JP7396327B2 - Steel pipe workability evaluation method - Google Patents

Description

The present invention relates to a method for evaluating the workability of steel pipes.

Steel pipes are manufactured by various methods and then subjected to processing such as bending and flattening before use. A flattening test is a method for evaluating the workability of such steel pipes. In this test, a steel pipe is crushed between two flat plates, and the ratio of the distance D' between the flat plates until a crack appears on the surface of the steel pipe to the outer diameter D of the steel pipe before flattening (D'/D, also known as the flattening ratio). (described below) evaluates the workability of steel pipes (see Patent Document 1).

Patent No. 5732999

In the above-mentioned flattening test, the testing machine is stopped when a crack occurs during the test, and the distance D' between the flat plates is recorded.However, conventionally, the presence or absence of cracks on the surface of the steel pipe can be determined visually by the tester. It was judged by.

However, in this judgment method, the criteria for judging cracks differ depending on the tester. For example, even if a flattening test is performed on the same steel pipe, there is a problem in that the flattening ratio measured by different testers may vary greatly. Furthermore, the ease with which cracks occur varies greatly depending on the steel type and size of the steel pipe to be subjected to the flattening test, so it has been extremely difficult to standardize the method for determining cracks. Additionally, depending on the size of the steel pipe, it may be difficult to visually determine cracks, and cracks may be overlooked. Thus, there has been a desire to improve the measurement accuracy of workability evaluation methods such as flattening tests.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for evaluating the workability of steel pipes with improved measurement accuracy.

In order to solve the above problem, the present inventors have made extensive studies and have developed an acoustic emission sensor (hereinafter also referred to as AE sensor) that uses an acoustic emission wave signal (hereinafter also referred to as AE signal) generated in the flattening test of steel pipes. I came up with the idea of detecting this by using The inventors have also discovered that it is possible to accurately determine the presence or absence of cracks in a steel pipe based on the AE signal generated by the flattening test.

Specifically, first, in this flattening test, cracks were conventionally judged by visual inspection, but if the diameter of the steel pipe is large or long enough to make it difficult to visually confirm cracks, Furthermore, when cracks occur on the inner surface of the steel pipe, the cracks occurring in the steel pipe may be overlooked. To address this problem, flattening tests have been monitored using cameras, but the location of cracks may change from test to test. In this case, it becomes necessary to adjust the focus of the camera for each test. Furthermore, as the size of the steel pipe becomes larger, it becomes more complicated to adjust the installation position of the camera and the monitoring monitor. Furthermore, if a camera is inserted inside the steel pipe to check for cracks on the inner surface of the steel pipe, there is a risk that the camera will be damaged during the test.
In this regard, by using an AE sensor in the flattening test, even if the size of the steel pipe is difficult to visually inspect or photograph with a camera, or if cracks occur on the inner surface of the steel pipe due to the flattening test, The present inventors have discovered that it is possible to measure cracks easily and with high precision even when using a conventional method.

Furthermore, after further investigation, it was discovered that the AE signal output per unit time increases immediately before a crack occurs in the steel pipe.

The present invention has been completed through further study based on such knowledge, and the gist thereof is as follows.
[1] A squeezing process of squeezing the steel pipe,
a detection step of detecting acoustic emission waves generated by the pinching pressure;
A method for evaluating the workability of a steel pipe, the method comprising: determining whether or not cracks have occurred in the steel pipe based on the detected acoustic emission waves.
[2] In the determination step,
In the detected acoustic emission wave signal,
The steel pipe workability evaluation method according to [1] above, wherein it is determined that a crack has occurred in the steel pipe when the number of signals exceeding a predetermined threshold per unit time is equal to or greater than a predetermined number.
[3] The steel pipe workability evaluation method according to [2] above, wherein the threshold value is determined based on the magnitude of a signal of an acoustic emission wave detected from the steel pipe under no load.

Here, acoustic emission (AE) refers to a phenomenon in which when a material deforms or breaks, the strain energy stored inside the material is released as an elastic wave. The acoustic emission wave (AE wave) referred to in the present invention is an elastic wave emitted by squeezing a steel pipe in a flattening test or the like in a squeezing process.

According to the present invention, a steel pipe workability evaluation method with improved measurement accuracy is provided.

FIG. 2 is a flow diagram illustrating a method for evaluating the workability of steel pipes according to the present invention. FIG. 2 is a schematic diagram of a workability evaluation device. FIG. 3 is a diagram for explaining the mounting position of an AE sensor. It is a graph showing the relationship between the AE signal and the strain on the surface of the steel pipe. It is a graph showing the relationship between the AE event rate and the flattening ratio of steel pipes.

