DE19982613B3 - High-temperature bearings part - Google Patents

Abstract

A high temperature rolling bearing member having an inner ring, an outer ring and a rolling member made of a steel product containing: at least 0.6% and not more than 1.3% C, at least 0.5% and not more than 3 , 0% Si, at least 0.2% and not more than 1.5% Mn, not more than 0.03% P, not more than 0.03% S, at least 0.3% and not more than 5.0 % Cr, at least 0.1% and not more than 3.0% Ni, not more than 0.050% Al, not more than 0.003% Ti, not more than 0.0015% O, not more than 0.015% N, and at least 0.05% and at most 0.25% Mo, wherein the proportions of the alloying elements are given in mass% and the balance of Fe and unavoidable impurities and having a structure which is subjected to a heat treatment after quench hardening, the hardness after the heat treatment is at least HRC 58 and the maximum carbide size is not more than 8 μm.

Description

Technical area

The present invention relates to a rolling bearing member which is used in a power transmission device or in the engine compartment of a motor vehicle, an airplane, a ship or in an industry-used machine, and in particular a cheap high-temperature rolling bearing part which has excellent rolling contact fatigue strength even under ambient conditions has, with foreign matter, z. B. contaminated with dust or waste, and under such environmental conditions, in which the temperature of the atmosphere between room temperature and 300 ° C.

State of the art

A rolling bearing used in an area of a power transmission device or in the engine compartment of a motor vehicle, aircraft, ship or industrial machine and used under severe environmental conditions must have excellent rolling contact endurance and reliability even under these environmental conditions preserve. Especially when the rolling bearing, which is used for the above tasks, with foreign matter such. As dust, waste or iron powder is contaminated, it is known that the rolling contact fatigue strength falls significantly compared to the use in a clean environment. As a countermeasure, therefore, some time ago a working method for carbonitriding of high-carbon chromium bearing steel, z. B. of SUJ2, or by case steel, z. SCM420, SNCM420 or SNCM815, and to produce a suitable level of retained austenite immediately below the rolling surface, and seeks to increase the lifetime of contaminant contamination.

However, the conventional carbonitriding is a longer-term treatment compared with the quenching and heat treatment applied to SUJ2 steel or the like. Therefore, in such carbonitrided rolling bearings, there is a problem that manufacturing costs considerably increase as compared with rolling bearings made by a usual quenching and heat treatment. Further, the rolling bearing used in a motor vehicle or an airplane is used in a high-temperature environment, and therefore it is required to have an excellent rolling contact fatigue strength under extremely difficult environmental conditions, that is, a high rolling contact fatigue strength. H. in an environment contaminated with foreign matter and having a high temperature. In general, the heat treatment in a rolling bearing used at a high temperature is carried out at a high temperature of at least 300 ° C. to provide dimensional stability after quench hardening is carried out on a high carbon chromium bearing according to SUJ2 or the like or after carburizing / Quench hardening of case hardened steel, e.g. From SCM420 or SNCM815.

However, the hardness considerably decreases when these materials are heat-treated at a high temperature, and thereby the prescribed hardness required for a rolling bearing can not be achieved, and the bearing contact fatigue life and wear resistance are lowered. Therefore, a precipitation hardening steel, e.g. As M50, used as a bearing steel in the high temperature range, it has been impossible that such a steel grade could meet the aforementioned requirements, because the manufacturing cost and material costs are high and the working temperature range is limited.

In a rolling bearing subjected to carbonitriding, quenching austenite is generated in the heat treatment immediately below the rolling surface, with nitrogen permeating into the steel. A stress concentration resulting from contaminant contamination dissolves due to the effect of retained austenite, while resistance to softening by heat due to the action of the nitrogen entering the steel is increased, and a change in structure is caused by the process of aging of the rolling contact is suppressed, whereby an improvement of the rolling contact fatigue strength is achieved.

