Manufacturing method of standard test piece, creating calibration curve, and sensitivity calibration method of ultrasonic flaw detector

Abstract

To precisely adjust a distance to an artificial inclusion in a standard test piece in which the artificial inclusion whose size is previously measured is buried and whose base material is formed of steel.SOLUTION: There is provided a method of manufacturing a standard test piece TP used for acquiring information indicating a correlation between a size of a nonmetallic inclusion and the reflection echo intensity of ultrasonic flaw detection and having an inspection surface S1 where the ultrasonic flaw detection is performed at a first frequency F1. An artificial inclusion AI (whose size is previously measured) is buried such that a base material BM closely adheres to a pre-processing test piece PP whose base material BM is steel. The ultrasonic flaw detection is performed at a second frequency F2 to the pre-processing test piece PP. A burying distance D2 from a surface equivalent to the inspection surface S1 or a surface located on the opposite side to the inspection surface S1 to the artificial inclusion AI is measured. The inspection surface S1 is formed by performing removal processing to the pre-processing test piece PP such that a distance D1 from the inspection surface S1 to the artificial inclusion AI becomes a target distance Dtarg on the basis of the measured burying distance D2.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a standard test piece used to create a calibration curve showing the correlation between the size of non-metallic inclusions contained in steel and the intensity of reflected echoes obtained by ultrasonic flaw detection, a method for creating a calibration curve using this standard test piece, and a method for calibrating the sensitivity of an ultrasonic flaw detection device using this standard test piece. [Background technology]

[0002] In Patent Document 1, a standard specimen for ultrasonic flaw detection testing (hereinafter simply referred to as a "standard specimen") is produced by forming a hole of a predetermined depth in a base made of thermoplastic resin, pouring particles for forming artificial defects into the hole, and then filling the hole with thermoplastic resin. This standard specimen is used to understand the relationship between the diameter of defects present in steel and the intensity of ultrasonic reflected waves when ultrasonic flaw detection is performed (the so-called calibration curve).

[0003] According to Patent Document 1, the transparent base material (thermoplastic resin) of the standard specimen allows the distance between the surface onto which ultrasonic waves are incident during ultrasonic testing (hereinafter referred to as the "ultrasonic wave incident surface") and the particles for forming artificial defects embedded in the standard specimen to be confirmed from the outside. By performing grinding or polishing on the surface of the standard specimen while checking this distance, the distance between the ultrasonic wave incident surface and the particles for forming artificial defects can be precisely adjusted. This improves the accuracy of the calibration curve described above. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-325831 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, the base material of the standard sample is formed from a thermoplastic resin, but the target for defect evaluation using actual ultrasonic flaw detection is a steel material. Because different materials result in different ultrasonic attenuation characteristics during ultrasonic flaw detection, a calibration curve created using a standard sample (thermoplastic resin) may deviate from the original calibration curve when a steel material is used. Furthermore, if the base material of the standard sample is formed from steel, it becomes impossible to externally confirm the distance from the ultrasonic wave incidence surface to the particles for artificial defect creation, making it impossible to precisely adjust this distance.

[0006] An object of the present invention is to provide a method for manufacturing a standard test piece having an artificial inclusion, the size of which has been measured in advance, embedded in the standard test piece, the base material of which is made of steel, which allows for precise adjustment of the distance from the surface of the standard test piece to the artificial inclusion, the size of which has been measured in advance. [Means for solving the problem]

[0007] The present invention relates to a method for manufacturing a standard test piece. The standard test piece is used to obtain information showing the correlation between the size of non-metallic inclusions contained in steel and the intensity of reflected echoes obtained by ultrasonic testing using ultrasonic waves of a first frequency. The standard test piece has an inspection surface on which ultrasonic testing using ultrasonic waves of the first frequency is performed.

[0008] In the manufacturing method of a standard test piece, a pre-machined test piece is prepared, the base material of which is formed from steel, before being machined into a standard test piece. An artificial inclusion corresponding to a non-metallic inclusion is embedded in this pre-machined test piece so as to be in close contact with the base material of the pre-machined test piece. The size of the artificial inclusion can be measured in advance. Next, ultrasonic testing is performed on the pre-machined test piece using ultrasonic waves of at least one type of second frequency to measure the embedding distance from the surface corresponding to the test surface or the surface opposite the test surface to the artificial inclusion. Next, based on the measured embedding distance, the pre-machined test piece is subjected to removal processing so that the distance from the test surface to the artificial inclusion is the target distance, thereby forming the test surface. This results in a standard test piece.

[0009] The following three methods are available for measuring the buried distance.

[0010] In the first method, a probe is placed away from the surface of the raw test piece, and ultrasonic testing (specifically, the water immersion method; JIS Z2344) is performed toward the surface of the raw test piece. The embedding distance can then be calculated based on the distance from the probe to the artificial inclusion and the distance from the probe to the surface of the raw test piece measured by this ultrasonic testing.

[0011] In the second method, ultrasonic testing (specifically, the water immersion method; JIS Z2344) is performed while moving the probe in a direction perpendicular to the surface of the raw test piece. The distance of movement of the probe from the time the surface of the raw test piece is detected by this ultrasonic testing until the artificial inclusion is detected can be regarded as the embedding distance.

[0012] In the third method, the buried distance can be measured by performing ultrasonic testing with the probe in contact with the surface corresponding to the test surface or the surface opposite the test surface (direct contact method; JIS Z2344).