Embodiments of the present invention will be described below based on the drawings.
FIG. 1 is a flow diagram illustrating the method for evaluating the workability of steel pipes according to the present invention.
The steel pipe workability evaluation method of the present invention includes a clamping step S1 of clamping the steel pipe, a detection step S2 of detecting acoustic emission waves generated by the clamping, and a method for determining cracks in the steel pipe based on the detected acoustic emission waves. and a determination step S3 of determining the presence or absence of occurrence of.
FIG. 2 is a schematic diagram of a suitable workability evaluation apparatus for carrying out this workability evaluation method. Hereinafter, the method for evaluating the workability of a steel pipe according to the present invention will be described with reference to FIGS. 1 and 2.

(Pinching step S1)
In the present invention, first, the steel pipe 4 is compressed in a clamping step S1.
In this step, a flattening test can be performed.
Specifically, the steel pipe 4 is sandwiched between the two flat plates 2 and 3 of the flattening tester 1 shown in FIG. Compress it so that it becomes an ellipse. By operating the upper flat plate 2 and fixing the lower flat plate 3, or by fixing the upper flat plate 2 and operating the lower flat plate 3, the cross section of the steel pipe 4 is compressed into an elliptical shape. can do.
The flattening tester 1 is not particularly limited as long as it can perform flattening work on steel pipes using the flat plates 2 and 3 as described above, and any known flattening tester 1 can be used. Furthermore, the compression speed by the flat plates 2 and 3 is not particularly limited.

(Detection step S2)
In the present invention, after the clamping process S1, in the detection process S2, acoustic emission waves (AE waves) generated by the clamping process are detected.
The AE wave can be detected by the AE sensor 6 shown in FIG. FIG. 3 is a diagram for explaining an example of the mounting position of the AE sensor 6. The AE sensor 6 can be installed on the flat plates 2 and 3 of the testing machine 1, on the steel pipe 4, etc., and AE waves can be detected in either of them. In order to make it possible to measure small AE signals, it is preferable to use a portion directly above the steel pipe 4 on the flat plate 2, as shown in FIG.

Further, it is preferable to apply grease between the AE sensor 6 and the flat plate 2 in order to improve the AE signal detection sensitivity. The AE sensor 6 is not particularly limited in terms of device configuration, as long as it can detect AE waves generated by squeezing the steel pipe 4.

Furthermore, in the present invention, the strain occurring in the steel pipe 4 can be detected in parallel with the detection step S2. Specifically, a strain sensor 5 is attached to the steel pipe 4, and the strain sensor 5 can detect strain caused by performing a flattening test. The strain sensor 5 is installed at the 3 o'clock or 9 o'clock position (also called the 90° position) when the upper flat plate 2 is set at 0 o'clock and the lower flat plate 3 is set at 6 o'clock in a vertical cross-sectional view of the steel pipe 4 in the tube axis direction. ) is preferably the outer surface of the steel pipe. This is because when a flattening test is performed on the steel pipe 4, the maximum tensile strain occurs at the 3 o'clock position and the 9 o'clock position.
This strain sensor 5 measures the amount of strain occurring on the surface of the steel pipe flat plate. The strain sensor 5 is not particularly limited in terms of its device configuration, as long as it can detect the strain generated by squeezing the steel pipe 4.

(Judgment step S3)
In the present invention, after the detection step S2, in the determination step S3, it is determined whether cracks have occurred in the steel pipe based on the detected acoustic emission waves (AE waves). Thereby, the presence or absence of cracks can be determined with high accuracy.

In the determination step S3, as an example, first, the AE signal transmitted from the AE sensor 6 shown in FIG. 2 is amplified by the AE sensor amplifier device 7. Then, the arithmetic unit 8 calculates the AE event rate and the like in the AE signal. Further, the AE signal display device 9 displays information regarding the AE signal, such as the AE event rate, and allows the user of the device to monitor the AE signal during the test.
The AE signal display device 9 may have a storage unit such as a data logger, but is not particularly limited thereto.
Note that the AE event rate refers to the number of AE signals (waveforms) exceeding a predetermined threshold per unit time.

Further, in the present invention, in parallel with the determination step S3, the strain detected by the strain sensor 5 can also be amplified as a signal by the strain sensor amplifier device 10 and recorded by the strain recording device 11.
The arithmetic device 8, AE signal display device 9, and strain recording device 11 described above can be part of an information processing device such as a computer (control device) having a CPU (Central Processing Unit).

In the determination step S3, the presence or absence of cracking in the steel pipe 4 can be determined based on the AE event rate. That is, in the detected acoustic emission wave (AE wave) signals, if the number of signals exceeding a predetermined threshold per unit time is a predetermined number or more, it can be determined that a crack has occurred in the steel pipe 4. can.

FIG. 4 is an example of a graph showing the relationship between the AE signal and the strain on the surface of the steel pipe. Moreover, FIG. 5 is an example of a graph showing the relationship between the AE event rate and the flattening ratio of the steel pipe. As shown in Figures 4 and 5, the number of AE signals that exceed a predetermined threshold per unit time when the tensile strain is greater than a predetermined value when the strain generated by the flattening test and the AE signal value that occurs simultaneously. It can be determined that cracking has occurred when (AE event rate) is greater than or equal to a predetermined number.

