Prediction method for maximum inclusion diameter in steel material using ultrasonic fatigue testing

Abstract

To provide an evaluation method for inclusions in a steel material that can detect quickly, at low cost, and with high accuracy relatively large non-metallic inclusions that exist at low frequencies in the steel material and can be used to predict the maximum inclusion diameter.SOLUTION: A prediction method for the maximum inclusion diameter in a steel material using ultrasonic fatigue testing includes collecting multiple test pieces each with a critical volume of 400 mm3 or more from the steel material to be evaluated, charging hydrogen into the test pieces, applying stress to each test piece using ultrasonic vibration to break the test pieces, measuring the diameters of non-metallic inclusions, which are fracture initiation points of fracture surfaces of the fractured test pieces, calculating the extreme value statistical distribution from the measured non-metallic inclusion diameters of each test piece, and predicting the maximum diameter of non-metallic inclusions present in a volume larger than the test pieces from this extreme value statistical distribution.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to a method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing.

Background Art

[0002] In high-strength steels used for bearing steels and the like, fatigue failure may occur with non-metallic inclusions inevitably contained in the steel as stress concentration sources. Inclusions in steel are mainly those that are inevitably generated in the steel manufacturing process and remain without being removed. And in order to ensure high cleanliness of steel, a technique for accurately evaluating the inclusion content in steel as described above is desired.

[0003] As methods for evaluating inclusions in steel, various methods have been proposed, such as a method for predicting the maximum inclusion diameter within a reference volume using a method combining observation with an optical microscope and extreme value statistics (for example, see Non-Patent Document 1), a method for predicting by observing inclusions that were the starting points after fatigue fracture with a servo fatigue testing machine using a fatigue test piece made of a metal material into which hydrogen has been introduced (for example, see Patent Document 1), and further, a prediction method by causing and observing fracture starting from inclusions using ultrasonic fatigue testing (for example, see Patent Documents 2 to 4), etc.

[0004] By the way, since the frequency of relatively large inclusions existing in steel is low, even when relatively large inclusions are actually contained in steel, if the cleanliness is evaluated with a small number of test pieces, it is difficult to contain inclusions and appropriate evaluation cannot be performed, and there may be a case where an actual part having a volume larger than the evaluated amount has a shorter life than expected and breaks (short life peeling). In order to suppress short life peeling of bearings, it is effective to reduce the size of relatively large inclusions that exist in steel at a relatively low frequency. However, as a prerequisite, it is important to correctly capture the presence of large inclusions and appropriately evaluate the state of the steel.

[0005] However, the method of evaluation using the extreme value statistical method based on the observation of inclusions with an optical microscope (microscopic examination extreme value statistical method) is an evaluation method based on two-dimensional observation. Therefore, even if it is converted into an evaluation volume considering the average thickness of the inclusions in the observation range, it is only a minute amount. Consequently, the probability of detecting relatively large inclusions that cause short-life delamination is extremely low, and moreover, it is a costly method.

[0006] In order to correctly evaluate relatively large inclusions, it is necessary to increase the dangerous volume of the test piece in order to increase the probability that a large inclusion is included in the dangerous volume of the test piece. Note that the dangerous volume refers to the volume of the evaluation target part per test piece. And, as a test method capable of evaluating a test piece with a large dangerous volume, a hydraulic servo fatigue test method is known. However, the repetition speed of the stress load of the test machine is only about 20 Hz to 1000 Hz, and because the repetition speed is slow, it takes a long time for the test piece to break.

[0007] On the other hand, in the method using an ultrasonic fatigue test machine, the repetition speed of the stress load is 20,000 Hz, and because the repetition speed is very fast, the time required for the test piece to break can be shortened to 10 minutes or less.

[0008] However, the ultrasonic fatigue test pieces shown in Patent Documents 2 to 4 are generally limited to those with a small dangerous volume. For example, in Patent Document 2, it is 14.14 mm 3 , in Patent Document 3, it is 33 mm 3 , and in Patent Document 4, it is 48.4 mm 3 . With an ultrasonic fatigue test piece having a dangerous volume only of this size, it is difficult to evaluate large non-metallic inclusions that exist at low frequencies in steel. On the other hand, in Patent Document 5, a large test piece with a dangerous volume of 400 mm 3 or more is shown.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non - Patent Document

[0010]

Non - Patent Document 1

Non - Patent Document 2

Non - Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] As described above, parts manufactured from steel containing relatively large inclusions may experience detachment in a shorter period than the expected lifespan. However, it is not easy to accurately grasp and predict the inclusion situation where large inclusions are mixed in. In the scope of the existing evaluation methods according to the invention, there is a limit to shortening the test time. For example, in the method of Patent Document 1, hydrogen is introduced into the test piece to promote hydrogen embrittlement, thereby shortening the test time. However, it takes several days from the hydrogen intrusion into the test piece to the fracture of the test piece, and further shortening of the test time is required. In the method of Patent Document 5, in order to shorten it, cathode charging is used to enable hydrogen intrusion in 24 hours, but there is still room for further shortening of the time required for the fatigue test.

