JP5224424B1 - Steel with excellent rolling fatigue life - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide excellent rolling fatigue life for bearings, gears, hub units, toroidal type CVT devices, constant velocity joints, crank pins, etc., and for surface fatigue of 58HRC or more to be used for machine parts and devices. Provide steel.
SOLUTION: Steel for machine parts having a surface hardness of 58 HRC or more, a forging ratio of the steel material to the ingot of the base material is 22.0 or more, an oxygen content in the steel is 6 ppm or less, and a sulfur content. Is 0.003 mass% or less, and by the ultrasonic flaw detection method with a probe frequency of more than 25 MHz to 125 MHz, the detected inclusion diameter is 20 μm or more and the inclusion diameter is 20 μm or more per 1000 mm ³ of the steel material, The closest distance between non-metallic inclusions is less than 40 μm, the number of non-metallic inclusion pairs is 2.0 or less, and the total volume of steel is 3000 mm ³ by ultrasonic flaw detection with a probe frequency of 25 MHz to 125 MHz. A steel excellent in rolling fatigue life in which the maximum diameter of non-metallic inclusions detected above is 100 μm or less.
[Selection figure] None

Description

  The present invention relates to a mechanical component used for curing a surface hardness of 58 HRC or more, such as a bearing, a gear, a hub unit, a toroidal type CVT device, a constant velocity joint, a crankpin, etc. The present invention relates to steel applied as a device.

  In recent years, due to the improvement in performance of various mechanical devices, the use environment of mechanical parts and devices that require a rolling fatigue life has become severe. Along with this, there are increasing demands for improving the life and reliability of these components and devices. In response to such demands, measures for steel surfaces include optimization of steel components and reduction of impurity elements that are harmful to rolling fatigue life, thereby improving life and improving reliability.

  Among the impurity elements contained in the steel composition, for example, oxygen is an element that constitutes oxide inclusions that can be a starting point of damage such as alumina. Therefore, especially oxygen with high toxicity has been reduced to the ppm order. When higher quality is required, the oxygen amount may be further reduced by special dissolution such as VAR and ESR. In addition, measures are taken to prevent adverse effects of other impurity elements by reducing their content to the order of 0.01% by mass.

Various proposals have been made for high cleanliness steel with a low oxygen content in the steel. Among these proposals, the oxygen content in the steel is less than 10 ppm, and the exposed surface area of the oxide inclusions floated and aggregated by the electron beam melting method is 20 μm ² or less per gram. Has been proposed (see, for example, Patent Document 1). Furthermore, a general-purpose long-life steel has been proposed without being subjected to refining process restrictions and without causing an increase in manufacturing cost (see, for example, Patent Document 2). Furthermore, steel that guarantees the size of the maximum inclusion that is expected to exist in a predetermined volume, which is necessary for designing parts that require fatigue strength, has been proposed (see, for example, Patent Document 3). . The extreme statistical method is applied to inclusions with a maximum inclusion diameter of approximately 100 μm or less, and the ultrasonic flaw detection method with a flaw detection frequency of 5 to 25 MHz is applied to inclusions with a maximum inclusion diameter of approximately 100 μm or more. There is a proposal of an evaluation method that uses both methods such as (see, for example, Patent Document 4). In addition, by evaluating the inclusion having a diameter of less than 100 μm by an ultrasonic flaw detection method with a flaw detection frequency of 20 to 125 MHz, a steel in which the number and size of inclusions as steel having excellent rolling fatigue life is specified. It has been proposed (see, for example, Patent Document 5).

JP-A-6-192790 JP 2002-220638 A JP-A-11-194121 JP 2006-317192 A JP 2008-121035 A

In order to suppress extremely early damage to the calculated life of mechanical parts that require excellent rolling fatigue life, the inventors have used L ₁ life (test specimens of the same lot under the same conditions) as a measure of reliability. In this case, 99% of the specimens are cycled without peeling, and the objective is to evaluate the evaluation results of non-metallic inclusions in steel by ultrasonic flaw detection with the aim of improving the number of specimens. We conducted intensive research. First, regarding the detection of non-metallic inclusions in steel by ultrasonic flaw detection, among non-metallic inclusions, oxides are significantly harder than the parent phase steel in any processing temperature range from cold to hot. Since it is an inclusion and does not easily deform following the parent phase during rolling or forging, it may have a part that is not in close contact with the parent phase after processing, so it can be detected relatively easily by ultrasonic flaw detection. Is possible. By the way, in order to reduce the adverse effect of oxide inclusions on the rolling fatigue life, it has been found that at least the oxygen content in the steel needs to be 6 ppm or less.

