JP5998631B2 - Rolling bearing - Google Patents

Description

  The present invention is intended to improve the durability of various rolling bearings, including general rolling bearings such as radial bearings and thrust bearings, and special rolling bearings such as linear motion bearings (linear guides) and ball screws. Specifically, the structural change due to the inclusions present in the steel constituting the bearing part constituting the rolling bearing is suppressed, and the durability of the rolling bearing including the bearing part is improved.

  For example, a radial ball bearing 1 as shown in FIG. 1 is incorporated in a rotation support portion of various rotary machine devices. The radial ball bearing 1 includes an outer ring 3 having an outer ring raceway 2 on an inner peripheral surface, an inner ring 5 having an inner ring raceway 4 on an outer peripheral surface, and an outer ring raceway 2 and an inner ring raceway 4 provided between the outer ring raceway 2 and the inner ring raceway 4. A plurality of balls 6 and 6 which are moving bodies are provided. These balls 6, 6 are held by a cage 7 so as to be able to roll while being arranged at equal intervals in the circumferential direction. Further, a radial tapered roller bearing 8 using a tapered roller as a rolling element as shown in FIG. 2, for example, is incorporated in the rotation support portion to which a large radial load is applied. The radial tapered roller bearing 8 includes an outer ring 3a having a conical concave outer ring raceway 2a on an inner peripheral surface, an inner ring 5a having a conical convex inner ring raceway 4a on an outer peripheral surface, the outer ring raceway 2a and the inner ring raceway 4a. Are provided with a plurality of tapered rollers 9, 9 each being a rolling element provided so as to be able to roll while being held by the cage 7 a. Further, out of both ends of the outer peripheral surface of the inner ring 5a, a large-diameter side flange 10 is formed at the large-diameter end, and a small-diameter flange 11 is formed at the small-diameter end. The small diameter side flange 11 may be omitted. Such a radial ball bearing 1 and a radial tapered roller bearing 8 are configured such that, for example, the outer rings 3 and 3a are fitted and fixed to the housing, and the inner rings 5 and 5a are fitted and fixed to the rotating shaft. The housing is rotatably supported.

  For example, a rolling bearing such as the radial ball bearing 1 or the radial tapered roller bearing 8 as described above is often used for a long period under a large load. With such use, metal fatigue occurs in the steel constituting the bearing parts such as the outer rings 3 and 3a, the inner rings 5 and 5a, and the rolling elements (balls 6 and 6 or tapered rollers 9 and 9). The surface of the part may peel off. Such delamination occurs on the raceway surfaces of the race rings such as the outer rings 3, 3 and the inner rings 5, 5a, which are more susceptible to stress than the rolling surfaces of the rolling elements. easy. The types of delamination that occur on the surface of rolling bearing components such as this are "inclusion-origin-type delamination" that starts from inclusions inside the material, and indentations that contain foreign objects such as dust. There are “surface-originating exfoliation” that occurs, and “white structure exfoliation” that originates from a structural change called white structure in which hydrogen has entered the steel and has become hydrogen brittle. Since each of these peelings occurs by different mechanisms, different measures are required for each. Since the present invention aims to suppress inclusion-initiated type peeling among these, the mechanism of this inclusion-initiated type peeling occurrence and the prior art considered to suppress the occurrence of inclusion-initiated type peeling First, I will explain.

  If there is an oxide-based non-metallic inclusion in the steel constituting the bearing part that is much harder than the original hardness of this steel, stress concentration occurs in the peripheral portion of this inclusion, and a butterfly type is formed in this peripheral portion. Organizational changes occur. When such a butterfly-type structure change occurs, a crack due to metal fatigue occurs along the interface of the part where the structure has changed, and this crack further develops, leading to inclusion-origin type separation. In order to suppress the occurrence of such inclusion-initiated type separation, conventionally, it has been proposed to reduce the number or the number of oxide-based non-metallic inclusions that are the origin of separation.

For example, Patent Document 1 discloses a bearing steel in which the maximum diameter of oxide-based nonmetallic inclusions included in an estimated area of 30000 mm ^{2 is} predicted using an extreme value statistical method, and the maximum diameter is controlled to 5 μm or less. A technique for extending the life of a rolling bearing by using it is disclosed. However, as described in Patent Document 1, in order to manufacture a steel material with extremely high cleanliness, it is indispensable to reduce the amount of oxygen and sulfur in the steel. The existing equipment has already reached its limit, and in order to further reduce the amount of oxygen and sulfur, it is necessary to improve the equipment and processes (substantial improvements in steelmaking equipment and process changes) Inevitable rise. Therefore, it may be difficult considering the price of steel and productivity.

Patent Document 2 discloses that the number of sulfide-based nonmetallic inclusions having a thickness of 1 μm or more included in a test area of 320 mm ² is 1200 or less, and the maximum diameter of oxide-based nonmetallic inclusions is 10 μm. In the following, there are described inventions related to bearing steels that can extend the life of rolling bearings by controlling each of them.
In Patent Document 3, the number of oxide nonmetallic inclusions contained in 320 mm ² is set to 100 to 200, and the amount of Sb as an impurity element is suppressed to 0.001% by mass or less, thereby increasing the length of the rolling bearing. An invention relating to bearing steel that achieves a long life is described.
However, in the actual operation state of the rolling bearing, peeling occurs starting from the largest nonmetallic inclusion among the nonmetallic inclusions present in the high stress portion of the rolling bearing. For this reason, even if the number and size of the nonmetallic inclusions contained in the minute area of the steel material is limited as in the inventions described in Patent Documents 2 and 3 as described above, the origin is not necessarily the nonmetallic inclusions. It is not always possible to extend the peeling life. In other words, unexpected starting point peeling occurs, and there is a possibility that an individual whose peeling life does not extend as intended is generated.

