JP2013221180A - Rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolling bearing with an excellent corrosion resistance.SOLUTION: An inner ring and an outer ring are manufactured by performing a gas nitriding treatment and quenching/annealing after forming an alloy steel with a carbon content of 0.10-0.35 mass% into a predetermined shape to make a nitrogen content [N] on a surface layer portion 0.05-0.50 mass%, a total content of nitrogen and carbon [C+N] 0.40 mass% or higher, a surface hardness HRC58 or higher, and a pitting resistance equivalent (PRE) represented by Formula (1) 20.0 or higher. PRE=[Cr]+3.3[Mo]+16[N]...(1).

Description

  The present invention relates to a rolling bearing.

The rolling bearing has an inner ring, an outer ring, and a rolling element. The rolling elements are arranged so as to roll freely between the raceways of the inner ring and the outer ring.
Medical devices are used as rolling bearings. Medical devices are immersed in a disinfecting solution for the purpose of sterilization and sterilization, and are dried after sterilization. Therefore, they are used in an environment in which wetting and drying are repeated. In addition, a strong alkaline solution may be used as a disinfectant in order to enhance the sterilization / sterilization effect of blood or protein. As described above, the use environment of the medical device is a very severe corrosive environment as compared with the use environment in which the medical device is simply washed with water. When rust is generated in a medical device, it becomes a factor of performance deterioration. Therefore, a rolling bearing for a medical device that requires high reliability is required to have corrosion resistance.

  Conventionally, martensitic stainless steel represented by SUS440C has been used as a steel material for rolling support devices (such as linear guide devices, ball screws, and rolling bearings) that require corrosion resistance and wear resistance. Ferritic and austenitic stainless steel sheets are superior in corrosion resistance to martensitic stainless steels, but are insufficient in strength as steel materials for rolling support devices.

Patent Document 1 discloses a specific steel material (weight%, C content is 0.26 to 0.40%, Si content is 1% or less, Mn content is 1% or less, and P content is 0.04. %, S content is 0.03% or less, Cr content is 12-14%, N content is 0.02% or less, B content is 0.0005-0.002%, the balance is Fe and inevitable Steel material) is heated in a nitrogen atmosphere to make the surface layer nitrogen concentration 0.25 to 0.3%, and then water-quenched, so that it is more corrosion resistant than conventional martensitic stainless steel It is described that a steel having excellent strength and equivalent strength is obtained.
The steel material is based on the composition of SUS420J2 and B is added in an amount of 0.0005 to 0.002%.

  In addition, the steel obtained is superior in corrosion resistance and cold workability than the conventional martensitic stainless steel, and also has sufficient hardness so that it has excellent wear resistance. It is described that if it is used as a component material such as a bolt, a mold, a strain gauge strain body, a blade, etc., a low-cost, highly reliable and long-life component can be obtained. However, in Patent Document 1, there is no description regarding performance when actually applied to an inner ring and an outer ring of a rolling bearing.

  Patent Document 2 describes martensitic stainless steel that satisfies the following configurations (a) to (c). The object of the invention described in Patent Document 2 is to have inherent corrosion resistance in an atmospheric medium (ocean or urban environment) where the external hydrogen source is cut off, and at the same time high tensile resistance (on the order of 1800 MPa and higher) And to provide a martensitic stainless steel exhibiting toughness equal to that of a slightly alloyed carbon steel having very high strength.

(a) When the composition is% by weight, 9% ≦ Cr ≦ 13%, 1.5% ≦ Mo ≦ 3%, 8% ≦ Ni ≦ 14%, 1% ≦ Al ≦ 2%. 0.5% ≦ Ti ≦ 1.5% under the condition of Al + Ti ≧ 2.25%. Measurement limit ≤ Co ≤ 2%. Measurement limit value ≦ W ≦ 1% under the condition of Mo + (W / 2) ≦ 3%. Measurement limit value ≦ P ≦ 0.02%, Measurement limit value ≦ S ≦ 0.0050%, Measurement limit value ≦ N ≦ 0.0060%, Measurement limit value ≦ C ≦ 0.025%, Measurement limit value ≦ Cu ≦ 0.5%, measurement limit value ≦ Mn ≦ 3%, measurement limit value ≦ Si ≦ 0.25%, measurement limit value ≦ O ≦ 0.0050%.

