JP7202129B2 - Test piece, manufacturing method of the test piece, and test method using the test piece - Google Patents

Description

TECHNICAL FIELD The present invention relates to a test piece, a method for manufacturing the test piece, and a test method using the test piece.

Despite being used under proper lubrication conditions, short-life flaking, which causes bearings to break earlier than expected, may occur, hindering the realization of smaller and lighter bearing designs. Such flaking is caused by non-metallic inclusions contained in steel. Non-metallic inclusions are inevitably generated during the refining, casting, and solidification processes of steel, and those that cannot be completely removed during these processes will be included in the bearing material that has undergone subsequent rolling, forging, and the like. This inclusion-initiated delamination usually originates somewhat inside the part rather than on the surface. This is because a high shear stress is generated slightly inside the bearing ring when the bearing ring and rolling elements (balls, rollers, etc.) are in rolling contact with each other.

Due to the fact that fatigue progresses inside the parts, it is difficult to directly observe rolling fatigue. In addition, it was rare to find inclusions on the fractured surface that were the origin of the flaking after delamination. Therefore, although there is no doubt that inclusions influence bearing life, the direct relationship between inclusions and life has not yet been clarified. The _L10 life is often used as a life index for rolling bearings. _L10 life refers to the life that 90% of test pieces do not flake when multiple samples are tested under the same conditions. That is, bearing life is usually evaluated probabilistically. Breaking through this and directly verifying the relationship between inclusions, service life, and rolling fatigue is necessary in order to realize steel that can avoid short-life flaking.

On the other hand, the present inventors artificially produced SUJ2 steel in which many cavities remain and are dispersed in the steel, and observed the rolling fatigue crack behavior of these cavities. From the result of comparing the stress simulation for the object, it is found that the harmfulness is promoted when there is a gap (void) between the inclusion and the matrix (see, for example, Non-Patent Document 1).

In relation to this, it has been confirmed that rolling fatigue life can be significantly improved by applying hot isostatic pressing (HIP) to steel to close the gap between inclusions and the matrix. (For example, see Non-Patent Document 2).

Takeshi Fujimatsu, Kazuhiko Hiraoka, Atsuyuki Yamamoto, "Crack Initiation Behavior from Internal Defects in Rolling Fatigue of High Carbon Chromium Bearing Steel", Tetsu-to-Hagane, The Iron and Steel Institute of Japan, Vol. 94, No. 1 ( 2008), pp. 13-20.

K. Hashimoto, T. Fujimatsu, N. Tsunekage, K. Hiraoka, K. Kida, E. C. Santos, “Internal Fracture Type Rolling Fatigue Life Effect of inclusion/matrix interface cavities on internal-fracture-type rolling contact fatigue life, Materials & Design, Elsevier B.V., (Netherlands) ), Vol. 32, Issue 10, December 2011, pp. 4980-4985

According to Non-Patent Document 2, it is clear that the state of the inclusion-mother phase interface is a factor in changing the life. However, it was practically difficult to verify the presence or absence of gaps around inclusions after flaking had occurred, and it was not possible to estimate the contribution of gaps to the length of life.

Therefore, in clarifying the relationship between the size of inclusions in practical use and life and rolling fatigue, there is a method for preparing test pieces for conducting rolling fatigue tests, taking into account the effect of gaps between inclusions in steel and the matrix phase. And the invention of the rolling fatigue test method using it was demanded.

The present invention has been made in view of the above problems. To provide a new rolling fatigue test method that does not rely on the condition of the gap between inclusions and the surrounding matrix, which may occur in the surroundings during the manufacturing process, but whose degree of formation is difficult to estimate in advance. aim.

The present invention has a test strip body, a non-metallic inclusion located at a predetermined position inside the test strip body, and a gap located between the periphery of the non-metallic inclusion and the test strip body. However, the non-metallic inclusion is located under the raceway when used for the thrust type rolling fatigue test, and is located at any position under the raceway when used for the thrust type rolling fatigue test, other This is a test piece with non-metallic inclusions .

In the present invention, a sample made of steel containing non-metallic inclusions is melted, the sample is hot-worked and drawn to produce an intermediate material, and the intermediate material is stretch-worked in a predetermined direction to thrust. When used for type rolling fatigue test Apply a predetermined stress to a predetermined position under the raceway, form a gap around at least a part of the non-metallic inclusion at the predetermined position, and use it for a thrust type rolling fatigue test. A test piece manufacturing method that does not apply a predetermined stress to an arbitrary position under a track and does not form a gap around other non-metallic inclusions in an arbitrary position .