Furthermore, Patent Document 4 uses an extreme value statistical method to set the predicted maximum diameter of sulfide-based nonmetallic inclusions contained in 30000 mm ² to 40 μm or less, and further includes oxide, sulfide, and nitride inclusions. An invention relating to a steel for bearings is described in which the predicted maximum diameter of an object is obtained, and the maximum value is suppressed to 60 μm or less, thereby extending the life of a rolling bearing.
However, as in the invention described in Patent Document 4, when a non-metallic inclusion of a certain size such as 60 μm at the maximum is allowed, the usage condition of the rolling bearing is If it is severe, butterfly structure changes may occur from this non-metallic inclusion as a starting point, which may lead to peeling.

JP 2003-232367 A JP-A-9-291340 JP 2000-144326 A JP 2006-63402 A

  In view of the circumstances as described above, the present invention reduces the non-metallic inclusions present in the steel constituting the bearing part to such an extent that it causes an excessive increase in cost (as compared to currently practiced). Rather than adding a proper amount of Si, Mn, Cr, and Mo to the steel, making it difficult for butterfly structure changes to occur around non-metallic inclusions (delaying the occurrence of butterfly structure changes) The invention was invented to realize a rolling bearing capable of extending the service life.

The rolling bearing of the present invention includes the radial ball bearing 1 or the radial tapered roller bearing 8 shown in FIGS. 1 and 2 described above, and is similar to various types of conventionally known rolling bearings. A first raceway having a raceway surface, a second raceway having a second raceway surface on a surface opposite to the first raceway surface, and between the first and second raceway surfaces. A plurality of rolling elements provided so as to freely roll.
In particular, in the rolling bearing of the present invention, a bearing component that is at least one member of the first race ring, the second race ring, and each of these rolling elements,
C is 0.85 to 1.15% by mass, Si is 0.40 to 0.90% by mass, Mn is 0.55 to 1.20% by mass, Cr is 1.30 to 1.90% by mass, Mo is 0.30 mass% or less, Ni 0.30 mass% or less, Cu 0.20 mass% or less, S 0.025 mass% or less, P 0.020 mass% or less, O 15 mass ppm or less, Each containing, the remainder as Fe and inevitable impurities,
When the maximum value regarding the size of oxide inclusions existing in an area of 30000 mm ² is predicted by the extreme value statistical method, the square root of the area of the maximum oxide inclusions is 22 μm or more and 50 μm or less. It is composed.
The hardness of the at least one member after quenching and tempering is Hv697-800. Since this member is subjected to so-called quenching, the hardness of this member is substantially the same from the surface to the core.
Further, the amount of the spheroidized carbide remaining in the steel after quenching and tempering of the at least one member is set to 5 to 9% by mass.
Furthermore, [Si], [Mn], [Cr], [Mo] is a numerical value representing the content of each alloy component (element) in the steel in mass%, and [MC] When the percentage of spheroidized carbide remaining in the steel after tempering is a numerical value expressed in mass%,
2.5 ≦ 2 [Si] + [Mn] + ([Cr] −7 [MC] / 100) / (1− [MC] / 100) +3 [Mo] ≦ 3.8
Meet.

  According to the present invention configured as described above, a butterfly structure is formed around the nonmetallic inclusions without reducing the nonmetallic inclusions present in the steel constituting the bearing component to an excessive cost increase. Changes are less likely to occur and the life of the rolling bearing can be extended. This reason will be described below together with the reasons for limiting each numerical value.

  As described above, “inclusion starting type separation” which is intended to suppress the occurrence according to the present invention is based on the stress concentration generated around the non-metallic inclusions existing in the steel constituting the bearing part. This is a phenomenon in which a butterfly structure change occurs in the peripheral portion, fatigue cracks generated along the interface of the changed structure develop, and lead to delamination. In the case of the inventions described in Patent Documents 1 to 4 described above, it is intended to suppress inclusion-origin type peeling by limiting the size and amount of non-metallic inclusions that are the starting point of such peeling. Was. On the other hand, in the case of the present invention, inclusion starting type peeling is suppressed as follows.

  The butterfly structure change is the following phenomenon. When a large load is applied to the bearing part, stress concentrates around the non-metallic inclusions existing in the steel constituting the bearing part. The large shear stress generated by this stress concentration is repeatedly applied to the martensitic structure of the steel base, so that dislocations and solute carbon in the martensitic structure are moved, and the martensitic structure gradually collapses. , Changes to an ultrafine ferrite structure. The present invention stabilizes the martensitic structure in the base by adding an optimum amount of Si, Mn, Cr, and Mo as alloy components in the steel in order to delay such a butterfly structure change. Is. That is, the present invention stabilizes the martensite structure, thereby making it difficult for dislocations and solute carbons to move in the martensite structure, delaying the butterfly structure change, and extending the life of the bearing component. Is.

  However, the addition of Si, Mn, Cr, and Mo does not completely prevent the butterfly structure change, but only delays it. Therefore, the addition of Si, Mn, Cr, and Mo can delay the butterfly structure change. However, if the size of the nonmetallic inclusions is too large, the butterfly structure change starting from the nonmetallic inclusions will occur. The occurrence of this is not always sufficiently suppressed. On the other hand, since the occurrence of this butterfly structure change can be delayed, the size and number of non-metallic inclusions in steel are particularly high in steelmaking costs, as in the inventions described in Patent Documents 1 to 4 described above. There is no need to make it smaller or smaller. In the case of the present invention, since the square root of the area of the maximum oxide inclusion is regulated to 22 μm or more and 50 μm or less, the butterfly structure change starting from the nonmetallic inclusion is suppressed while suppressing the steelmaking cost. Occurrence can be suppressed sufficiently.

