JP2017160474A - Bearing component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bearing component excellent in abrasion resistance and rolling motion fatigue properties.SOLUTION: A bearing component has a steel material containing 0.30 mass% to 0.90 mass% of C, 0.05 mass% to 1.00 mass% of Si, 0.10 mass% to 1.50 mass% of Mn, 0.003 mass% to 0.030 mass% of P, 0.001 mass% to 0.020 mass% of S and 0.10 mass% to 0.70 mass% of Nb and the balance Fe with inevitable impurities as a raw material. It also has a metallic structure after a refining heating treatment in which Nb-containing carbide is dispersed. Further the number of the Nb-containing carbide having particle diameter of 1.0 μm is 200/mmor more when the particle diameter of the particle is defined as a square root of an area of Nb-containing carbide particles observed by a cross section structure observation and Dmax which is maximum particle diameter of the Nb-containing carbide particles in 10mmestimated by an extreme statistical process is 16.0 μm or less.SELECTED DRAWING: None

Description

  The present invention relates to a bearing component used, for example, in a wear environment.

  In recent years, many automobile engines have adopted technologies such as turbo and direct injection for the purpose of increasing output and improving fuel efficiency.

  These techniques are believed to increase the generation of fuel combustion intermediates, etc., that contain a large amount of soot that would be caused by incomplete combustion.

  When hard and fine foreign matter such as a combustion intermediate product of fuel is mixed into the engine oil, wear damage of parts lubricated with the engine oil is promoted. In particular, since a relatively high load is applied to the bearing component, the degree of wear damage is remarkably high and excellent wear resistance is required.

  In addition, the use of diesel engines is also increasing, and there is a demand for bearing parts with excellent wear resistance.

  As a technique for improving this type of wear resistance, for example, techniques described in Patent Documents 1 to 5 are known.

  In Patent Documents 1 to 3, by adding a large amount of alloy elements, the hardness is increased and the wear resistance is improved by solid solution strengthening and precipitation strengthening.

  In Patent Document 4, wear resistance is improved by applying a hard coating on the rolling surface of the bearing component.

  In Patent Document 5, wear resistance is improved and toughness is ensured by adjusting the density of Nb carbide.

Japanese Patent Laid-Open No. 62-142726 JP-A 63-169359 Japanese Patent Laid-Open No. 1-142023 JP 2009-257567 A JP2013-136820A

  However, the bearing parts are generally used in the vicinity of the upper limit of the hardness level achievable with steel materials, and the technical idea of increasing the hardness as in the methods of Patent Documents 1 to 3 cannot be applied. There is a case.

  Further, as in Patent Documents 1 to 3, when a large amount of alloy element is added and the hardness is increased by solid solution strengthening or precipitation strengthening, as a result, the manufacturability and workability of the material are lowered and the manufacturing cost is reduced. As a result, there is a problem in that it cannot be efficiently manufactured.

  Although applying a hard film as in Patent Document 4 is effective in improving the wear resistance, there is a problem that the manufacturing cost increases and the manufacturing cannot be efficiently performed.

  In patent document 5, it cannot be said that it is appropriate when it is caused by rolling fatigue which is the most important characteristic of the bearing component among the fractures. In other words, the bearing parts are required to have rolling fatigue characteristics as well as wear resistance, but Nb carbide may be the starting point of fatigue failure, and the rolling fatigue characteristics may be reduced.

  Accordingly, there has been a demand for a bearing component having excellent wear resistance and rolling fatigue characteristics that can be used in a wear environment such as a lubricating oil mixed with hard foreign matters.

  The present invention has been made in view of these points, and an object thereof is to provide a bearing component that is excellent in wear resistance and rolling fatigue characteristics.

The bearing parts described in claim 1 are: C: 0.30 mass% or more and 0.90 mass% or less, Si: 0.05 mass% or more and 1.00 mass% or less, Mn: 0.10 mass% or more 1 .50 mass% or less, P: 0.003 mass% or more and 0.030 mass% or less, S: 0.001 mass% or more and 0.020 mass% or less, and Nb: 0.10 mass% or more and 0.70 mass% or less The balance of the square root of the area of the Nb-containing carbide particles observed by cross-sectional structure observation. The number of Nb-containing carbide particles having a particle size of 1.0 μm or more is 200 pieces / mm ² or more, and the Nb-containing carbide particles in 10 ³ mm ³ estimated by an extreme value statistical method are used. The maximum particle size Dmax is 16.0μ It is not more than.