[0012] Here, in order to evaluate relatively large inclusions, a method of increasing the test piece size with the aim of increasing the probability that the test piece contains relatively large inclusions by taking a large dangerous volume can be considered. However, there is a limit to the ultrasonic output that can be applied by a generally available ultrasonic fatigue testing machine. Therefore, the maximum stress (loading stress) that can be applied to the test piece is insufficient, and the test piece cannot be broken, making the measurement itself difficult in some cases.

[0013] From this perspective, in order to be able to break with a generally available ultrasonic fatigue testing machine, the size of the test piece must be small (in other words, the dangerous volume is small). However, if the dangerous volume of the test piece is too small, even if a rapid test can be realized, it becomes impossible to appropriately capture relatively large inclusions that exist at a low frequency. Therefore, since the situation of the entire steel is predicted based on insufficient test results, accurate speculation becomes difficult, and it becomes difficult to appropriately predict non-metallic inclusions that may be contained in the dangerous volume of actual steel parts or in the dangerous volume assuming a plurality of these actual parts.

[0014] In order to break through the above situation, the present invention provides a method for evaluating relatively large non-metallic inclusions in steel contained in a volume larger than the test piece volume by using a large test piece, and further aims at high-precision, low cost due to rapid testing in such a method. That is, while using a large-sized test piece, hydrogen charging is performed and then ultrasonic fatigue testing is carried out to break it quickly, and the results obtained by observing the non-metallic inclusions appearing at the starting point of the fracture surface of the broken test piece are statistically analyzed, so as to provide a method capable of appropriately estimating and evaluating the diameter of non-metallic inclusions contained in a steel volume larger than the individual test piece volume.

Means for Solving the Problems

[0015] The inventors of the present invention increased the dangerous volume of the ultrasonic fatigue test piece from the conventional about 14.14 mm 3 ~48.4 mm 3 to 400 mm 3 or more (for example, 770 mm 3 ), and established means for quickly breaking it at the same time, making it easier to quickly capture and evaluate relatively large non-metallic inclusions in steel that exist at low frequencies and were difficult to evaluate conventionally. When the volume of the evaluation target part per test piece is taken as the dangerous volume, the sum of the dangerous volumes when a plurality of test pieces are measured is called the evaluation volume. The volume of the steel material to be estimated based on this evaluation volume is called the prediction volume. Therefore, the prediction volume is a volume larger than the evaluation volume.

[0016] However, since generally available ultrasonic fatigue testing machines have limitations on the maximum output, when the test piece is enlarged, it cannot be broken and cannot be evaluated. That is, when the test piece is enlarged, conversely, the maximum stress (loading stress) that can be applied to the test piece becomes small, so it may happen that fatigue failure cannot be caused as it is.

[0017] Therefore, the inventors established a method of performing hydrogen charging after previously etching a large-sized test piece. By performing etching, the steel test piece can be pre-hydrogen embrittled by utilizing the action of the infiltrated hydrogen. In addition to this, it was newly found that when hydrogen is infiltrated, the internal friction in the fatigue test is reduced. As a result, by charging hydrogen, it became possible to reduce the temperature rise during the test as compared with the case where hydrogen is not charged. Thereby, the vibration time (the time when the test piece is actually vibrated by ultrasonic waves) in the ultrasonic fatigue test performed intermittently can be increased, and even a large test piece can be broken in a short time by an ultrasonic fatigue testing machine.

[0018] Then, the inventors observed the diameters of non-metallic inclusions that became the fracture initiation points appearing on the fracture surfaces of ultrasonic fatigue test pieces obtained in a plurality of ultrasonic fatigue tests, and applied the polar statistical method to these data. The inventors found that by actually evaluating a volume that is impossible to evaluate by the prior art quickly and at low cost, it is possible to predict the diameter of the largest non-metallic inclusion that can exist in a large volume with higher accuracy than the prior art.

[0019] Therefore, a first means for solving the problems of the present invention is to collect a plurality of test pieces with a dangerous volume of 400 mm 3 or more from the steel material to be evaluated, charge hydrogen into these test pieces, then apply stress by ultrasonic vibration to each test piece to break the test piece, measure the diameter of the non-metallic inclusion that is the fracture initiation point of the fracture surface of each broken test piece, obtain an extreme value statistical distribution based on the data group of the non-metallic inclusion diameters measured for each test piece, and use this extreme value statistical distribution to predict the diameter of the largest non-metallic inclusion existing in a larger volume of the evaluation steel material. This is a method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing. The "larger volume of the evaluation steel material" is an arbitrary volume and corresponds to the above-mentioned "prediction volume". Note that the dangerous volume per test piece here refers to the volume of the parallel part at the center of the ultrasonic fatigue test piece with a parallel part as illustrated in Fig. 2. Even when the central part of the test piece is a tapered part instead of a parallel part, the volume of the region where 90% or more of the load stress of the test piece acts is defined as the dangerous volume.

[0020] The second means is a method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to the first means, characterized in that the load stress applied to the test piece in the ultrasonic fatigue test is 550 MPa or more.

