JP2014122378A - Rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve life prolongation of a rolling bearing by preventing generation of changes of a butterfly type structure around non-metallic inclusion while preventing cost increase.SOLUTION: An alloy steel constituting a rolling bearing contains C of 0.85 to 1.15 mass%, Si of 0.40 to 0.90 mass%, Mn of 0.55 to 1.20 mass%, Cr of 1.30 to 1.90 mass%, Mo of 0.30 mass% or less, Ni of 0.30 mass% or less, Cu of 0.20 mass% or less, S of 0.025 mass% or less, P of 0.020 mass% or less, O of 15 mass ppm or less and the balance Fe with inevitable impurities. A square root of area of an oxide-based inclusion existed in area of 30000 mmof 22 μm to 50 μm in terms of the rolling bearing. Also a maximum thickness of sulfide-based inclusion is 15 μm or less. A hardness after hardening and tempering is Hv 697 to 800.

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.

  In addition, a radial cylindrical roller bearing 9 using cylindrical rollers 8 and 8 as rolling elements is incorporated in the rotation support portion to which a large radial load is applied, for example, as shown in FIG. The radial cylindrical roller bearing 9 includes an outer ring 3a having a cylindrical concave outer ring raceway 2a on an inner peripheral surface, an inner ring 5a having a cylindrical 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 cylindrical rollers 8, 8 provided so as to be freely rollable while being held by the cage 7 a. Further, inward flange portions 10 and 10 are formed on the inner peripheral surfaces of both ends of the outer ring 3a, and outward flange portions 11 are formed on the outer peripheral surface of one end portion of the inner ring 5a.

  In addition, a radial tapered roller bearing 13 using tapered rollers 12 and 12 as rolling elements, for example, as shown in FIG. 3 is incorporated in a rotation support portion to which a thrust load is applied in addition to a large radial load. The radial tapered roller bearing 13 includes an outer ring 3b having a tapered outer ring raceway 2b on the inner peripheral surface, an inner ring 5b having a tapered outer ring raceway 4b on the outer peripheral surface, the outer ring raceway 2b and the inner ring raceway 4b. In between, there are provided a plurality of tapered rollers 12, 12 provided so as to be able to roll while being held by the cage 7 b. Further, out of both ends of the outer peripheral surface of the inner ring 5b, a large-diameter side flange 14 is formed at the large-diameter end, and a small-diameter flange 15 is formed at the small-diameter end. The small diameter side flange 15 may be omitted.

  Furthermore, under the condition that it is operated under a large radial load and it is difficult to keep the center axis of the outer ring and the center axis of the inner ring in exact agreement, a self-aligning roller as shown in FIG. A bearing 16 is used. In this self-aligning roller bearing 16, spherical rollers 17 and 17 are used as rolling elements, the bus bar shape of which is a partial arc shape, and the whole is a beer barrel type. Further, the outer ring raceway 2c provided on the inner peripheral surface of the outer ring 3c has a partially spherical shape with a point on the central axis of the outer ring 3c as the center of curvature. In addition, double-row inner ring raceways 4c and 4c are provided on the outer peripheral surface of the inner ring 5c. The bus bar shapes of the inner ring raceways 4c and 4c are symmetric with respect to the bus bar shape of the outer ring raceway 2c with respect to the central axis of the spherical rollers 17 and 17, respectively. Further, each of the spherical rollers 17 and 17 is rotatably provided between the outer ring raceway 2c and the inner ring raceways 4c and 4c while being held by the cages 7c and 7c, respectively.

