameter of 0.1 mm to 0.5 mm. The ultrasonic flaw detection method according to the present invention is also suitable for detecting an internal defect shorter than the length of the vibrator provided in the probe.

  In this ultrasonic flaw detection method, the sound pressure of the ultrasonic wave in the vicinity of the focal point is large. Therefore, the energy of the reflected wave generated by the internal flaw is large. In this ultrasonic flaw detection method, minute internal defects can be detected.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

  FIG. 1 is an explanatory view showing a state in which a round steel bar S is inspected by an ultrasonic flaw detection method according to an embodiment of the present invention. In this method, a probe 14 is used. The probe 14 includes a large number of transducers 16, a first lens 18, and a second lens 20. The vibrator 16 is arranged in parallel in the longitudinal direction of the round steel bar S. The vibrator 16 is positioned above the first lens 18 and is laminated with the first lens 18. The second lens 20 is separated from the first lens 18 and is located below the first lens 18.

  2A is a cross-sectional arrow view along the line AA in FIG. 1, and FIG. 2B is a cross-sectional arrow view along the line BB in FIG. FIG. 2A shows a cross section of the round bar steel S, and FIG. 2B shows a vertical cross section of the round bar steel S.

  As is apparent from FIG. 2, the first lens 18 is a concave lens. The lower surface of the first lens 18 is recessed upward in the cross section. The lower surface of the first lens 18 is flat in the longitudinal section. The second lens 20 is also a concave lens. The lower surface of the second lens 20 is flat in cross section. The lower surface of the second lens 20 is recessed upward in the longitudinal section.

  An ultrasonic wave U is emitted downward from the probe 14 in FIG. 2, and this ultrasonic wave U is incident on the round bar steel S. In FIG. 2A, the ultrasonic wave U is focused on the focusing point Fa by the first lens 18. In FIG. 2B, the ultrasonic wave U is focused on the focusing point Fb by the second lens 20. As is clear from the comparison between FIG. 2A and FIG. 2B, the depth Da of the focal point Fa and the depth Db of the focal point Fb are substantially the same. In other words, the focal point Fa and the focal point Fb are substantially the same. In the present invention, when the absolute value of the difference (Da−Db) between the depth Da and the depth Db is 25% or less, further 15% or less, particularly 10% or less of the radius of the round steel bar S, “ The converging point Fa and the converging point Fb are substantially the same. "

  When the inner rod exists in the round steel bar S, the ultrasonic wave U collides with the inner rod and a reflected wave is generated. The reflected wave travels upward in FIG. 2 and is received by the probe 14. When the energy of the reflected wave is large enough to be distinguished from noise, the presence of an internal flaw is detected by the probe 14. In the ultrasonic flaw detection method shown in FIGS. 1 and 2, the sound pressure of the ultrasonic wave U in the vicinity of the focal point is high. Therefore, the energy of the reflected wave from the internal ridge existing near the focal point is large. Therefore, detection is possible even if the internal wrinkles present in the vicinity of the focusing point are very small. For example, a granular wrinkle having a diameter of 100 μm or a columnar wrinkle having a diameter of 25 μm and a length of 400 μm can be detected.

  The round steel bar S rotates in the direction indicated by the arrow R in FIG. By this rotation, flaw detection over the entire cross section can be performed. The round steel bar S may not rotate, and the probe 14 may rotate around the round steel bar S. The round steel bar S proceeds in the direction indicated by the arrow A in FIG. By this progression, the entire lengthwise flaw detection can be performed.

The sound pressure PF of the reflected wave received by the probe 14 is expressed by the following formula (I).
PF = K ₁ · (Po · Ae · AF / (λ ² · x ² )) / (a · b) (I)
In this equation, Po represents the sound pressure of the ultrasonic wave U generated by the vibrator 16, Ae represents the effective area of the vibrator 16, AF represents the area of the inner eyelid, λ represents the wavelength of the ultrasonic wave U, x represents the distance between the transducer 16 and the inner eyelid, a represents the width of the ultrasound group at the focusing point Fa, and b represents the length of the ultrasound group at the focusing point Fb. In the above mathematical formula (I), K ₁ is a coefficient. The effective area Ae is a product (W · L) of the width of the vibrator 16 (represented by a double arrow W in FIG. 2) and the entire length (represented by a double arrow L in FIG. 2). From the viewpoint of the balance between detection accuracy and efficiency, a and b are set to about 1.0 mm.