According to the test piece of the present invention, the test piece manufacturing method, and the rolling fatigue test method using the test piece, the harmfulness of inclusions (effect on life and rolling fatigue) considering the effect of gaps is elaborated. can be verified.

It is a figure which shows the structure of the test piece of embodiment. 4 is a flow chart showing the manufacturing process of the test piece of the embodiment. It is a figure which shows the structure of the intermediate|middle material of embodiment which gives tensile processing. It is a figure which illustrates the clearance gap formed in a nonmetallic inclusion in a test piece, and a part of circumference|surroundings of it. Observation of the cross section of the thrust type rolling fatigue test piece shows the state of the gap formed between the sulfide (MnS) and the surrounding matrix (a), and the gap between the surrounding matrix in the same test piece. 2 is a photograph of observation of a state (b) of a portion where no sulfide (MnS) is formed in the substrate.

Hereinafter, a test piece, a method for manufacturing the test piece, and a rolling fatigue test method using the test piece, which are embodiments of the present invention, will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing the configuration of the test piece of this embodiment. 1(a) is a left side view (including an enlarged cross-sectional view of part A), FIG. 1(b) is a front view, and FIG. 1(c) is a right side view (including an enlarged cross-sectional view of part B). , and FIG. 1D is a bottom view (including cross-sectional enlarged views of parts A and B). In addition, in order to facilitate the explanation, FIG. 1 partially exaggerates the dimensional relationship.

The test piece 100 of the embodiment includes a test piece main body 102 which is a hollow elliptical disk-shaped member having an inner diameter hole 101 in the center, and a polishing surface of the test piece main body 102 corresponding to a position A below the thrust test trajectory C. has non-metallic inclusions 103 and gaps 104 provided between the matrix phase surrounding the non-metallic inclusions 103 . Although the single nonmetallic inclusion 103 is shown in FIG. 1, a plurality of nonmetallic inclusions having similar gaps may be distributed around the nonmetallic inclusion 103 . In particular, as will be described later, when the sulfur (S) content and oxygen content are intentionally increased, a plurality of nonmetallic inclusions are distributed close to each other.

The test piece main body 102 of the embodiment has a substantially elliptical disk shape with a major axis parallel to the tensile direction X described later. However, the shape of the test strip main body 102 is not limited to this, and may be processed into an appropriate shape (for example, disk shape) depending on the test.

The non-metallic inclusions 103 of the embodiment are contained in the test piece main body 102 via the gaps 104 . A non-metallic inclusion 103 existing at a position A below the thrust test track C on the polished surface of the test piece main body 102 has a gap 104 in front and rear in the tensile direction X. As shown in FIG. On the other hand, the non-metallic inclusions 103 existing at the position B under the thrust test track C under the polished surface of the test piece main body 102 are contained in the test piece main body 102 without the gap 104 intervening therebetween. A method of manufacturing a test piece that can define the positions A and B will be described later.

The non-metallic inclusions 103 of the embodiment are MnS. However, the nonmetallic inclusions 103 are not limited to this, and may be oxides or other sulfides.

FIG. 2 is a flow chart showing the manufacturing process of the test piece of this embodiment.

First, a sample is melted (STEP 1). This is the material of the test piece 100 shown in FIG. 1, and the material of the intermediate material 100a for tensile processing during the process of processing the test piece shown in FIG. As a sample, a steel ingot melted in a mass production process, a steel billet continuously cast, or a vacuum-melted test steel ingot can be used.

Here, a steel ingot with the base composition of SUJ2 steel was melted by vacuum induction melting with the amount of sulfur (S) increased to 0.06%, which is adjusted to 0.010% or less in mass-produced steel. made. In the solidified state of the steel ingot, sulfur (S) combines with Mn contained in SUJ2 and exists in the form of sulfide (MnS). In addition, in this embodiment, sulfide (MnS) corresponds to the nonmetallic inclusions 103 .

After that, the sample was heated to 1150° C. and hot-stretched to a diameter of 65 mm (STEP 2). This step may be substituted by hot rolling. The forging ratio at this time is 7s with respect to the original steel ingot. However, the forging ratio in the present embodiment is not limited to this. In this hot forging process, the sulfide (MnS) (non-metallic inclusions 103) is stretched in the stretching direction of the material (rolling direction in the case of rolling) to take a spindle shape. This shape depends on the forging ratio. If the forging ratio is small, the difference between the length of the sulfide in the stretching direction and the width in the direction perpendicular to it is small. The length of the direction becomes larger than the width.

This sample was subjected to heat treatment such as normalizing by air cooling after being held at 865°C for 1 hour, and spheroidizing annealing by setting the maximum heating temperature to 800°C and slowly cooling after being held at that temperature (STEP 3).

