Method for evaluating the cleanliness of steel materials

Abstract

This invention provides a method for evaluating the cleanliness of steel materials that can detect the number and size of inclusions in the steel material with greater accuracy, and is also applicable to steel materials with large diameters. [Solution] The method for evaluating the cleanliness of steel materials involves performing ultrasonic testing to inspect at least a portion of the steel material, and evaluating the cleanliness based on the dimensions and number of inclusions in the steel material obtained by the ultrasonic testing, taking into consideration all of the following (1) to (5). This method can also be used for large-diameter steel materials with a diameter of 80 mm or more. (1) Echo intensity, (2) Sample noise, (3) Focal length in the steel material, (4) Echo area of ​​the inclusion, (5) Sample diameter

Description

[Technical Field]

[0001] This invention relates to a method for evaluating the cleanliness of steel materials. [Background technology]

[0002] It has long been known that the rolling fatigue life of steel materials used in applications where rolling fatigue occurs, such as bearing steel, is strongly correlated with the amount of non-metallic inclusions in the steel, particularly oxide inclusions. Therefore, the amount of non-metallic inclusions in steel materials has been measured using the JIS method (JIS-G0555) or the ASTM (American Society for Testing and Materials): E45 method, and the results have been fed back into the steelmaking process to produce steel materials of a certain quality, or even higher quality.

[0003] Furthermore, since the amount of oxide inclusions in steel naturally correlates strongly with the oxygen content of the steel, various steelmaking methods effective in improving cleanliness, such as molten steel degassing, vacuum refining, and ladle refining, are employed in the production of steel requiring high cleanliness, such as bearing steel, to reduce the oxygen content in the steel. However, while recent advances in steelmaking technology have made it possible to reduce the oxygen content in steel to the greatest extent possible, when the oxygen content in steel is at a low level, nonmetallic inclusions in the steel become very few and small. As a result, measurements using the JIS method and ASTM method mentioned above are in the realm of detection limits, and in some cases, the evaluation of cleanliness levels may not be appropriate.

[0004] On the other hand, the extreme value statistical method is used as a method for evaluating small inclusions of several tens of micrometers, offering higher measurement accuracy than the JIS method and the ASTM method. The extreme value statistical method is currently a common evaluation method, and its evaluation method is for a unit area of ​​100-200 mm². 2 This method involves observing 15 to 30 samples, recording the largest inclusion diameter for each sample, and then using statistics to estimate the largest inclusion diameter present in the assumed area.

[0005] However, extreme value statistics is an evaluation method that uses statistics to assess a certain area of ​​steel material and predict the largest inclusion diameter present in the assumed area. As such, it has the problem that it requires a great deal of time and processing power for sample preparation, evaluation, and analysis.

[0006] In response to the problems with conventional cleanliness evaluation methods such as the JIS method, ASTM method, and extreme value statistical method, the applicant has proposed a method for evaluating the cleanliness of steel materials in a short time and with high measurement accuracy using ultrasonic testing, as shown in Patent Document 1. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 7092101 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the demand for evaluating the cleanliness of steel materials is increasing, and greater accuracy is required.

[0009] Furthermore, Patent Document 1 evaluates the cleanliness based on the dimensions and number of inclusions in the obtained steel material, but it targets round bars with a diameter of 42 mm to 82 mm as the steel material. However, depending on the application, it may be necessary to process thicker round bars, and there is a risk that sufficient evaluation cannot be obtained for thicker round bars.

[0010] This invention has been made in view of the above problems, and aims to provide a method for evaluating the cleanliness of steel materials that can detect the number and size of inclusions in steel materials more accurately, and that can also be used for steel materials with large diameters. [Means for solving the problem]

[0011] The method for evaluating the cleanliness of steel materials according to the present invention has the configuration shown in [1] below.