For example, in the examples shown in FIGS. 4 and 5, it can be determined that cracking has occurred when the tensile strain is 2.0% or more and the AE event rate is 10 or more. In this example, by determining cracking when the tensile strain is 2.0% or more, the AE waves generated due to the sliding between the steel pipe 4 and the testing machine 1 immediately after the start of the test, and the AE waves generated during elastic deformation. By detecting this, it is possible to avoid erroneously determining that a crack has occurred in the steel pipe 4.
In addition, in this example, by setting in advance that cracking has occurred when the AE event rate is 10 or more, AE waves caused by plastic deformation or slipping between the steel pipe 4 and the testing machine 1 can be excluded, and the cracking can be done more accurately. can be determined.

Moreover, the above-mentioned threshold value can be determined based on the magnitude of the acoustic emission wave signal (AE signal) detected from the steel pipe under no load.
For example, in the examples shown in Figures 4 and 5, the AE waves generated when a crack occurs on the surface of a steel pipe are more noticeable than the AE waves (also referred to as noise) caused by machine oil pressure, servo motor vibration, etc. that occur when there is no load. Indicates a large value. From the viewpoint of accurately detecting AE waves caused by cracks without erroneously detecting this noise, for example, the threshold value of the AE signal is set to be twice or more than twice the average absolute value of the AE signal under no load. be able to. Note that the absolute value average value herein refers to the average value of all the AE signal values (wave amplitude values) of the AE waves measured during no-load, as absolute values.

Note that the steel pipes whose workability is evaluated in the present invention are not particularly limited, and include electric resistance welded steel pipes, forge-welded steel pipes, seamless steel pipes, UOE steel pipes, and the like. Moreover, the size and steel type of the steel pipe are not particularly limited.

According to the present invention, since the presence or absence of cracking is determined based on AE waves, the occurrence of cracking can be determined with high accuracy.
Further, according to the present invention, since it is possible to quantitatively evaluate a steel pipe during a flattening test, it is possible to prevent variations in evaluation by testers. Moreover, since monitoring can be performed using an AE sensor, it is possible to prevent cracks from being overlooked even in a flattening test of a steel pipe whose size is difficult to judge visually.

This example was carried out using the apparatus shown in FIG. Table 1 shows the implementation conditions.
A strain sensor was attached to each steel pipe at a 90° position, and after applying grease between the AE sensor and the flat plate, the AE sensor was attached to the upper flat plate of the flattening tester.
The mounting position of the AE sensor on the flat plate was directly above the steel pipe, as shown in FIG. In the example of the present invention, the lower flat plate was fixed and the upper flat plate was operated to compress the steel pipe.
In the test, the test was first stopped for 5 seconds when the flat plate was brought into contact with the steel pipe, and the average absolute value of the AE signal under no load was measured.
Thereafter, compression was started, and the strain produced on the surface of the steel pipe and the AE signal were recorded.
A recording example is shown in FIG. Further, FIG. 5 shows a measurement example of the AE event rate (the number of AE signals (waveforms) exceeding a predetermined threshold per unit time (1 second)) and the flattening rate of a steel pipe. The records in FIGS. 4 and 5 are No. This is the result of 1.
The above-mentioned threshold value for determining that a crack has occurred was twice the absolute average value (V) of the AE signal at no load.
During the test, a strain of 2.0% or more was recorded by a strain sensor, and when the AE event rate reached 10 or more, the testing machine was stopped and the steel pipe surface was observed, and cracks were observed.

Table 1 shows the evaluation results under each implementation condition. In addition, in the table, the maximum AE event rate refers to the maximum value of the AE event rates measured in each evaluation.

According to the present invention, it has been found that it is possible to perform a flattening test, detect acoustic emission waves (AE waves) generated by the flattening test, and determine whether cracks have occurred in the steel pipe based on the detected AE waves. Ta.

1 flattening tester 2 flat plate (upper side)
3 Flat plate (lower side)
4 Steel pipe 5 Strain sensor 6 AE sensor 7 AE sensor amplifier device 8 Arithmetic device 9 AE signal display device 10 Strain sensor amplifier device 11 Strain recording device

Claims (2)

A clamping process of clamping the steel pipe;
a detection step of detecting acoustic emission waves generated by the pinching pressure;
a determination step of determining whether or not a crack has occurred in the steel pipe based on the detected acoustic emission wave;
including;
In the determination step,
In the detected acoustic emission wave signal,
A method for evaluating the workability of a steel pipe, in which it is determined that a crack has occurred in the steel pipe when the number of signals exceeding a predetermined threshold per unit time is equal to or greater than a predetermined number . The method for evaluating workability of a steel pipe according to claim 1 , wherein the threshold value is determined based on the magnitude of a signal of an acoustic emission wave detected from the steel pipe under no load.

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