In short, by adding Si, Mn, Cr, and Mo to the steel, even if non-metallic inclusions of a certain size exist in the steel, inclusion starting type peeling occurs in the bearing parts. It is difficult to improve the durability of the rolling bearing incorporating the bearing component.
Next, the reason why the elements added to the steel constituting the bearing part, the content thereof, and the size of the oxide inclusions are regulated will be described below.

[C: 0.85 to 1.15% by mass]
C is an element that dissolves in the base by quenching and improves the hardness, so it is added to ensure the necessary hardness for the bearing parts. When the amount of C in the alloy component is less than 0.85% by mass, the hardness after quenching is insufficient, and wear resistance and rolling fatigue life are reduced. Therefore, C is contained in an amount of 0.85% by mass or more. In order to obtain these wear resistance and rolling fatigue life more stably, the C content is preferably 0.95% by mass or more. On the other hand, if the C content exceeds 1.15% by mass, the resulting bearing part becomes too hard, resulting in a decrease in grindability and a decrease in fracture toughness value. Therefore, the C content is suppressed to 1.15% by mass or less. In order to further stabilize the grindability, the C content is preferably 1.10% by mass or less.

[Si: 0.40-0.90 mass%]
Si has the effect of improving the hardenability and temper softening resistance by dissolving in the base and is added to ensure the necessary hardness for the bearing parts. Moreover, Si has an effect of suppressing the occurrence of inclusion-origin separation, which is an important object of the present invention. That is, Si stabilizes martensite in the base structure, delays butterfly-type structural changes that occur around non-metallic inclusions, and suppresses (delays) the occurrence of inclusion-origin-type separation in bearing parts. This contributes to extending the life of rolling bearings incorporating this bearing component. Such a life extension effect due to the butterfly structure change delay cannot be sufficiently obtained when the Si amount is less than 0.40 mass%. On the other hand, when the Si content exceeds 0.90% by mass, the hardness after spheroidizing annealing is increased, so that the turning property and the cold workability are lowered. In order to suppress the hardness after spheroidizing annealing to an appropriate range and obtain stable turning and cold workability, the Si content is preferably suppressed to 0.70% by mass or less.

[Mn: 0.55 to 1.20 mass%]
Mn has the effect of improving the hardenability by solid solution in the base, so Mn is added to ensure the necessary hardness for the bearing parts. Mn also has the effect of suppressing the occurrence of inclusion-origin separation, which is an important object of the present invention, as in the case of Si described above. That is, Mn also stabilizes the martensite in the base structure, delays the butterfly structure change that occurs around the non-metallic inclusions, and suppresses the occurrence of inclusion-origin separation in the bearing parts. Contributes to extending the life of rolling bearings incorporating Further, Mn has an effect of easily generating the amount of retained austenite after the heat treatment. Residual austenite is a relatively sticky structure, and suppresses the above-described surface-origin separation, and contributes to extending the life of a rolling bearing incorporating the bearing component from another viewpoint. Such an effect cannot be sufficiently obtained when the Mn content is less than 0.55% by mass. On the other hand, if the content of Mn exceeds 1.20% by mass, the deformation resistance during hot forging increases and the hot forgeability decreases. Further, the retained austenite in the steel constituting the bearing part is decomposed little by little with the use of the rolling bearing, and the volume expands, albeit slightly, with the decomposition. For this reason, when the amount of retained austenite becomes excessive by increasing the content of Mn, the stability of the shape and dimensions of the bearing component is lowered. Therefore, the amount of Mn in the steel constituting this bearing part is set in the range of 0.55 to 1.20% by mass. In order to further stabilize the hot forgeability and dimensional stability, the Mn content is preferably suppressed to 0.85% by mass or less.

[Cr: 1.30 to 1.90 mass%]
Cr is distributed into a part that dissolves in the base martensite and a part that dissolves in the spheroidized carbide. Cr dissolved in the base martensite has the effect of improving the hardenability and ensuring the hardness of the surface of the bearing component. Cr also has the effect of suppressing the occurrence of inclusion-origin separation, which is an important object of the present invention, as in the case of Si and Mn. That is, Cr also stabilizes martensite in the base structure, delays the butterfly structure change that occurs around non-metallic inclusions, and suppresses the occurrence of inclusion-origin separation in the bearing part. Contributes to extending the life of rolling bearings incorporating Such an effect cannot be sufficiently obtained when the Cr content is less than 1.30% by mass. On the other hand, if the Cr content exceeds 1.90 mass%, the hardness after spheroidizing annealing is increased, so that the turning property and the cold workability are lowered. Therefore, the amount of Cr in the steel constituting the bearing part is set to a range of 1.30 to 1.90% by mass. In order to further stabilize the turning property and the cold workability, the Cr content is preferably 1.70% by mass or less.

[Mo: 0.30 mass% or less]
Mo dissolves in the base and improves the hardenability and temper softening resistance, and has the effect of ensuring the hardness of the bearing component surface. Further, Mo also has an effect of suppressing the occurrence of inclusion-origin type peeling, which is an important object of the present invention, as in the case of Si, Mn, and Cr described above. In other words, Mo also stabilizes the martensite of the base structure, delays the butterfly structure change that occurs around the non-metallic inclusions, and suppresses the occurrence of inclusion-origin separation from the bearing parts. Contributes to extending the life of the built-in rolling bearing. However, if the Mo content exceeds 0.30% by mass, a part of Mo forms a hard carbide, which reduces grindability. Moreover, since it is a very expensive element, it causes a high manufacturing cost of the rolling bearing including the bearing component. Therefore, the Mo content is set to 0.30% by mass or less. Preferably, the Mo content is 0.15% by mass or less. In addition, although the lower limit of content of Mo is controlled from the surface of manufacturing cost, it is preferable to set it as 0.01 mass% or more.