As an example of the sample, we explained the case of using SUJ2 steel with an increased amount of S added, but it is possible to use a steel with an intentionally increased S content like this, or use other steels. can be done. In that case, for example, steel with a high oxygen content may be intentionally melted and used. In that case, the steel has a large amount of oxides dispersed as non-metallic inclusions. The use of steel containing a large amount of sulfides and oxides increases the frequency of observation of rolling fatigue behavior around non-metallic inclusions by increasing the distribution frequency of inclusions, enabling detailed investigation of the phenomenon. It's for.

Since oxides are difficult to deform during hot working, they tend to remain in a state close to a sphere or in a massive state even after hot forging or rolling. However, if it is produced in a refining process that produces silicate-based oxides, the oxides will become thermoplastic and may deform during processing and take on a spindle shape, like the sulfide (MnS) mentioned above. be. Regarding sulfides, in the case of (Mn, Ca) S and (Mn, Ca, Mg) S, which contain Ca and Mg in MnS, their thermoplasticity is lower than that of MnS, and they are not as stretchable as MnS. It may take a form that is not

When oxides or sulfides with poor thermoplasticity are contained in the steel, gaps may be formed between the nonmetallic inclusions and the surrounding matrix during hot forging or rolling. , this may hinder the determination of the effects of artificially introduced clearances on rolling fatigue behavior and rolling fatigue life, which will be described later.

Therefore, when such non-metallic inclusions are included, following forging (or rolling) (STEP 2), hot isostatic pressing is performed to close the gap before performing STEP 3. may be added and applied. As a condition for this, hot isostatic press working of 110 MPa or more at a temperature of 1160° C. or more may be applied. A more desirable hot isostatic pressing pressure is 140 MPa or higher.

Regarding normalizing and spheroidizing annealing in STEP 3, when using a steel grade other than SUJ2 or a similar SUJ2 base steel, select conditions that match the selected steel grade, or may omit it depending on the steel grade. shall be

Subsequently, an intermediate member 100a having a shape shown in FIG. 3 was produced from the sample (STEP 4). This is a shape for performing tensile work to artificially form a gap 104 in a part around the non-metallic inclusion 103 in the steel. At this time, the MnS stretching direction (matching the rolling direction) is perpendicular to the wide surface of the intermediate material 100a, but this direction may be changed according to the purpose. . The intermediate member 100a of the embodiment is a hollow disk-shaped member having an inner diameter hole 105 in the center. The inner diameter hole portion 105 has protrusions 105a and 105b at symmetrical positions centering on the inner diameter hole portion 105 in a direction parallel to the tensile direction (X in the figure) of subsequent tensile processing. Moreover, the inner diameter hole portion 105 has projections 105c and 105d at symmetrical positions centering on the inner diameter hole portion 105 in the direction perpendicular to the drawing direction X of the drawing process.

As for the pulling process, in this embodiment, a high-hardness steel pin is used to radially pull the inner diameter portion 105 of the intermediate member 100a (STEP 5).

A set of pins was prepared by grinding part of a SUJ2 round bar with a diameter of 12 mm in the longitudinal direction and then adjusting it to about 60 HRC by quenching and tempering. Pins are passed through the positions of the protrusions 105a and 105b of the intermediate member 100a shown in FIG. After being fixed in a die (to which a matching hole is applied), cold drawing was applied via a pin fixed to the die.

As in the present embodiment, the intermediate material 100a in the spheroidized and annealed state of SUJ2 is subjected to tensile work to form a gap 104 in part between the nonmetallic inclusion 103 particles and the matrix. Assuming that the tensile strength of the spheroidized intermediate material 100a is 1, a stress of at least about 0.85 times or more is applied to the vicinity of the portion where the non-metallic inclusion 103 exists. It is preferable to perform tensile processing as follows.

For example, when the shape of the intermediate member 100a shown in FIG. 3 is used, the stroke amount of the stretching process should be at least 5.3 mm or more. Assuming that the tensile strength of the intermediate material at the time of stretching is 1, the stress concentration effect caused by the presence of non-metallic inclusions was not taken into consideration in the point that a stress of about 0.85 times or more was required. Absent. However, in practice, the gap 104 is formed only around the nonmetallic inclusion particles 103 through the stress exceeding the tensile strength acting around the nonmetallic inclusions with the assistance of the stress concentration action around the nonmetallic inclusions. can be formed.