  On the other hand, among the non-metallic inclusions, the inventors have found that the sulfide is a soft inclusion that tends to deform relatively well following the parent phase steel during hot or cold rolling or forging. It was found that sulfides are inclusions that are difficult to detect by ultrasonic flaw detection compared to oxides due to the characteristics of good adhesion to the matrix. It has been found that the presence of some sulfide inclusions that are actually present in steel but cannot be detected by the ultrasonic flaw detection method may contribute to a reduction in rolling fatigue life. Then, as a result of examining the sulfur content in the steel, it was found that the sulfur content in the steel needs to be regulated to at least 0.003% by mass or less.

Furthermore, according to the earnest study by the inventors, in order to obtain a steel having an excellent rolling fatigue life (L ₁ life), the oxygen content in the steel is set to 6 ppm or less and the sulfur content in the steel is as described above. In addition to restricting the amount to 0.003 mass% or less, in the case where a steel material volume of 3000 mm ³ or more is detected by ultrasonic flaw detection at a probe frequency exceeding 25 MHz and 125 MHz or less, the inclusion diameter is 20 μm. The number of non-metallic inclusion pairs in which the closest distance between non-metallic inclusions whose inclusion diameter is 20 μm or more is less than 40 μm is 2.0 or less per 1000 mm ³ of steel volume. I knew that I should do it. Inclusions having an inclusion diameter of about 20 μm are not large enough to cause separation with an extremely short life, but even non-metallic inclusions having a size of about 20 μm have such a size. When the closest distance between inclusions is less than 40 μm, it behaves as if it is a large inclusion, resulting in separation with a very short life, so the presence of such non-metallic inclusion pairs in steel It has been found that if the frequency is 2.0 or less per 1000 mm ^3, it is possible to obtain a steel having an excellent rolling fatigue life (L ₁ life).

In addition, non-metallic inclusions whose inclusion diameter exceeds 100 μm may cause separation with an extremely short life even if they are used alone. Therefore, an ultrasonic wave having a probe frequency exceeding 25 MHz and not exceeding 125 MHz. The maximum diameter of non-metallic inclusions detected when a steel material volume of 3000 mm ³ or more is detected by the flaw detection method needs to be 100 μm or less. In addition, for sulfide inclusions that are relatively difficult to detect by ultrasonic flaw detection, in order to avoid the proximity inclusion dispersion state that is harmful to the rolling fatigue life, = Cross sectional area of base material ingot / cross sectional area of steel material) must be at least 22.0 or more.

  In the steels described in the above cited references 1 to 3, when the occurrence frequency is rare, it is possible to stably provide a steel excellent in rolling fatigue life, which is suppressed in practical use and has an extremely short life, and is excellent in rolling fatigue life. The inclusion diameter is 20 μm or more, and the closest distance between the inclusions having a diameter of 20 μm or more is 40 μm, which causes an extremely early accidental peeling for the calculated life in a part requiring rolling fatigue. Such non-metallic inclusion pairs that are less than are present in steel with a very low probability, so it is very difficult to detect their presence or absence. In addition, depending on the conventional technique mainly based on two-dimensional observation, since the test area is small, it takes a lot of time to inspect a large volume of the steel material, and the quality of the steel material cannot be determined. Further, in the steel described in Patent Document 1, since inclusions melt and aggregate, the exact inclusion diameter and the interval between the inclusions cannot be evaluated.

In the method described in the cited document 4, the extreme value statistical method is applied to nonmetallic inclusions whose maximum inclusion diameter is less than 100 μm, and the flaw detection frequency is used for nonmetallic inclusions whose inclusion diameter is 100 μm or more. An evaluation method based on a combination of, for example, applying an ultrasonic flaw detection method with a frequency of 5 to 25 MHz is proposed. However, since the extreme value statistical method is an evaluation method based on two-dimensional observation, as in the case described above, the non-metallic inclusions having a small test area and a low inclusion frequency in steel with a diameter of 20 μm or more are observed. It is hard to say that the quality of steel can be judged sufficiently. On the other hand, in the ultrasonic flaw detection method with a flaw detection frequency of 5 to 25 MHz, since the detectable inclusion diameter is about 100 μm or more, the inclusion diameter is 20 μm or more, and the inclusion diameter is 20 μm or more. the number of non-metallic inclusions pairs as spacing nonmetallic inclusions each other is less than 40 [mu] m, since the ultrasonic beam band of the flaw detection frequency is difficult evaluation, excellent steel L ₁ life It is hard to say that the evaluation method can be provided stably.