[Ni: 0.30 mass% or less]
Ni is an element having an effect of improving hardenability and an effect of stabilizing austenite. Further, when added in a large amount, Ni improves toughness. However, since it is a very expensive element, it causes an increase in the manufacturing cost of the rolling bearing including the bearing component. Therefore, Ni was not actively added, and its content was set to 0.30% by mass or less. Preferably, the Ni content is 0.18% by mass or less. In addition, although the lower limit of content of Ni is controlled from the surface of manufacturing cost, it is preferable to set it as 0.01 mass% or more.

[Cu: 0.20% by mass or less]
Cu has the effect of improving the hardenability and the effect of improving the grain boundary strength. However, when the Cu content increases, hot forgeability decreases. Therefore, Cu is not actively added, and its content is set to 0.20% by mass or less. However, regarding Cu, there is an advantage by adding, so 0.01% by mass or more is preferably added.

[S: 0.025% by mass or less]
Since S forms MnS and acts as an inclusion, the smaller the amount of S contained in the steel, the better. However, S is an element that exists abundantly in the natural world, and trying to keep the S content extremely low reduces the productivity of the steel material and increases the manufacturing cost of the steel material, so it can be widely used industrially. It becomes difficult. On the other hand, even if S is contained in an amount of about 0.025% by mass, durability required for the bearing component can be ensured by making the content of other elements and the heat treatment method appropriate. Therefore, the upper limit of the S content is set to 0.025% by mass.

[P: 0.020% by mass or less]
P is preferably as small as possible because it segregates at the grain boundaries and lowers the grain boundary strength and fracture toughness value. However, P is also an element that exists in a large amount in nature. If an attempt is made to keep the P content extremely low, the manufacturing cost of the steel material will increase. On the other hand, even if P is contained in an amount of about 0.020% by mass, durability required for bearing parts can be ensured by making the content of other elements and the heat treatment method appropriate. Therefore, the upper limit of the P content is set to 0.020% by mass.

[O: 15 mass ppm or less]
O forms oxide-based non-metallic inclusions such as Al ₂ O ₃ in the steel. Oxide-based non-metallic inclusions serve as a starting point for peeling and adversely affect the rolling fatigue life. Therefore, the smaller the O content, the better. However, with regard to O as well, the steel material cost increases when the content is extremely reduced. On the other hand, even if O is contained in an amount of about 15 ppm by mass, the content of other elements and the heat treatment method can be made appropriate. The required durability can be ensured. Therefore, the upper limit of the O content was set to 15 ppm by mass.

[The square root of the area of the largest oxide inclusion is 22 μm or more and 50 μm or less]
The extreme value statistical method used to define the square root value in the present invention is a method for predicting extreme values such as maximum value and minimum value for a set according to normal distribution, exponential distribution, logarithmic distribution, etc. It is an effective method for predicting the maximum diameter of non-metallic inclusions contained in steel. In addition, in the inclusion-origin type separation due to non-metallic inclusions existing in the steel of the bearing parts constituting the rolling bearing, it is good between the maximum inclusion diameter predicted by the extreme value statistical method and the rolling fatigue life. Correlation is seen. In particular, it is known that oxide-based non-metallic inclusions have the most adverse effect on the lifetime.

Therefore, in the case of the present invention, when the size of the maximum oxide inclusions existing in the area of 30000 mm ² is predicted by the extreme value statistical method, the rolling bearing has the maximum area of the oxide inclusions. Is set to 22 μm or more and 50 μm or less. When the value regarding this square root exceeds 50 μm, even when subjected to rolling fatigue, a butterfly structure change does not occur, and fatigue cracks are generated directly from oxide inclusions. For this reason, even if the alloy component (composition) is within the range specified in the present invention, the effect of extending the life of the bearing component cannot be obtained. On the other hand, it is preferable to set the value relating to the square root to less than 22 μm from the viewpoint of preventing damage due to rolling fatigue, but it is necessary to discard a portion containing large inclusions in the steel material, so that the yield of the steel material is reduced. However, the cost of the steel material increases, making it difficult to use widely in the industry. Therefore, the square root of the area of the maximum oxide inclusion is set to 22 μm or more and 50 μm or less.

Note that when the value of the square root is regulated in the present invention, the area of the oxide inclusions may be determined approximately assuming that the oxide inclusions are rectangular. In addition, when performing extreme value statistics, a method proposed by the “Study Group for Evaluation Method of Nonmetallic Inclusions in Bearing Steel (abbreviated as EIBS Study Group)” of the Japanese Society of Tribology is preferable. That is, the maximum oxide inclusions included in the observation area 100 mm ² of the steel material cross section are obtained, and the maximum oxide inclusions included in 30000 mm ² are obtained by statistical processing. It is preferable to determine the size.

[Hardness after quenching / tempering: Hv697 to Hv800]
The inclusion-origin-type peeling to be suppressed by the present invention occurs as a crack progresses due to metal fatigue along the interface of the butterfly-type structure change portion as described above. In addition, as described above, this butterfly-type structure change is caused by a large shear stress generated by stress concentration around oxide inclusions being repeatedly applied to the base martensite structure. This is a phenomenon in which dislocations and solute carbon are moved to change to an ultrafine ferrite structure. Improving the hardness of the matrix structure makes it difficult for dislocations and solute carbons to move in the martensite structure even when shear stress is applied to the matrix structure, and delays the occurrence of butterfly structure changes. effective. If the hardness is less than Hv697, the above effects are insufficient, and a butterfly structure change is likely to occur, resulting in a reduced rolling fatigue life. On the other hand, if the hardness exceeds Hv 800, the grindability and fracture toughness value of the bearing parts are reduced. Therefore, the hardness value is regulated within the range of Hv697 to Hv800. As described above, the hardness is substantially the same from the surface to the core.

