JP2022133622A - Measurement method of diameter of included object included in steel product - Google Patents

Abstract

To provide a method for predicting a maximum diameter of an included object in a steel product efficiently and accurately.SOLUTION: A prediction method of a maximum size comprises: (A) a step of collecting a first test piece from a steel product; (B) a step of subjecting the first test piece to an ultrasonic flaw detection test to identify a detection position being a position where existence of an included object is predicted; (C) a step of collecting a second test piece including the detection position from the first test piece; (D) a step of subjecting the second test piece to an ultrasonic fatigue test to break the second test piece and acquire a fracture surface; (E) a step of measuring a size of the included object existing on a starting part on the fracture surface; (F) a step of, on the basis of the size, by a statistical method, deriving a size of a maximum included object in the steel product.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for predicting the maximum diameter (maximum size) of inclusions present in steel. More particularly, the present invention relates to a method of predicting the maximum size of inclusions by a combination of ultrasonic testing and ultrasonic fatigue testing.

Bearings made of high-strength steel can break during use. One of the causes of this fracture is that stress concentration occurs in non-metallic inclusions in steel due to repeated fatigue called rolling fatigue during use of the bearing. These non-metallic inclusions are inevitably generated in the steel manufacturing process. It is thought that the influence range of stress concentration around inclusions correlates with the size of non-metallic inclusions. Therefore, it is important to grasp this size when estimating the service life of the bearing. It is also important to know the size of non-metallic inclusions when estimating the service life of steel parts other than bearings. In particular, it is important to grasp the diameter (maximum diameter) of the largest non-metallic inclusion among a plurality of existing non-metallic inclusions from the viewpoint of ensuring reliability of parts.

In 1993, "Metal Fatigue: Effects of Micro Defects and Inclusions (by Yokendo and Takanobu Murakami)" describes a method of predicting the maximum diameter of inclusions in a reference volume using speculum and extreme value statistics. is disclosed.

Japanese Unexamined Patent Application Publication No. 2009-281738 discloses a method for predicting the maximum diameter of inclusions in steel. In this method, the diameter of inclusions existing on the fracture surface is actually measured by the ultrasonic fatigue test. Based on this diameter value, the maximum diameter is calculated by extreme value statistics.

Japanese Patent Application Laid-Open No. 2020-34292 also discloses a method of predicting the maximum diameter of inclusions by an ultrasonic fatigue test and an extreme value statistical method. In this method, a hydrogen-charged specimen is subjected to an ultrasonic fatigue test. Hydrogen charging is performed for the purpose of promoting fracture of the test piece using hydrogen embrittlement.

Japanese Patent Application Laid-Open No. 2009-281738 Japanese Patent Application Laid-Open No. 2020-34292

"Metal Fatigue: Influence of Micro Defects and Inclusions (by Yokendo and Takanobu Murakami)" published in 1993

In steel parts such as bearings, fatigue failure may occur earlier than expected life based on the maximum diameter of non-metallic inclusions predicted using the extreme value statistics method. Such fatigue failures occur when the volume affected by fatigue in mass-produced steel parts is greater than the volume examined during prediction, and includes non-metallic inclusions with diameters greater than the predicted maximum diameter. This is because there are cases where Therefore, conventional methods of predicting the maximum diameter have room for improvement in terms of accuracy.

If a large number of cross-sectional observations are performed and the diameter of inclusions is actually measured, the maximum diameter can be predicted with high accuracy. However, it takes time and effort to actually measure diameters in a large number of cross sections, which is not realistic.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method by which the maximum size of inclusions in a large volume of steel can be predicted efficiently and accurately.

The maximum size prediction method according to the present invention includes:
(A) collecting a first test piece from the steel;
(B) a step of subjecting the first test piece to an ultrasonic flaw detection test to identify detection points where inclusions are suspected to exist;
(C) collecting a second test piece containing the detection point from the first test piece;
(D) subjecting the second test piece to an ultrasonic fatigue test to fracture the second test piece to obtain a fracture surface;
(E) measuring the size of inclusions present at the starting point on the fracture surface;
and (F) the above steps (A) to (E) are performed multiple times, and based on the sizes of the plurality of inclusions obtained, a statistical method is used to perform flaw detection by ultrasonic testing. Compared to the volume deriving the size of the largest inclusion present in a large volume of said steel.

Preferably, the statistical method in step (F) is extreme value statistics.