  The various rolling bearings as shown in FIGS. 1 to 4 are often used for a long period of time under a large load. With such use, the outer rings 3, 3a, 3b, 3c, the inner rings 5, 5a, 5b, 5c, rolling elements (balls 6, 6, cylindrical rollers 8, 8, tapered rollers 9, 9, spherical rollers In some cases, the steel constituting the bearing parts such as 17 and 17) is subjected to metal fatigue, and the surface of the bearing parts is peeled off. Such peeling is more likely to generate a larger stress than the rolling surface of the rolling element, such as the outer rings 3, 3a, 3b, 3c and the inner rings 5, 5a, 5b, 5c. It tends to occur on the raceway surface (especially the inner raceway 4, 4a, 4b, 4c) of the raceway. 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 a white structure, in which hydrogen enters the steel and causes hydrogen embrittlement. 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 are non-metallic inclusions such as oxide inclusions in the steel constituting the bearing parts that are much harder than the original hardness of the steel, stress concentration will occur in the peripheral parts of these inclusions, A butterfly type tissue change occurs in the part. When such a butterfly 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. Conventionally, in order to suppress the occurrence of such inclusion-initiated type peeling, it has been proposed to reduce or reduce the number of non-metallic inclusions such as oxide inclusions that are the starting point of peeling. .

For example, in Patent Document 1, the content of C, Si, Mn, Cr, P, S, O, Mo, and Sb in the alloy steel is regulated, and the maximum diameter of the oxide-based nonmetallic inclusion is 10 μm or less. Further, it is described that the rolling fatigue life of bearing parts is improved by suppressing the number of sulfide-based nonmetallic inclusions having a thickness of 1 μm or more to 1200 or less per 320 mm ² .
In Patent Document 2, the content of C, Cr, O, Sb, Si, Mn, Mo, Ni, Nb, V, Cu, and Al in the alloy steel is regulated, and oxide-based nonmetallic inclusions are also disclosed. It is described that the rolling fatigue life of bearing parts is improved by restricting the number of bearings to 100 to 200 per 320 mm ² .
Patent Document 3 also improves the rolling fatigue life of bearing parts by regulating the maximum diameter of oxide-based non-metallic inclusions contained in 30000 mm ² estimated to be 5 μm or less, which is estimated by the extreme value statistical method. It is described that
Further, in Patent Document 4, the surface hardness is set to HRC58 or more, and the predicted maximum diameter of sulfide-based nonmetallic inclusions contained in 30000 mm ² estimated by the extreme value statistical method is suppressed to 40 μm or less, Alternatively, it is described that the rolling fatigue life of bearing parts is improved by suppressing the predicted maximum diameter of each oxide, sulfide, and nitride inclusion to 60 μm or less under the same conditions.

However, depending on the prior arts described in the above-mentioned Patent Documents 1 to 4, the durability of the rolling bearing used under severe conditions cannot always be improved sufficiently, or the cost increases.
For example, even if the number and size of non-metallic inclusions contained in a minute area in the alloy steel are limited to the extent described in Patent Documents 1 and 2, the rolling bearing is actually used under a high load. When used under such severe conditions, peeling may occur starting from the largest non-metallic inclusion present in the high stress portion of the bearing component. In such a case, the rolling fatigue life of the rolling bearing may be unexpectedly shortened due to separation starting from non-metallic inclusions.
In addition, as in the prior art described in Patent Document 3, it is costly to produce an alloy steel with extremely high cleanliness so as to restrict the maximum diameter of oxide-based nonmetallic inclusions to 5 μm or less. difficult. That is, in order to produce such a steel alloy with a high degree of cleanness, it is indispensable to reduce the amount of O and S in the alloy steel. In order to further reduce the amount of O and S, further improvements in equipment and processes are required. As a result, the price of steel materials rises, making it difficult to use widely industrially.
Furthermore, as in the prior art described in Patent Document 4, when the presence of non-metallic inclusions of a certain size is allowed, when the use conditions become severe, the non-metallic inclusions are used as starting points. A butterfly structure change occurs, leading to separation, and the durability of the rolling bearing cannot be improved sufficiently.