[Amount of each element in steel and ratio of spheroidized carbide remaining in steel after quenching and tempering]
[Si], [Mn], [Cr], [Mo] is a numerical value representing the content of each alloy component in the steel constituting the bearing part in mass%, and [MC] When the percentage of spheroidized carbide remaining in the steel after tempering is a numerical value expressed in mass%,
2.5 ≦ 2 [Si] + [Mn] + ([Cr] −7 [MC] / 100) / (1− [MC] / 100) +3 [Mo] ≦ 3.8
Regarding the points to satisfy.
In order to explain this point, in “2 [Si] + [Mn] + ([Cr] −7 [MC] / 100) / (1− [MC] / 100) +3 [Mo]” in the inequality, The represented value is defined as “calculated value”.

  The present inventors conducted a rolling fatigue life test using a plurality of types of steel materials having different components, and quantified the effect of extending the life of each alloy element on the peel life due to the butterfly structure change. As a result, the effect of extending the life of each alloy element, which contributes to improving the peel life by suppressing the butterfly structure change based on the addition of Si, Mn, Cr, and Mo, is 2: 1: It turned out to be 1: 3. In addition, it can be seen that the larger the calculated value, the greater the amount of alloying elements dissolved in the base martensite structure, the base structure becomes more stable, the occurrence of butterfly structure changes delays, and the life becomes longer. It was. Si and Mo have a longer life-spanning effect than Mn and Cr because Si and Mo have a large difference in atomic radius between Fe and Si. It is presumed that the effect of making dislocation and solute carbon difficult to move in the structure is greater.

However, Cr is distributed into a part that dissolves in the matrix martensite structure and a part that dissolves in the spheroidized carbide, and in the spheroidized carbide, Cr is concentrated to about 7% by mass. Therefore, if the ratio of spheroidized carbides is large, Cr is distributed more in the spheroidized carbides, so the amount of Cr in the matrix martensite structure becomes low. After spheroidizing annealing, about 15% by mass of carbide in the steel is present, but by quenching, a part of the spheroidizing carbide is dissolved in the matrix martensite structure. That is, the amount of Cr dissolved in the base structure is determined by the amount of Cr in the steel material and the amount of spheroidized carbide remaining after quenching and tempering ((Cr) -7 [MC] / 100 ) / (1- [MC] / 100). In the case of the present invention, the amount of spheroidized carbide remaining in the steel of the bearing component of the rolling bearing is 5 to 9% by mass in the state after quenching and tempering. The reason is as follows. First, when the residual amount of spheroidized carbide exceeds 9% by mass, Cr is distributed in a large amount in the spheroidized carbide, so that the amount of Cr dissolved in the base structure is insufficient, and the butterfly structure change is suppressed. The effect is reduced. On the other hand, when the residual amount of the spheroidized carbide is less than 5% by mass, the amount of the spheroidized carbide dissolved in the base becomes excessive, and the C amount (carbon amount) in the base becomes excessive. As a result, the amount of retained austenite becomes excessive and the dimensional stability decreases.

  When the calculated value defined as described above is less than 2.5, it is not possible to sufficiently obtain a long life effect related to the peeling life based on the butterfly structure change. On the other hand, if the calculated value exceeds 3.8, the amount of alloying elements dissolved in the base martensite becomes excessive, so that the grindability decreases or the retained austenite becomes excessive, resulting in shape stability. And the dimensional stability is reduced. Preferably, in order to stably improve the rolling fatigue life and obtain good grindability, the calculated value is set to 2.8 or more and 3.8 or less.

1 is a partially cut perspective view of a radial ball bearing which is a kind of rolling bearing that is an object of the present invention. FIG. The partial cut perspective view of a radial tapered roller bearing. The graph which shows the relationship between the calculated value regarding the amount of each element in steel, the ratio of the spheroidized carbide which remains in this steel, and the rolling fatigue life of a bearing component. The microscope picture which shows two examples of the oxide type nonmetallic inclusion observed in the vicinity of the peeling part.

  The feature of the present invention is to appropriately regulate the components of steel materials, the size of oxide inclusions, and the hardness after quenching and tempering for some or all of the bearing parts constituting the rolling bearing. Therefore, it is in the point which suppresses peeling which generate | occur | produces from an oxide type inclusion. Regarding the structure appearing in the drawings, the radial ball bearing 1 shown in FIG. 1 and the radial tapered roller bearing 8 shown in FIG. 2 are the same as the rolling bearings of various structures conventionally known. In the following, a description will be given of a technique that can improve the life of a rolling bearing by carrying out the present invention together with the present invention.

[Appropriate amount of retained austenite after quenching and tempering]
Residual austenite contained in the steel is softer than martensite, which is a base structure, and therefore relieves stress concentration at the edge of the indentation caused by biting hard foreign matter such as iron powder. And generation | occurrence | production of the crack which started from the edge part of this indentation can be suppressed, and there exists an effect which extends a surface origin type | mold peeling life. In order to sufficiently obtain such an effect in combination with the present invention, the amount of retained austenite is preferably 11% by volume or more. As described above, the present invention is intended to suppress inclusion-initiated type peeling, and the surface-initiated type peeling life by securing the amount of retained austenite as described above is greater than the inclusion-initiated type peeling life. Is preferred. When the amount of retained austenite in the steel is less than 11% by volume, there is a higher possibility that surface-initiated separation will occur earlier than inclusion-initiated separation. On the other hand, when the amount of retained austenite exceeds 20% by volume, shape stability and dimensional stability are lowered due to the reasons described above.

In consideration of these, in combination with the present invention, when the amount of retained austenite after quenching and tempering is regulated, it is preferably set to 11% by volume or more and 20% by volume or less (described in claim 2). invention). More preferably, the amount of retained austenite in the steel is 11% by volume or more and 16% by volume or less in order to obtain good shape stability and dimensional stability and surface-origin type peel life. The amount of retained austenite is measured by cutting out a part of the bearing part (for example, a part of the raceway surface), then electrolytically polishing the part of the surface (for example, the surface of the raceway surface), and using an X-ray diffractometer. Do.
However, when carrying out the present invention, the amount of retained austenite is not necessarily 11% by volume or more. For example, when the present invention is applied to a rolling bearing used under high temperature conditions in which decomposition of retained austenite is likely to proceed, the amount of retained austenite is set to less than 11% by volume with emphasis on shape stability and dimensional stability. It may be preferable to use it.