(b) Ms (° C.) = 1302-42Cr-63Ni-30Mo + 20Al-15W-33Mn-28Si-30Cu-13Co + 10Ti ≧ 50.
(c) Cr equivalent / Ni equivalent ≦ 1.05 under the condition of Cr equivalent (%) = Cr + 2Si + Mo + 1.5Ti + 5.5Al + 0.6W, Ni equivalent (%) = 2Ni + 0.5Mn + 30C + 25N + Co + 0.3Cu.
As an index indicating the corrosion resistance of steel, a pitting resistance index (PRE) represented by the following equation (1) is known. A higher pitting corrosion index indicates higher corrosion resistance.
PRE = [Cr] +3.3 [Mo] +16 [N] (1)
In the formula, [Cr] is the chromium content (mass%) of the steel, [Mo] is the molybdenum content (mass%) of the steel, and [N] is the nitrogen content (mass%) of the steel.
The pitting corrosion index of the steel of Patent Document 1 is 16.0 to 18.8. The pitting corrosion index of the steel of Patent Document 2 is 13.5 to 22.996. Patent document 2 states that "chromium and molybdenum contents should preferably be able to obtain a pitting index of at least 16.5".

JP 2010-138425 A Special table 2008-546912 gazette

  The subject of this invention is providing the rolling bearing excellent in corrosion resistance.

In order to solve the above problems, a rolling bearing of the present invention is a rolling bearing defined by the following configuration (1), wherein the inner ring and the outer ring have the following configurations (2) to (6). And
(1) It has an inner ring, an outer ring, and a rolling element disposed so as to be freely rollable between the raceways of the inner ring and the outer ring.
(2) The carbon content [C] is 0.10% by mass to 0.35% by mass, the chromium content [Cr] is 12.5% by mass to 18.0% by mass, and the molybdenum content [Mo] is 0.80 mass% to 5.0 mass%, manganese content [Mn] is 0.1 mass% to 1.5 mass%, and silicon content [Si] is 0.1 mass% to 2.0 mass% %, Nickel content [Ni] is 0.05 mass% or more and 4.0 mass% or less, copper content [Cu] is 0.2 mass% or less, and sulfur content [S] is 0.01 mass% or less. Then, after processing a material made of alloy steel having a phosphorus content [P] of 0.02 mass% or less, an oxygen content [O] of 10 ppm or less, and the balance being iron (Fe) and inevitable impurities, into a predetermined shape, It is obtained by performing gas nitriding treatment and quenching and tempering.

(3) The nitrogen content [N] in the surface layer (in the range of 200 μm depth from the surface) is 0.10% by mass or more and 0.50% by mass or less.
(4) The total content [C + N] of nitrogen and carbon in the surface layer portion (in the range of 200 μm depth from the surface) of the raceway surface is 0.40 mass% or more and 0.85 mass% or less.
(5) The surface hardness of the raceway surface is HRC58 or more and 62 or less.
(6) The pitting corrosion index (PRE) represented by the following formula (1) is 19.0 or more and 42.5 or less in the surface layer portion.
PRE = [Cr] +3.3 [Mo] +16 [N] (1)
In the rolling bearing of this invention, the carbon content of the alloy steel to be used is reduced, and the corrosion resistance of the inner ring and the outer ring is improved by making the PRE of the surface layer portion 20.0 or more.

Further, the hardness of the raceway surface, which is reduced due to the low carbon content of the alloy steel used, is compensated by nitrogen introduced into the surface layer portion by gas nitriding treatment. And in order to obtain “surface hardness HRC58 or more” necessary for the raceway surfaces of the inner and outer rings of the rolling bearing, the total content [C + N] of nitrogen and carbon in the surface layer portion of the raceway surface is 0.40% by mass or more. I have to.
In order to obtain a “pitting corrosion index of 20.0 or more” in the surface layer portion, the nitrogen content [N] in the surface layer portion needs to be 0.05 mass% or more. If the nitrogen content of the surface layer exceeds 0.50% by mass, the amount of retained austenite increases excessively during quenching, and sufficient hardness cannot be obtained.
In addition, when the nitrogen nitriding treatment temperature is low and nitrogen is hardly absorbed, or when the nitrogen gas pressure is too high and excessive nitrogen is absorbed, nitrides such as CrN and Cr ₂ N are precipitated. Since the corrosion resistance is lowered, the gas nitriding treatment temperature is preferably higher than 1100 ° C. (1373 K), and the nitrogen gas pressure is preferably 0.05 to 0.1 MPa.