FIG. 4 is an enlarged view of part A in FIG. 1, and is a schematic diagram showing the state of a gap formed around non-metallic inclusions in steel. The upper surface of FIG. 4 corresponds to the wide surface of the test piece 100 of FIG. When it is desired to adjust the degree of the gap 104, processing with a stroke amount greater than the above is also included in the scope of the present invention. When the tensile strength is 1, it must be 0.95 times or less. More desirably, it is 0.93 times or less. The reason why the upper limit is set for the stress applied in the stretching process is to avoid the formation of voids in locations other than the periphery of the non-metallic inclusions 103, which would affect the rolling fatigue behavior and life.

In this example, the stroke amount of the stretching process was set to 7 mm. At this time, the protrusions 105c and 105d of the shape of the intermediate member 100a shown in FIG. 3 were changed to 4.5 mm instead of the 3 mm shown in FIG. 3 for the outer diameter of R7. Also, the thickness of the intermediate member 100a was set to 6.2 mm. After the tensioning, the inner diameter hole 105 of the intermediate member 100a after the tensioning was processed again into a circular shape (27 mm in diameter), although this is not an essential step, in order to facilitate the adjustment of the orbital position in the thrust test described later.

The shape of the intermediate member 100a is not limited to the shape illustrated in FIG. It shall be allowed to do so. Also, the thickness of the intermediate member 100a is not limited to the thickness of this embodiment. However, the shape and thickness of the intermediate member 100a must be such that the processing pin is not plastically deformed during the tensile processing.

Next, the hardness of the intermediate material was adjusted for the thrust test described later. In the case of the SUJ2 thrust test piece in this example, quenching and tempering (835 ° C - 0.5 h, oil cooling → 180 ° C - 1.5 h, air cooling) was performed to adjust the hardness of the test piece to about 62 HRC ( STEP6).

At this time, for the purpose of this embodiment for investigating the rolling fatigue behavior taking into account the effect of the gap 104 artificially introduced around the non-metallic inclusion 103, the test piece 100 (after hardness adjustment in STEP 6 The hardness of the intermediate material 100a) is preferably 55 HRC or more. If the hardness is lower than this, not only the defect and its surroundings but also the matrix phase undergoes rolling fatigue, making it difficult to distinguish and verify the harmfulness of the defect itself. A more desirable hardness of the test piece 100 is 58 HRC or more. More desirably, it is 60 HRC or more.

Subsequently, the oxide scale on the surface of the intermediate material 100a during the heat treatment was removed by surface grinding, and then the sample was finished by buffing. Thereby, the test piece 100 is completed.

In conducting a thrust type rolling fatigue test using the test piece 100, it is necessary to adjust the thrust test track C (STEP 7). That is, in the above-mentioned buffed sample, that is, the sample in which the inner diameter was reprocessed to a diameter of 27 mm after being subjected to tensile processing during the test piece preparation process, and the hardness was adjusted by quenching and tempering. Assuming that the stroke of the tensile processing is at least 5.3 mm or more, and the tensile strength of the sample at the time of tensile processing is 1, the position where a stress of about 0.85 times or more is applied passes through the center of the test piece 100. , and is perpendicular to the direction of tension, and is located near the position of 4 mm from the inner diameter side surface of which the diameter B is machined to 27 mm (A portion in FIG. 1).

Therefore, when conducting a thrust type rolling fatigue test, the fixed position of the test piece 100 to the thrust type rolling fatigue tester is adjusted so that the thrust test track C passes near that position (part A in FIG. 1). to

In addition, when the diameter of the track C in the thrust test to be carried out is 38.5 mm, by making the thrust test track C pass through a point about 4 mm from the inner diameter side surface, the position (B part in Fig. 1 ) and the opposite side through the center of the test piece will be more than 4 mm away from the inner diameter surface. As a result, at that position (part B in Fig. 1), if the stroke amount of the tensile processing is at least 5.3 mm and the tensile strength is 1, a stress less than 0.85 times that is applied. A gap around inclusions does not occur.

As a result, in the same test piece 100 in which the distribution of non-metallic inclusions in the steel is the same, the rolling is Since the difference in fatigue behavior can be confirmed at the same time, it will be possible to obtain a rolling fatigue test piece and a test method that have not existed in the past, which can clarify the effect of the gap around the non-metallic inclusion.

When actually conducting a thrust type rolling fatigue test, regarding the arrangement of the test piece 100, the upper plate is a SUJ2 single-direction thrust bearing race (model number 51305), the lower plate is the test piece 100, and the upper plate and the lower Three SUJ2 steel balls with a diameter of 3/8 inch should be equally distributed between the plates as rolling elements at a pitch of 120°. The number of rolling elements during the test may be three, or may be increased depending on the load capacity of the thrust tester.