[Suitable heat treatment conditions]
In carrying out the rolling bearing according to the present invention, when the bearing part satisfying the conditions described in the claims is a bearing ring, the part is subjected to hot working and turning in order, and the part. After making the shape close to the completed shape of the raceway and making it an intermediate material, the intermediate material is subjected to quenching and tempering treatment to obtain a second intermediate material. After that, at least the raceway surface portion of the second intermediate material is ground to finish the finished shape. The above-mentioned hardness, residual austenite amount, and the ratio of remaining spheroidized carbide are realized by using steel materials that satisfy the conditions described in the claims, and by properly regulating the quenching and tempering conditions. it can.

  In order to make the productivity equivalent to 2 types of high carbon chrome bearing steel (SUJ2, JIS G 4805), which is generally used as bearing parts, the steel material can be quenched under the same conditions as SUJ2. preferable. That is, preferably, quenching is oil-cooled after being held at 820 to 860 ° C. for a predetermined time. More preferably, the holding temperature is set to 830 to 850 ° C. in order to stably bring the ratio of hardness, retained austenite and remaining spheroidized carbide to a suitable range.

Tempering is also preferably performed under the same conditions as SUJ2. That is, it is preferable to hold at 160 to 200 ° C. for a predetermined time, and then air or furnace cool. When the tempering temperature is less than 160 ° C., the amount of retained austenite becomes excessive, and the shape stability and dimensional stability are lowered. On the other hand, when the tempering temperature exceeds 200 ° C., the amount of retained austenite decreases, and the effect of alleviating the stress concentration at the indentation edge, which causes the above-described surface-origin separation, cannot be obtained sufficiently.
However, as described above, when the present invention is applied to a rolling bearing that uses the present invention under high temperature conditions, emphasis is placed on shape stability and dimensional stability. Tempering may be performed at a temperature so that the amount of retained austenite is less than 11% by volume.

[Suitable raceway groove shape]
The rolling bearing of the present invention is a ball bearing such as a deep groove type ball bearing, an angular type ball bearing, a thrust ball bearing, a roller bearing such as a cylindrical roller bearing, a tapered roller bearing, a self-aligning roller bearing, or a needle bearing. It is applicable without being limited to the type of. Among these, in the case of the most general ball bearing, it is preferable to restrict the shape of the raceway groove of the bearing ring as follows from the viewpoint of ensuring both the durability of the ball bearing and the reduction in torque.

  That is, in rolling bearings, not only durability (life) but also low rotational resistance (dynamic torque) (reduction in torque) is often required. In order to reduce the dynamic torque of the ball bearing, the ratio of the radius of curvature of the raceway groove (the bus bar shape) to the ball diameter is increased, and the contact area between the rolling surface and raceway surface of each ball (contact ellipse) It is effective to reduce. However, in general, when the ratio of the radius of curvature of the raceway groove to the diameter of each ball is increased and the contact area is reduced, the surface pressure of the contact portion increases and stress generated in the vicinity of the surface of the raceway surface. Becomes larger. For this reason, butterfly structure changes are likely to occur starting from non-metallic inclusions present in the steel constituting the bearing ring, and the life of the rolling bearing including the bearing ring is reduced.

On the other hand, the rolling bearing of the present invention makes it difficult for the butterfly structure change to occur by providing the above-described requirements. Therefore, the ratio of the radius of curvature of the raceway groove to the diameter of each ball is set. Even if it is increased, the life is not likely to decrease. Considering these matters, the rolling bearing of the present invention is suitable for applications requiring rotation with low torque, for example, motor bearings, automobile transmission bearings, machine tool bearings, and the like.
The ratio of the radius of curvature of the raceway groove to the diameter of the ball is generally about 51 to 52%. However, in the case of the rolling bearing of the present invention, the ratio of the radius of curvature of the raceway groove to the diameter of the ball is 53%. As described above, even when the ratio is 54% or less, the same life as a bearing having a ratio of the curvature radius of the raceway groove to the diameter of a ball made of general steel of 52% can be obtained.
In consideration of these matters, when the present invention is implemented as a ball bearing, the ratio of the radius of curvature of the raceway groove to the diameter of each ball is preferably 53% or more and 54% or less. 3).

  An experiment conducted in the process of forming the present invention will be described. In the experiment, for the A to Q17 kinds of steel materials shown in Table 1 below, a turning evaluation test for knowing the ease of processing, and whether or not a desired hardness and a retained austenite amount can be obtained. 4 of the heat treatment test to know, the bearing prototype test to know if the surface roughness can be finished as desired, and the bearing life test to know the durability of the actual rolling bearing A variety of tests were conducted.

In Table 1, numerical values enclosed in parentheses indicate that the numerical values are outside the technical scope of the present invention. Steel grade O is SUJ2.
[Turability evaluation test]
Using a steel material having the composition shown in Table 1, spheroidizing annealing was performed, and then a turning test was performed. This turning test was performed by measuring the amount of wear on the flank of this cutting tool after turning the outer periphery of the bar for 20 minutes with a cutting tool (bite). Test conditions are shown below.
Cutting tool: Carbide (P20)
Peripheral speed of part to be cut: 150 m / min
Cutting depth: 1.0mm
Cutting tool feed rate: 0.2 mm / rev
Lubrication condition: Dry type

The results of the experiment conducted under such conditions are shown in the “Turning test wear amount” column in the following Table 2.