[Composition of alloy steel used in configuration (2)]
The reason why [C] is 0.10 mass% or more and 0.35 mass% or less is as follows.
Carbon (C) is an element that strengthens steel by solid solution in matrix (matrix) by quenching and martensifying the structure. Moreover, it has the effect | action which combines with another alloy element (Cr and Mo), forms a hard carbide | carbonized_material in steel, and improves abrasion resistance.
However, due to the formation of carbides, the abundance of Cr and Mo in the vicinity of carbides that contribute to corrosion resistance is reduced, thereby reducing corrosion resistance. Moreover, when there is much carbon content, it will become easy to form a coarse eutectic carbide in steel, and toughness and workability (grindability) will fall. Therefore, in order to obtain good corrosion resistance and workability, the carbon content is set to 0.35 mass% or less.
Moreover, in order to obtain hardness required for the inner ring and outer ring of the rolling bearing, the carbon content is set to 0.10% by mass or more.

The reason why [Cr] is 12.5 mass% or more and 18.0 mass% or less is as follows.
Chromium (Cr) is an element that contributes to corrosion resistance. Moreover, it has the effect | action which improves hardenability by making it dissolve in a base. Moreover, it has the effect | action which combines with carbon and forms a hard carbide | carbonized_material in steel, and improves abrasion resistance. In order to obtain the corrosion resistance required for the inner and outer rings of the rolling bearing for medical equipment, the chromium content needs to be 12.5% by mass or more.
However, if the chromium content increases, the workability (turnability) decreases, so the content is made 18.0% by mass or less.

The reason why [Mo] is 0.80 mass% or more and 5.0 mass% or less is as follows.
Molybdenum (Mo) is an element that contributes to corrosion resistance. Moreover, it has the effect | action which improves the hardenability and temper softening resistance by making it dissolve in a base. Moreover, it has the effect | action which combines with carbon and forms a hard carbide | carbonized_material in steel, and improves abrasion resistance.
In order to obtain the corrosion resistance required for the inner ring and outer ring of a rolling bearing for medical equipment, the molybdenum content needs to be 0.80% by mass or more.
However, if the molybdenum content exceeds 5.0% by mass, the cold workability and the machinability become insufficient, and the productivity decreases.

The reason why [Mn] is 0.1 mass% or more and 1.5 mass% or less is as follows.
Manganese (Mn) has the effect of improving the hardenability by dissolving in the matrix. It also acts as a deoxidizing material. If it is less than 0.1% by mass, these effects cannot be substantially obtained.
However, if the manganese content increases, the forgeability and machinability may decrease, and the durability may decrease due to coexistence with sulfur (S) and phosphorus (P). 1.5% by mass or less. Moreover, since the amount of retained austenite increases and sufficient hardness may not be obtained when the manganese content increases, the manganese content is preferably set to 0.9 mass% or less.

The reason why [Si] is 0.1 mass% or more and 2.0 mass% or less is as follows.
Silicon (Si) acts as a deoxidizer during refining and has the effect of improving the hardenability by dissolving in a matrix. It also has the effect of increasing the temper softening resistance. When the silicon content is less than 0.2% by mass, these effects are not substantially obtained.
However, when the silicon content exceeds 2.0% by mass, the toughness, cold workability and machinability become insufficient. Preferably, the silicon content is 0.5% by mass or less.

The reason why [Ni] is 0.05 mass% or more and 4.0 mass% or less is as follows.
Nickel (Ni) is an element that contributes to corrosion resistance. Moreover, it has the effect | action which dissolves in a base and improves hardenability and toughness. When the nickel content is less than 0.05% by mass, these effects are not substantially obtained.
However, if the nickel content exceeds 4.0% by mass, cold workability and machinability become insufficient. Preferably, the nickel content is 0.9 mass% or less.

The reason for making [Cu] 0.2 mass% or less is as follows.
Copper (Cu) has a function of improving the hardenability and grain boundary strength by dissolving in a matrix. Copper is not an essential component, but if the content is less than 0.02% by mass, these effects cannot be substantially obtained. Therefore, when copper is contained, the content is 0.02% by mass or more. And
However, if the copper content exceeds 0.20% by mass, the hot forgeability becomes insufficient and the productivity decreases.

The reason why [S] is 0.01% by mass or less is as follows.
Sulfur (S) combines with manganese (Mn) to form MnS and become inclusions, so the content is made 0.01% by mass or less.
The reason why [P] is 0.02 mass% or less is as follows.
Phosphorus (P) segregates at the crystal grain boundaries and lowers the grain boundary strength and fracture toughness, so the content is made 0.02% by mass or less.