JP 09-291340 A JP 2000-144326 A JP 2003-232367 A JP 2006-066342 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 according to the present invention includes the radial ball bearing 1, the radial cylindrical roller bearing 9, the radial tapered roller bearing 13, and the self-aligning roller bearing 16 shown in FIGS. As with the bearing, a first race ring having a first raceway surface on any surface, a second race ring having a second raceway surface on a surface opposite to the first raceway surface, and these A plurality of rolling elements provided between the first and second raceway surfaces so as to be freely rollable.
According to the present invention, among the rolling bearings, the first and second raceway surfaces are surfaces of raceway grooves having a partial arc shape on the generatrix, the rolling elements are balls, and both raceways It can be preferably implemented for a ball bearing in which the maximum diameter (in the case of a radial ball bearing, the outer diameter of the outer ring) of each part is 180 mm or less and the diameter of each ball is 30.2 mm or less. As will be apparent from the following description, in the case of a small-diameter rolling bearing, it can be carried out without any particular limitation on the lower limit value.

  For example, in the present invention that is intended for a rolling bearing such as a ball bearing having the above-described size, at least one of the first race ring, the second race ring, and the rolling elements. A bearing component (most preferably an inner ring of a radial ball bearing) which is a seed member, C is 0.85 to 1.15 mass%, Si is 0.40 to 0.90 mass%, and Mn is 0.55 to 1. 20 mass%, Cr 1.30 to 1.90 mass%, Mo 0.30 mass% or less, Ni 0.30 mass% or less, Cu 0.20 mass% or less, and S 0.025 mass% % Or less, P is 0.020 mass% or less, O is 15 mass ppm or less, and the balance is made of steel with Fe and inevitable impurities.

Further, regarding the bearing part made of this steel, 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, When the maximum thickness of sulfide inclusions having an area square root of 22 μm or more and 50 μm or less and also having an area of 30000 mm ² is predicted, the maximum thickness is 15 μm or less, and quenching / tempering is performed. The latter hardness is Hv697-800. Since the bearing part is subjected to so-called quenching, the hardness of the bearing part is substantially the same from the surface to the core part.

  According to the present invention configured as described above, the oxide inclusions and sulfide inclusions existing in the steel constituting the bearing part can be reduced (reduced) without causing excessive cost increase. However, a butterfly structure change hardly occurs around these inclusions, 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, the “inclusion-origin-type peeling” that suppresses the occurrence according to the present invention is generated around the oxide inclusions and sulfide inclusions existing in the steel constituting the bearing part. This is a phenomenon in which a butterfly structure change occurs in the peripheral portion based on the stress concentration, and 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 component, stress concentrates around the oxide inclusions and sulfide inclusions present in the steel constituting the bearing component. 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. It is.

  However, the addition of Si, Mn, Cr, and Mo does not completely prevent the butterfly structure change and only delays it. Therefore, although the butterfly structure change can be delayed by addition of Si, Mn, Cr, and Mo, if the oxide inclusions and sulfide inclusions are too large, these oxide inclusions and The occurrence of a butterfly structure change starting from sulfide inclusions 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 oxide inclusions and sulfide inclusions in steel as in the inventions described in Patent Documents 1 to 4 described above. It is not necessary to reduce or reduce the amount of steel as the steelmaking cost increases. In the case of the present invention, since the square root of the area of the maximum oxide inclusions is regulated to 22 μm or more and 50 μm or less, and the maximum thickness of the sulfide inclusions is regulated to 15 μm or less, respectively, the steelmaking cost is reduced. While suppressing, it is possible to sufficiently suppress the occurrence of butterfly structure changes starting from oxide inclusions and sulfide inclusions.

In short, by adding Si, Mn, Cr, and Mo to the steel, even if oxide inclusions and sulfide inclusions of a certain size exist in the steel, they intervene in bearing parts. It is possible to improve durability of a rolling bearing in which the bearing part is incorporated by making it difficult to cause product-origin separation.
Next, the elements added to the steel constituting the bearing component of the rolling bearing subject to the present invention and its content, the size of oxide inclusions, the maximum thickness of sulfide inclusions, The reason why the hardness, the size of a rolling bearing that is preferable as an object of implementation of the present invention, the preferable amount of retained austenite, and the like 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 delay of the butterfly structure change 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 retained austenite after heat treatment. Residual austenite is a relatively soft structure, and suppresses the above-described surface-origin type peeling, 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 improve the surface-origin type peeling life, the Mn content is preferably 0.80 to 1.20% by mass.