However, when using a rolling bearing at a high temperature, a high-temperature heat treatment must be performed to ensure the dimensional stability as described above. When this high-temperature heat treatment is performed, the effect can not be predicted because the retained austenite decomposes and its amount decreases, while prevention of heat softening due to nitrogen is also limited, and thereby a suitable application in a high-environment environment Temperature contaminated with foreign matter, can not be guaranteed.

On the other hand, although high miniaturization at the output of an engine in a motor vehicle or the like is advancing rapidly, on the other hand, the motor housing which is used under the more severe conditions increases due to the environmental conditions for the rolling bearing. It is expected that the working temperature range, which is about 130 ° C at room temperature, may suddenly rise up to 160 ° C in a rolling bearing used in an engine compartment. It is foreseen today that the working temperature range of a rolling bearing at room temperature rises to about 160 ° C, and further, when the output of the motor increases, it may suddenly exceed 200 ° C. Therefore, it is expected that, if the output power is increased or further weight reduction of the engine occurs, an improvement of the rolling contact fatigue strength at ambient conditions contaminated with foreign matter, as well as at high temperature environment is required.

However, the presently available high carbon chromium bearing steel or rolling bearing subjected to hardening or carbonitriding does not have sufficient hot strength and can provide proper rolling contact under the expected environmental conditions contaminated with foreign matter and under high temperature environmental conditions. Not guarantee fatigue strength. In the bearing steel produced by precipitation hardening, z. M 50, on the other hand, the problem arises that the cost is high, and in this situation, a cheap rolling bearing having excellent rolling contact fatigue strength can not be produced.

document JP 07 126 804 A discloses a bearing steel with a high rolling fatigue strength.

The publication US 5,413,643 discloses a rolling bearing made of a bearing steel, which is subjected to a hardening process after carburizing or after carbonitriding.

The publication JP 06 293 939 discloses rolling bearing elements having a high rolling fatigue strength obtained after a quenching and annealing process.

The present invention proposes to solve the above-mentioned problems, and has as its object to provide a high-temperature rolling bearing part having excellent rolling contact fatigue strength even under environmental conditions contaminated with foreign matter and under high-temperature environmental conditions and cheap compared to the prior art products.

Disclosure of the invention

The inventors have conducted in-depth studies to find a combination of composite elements capable of producing a low-cost high-temperature rolling bearing part having excellent rolling fatigue strength under ambient conditions contaminated with foreign matter and under high-temperature environmental conditions owns every share of it.

A high-temperature rolling bearing part according to the invention is thus a high-temperature rolling bearing part having an inner ring, an outer ring and a rolling element consisting of a steel product containing: at least 0.6% and not more than 1.3% C (carbon) , at least 0.5% and not more than 3.0% Si (silicon), at least 0.2% and not more than 1.5% Mn (manganese), not more than 0.03% P (phosphorus), not more than 0.03% S (sulfur), at least 0.3% and not more than 5.0% Cr (chromium), at least 0.1% and not more than 3.0% Ni (nickel), not more than 0.050% Al (aluminum), not more than 0.003% Ti (titanium), not more than 0.0015% O (oxygen) and not more than 0.015% N (nitrogen), the proportions of the alloying elements being in mass% and the balance is Fe (iron) and unavoidable impurities, and has a structure which is subjected to heat treatment after quench hardening, the hardness after heat e treatment is at least HRC 58 and the maximum carbide size is not more than 8 μm.

In the rolling bearing according to the invention is an excellent rolling contact fatigue strength under environmental conditions contaminated with foreign matter, when subjected to quenching and heat treatment, even if it is not subjected to carbonitriding, because it has the aforementioned structure. Therefore, the carbonitriding can be omitted, and the manufacturing cost can be reduced in this way.

Although it is preferable to omit the carbonitriding from the viewpoint of reducing the manufacturing cost as described above, an excellent rolling contact fatigue strength under environmental conditions contaminated with foreign matter can also be achieved when carbonitriding is carried out in place of quench-hardening.