The reason why [O] is 10 mass ppm or less is as follows.
Oxygen (O) combines with aluminum (Al), magnesium (Mg), calcium (Ca) and the like to form oxides such as Al ₂ O ₃ , MgO and CaO. Since these oxides become inclusions and serve as starting points for peeling, the content is set to 10 mass ppm or less.
The inner ring and outer ring constituting the rolling bearing of the present invention have higher corrosion resistance and better mechanical strength than the inner ring and outer ring formed of martensitic stainless steel described in Patent Documents 1 and 2.

  According to the present invention, a rolling bearing having excellent corrosion resistance is provided.

It is sectional drawing which shows the rolling bearing corresponded to one Embodiment of this invention. It is a figure which shows the state of the outer peripheral surface of the inner ring | wheel of No. 5 before and behind a corrosion resistance test, Comprising: (a) shows the state before a test, (b) shows the state after a test. It is a graph which shows the relationship between the number of rust generation | occurrence | production, and a pitting corrosion index (PRE) in the inner ring | wheels No. 1-10 produced by embodiment. It is a graph which shows the relationship between the number of rust generation | occurrence | production, and the carbon content rate [C] of steel materials in the inner ring | wheels of No. 11-16 produced by embodiment. In the inner rings of No.1, No.2, and Nos.6-8, in which the carbon content [C] of the steel material produced in the embodiment is substantially the same, the surface hardness (HRC) and the carbon and nitrogen of the surface layer portion It is a graph which shows the relationship with total content rate [C + N].

Embodiments of the present invention will be described below.
FIG. 1 is a view showing a rolling bearing corresponding to an embodiment of the present invention. This rolling bearing has an inner ring 1, an outer ring 2, a ball (rolling element) 3, a cage 4, and a seal 5.
A raceway surface 11 is formed on the outer peripheral surface of the inner ring 1. A raceway surface 21 is formed on the inner peripheral surface of the outer ring 2. The ball 3 is disposed between the raceway surfaces 11 and 21 of the inner ring 1 and the outer ring 2 so as to be able to roll while being held by the cage 4.

As a rolling bearing having the shape shown in FIG. 1, a rolling bearing “Nippon No. SR144” (inner diameter 3.175 mm, outer diameter 6.35 mm, width 2.38 mm, ball diameter 1.00 mm) manufactured by NSK Ltd. is used as follows. Assembled.
First, the materials made of alloy steel Nos. 1 to 16 having the compositions shown in Tables 1 and 2 below were turned into the shapes of the inner ring 1 and the outer ring 2. Next, No. 1 and No. 3 to 5 were subjected to the following heat treatment A, and No. 2 and No. 6 to 16 were subjected to the following heat treatment A. Next, grinding finishing was performed to obtain No. 1 to No. 16 inner ring 1 and outer ring 2.

<Heat treatment a>
A heat treatment consisting of quenching and tempering was performed.
Quenching was performed by oil cooling after holding in a vacuum of 1000 ° C. to 1100 ° C. for 0.5 hours.
Tempering was performed by holding at 160 ° C. to 200 ° C. for 1.5 hours.

<Heat treatment a>
A heat treatment including gas nitriding, quenching, and tempering was performed.
In the gas nitriding treatment, an inner ring and an outer ring are placed in an electric furnace, the temperature in the furnace is maintained at 1200 ° C. (1473 K), nitrogen gas is introduced, and the pressure of the nitrogen gas is maintained at 0.05 to 0.1 MPa for 12 hours. It was done by doing. By changing the pressure of nitrogen gas, the nitrogen content in the surface layer portion was adjusted to the values shown in Tables 1 and 2.

Quenching was performed by oil cooling.
Tempering was performed by holding at 160 ° C. to 200 ° C. for 1.5 hours.
No. 1 to 16 rolling bearings were assembled using the obtained inner ring 1 and outer ring 2, balls 3 made of silicon nitride (ceramics), cage 4 made of polyamide, and seal 5 made of SUS304.
Each rolling bearing of No. 1 to 16 was immersed in a 6% by mass hypochlorous acid aqueous solution (pH 12) at a temperature of 30 ° C. for 168 hours, and then the number of locations where rust was generated by visual appearance inspection ( Corrosion resistance test). Hypochlorous acid aqueous solution is used as a disinfectant, and the environment of this corrosion resistance test is a severer environment than the actual disinfectant environment.
As a result, the outer peripheral surface of No. 5 inner ring was in the state shown in the photograph in FIG. FIG. 2A is a photograph showing a state before dipping in an aqueous hypochlorous acid solution. In FIG.2 (b), the location where rust generate | occur | produced is shown by (circle).