Further, in the method described in the cited document 5, the non-metallic inclusions having a sulfur content of 0.008 mass% or less and an inclusion diameter detected per 300 mm ³ of steel material volume by an ultrasonic flaw detection method is 20 μm or more. Steel having an excellent rolling fatigue life specified to be 12 or less per 300 mm ³ (in a thrust type rolling fatigue test, the maximum hertz stress P _max = 5.3 GPa and the L ₁₀ life> 1. A steel capable of obtaining 0 × 10 ⁷ cycle) and its evaluation method are proposed. However, the regulation of the sulfur content is not sufficient, and the inclusion diameter is 20 μm or more, and the inclusion diameter is 20 μm or more, which is an index of reliability against accidental damage to the bearing in use extremely early than the calculated life. since the closest distance of inclusions each other in diameter is not evaluated the number of nonmetallic inclusions pairs as less than 40 [mu] m, it is hard to say that the steel which can stably provide an excellent steel L ₁ life.

  The present invention has been made in order to solve such a conventional problem, and the problem to be solved by the present invention is to secure a forging ratio of the steel material at a certain value or more, and the oxygen content in the steel. In addition, the non-metallic inclusions in steel are detected in a large volume by ultrasonic flaw detection, and the closest distance between non-metallic inclusions with inclusion diameters of a certain size or more is constant. It is used for machine parts with excellent rolling fatigue life by limiting the number of non-metallic inclusion pairs as follows and limiting the maximum size of non-metallic inclusions in steel by ultrasonic flaw detection. Is to provide steel.

The means of the present invention for solving the above-mentioned problems is a steel used for a machine part having a surface hardness of 58 HRC or higher in the first means, wherein the steel material has a forging ratio of 22.2. Steel material by an ultrasonic flaw detection method in which the oxygen content in steel is 6 ppm or less, the sulfur content is 0.003 mass% or less, the probe frequency exceeds 25 MHz and is 125 MHz or less. The number of inclusions detected per volume of 1000 mm ³ is 20 μm or more, and the number of non-metallic inclusion pairs in which the closest distance between nonmetallic inclusions having a diameter of 20 μm or more is less than 40 μm is 2.0. less and, and, beyond the probe frequency 25 MHz, the maximum diameter of nonmetallic inclusions detected by 125MHz following ultrasonic flaw detection method of steel total volume 3000 mm ³ or more is 100μm or less, rolling is It is an excellent steel in fatigue life.

In the second means, steel used for machine parts having a surface hardness of 58 HRC or higher, the forging ratio of the steel to the base metal ingot is 22.0 or higher, and the oxygen content in the steel is a mass ratio. 6 ppm or less, the sulfur content is 0.003 mass% or less, the inclusion diameter detected per volume of 1000 mm ^{3 of} steel is 20 μm or more by an ultrasonic flaw detection method with a probe frequency exceeding 25 MHz and 125 MHz or less. And the number of non-metallic inclusion pairs whose closest distance between non-metallic inclusions having a diameter of 20 μm or more is less than 40 μm is 2.0 or less, and the probe frequency exceeds 25 MHz, and 125 MHz. The maximum diameter of non-metallic inclusions detected at a steel material total volume of 3000 mm ³ or more by the following ultrasonic flaw detection method is 100 μm or less, and the inclusion diameter is 20 μm or more, and 20 μm or more thereof. The number of non-metallic inclusions pair closest distance is less than 40μm of non-metallic inclusions each other in diameter, in a total volume of 3000 mm ³ or more by ultrasonic flaw detection, and by testing the steel is 300000Mm ³ or less What was evaluated and evaluated by testing a steel material having a total volume of 300,000 mm ³ or less in ultrasonic testing for evaluating that the maximum diameter of the non-metallic inclusions is 100 μm or less. It is a steel excellent in rolling fatigue life.