[0013] The surface of the pre-processed test piece on which ultrasonic testing at the second frequency is performed can be the surface opposite to the surface on which removal processing is performed. In this case, the processing distance for performing removal processing can be determined based on the embedding distance measured by ultrasonic testing at the second frequency, the thickness of the pre-processed test piece, and the target distance.

[0014] In the pre-processing test piece, the surface on which ultrasonic testing at the second frequency is performed can be the same as the surface on which removal processing is performed. In this case, the processing distance for performing removal processing can be determined based on the embedding distance measured by ultrasonic testing at the second frequency and the target distance.

[0015] The first frequency is preferably 10 MHz or higher. The second frequency is preferably 10 MHz or higher. The diameter of the artificial inclusion can be 30 μm or higher and equal to or smaller than the beam diameter of the ultrasonic wave of the first frequency.

[0016] After the removal process, a predetermined heat treatment is performed on the steel, and a structure corresponding to the heat treatment can be formed. The processing distance during the removal process can be adjusted depending on the reduction in the distance from the inspection surface to the artificial inclusion due to the heat treatment.

[0017] By using the standard test piece manufactured by the manufacturing method of the present invention, it is possible to create a calibration curve showing the correlation between the size of non-metallic inclusions contained in steel and the intensity of reflected echoes obtained by ultrasonic flaw detection using ultrasonic waves of a first frequency. Here, the frequency of the ultrasonic waves used in the ultrasonic flaw detection when creating the calibration curve (the above-mentioned first frequency) can be 10 MHz or higher.

[0018] By performing ultrasonic flaw detection using the standard test piece manufactured by the manufacturing method of the present invention, the sensitivity of the ultrasonic flaw detection device can be calibrated so that the reflected echo intensity becomes the reference intensity. [Effects of the Invention]

[0019] According to the present invention, the base material of the standard test piece is the steel material to be evaluated for non-metallic inclusions, and the artificial inclusions are embedded in the base material in a state of being in close contact with the base material. This prevents the standard test piece and the steel material to be evaluated from exhibiting different ultrasonic attenuation characteristics when ultrasonic flaw detection is performed.

[0020] Furthermore, by performing ultrasonic testing using ultrasonic waves of the second frequency on the surface of the unprocessed test piece that corresponds to the test surface or the surface located opposite to the test surface, it is possible to precisely measure the embedded distance from the surface of the unprocessed test piece to the artificial inclusion. By forming the test surface by removing the unprocessed test piece based on this measured embedded distance, it is possible to precisely adjust the distance from the test surface to the artificial inclusion to the target distance.

[0021] If a calibration curve is created using the standard test piece manufactured in this way, the accuracy of the calibration curve can be improved. In addition, if the sensitivity of the ultrasonic flaw detector is calibrated using the standard test piece, the accuracy of this calibration can be improved. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a schematic diagram showing the internal structure of a standard test piece. [Figure 2] FIG. 1 is a diagram showing the appearance of a standard test piece. [Figure 3] 1 is a flowchart illustrating a method for manufacturing a standard test piece. [Figure 4] FIG. 2 is a schematic diagram showing the internal structure of a test piece before processing. [Figure 5] FIG. 10 is a diagram illustrating a first method for measuring a distance D2. [Figure 6] FIG. 10 is a diagram illustrating a second method for measuring the distance D2. [Figure 7] FIG. 1 is a diagram illustrating a method for producing a standard test piece from a pre-processed test piece. [Figure 8] 10A and 10B are diagrams illustrating another method for producing a standard test piece from an unprocessed test piece. [Figure 9] 10A and 10B are diagrams illustrating another method for producing a standard test piece from an unprocessed test piece. [Figure 10] 1 is a flowchart illustrating a method for creating a calibration curve using a standard test piece. DETAILED DESCRIPTION OF THE INVENTION

[0023] This embodiment is a method for producing a standard test piece. This standard test piece is used to obtain information (hereinafter referred to as "correlation information") showing the correlation between the size of non-metallic inclusions contained in steel and the reflected echo intensity obtained by ultrasonic flaw detection. This correlation information can be used to create a calibration curve for estimating the size of non-metallic inclusions, or to calibrate the sensitivity (Sr) of an ultrasonic flaw detection device. The calibration curve is a graph showing the reflected echo intensity according to the size of non-metallic inclusions. A specific description will be given below.

[0024] (Standard test piece) As shown in Figure 1, the standard test piece TP has an artificial inclusion AI (spherical) simulating a non-metallic inclusion embedded in the base material BM. The standard test piece TP also has a surface S1 (hereinafter referred to as the "inspection surface") on which ultrasonic testing is performed to obtain the above-mentioned correlation information, and a surface S2 (hereinafter referred to as the "distance measurement surface") that serves as a reference when measuring the distance to the artificial inclusion AI.

[0025] The distance between the inspection surface S1 and the distance measurement surface S2 corresponds to the thickness T1 of the standard test specimen TP. The distance D1 is the distance from the inspection surface S1 to the artificial inclusion AI in a direction perpendicular to the inspection surface S1. The distance D2 is the distance from the distance measurement surface S2 to the artificial inclusion AI in a direction perpendicular to the distance measurement surface S2 (corresponding to the "embedded distance" in this invention). Here, the position of the artificial inclusion AI when determining the distances D1 and D2 is the position where the size of the artificial inclusion AI is largest when viewed from the inspection surface S1, that is, the center of the artificial inclusion AI (spherical). The sum of the distances D1 and D2 is the thickness T1 of the standard test specimen TP.