[0021] The third means is a method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to the first means, characterized in that the load stress applied to the test piece in the ultrasonic fatigue test is 600 MPa or more.

[0022] The fourth means is a method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to the first means, characterized in that the load stress applied to the test piece in the ultrasonic fatigue test is 650 MPa or more.

[0023] The fifth means is that the means for charging hydrogen into the test piece is by electrolytic charging by electrolysis with the test piece as the cathode in an electrolytic solution, and further, the temperature of the electrolytic solution is 20°C or more and 80°C or less. It is a method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to any one of the first to fourth means. When the temperature of the electrolytic solution increases, the amount of hydrogen charged into the test piece by hydrogen charging can be increased as will be described later.

[0024] In addition, a parallel portion is provided at the center in the longitudinal direction of the test piece for the purpose of increasing the dangerous volume as much as possible. Before performing hydrogen charging, nitriding treatment is carried out using nitriding to remove protrusions associated with minute unevenness on the surface generated during test piece processing with respect to this parallel portion. As an example, 5% nitriding can be used. This can suppress breakage from the surface due to the unevenness of the test piece surface and promote breakage from inclusions in the test piece.

[0025] When intermittently performing an ultrasonic fatigue test while cooling the test piece, the vibration time of the ultrasonic wave can be 120 msec or more. This can promote rapid fracture of the test piece. On the other hand, in order to prevent the test piece from overheating and becoming unable to continue the test even while cooling is being performed, the vibration time is preferably 240 msec or less, and more preferably 220 msec or less.

Advantages of the Invention

[0026] According to the present invention, by previously charging hydrogen into the test piece to embrittle the steel material with hydrogen, not only can the breaking stress be reduced and it can be easily broken from inclusions in the test piece even for a relatively large test piece, but also the temperature rise of the test piece during vibration with ultrasonic waves can be reduced. Therefore, by extending the vibration time, the test time can be shortened, enabling rapid evaluation compared to conventional methods. Thus, even a large test piece that normally cannot be broken by an ultrasonic fatigue test can be broken with a small load stress and in a short time. That is, the present invention is a low-cost evaluation method that does not require special equipment development, and can realize rapid fatigue fracture using a normal ultrasonic fatigue testing machine even for a test piece having a large dangerous volume (measurement site) that was difficult to break and could not be used in a normal ultrasonic fatigue test. It becomes possible to more appropriately evaluate the diameter of relatively large inclusions in the actual steel material, which is the fracture origin of the evaluated test piece, and use the data to estimate the maximum non-metallic inclusion diameter existing in a larger volume of steel material. Therefore, this method can predict the maximum non-metallic inclusion diameter present in a larger volume based on the diameter of larger non-metallic inclusions contained within a larger test volume than the conventional test method. As such, it is a method with higher prediction accuracy than the conventional method and is expected to enable low-cost and rapid evaluation.

Brief Description of the Drawings

[0027]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 8

Modes for Carrying Out the Invention

[0028] An embodiment of the prediction method according to the present invention will be described in order with reference to the flowchart of the test method shown in FIG. 1. In this embodiment, SUJ2 steel used for bearing steel will be described as the evaluation steel material. However, SUJ2 steel is a representative example, and the prediction method of the present invention does not limit the steel type. In other steel materials, test pieces can be measured according to the characteristics of the steel materials, and the maximum inclusion diameter in the steel can be appropriately predicted.

[0029] (Regarding Step A: Sampling and Adjustment of Ultrasonic Fatigue Test Pieces) When fabricating ultrasonic fatigue test pieces, for the steel material to be evaluated, appropriate heat treatment is carried out so as to be easy to rough machine and as a pretreatment when quenching is performed after rough machining, and then rough machining is performed into the test piece shape. As the heat treatment of the steel material prior to rough machining, normalizing, annealing, spheroidizing annealing, etc. can be selected. FIG. 2 is an example of a rough-machined test piece. The rough-machined test piece is quenched and tempered and then finish-machined to become a test piece. A plurality of such test pieces, for example, about 10 pieces, are fabricated. If it seems possible to ensure the hardness required for fracture by ultrasonic fatigue described later, the hardness may be adjusted by a method other than quenching and tempering. For example, high-frequency quenching etc. can be used. Here, since the test piece is intended to increase the dangerous volume to be evaluated to 400 mm 3 or more, as illustrated in FIG. 2, it is preferable to provide a parallel portion at the central part of the test piece. In this case, as described above, the dangerous volume is the volume of the parallel portion at the central part of the test piece. Since the diameter of the parallel portion is 7.0 (mm) and the length is 20.0 (mm), the dangerous volume per test piece in this embodiment is about 770 mm 3 is. The test piece is not limited to the shape illustrated in FIG. 2. The dangerous volume (the region where 90% or more of the load stress acts) changes according to the shape of the test piece, but the dangerous volume can be grasped for each shape by performing stress analysis etc. using a computer. For example, for a test piece in which the central part of the test piece is formed by a tapered portion instead of a parallel portion, based on stress analysis, the volume near the center of the test piece can be set as the dangerous volume.