[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 is dissolved in the base and has the effects of improving the hardenability and temper softening resistance and 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 martensite in the base structure, delays butterfly-type structure changes that occur around oxide inclusions and sulfide inclusions, and causes inclusion origin-type separation in bearing parts. This contributes to extending the life of rolling bearings incorporating this bearing component. 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 an expensive element, it causes an increase in the 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 an 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 nature, and trying to keep the S content extremely low reduces the productivity of the alloy material (steel material) for making bearing parts, and the manufacturing cost of the steel material. Increases, making it difficult to use widely in industry. 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 inclusions such as Al ₂ O ₃ in the steel. Oxide inclusions are hard and serve as starting points for peeling and have a great adverse effect on rolling fatigue life, so 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 effective as a method for predicting the maximum diameter of inclusions contained in steel. In addition, in the inclusion-origin separation due to oxide inclusions existing in the steel of the bearing parts that make up a rolling bearing, the maximum inclusion diameter predicted by the extreme statistical method and the rolling fatigue life are between. There is a good correlation. In particular, it is known that oxide inclusions are hard and thus 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. If the value regarding this square root exceeds 50 μm, fatigue cracks are generated directly from oxide inclusions before the butterfly structure change occurs when rolling fatigue is applied. 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 many portions containing large inclusions in the steel material. It decreases excessively, and the cost of the steel material increases, making it difficult to use it widely industrially. 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, determine the maximum oxide inclusions contained in the observation area 100 mm ² of steel cross-section, it was carried out a cross section of 30 points of the steel material, the largest oxide inclusions present in the 30,000 mm ² by statistical processing It is preferable to determine the size.

[Maximum thickness of sulfide inclusions is 15 μm or less]
Sulfide inclusions such as MnS have a low hardness and are elongated in the rolling direction of the alloy steel material. For this reason, it is considered that the adverse effect (harmfulness) on the rolling fatigue life of bearing parts is smaller than that of oxide inclusions. However, when the inclusion has a thick sulfide diameter with a thickness exceeding 15 μm, the degree of harmfulness increases, and the influence on the rolling fatigue life shortening (deterioration) of the bearing part cannot be ignored. Therefore, in the case of the present invention, when the maximum thickness of the sulfide inclusion existing at 30000 mm ² is predicted by statistical processing, the maximum thickness of the sulfide inclusion is suppressed to 15 μm or less. . The thickness of the sulfide inclusion is determined by measuring the length of the short side as the thickness by the same method as the above-described extreme value statistics of the oxide inclusion.

[Hardness after quenching and tempering is Hv697-800]
The inclusion-origin type separation to be suppressed by the present invention occurs as a crack progresses due to metal fatigue along the interface of the butterfly structure change portion as described above. In addition, as described above, this butterfly structure change is caused by repeatedly applying a large shear stress caused by stress concentration around oxide inclusions and sulfide inclusions to the base martensite structure. This is a phenomenon in which dislocations and solute carbon in the martensite structure are moved to change to an ultrafine ferrite structure. Improving the hardness of the matrix structure has the effect of making it difficult for dislocations and solute carbons to move in the martensite structure even when shear stress is applied to the matrix structure, and delaying the occurrence of butterfly structure changes. There is. 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 800. As described above, the hardness is substantially the same from the surface to the core.

[Maximum diameter of raceway is 180mm or less, diameter of each ball is 30.2mm or less]
Rolling bearings of ball bearings and other rolling bearings are subjected to internal shear stress due to the load from each rolling element such as balls, and the volume of the region where the shear stress increases is the same as the outer diameter of the rolling bearing. Strong correlation with moving body diameter. In the case of a ball bearing such as a radial ball bearing, if the maximum diameter of the race (in the case of a radial ball bearing, the outer diameter of the outer ring) is 180 mm or less and the diameter of each ball is 30.2 mm or less, the race The volume of the portion of the ring where the shear stress increases is reduced, and the possibility of including large inclusions more than predicted by the extreme value statistical method can be suppressed (the invention described in claim 2). In other words, in the case of a large-sized rolling bearing where the maximum diameter of the raceway exceeds 180 mm or the diameter of each rolling element exceeds 30.2 mm, the oxide is predicted by the extreme value statistical method. There is a possibility that the inclusions of the system and sulfide system become large, and there is a possibility that the durability of the rolling bearing cannot be ensured sufficiently. In the case of a small-diameter rolling bearing, since the predicted value of the extreme value statistical method is smaller, it can be carried out without any particular limitation on the lower limit value.