  The bearing component according to claim 2 is the bearing component according to claim 1, wherein Cr: 1.50 mass% or less, Mo: 0.50 mass% or less, V: 0.50 mass% or less, Ni: 2 It contains at least one of 0.000 mass% or less, Ti: 0.10 mass% or less, and B: 0.0050 mass% or less.

According to the present invention, in a predetermined component range, the number of Nb-containing carbides having a particle size of 1.0 μm or more is 200 / mm ² or more, and in 10 ³ mm ³ estimated by an extreme value statistical method. Since Dmax, which is the maximum particle size of the Nb-containing carbide particles, is 16.0 μm or less, wear resistance and rolling fatigue characteristics can be improved.

It is description which shows typically the structure of the high temperature melting furnace used in the Example. It is explanatory drawing which shows typically the structure of the rolling fatigue test in an Example.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  The bearing component according to an embodiment includes 0.30 mass% or more and 0.90 mass% or less of C (carbon), 0.05 mass% or more and 1.00 mass% or less of Si (silicon), 0.10 mass. % Mn (manganese), 0.003 mass% to 0.030 mass% P (phosphorus), 0.001 mass% to 0.020 mass% S (sulfur), And the steel material which contains 0.10 mass% or more and 0.70 mass% or less Nb (niobium), and remainder consists of Fe and an unavoidable impurity is used as a raw material.

  Further, if necessary, 1.50 mass% or less of Cr (chromium), 0.50 mass% or less of Mo (molybdenum), 0.50 mass% or less of V (vanadium), or 2.00 mass% or less of You may contain at least 1 sort (s) of Ni (nickel), 0.10 mass% or less Ti (titanium), and 0.0050 mass% or less B (boron).

  C is an element that has the effect of improving the tempering hardness, strength, and wear resistance. To ensure wear resistance as a bearing part, it is necessary to add 0.30% by mass or more. It is preferable to add 32% by mass or more. On the other hand, when C is added excessively exceeding 0.90 mass%, a large eutectic carbide is generated in the casting process, and there is a possibility that material characteristics such as fatigue characteristics are deteriorated. Therefore, the C content is set to 0.30 mass% or more and 0.90 mass% or less.

  Si acts as a denitrifying element of molten steel and is an element that increases temper softening resistance. To achieve these effects, it is necessary to add 0.05% by mass or more. On the other hand, when Si is added excessively exceeding 1.00 mass%, since a hot-rolled sheet and a cold-rolled sheet become hard, manufacturability may deteriorate. Therefore, Si content shall be 0.05 mass% or more and 1.00 mass% or less.

  Mn is an element effective for improving the hardenability, and it is necessary to add 0.10% by mass or more in order to improve the hardenability. On the other hand, when Mn is added excessively exceeding 1.50 mass%, since a hot-rolled sheet and a cold-rolled sheet become hard, manufacturability may deteriorate. Therefore, Mn content shall be 0.10 mass% or more and 1.50 mass% or less.

  P precipitates at the austenite grain boundary during quenching, lowers the grain boundary strength, and lowers the fatigue characteristics and toughness. Therefore, the content is preferably reduced as much as possible. Therefore, the P content is set to 0.003 mass% or more and 0.030 mass% or less.

  Since S forms MnS that becomes the starting point of impact fracture or fatigue fracture in steel and lowers fatigue characteristics and toughness, the content is preferably reduced as much as possible. Then, S content shall be 0.001 mass% or more and 0.020 mass% or less.

  Nb precipitates as very hard Nb carbide in the steel after casting and contributes to improvement of wear resistance, and Nb re-dissolved after casting contributes to grain refinement during quenching, Improve toughness. And in order to improve abrasion resistance, it is necessary to add 0.10 mass% or more of Nb, and adding 0.15 mass% or more is preferable. On the other hand, when Nb is added in excess of 0.70% by mass, coarse Nb carbide precipitates, which may become a starting point of fatigue failure and may deteriorate fatigue characteristics. Therefore, Nb content shall be 0.10 mass% or more and 0.70 mass% or less.

  Cr, like Mn, is an element that improves the hardenability, suppresses the coarsening of carbides during annealing, and improves the impact value, and is added as necessary. And in order to improve hardenability, it is preferable to add Cr 0.10 mass% or more. On the other hand, if Cr is added in excess of 1.50% by mass, the toughness may decrease. Therefore, the Cr content in the case of adding Cr is 1.50% by mass or less.

  Mo is an element effective for improving toughness, and is added as necessary. And in order to improve toughness, it is preferable to add 0.10 mass% or more of Mo. On the other hand, Mo is relatively expensive, so excessive addition increases the raw material cost. Therefore, when Mo is added, the Mo content is 0.50% by mass or less.