[0026] 1, the distance measurement surface S2 is located on the opposite side of the inspection surface S1, but this is not limiting, and the distance measurement surface S2 may be the same as the inspection surface S1. Here, the inspection surface S1 and the distance measurement surface S2 form parallel surfaces.

[0027] The inspection surface S1 does not have to be one surface of the standard test piece TP, and multiple surfaces of the standard test piece TP can be used as the inspection surface S1. When multiple inspection surfaces S1 are provided on the standard test piece TP, the above-mentioned correlation information can be obtained by performing ultrasonic testing on each inspection surface S1. Furthermore, when multiple surfaces of the standard test piece TP are used as the inspection surfaces S1, the same surface as each inspection surface S1 or a surface located on the opposite side of each inspection surface S1 (a surface parallel to the inspection surface S1) can be used as the distance measurement surface S2.

[0028] Fig. 2 shows an external appearance (one example) of the standard test piece TP. As shown in Fig. 2, the standard test piece TP is a cube and has six faces S11 to S16. At least one face (any face) of the six faces S11 to S16 can be used as the inspection face S1. For example, when face S11 (front face) is used as the inspection face S1, face S11 or face S12 (rear face) located opposite face S11 can be used as the distance measurement face S2.

[0029] The correlation information obtained using the standard test piece TP represents the correlation between the size of the artificial inclusion AI and the reflected echo intensity measured by ultrasonic testing. The reflected echo intensity here refers to the maximum value (peak value) of the reflected echo intensity measured by ultrasonic testing. This correlation can be defined, for example, by a linear function with the size (projected area, described below) of the artificial inclusion AI and the reflected echo intensity as variables.

[0030] When ultrasonic testing is performed on steel material to be evaluated for nonmetallic inclusions, the reflected echo intensity can be measured. If a calibration curve is previously calculated based on correlation information obtained using a standard test piece TP, the size of nonmetallic inclusions corresponding to the reflected echo intensity (measured value) can be determined based on this calibration curve. In other words, the size of nonmetallic inclusions present in the steel material (evaluation target) can be estimated.

[0031] The above-mentioned sizes may be defined in advance. For example, the maximum diameter (diameter or radius) or the projected area may be used as the size. The projected area is the projected area of ​​the artificial inclusion AI or the non-metallic inclusion when viewed from a predetermined direction (for example, a direction perpendicular to the inspection surface S1). In determining the correlation between the size of the artificial inclusion AI and the reflected echo intensity, the above-mentioned projected area can be used as the size of the artificial inclusion AI. Furthermore, when estimating the size of a non-metallic inclusion, the diameter or radius can be used as this size. When the artificial inclusion AI or the non-metallic inclusion is considered to be a sphere, the diameter or radius can be determined from the projected area, or the projected area can be determined from the diameter or radius.

[0032] The base metal BM of the standard test piece TP is made of a steel material having the same chemical composition as the steel material to be evaluated for non-metallic inclusions. For example, when the steel material to be evaluated is a bearing steel, the base metal BM can be made of the same chemical composition as the steel grade of this bearing steel (SUJ2, SUJ3, etc. as specified in JIS G4805).

[0033] The artificial inclusions AI may be any material that can simulate non-metallic inclusions present in steel, and for example, spherical Al2O3 particles can be used.

[0034] In the standard test specimen TP, the gaps at the interface between the artificial inclusions AI and the base material BM are closed, and the artificial inclusions AI are embedded in close contact with the base material BM. "Close contact" here refers to the absence of gaps or voids at the interface between the artificial inclusions AI and the base material BM. In actual steel, ultrasonic testing is often performed from the side of the rolled steel. When testing from this direction, nonmetallic inclusions and the base material are in close contact with each other. Therefore, in the standard test specimen TP, the artificial inclusions AI must be placed in close contact with the base material BM. This ensures that the correlation information obtained using the standard test specimen TP reflects the internal state of the actual steel (i.e., the state of contact between the nonmetallic inclusions and the base material). The accuracy of the size estimation of nonmetallic inclusions can be improved by using a calibration curve created from this correlation information. Furthermore, the accuracy of the sensitivity calibration of the ultrasonic testing instrument can be improved by calibrating the sensitivity (Sr) based on the correlation information.

[0035] The reflectivity of ultrasonic waves (in other words, the intensity of the reflected echo) during ultrasonic testing varies depending on whether or not there is a gap at the interface between the artificial inclusion AI and the base metal BM. Therefore, if correlation information is acquired when a gap exists at the interface between the artificial inclusion AI and the base metal BM, this correlation information will be less likely to reflect the actual internal state of the steel (i.e., the adhesion state between the non-metallic inclusion and the base metal). In this case, the accuracy of estimating the size of non-metallic inclusions based on a calibration curve created from the correlation information will decrease. Furthermore, the accuracy of calibrating the sensitivity (Sr) of the ultrasonic testing device based on the correlation information will decrease.

[0036] The diameter of the artificial inclusions AI can be 30 μm or more. Non-metallic inclusions with a size (diameter) of less than 30 μm can be measured based on the microscope testing method (JIS G0555), and therefore are not suitable for estimating the size of non-metallic inclusions by ultrasonic flaw detection as in this embodiment.