Preferably, in the ultrasonic flaw detection test of step (B), ultrasonic waves having a frequency of 10 MHz or more and 25 MHz or less are incident on the first test piece.

Preferably, in step (B), the evaluation volume of the first test piece subjected to the ultrasonic testing is 100,000 mm ³ or more.

Preferably, in the ultrasonic fatigue test of step (D), a load having a stress of 550 MPa or more is applied to the second test piece.

According to another aspect, the test piece manufacturing method according to the present invention comprises:
obtaining a first specimen from the steel;
A step of subjecting the first test piece to an ultrasonic flaw detection test to identify detection points where inclusions are expected to be present;
and obtaining from the first test strip a second test strip containing the detection point.

By the method according to the present invention, the maximum size of inclusions in steel can be accurately predicted. In addition, this method is highly efficient as a large-volume evaluation means.

FIG. 1 is a flow chart showing a prediction method according to one embodiment of the present invention. 2 is a perspective view showing an example of a first test piece used in the prediction method of FIG. 1. FIG. 3 is a perspective view showing an example of a second test piece used in the prediction method of FIG. 1. FIG. FIG. 4 is an enlarged cross-sectional view taken along line IV--IV of FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail based on preferred embodiments with appropriate reference to the drawings.

In the prediction method shown in FIG. 1, a steel bar is first prepared (STEP1). This steel bar is the target for which the maximum size of inclusions is expected. A steel material having another shape may be prepared instead of the steel bar.

A first test piece 2 is taken from this steel bar (STEP 2). This first test piece 2 is shown in FIG. This first test piece 2 has a cylindrical shape. The first specimen 2 of the example shown here has a diameter of 63 mm and a length of 250 mm. The evaluation volume of the first test piece 2 is determined by the diameter and length of the test piece, the detectable length of the ultrasonic flaw detector, and the flaw detection area (according to the flaw detection area excluding the dead zone on the steel surface in ultrasonic flaw detection). be done. This evaluation volume is used as a reference evaluation volume. The reference evaluation volume may be changed as appropriate for implementation. The first test piece 2 is obtained by subjecting a steel bar or the like as a raw material to cutting, plastic working, or the like. Prior to processing, the steel bar or the like may be subjected to heat treatment for facilitating processing or for suppressing attenuation of ultrasonic waves during ultrasonic flaw detection. The heat treatment is, for example, normalizing, and may be annealing or spheroidizing annealing. These heat treatment conditions are determined based on the chemical composition of the steel to be inspected. As a means for determining the conditions, calculation of equilibrium diagrams by thermodynamic calculation software and verification by preliminary experiments can be used, and those conditions can be used for steel types whose conditions are already known in literature and materials. The shape of the first test piece 2 is not limited to a cylinder. Various shapes can be adopted for the first test strip 2 .

This first test piece 2 is subjected to an ultrasonic flaw detection test (STEP 3). An apparatus for ultrasonic testing is equipped with an ultrasonic probe. Ultrasonic waves are emitted from this probe. This ultrasonic wave is incident on the first test piece 2 . If the first test piece 2 has internal defects such as non-metallic inclusions, ultrasonic waves are reflected by these internal defects. An internal defect is detected by detecting a reflected wave (echo from an internal defect) generated by this reflection with a probe. By emitting ultrasonic waves while the probe moves relative to the first test piece 2, a wide range of the first test piece 2 is inspected, and a single internal defect or a plurality of internal defects are detected. . If multiple internal defects are detected, one of the largest internal defects within the measurement volume is identified based on the reflected wave intensity (echo height) from the defect.

A direct contact method and a water immersion method can be employed as ultrasonic testing. Here, the water immersion flaw detection method is preferable from the viewpoint that ultrasonic waves can be transmitted and received stably. The frequency of the ultrasonic waves emitted from the probe in the ultrasonic testing is preferably 10 MHz or more and 25 MHz or less. More desirably, it is 20 MHz or more with high detection accuracy of internal defects. From the surface of the first test piece 2, the focal depth in the steel of the ultrasonic probe is, as described later, by processing the first test piece 2 followed by the second test piece 4, and the test piece In view of including the internal defects detected in the first test piece 2, it is preferable to set the depth to about 5 mm or more from the surface of the steel material.