  V is an element effective for improving toughness, and is added as necessary. And in order to improve toughness, it is preferable to add 0.1 mass% or more of V. On the other hand, since V is relatively expensive, excessive addition increases the raw material cost. Therefore, the V content when V is added is 0.50 mass% or less.

  Ni is an element effective for improving the hardenability, and is added as necessary. And in order to improve hardenability, it is preferable to add 0.1 mass% or more of Ni. On the other hand, since Ni is relatively expensive, excessive addition increases the raw material cost. Therefore, when Ni is added, the Ni content is 2.00% by mass or less.

  Since Ti has a strong binding force with N (nitrogen), it prevents the generation of BN and contributes to the effect of improving the hardenability of B. Moreover, it precipitates as very hard Ti carbide in the steel after casting, and contributes to the improvement of wear resistance. Further, after casting, the solid-dissolved Ti contributes to crystal grain refinement during quenching and improves toughness. Therefore, it is preferable to add 0.01% by mass or more in order to add as necessary and prevent the formation of BN. On the other hand, the BN generation preventing action by Ti is saturated by Ti content up to 0.10 mass, and if added excessively, toughness may be lowered. Therefore, when Ti is added, the Ti content is 0.10% by mass or less.

  B is an element effective for improving the hardenability, and is added as necessary. And in order to improve hardenability, it is preferable to add B 0.0005 mass% or more. On the other hand, the hardenability improving effect by B is saturated when the B content is 0.0050 mass%. Accordingly, the B content when B is added is 0.0050 mass% or less.

  Here, when a foreign matter is present on the surface roughness of the mating friction surface or the friction surface, abrasive wear occurs due to the surface being scraped off. This abrasive wear occurs when the surface roughness of the friction counterpart material is large and hard, or when foreign matter present on the friction surface is hard against the friction target member. That is, foreign matters present on the convex portions and the friction surface on the surface having a large surface roughness become abrasive grains, and the friction target member is worn.

  Under such a wear environment, the wear resistance is improved by dispersing and precipitating Nb-containing carbide having a particle size of several μm in the friction member matrix over a predetermined amount. In other words, since the hardness of the Nb-containing carbide is 2000 HV or more, it is considered that the hard Nb-containing carbide dispersed and precipitated acts as a resistance to wear, thereby reducing the wear amount of the friction target member.

  And the wear form that occurs when fine and hard foreign matter such as soot is mixed is also considered to be the same as the above-mentioned abrasive wear, so the lubricating oil such as engine oil mixed with combustion intermediate products such as soot etc. For improving the wear resistance of bearing parts used in the environment, for example, the action of Nb-containing carbides mainly composed of NbC is effective.

  Furthermore, various experiments were performed in consideration of steel components and preparation conditions, and the relationship between the state of hard Nb-containing carbide dispersed and precipitated in the matrix and the wear resistance and rolling fatigue characteristics was examined. Nb-containing carbide Among the dispersion distribution states, it is effective to improve the wear resistance to define the density (number) of Nb-containing carbides having a predetermined particle size or more, and the maximum particle size of Nb-containing carbides is the most effective in rolling fatigue characteristics. I found it to have an impact. That is, the number of Nb-containing carbides is specified to improve the wear resistance by the hard Nb-containing carbides, while the maximum particle size of the Nb-containing carbides is regulated so that the Nb-containing carbides do not become the starting point of fatigue fracture. This is very important.

Specifically, in order to ensure good wear resistance as a bearing component, the number of Nb-containing carbides having a particle size of 1.0 μm or more in the metal structure after tempering heat treatment in which Nb-containing carbides are dispersed is 200 / The distribution state of the Nb-containing carbide is adjusted to be mm ² or more. Further, in order to ensure good rolling fatigue characteristics as a bearing part, Dmax, which is the maximum particle diameter of Nb-containing carbide particles in 10 ³ mm ³ estimated by the extreme value statistical method, is 16.0 μm or less. Next, the distribution state of the Nb-containing carbide is adjusted. The particle diameter of the Nb-containing carbide is defined as the square root of the area of the Nb-containing carbide particles observed by cross-sectional structure observation.

  Such a metallographic state can be obtained by controlling the cooling rate during casting and the heating temperature during slab heat treatment. That is, since Nb-containing carbide is precipitated by cooling after casting, the cooling rate after casting depends on the C content and Nb content in the steel and the heating temperature in the slab heat treatment in the subsequent process. It is important to control.

Further, refining heat treatment is a process of hardening the metal structure by transformation process including a step of rapidly cooling from the austenite temperature zone to A temperature range of less than ₁ transformation point, for example, quenching and tempering treatment, and, there is austempering etc. .