[0037] Furthermore, the diameter of the artificial inclusion AI can be set to be equal to or smaller than the ultrasonic beam diameter in ultrasonic flaw detection. This is because if the diameter of the artificial inclusion AI is larger than the ultrasonic beam diameter, there will be no correlation between the projected area of ​​the artificial inclusion AI and the reflected echo intensity. Therefore, it is preferable that the diameter of the artificial inclusion AI be equal to or smaller than the ultrasonic beam diameter.

[0038] Here, ultrasonic flaw detection is divided into a type that focuses an ultrasonic beam (hereinafter referred to as a "focused type") and a type that does not focus an ultrasonic beam (hereinafter referred to as a "non-focused type").Furthermore, the focused type is divided into a type that focuses the beam to a single point (hereinafter referred to as a "point focusing type") and a type that focuses the beam in a straight line (hereinafter referred to as a "line focusing type").

[0039] When using a focusing type (point focusing type or line focusing type), the beam diameter used to determine the diameter of the artificial inclusion AI can be the beam diameter at the position where the artificial inclusion AI exists (in other words, the beam diameter corresponding to the distance from the inspection surface S1 to the artificial inclusion AI). If the beam focusing state is known in advance, the beam diameter at the position where the artificial inclusion AI exists can be identified. Then, the diameter of the artificial inclusion AI can be determined so that it is equal to or smaller than this beam diameter. Note that because the beam focusing state differs depending on the type of probe (focusing type), the diameter of the artificial inclusion AI can be determined taking into account the probe (focusing type) used in ultrasonic flaw detection.

[0040] When using a non-focused type, the beam diameter for determining the diameter of the artificial inclusion AI can be the beam diameter at the inspection surface S1 of the standard test piece TP. With a non-focused type, a circular ultrasonic beam is emitted from the probe and travels in one direction, resulting in a columnar trajectory. Therefore, the diameter of the artificial inclusion AI can be determined by determining the beam diameter at the inspection surface S1. Note that because the beam diameter varies depending on the type of probe (non-focused type), the diameter of the artificial inclusion AI can be determined by taking into account the type of probe (non-focused type) used in ultrasonic flaw detection.

[0041] On the other hand, when the steel material being evaluated for nonmetallic inclusions is, for example, a high-cleanliness steel, the diameter of the artificial inclusions AI can be set to 500 μm or less. Since the frequency of nonmetallic inclusions larger than 500 μm in size (diameter) is extremely low in high-cleanliness steel, when the steel material being evaluated is a high-cleanliness steel, the diameter of the artificial inclusions AI can be set to 500 μm or less. Examples of high-cleanliness steel include steels with an oxygen content of 5 ppm or less and steels with impurity contents of P, S, and Ti below predetermined values ​​(P≦0.020% by mass, S≦0.008% by mass, Ti≦0.004% by mass). This high-cleanliness steel is suitable for use as bearing steel, particularly high-carbon chromium bearing steel.

[0042] The artificial inclusion AI is embedded at a predetermined position inside the standard test piece TP. In the example shown in Fig. 1, the artificial inclusion AI is located closer to the distance measurement surface S2 than to the inspection surface S1. Here, the distance D1 from the inspection surface S1 to the artificial inclusion AI is preferably 4 mm or more and 9 mm or less.

[0043] When ultrasonic testing is performed on the inspection surface S1 of the standard test piece TP to obtain the above-mentioned correlation information, the ultrasonic frequency F1 (corresponding to the "first frequency" in the present invention) can be set to 10 MHz or higher. The frequency F1 is preferably 25 MHz. When ultrasonic waves in this frequency range are used, if the distance D1 is less than 4 mm, noise is likely to be included in the detection data obtained when ultrasonic testing is performed, and the accuracy of the obtained correlation information is likely to decrease due to the influence of noise. Furthermore, if the distance D1 is longer than 9 mm, the ultrasonic waves are likely to be attenuated, making it difficult to identify the reflected echo intensity. Therefore, when the frequency F1 is set to 10 MHz or higher, setting the distance D1 to 4 mm or more and 9 mm or less can prevent a decrease in the accuracy of the obtained correlation information. Note that by using a filter that reduces noise included in the detection data obtained when ultrasonic testing is performed, the range of the distance D1 can be wider than the above-mentioned range (4 mm or more and 9 mm or less).

[0044] The standard test piece TP may be one that has undergone a specific heat treatment, such as that performed on steel for hardening, and has a microstructure corresponding to that heat treatment. For example, actual parts and steel materials may undergo heat treatments such as carburizing and induction hardening to harden their surfaces, resulting in a microstructure corresponding to that heat treatment (e.g., a carburized and quenched structure). Since the attenuation characteristics of ultrasonic waves used in flaw detection generally vary depending on the microstructure of the material, accurate sensitivity calibration is not possible if the microstructures of the standard test piece TP and the actual part being detected differ. Therefore, using a standard test piece TP with a similar microstructure to the actual part being detected by the same heat treatment as the actual part can provide more accurate correlation information, enabling more accurate sensitivity calibration and the determination of a threshold for detecting defects in flaw detection. Furthermore, when detecting flaws in an actual part with a carburized and quenched structure, for example, using an ultrasonic flaw detection system whose sensitivity has been calibrated using a standard test piece TP with a similar carburized and quenched structure can enable more accurate flaw detection.