[0030] In order to ensure fracture starting from inclusions, it is desirable to have a microstructure with low plastic deformation ability (brittle). For example, the hardness of the test piece can be used as a criterion, and it is desirable that the hardness is at least 400 HV or more. However, when the hardness is about 400 HV, when a crack starting from an inclusion progresses to a certain extent, the length of the test piece does not satisfy the resonance frequency, so the crack does not progress, and the ultrasonic fatigue test may end without complete fracture. In order to completely fracture the test piece, it is more desirable that the hardness of the test piece is 500 HV or more. Also, as a criterion for obtaining a microstructure that is easily brittlely fractured, the Ms point temperature (martensite transformation start temperature), which is a factor related to the martensite structure morphology, can be used. In this case, steel grades with an Ms point temperature of 450°C or lower are preferable for fracture by this method.

[0031] When performing an ultrasonic fatigue test on a finished test piece, the test piece needs to resonate according to the oscillation frequency of the ultrasonic fatigue testing machine used. Therefore, it is desirable to confirm the resonance frequency of the pre-processed test piece and adjust it as appropriate during the test. When using an ultrasonic testing machine with an oscillatable frequency of 20000 Hz ± 500 Hz, the resonance frequency of the test piece should be made as close as possible to the resonance frequency in order to facilitate leading to fracture after crack generation starting from inclusions in the test piece. Preferably, it is within 20000 Hz ± 200 Hz, and more preferably within 20000 Hz ± 30 Hz. When the test piece does not satisfy such a resonance frequency, the resonance frequency of the test piece can be adjusted by adjusting the length of the test piece.

[0032] The inventors have confirmed in the following experiment that when the test piece shown in FIG. 2 is subjected to an ultrasonic fatigue test generally used without hydrogen charging, it cannot be fractured in a short time. This will be specifically described below. A rolled material of φ65 mm made of SUJ2 steel was prepared. The SUJ2 steel was normalized and spheroidized by holding at 865°C for 1 hour, air-cooling, holding at the highest heating temperature (800°C), and slow cooling, and then roughly machined into the test piece shape shown in Fig. 2. The roughly machined test piece was quenched and tempered (holding at 835°C for 30 minutes, oil-cooling (OQ), holding at 180°C for 1.5 hours, and air-cooling (AC)), and then finish-machined. "OQ" is the symbol for oil-cooling, and "AC" is the symbol for air-cooling. The test piece after finish-machining is hereinafter also referred to as "example test piece". As mentioned above, the risk volume of one example test piece was about 770 mm 3 It is. In addition, after finishing the surface of the test piece by cutting, the surface unevenness of the test piece is reduced by utilizing the corrosive action of nital as described below, so mirror finishing by buffing or the like may be omitted.

[0033] The ultrasonic fatigue tester used in the implementation of the invention can oscillate at a frequency of 20,000 Hz ± 500 Hz, whereas the resonance frequency of the example test piece is 19,980 Hz, and a test piece was obtained that satisfies the above-mentioned "more preferable conditions". Note that, when the size of the test piece is increased to increase the risk volume relative to the rated output of the ultrasonic fatigue tester, the maximum stress that can be applied to the test piece decreases with the size of the risk volume. For example, in the case of the test piece of the example, the maximum stress that can be applied to the test piece is 870 MPa in the general ultrasonic fatigue tester used.

[0034] When the ultrasonic fatigue test was performed on the example test piece at a test stress of 840 MPa, which is close to the maximum stress that can be applied by the ultrasonic fatigue tester, the fatigue strength of 5.0 × 10 9 The test piece did not break even after the cycles. As described later, this test was an intermittent test in which ultrasonic oscillation and pause were repeated. 9The cycle corresponds to a test time of 6.75 days. That is, the test specimens of the examples could not be broken by the ultrasonic testing machine even after about one week. Also, since the specimens could not be broken, it was not possible to predict the diameter of non-metallic inclusions in the evaluated steel by subsequent analysis.

[0035] As is clear from the above experimental results, when using specimens of the size shown in Fig. 2, the load stress that can be applied by a generally available ultrasonic fatigue testing machine becomes small, making it difficult to break them. That is, it was concluded that it is difficult to break them only by ultrasonic fatigue testing. That is, even when using an ultrasonic testing machine, simply increasing the size of the specimen makes it practically difficult to break and evaluate the inclusions, or it was found that the test time required to break them becomes extremely long and rapid evaluation is impossible. Therefore, in order to promote fracture, in process C, hydrogen charging was used.

[0036] (Process B: Corrosion process of specimen surface) For the central part in the longitudinal direction of the specimen finished by machining (corresponding to the parallel part in this example), before performing hydrogen charging, a nitriding corrosion treatment is carried out using nitriding to remove the protrusions associated with the minute unevenness on the surface generated during specimen machining. As an example, a concentration of 5% nitriding can be used. This can prevent fracture from the surface caused by the unevenness of the specimen surface and promote fracture from inclusions in the specimen.

[0037] (Process C: Method of hydrogen charging the specimen) Hydrogen charging is carried out on the specimen to embrittle the specimen. By embrittling the specimen, the fracture stress of the specimen can be reduced, and even a large specimen can be broken by a short-time ultrasonic fatigue test.