[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 10% 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 10% by volume, there is a higher possibility that surface-initiated separation will occur earlier than inclusion-initiated separation, and this will not lead to an increase in the life of rolling bearings. . 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 10% by volume or more and 20% by volume or less (described in claim 3). invention). More preferably, in order to obtain good shape stability and dimensional stability and a surface-origin type peel life, the amount of retained austenite in the steel is set to 10% by volume or more and 15% by volume or less. 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 10% 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 10% 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 10% by volume.
Note that depending on the combination of the alloy components containing elements that tend to harden, the quenching temperature, and the tempering temperature, even if these are all within the range specified by the present invention, the hardness value increases (this This is out of the scope of the invention) and may deteriorate productivity. For this reason, in the case of the present invention, the hardness after heat treatment is also defined.

[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 to generate a butterfly structure change 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 determined. Even if it is increased, the lifetime is unlikely 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%. Even when the ratio is 54% or less, the same life as a bearing made of general steel and having a ratio of the radius of curvature of the raceway groove to the ball diameter 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. 4).

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 cutaway perspective view of a radial cylindrical roller bearing. The partial cut perspective view of a radial tapered roller bearing. The partial cutaway perspective view of a self-aligning roller bearing. The microscope picture which shows two examples of the nonmetallic inclusion observed in the vicinity part of a track surface.

  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 the peeling which generate | occur | produces from an oxide type inclusion and a sulfide type inclusion. Regarding the structure appearing in the drawings, various types of conventionally known rolling elements including the radial ball bearing 1, the radial cylindrical roller bearing 9, the radial tapered roller bearing 13, and the self-aligning roller bearing 16 shown in FIGS. Since it is the same as that of a bearing, the overlapping description is abbreviate | omitted.

  An experiment conducted in the process of forming the present invention will be described. The experiment was conducted on 15 kinds of steel materials A to O shown in Table 1 below, and a turning evaluation test for knowing the ease of processing, and whether a desired hardness and a retained austenite amount can be obtained. A heat treatment test to determine whether or not the roughness of the raceway surface can be finished as desired and a bearing test to determine the durability of the actual rolling bearing. Four types of tests were conducted.

In Table 1, the unit of each numerical value is mass%. However, only O is mass ppm. A numerical value enclosed in parentheses indicates that the numerical value deviates from the technical scope of the present invention. Steel grade M is SUJ2.

[Turability evaluation test]
Using a steel material having the composition shown in Table 1, a 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 the 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.
In Table 2, the unit of each numerical value is as described above for each. The unit of turning wear amount and groove roughness is μm. The numerical value enclosed in parentheses indicates that the numerical value deviates from the technical range or the preferable range of the present invention, or is not preferable in terms of performance.

As apparent from Table 2, since Examples 1 to 8 belonging to the technical scope of the present invention have a suitable composition of steel materials, the turning property after spheroidizing annealing is JIS-SUJ2 (Comparative Example 5). ) 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, rR) 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 apparent from Table 2, Examples 1 to 8 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, it cannot be said that sufficient shape stability and dimensional stability can be obtained in consideration of long-term use as a rolling bearing.
In Comparative Example 8, the same steel material D as in Example 4 is used, but due to the difference in quenching temperature, the hardness is higher than the range specified in the present invention. It is disadvantageous from the aspect of securing.
Moreover, although the comparative example 9 uses the same steel material F as Example 6, since hardness is lower than the range prescribed | regulated by this invention by the difference in quenching temperature, it is disadvantageous from the surface of ensuring rolling fatigue life. Become.
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 oxide inclusion diameter (unit: μm) in Table 2. Similarly, for sulfide inclusions, the maximum thickness prediction of sulfide inclusions contained in 30000 mm ² is the same as the maximum sulfide inclusion inclusion thickness in Table 2 (unit: μm). It described in the column of.