[0045] (Manufacturing method of standard test pieces) In this embodiment, the artificial inclusions AI are embedded in predetermined positions with precision in the standard test piece TP. If the embedding positions of the artificial inclusions AI vary, the accuracy of the acquired correlation information will decrease, which will in turn decrease the accuracy of the calibration curve created from the correlation information and decrease the accuracy of the sensitivity calibration of the ultrasonic flaw detector based on the correlation information. If the artificial inclusions AI are embedded in predetermined positions with precision, it is possible to prevent the accuracy of the acquired correlation information from decreasing, improve the accuracy of the calibration curve for estimating the size of non-metallic inclusions, and improve the accuracy of calibrating the sensitivity (Sr) of the ultrasonic flaw detector.

[0046] A method for manufacturing a standard test piece TP in which artificial inclusions AI are embedded at predetermined positions with high precision will be described below with reference to the flowchart shown in FIG.

[0047] In step S101, a pre-machined test specimen PP is prepared before embedding the artificial inclusions AI. The pre-machined test specimen PP is a test specimen before manufacturing the standard test specimen TP. In manufacturing the pre-machined test specimen PP, a steel material having the same chemical composition as the steel material to be evaluated is prepared and machined into a predetermined shape. In manufacturing the pre-machined test specimen PP, the same heat treatment (e.g., normalizing and spheroidizing annealing) as used in manufacturing the steel material to be evaluated can be performed.

[0048] As shown in FIG. 4, the pre-machined test piece PP with the embedded artificial inclusion AI has a machining surface S3, which is machined to generate the inspection surface S1 of the standard test piece TP, and a distance measurement surface S2 located opposite the machining surface S3. The distance between the machining surface S3 and the distance measurement surface S2 corresponds to the thickness T2 of the pre-machined test piece PP. The thickness T2 of the pre-machined test piece PP is thicker than the thickness T1 of the standard test piece TP. The thickness T2 can be, for example, 12 mm.

[0049] In step S102, the size of the artificial inclusion AI prepared in advance is measured using a microscope or the like. For example, if the diameter r of the artificial inclusion AI (spherical) is measured, the projected area (=π×(r / 2)) of the artificial inclusion AI can be calculated. 2 ) can be obtained.

[0050] In step S103, the artificial inclusions AI whose sizes were measured in step S102 are embedded in the raw test piece PP. Specifically, first, artificial inclusions AI having the predetermined diameter (e.g., 30 μm or more) described above are prepared. Then, a drill hole is formed in the raw test piece PP using a microdrill or the like, and the artificial inclusions AI are placed at the bottom of the drill hole. The depth of the drill hole can be determined as appropriate. After the artificial inclusions AI are placed at the bottom of the drill hole, the drill hole is filled, and the artificial inclusions AI are adhered to the base material BM of the raw test piece PP by HIP (Hot Isostatic Pressing) or the like.

[0051] Specifically, when HIP processing is performed in step S103, the artificial inclusions AI placed in the drilled holes are first secured, the pre-machined test piece PP is placed in a low-carbon steel container, a mandrel is inserted into the bore hole formed in the pre-machined test piece PP, and the container is then sealed. The container is then evacuated, and the pre-machined test piece PP is then held at a predetermined pressure (e.g., 147 MPa) and a predetermined temperature (e.g., 1170°C) for a predetermined time (e.g., 5 hours), followed by slow cooling, thereby tightly bonding the artificial inclusions AI and the matrix BM. This intentionally creates a gap-free interface between the artificial inclusions AI and the matrix BM.

[0052] In step S104, the pre-machined test piece PP having the artificial inclusions AI embedded therein is machined into a predetermined shape. The predetermined shape here refers to a shape that allows ultrasonic testing in step S105, which will be described later. Any shape is acceptable as long as it allows ultrasonic testing in step S105, and it can be a disk shape or a cube shape, for example. Note that when HIP processing is performed in the processing of step S103, the processing in step S104 includes turning to remove the container that holds the pre-machined test piece PP and surface polishing to allow ultrasonic testing in step S105, which will be described later.

[0053] After the process of step S104, the as-processed test specimen PP having the artificial inclusions AI embedded therein may be subjected to a heat treatment. This heat treatment is the same as that used when preparing the as-processed test specimen PP in the process of step S101. Examples of this heat treatment include normalizing and spheroidizing annealing.

[0054] In step S105, ultrasonic testing is performed on the as-machined test piece PP in which the artificial inclusion AI is embedded, using ultrasonic waves having a predetermined frequency F2 (corresponding to the "second frequency" in the present invention), thereby measuring the distance D2 from the distance measurement surface S2 to the artificial inclusion AI. Note that the distance D2 in the as-machined test piece PP is the same as the distance D2 in the standard test piece TP (see FIG. 1).

[0055] The distance D2 can be measured, for example, by ultrasonic testing with the unprocessed test piece PP and the probe immersed in water (water immersion method; JIS Z2344). The distance D2 can be measured using the water immersion method in the following two ways.

[0056] In the first method, as shown in Figure 5, the probe P is placed at a position a predetermined distance D3 away from the distance measurement surface S2. The orientation of the probe P is determined so that ultrasonic waves are emitted toward the distance measurement surface S2. When ultrasonic flaw detection is performed in the state shown in Figure 5, the apex (center) coordinates of the artificial inclusion AI can be confirmed from the obtained distribution of reflected echo intensity (maximum reflected echo intensity), and D4 can be accurately measured. Distance D2 can be determined by subtracting distance D3 from distance D4.

