Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-11-01
2002-11-05
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S602000, C073S614000, C073S159000
Reexamination Certificate
active
06474163
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an ultrasonic flaw detection method and an instrument therefor. The present invention is specifically suitable for detecting internal flaws such as nonmetallic inclusions in a rolled metallic sheet including a steel sheet. By use of the present invention, flaw detection at a time of a linear region with definite width is possible.
BACKGROUND ART
An internal flaw such as microscopic nonmetallic inclusion of approximately 50 &mgr;m in diameter in a rolled metallic sheet may cause a crack when the rolled metallic sheet is pressed or drawn. Therefore, it is required for the internal flaw inspection of a rolled metallic sheet to detect an extremely small internal flaw.
Generally, the ultrasonic flaw detection method is most frequently applied to internal flaw inspection of rolled metallic materials. In this method, ultrasonic waves are propagated into rolled metallic materials so as to detect discontinuity in ultrasound propagation caused by the internal flaw. As an applied example of this method, there is a method for flaw inspection of entire volume of the rolled metallic sheet at a transfer line of the rolled metallic sheet. In Japanese Unexamined Patent Publication No. 7-253414, for example, the following ultrasonic flaw detection method and the instrument therefor are proposed. That is, in medium, a line-focused ultrasonic transmitting probe and a linear probe array are arranged face to face with a sheet being inspected (a rolled metallic sheet) between them. A line-focused ultrasonic beam transmitted from the transmitting probe propagates into the sheet being inspected approximately in a perpendicular direction thereto, so that part of ultrasound reflected at an internal flaw in the sheet being inspected will be received by the linear probe array. After the ultrasonic signal which had been received and transformed into the electrical signal was amplified and only the echo from internal flaw was picked up therefrom, any signal greater than a predetermined threshold voltage is detected.
However, in order to detect flaws effectively by use of the above-mentioned ultrasonic flaw detection method and the instrument therefor, the gap length “Ls” (mm) between the line-focused ultrasonic transmitting probe and the linear probe array is required to satisfy the following equation. In this equation, “F” (mm) represents a focal length in medium, of the line-focused ultrasonic transmitting probe, and “t” (mm) denotes a thickness of the sheet being inspected.
Ls≦F−{(CS/CW)−1}t+5.5
(Provided that: “CS”; ultrasonic velocity (m/sec) in the sheet being inspected, “CW”; ultrasonic velocity (m/sec) in the medium) Accordingly, when a steel sheet of 4.5 mm in thickness is inspected and the focal length in the medium of the line-focused ultrasonic transmitting probe “F”=38 mm, the gap length “Ls” between the transmitting probe and the receiving probe is required to be less than 31 mm.
A problem with this method is that there may be cases that the sheet being inspected in on-line inspection has a wavy shape in its edge or side portion. When the sheet having such a wavy shape is passed through between the transmitting probe and the receiving probe with the gap length of less than 31 mm, it may frequently hit the housing of the probe to be scratched thereon. The impact of the hit on the probe shortens probe's useful life. In the worst case, the probe is broken.
It is an object of the present invention to provide an ultrasonic flaw detection method and an instrument therefor, having such an enough gap length between the transmitting probe and the receiving probe to be passed through by the sheet being inspected having a wavy shape that the sheet will not hit the probes and moreover being reliably detectable the internal flaw such as a microscopic nonmetallic inclusion.
DISCLOSURE OF INVENTION
The inventors have ardently studied conventional ultrasonic flaw detection methods, so that the present invention has been made based on a new knowledge that the gap length between a line-focused ultrasonic transmitting probe and a line-focused ultrasonic receiving probe is determined by the height of a flaw echo which is a function of a focal length in a coupling medium of the line-focused ultrasonic beam of the line-focused ultrasonic transmitting probe and a focal length in a coupling medium of the line-focused receiving beam of the line-focused ultrasonic receiving probe, and so forth. That is, summarized configurations of the present invention are as follows.
(1) An ultrasonic flaw detection method comprising the steps of: transmitting ultrasonic waves into the sheet being inspected approximately in a perpendicular direction to the sheet through a coupling medium with a line-focused ultrasonic transmitter; receiving ultrasonic waves reflected at an internal flaw through the coupling medium with a line-focused ultrasonic receiver; amplifying the received ultrasonic signals which have been transformed into electrical signals; picking up amplified signals of the echo from the internal flaw; and detecting the flaw by detecting the signal more than a predetermined threshold amplitude, wherein the transmitter and the receiver are arranged face to face with the sheet being inspected between them, and wherein the gap length (L) between the transmitter and the receiver is near the minimum value (Lp) in which the height (f(L))of the echo from the internal flaw takes the maximum value.
(2) The method in the above (1), wherein Lp is determined by a focal length (FT) of the line-focused ultrasonic transmitter in the coupling medium, a focal length (FR) of the line-focused ultrasonic receiver in the coupling medium, the velocity (CS) of ultrasonic waves in the sheet being inspected, the velocity (CW) of ultrasonic waves in the coupling medium, and the thickness (t) of the sheet being inspected.
(3) The method in the above (2), wherein when Lp
1
and Lp
2
(Lp
1
<Lp
2
) are the values of L in which f(L) gives f(L)/f(Lp)=−3 dB, L is more than Lp1 and less than Lp
2
.
(4) The method in the above (2), wherein the coupling medium is a liquid, and wherein Lp satisfies Lp=0.75(FT+FR)−{(CS/CW)−1}t.
(5) The method in the above (3), wherein the coupling medium is a liquid, and wherein Lp satisfies Lp=0.75(FT+FR)−{(CS/CW)−1}t.
(6) The method in the above (5), wherein Lp
1
and Lp
2
satisfy Lp
1
=0.68(FT+FR)−{(CS/CW)−1}t, Lp
2
=0.81(FT+FR)−{(CS/CW)−1}t, respectively.
(7) An ultrasonic flaw detecting instrument comprising: a line-focused ultrasonic transmitter transmitting ultrasonic waves into the sheet being inspected approximately in a perpendicular direction to the sheet through a coupling medium; a line-focused ultrasonic receiver receiving ultrasonic waves reflected at an internal flaw through the coupling medium; a receiving amplifier amplifying the received ultrasonic signals which have been transformed into electrical signals; a gating means for picking up amplified signals of the echo from the internal flaw; and a comparator detecting the signals of the echo from the internal flaw which is more than or equal to a predetermined threshold amplitude, wherein the transmitter and the receiver are arranged face to face with the sheet being inspected between them, and wherein the gap length (L) between the transmitter and the receiver is near the minimum value (Lp) in which the height (f(L)) of the echo from the internal flaw takes the maximum value.
(8) The instrument in the above (7), wherein Lp is determined by a focal length (FT) of the line-focused ultrasonic transmitter in the coupling medium, a focal length (FR) of the line-focused ultrasonic receiver in the coupling medium, the velocity (CS) of ultrasonic waves in the sheet being inspected, the velocity (CW) of ultrasonic waves in the coupling medium, and a thickness (t) of the sheet being inspected.
(9) The instrument in the above (8
Takada Hajime
Torao Akira
Yarita Ikuo
Kawasaki Steel Corporation
Larkin Daniel S.
Miller Rose M.
Oliff & Berridg,e PLC
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