[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 (unit: μm) regarding the surface roughness thus obtained are listed in the “groove roughness” column of Table 2 above.

As is clear from the description of Table 2 above, Examples 1 to 8 belonging to the technical scope of the present invention have a composition and a hardness after quenching / tempering that are in a suitable range. The roughness is the same value as that of Comparative Example 5 having the same component as SUJ2, and is excellent in grindability.
On the other hand, since the hardness after hardening and tempering is higher than the range prescribed | regulated by this invention in the comparative example 8, the surface roughness of a raceway groove | channel is large (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. Similarly, a SUJ2 ball (diameter 30.162 mm) is incorporated between an inner ring and an outer ring of a ball bearing (inner diameter 85 mm, outer diameter 180 mm, width 41 mm) having a nominal number 6317, 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.
“Regarding the reference number 6206”
Radial load: 13818N
Rotational speed: 3900 min ^-1
Rotating condition: Inner ring rotation Lubricating oil: Mineral oil equivalent to ISO-VG68 (forced circulation method)
“Regarding reference number 6317”
Radial load: 66150N
Rotational speed: 2000 min ^-1
Rotating condition: Inner ring rotation Lubricating oil: Mineral oil equivalent to ISO-VG68 (forced circulation method)
In addition, with respect to Comparative Examples 1, 3, and 8 that were determined to be defective in the above-described tests and turning evaluation tests, and Comparative Example 2 that was determined to have a large amount of retained austenite and poor dimensional stability, bearings The 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. For any test piece, even when the surface of the peeled portion was observed after the end of the life test, no indentation that would cause peeling was found on the surface of the peeled portion. 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 near 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 5 using SUJ2 as 1, and each life is expressed as a ratio with Comparative Example 5. The following can be understood from the description in Table 2 showing the results of the experiment conducted in this manner.

  For Examples 1 to 8 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. Longer life against inclusion-origin separation. In particular, in Examples 1 to 3, the alloy composition is in a more preferable range, and further, the amount of retained austenite after quenching and tempering is in a more preferable range (10 to 15% by volume), so the life is long. . Since the ball bearing with the bearing number 6206 is smaller than the ball bearing 6317, the volume of the portion of the bearing ring where the shear stress increases is reduced, and the oxide predicted by the extreme statistical method is used. Since the size of each of the system and sulfide system inclusions is small, the durability improvement effect by implementing the present invention is increased.

On the other hand, in Comparative Examples 4 and 5, the composition of the alloy steel was outside the range specified in the present invention, so that the butterfly structure change was likely to occur, and the rolling fatigue life was shortened.
In Comparative Examples 6 and 7, the size of the oxide inclusions or sulfide inclusions predicted by the extreme value statistical method is larger than the range defined in the present invention, and a large oxide type is directly below the raceway surface. It is presumed that inclusions or sulfide inclusions are present, and it is considered that the lifetime is shortened based on the presence of these oxide inclusions or sulfide inclusions.
Further, in Comparative Example 9, the hardness is lower than the range defined in the present invention, and therefore, it is considered that the butterfly structure change is likely to occur due to the stress concentration around the inclusions and the life is shortened.

  In FIG. 5, the microscope picture of the cut surface of the sample after a lifetime test is shown. FIG. 5A shows the oxide inclusions observed in the vicinity of the peeled portion of Comparative Example 5. FIG. A butterfly structure change that appears white occurs around the oxide inclusions. FIG. 5B shows oxide inclusions observed in the vicinity of the peeled portion of Example 2. As far as the size of the oxide inclusions is concerned, there is no significant difference from the case of Comparative Example 5, but the butterfly structure change is small. From this, it is considered that due to the difference in the composition of the alloy steel, the structural change was delayed, and the life regarding the inclusion-origin separation was extended.