Furthermore, the aforementioned composition may have a high hardness of at least HRC 58 even when subjected to a heat treatment at a high temperature (eg, 350 ° C). The amount of retained austenite can be reduced by carrying out this heat treatment at a high temperature, thereby achieving dimensional stability in a high temperature environment while maintaining a high hardness of at least HRC 58. Therefore, the rolling contact fatigue strength and the wear resistance in a high-temperature environment can be improved over the prior art.

Furthermore, the steel of the aforementioned composition is cheaper than a precipitation-hardening bearing steel, such as. Eg M50.

Thus, it is possible to produce a high-temperature rolling bearing part at a low cost, which has excellent rolling contact fatigue strength under environmental conditions contaminated with foreign matter and having a high temperature.

The heat treatment temperature is preferably at least 180 ° C and not more than 350 ° C. The rolling bearing is generally used at a temperature of about 100 ° C, and therefore, the heat treatment temperature must be at least 180 ° C.

The reasons for the limitation of the chemical components of the rolling bearing part according to the invention for a high temperature will now be described.

(1) The C content (at least 0.6% and not more than 1.3%)

C is an element important for ensuring the strength of the rolling bearing, and it must be contained at least 0.6% in order to maintain the prescribed hardness after the heat treatment, and therefore, the lower limit of the C content became 0.6% set. Although a carbide plays an important role in the rolling contact fatigue strength in the present invention, as will be described later, it has been found that a larger carbide causes a reduction in the rolling contact fatigue strength, when C with a content of over 1.3%, and therefore the upper limit of the C content is set at 1.3%.

(2) Si content (at least 0.5% and not more than 3.0%)

Si has a function of suppressing softening in a high temperature region and improving the heat resistance of the rolling bearing, and therefore, it is preferably added. However, these effects can not be obtained when the Si content is less than 0.5%, and therefore the lower limit of the Si content is set at 0.5%. Although the heat resistance increases with increasing Si content, the effect comes to a standstill, and deterioration of the hot workability and the machinability occurs when Si is contained with a large proportion exceeding 3.0%, and therefore becomes the upper limit of the Si content is set at 3.0%.

(3) Mn content (at least 0.2% and not more than 1.5%)

Mn is an element used in steel making for deoxidation and an element which enhances quenching curability, and it must be contained at least 0.2% to effect this effect, and therefore the lower limit of Mn content becomes set at 0.2%. However, because the machinability noticeably decreases when Mn is contained in a large amount exceeding 1.5%, therefore, the upper limit of the Mn content is set at 1.5%.

(4) P content (not more than 0.03%)

P precipitates on the austenite grain boundaries of the steel and causes a reduction in toughness and rolling contact fatigue strength, and therefore, the content was limited to at most 0.03%.

(5) S content (not more than 0.03%)

S damages the machinability of steel, forms a nonmetallic inclusion in the steel and lowers toughness and rolling contact fatigue strength, and therefore 0.03% was set as the upper limit of the S content. S, though having the aforementioned harmful effects, causes an improvement in machinability, and therefore a content of up to 0.005% is allowed, although a content of the lowest level is desired.

(6) Cr content (at least 0.3% and not more than 5.0%)

Cr is an element which fulfills an important function in the present invention, and is used for improving the quenching curability, for securing the hardness by a carbide, and for increasing the life. An addition of at least 0.3% is required to produce the described carbide, and therefore the lower limit of the Cr content is set at 0.3%. However, since a large-size carbide results in a reduction in rolling contact fatigue strength, when Cr is present in a large proportion exceeding 5.0%, the upper limit of the Cr content is set at 5.0%.

(7) Al content (not more than 0.050%)

Although Al is used as a deoxidant in steelmaking, it is desirable to reduce the proportion because Al forms hard oxide inclusion and reduces rolling contact fatigue strength. A remarkable reduction in the rolling contact fatigue strength was found when Al was present in a proportion exceeding 0.050%, and therefore the upper limit of the Al content is set at 0.050%.