And according to the said bearing component, in the range of a predetermined component, since the number of Nb containing carbide | carbonized_materials with a particle size of 1.0 micrometer or more is 200 pieces / mm < ² > or more, while being able to improve abrasion resistance, extreme value statistics Since the maximum particle diameter Dmax of the Nb-containing carbide particles in 10 ³ mm ³ estimated by the method is 16.0 μm or less, the Nb-containing carbide is less likely to be the starting point of fatigue failure, and the rolling fatigue characteristics of the Nb-containing carbide Reduction can be effectively suppressed. Therefore, a bearing component having excellent wear resistance and rolling fatigue characteristics can be obtained.

  Hereinafter, this example and a comparative example will be described.

  A wear test and a rolling fatigue test were performed on each steel material having the components shown in Table 1.

  In the wear test, each steel type shown in Table 1 was cast and subjected to hot forging, hot rolling, surface grinding, annealing, cold rolling, and quenching and tempering (tempering heat treatment), and then processed into a test piece.

  In the rolling fatigue test, each steel shown in Table 1 was cast and subjected to hot forging, hot rolling, surface grinding, annealing and quenching in order, and then processed into a test piece.

  In casting, a high-temperature melting furnace 1 having the configuration shown in FIG. 1 was used. In this high-temperature melting furnace 1, a crucible 3 is installed in an internal space covered with a heat insulating material 2, and a steel block is melted by heat generated by a heater 4 in the crucible 3 to obtain a molten steel 5. The crucible 3 is placed on a lifting means 7 that can be lifted and lowered via a refractory brick 6. The crucible 3 in which the molten steel 5 heated by the heater 4 at a heating temperature of 1700 ° C. was accommodated was moved to the cooling zone in which the water cooling coil 8 was disposed by the descending of the elevating means 7 to solidify the molten steel 5. At that time, the temperature of the molten steel 5 and the solidified ingot solidified by the thermocouple 9 installed in the center of the crucible 3 is monitored, and the average value of the cooling rate from 1500 ° C. to 1000 ° C. is 0.5 to 20 ° C. / Min, the descending speed of the elevating means 7, the amount of heat generated from the heater 4, and the amount of heat removed by the water cooling coil 8 were adjusted. That is, in the high-temperature melting furnace 1, the average value of the cooling rate when the molten steel 5 and the solidified ingot solidified from the molten steel 5 are cooled from 1500 ° C to 1000 ° C is calculated from 1500 ° C to 1000 ° C at the center of the slab during casting. Thus, the maximum value of the NbC particles was controlled by controlling the cooling rate at the center of the steel slab.

  In the hot casting, a slab having a shape of 100 mm × 30 mm × L was used, and the heating temperature was 1250 ° C.

  In the hot rolling, the heating temperature is 1250 ° C., the finishing temperature is 850 ° C., the coiling equivalent temperature is 550 ° C., the thickness for the wear test is 3.5 mm, and the thickness for the rolling fatigue test is 10 mm. .

  Annealing was performed at 690 ° C. for 15 hours, cold rolling was performed to a plate thickness of 1.5 mm, and finish annealing was performed at 670 ° C. for 15 hours.

  In addition, the quenching and tempering treatment was performed by heating at 820 ° C. for 15 minutes, followed by oil quenching at 60 ° C. and tempering at a predetermined temperature for 30 minutes according to the composition so that the tempered hardness was 600 ± 15 HV. .

  The structure of each steel material was observed, and the maximum particle size Dmax of the Nb-containing carbide was estimated by the extreme value statistical method.

  Dmax is based on Takashi Murakami, “Effects of Metal Fatigue Microdefects and Inclusions”, Yokendo, 1993, Chapter A3 “Procedure for Estimating √areamax of Maximum Inclusions Contained in Constant Volume”. This was measured by replacing the inclusions of Nb with carbides containing Nb and performing statistical processing.

In the tissue observation, an optical microscope was used, the observation magnification was 100 to 1000 times, the inspection reference area (V ₀ ) was 100 mm ² , the number of inspections (n) was 30 times, and the predicted volume (V) was 1000 m ³ . This maximum particle size Dmax (μm) is shown in Table 2.

Further, for each steel material, the L cross section was observed with an analytical scanning electron microscope, and among the Nb-containing carbide particles present in the observation area 61 × 61 μm ² × 20 field of view, the number of carbide particles having a particle size of 1.0 μm or more was determined. Counted and converted to the number per mm ² . The particle size is the square root of the particle area, and particles having a particle size of 1.0 μm or more were picked up by image processing. Table 2 shows the number of particles of 1.0 μm or more (number / mm ² ).