Ultrasonic testing identifies locations where internal defects exist. In other words, the location (detection location) where the existence of the inclusion is presumed is specified. Marking is performed at the detection point which is considered to be the largest among them (STEP 4). A marking can be drawn on the surface of the first test strip 2 . Marking may be accomplished by electronic recording to identify the location. This is used when the information on the marking position of the first test piece 2 can be reflected as the processing position information of the processing device for processing the subsequent second test piece 4 .

A second test piece 4 is obtained from the first test piece 2 (STEP 5). In this sampling, the first test piece 2 is processed while referring to the marking positions described above. Specifically, processing is performed so that the detection point is included in the second test piece 4 . From one first test piece 2, one second test piece 4 can be taken. Specific examples of working procedures include roughing, quenching and tempering, and finishing. In the rough processing, the first test piece 2 is subjected to cutting or the like. As a guideline for the hardness by quenching and tempering of the second test piece 4, it is preferable to secure 450 HV or more. This is because there are cases where it cannot be broken from an object. More preferably, it is 500 HV or more. It should be noted that there may be a case where the second test piece 4 cannot be obtained from the first test piece 2 depending on the location where the internal defect exists, but in that case the first test piece 2 is not included in the evaluation.

An example of a second specimen 4 subjected to ultrasonic fatigue testing is shown in FIGS. The second test piece 4 has a dumbbell shape with a straight portion. The second test piece 4 has a central portion 6, a pair of large-diameter portions 8, and a male screw portion at one end. The male threaded portion is a portion connected to a horn of an ultrasonic fatigue tester in an ultrasonic fatigue test described later. The central portion 6 has a straight portion 10 and a pair of tapered portions 12 . In collecting the second test piece 4 shown in FIGS. 3 and 4, the straight portion 10 is processed so that the detection point is included. Since the second test piece 4 having the straight portion 10 contains the largest internal defect that was included in the first test piece 2, the actual evaluation volume in this case is the second test piece It can be regarded as the evaluation volume (reference evaluation volume) in the ultrasonic flaw detection test of the first test piece 2 instead of the volume of the straight part of 4. In addition, the shape of the second test piece 4 is not limited to the shape of the present embodiment, and other shapes may be employed.

Subsequently, the resonance frequency of this second test piece 4 is confirmed (STEP 6). For the second test piece 4 connected to the ultrasonic horn of the ultrasonic fatigue tester, the resonance frequency is compared with the reference frequency to determine pass/fail (STEP 7). The reference frequency is the frequency of ultrasonic vibration given to the second test piece 4 in the ultrasonic fatigue test which will be detailed later. A pass/fail determination is made based on the degree of divergence between the resonance frequency and the reference frequency. For example, when the reference frequency of the ultrasonic fatigue tester is 20,000 Hz, the second test piece 4 with a resonance frequency of 19,800 Hz or more and 20,200 Hz or less passes the test. The second test piece 4 with a resonance frequency of less than 19,800 Hz and the second test piece 4 with a resonance frequency of more than 20,200 Hz fail. According to a more stringent standard, the second test piece 4 with a resonance frequency of 19,970 Hz or more and 20,030 Hz or less is considered acceptable. The rejected second test piece 4 is corrected by processing (STEP 8). A typical modification is changing the length of the second test strip 4 . The change in length causes the resonance frequency of this second test piece 4 to fluctuate. The corrected resonance frequency of the second test piece 4 is checked again (STEP 6), and a pass/fail judgment is made (STEP 7). Second test strips 4 that fail may be discarded without modification.

The passed second test piece 4 is charged with hydrogen (STEP 9). Hydrogen embrittlement may occur in the second test piece 4 due to this charging. The second test piece 4 in which hydrogen embrittlement has occurred can break with a small stress and a small number of repeated cycles in an ultrasonic fatigue test, which will be detailed later. Hydrogen charging is performed for the purpose of facilitating and speeding up the fracture by this ultrasonic fatigue test.

As a method of charging hydrogen,
(1) Immersion of the second test piece 4 in the electrolytic solution (2) Exposure of the second test piece 4 to high-pressure hydrogen gas and (3) Electricity using the second test piece 4 as a cathode in the electrolytic solution Decomposition is exemplified. Other methods may be employed for hydrogen charging. In the case of electrolysis, for example, an electrolytic solution obtained by adding 3% sodium chloride and 0.3% ammonium thiocyanate to pure water can be used. The diffusion coefficient of hydrogen atoms into steel has temperature dependence. Therefore, if an electrolyte solution at a temperature higher than room temperature is used, charging can be performed with high efficiency, ie, in a shorter time. If a sufficiently large internal defect is included in the second test piece 4, the charging of hydrogen (STEP 9) may be omitted because the fracture is facilitated.