[0012] [1] Perform ultrasonic flaw detection to detect at least a part of the steel material, and based on the dimensions and number of inclusions in the steel material obtained by the ultrasonic flaw detection, A method for evaluating the cleanliness of a steel material, which evaluates the cleanliness in consideration of all of the following (1) to (5). (1) Echo intensity (2) Sample noise (3) Focal distance in the steel material (4) Echo area of the inclusions (5) Sample diameter

[0013] Moreover, a preferred embodiment of the method for evaluating the cleanliness of a steel material according to the present invention is as follows in the configurations of [2] to [3] below.

[0014] [2] The method for evaluating the cleanliness of a steel material according to [1], wherein the diameter of the steel material is 80 mm or more. [3] The method for evaluating the cleanliness of a steel material according to [1] or [2], wherein the steel material is a steel material used for a rolling bearing, a ball screw, a linear guide, a traction drive transmission, or an electric brake. [Advantages of the Invention]

[0015] According to the present invention, inclusions in a steel material can be detected in a shorter time using the ultrasonic flaw detection method, and more reliable data can be obtained regarding the size and number of the inclusions. Also, it can be applied to steel materials with a larger diameter. [Brief Description of the Drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing an example of an ultrasonic flaw detection inspection apparatus used in the present invention. [Figure 2] FIG. 2 is a graph showing the results of Example 1. [Figure 3] FIG. 3 is a graph showing the results of Comparative Example 1. [Modes for Carrying Out the Invention]

[0017] As a result of intensive research to solve the above problems, the inventors have found that when inspecting steel materials by ultrasonic flaw detection, by adjusting five factors: (1) echo intensity, (2) sample noise, (3) focal distance in the steel material, (4) echo area of inclusions, and (5) sample diameter, inclusions in the steel material can be detected more accurately, and it is also possible to fully handle even thicker round bars. The present invention has been made based on this finding. Specifically, the influence of each influencing factor on the dimensions of actual inclusions is experimentally investigated. If it is a positive relationship, it is proportional, and if it is a negative relationship, it is inversely proportional. Also, considering the interaction between the influencing factors, the influencing factors are multiplied or divided to reflect them in the dimensions of the inclusions.

[0018] Hereinafter, the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments described below, and can be arbitrarily modified and implemented without departing from the gist of the present invention.

[0019] In the present invention, ultrasonic flaw detection is performed on a steel material. For example, the ultrasonic flaw detection inspection device 1 shown in FIG. 1 can be used. That is, the ultrasonic flaw detection inspection device 1 is a device for evaluating the cleanliness of a round bar 2 which is a steel material. As shown in FIG. 1, it includes a probe 11, a liquid tank 12 storing an ultrasonic transmission medium, three motors 13 to 15, a motor controller 16, a flaw detector 17, a calculation unit 18, and a chuck device 19.

[0020] As the transmission medium, water may be used, but in order to prevent rusting of the round bar 2, it is preferable to use kerosene, hydrocarbons other than kerosene, or industrial cleaning agents. Among them, industrial cleaning agents are widely used in the manufacturing process of bearings, and it can be said that they are particularly preferable as the transmission medium because there is no need to prepare them separately like kerosene and hydrocarbons.

[0021] Furthermore, there are no restrictions on the type of steel used for the round bar 2. In addition to bearing steel, other steel materials that may cause defects due to non-metallic inclusions (for example, high-cleanliness steel such as case-hardened steel) can be used, such as steel used in rolling bearings, ball screws, linear guides, traction drive transmissions, electric brakes, etc. There are also no restrictions on the diameter of the round bar 2. However, according to the present invention, it becomes possible to evaluate the cleanliness of larger diameters that are not covered in Patent Document 1, specifically φ80 mm or more, preferably φ83 mm or more, more preferably φ90 mm or more, and especially preferably φ100 mm or more. However, regarding the upper limit of the diameter, a diameter of around φ180 mm is appropriate, taking into consideration the size of the liquid tank 12 and the load on the chuck device 19.