  In the third means, the steel having excellent rolling fatigue life as described above includes high carbon chromium bearing steel (SUJ) defined in JIS standard, 52100 defined in SAE standard or ASTM standard A295, and DIN standard. Among the steels for mechanical structure defined in JIS, carbon steel (SC) for mechanical structure (SC), and alloy steel for mechanical structure, chromium steel (SCr), chromium molybdenum steel (SCM) and nickel chromium molybdenum steel Any one kind of steel selected from (SNCM) is mentioned.

  The steel excellent in rolling fatigue life of the present invention was made to solve such a conventional problem, and regulates the forging ratio of the steel material and regulates the oxygen content and sulfur content in the steel. In addition, non-metallic inclusions in steel are detected in a large volume by ultrasonic flaw detection, and the closest distance between non-metallic inclusions having inclusion diameters greater than a certain size is less than a certain distance. To provide steel used for machine parts with excellent rolling fatigue life by limiting the number of metal inclusion pairs and limiting the maximum size of nonmetallic inclusions in steel by ultrasonic flaw detection. Is possible.

  The steel excellent in rolling fatigue life according to the embodiment of the present invention will be described in detail with reference to the following table.

The steel excellent in rolling fatigue life according to the embodiment of the present invention is steel used for machine parts having a surface hardness of 58 HRC or more, and has a forging ratio of 22.0 to the ingot as a base material. Thus, the oxygen content in the steel is 6 ppm or less by mass and the sulfur content is 0.003 mass% or less. Furthermore, by the ultrasonic flaw detection method exceeding the probe frequency 25 MHz and 125 MHz or less, the inclusion diameter detected per 1000 mm ³ of the steel material is 20 μm or more, and the nonmetallic inclusions having a diameter of 20 μm or more are used. The closest distance is less than 40 μm, the number of non-metallic inclusion pairs is 2.0 or less, and the total frequency of steel is 3000 mm ³ or more by ultrasonic flaw detection with a probe frequency exceeding 25 MHz and 125 MHz or less. The maximum diameter of non-metallic inclusions detected in (1) is 100 μm or less.

  In parts subjected to rolling fatigue, it is important to improve properties to reduce non-metallic inclusions of a certain size or more from steel. If non-metallic inclusions with a detrimental size exist under the rolling surface of the bearing, separation will occur, so the size of non-metallic inclusions that appear in the hazardous area under the rolling surface of the bearing will be reduced. This is extremely important for improving the life of the bearing. As inclusion diameters that cause separation very early without reaching the calculated life, non-metallic inclusions having an inclusion diameter of 20 μm are not harmful by themselves. However, even if it is a non-metallic inclusion of a 20 μm class, if the three-dimensional closest distance to another non-metallic inclusion having a size equal to or larger than that is less than 40 μm, it causes early separation. Act as a harmful inclusion. In addition, non-metallic inclusions exceeding 100 μm alone cause early peeling. Therefore, the number of nonmetallic inclusion pairs whose inclusion diameter is 20 μm or more and the closest distance between the nonmetallic inclusions of 20 μm or more is less than 40 μm, and the nonmetallic inclusions whose inclusion diameter exceeds 100 μm. By using steel that has a very low existence frequency and is guaranteed to be evaluated, the probability that inclusions will be present in dangerous parts (that is, parts that are subject to rolling fatigue) under the transfer surface of parts is extremely low. This makes it possible to suppress early peeling.

Therefore, the ultrasonic flaw detection method is applied as an evaluation method for assuring the characteristics of the steel having excellent rolling fatigue life according to the first embodiment of the present invention. In this ultrasonic flaw detection method, the inclusion diameter of the nonmetallic inclusion detected per 1000 mm ³ of the volume of the steel material is 20 μm or more by the ultrasonic flaw detection method exceeding the probe frequency 25 MHz and 125 MHz or less, and 20 μm. The closest distance between the non-metallic inclusions is less than 40 μm, the number of non-metallic inclusion pairs is 2.0 or less, and the probe frequency exceeds 25 MHz and the ultrasonic flaw detection method is 125 MHz or less. The maximum diameter of non-metallic inclusions detected with a steel material total volume of 3000 mm ³ or more is regulated to 100 μm or less.

Further, in the steel having an excellent rolling fatigue life according to the embodiment of the second means, the inclusion diameter of the nonmetallic inclusion in the ultrasonic flaw detection method applied as a method for guaranteeing the characteristics is 20 μm or more, and The number of non-metallic inclusion pairs in which the closest distance between non-metallic inclusions of 20 μm or more is less than 40 μm, and the point that the maximum diameter of the non-metallic inclusion of the present embodiment is 100 μm or less are ultrasonic flaw detection in a total volume of 3000 mm ³ or more by law, and those that have been evaluated by testing the 300000Mm ³ below.