The second test piece 4 after charging with hydrogen is subjected to an ultrasonic fatigue test (STEP 10). The hydrogen introduced into the second test piece 4 by charging (STEP 9) is gradually released to the atmosphere. Due to the release, the amount of hydrogen contained in the second test piece 4 gradually decreases. However, if the ultrasonic fatigue test (STEP 10) is performed in a short time after charging (STEP 9), the second test piece 4 containing sufficient hydrogen can be subjected to the ultrasonic fatigue test. Therefore, the test can be conducted in a state where hydrogen embrittlement occurs.

In the ultrasonic fatigue test, a tensile load and a compressive load due to ultrasonic vibration are alternately and repeatedly applied to the second test piece 4 connected to the ultrasonic horn. The acting direction of these loads is the axial direction of the second test piece 4 . Stress is generated in the second test piece 4 as the load is applied. At this time, preferably, a load is applied to the second test piece 4 so as to generate a stress of 550 MPa or more. Note that this stress may be adjusted to a higher stress in order to promote rapid fracture on the premise that heat generation during the ultrasonic fatigue test of the second test piece 4, which will be described later, is suppressed.

During the ultrasonic fatigue test, heat is generated by internal friction caused by repeated stress loading, and the temperature of the second test piece 4 rises. This temperature rise promotes the release of hydrogen from the second test piece 4 and inhibits brittle fracture of the test piece, preventing completion of the test. Therefore, temperature rise needs to be suppressed. By blowing cooled air onto the second test piece 4, the temperature rise can be suppressed. Furthermore, the temperature rise can be suppressed by intermittently inputting ultrasonic vibrations to the second test piece 4 . Since the degree of temperature rise due to friction depends on the hardness of the test piece and the composition of the alloy, it is also possible to suppress frictional heat and temperature rise by selecting and applying an appropriate load. Usually, both air blowing, intermittent input of ultrasonic vibration, and appropriate load selection of the test load are performed.

Stress concentrates on the non-metallic inclusions in the second test piece 4 to which the load is applied. By repeating the loading of the load, fatigue from the largest non-metallic inclusion contained in the evaluation volume of the second test piece 4 (refers to the volume of the straight part with a diameter of 6 mm in the case of the test piece illustrated in FIG. 3) destruction occurs. The second test piece 4 is fractured by this fatigue failure, and a fracture surface is obtained. Breakage usually occurs in a plane perpendicular to the load direction (the axial direction of the second test piece 4). By observing the starting point of this fracture, the non-metallic inclusion that caused the fracture can be observed. There may be cases where non-metallic inclusions do not appear at the starting point, but the test results in such cases are not included in the analysis data by the extreme value statistical method performed later.

The diameter of the non-metallic inclusion present at the starting point on this fracture surface is measured (STEP 11). This measurement can usually be made by scanning electron microscopy (SEM). As a method for measuring the diameter (size) of non-metallic inclusions, the square root of the product of the major axis and the minor axis of the inclusion in the observed cross section ((major axis × minor axis) ^1/2 ) may be used. The cross-sectional area of inclusions may be calculated by image analysis and converted to a circle equivalent diameter. In addition to measuring the diameter, the chemical composition of inclusions may be analyzed using an energy dispersive X-ray spectrometer or a wavelength dispersive X-ray spectrometer attached to a scanning electron microscope. This can be utilized as data indicating the characteristics of inclusions contained in steel.

By performing an ultrasonic fatigue test (STEP 10) on a plurality of second test pieces 4 taken from a plurality of first test pieces 2, a plurality of fracture surfaces are obtained. By observing these, the diameters of a plurality of nonmetallic inclusions are measured. Based on the diameter data group of these non-metallic inclusions and the standard evaluation volume (evaluation volume per first test piece 2 by ultrasonic testing), the maximum inclusion present in the steel bar by a statistical method The diameter of the object is derived (STEP 12). Here, extreme value statistics are used as a typical statistical method. Based on the extreme value distribution model, the extreme value statistical method derives the maximum diameter of non-metallic inclusions within a conveniently defined predicted volume. In other words, the diameter of the largest inclusion that can be included in the steel bar is predicted.