[0038] The method of hydrogen charging is not particularly limited, but it is desirable to use a cathode charging method such as electrolysis with the test piece as the cathode in the electrolyte. According to this method, it is possible to perform hydrogen charging on a test piece with a parallel part diameter of φ7 mm in 24 hours, which is effective in shortening the test period. In the cathode charging method, for example, an electrolyte obtained by adding 3% sodium chloride + 0.3% ammonium thiocyanate to pure water can be used. In addition, for example, a method of immersing the test piece in various electrolytes or a method of exposing the test piece in high-pressure hydrogen gas can also be used.

[0039] Also, by utilizing the fact that the diffusion coefficient of hydrogen in steel increases with the increase in temperature, hydrogen charging can be performed more efficiently (in other words, the amount of charged hydrogen can be increased) by raising the temperature of the electrolyte. Utilizing this, it becomes possible to further accelerate the fracture during the ultrasonic fatigue test. At the same time, the time required for preparing the test piece can also be shortened. For example, in the cathode charging method, if the electrolyte is set at a temperature higher than room temperature, hydrogen charging can be performed efficiently. As the temperature higher than room temperature, a temperature range of 20°C or higher and 80°C or lower can be selected. If the temperature exceeds 80°C, corrosion of the test piece surface progresses, which is not preferable.

[0040] By performing efficient hydrogen charging, it is possible to quickly reach the fracture of a large test piece. By realizing this fracture process, it becomes possible to quickly evaluate relatively large non-metallic inclusions that were conventionally overlooked and present in the evaluated steel material. As a result, from the diameter of the non-metallic inclusion that became the fracture origin, the maximum non-metallic inclusion diameter that can be contained in an arbitrary volume larger than the test piece can be accurately estimated by the extreme value statistical method, and a more reliable evaluation can be stably performed.

[0041] (Process D: Regarding ultrasonic fatigue test) The hydrogen-charged specimen is subjected to ultrasonic fatigue testing. Here, the hydrogen charged in the specimen is gradually released from the specimen surface after the charging is completed. Therefore, it is desirable to promptly subject the specimen after hydrogen charging to ultrasonic fatigue testing.

[0042] Ultrasonic fatigue testing is a test in which repeated tensile and compressive loads are applied to the specimen by ultrasonic vibration in the axial direction of the specimen. When increasing the stress applied to the specimen in ultrasonic fatigue testing, it is possible to break the specimen in a shorter time in combination with the embrittlement promotion effect due to hydrogen charging.

[0043] Here, when continuously vibrating the specimen by ultrasonic waves, it is known that frictional heat is generated due to the high-speed repetition of tension and compression, and the specimen heats up. Due to the influence of heat generation, when the resonance frequency cannot be satisfied, the test cannot be continued. Also, the effect of heat generation is more likely to occur significantly the lower the hardness. Therefore, depending on the steel material, it may be difficult to increase the output, but it is possible to break the specimen without excessively increasing the output (loading stress) due to hydrogen charging, and as described above, it is also possible to reduce the temperature rise of the specimen itself due to hydrogen charging. Thus, the test can be performed without sacrificing the rapidity of ultrasonic fatigue too much.

[0044] Also, considering the suppression of heat generation of the specimen, if necessary, the ultrasonic fatigue test may be performed while cooling the specimen, or intermittent operation may be performed. The cooling means may be, for example, a method of blowing compressed air cooled by a chiller onto the specimen. Intermittent operation refers to an operation method in which the oscillation and stop of ultrasonic waves are repeated, and by performing intermittent operation, heat generation of the specimen can be suppressed. When performing the ultrasonic fatigue test intermittently while cooling the specimen, it is desirable that the vibration time of the ultrasonic waves be 120 msec or more. Thereby, rapid fracture of the specimen can be promoted. On the other hand, in order to prevent the specimen from heating up and making the continuation of the test impossible because the cooling of the specimen cannot catch up even when cooling is performed, the vibration time is preferably 240 msec or less, and more preferably 220 msec or less.

[0045] Here, in the ultrasonic fatigue test by intermittent operation, extending the vibration time of the ultrasonic wave results in shortening the test time. Therefore, by using hydrogen charging, it becomes possible to evaluate quickly as compared with the case without hydrogen charging. That is, the present inventors have found that hydrogen charging has a useful effect of delaying the temperature rise of the test piece during the ultrasonic fatigue test in addition to the effect of making the steel material more susceptible to brittle fracture. If the test piece is pre-charged with hydrogen before the ultrasonic fatigue test by intermittent operation, the oscillation time can be increased or the stop time can be shortened, so that evaluation can be performed more quickly as compared with the case without hydrogen charging. Such an effect will be clarified in the examples described later.

[0046] When evaluating a steel material with low hardness, the internal friction associated with ultrasonic vibration during the test increases, so that the test piece becomes more likely to generate heat. That is, in order to realize a rapid test, it is desirable to apply the maximum stress that can be applied by the ultrasonic fatigue testing machine, but considering heat generation and the like, there may be cases where the maximum stress cannot be applied. In this case, it is desirable to appropriately select the test stress according to the hardness of the test piece, that is, the evaluation steel material.