The production cost of the steel increases if the Al content is to be reduced to less than 0.005%, and therefore it is preferable to set the lower limit of the Al content at 0.005%.

(8) The content of Ti (not more than 0.003%), O (not more than 0.0015%) and N (not more than 0.015%)

Ti, O and N, which form oxides and nitrides in steel, develop as non-metallic inclusions to starting points of fatigue fracture and reduce the rolling contact fatigue strength, and therefore, Ti at 0.003%, O at 0.0015% and N at 0.015% Upper limits of the respective elements.

(9) Ni content (at least 0.1% and not more than 3.0%)

Ni is an element that performs an important function in the present invention, it suppresses a change in the structure by the rolling contact endurance stress, particularly in a high-temperature environment, and suppresses the lowering of the hardness in a high temperature range as well as a Improvement of the rolling contact fatigue strength. In addition, Ni increases the toughness and hence the life in an environment of foreign matter and also causes an increase in the resistance to corrosion. Therefore, Ni must be contained at least 0.1%, and the lower limit of the Ni content has therefore been set at 0.1%. When Ni is contained in a large proportion exceeding 3.0%, however, in the quenching hardening, a large amount of retained austenite is formed, and the prescribed hardness can not be achieved while the cost of the steel product increases, and therefore the upper limit of the Ni content set at 3.0%.

Now, the tempering hardness of the rolling bearing part according to the invention for a high temperature and the carbides are to be described.

(10) temper hardness

It is common that a bearing used in a high temperature range is subjected to a heat treatment at a temperature exceeding the ambient temperature to stabilize the dimensional stability in the functional environment. The inventors have made detailed studies on the tempering hardness and the rolling contact fatigue strength at an ambient temperature of 200 ° C to find out whether there is a correlation between the tempering hardness and the rolling contact fatigue strength, and whether the rolling contact fatigue strength gives a longer life, when the tempering hardness is high. It has been found that, especially with identical tempering hardness, a bearing in which the heat treatment was carried out at a high temperature has a longer life and a bearing whose temper is high, also has a longer life when the heat treatment is performed at a high temperature. Furthermore, it has been found that the life decreases abruptly and a spread of the life increases when the hardness after heat treatment is less than HRC 58. In order to increase the life at a high temperature and to reduce the scattering, it is necessary to achieve a hardness of at least HRC 58, and the heat treatment temperature should preferably be as high as possible at the same time.

(11) Carbides

It has been found that the carbides cause the hardness to be maintained in the heat treatment while suppressing a structural change by the rolling contact permanent load, thus causing an increase in the rolling contact fatigue strength. As a result of the investigation of the maximum size of the carbides at a depth of 0.1 mm from the surface of the bearing and the simultaneous determination of the rolling contact fatigue strength, it has been found that the lifetime is reduced when a large-sized carbide is present and It has become apparent that a sudden decrease in life occurs when a large carbide whose maximum size exceeds 8 μm exists, and therefore, the maximum size of the carbides is set to 8 μm.

The steel in the rolling bearing part described above preferably contains at least one of the elements in mass%: at least 0.05% and less than 0.25% Mo and at least 0.05 and not more than 1.0% V.

Thereby, the rolling contact fatigue strength can be further improved under an environmental condition contaminated with impurity and under a high temperature environmental condition, and the hardness after the heat treatment can be increased.

The reasons for the limitation of the above-mentioned chemical components will now be described.

(12) Mo content (at least 0.05% and less than 0.25%)

Mo enhances the quench hardenability of the steel, and it prevents the softening in the heat treatment because it is firmly dissolved in the carbides. In particular, Mo is added because it has been shown to improve the rolling contact fatigue strength in a high temperature range. However, the cost of the steel product increases while the hardness does not decrease, but the machinability noticeably deteriorates in softening for ease of cutting when Mo is contained in a large proportion of more than 0.25%, and therefore, the modulus Share fixed at less than 0.25%. With respect to carbide formation, no effect is exhibited when the content of Mo is less than 0.05%, and therefore, the lower limit of the Mo content is set to 0.05%.