  The wear resistance test was conducted with a pin-on-disk wear tester. The test piece used was a columnar one with a friction surface of 1.5 mm square, and a VC (vanadium carbide) film was used as the friction counterpart. Further, under the lubrication conditions in which engine oil was dropped, the friction counterpart material was rotated, the test load was 500 N, the friction speed was 1 m / sec, and the friction distance was 3600 m.

  Then, when the specific wear amount is C, the wear loss is W, the test load is F, and the friction distance is L, the specific wear amount is calculated based on the formula C = W / (F × L). . This specific wear amount is shown in Table 2.

The rolling fatigue test was performed with a thrust type rolling fatigue tester 11 shown in FIG. 2, and the test piece 12 was a disk-shaped one having a diameter of 63 mm × t9 mm (rolling diameter: 38 mm). Also, three steel balls 13 (SUJ2) of φ3 / 8 inch are used, the load is 325 kgf (maximum contact stress 4903 N / mm ² ), the rotational speed is 1800 cpm, the turbine 14 is used as the lubricating oil 14, and the maximum number of repetitions is 10 ⁷ times and the number of evaluations was 10 points per level.

In the rolling fatigue test, when a majority of 5 or more points out of 10 did not break, it was evaluated that the rolling fatigue characteristics were sufficient from the viewpoint of suppressing the fracture of the bearing parts. Also, when the specific wear amount is 0.35 × 10 ⁻⁷ mm ³ / N · m or less, it is evaluated as excellent wear resistance as a bearing component, and both the rolling fatigue test and the wear resistance test are standards. When it is satisfied, it is indicated by a circle as an evaluation as a bearing part.

As shown in Table 2, in the predetermined component range, the number of particles of Nb-containing carbide having a particle size of 1.0 μm or more is 200 particles / mm ² or more, and this example in which Dmax is 16.0 μm or less Also had excellent wear resistance and rolling fatigue characteristics.

  On the other hand, steel No. as a comparative example. Since Nos. 1 to 3 use steel type A having a C content greater than 0.90% by mass, rolling fatigue characteristics are insufficient. That is, since the C content is large, iron-based coarse eutectic carbides are produced in casting, which are the starting points of fracture, and it is considered that the rolling fatigue characteristics are lowered.

  Steel No. as a comparative example. Since No. 4 uses a steel type B having a C content of less than 0.30% by mass, the specific wear amount is higher than the standard and the wear resistance is insufficient.

  Steel No. as a comparative example. 5 and 6 are those using steel type C having an Nb content greater than 0.70% by mass, and thus wear resistance is good, but coarse Nb-containing carbides are formed and transfer fatigue properties are insufficient. Met.

  Steel No. as a comparative example. No. 7 uses a steel type D having a Nb content lower than 0.10% by mass. No. 8 uses steel type E to which Nb is not added. Therefore, the Nb-containing carbide cannot effectively improve the wear resistance, and the specific wear amount is large and the wear resistance is insufficient.

  Steel No. as a comparative example. Since Nos. 9 and 10 use steel type F having a Ti content greater than 0.10% by mass, coarse Nb and Ti carbides are generated, and fatigue characteristics are insufficient.

  Steel No. as a comparative example. 13, 17, 23, 27, 30, 32, 33, 35, 37 and 39 have Dmax exceeding 16.0 μm and good wear resistance, but rolling fatigue characteristics are insufficient. It was.

Claims (2)

C: 0.30% by mass to 0.90% by mass, Si: 0.05% by mass to 1.00% by mass, Mn: 0.10% by mass to 1.50% by mass, P: 0.003 Contains from mass% to 0.030 mass%, S: 0.001 mass% to 0.020 mass% and Nb: 0.10 mass% to 0.70 mass%, with the balance being Fe and inevitable impurities Consists of
Having a metal structure after tempering heat treatment in which Nb-containing carbides are dispersed;
When the square root of the area of Nb-containing carbide particles observed by cross-sectional structure observation is the particle diameter of the particles, the number of Nb-containing carbides having a particle diameter of 1.0 μm or more is 200 pieces / mm ² or more, and A bearing component, wherein Dmax, which is the maximum particle size of Nb-containing carbide particles in 10 ³ mm ³ estimated by an extreme value statistical method, is 16.0 μm or less. Cr: 1.50 mass% or less, Mo: 0.50 mass% or less, V: 0.50 mass% or less, Ni: 2.00 mass% or less, Ti: 0.10 mass% or less, and B: 0.0050 The bearing component according to claim 1, comprising at least one of mass% or less.

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