The volume of the area where the stress of 90% or more of the load stress in the ultrasonic fatigue test of one second test piece 4 acts ("evaluation volume of ultrasonic fatigue test") is originally regarded as the evaluation volume. However, as described above, here, the second test piece 4 is removed from the original first test piece 2 so that the largest inclusion in the evaluation volume in the ultrasonic flaw detection test is included. Since 4 is taken out, the evaluation volume in the ultrasonic flaw detection test per first test piece 2 can be regarded as a substantial "ultrasonic fatigue test evaluation volume." This evaluation volume becomes a reference evaluation volume for prediction of the diameter of the largest inclusion.

In the present invention, regarding the "evaluation volume of ultrasonic fatigue test", which is originally only a very small volume, the steel bar is subjected to ultrasonic flaw detection test (STEP 3), and out of the extremely large volume, ultrasonic fatigue test (STEP 10) The target location (location where the presence of the largest inclusion is expected) is specified, and the ultrasonic fatigue test piece is taken out from there. It will be the target area of the evaluation.

In this way, by devising the "evaluation volume of the ultrasonic flaw detection test", which can be easily made extremely large, as an evaluation target area for the purpose of predicting the maximum inclusion diameter, in the prediction method according to the present invention, the maximum inclusion The diameter of an object can be accurately predicted. On the other hand, since the ultrasonic fatigue test piece used for evaluating it can be made small, it is possible to use equipment that is already commercially available for ultrasonic fatigue testing, and therefore it can be easily realized. In addition, as for the test time (the time required for breaking the test piece), the ultrasonic fatigue test and the embrittlement effect of charging the test piece with hydrogen can be used together, so that the test piece can be broken quickly. Therefore, the prediction method according to the present invention efficiently predicts the diameter of the largest inclusion. Furthermore, the prediction method according to the present invention is excellent in both accuracy and efficiency.

From the viewpoint of accuracy, the evaluation volume of the first test piece 2 subjected to the ultrasonic flaw detection test is preferably 100,000 mm ³ or more. More preferably, it is 200,000 mm ³ or more.

The present invention is also directed to a method of manufacturing test strips. This manufacturing method
A step of collecting the first test piece 2 from the steel material,
A step of subjecting the first test piece 2 to an ultrasonic flaw detection test to identify detection points where inclusions are expected to be present;
and from this first test piece 2 a step of taking a second test piece 4 containing the detection point.

A test piece can be easily obtained by this manufacturing method. This test piece can contribute to highly accurate inclusion size evaluation.

The prediction method according to the invention can be applied to steel products.

2... First test piece 4... Second test piece 6... Center part 8... Large diameter part 10... Straight part 12... Taper part

Claims (6)

(A) collecting a first test piece from the steel;
(B) a step of subjecting the first test piece to an ultrasonic flaw detection test to identify detection points where inclusions are suspected to exist;
(C) collecting a second test piece containing the detection point from the first test piece;
(D) subjecting the second test piece to an ultrasonic fatigue test to fracture the second test piece to obtain a fracture surface;
(E) measuring the size of inclusions present at the starting point on the fracture surface;
and (F) a plurality of steps (A) to (E) above are performed, and based on the obtained plurality of inclusion sizes, a statistical method is used to perform flaw detection by ultrasonic testing. A method for predicting the maximum size, comprising deriving the size of the largest inclusion present in the steel. 2. The prediction method according to claim 1, wherein the statistical technique in step (F) is extreme value statistics. The prediction method according to claim 1 or 2, wherein in the ultrasonic flaw detection test in step (B), ultrasonic waves having a frequency of 10 MHz or more and 25 MHz or less are incident on the first test piece. The prediction method according to any one of claims 1 to ³ , wherein in the step (B), the evaluation volume of the first test piece subjected to the ultrasonic flaw detection test is 100,000 mm3 or more. The prediction method according to any one of claims 1 to 4, wherein a load having a stress of 550 MPa or more is applied to the second test piece in the ultrasonic fatigue test of step (D). obtaining a first specimen from the steel;
A step of subjecting the first test piece to an ultrasonic flaw detection test to identify detection points where inclusions are expected to be present;
and a method for manufacturing a test strip, comprising the step of extracting a second test strip including the detection point from the first test strip.

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