  Still further, the steel excellent in rolling fatigue life according to the embodiment of the third means is preferably a steel type used for applications requiring rolling fatigue life such as bearings. Specifically, high carbon chromium bearing steel (SUJ) specified in JIS standard, 52100 specified in SAE standard or ASTM standard A295, 100Cr6 specified in DIN standard, and specified in JIS standard Carbon steel for machine structure (SC), and steel selected from chrome steel (SCr), chrome molybdenum steel (SCM) and nickel chrome molybdenum steel (SNCM) among alloy steels for machine structure . For example, the present invention can be applied to foreign standard steels corresponding to JIS standards such as SAE standards 4320, 5120, 4140, 1053, and 1055.

  In the ultrasonic flaw detection method, various types of ultrasonic flaw detectors and probes have already been put into practical use, and these can be used in the present invention. As a preferred probe, a focus type high frequency probe and the like can be cited. The detection capability of the flat probe is said to be ½ wavelength, but the focus probe is ¼ wavelength. Therefore, the focus probe is suitable for accurate evaluation. is there. Note that the probe frequency in this embodiment is preferably greater than 25 MHz and not greater than 125 MHz, and particularly preferably about 30 to 100 MHz.

In the steel having an excellent rolling fatigue life according to the present embodiment, the nonmetallic inclusions whose nonmetallic inclusions have an inclusion diameter of 20 μm or more and the closest distance between the nonmetallic inclusions of 20 μm or more is less than 40 μm. the number of objects pair, and the maximum diameter of the nonmetallic inclusions is 100μm or less, as already described above, the ultrasonic flaw detection method in a total volume of 3000 mm ³ or more, and testing the 300000Mm ³ below, non-metallic inclusions Is preferably detected.

In ultrasonic flaw detection, the number of nonmetallic inclusion pairs whose inclusion diameter is 20 μm or more and the closest distance between the nonmetallic inclusions of 20 μm or more is less than 40 μm, and the maximum nonmetallic inclusion diameter is 100 μm or less. flaw detection total volume to ensure that it is in 3000 mm ³ or more, and the reason for the 300000Mm ³ or less, the steel is excellent in rolling fatigue life can be suppressed peeling at very short life for the L ₁ life and performance index This is because it is necessary to obtain a satisfactory evaluation result in terms of evaluation accuracy. In addition, the evaluation volume in the ultrasonic flaw detection method of the present embodiment cannot be evaluated practically because the processing time is enormous in the conventional evaluation method mainly based on microscopic observation. Furthermore, as a general-purpose means, it is impossible to three-dimensionally evaluate the closest distance between inclusions existing in a large volume steel material by a method other than the ultrasonic flaw detection method. In performing ultrasonic flaw detection, the dead zone region from the surface of the specimen to the depth corresponding to the probe frequency is excluded from the evaluation volume, and if necessary, tissue abnormalities due to heat treatment etc. and measurement noise in ultrasonic flaw detection are detected. Evaluation volume in ultrasonic flaw detection based on the underwater focal length range according to the frequency and performance of the probe after excluding the end of the sensitive specimen from the flaw detection range of the ultrasonic beam at the focal position the need to ensure at 3000 mm ³ or more, and 300000Mm ³ below. In addition, it is necessary to set an appropriate flaw detection pitch so that the interval between inclusions having a threshold value of 40 μm can be identified.

  As described above, according to the steel having an excellent rolling fatigue life according to the present embodiment, the forging and forming ratio of the steel material is regulated, and the oxygen content and the sulfur content in the steel are regulated, and the ultrasonic wave Detecting non-metallic inclusions in steel in a large volume by flaw detection method, and the non-metallic inclusion pairs whose non-metallic inclusions having inclusion diameters above a certain size are less than a certain distance By limiting the number and limiting the maximum size of non-metallic inclusions in steel by ultrasonic flaw detection, it is possible to provide steel used for machine parts having excellent rolling fatigue life.

  Next, the specimens 1 to 25 as examples and the specimens 26 to 36 as comparative examples are listed in Tables 1 and 2, and the steel excellent in rolling fatigue life of the present invention will be described more specifically. . However, the present invention is not limited to these examples.