(13) V content (at least 0.05% and not more than 1.0%)

V has the effect of combining with the carbon and forming a fine carbide, which leads to a refinement of the crystal grain and increases the strength and toughness, with an effect to increase the heat resistance of the steel product by the content of V by Softening is suppressed after a high-temperature heat treatment, whereby the rolling contact fatigue strength increases and a spread of the life is reduced. The content of V at which this effect is obtained is at least 0.05%, and therefore the lower limit of the V content is set to 0.05%. However, the machinability and hot workability decrease when V is contained in a large proportion exceeding 1.0%, and therefore the upper limit of the V content is set at 1.0%.

Best way to implement the invention

Now, examples of the present invention will be described.

The steels having the chemical compositions shown in Table 1 were melted in a vacuum induction furnace, cast into steel blocks of 150 kg, and then heated and kept at a temperature of 1200 ° C for three hours to hot forging to produce round bars of 50 mm diameter. The round rod material was subjected to a treatment by normalizing at 850 ° C for one hour, and then it was subjected to normalizing Air cooled and further softened by keeping it at 790 ° C for six hours, and then it was cooled to 650 ° C at a rate of 10 ° C / hour and further brought to room temperature by air cooling to make it easier to clean To soften cutting and produce materials for the various investigations.

hardness investigation

In order to measure the hardness after heat treatment by quench hardening and after carbonitriding, specimens of 20 mm in diameter and 100 mm in length were made of the material of 50 mm in diameter by machining.

The quench hardening was carried out by heating in a molten salt furnace, by annealing at 850 ° C for 30 minutes, and then quench-hardening the specimen in oil at 80 ° C. Thereafter, the heat treatment was carried out by reheating in a salt oven, holding the sample at 350 ° C for two hours, and then cooling it in air.

For carbonitriding, a gas atmosphere furnace as used in production was used to introduce the carbon content to 1.0 to 1.2% and an additional content of NH ₃ of 5 to 10% in an RX gas atmosphere. in which the specimens were kept at 850 ° C for 60 minutes, and then the specimens were quenched in oil. Then, a heat treatment was carried out at 350 ° C for 120 minutes.

Disk-shaped specimens of 10 mm thickness were cut out from the central portions of the specimens subjected to these quenching and heat treatments or the specimens subjected to the heat treatment after carbonitriding, and both surfaces were wet-polished to prepare specimens for to get the hardness measurement.

To determine the hardness, a Rockwell hardness meter was used to make a hardness measurement at locations 2 mm deep from the surfaces in sections of the test piece, and averages of seven points were determined as the tempering hardness.

Rolling contact fatigue life test

To confirm the suitability as rolling bearing part, a fatigue test was carried out with. performed a shear-rolling contact fatigue tester and determines the life expectancy of the respective materials.

Life expectancy specimens were 47 mm outside diameter, 29 mm inside diameter, and 7 mm thickness annular shear contact fatigue strength specimens roughly machined from 50 mm diameter round bar materials by turning.

After the rough machining, the samples were subjected to quenching and tempering treatment and carbonitriding, respectively. An experimental furnace used for general production tasks was used.

For quenching and heat treatment, a gas atmosphere furnace was used to hold the samples at 850 ° C for 30 minutes, controlling the carbon potential in an RX gas atmosphere so that no decarburization or carburization corresponding to the carbon content of each steel occurred, and the specimens were then quenched in oil. The subsequent heat treatment of 120 minutes was carried out at 350 ° C.

The carbonitriding and the heat treatment were treated under the same conditions as in the aforementioned hardness test specimens.

After completion of the heat treatment, both surfaces of the samples were polished and mirror-finished. For the samples subjected to carbonitriding, processing limits for polishing at 0.1 mm were set on both surfaces.