[0022] The probe 11 is a focused ultrasonic probe that emits ultrasonic waves and detects the reflected echoes to inspect the round bar 2 for defects. In this embodiment, the ultrasonic testing method is used, and the probe 11 is positioned directly above the center line of the round bar 2 immersed in the liquid tank 12, with the direction of ultrasonic wave emission set to downward in the vertical direction.

[0023] The three motors 13-15 receive control signals from the motor controller 16 and move the probe 11 in directions parallel to the x, y, and z axes, respectively. The x, y, and z axes are orthogonal axes, with the x and y axes parallel to the horizontal direction and the z axis parallel to the vertical direction. The y axis is also parallel to the longitudinal direction of the round bar 2.

[0024] The motor controller 16 is controlled by input to the calculation unit 18, and controls the positional relationship between the probe 11 and the round bar 2 by controlling the rotation direction, rotation speed, and rotation angle of the three motors 13 to 15.

[0025] The flaw detector 17 is connected to the probe 11 and monitors the flaw detection frequency of the probe 11, the reflected echo, and the size of the inclusion detected from the intensity of the reflected echo, and stores these measurement results in the calculation unit 18.

[0026] The calculation unit 18 is a computer system such as a personal computer that has calculation processing capabilities, and is configured with ROM, RAM, CPU, etc. It implements the various functions described later in software by executing various dedicated programs that are pre-stored in the ROM, etc. The calculation unit 18 also stores the measurement results calculated by the flaw detector 17.

[0027] The chuck device 19 is a tool for fixing the round bar 2, and it fixes both ends of the round bar 2 in the longitudinal direction. The chuck device 19 rotates by receiving the driving force of a rotary drive device (not shown), thereby rotating the round bar 2 around its centerline. The rotation of the chuck device 19 may be controlled by a motor controller 16.

[0028] Using the ultrasonic flaw detection apparatus 1 described above, the round bar 2 is first rotated in a certain direction by the chuck device 19, and the round bar 2 is ultrasonically inspected with the probe 11. At this time, there is no limit to the frequency of the ultrasonic waves emitted from the probe 11, but it is preferable to set it to 30 MHz or more and 70 MHz or less. By setting the ultrasonic flaw detection frequency within this range, it is possible to detect inclusions with a minimum size of about 30 μm.

[0029] In ultrasonic testing, reflected ultrasonic waves (echoes) from inclusions are detected. Typically, a cluster of cells of a suitable size is considered to represent the size of the inclusion. The number of cells in the target cluster can then be used to determine the size (area) of the target inclusion, and "√area" can be used as an indicator of the size of the target inclusion. Alternatively, the major axis of the target cluster may also be used as an indicator of the size of the target inclusion.

[0030] Increasing the ultrasonic frequency allows for the detection of smaller inclusions. However, this also increases the attenuation of the ultrasound, resulting in longer measurement times per unit volume. Lowering the frequency reduces the attenuation of the ultrasound, which is advantageous in terms of measurement time, but it increases the minimum size of detectable inclusions, potentially leading to misjudgments of cleanliness. Specifically, larger inclusions are fewer in number per unit volume, so if only large inclusions can be detected, or if only large inclusions are targeted for measurement, the evaluation volume must be increased to avoid missing large inclusions even if they are present. While there is a trade-off with the evaluation volume, using the above frequency range allows for accurate cleanliness evaluation with an appropriate measurement time. More preferably, the ultrasonic frequency is set to 40 MHz or higher and 60 MHz or lower, and more preferably to 50 MHz.

[0031] Ultrasonic testing is performed on the round bar 2 while it is rotating, continuously measuring a specific depth range from the surface of the round bar 2 in the circumferential direction. Preferably, the specific depth range of the round bar 2 where ultrasonic testing is performed is between 90% and 100% in the radial direction. The radial position refers to the position of the round bar 2 in the radial direction, where the center of the round bar 2 is 0% and the surface of the round bar 2 is 100%, in a cross-section perpendicular to the longitudinal direction of the round bar 2. Ultrasonic testing may be performed over the entire range, or only in a portion of it. In other words, ultrasonic testing is performed in at least a portion of the range between 90% and 100% in the radial direction. In the range between 90% and 100% in the radial direction of the round bar 2, differences in the cleanliness of the steel material become significantly apparent. Therefore, by testing this range, highly reliable cleanliness data can be obtained.