There are crystal grains in the steel. The probe 14 also receives noise due to crystal scattering echoes. The sound pressure PN of noise is expressed by the following formula (II).
PN = (K ₂ · Gs) / λ (II)
In this equation, Gs represents the crystal grain size of the steel material. In the above formula (II), _{K 2} are coefficients.

An internal defect can be detected when the reflected wave from the internal defect is sufficiently larger than the noise. The detectability correlates with (PF / PN). From the above formulas (I) and (II), the detectability is expressed by the following formula (III).
(PF / PN) = K ₃ · Po · Ae · AF / (Gs · λ · x ² · a · b) (III)

As is clear from the above formula (III), even if the area AF of the internal ridge is small, the internal fold can be detected when the effective area Ae of the vibrator 16 is sufficiently large or when the distance x is sufficiently small. From the viewpoint of detection accuracy, (Ae / x ² ) is preferably 0.03 or more, more preferably 0.05 or more, and particularly preferably 0.07 or more. When (Ae / x ² ) is too large, the apparatus becomes large, and (Ae / x ² ) is preferably 1.0 or less.

  As is clear from the above formula (I), if the wavelength λ of the ultrasonic wave U is sufficiently small, the internal defect can be detected even if the area AF of the internal defect is small. The wavelength λ is represented by (C / f) when the speed of sound in the medium of the ultrasonic wave U is C and the frequency of the ultrasonic wave U is f. In other words, if the frequency f is sufficiently large, the detection accuracy is high. From the viewpoint of detection accuracy, the frequency f is preferably 6 MHz or more, more preferably 8 MHz or more, and particularly preferably 10 MHz or more. The frequency f is usually 30 MHz or less.

  FIG. 3 is an explanatory view showing a state in which the round steel bar S is inspected by the ultrasonic flaw detection method according to another embodiment of the present invention. Although not shown, a probe is also used in this method. This probe also includes a second lens 20 similar to the probe 14 shown in FIG. Also in this probe, the depth Da of the converging point F in the transverse section and the depth Db of the converging point F in the longitudinal section are substantially the same. In this probe, the focal length is variable. The round steel bar S advances in the longitudinal direction while rotating in the circumferential direction.

  In FIG. 3A, the focal point F of the ultrasonic wave U is the center of the round steel bar S. In FIG. 3A, the density of the ultrasonic wave U is particularly high inside the virtual circle C1. In FIG. 3 (a), the detection accuracy of the internal flaw existing inside the virtual circle C1 is particularly high.

  In FIG. 3 (b), the focal point F of the ultrasonic wave U is above the center of the round steel bar S. The distance between the focusing point F and the center of the round steel bar S is about 28% of the radius r of the round steel bar S. In FIG. 3B, the density of the ultrasonic wave U is particularly high in a region surrounded by the virtual circle C1 and the virtual circle C2. In FIG. 3 (b), the detection accuracy of the inner eyelid existing in the region surrounded by the virtual circle C1 and the virtual circle C2 is particularly high.

  In FIG. 3C, the focal point F of the ultrasonic wave U is further above the center of the round steel bar S. The distance between the focal point F and the center of the round steel bar S is about 57% of the radius r of the round steel bar S. In FIG. 3C, the density of the ultrasonic wave U is particularly high in the region surrounded by the virtual circle C2 and the virtual circle C3. In FIG. 3 (c), the detection accuracy of the inner eyelid existing in the region surrounded by the virtual circle C2 and the virtual circle C3 is particularly high.

  In FIG. 3D, the focal point F of the ultrasonic wave U is further above the center of the round steel bar S. The distance between the focusing point F and the center of the round bar steel S is about 86% of the radius r of the round bar steel S. In FIG. 3D, the density of the ultrasonic wave U is particularly high in a region surrounded by the virtual circle C3 and the surface of the round steel bar S. In FIG. 3 (d), the accuracy of detecting internal defects present in the region surrounded by the virtual circle C 3 and the surface of the round steel bar S is particularly high.