The rolling contact fatigue test was performed with a rolling contact fatigue tester. Table 2 shows the conditions of the experiment. The experiment was conducted in a room temperature environment and an environment of 200 ° C, and the experiment was further conducted in an environment tracking an impurity contaminated environment. Table 2 Test conditions of the rolling contact endurance test tester Thrust rolling contact Dauerfestigkeitsprüfgerät contact pressure 5.0 GPa rotation speed 2000 rpm Test temperature Room temperature, 200 ° C lubricant Turbo oil Amount of foreign substances 0.4 g / 1000 cm ³

For the fatigue test, 15 repeated tests were conducted under the same conditions for determining such a life that a cumulative probability of failure in a Weibull probability reaches 10% as the life of each material. Comparative Example No. 13 in Table 2 is a general-purpose material SUJ2, and the life of each material has been described with a probability assuming that the life of this quench-hardened and heat-treated material is 1.0.

carbides

The shear-to-rolling fatigue strength test specimens were used to determine the carbides present in the steel. Into the samples processed by various types of heat treatment in shear-rolling-contact fatigue strength test pieces, annular cross-sections were cut to prepare microstructures for detecting the structure. The specimens were mirror polished and further corroded with a Pikral corrosion solution to detect the carbides. In the case of the micro-test pieces, the carbides were detected at 0.1 mm depth from the surface layer of the rolling surface with an optical microscope, and the largest carbides were measured on a detectable area of 50 mm ² .

The results of the aforementioned temper hardness at 350 ° C, the rolling fatigue strength at room temperature and 200 ° C, the rolling contact fatigue strength in an environment of impurity and the maximum carbide sizes are shown in Table 3 for the samples of the present invention and in Table 4 for the comparative samples shown.

From the results shown in Tables 3 and 4, it can be seen that the hardness in the examples of the present invention having the composition of the present invention reaches at least the hardness HRC 58, even if heat treatment is carried out at 350 ° C. Further, from the examples of the present invention, the rolling contact fatigue strength at room temperature and at 200 ° C. and the rolling contact fatigue strength improved under the foreign substance conditions in comparison with the comparative examples, even if only quenching and heat treatment (HT) were carried out. It has also been found that an excellent rolling contact fatigue strength results when carbonitriding is carried out in place of quench hardening. In the examples according to the invention, it is further shown that the maximum size of the carbides at a depth of 0.1 mm from the surface layer of the rolling surfaces is not more than 8, 0 μm.

The examples presented are for illustration only and must not be construed as limiting. The scope of the present invention is determined not by the foregoing description but by the scope of the claims, and all modifications with respect to the meaning and the scope equivalent to the scope of the claims are included.

As described above, the inventors have found out the optimum composition of the elements and their content, whereby an excellent rolling fatigue strength under environmental conditions can be achieved with contaminant contamination without performing carbonitriding, thereby making it possible to obtain a low-cost high temperature rolling bearing part which achieves high hardness even if the heat treatment is performed at a high temperature (eg, 350 ° C).

Industrial applicability

The present invention is advantageously applicable to a rolling bearing member which can be advantageously used at a high temperature under an environmental condition contaminated with foreign matter and having a high temperature.

Claims (2)

High temperature rolling bearing part having an inner ring, an outer ring and a rolling element, consisting of a steel product containing: at least 0.6% and not more than 1.3% C, at least 0.5% and not more than 3.0% Si, at least 0.2% and not more than 1.5% Mn, not more than 0.03% P, not more than 0.03% S, at least 0.3% and not more than 5.0% Cr, at least 0.1% and not more than 3.0% Ni, not more than 0.050% Al, not more than 0.003% Ti, not more than 0.0015% O, not more than 0.015% N, and at least 0.05% and at most 0.25% Mo, wherein the proportions of the alloying elements are given in mass% and the balance of Fe and unavoidable impurities and has a structure which is subjected to a heat treatment after quench hardening, wherein the hardness after the heat treatment is at least HRC 58 and the maximum carbide size not more than 8 microns. A high temperature rolling bearing part according to claim 1, wherein the steel product further has at least 0.05% and at most 1.0% V in mass%.