[0047] (Step E: Confirmation of inclusions that became fracture initiation points by SEM observation) Confirmation of non-metallic inclusions (initiation inclusions) that became fracture initiation points of the test piece is performed by a scanning electron microscope (SEM). The composition of the non-metallic inclusions that became fracture initiation points can be analyzed based on characteristic X-rays obtained by an energy dispersive X-ray spectrometer (EDS). Also, the inclusion diameter can be taken as the square root of the projected area (√area).

[0048] (Steps F and G: Extreme value statistical evaluation based on the measured non-metallic inclusion diameter) Data on the inclusion diameters that became fracture initiation points observed from the fracture surfaces of the respective test pieces are acquired, an extreme value statistical distribution is obtained by the extreme value statistical method, and based on this extreme value statistical distribution, the maximum inclusion diameter contained in the volume of any steel material larger than the test piece is estimated.

[0049] (Example) Examples are shown to specifically describe the present invention. For the above-described example test pieces, hydrogen charging by the cathode charging method was continuously performed for 24 hours. As the electrolytic solution, a solution obtained by adding 3% sodium chloride + 0.3% ammonium thiocyanate to pure water was used. The average current density of the current flowing through the example test pieces was set to 1.0 mA / cm 2 . The temperature of the electrolytic solution was set to two levels, 25°C or 50°C, and the results of hydrogen charging were measured by temperature-programmed desorption analysis using a gas chromatograph.

[0050] The hydrogen charge was 3.5 wt·ppm when the temperature of the electrolytic solution was 25°C and 6.8 wt·ppm when it was 50°C. Thus, it was confirmed that the amount of hydrogen charge can be increased by raising the temperature of the electrolytic solution.

[0051] The hydrogen-charged example test pieces (hardness 720 HV) were subjected to ultrasonic fatigue tests. In the ultrasonic fatigue tests, intermittent operation was performed in which ultrasonic vibration for 0.11 sec and stoppage of vibration by ultrasonic waves for 0.40 sec were repeated while blowing compressed air onto the test pieces as a cooling means. By such intermittent operation, the test could be appropriately performed without excessive heat generation even at a load stress of 840 MPa. As a comparative example, ultrasonic fatigue tests were also performed on the example test pieces that were not hydrogen-charged.

[0052] Figure 3 is a graph plotting the number of cycles (fatigue cycle number) until the example test pieces break and the load stress. The "double circle" represents the example test pieces that were not hydrogen-charged, the "square" represents the example test pieces hydrogen-charged with an electrolytic solution at 25°C, and the "triangle" represents the example test pieces hydrogen-charged with an electrolytic solution at 50°C. The example test pieces that were not hydrogen-charged did not break. In contrast, the hydrogen-charged example test pieces could break the example test pieces not only at a load stress of 840 MPa but also at a load stress of 750 MPa. In addition, in the case of the hydrogen-charged example test piece, it broke at a short number of cycles under any load stress, and it was also found that by increasing the temperature of the electrolytic solution, the number of cycles required for breakage could be made shorter.

[0053] In addition, an ultrasonic fatigue test was carried out by replacing the evaluation steel material with SCM420 steel (hardness 420 HV). The shape of the test piece was the same as that for SUJ2 steel. That is, a test piece with a dangerous volume of about 770 mm shown in FIG. 2 was used. However, in the case of SCM420 steel, even when the output was slightly suppressed so that the load stress became 750 MPa, significant heat generation of the test piece was recognized only by performing the same cooling as in the case of SUJ2 steel. As described above, depending on the hardness of the evaluation steel material, it is necessary to carry out the test at a lower load stress for a test piece with a lower hardness. In the case of SCM420, by further suppressing the output to a load stress of 710 MPa at maximum, it was possible to break in a short time while suppressing heat generation. 3 As described above, the inventors have newly found that hydrogen charging has the effect of delaying the temperature rise of the test piece during the ultrasonic fatigue test.

[0054] As described above, the inventors have newly found that hydrogen charging has the effect of delaying the temperature rise of the test piece during the ultrasonic fatigue test. FIG. 4 is a graph plotting the temperature rise process of a test piece accompanying an ultrasonic fatigue test of a general bearing steel as a representative example. The open white circles correspond to the case where hydrogen charging was not carried out, and the filled black circles correspond to the case where hydrogen charging was carried out. In both cases, the ultrasonic fatigue test was carried out by continuous operation without including the stop time. Also, no cooling treatment was carried out during the ultrasonic test. These tests were conducted to clarify the difference in the temperature rise behavior of the test piece depending on the presence or absence of hydrogen charging. For example, when comparing the time when the test piece temperature reached 150 °C, it was found that it was delayed by about 4.8×10 6 cycles due to hydrogen charging.

