SE1051359A1 - Förfarande för produktion av maskindelar med överlägsen hållbarhet vid rullkontakt - Google Patents

Description

10 15 20 25 30 35 2 amount of non-metallic inclusions, i.e., have a high cieaniiness and have an extremeiy small amount of iarge oxide inclusions having the inclusion size of 20 pm or more (For example, see Japanese Patent Laid-Open Pubiication No. 2006-63402 and Japanese Patent Laid-Open Publication No. H6(1994)~192790).

[0005] Even the use of a steel materia! with such high cleanliness cannot sufficiently prevent a failure in a short life.

Therefore, developments are activeiy conducted to reduce the amount of the non-metallic inclusions in the steel materials and to reduce the size of the non-metallic inclusions.

SUMMARY OF THE INVENTION

[0006] The inventors have currently found that, even without reciucing the amount and size of the non-metailic inclusions at the time of producing steel, it is possible to provide a machine component which has a surface hardness of 58 HRC or more, prevents flaking, and has a superior roliing contact fatigue life, by providing a steel material with a condition where the cavity between the non-metallic inclusions and the matrix phase in the steei is closed.

[0007] That is, in order to improve roliing contact fatigue iife in bearlngs and other machine components, it is important to reduce the amount of the non~metallic inclusions in the steei materials for these machine components. It is also known that, since the existence of a large inciusion under the roiling track surface of the hearing or other machine component may cause flaking in the machine component to lead to failure, it is particularly important to reduce the size of the non-metaliic inclusions which may be an origin of flaking under the roliing track surface of the bearing or other machine component for improving the service life of the hearing and other machine component. Aithough a iarge number of inventions were made for reducing the size of the inclusions in the mass production process, it was difficult to stabiy reduce the size of the non-metallic inclusions.

[0008] The inventors extensively investigated the process 10 15 20 25 30 35 3 which leads to failure in roliing contact fatigue, i.e., flaking, by observing the cracks with use of materiais containing artificiai pore defects. It has been found that, in the process that cracks originated from the non-metallic inclusion lead to the generation of flaking through their growth, the process undergoes the crack initiation stage with a crack being displaced (hereinafter, "Mode I-type initial crack") by stress concentration effect around the non-metallic inclusions. The process then undergoes the propagation of the cracks depending on shearing stress to lead to failure, as conventionaliy known. This means that, without generation of a Mode I-type initial crack that the inventors have found, the subsequent crack propagation and failure will not occur. In addition, the Mode I-type initial cracks occur on the premise that a physical cavity forms at the interface between the non-metallic inclusions and the matrix. It is confirmed by stress anaiysis that an Model I-type initial crack does not occur, without formation of a physical cavity (see, Iron and Steel (Tetsu-to-i-iagane), 94 (2008), p.13; and a doctoral dissertation of University of Hyogo in 2008 written by Kazuhiko Hiraoka (January 2008), which are incorporated herein by reference.)

[0009] Moreover, it has been also found that the physical cavity is formed by some plastic working which is necessarily conducted in the process of producing a steel materiai or the process of shaping the steel material into a component, i.e., hot rolling, cold rolling, hot forging, warm forging, cold forging, rolling forging, cold rolling, coid header, and drawing. Fig. 1 shows a conceptual view of an image in which existence/absence of a cavity around the inclusions is observed by means of scanned electronic microscopy (FE-SEN) after cutting out of a hot-rolled steel material and conducting ion milling. In Fig. 1, reference number 2 indicates AlzOg, while reference number 3 indicates a cavity. In particular, in the machine structural steel, Ai is normally used as oxide formation element for deoxidization. It has been confirmed that the Al203 inciusion tends to form a cavity particularly at the interface with the matrix due to the difference in deforrnability with steel and 10 15 20 25 30 35 4 the shape. 'The presentinvenüon has beenrnade on the basß of the above new findings.