  By using the vibrator 16 having a variable focal length, the internal flaw can be detected with extremely high accuracy from the vicinity of the center of the round steel bar S to the vicinity of the surface as shown in FIG. In the example of FIG. 3, four converging points F are employed. The number of converging points F is determined according to the diameter and required accuracy of the round steel bar S. In the case of the round steel bar S having a radius of r (mm), the number of converging points F is preferably (r / 15) or more, particularly preferably (r / 10) or more. The inspection need not be performed over the entire region from the vicinity of the center to the vicinity of the surface. For example, in the case of a round steel bar S that is hollowed out and used as a pipe, the inspection near the center is omitted. By omitting, it is possible to prevent the round steel bar S from being judged to be defective (so-called false information) due to the presence of the internal defects near the center, even though the internal defects near the center are permitted.

  A large number of excitation elements may be used as the probe. An excitation pulse delay circuit is connected to the excitation element. From the excitation pulse delay circuit, an excitation pulse is emitted toward each excitation element. When receiving the excitation pulse, the excitation element emits an ultrasonic wave U. The group of the ultrasonic waves U is focused by shifting the excitation pulse reception timing between the excitation elements.

  Hereinafter, the effects of the present invention will be clarified by examples. However, the present invention should not be construed in a limited manner based on the description of the examples.

Three types of probes shown in Table 1 below were prepared. In any of the probes, the effective area of the transducer is 600 mm ² , and the ultrasonic frequency is 5 MHz.

  First, by using the probe of the example, ultrasonic flaw detection was performed on a round bar steel having a radius of 70 mm and a material, and a round bar steel having an internal flaw was found. Ultrasonic flaw detection was performed on the round bar steel using the probes of Comparative Examples 1 and 2, but no internal defects were detected. A test piece was prepared by cutting out the vicinity of the round bar steel where the internal flaw was found with the probe of the example, and the size of the internal flaw was observed with a microscope while polishing the test piece little by little. By microscopic observation, it was found that the internal wrinkles were almost granular and had a diameter of about 100 μm. From this evaluation result, the superiority of the present invention is clear.

  The present invention can be applied not only to round steel bars but also to various bars having a circular cross section.

FIG. 1 is an explanatory view showing a state in which a round bar steel is inspected by an ultrasonic flaw detection method according to an embodiment of the present invention. 2A is a cross-sectional arrow view along the line AA in FIG. 1, and FIG. 2B is a cross-sectional arrow view along the line BB in FIG. FIG. 3 is an explanatory view showing a state in which a round bar steel is inspected by an ultrasonic flaw detection method according to another embodiment of the present invention. FIG. 4 is an explanatory view showing a state in which a round bar steel is inspected by a conventional ultrasonic flaw detection method. FIG. 5 is an explanatory view showing a state in which a round bar steel is inspected by another conventional ultrasonic flaw detection method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 14 ... Probe 16 ... Vibrator 18 ... First lens 20 ... Second lens F ... Focusing point S ... Round bar steel U ... Ultrasonic

Claims (5)

An ultrasonic flaw detection method in which ultrasonic waves emitted from a probe are incident on a metal round bar from the side and focused inside the metal round bar, and the presence of internal flaws is inspected by reflected waves. There,
The probe includes a transducer that emits ultrasonic waves, a first lens positioned between the transducer and a round bar, and a second lens positioned between the first lens and the round bar. With
This probe is configured so that the ultrasonic wave emitted from the vibrator enters the round bar after passing through the first lens and the second lens,
An ultrasonic flaw detection method characterized in that an ultrasonic focusing point in a transverse cross section of a round bar material substantially coincides with an ultrasonic focusing point in a longitudinal cross section of the round bar material. When the probe includes a transducer, the effective area of the transducer is Ae (mm ² ), and the distance between the transducer and the internal rod is xmm, the value of (Ae / x ² ) The ultrasonic flaw detection method according to claim 1, wherein is 0.03 or more.   The ultrasonic flaw detection method according to claim 1, wherein a frequency of the ultrasonic wave is 6 MHz or more.   The ultrasonic flaw detection method according to any one of claims 1 to 3, wherein a granular internal flaw having a diameter of 0.1 mm to 0.5 mm is detected by the probe.   The ultrasonic flaw detection method according to any one of claims 1 to 3, wherein an internal defect shorter than a length of a vibrator included in the probe is detected by the probe.

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