[0032] There is no limit to the evaluation volume, which is the volume of round bar 2 to be ultrasonically inspected, but 60,000 mm 3 The above is preferable, 100,000 mm 3 The above is more preferable. The evaluation volume is the volume of the area inspected on the round bar 2 by ultrasonic testing performed continuously while the round bar 2 is rotating. The evaluation volume is 60,000 mm³.3 If it is less than this value, the variation in the number of non-metallic inclusions, which is the cleanliness, becomes large, and thus the cleanliness cannot be accurately evaluated. Therefore, by setting the evaluation volume to 60000 mm 3 or more, the cleanliness can be accurately evaluated, and by setting the evaluation volume to 100000 mm 3 or more, the cleanliness can be evaluated with higher accuracy. Also, although the upper limit of the evaluation volume is not particularly limited, it is preferably 200000 mm 3 When the evaluation volume exceeds 200000 mm 3 the time required for flaw detection becomes long, and there is no significant difference in the evaluation accuracy of the cleanliness compared to the case where the evaluation volume is 100000 mm 3 or more and 200000 mm 3 or less.

[0033] The flaw detection result by the probe 11 is sent to the flaw detector 17, and the flaw detection result in the flaw detector 17 is recorded in the arithmetic unit 18, and the size of the detected inclusions is calculated. That is, in the arithmetic unit 18, the size and number of inclusions included in the evaluation volume are recorded. Note that by setting the ultrasonic frequency to 30 MHz or more and 70 MHz, the number and size of inclusions with a minimum size of about 30 μm or more that are detected are recorded as a result.

[0034] After the ultrasonic flaw detection is completed, the arithmetic unit 18 evaluates the cleanliness by calculating the cleanliness index, which is the cleanliness evaluation index, from the flaw detection result. Although the cleanliness should have a strong correlation with the bearing life, the inclusion size and the number of inclusions are cited as the main factors among the cleanliness parameters that are correlated with the life. Therefore, the cleanliness index is an index calculated using the size and number of inclusions. The method for calculating the cleanliness index is not particularly limited and may be appropriately set according to the application and the type of steel material. For example, the cleanliness index may be set such that the higher the value, the worse the cleanliness, and the higher the value as the number of larger inclusions increases. Also, for example, as the cleanliness index, the square root √area of the area of the inclusions used in the extreme value statistical method may be calculated from the size of the detected inclusions, and an index determined from the size and number of inclusions for which this √area is greater than or equal to a predetermined value may be used.

[0035] In this invention, by adjusting five factors—(1) echo intensity, (2) sample noise, (3) focal length in the steel material, (4) echo area of ​​the inclusion, and (5) sample diameter—based on the dimensions and number of inclusions obtained by the ultrasonic testing described above, it is possible to detect inclusions in steel materials more accurately, and furthermore, it is suitable for large-diameter round bars as described above.

[0036] (1) Echo intensity Echo intensity is the intensity of the reflected wave from the defect, and the larger the defect, the greater the echo intensity. Also, as the diameter of the steel material increases, the curvature changes, increasing the amount of ultrasonic waves penetrating from the transducer 11, and consequently increasing the intensity of the reflected wave from the defect.

[0037] (2) Sample noise The microstructure of steel materials differs depending on the type of steel, which affects the transmission pattern of ultrasonic waves from the transducer 11. In this invention, as mentioned above, there are no restrictions on the type of steel, but it is necessary to consider the sample noise according to the type of steel. Also, as mentioned above, the curvature changes as the diameter of the steel increases, and since the amount of ultrasonic wave penetration differs depending on the curvature, the echo reflection due to material causes such as particle size also changes.