Regarding the layout of the track, it is acceptable to deviate from the above recommended position within the testable range. In that case, since the stress applied to the test piece during tensile processing varies depending on the location of the test piece, it is sufficient to estimate the acting stress at the point corresponding to the trajectory of the thrust test by stress simulation using FEM. Furthermore, if necessary, we actually observe the cross section of the test piece to check the condition of the gap, and based on the feedback of the observation information, we use another test piece for tensile processing to determine the amount of gap around the non-metallic inclusion. You may also choose to adjust the stroke amount of the drawing process so that it is adjusted.

Figure 5 shows the cross section of the completed SUJ2 base steel thrust test piece with increased S content, and the state of the gap artificially formed by tensile working between the matrix around the sulfide (MnS) and the was observed. It is clear that a gap is formed around the sulfide in the cross section around 4 mm from the inner diameter side surface (part A in FIG. 1). No gaps are seen around the sulfide in the cross section about 8 mm from the inner diameter side surface (part B in Fig. 1), which is equivalent to the position under the same orbit with a diameter of 38.5 mm. That is, by the method of the present invention, it is possible to artificially form a gap in a part around the non-metallic inclusion contained in the steel, and at the same time, the sulfide under the same thrust test track opposite to that part It is confirmed that it is possible to prevent the formation of gaps in the

As described above, in the present embodiment, a method for producing a test piece in which artificial gaps are provided around non-metallic inclusions in steel and a method for testing rolling fatigue have been described. In this method, a steel sample prepared in advance (a sample that intentionally contains a large amount of nonmetallic inclusions may be selected or produced so that the nonmetallic inclusions can be easily observed when observing the cross section), It is possible to simultaneously create a portion where a gap is formed and a portion where a gap is not formed in a portion of the periphery of the non-metallic inclusions contained therein.

As a result, it is possible to conduct a thrust test that passes through both of these points at the same time. A rolling fatigue test was conducted taking into account the effect of the gaps around inclusions. Since the comparison can be performed under the same test conditions, it is possible to realize a precise investigation that has not been possible in the past.

Therefore, the rolling fatigue behavior around non-metallic inclusions and the applied It can also be expected to easily obtain knowledge about the behavior of the gap, which has been difficult to obtain in the past. As an observation method, a conventional cross-sectional observation method can be used, or a non-destructive observation method can be applied.

Although the embodiment has been described above, this embodiment is presented as an example and is not intended to limit the scope of the invention. This novel embodiment can be embodied in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. This embodiment and its modifications are included in the scope and gist of the invention, and are included in the scope of the invention described in the claims and its equivalents.

100: test piece 101: inner diameter hole portion 102: test piece body portion 103: nonmetallic inclusion 104: gap

Claims (8)

a test piece main body;
a non-metallic inclusion positioned at a predetermined position inside the test piece main body;
a gap positioned between the non-metallic inclusion and the test piece main body;
has _
The non-metallic inclusion is under the raceway when used for a thrust type rolling fatigue test,
A test piece in which other non-metallic inclusions not having the above-mentioned gap exist at any position under the raceway when used for the above-mentioned thrust type rolling fatigue test . The test piece according to claim 1, wherein the gap is formed partly around the non-metallic inclusion. The test piece according to claim 1 or 2, wherein the hardness of the test piece is 55 HRC or more. The test piece according to any one of claims 1 to 3, wherein said non-metallic inclusions are sulfides or oxides. A test method, wherein a thrust-type rolling fatigue test is performed using the test piece according to any one of claims 1 to 4 . Melting a sample made of steel containing non-metallic inclusions,
Hot working and stretching the sample to produce an intermediate material,
The intermediate material is subjected to tensile processing in a predetermined direction, a predetermined stress is applied to a predetermined position under the raceway when used in a thrust type rolling fatigue test, and at least one portion around the nonmetallic inclusion at the predetermined position Form a gap in the part,
The predetermined stress is not applied to an arbitrary position under the raceway when used in the thrust type rolling fatigue test, and no gap is formed around other non-metallic inclusions at the arbitrary position. Method of manufacturing test pieces. The tensile work is performed by applying a stress of 0.85 to 0.95 times the tensile strength of the intermediate material at a predetermined position of the intermediate material, where the tensile strength of the intermediate material at the time of the tensile work is set to 1. The method for producing a test piece according to claim 6 , wherein the test piece is loaded. 8. The method for producing a test piece according to claim 6 , further comprising the step of adjusting the hardness of said test piece to 55 HRC or more by quenching and tempering .

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