[0055] The temperature rise of the test piece associated with the ultrasonic fatigue test is due to the heat generated by the internal friction of the test piece vibrating by ultrasonic waves. Hydrogen is considered to play a role in promoting the movement of dislocations by reducing the elastic interaction acting between dislocations and solute atoms. Therefore, by performing hydrogen charging, it is presumed that dislocations become more mobile and the internal friction, which is the resistance to the movement of dislocations, is reduced. This is the mechanism by which heat generation decreases due to hydrogen charging, and hydrogen charging becomes a means of reducing the heat generation of the test piece in the ultrasonic fatigue test.

[0056] Also, the test time when intermittent operation and hydrogen charging were used in combination was evaluated. The evaluation material was the same SUJ2 steel as the test piece of the above-described example. The electrolytic solution used for hydrogen charging was the same as above, and the temperature of the electrolytic solution was set at 50°C. As shown in Table 1, when compared with the case of performing intermittent operation without hydrogen charging, the test time could be shortened by 31% in the case of intermittent operation (vibration time: 110 msec, stop time: 400 msec) + with hydrogen charging. Furthermore, by making it intermittent operation (vibration time: 220 msec, stop time: 900 msec) + with hydrogen charging, the test time could be shortened by 35%. The extension of the vibration time has a higher effect of accelerating the evaluation when evaluating steel grades with a lower carbon content than SUJ2 steel, that is, steel grades with a lower hardness of the test piece. This is because the lower the hardness of the test piece, the more the load stress has to be reduced to suppress the temperature rise of the test piece, and the effect of improving the time-consuming evaluation is large. This beneficial effect on rapid evaluation was achieved by finding that hydrogen charging reduces the temperature rise of the test piece and pursuing test conditions that actively utilize it. In any test, while performing the ultrasonic fatigue test, cooling treatment is carried out by blowing compressed air onto the test piece. Here, the shortening of the test time by utilizing the suppression of the temperature rise of the test piece during the ultrasonic fatigue test of the hydrogen-charged test piece is not limited to the increase in the vibration time in the intermittent operation shown in the example, and may be due to the shortening of the stop time in the intermittent operation.

Table 1

[0057] Figure 5 is an SEM image of the fracture surface of a test specimen of a certain example after the ultrasonic fatigue test. The fracture morphology presented a fish-eye pattern starting from non-metallic inclusions inside the test specimen as shown in the area of the broken line in the figure.

[0058] Figure 6 is an enlarged SEM image of the non-metallic inclusion that became the fracture origin from the area of the broken line shown in Figure 5. From the elemental analysis by EDS, the starting inclusion in this case was an oxide composed of CaO - Al2O3 and CaS. Also, the measurement of the diameter of the non-metallic inclusion was obtained as the square root of the projected area (√area).

[0059] Next, examples corresponding to Process E and Process F will be described. Table 2 shows the results of 11 test specimens subjected to hydrogen charging and ultrasonic fatigue tests.

Table 2

[0060] Hydrogen charging was performed on 11 test specimens of the example described in Figure 2. However, prior to performing hydrogen charging, corrosion treatment with a nital etching solution was performed on the parallel part. This is a necessary process to relieve the unevenness formed on the surface of the test specimen finished by cutting and to suppress fracture starting from the surface rather than inclusions. Hydrogen charging was continuously performed for 24 hours by the cathode charging method. As the electrolytic solution, a solution obtained by adding 3% sodium chloride + 0.3% ammonium thiocyanate to pure water was used. The average current density of the current flowing through the test specimen of the example was set to 1.0 mA / cm 2 and the temperature of the electrolytic solution was set to 50°C. Although the load stress during the test is related to the test time required until fracture, when delamination starting from inclusions occurs, the difference in load stress does not affect the inclusion diameter that appears.

[0061] After hydrogen charging, ultrasonic fatigue tests were carried out by intermittent operation (repeating vibration time of 220 msec and stop time of 900 msec) to fracture each test piece. The test pieces were cooled by blowing compressed air onto the test pieces. The test stresses are shown in Table 2. Regarding the non-metallic inclusions that became the fracture initiation points observed on the fracture surfaces of each test piece, their diameters were determined from the square root (√area) of the projected area on the observation surface, and the extreme value statistical distribution was obtained by extreme value statistical analysis. The obtained extreme value statistical distribution (in other words, the extreme value statistical graph) is shown in Fig. 7. The approximate curve of the extreme value statistical plot was obtained using least squares approximation.

[0062] The standardized variable (Y) in the extreme value statistical graph is a numerical value expressed by Y = -ln(-ln(F)), where F represents the standardized coefficient at this time. The standardized coefficient F is expressed by F = (T - 1) / T, and T represents the recurrence period. Furthermore, the recurrence period T is expressed by T = (V + V0) / V0. Here, V is the volume of the steel material to be estimated, and V0 is the dangerous volume per test piece.

[0063] As an example, when the recurrence period is 300, that is, the dangerous volume is 770 mm 3 the volume corresponding to 300 ultrasonic fatigue test pieces (231000 mm 3 ), the diameter of the non-metallic inclusions present in the steel is estimated to be at most 81 μm as shown in Fig. 7. This predicted volume (the volume of the steel material to be estimated) is selected according to the purpose, and is not limited to the prediction only for the volume corresponding to 300 test pieces exemplified here.