[0038] (3) Focal length in the material The curvature changes depending on the diameter of the steel material, which alters the incident and refraction angles of the ultrasonic waves from the probe 11, thus changing the focal length of the flaw detector 11. The larger the diameter of the steel material, the greater the effect. Furthermore, the influence of focal length on the actual dimensions of inclusions varies depending on the radial depth of the inclusion from the steel surface; therefore, the inclusion depth must also be considered when determining the degree of influence.

[0039] (4) Echo area of ​​the inclusion It is expected that changes in the curvature of the steel material will alter the way the ultrasonic waves from the flaw detector 11 are refracted. Consequently, the number of echo masses detected as inclusions, i.e., the echo area, will also be affected. (5) Sample diameter This refers to the diameter of the object being measured.

[0040] Thus, by considering all of the factors (1) to (5) above, the cleanliness of the steel material can be evaluated more accurately. Below, Example 1 and Comparative Example 1 will be given to clearly demonstrate the significance of considering all of the factors (1) to (5) above in accordance with the present invention. [Examples]

[0041] (Example 1) A φ85mm round bar made of bearing steel was prepared as the steel material, and ultrasonic flaw detection was performed by mounting it on the ultrasonic flaw detection device shown in Figure 1. The probe used had a frequency of 50MHz, a transducer diameter of 3mm, and an underwater focal length of 12.5mm.

[0042] Then, based on the dimensions and number of inclusions obtained in the steel material, the "estimated inclusion dimensions (μm)" (√area) was calculated by considering all five factors (flaw detection results): (1) echo intensity, (2) sample noise, (3) focal length in the steel material, (4) echo area of ​​the inclusion, and (5) sample diameter.

[0043] Furthermore, the steel material was polished to detect inclusions, and their actual dimensions were measured to determine the "actual inclusion dimensions (μm)" (√area).

[0044] Figure 2 shows a graph where the correlation between "estimated inclusion size (μm)" and "actual inclusion size (μm)" is calculated on the vertical axis and the horizontal axis is calculated on the horizontal axis. The correlation coefficient was a high value of 0.90.

[0045] (Comparative Example 1) Based on the dimensions and number of inclusions in the steel obtained in Example 1, (1) the "estimated inclusion dimensions (μm)" (√area) were calculated considering only the echo intensity.

[0046] Next, the correlation between the "estimated inclusion dimensions" and the "actual inclusion dimensions" obtained in Example 1 was calculated. The results are shown in Figure 3, where the correlation coefficient was 0.70, which is considerably lower than that of Example 1.

[0047] Thus, by considering all five factors—(1) echo intensity, (2) sample noise, (3) focal length in the steel, (4) echo area of ​​the inclusion, and (5) sample diameter—based on the dimensions and number of inclusions in the steel obtained by ultrasonic testing, it is possible to increase the correlation coefficient between "estimated inclusion dimensions" and "actual inclusion dimensions," and as a result, the cleanliness of the steel can be evaluated more accurately. [Explanation of symbols]

[0048] 1. Ultrasonic flaw detection device 2 round bars 11 Probe 12 aquariums 13-15 Motor 16 Motor Controller 17 Flaw detector 18 Arithmetic section 19. Chuck device

Claims

1. Ultrasonic testing is performed to inspect at least a portion of the steel material, and based on the dimensions and number of inclusions in the steel material obtained by the ultrasonic testing, A method for evaluating the cleanliness of steel materials, which considers all of the following (1) to (5). (1) Echo intensity (2) Sample noise (3) Focal length in the steel material (4) Echo area of ​​the inclusion (5) Sample diameter

2. The method for evaluating the cleanliness of a steel material according to claim 1, wherein the diameter of the steel material is 80 mm or more.

3. The method for evaluating the cleanliness of a steel material according to claim 1 or 2, wherein the steel material is a steel material used in rolling bearings, ball screws, linear guides, traction drive transmissions, or electric brakes.

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