  Table 1 shows the component composition of each sample material of the example and the comparative example. For Specimens 1 to 7 and Specimens 26 to 33 in Table 1, JIS SUJ2 steel, which is a high carbon chromium bearing steel, for Specimens 8 and 34, JIS SCr420 steel, for Specimen 9 Is JIS SNCM420 steel, JIS S53C steel is used for specimen 10, JIS SCM420 steel is used for specimen 11, specimen 35 and specimen 36, specimen 12 and specimen 13 SAE 52100, for specimens 14 and 15, ASTM 52100, for specimens 16 and 17, DIN 100Cr6, for specimen 18 JIS SUJ3 steel, Specimen 19 is JIS SUJ5 steel, Specimen 20 is SAE 4320 steel, Specimen 21 is SAE 5120 steel, Specimen 22 is JIS SCM435 steel. Material 23 to 4140 steel SAE, test materials 24 to S55C steel JIS, test material 25 was used 1053 steel SAE. For specimens 1-5, specimens 8-9, specimen 12, specimen 14, 14, specimen 16, specimen 18, specimens 20-30, specimen 36, arc melting The ingot is smelted in a furnace, then smelted in a ladle, and further degassed with a vacuum degassing apparatus to produce an ingot by continuous casting.

  Specimen 6, Specimen 7, Specimen 10, Specimen 11, Specimen 13, Specimen 15, Specimen 17, Specimen 19, Specimen 31-35 It is made by melting in a melting furnace, then refining a ladle, and degassing with a vacuum degassing device to produce an ingot with an ingot.

  The ingot obtained above was hot-worked to obtain a steel material having a diameter of 65 mm in which a steel forging ratio of 22.0 or more was ensured.

(Thrust type rolling fatigue test)
The steel materials of the test materials 1 to 7, the test materials 12 to 19, and the test materials 26 to 33 are subjected to spheroidizing annealing at 800 ° C., and the outer diameter is 60 mm and the inner diameter is perpendicular to the longitudinal direction of the steel materials. A test piece made of a disk having a thickness of 20 mm and a thickness of 5.8 mm was produced. After holding this test piece at 835 ° C. for 20 minutes, it was quenched by oil cooling, and then tempered at 170 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher, and then subjected to surface polishing. A thrust type rolling fatigue test was conducted. The steel materials of Specimen 8, Specimen 9, Specimen 11, Specimen 20, Specimen 21, and Specimens 34 to 36 were subjected to normalization at 925 ° C. The steel materials of the test materials 22 and 23 were subjected to normalization at 870 ° C., and then a test piece made of a disk having an outer diameter of 60 mm, an inner diameter of 20 mm, and a thickness of 8.3 mm perpendicular to the longitudinal direction of the steel material. Was made. This test piece was carburized at 930 ° C., then quenched by oil cooling, and then tempered at 180 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher, followed by surface polishing and thrust. A mold rolling fatigue test was conducted. The steel material of the test material 10, the test material 24, and the test material 25 is subjected to normalization at 870 ° C., and is a disk having an outer diameter of 60 mm, an inner diameter of 20 mm, and a thickness of 8.3 mm perpendicular to the longitudinal direction of the steel material. A test piece consisting of: After this test piece was induction-quenched, tempering was then performed at 180 ° C. for 90 minutes to obtain the desired hardness of 58 HRC or higher, and then surface polishing was performed to perform a thrust type rolling fatigue test. The thrust type rolling fatigue test was performed at a maximum hertz stress P _{max of} 5.3 GPa. In order to obtain the L ₁ life, a censoring test with about 1.5 × 10 ⁷ cycles was used to shorten the test evaluation time.