[0057] In the second method, the orientation of the probe P is set so that ultrasonic waves are emitted in the direction indicated by the arrow (to the right in FIG. 6), as shown in FIG. 6. Here, the unprocessed test piece PP is not positioned below the probe P, but is shifted to the right in FIG. 6 from the position below the probe P.

[0058] When ultrasonic testing is performed while moving the probe P downward from the position shown in Figure 6, the edge of the distance measurement surface S2 is first detected, and when the probe P is further lowered, the artificial inclusion AI is detected. Here, the direction in which the probe P is moved is perpendicular to the distance measurement surface S2. By the above-mentioned ultrasonic testing, a coordinate Z1 of the probe P when the edge of the distance measurement surface S2 is detected (position information in the movement direction of the probe P) and a coordinate Z2 of the probe P when the artificial inclusion AI is detected (position information in the movement direction of the probe P) can be obtained. Based on these coordinates Z1 and Z2, the distance D2 can be determined. In other words, the amount of movement (lowering amount) of the probe P from the coordinate Z1 to the coordinate Z2 is the distance D2.

[0059] Here, the frequency F2 used when performing ultrasonic flaw detection in the process of step S105 may be one type of frequency or multiple types of frequencies. When multiple types of frequencies are used, ultrasonic waves of each frequency can be generated to measure the distance D2 respectively. Then, the distance D2 can be determined from the measurement results at the multiple types of frequencies.

[0060] When ultrasonic testing is performed in the process of step S105, a distance measurement plane S2 is determined in the unprocessed test piece PP, and when determining the distance measurement plane S2, the distance from the distance measurement plane S2 to the artificial inclusion AI (i.e., the measured distance D2) can be taken into consideration. Here, the distance from the distance measurement plane S2 to the artificial inclusion AI can be roughly determined based on the depth of the drill hole formed in the process of step S103.

[0061] If the distance from the distance measurement surface S2 to the artificial inclusion AI is too short, noise is likely to be included in the detection data when ultrasonic testing is performed, and the noise may reduce the accuracy of the correlation information obtained or make it difficult to determine the reflected echo intensity. Furthermore, if the distance from the distance measurement surface S2 to the artificial inclusion AI is too long, ultrasonic waves are likely to be attenuated, making it difficult to determine the reflected echo intensity. By taking these points into consideration when determining the distance measurement surface S2 for the unprocessed test piece PP, it is possible to eliminate the effects of noise and suppress ultrasonic attenuation.

[0062] By performing ultrasonic testing on the distance measurement surface S2 determined in consideration of the above points, the distance D2 can be measured with high accuracy. Note that, because the influence of noise and the attenuation of ultrasonic waves described above depend on the frequency F2, the distance measurement surface S2 can be determined taking into consideration the influence of noise and the attenuation of ultrasonic waves according to the frequency F2.

[0063] The frequency F2 is set to a frequency at which the distance D2 can be measured. Since the distance D2 varies depending on the position of the artificial inclusion AI inside the as-machined test piece PP, a frequency that includes the distance D2 within the range of measurable distances can be set as the frequency F2. When multiple distance measurement surfaces S2 are set in the as-machined test piece PP as described above, the distance D2 may differ depending on the distance measurement surface S2. In this case, the frequency F2 can be set to be different depending on the distance measurement surface S2. Note that the frequency F2 may be the same as or different from the above-mentioned frequency F1.

[0064] In step S106, as shown in FIG. 7, removal processing is performed on the processed surface S3 of the unprocessed test piece PP to form the test surface S1 of the standard test piece TP. The removal processing includes cutting, grinding, and polishing. In the removal processing, the distance D1 is adjusted to the target distance Dtarg. In this way, the standard test piece TP is obtained.

[0065] In adjusting the distance D1 by removal processing, the thickness T2 of the pre-processed test piece PP is measured in advance before performing removal processing on the processing surface S3. As shown in Figure 7, the distance ΔD (ΔD = T2 - (D1 + D2)) obtained by subtracting the sum of the distance D1 (corresponding to the target distance Dtarg) and the distance D2 (D1 + D2) from the thickness T2 is the processing distance when performing removal processing on the processing surface S3.

[0066] In the removal process for the processing surface S3, for example, the processing surface S3 can be cut by a distance shorter than the distance ΔD, and then polished until the distance ΔD is reached, thereby forming the inspection surface S1. As described above, the distance ΔD can be determined from the accurately measured distance D2, so by performing the removal process for the distance ΔD on the processing surface S3, the distance D1 can be made to match the target distance Dtarg with high precision.

[0067] By accurately matching the distance D1 with the target distance Dtarg, it is possible to eliminate variations in the distance D1, thereby improving the accuracy of the correlation information obtained by ultrasonic testing of the standard test piece TP. In other words, it is possible to suppress variations in the correlation information due to variations in the distance D1, thereby improving the accuracy of the correlation information. If the accuracy of the correlation information is improved, it is possible to improve the accuracy of the calibration curve created from the correlation information and to improve the accuracy of the sensitivity calibration of the ultrasonic flaw detection device based on the correlation information. Then, by estimating the size of non-metallic inclusions in steel using the calibration curve with improved accuracy, the accuracy of this estimation can be improved.