Moreover, the raw material which consists of alloy steel No. 1-16 was processed into the shape of the test piece for various tests, and 16 types of test pieces which respectively performed the same heat processing as alloy steel No. 1-16 were prepared. Using these test pieces, the nitrogen content [N] of the surface layer portion, the total content of carbon and nitrogen [C + N] of the surface layer portion, and the surface Rockwell hardness (HRC) were measured by the following methods.
The nitrogen content [N] of the surface layer and the total content [C + N] of carbon and nitrogen were measured using an electron beam microanalyzer (EPMA) under the condition of an acceleration voltage of 15 kV. The surface Rockwell hardness (HRC) was measured by a method according to “JIS Z2245”.
These measurement results are also shown in Tables 1 and 2.

  In Nos. 1 to 10 shown in Table 1, the number of rust occurrences is shown as a relative value with the number of rust occurrences of No. 5 being 1. FIG. 3 is a graph showing the relationship between the number of rust occurrences and the pitting index (PRE) in the inner rings of Nos. 1 to 10. In the inner rings of Nos. 3 to 5, the carbon content of the steel material used exceeds 0.35 mass%.

From the graph of FIG. 3, it can be seen that the number of rust occurrences can be significantly reduced by setting the pitting corrosion index to 20.0 or more except for the result of No. 4. The reason for the large number of rust occurrences on the inner ring of No. 4 is considered to be because the carbon content of the steel material used was excessive.
In No.11-16 shown in Table 2, the number of rust generation | occurrence | production is shown by the relative value which made the number of rust generation | occurrence | production locations of No.16 1 The inner rings of Nos. 11 to 16 have substantially the same pitting corrosion index and differ in the carbon content [C] of the steel material. FIG. 4 is a graph showing the relationship between the number of rust occurrences and the carbon content [C] of the steel material in No. 11 to 16 inner rings. From the graph of FIG. 4, it is understood that the carbon content of the steel material to be used needs to be 0.35% by mass or less in order to make the number of rust generation 20% or less of No. 16.

FIG. 5 shows the surface hardness (HRC) and the sum of carbon and nitrogen in the surface layer in the inner rings of No. 1, No. 2, and Nos. 6 to 8 where the carbon content [C] of the steel material is substantially the same. It is a graph which shows the relationship with content rate [C + N]. From the graph of FIG. 5, it can be seen that the surface hardness can be made HRC58 or more by setting the total content of carbon and nitrogen in the surface layer part to 0.40% by mass or more.
From the above results, the rolling bearings of No. 6 to 12 satisfying the scope of the present invention are excellent in corrosion resistance compared with the rolling bearings of No. 1 to 5 and No. 13 to 16 that do not satisfy the scope of the present invention. I understand that

DESCRIPTION OF SYMBOLS 1 Inner ring 11 Race surface of inner ring 2 Outer ring 21 Rolling surface of outer ring 3 Ball (rolling element)
4 Cage 5 Seal

Claims (1)

An inner ring, an outer ring, and a rolling element that is arranged to freely roll between the raceways of the inner ring and the outer ring,
The inner ring and the outer ring are
The carbon content [C] is 0.10% by mass to 0.35% by mass, the chromium content [Cr] is 12.5% by mass to 18.0% by mass, and the molybdenum content [Mo] is 0.80. Mass% to 5.0 mass%, manganese content [Mn] is 0.1 mass% to 1.5 mass%, silicon content [Si] is 0.1 mass% to 2.0 mass%, Nickel content [Ni] is 0.05 mass% or more and 4.0 mass% or less, copper content [Cu] is 0.2 mass% or less, sulfur content [S] is 0.01 mass% or less, phosphorus content Gas nitriding treatment is performed after processing a material made of alloy steel having a rate [P] of 0.02 mass% or less, an oxygen content [O] of 10 ppm or less, and the balance of iron (Fe) and inevitable impurities into a predetermined shape And obtained by quenching and tempering,
The nitrogen content [N] in the surface layer part is 0.05% by mass or more and 0.50% by mass or less,
The total content [C + N] of nitrogen and carbon in the surface layer portion of the raceway surface is 0.40 mass% or more and 0.85 mass% or less,
The surface hardness of the raceway surface is HRC58 or more and 62 or less,
A rolling bearing having a pitting corrosion index (PRE) represented by the following formula (1) of 20.0 or more and 42.5 or less at the surface layer portion.
PRE = [Cr] +3.3 [Mo] +16 [N] (1)

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