(Ultrasonic test)
The number of nonmetallic inclusion pairs whose nonmetallic inclusions have a diameter of 20 μm or more and the closest distance between the nonmetallic inclusions of 20 μm or more is less than 40 μm, and the maximum diameter of the nonmetallic inclusions is 100 μm. In evaluating the following, the steel materials of Specimens 1-7, Specimens 12-19, and Specimens 26-33 were subjected to spheroidizing annealing at 800 ° C., and L-section test pieces were cut out. After quenching and tempering, the steel materials of the test material 8, the test material 9, the test material 11, the test material 20, the test material 21, and the test materials 34 to 36 are tempered at 925 ° C. After the L section test piece was cut out and quenched and tempered, the steel materials of the test material 10 and the test materials 22 to 25 were subjected to normalization at 870 ° C., and the L cross section test piece was cut out. After performing quenching and tempering treatment, reduce ultrasonic transmission loss In order to decrease, it was surface grinding. Each test piece was finished to a thickness of 10 mm by surface polishing, and an ultrasonic flaw detection test was performed. For ultrasonic flaw detection, an ultrasonic flaw detector equipped with a focus type high-frequency probe (50 MHz) was used. The volume of ultrasonic flaw detection was set to 4000 mm ³ . From the data of the reflected wave by the obtained inclusions, the nonmetallic inclusions whose non-metallic inclusions per volume of 1000 mm ^{3 of} the steel material are 20 μm or more and the closest distance between the nonmetallic inclusions of 20 μm or more is less than 40 μm. The number of detected object pairs and the presence / absence of non-metallic inclusions having a maximum diameter exceeding 100 μm were determined.

About each test piece of these specimens, the surface hardness, the number of detected non-metallic inclusion pairs per 1000 mm ³ of steel volume by ultrasonic flaw evaluation evaluated with a 50 MHz focal high-frequency probe, the interposition exceeding 100 μm Table 2 shows the presence / absence of detection of the object and the L ₁ life by the thrust type rolling fatigue test.

In Table 2, the test materials 26 to 36 of the comparative example are 20 μm or more detected per 1000 mm ³ of steel material volume, and the closest distance between the nonmetallic inclusions of 20 μm or more is less than 40 μm. The detection of non-metallic inclusions with the number of pairs exceeding 2.0 and the maximum diameter exceeding 100 μm can be seen, which is outside the scope of the present invention. Moreover, the L ₁ life of the test materials 26 to 36 of these comparative examples is three times or less when the L ₁ life of the test material 33 of the comparative example is 1. In contrast to the test materials 26 to 36 of these comparative examples, the test materials 1 to 25 of the examples in the present invention have an inclusion diameter of 20 μm or more and 20 μm of the inclusion diameter detected per 1000 mm ³ of steel material volume by the ultrasonic flaw detection method. The number of non-metallic inclusion pairs in which the closest distance between the non-metallic inclusions is less than 40 μm is 2.0 or less, and there is no detection of non-metallic inclusions having a maximum diameter exceeding 100 μm. The first means of the present invention, that is, the invention according to claim 1, the second means, ie, the invention according to claim 2, and the third means, ie, the invention according to claim 3, are satisfied. It is four times or more the L ₁ life of the specimen 33 and is excellent in the L ₁ life.

Claims (3)

Steel used for machine parts having a surface hardness of 58 HRC or more, wherein the forging ratio of the steel material to the ingot as a base material is 22.0 or more, and the oxygen content in the steel is 6 ppm or less by mass ratio, The sulfur content is 0.003 mass% or less, the probe frequency exceeds 25 MHz, and the inclusion diameter detected per 1000 mm ³ of the volume of the steel material by an ultrasonic flaw detection method of 125 MHz or less is 20 μm or more and 20 μm The ultrasonic flaw detection method in which the closest distance between the nonmetallic inclusions is less than 40 μm, the number of nonmetallic inclusion pairs is 2.0 or less, the probe frequency exceeds 25 MHz, and the frequency is 125 MHz or less. A steel having excellent rolling fatigue life, wherein the maximum diameter of non-metallic inclusions detected at a total volume of 3000 mm ³ or more is 100 μm or less. The number of non-metallic inclusion pairs whose inclusion diameter is 20 μm or more and the closest distance between the non-metallic inclusions of 20 μm or more is less than 40 μm is 2.0 or less, and the maximum diameter of non-metallic inclusions is 100μm or less, excellent steel rolling fatigue life according to claim 1, characterized in that which is evaluated by testing the total volume 3000 mm ³ or more and 300000Mm ³ below by ultrasonic flaw detection method.   Steels with excellent rolling fatigue life are specified in high carbon chromium bearing steels specified in JIS standards, 52100 specified in SAE standards or ASTM standards A295, and 100Cr6 specified in DIN standards, and in JIS standards. Carbon steel material for machine structure and alloy steel material for machine structure specified in JIS standard, any one steel selected from chrome steel, chrome molybdenum steel and nickel chrome molybdenum steel A steel excellent in rolling fatigue life according to claim 1 or 2.