[0068] In the above-described manufacturing method of the standard test piece TP, the processing surface S3 and the distance measurement surface S2 are different surfaces, but this is not limiting, and the processing surface S3 and the distance measurement surface S2 may be the same surface. A manufacturing method of the standard test piece TP in this case will be described with reference to Figures 8 and 9. In this manufacturing method, the processes from steps S101 to S104 shown in Figure 3 are the same.

[0069] In the processing of step S105 shown in Fig. 3, as shown in Fig. 8, ultrasonic flaw detection is performed on the distance measurement surface S2 (in other words, the processed surface S3) of the unprocessed test piece PP using ultrasonic waves of frequency F2 to measure the distance D2. This makes it possible to measure the distance D2 with high accuracy, as described above. Note that the distance measurement surface S2 is the same surface as the processed surface S3, and corresponds to the inspection surface S1 formed by the removal processing described below.

[0070] Next, in the process of step S106 shown in Fig. 3, as shown in Fig. 9, removal processing is performed on the processing surface S3 of the unprocessed test piece PP (in other words, the distance measurement surface S2) to form the inspection surface S1. Specifically, removal processing is performed on the processing surface S3 by a distance (processing distance) ΔD corresponding to the difference between the distance D2 and the target distance Dtarg. Here, the target distance Dtarg must be shorter than the distance D2.

[0071] As a result, when the inspection surface S1 is formed, the distance from the inspection surface S1 to the artificial inclusion AI can be set to the target distance Dtarg. As described above, the distance ΔD can be calculated from the accurately measured distance D2, so by performing removal processing on the processing surface S3 by the distance ΔD, the distance from the inspection surface S1 to the artificial inclusion AI can be made to match the target distance Dtarg with high precision.

[0072] 3, the distance D2 is measured by using the water immersion method (JIS Z2344), but the method is not limited to this and the distance D2 can also be measured by using the direct contact method (JIS Z2344). Specifically, the distance D2 can be measured by bringing a probe into contact with the distance measurement surface S2 of the unprocessed test piece PP and generating ultrasonic waves having a predetermined frequency F2 (corresponding to the "second frequency" in the present invention).

[0073] In addition, when manufacturing a standard test piece TP having a structure corresponding to a predetermined heat treatment performed on steel for purposes such as hardening, the standard test piece TP can be manufactured having a desired microstructure corresponding to the heat treatment by further performing the necessary predetermined heat treatment after the removal processing in step S106. The heat treatment is not particularly limited as long as it corresponds to the microstructure that can be the target of flaw detection, but examples include carburizing and induction hardening, which harden the surface of the steel. Furthermore, if polishing is performed on actual parts after heat treatment, the standard test piece TP can also be polished in the same way.

[0074] The heat treatment may result in scale loss on the surface of the test piece, or in polishing the carburized abnormal layer formed by the heat treatment. This loss may reduce the distance D1 between the inspection surface S1 and the artificial inclusion AI by the amount of the loss. In this case, the processing distance for the removal process in step S106 can be adjusted (reduced) according to the reduction in distance D1. Specifically, the reduction in the distance between the inspection surface S1 and the artificial inclusion AI due to the loss is first determined by heat treating another test piece in advance and measuring it. Then, the removal process is performed using the distance ΔD' (= ΔD - reduced distance), which is calculated by subtracting the reduction in the distance ΔD shown in Figures 7 and 9 from the processing distance ΔD. This results in a standard test piece TP whose final thickness reduction due to the heat treatment after the removal process accurately matches the target distance Dtarg.

[0075] (How to create a calibration curve) Next, a method for creating a calibration curve using the standard test piece TP will be described with reference to the flowchart shown in FIG.

[0076] In step S201, ultrasonic flaw detection is performed by bringing a probe into contact with the inspection surface S1 of the standard test piece TP and generating ultrasonic waves with a frequency F1. This ultrasonic flaw detection obtains the correspondence between the size (projected area) of the artificial inclusion AI and the intensity of the reflected echo.

[0077] The process of step S201 is performed on a standard test piece TP in which a plurality of artificial inclusions AI with different diameters are embedded. Here, the plurality of artificial inclusions AI with different diameters may be embedded at different positions in one standard test piece TP (in other words, at different target distances Dtarg), or may be embedded in a plurality of standard test pieces TP. By performing ultrasonic flaw detection on the plurality of artificial inclusions AI with different diameters, a correspondence relationship between the size (projected area) of the artificial inclusion AI and the reflected echo intensity can be obtained for each of the plurality of artificial inclusions AI with different diameters.

[0078] In step S202, a calibration curve is created based on the correspondence relationship between the size (projected area) of the artificial inclusion AI and the reflected echo intensity obtained in the processing of step S201. Specifically, the correspondence relationship between the size (projected area) of the artificial inclusion AI and the reflected echo intensity obtained in the processing of step S201 is plotted in a coordinate system whose coordinate axes are the size (projected area) of the artificial inclusion AI and the reflected echo intensity, respectively. A regression equation can be obtained based on the plotted points in this way, and this regression equation can be used as the calibration curve.

[0079] (Sensitivity calibration of ultrasonic flaw detectors) Since there is variation (individual differences) in the reflected echo intensity between ultrasonic flaw detectors, it is necessary to calibrate the sensitivity (Sr) [dB] of the ultrasonic flaw detector to reduce this variation. The standard test piece TP manufactured by the manufacturing method of this embodiment can be used to calibrate the sensitivity (Sr) of the ultrasonic flaw detector.