As is apparent from Table 2, Examples 1 to 9 belonging to the technical scope of the present invention have a suitable steel composition, so that the turning property after spheroidizing annealing is JIS-SUJ2 (Comparative Example 6). ) And almost the same level.
On the other hand, the amount of Si in Comparative Example 1 and the amount of Cr in Comparative Example 3 are higher than specified in the present invention, respectively, so that the amount of flank wear of the tool in the turning test is large and the turning performance is inferior.

[Heat treatment test]
Using a steel material having the composition shown in Table 1, spheroidizing annealing was performed, and then a disk specimen having a diameter of 60 mm and a thickness of 6 mm was produced. These were subjected to quenching and tempering treatment at temperatures described in the columns of “Quenching temperature” and “Tempering temperature” in Table 2, and then the surface hardness was measured with a Vickers hardness meter. The amount of retained austenite (residual γ) was also measured. The holding time during quenching is 40 min, and the holding time during tempering is 2 hr. Such heat treatment conditions are almost the same as those of SUJ2. The measurement results are shown in the “hardness” column and the “residual γ” column in Table 2.

As is apparent from Table 2, Examples 1 to 9 belonging to the technical scope of the present invention are in a suitable range of the steel material. Therefore, even when heat-treated under the same conditions as SUJ2, good hardness and residual properties are obtained. The amount of austenite can be obtained.
On the other hand, Comparative Example 2 has a higher amount of retained austenite because the amount of Mn is higher than that defined in the present invention. Therefore, when used for a long time as a rolling bearing, sufficient shape stability and dimensional stability cannot be obtained.
Further, Comparative Example 9 uses the same steel material E as Example 5, but due to the difference in quenching temperature, the hardness is higher than the range specified in the present invention, so that the grinding and fracture toughness values of bearing parts are It is disadvantageous from the aspect of securing.
Further, Comparative Example 10 uses the same steel material G as in Example 7, but due to the difference in quenching temperature, the hardness is lower than the range specified in the present invention, which is disadvantageous from the viewpoint of securing the rolling fatigue life. Become.

Also, in order to know the ratio of spheroidized carbide ([MC]) remaining in the steel after quenching and tempering, picral etching is performed on the cross section of the specimen subjected to heat treatment for quenching and tempering. Was used to observe the metal structure, and the area ratio of carbide was measured. The measured area ratio converted to mass% is shown in Table 2 as the ratio of spheroidized carbide ([MC]).
In addition, the oxide inclusions were also observed, and the size of the largest oxide inclusion contained in 30000 mm ² was predicted by an extreme value statistical method. The predicted value of the square root of the maximum area of the oxide inclusions is shown in the column of the maximum diameter of oxide inclusions in Table 2.

[Bearing prototype (grindability evaluation) test]
Using the steel material having the composition described in Table 1, the inner ring and the outer ring of a single row deep groove type ball bearing (inner diameter 30 mm, outer diameter 62 mm, width 16 mm) having a nominal number of 6206 were manufactured in the following steps. . First, the steel material is subjected to spheroidizing annealing to be an intermediate material, and then this intermediate material is turned, quenched and tempered to become a second intermediate material, and finally the second intermediate material is ground and finished. Shaped. Grindability was evaluated by measuring the surface roughness of the raceway grooves (inner ring raceway and outer ring raceway) after grinding. The surface roughness is measured by measuring the three axially spaced positions on the inner ring raceway and outer ring raceway surface using the arithmetic average roughness Ra as an index (six kinds of measurements for each sample). The average value was calculated. The measurement results relating to the surface roughness thus obtained are listed in the “groove roughness” column of Table 2 above.

As is apparent from Table 2, Examples 1 to 9 belonging to the technical scope of the present invention are in a suitable range of composition and hardness after quenching and tempering. The value is comparable to that of Comparative Example 6 having the same components as SUJ2 and is excellent in grindability.
On the other hand, in Comparative Example 4, since the Mo content was higher than the range specified in the present invention, it was found that the surface roughness of the raceway groove was large (bad) and inferior in grindability.
Moreover, since the calculated value of the comparative example 8 was higher than the range prescribed | regulated by this invention, it turned out that the surface roughness of a track groove is bad and it is inferior to grindability.
Moreover, since the hardness of the comparative example 9 was higher than the range prescribed | regulated by this invention, it turned out that the surface roughness of a track groove is bad and it is inferior to grindability.

[Bearing life test]
A SUJ2 ball (diameter 9.525 mm) is incorporated between the inner ring and outer ring of the ball bearing 6206, which is manufactured as described in the above-mentioned bearing prototype test, and a crown shape made of polyamide resin. Each of the test bearings was held by a cage.
The life test conditions are as follows. In each example and each comparative example, a life test was performed with 4 to 8 ball bearings, and a life (L10 life) at which the cumulative failure probability was 10% was obtained.
Radial load: 13818N
Rotational speed: 3900 min ^-1
Rotating condition: Inner ring rotation Lubricating oil: Mineral oil equivalent to ISO-VG68 (forced circulation method)
In addition, each of the tests described above and Comparative Examples 1 and 3 determined as defective in the turning evaluation test, Comparative Example 2 determined as defective in the heat treatment test, and a bearing prototype (grindability) test determined as defective. For Comparative Examples 4, 8, and 9, the bearing life test was omitted.

  The bearing life test conducted under the above conditions shows that the surface of the raceway surface (inner ring raceway or outer ring raceway, but in many cases, the inner ring raceway) is peeled off and the vibration associated with the operation of the ball bearing increases. In addition, the test piece (ball bearing) reached the end of its life. In any of the test pieces, no indentation was observed on the surface of the peeled portion so as to cause peeling. Moreover, the white structure | tissue produced by hydrogen was not observed by the peeling part cross section. On the other hand, a butterfly structure change starting from inclusions was observed in the vicinity of the cross section of the peeled portion. Therefore, it is presumed that each test piece was peeled off from the inclusion as the starting point (inclusion starting point type peeling occurred). The results of the bearing life test conducted under such conditions are listed in the “Bearing test life ratio” column of Table 2 above. In addition, the numerical value described in the column of this bearing test life ratio represents L10 life of Comparative Example 6 using SUJ2 as 1, and each life is represented by a ratio with this Comparative Example 6.