Next, an experiment conducted to know the influence of the size of the rolling bearing will be described. In this experiment, a SUJ2 ball (diameter: 36.512 mm) was assembled between the inner ring and outer ring of a ball bearing (inner diameter: 100 mm, outer diameter: 215 mm, width: 47 mm) having a nominal number of 6320, and a steel press was held. A test bearing which is Comparative Examples 10 to 12 was held by a vessel (wave type cage). And about these comparative examples 10-12, a bearing life test is implemented similarly to the case of each Example and each comparative example shown in the said Table 2, and the lifetime (L10 life) in which a cumulative failure probability becomes 10%. Asked. The test conditions are as follows.
Radial load: 86730N
Rotational speed: 2000 min ^-1
Rotating condition: Inner ring rotation Lubricating oil: Mineral oil equivalent to ISO-VG68 (forced circulation method)

Table 3 below shows the processing conditions of the outer ring and the inner ring constituting the test bearings of Comparative Examples 10 to 12 and the results of the bearing life test.

As is apparent from Table 3, when the bearing size is increased as in Comparative Examples 10 to 12, even if the bearing ring made of the alloy steel having the same composition as the present invention is subjected to the same heat treatment as in the present invention, The bearing life cannot be extended. On the other hand, for ball bearings with an identification number smaller than 6206, the possibility of including large inclusions predicted by the extreme value statistical method becomes smaller, so it is clear that the effect of carrying out the present invention becomes larger.
From the experimental results described above, the composition of the alloy steel constituting the bearing parts, the size of the oxide inclusions predicted by the extreme value statistical method, the thickness of the sulfide inclusions, the hardness, the rolling bearing It can be seen that by setting the size of the material within the appropriate range, the butterfly structure change formed around the inclusions can be delayed to extend the inclusion-origin type peeling life.

  The bearing life test described above was performed when the present invention was applied to a deep groove type radial ball bearing. However, the present invention is not limited to other ball bearings such as angular ball bearings and thrust ball bearings, cylindrical roller bearings, conical bearings. The same effect can be obtained when applied to roller bearings such as roller bearings, self-aligning roller bearings, needle bearings, and special rolling bearings such as ball screws and linear guides. However, the size at which the effect can be obtained by carrying out the present invention differs depending on the type of rolling bearing.

DESCRIPTION OF SYMBOLS 1 Radial ball bearing 2, 2a, 2b, 2c Outer ring raceway 3, 3a, 3b, 3c Outer ring 4, 4a, 4b, 4c Inner ring raceway 5, 5a, 5b, 5c Inner ring 6 Ball 7, 7a, 7b, 7c Cage 8 Cylindrical roller 9 Radial cylindrical roller bearing 10 Inward flange 11 Outward flange 12 Tapered roller 13 Radial tapered roller bearing 14 Large diameter flange 15 Small diameter flange 16 Spherical roller bearing 17 Spherical roller

Claims (4)

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, made of steel with 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 for the bearing part made of this steel, the area of the maximum oxide inclusions is When the maximum thickness of sulfide inclusions having a square root of 22 μm or more and 50 μm or less and also having an area of 30000 mm ² is predicted, this maximum thickness is 15 μm or less, and the hardened material after quenching / tempering is hardened. Is a rolling bearing characterized by being Hv697-800.   The first and second raceway surfaces are surfaces of raceway grooves having a partial arc shape in the generatrix shape, the rolling elements are balls, and the maximum diameter of the diameters of each part of the raceways is 180 mm or less. The rolling bearing according to claim 1, wherein the diameter of each ball is 30.2 mm or less.   The rolling bearing according to any one of claims 1 to 2, wherein the amount of retained austenite in the steel of the bearing part made of steel satisfying the condition of claim 1 is 10% by volume to 20% by volume. .   The rolling bearing according to claim 2, wherein the ratio of the radius of curvature of the raceway groove to the diameter of each ball is 53% or more and 54% or less.