[0080] Ultrasonic testing can be performed on the inspection surface S1 of the standard test piece TP, and the sensitivity (Sr) of the ultrasonic flaw detector can be calibrated so that the reflected echo intensity becomes a reference intensity (e.g., 80%). In the standard test piece TP manufactured by the manufacturing method of this embodiment, the distance from the inspection surface S1 to the artificial inclusion AI (distance D1 shown in FIG. 1) precisely matches the target distance Dtarg, so that the sensitivity (Sr) of the ultrasonic flaw detector can be calibrated with high precision. [Explanation of symbols]

[0081] TP: Standard test piece, PP: Unprocessed test piece, BM: Base material, S1: Inspection surface, S2: Distance measurement surface, S3: Processed surface, AI: Artificial inclusion, D1: Distance, D2: Distance, T1: Thickness (standard test piece), T2: Thickness (unprocessed test piece)

Claims

1. A method for manufacturing a standard test piece used to obtain information indicating a correlation between the size of non-metallic inclusions contained in steel and the intensity of a reflected echo obtained by ultrasonic testing using ultrasonic waves of a first frequency, the standard test piece having an inspection surface on which the ultrasonic testing is performed, comprising: an artificial inclusion whose size is measured in advance and corresponds to the non-metallic inclusion is embedded in a pre-processed test piece, the pre-processed test piece having a base material made of steel and prior to being processed into the standard test piece, so as to be in close contact with the base material of the pre-processed test piece; measuring an embedding distance from a surface corresponding to the inspection surface or a surface located on the opposite side of the inspection surface to the artificial inclusion by performing ultrasonic flaw detection on the unprocessed test piece using ultrasonic waves of at least one type of second frequency; forming the inspection surface by performing removal processing on the unprocessed test piece so that the distance from the inspection surface to the artificial inclusion becomes a target distance based on the measured embedding distance; A method for manufacturing a standard test piece.

2. When measuring the buried distance, performing ultrasonic flaw detection toward the surface of the unprocessed test piece using a probe disposed away from the surface of the unprocessed test piece; The embedding distance is determined based on the distance from the probe to the artificial inclusion and the distance from the probe to the surface of the unprocessed test piece, which are measured by the ultrasonic flaw detection.

2. The method for manufacturing a standard test piece according to claim 1.

3. When measuring the buried distance, ultrasonic flaw detection is performed while moving a probe in a direction perpendicular to the surface of the unprocessed test piece; The distance of movement of the probe from when the surface of the unprocessed test piece is detected by this ultrasonic flaw detection until when the artificial inclusion is detected is defined as the embedding distance.

2. The method for manufacturing a standard test piece according to claim 1.

4. 2. The method for manufacturing a standard test piece according to claim 1, characterized in that, when measuring the embedding distance, ultrasonic testing is performed with a probe in contact with a surface corresponding to the inspection surface or a surface located opposite to the inspection surface.

5. In the unprocessed test piece, a surface on which ultrasonic flaw detection at the second frequency is performed is a surface opposite to a surface on which the removal processing is performed, A method for manufacturing a standard test piece according to any one of claims 1 to 4, characterized in that the processing distance when performing the removal processing is determined based on the embedding distance measured by ultrasonic flaw detection at the second frequency, the thickness of the pre-processing test piece, and the target distance.

6. In the unprocessed test piece, a surface on which ultrasonic flaw detection at the second frequency is performed is the same as a surface on which the removal processing is performed, A method for manufacturing a standard test piece according to any one of claims 1 to 4, characterized in that the processing distance when performing the removal processing is determined based on the embedding distance measured by ultrasonic testing at the second frequency and the target distance.

7. 5. The method for manufacturing a standard test piece according to claim 1, wherein the first frequency is 10 MHz or higher.

8. 5. The method for manufacturing a standard test piece according to claim 1, wherein the second frequency is 10 MHz or higher.

9. 5. The method for manufacturing a standard test piece according to claim 1, wherein the diameter of the artificial inclusion is 30 μm or more and is equal to or less than the beam diameter of the ultrasonic wave of the first frequency.

10. 5. The method for manufacturing a standard test piece according to claim 1, wherein after the removal processing, a predetermined heat treatment is performed on steel to form a structure corresponding to the heat treatment.

11. 11. The method for manufacturing a standard test piece according to claim 10, characterized in that a processing distance when performing the removal processing is adjusted in accordance with a decrease in the distance from the inspection surface to the artificial inclusion caused by the heat treatment.

12. 5. A method for creating a calibration curve, comprising: using the standard test piece manufactured by the manufacturing method according to any one of claims 1 to 4, creating a calibration curve showing a correlation between the size of non-metallic inclusions contained in steel and the intensity of reflected echoes obtained by ultrasonic flaw detection using ultrasonic waves of a first frequency.

13. 13. The method for creating a calibration curve according to claim 12, wherein the first frequency is 10 MHz or higher.

14. A method for calibrating the sensitivity of an ultrasonic flaw detection device, characterized in that ultrasonic flaw detection is performed using the standard test piece manufactured by the manufacturing method described in any one of claims 1 to 4, thereby calibrating the sensitivity of the ultrasonic flaw detection device so that the reflected echo intensity becomes a reference intensity.

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