  As apparent from Table 2, in Examples 1 to 9 belonging to the technical scope of the present invention, the composition, the size of oxide inclusions, the hardness, and the calculated values are all within the range defined by the present invention. Therefore, it has a long life against inclusion starting type peeling. In particular, in Examples 1 to 7 in which the calculated value is 2.8 or more, the effect of extending the life is stably obtained. Furthermore, in Examples 1 to 4, since the amount of Mn in the composition is in a more preferable range, the amount of retained austenite after quenching and tempering is in a more preferable range (11 to 16% by volume). Compared to Examples 5 to 7 in which the amount is in the range of 18 to 20% by volume, the shape stability and dimensional stability are also excellent.

On the other hand, in Comparative Example 5 and Comparative Example 6, since the calculated value is smaller than the range of the example of the present invention, the butterfly structure change is likely to occur and the life is short.
Further, in Comparative Example 7, the size of the oxide inclusions predicted by the extreme value statistics is larger than the range defined in the present invention, and the inner ring or the outer ring is directly below the raceway surface of the raceway where separation has occurred. It is estimated that there are large oxide inclusions. If the size of inclusions is large, fatigue cracks are generated directly from oxide inclusions without going through the process of changing the butterfly structure, so the effect of changing the composition cannot be obtained and the life is short. .
Furthermore, the hardness of Comparative Example 10 is lower than the range defined in the present invention. Therefore, a butterfly structure change is likely to occur due to stress concentration around oxide inclusions, and the life is short.

The test results of the bearing life test described above are summarized, and the relationship between the calculated value for each test piece (inner ring and outer ring constituting the ball bearing) and the L10 life ratio is shown in FIG. As can be seen from the description of FIG. 3, when the calculated value is 2.5 or more, the life is rapidly improved, and when the calculated value is 2.8 or more, a long life can be stably obtained.
FIG. 4A shows oxide-based nonmetallic inclusions observed in the vicinity of the peeled portion of Comparative Example 5. FIG. 4B shows oxide-based nonmetallic inclusions observed in the vicinity of the peeled portion in Example 2. In either case, a butterfly-type structural change that appears white occurs around the oxide inclusions. However, in Comparative Example 5 (calculated value = 2.4) and Example 2 (calculated value = 3.8), the size of the oxide-based non-metallic inclusions is approximately the same. As for the butterfly type | mold structure change part which generate | occur | produced around the system nonmetallic inclusion, Example 2 is smaller than the comparative example 5, and it is estimated that the structure | tissue change is delayed.

  From the results of the bearing life test described above, the composition of the steel material, the size and hardness of the oxide inclusions predicted by the extreme value statistical method, and the calculated value are within the range specified in the present invention. It can be seen that the formation of a butterfly structure change around the inclusions can be delayed to provide a long-life bearing against peeling of the inclusion origin.

  The bearing life test described above was performed when the present invention was applied to a deep groove type ball bearing. However, the present invention includes other ball bearings such as angular ball bearings and thrust ball bearings, cylindrical roller bearings, tapered rollers. The same effects can be obtained even when applied to roller bearings such as bearings, self-aligning roller bearings, needle bearings, and special rolling bearings such as ball screws and linear guides.

DESCRIPTION OF SYMBOLS 1 Radial ball bearing 2, 2a Outer ring raceway 3, 3a Outer ring 4, 4a Inner ring raceway 5, 5a Inner ring 6 Ball 7, 7a Cage 8 Radial tapered roller bearing 9 Tapered roller 10 Large diameter side collar 11 Small diameter side collar

Claims (3)

A first raceway having a first raceway surface on any surface, a second raceway having a second raceway surface on a surface opposite to the first raceway surface, and the first and second In a rolling bearing provided with a plurality of rolling elements provided between the raceway surfaces of the two rolling elements,
A bearing component which is at least one member of the first race ring, the second race ring, and each of the rolling elements,
C is 0.85 to 1.15% by mass, Si is 0.40 to 0.90% by mass, Mn is 0.55 to 1.20% by mass, Cr is 1.30 to 1.90% by mass, Mo is 0.30 mass% or less, Ni 0.30 mass% or less, Cu 0.20 mass% or less, S 0.025 mass% or less, P 0.020 mass% or less, O 15 mass ppm or less, Each containing, the remainder as Fe and inevitable impurities,
When the maximum value related to the size of oxide inclusions existing in an area of 30000 mm ² is predicted by the extreme value statistical method, the square root of the area of the maximum oxide inclusions is 22 μm or more and 50 μm or less. ,
The hardness after quenching and tempering is Hv697-800,
The amount of spheroidized carbide remaining in the steel after quenching and tempering is 5 to 9% by mass,
[Si], [Mn], [Cr], and [Mo] are numerical values representing the content of each alloy component in the steel in mass%, and [MC] is steel after quenching and tempering. When the ratio of the spheroidized carbide remaining in the numerical value expressed in mass%,
2.5 ≦ 2 [Si] + [Mn] + ([Cr] −7 [MC] / 100) / (1− [MC] / 100) +3 [Mo] ≦ 3.8
Rolling bearing characterized by satisfying   The rolling bearing according to claim 1, wherein the amount of retained austenite in the steel made of steel satisfying the conditions of claim 1 is 11% by volume to 20% by volume.   The first and second raceway surfaces are the surfaces of raceway grooves whose generatrix is partially arcuate, the rolling elements are balls, and the ratio of the radius of curvature of the raceway grooves to the diameter of each ball is 53. The rolling bearing according to claim 1, wherein the rolling bearing is at least% and at most 54%.