[0064] For comparison, the size of the test piece was such that the dangerous volume was 33 mm 3The extreme value statistical analysis results obtained from the ultrasonic fatigue test carried out with modifications to achieve this, and the extreme value statistical analysis results by the microscopy method using an optical microscope are shown together in Fig. 8 as comparative examples.

[0065] Fig. 8 shows the data superimposed based on the extreme value statistical distribution by the microscopy method. Since the microscopy method is a two-dimensional inspection, assuming that the average value of the inclusion diameters observed within the inspection reference area is the average thickness of the inclusions in the evaluated test piece, it is possible to consider it as a three-dimensional inspection with the product of the average thickness of the inclusions and the inspection reference area as the approximate evaluation reference volume.

[0066] As shown in Fig. 8, by performing the evaluation using a test piece with a significantly increased dangerous volume compared to the prior art, when evaluating a large volume exceeding 8700 mm 3 (equivalent to 8.0 in terms of the standardized variable for the evaluation volume per test piece of 0.8 mm 3 in the microscopy method of this comparative example), it is clear that the presence of larger non-metallic inclusion diameters can be predicted as shown in the graph of Fig. 8. Therefore, it has been found that by using a large test piece, the maximum inclusion diameter in a larger volume of steel, which has been difficult to estimate so far, can be accurately and stably estimated by this method.

[0067] As described above, even when using an ultrasonic fatigue testing machine that makes it difficult to break due to insufficient stress that can be applied when evaluating using a relatively large test piece, according to the present invention, by previously hydrogen charging the test piece, through the effect of becoming more likely to break due to hydrogen embrittlement and the effect of being able to extend the vibration time of the ultrasonic wave because the temperature rise due to the internal friction of the test piece is suppressed, it becomes possible to quickly break with only the load stress that can be applied to a large test piece without increasing the output of the testing machine or increasing the size of the testing machine. Furthermore, by raising the temperature of the electrolytic solution used for hydrogen charging, the ultrasonic fatigue test can be performed even more quickly. Therefore, according to the method of the present invention, since evaluation can be quickly performed with an ultrasonic fatigue testing machine with normal output, by performing evaluation using a relatively large test piece, it becomes possible to accurately estimate the maximum inclusion diameter that can be contained in a larger volume of steel.

Explanation of Signs

[0068] 1 Test piece 2 Dangerous volume part (measurement site)

Claims

1. Collect a plurality of test pieces with a dangerous volume of 400 mm or more from the steel material to be evaluated. After charging these test pieces with hydrogen, then apply stress by ultrasonic vibration to each test piece to break the test piece, measure the diameter of the non-metallic inclusions that are the fracture initiation points of the fracture surfaces of the broken test pieces, obtain an extreme value statistical distribution from the measured non-metallic inclusion diameters for each test piece, and predict the maximum non-metallic inclusion diameter existing in a volume larger than the test piece from this extreme value statistical distribution. A method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing. 3

2. The method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to Claim 1, characterized in that the stress applied to the test piece in the ultrasonic fatigue test is 550 MPa or more.

3. The method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to Claim 1, characterized in that the stress applied to the test piece in the ultrasonic fatigue test is 600 MPa or more.

4. The method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to Claim 1, characterized in that the stress applied to the test piece in the ultrasonic fatigue test is 650 MPa or more.

5. The means for charging the test piece with hydrogen is a method of performing electrolytic charging by electrolysis with the test piece as the cathode in an electrolytic solution, Further, the temperature of the electrolytic solution is 20°C or more and 80°C or less. The method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to any one of Claims 1 to 4.

6. The test piece has a parallel portion at the center in the longitudinal direction of the test piece, Before performing hydrogen charging, a corrosion treatment for removing surface protrusions generated during sampling of the test piece is performed on the parallel portion. The method for predicting the maximum inclusion diameter in steel by ultrasonic fatigue testing according to any one of Claims 1 to 4.

7. ​The test piece has a parallel portion at the center in the longitudinal direction of the test piece, The method for predicting the maximum inclusion diameter in a steel material by ultrasonic fatigue test according to claim 5, characterized in that, before performing hydrogen charging, a corrosion treatment for removing surface protrusions generated during sampling of the test piece is performed on the parallel portion.

8. The method for predicting the maximum inclusion diameter in a steel material by ultrasonic fatigue test according to any one of claims 1 to 4, characterized in that the load applied to the test piece in the ultrasonic fatigue test is performed by an intermittent test in which the ultrasonic vibration time is 120 msec or more while performing cooling.

9. The method for predicting the maximum inclusion diameter in a steel material by ultrasonic fatigue test according to claim 5, characterized in that the load applied to the test piece in the ultrasonic fatigue test is performed by an intermittent test in which the ultrasonic vibration time is 120 msec or more while performing cooling.

10. The method for predicting the maximum inclusion diameter in a steel material by ultrasonic fatigue test according to claim 6, characterized in that the load applied to the test piece in the ultrasonic fatigue test is performed by an intermittent test in which the ultrasonic vibration time is 120 msec or more while performing cooling.

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