[0010] Accordingly, it is an object of the invention to provide a method for producing a machine component which can stably exhibât a superior rolling contact fatigue life as compared to the steel material in which the amount and size of the non-metallic hufluaons are reduced at the thne of produdng steeh by improving the condition of the interface between the non-metallic inclusions and the matrix in the steel material, even without reducing the amount and size of the non-metallic inclusions at the time of producing steel. {0O11] Acconfing hathe presentinvenhon,thereis pnnnded a rnethod for prodtnïng a rnachine conwponent having a surface hardness of 58 l4RC or nfiona and a supenor roHing contact fatigue life by subjecting a part or the whole of a machine structurai steei to a quenching and tempering treatment, the nnethod conwpnsing the steps of: subjecüng the rnachine structural steel to a step for providing a shape as a steel material or a subsequent step for providing a shape as a machine component, wherein the steel is subjected to a plastic working; heating the steel, to which the plastic working has been conduded,to 780 °C or mgherto apmy a hydrofiatm pressure of 80 MPa or rnore, whereby bringing the non~metallic inclusion contained in the steel and the steel as a matrix into close contact with each other in an interface; and thereafter subjecting a part or the whole of the steel to a quenchirig and tempering treatment.

[0012] According to a first preferred embodiment of the present invenüon, there is prowded the above produfing method, wherein the heating is conducted at 800 °C or higher, and wherein the hydrostatic pressure is 100 MPa or higher.

[0013] According to a second preferred embodiment of the present invention, there is provided the above producing nnethod, nnwerein the nwachine structuralsteelto be subjected to the wormng has deoxkhzed by adding a plastic been 10 15 20 25 30 35 5 cleoxiclization agent containing Si in addition to generally-used Ai or by not adding a deoxidization agent containing Al.

[0014] According to a third preferred embociiment of the present invention, there is provided the above producing method, wherein the machine structural steel to be subjected to the piastic working has been deoxiciized by adding a deoxidization agent containing Ca in addition to generaily~used Al.

[0015] According to the method for producing a machine component, even without reducing the amount and size of the non~metaliic inclusions at the time of producing steel, when closing the physical cavity formed at the interface between the matrix and the non-metallic inclusions, which have been contained in the steel through some piastic working, it is possible to avoid fiaking which occurs due to rolling contact fatigue originated from a non-metallic inclusion, resulting in a significant improvement in service life.

BRIEF DESCRIPTION OF THE DRAWING

[0016] Fig.1 is a conceptual figure depicting an image in the surrounding of the non-metailic inclusion, which was observed with a scanning electron microscope (FE~SEM) after cutting out of the hot-rolled steei and conducting ion miiiing.

DESCRIPTION OF EMBODIMENTS

[0017] The machine structural steel of the present invention broadly includes steels that are required for machine components, such as bearings, gears, hub units, constantly variable transmissions, constant-velocity joints, piston pins and the like. Specifically, as such machine structural steel, there are generally used high carbon chromiurn bearing steeis defined in JIS G 4805, carbon steels for machine structural use defied in 315 G 4051, structure steels with specified hardenability bands defined in JIS G 4052 (H steel), low-alloyed steeis for machine structural use defined in JIS G 4053, alloy steel tubes for machine purposes defined in JIS G 3441, carbon steel tubes for 10 15 20 25 30 35 6 machine structural purposes defined in JIS G 3445, carbon steels for cold heading - the first part: wire rods defined in JIS G 3507-1, carbon steels for cold heading ~ the second part: wires defined ln JIS G 3507-2, low-alloyed steels for cold heading - the first part: wire rods defined in JIS G 3509-31., low-alloyed steels for cold heading - the second part: wlres defined in JIS G 3509-2, their relatlve foreign standard steels, steels having similar components, and steels having improved components. The disclosures of these JIS standards are incorporated herein by reference. The machine structural steel in the present invention includes steels satisfying the chemical components described in the above JIS standards.

[0018] The numerlcal range (wtß/a) of preferred compositions of the machine structural steel in the present invention are as follows: Preferred More Preferred Further Preferred Range Rande Ranoe C 0.10~1.10 0.95-1.10 0.95-1.10 Si 2.0 or less 0.15-0.70 0.15~0.35 Mn 3.0 or less 1.15 or less 0.50 or less P 0.025 or less 0.025 or less 0.025 or less S 0.100 or less 0.025 or less 0.025 or less Cr 15.0 or less 0.90-1.60 1.30~1.60 Mo 2.0 or less 0.25 or less 0.08 or less Ni 5.0 or less 0.25 or less 0.25 or less Cu 0.25 or less 0.25 or less 0.25 or less Balance Fe and unavoidable impurltles Remarks JIS G 4805 JIS G 4805 SUJ1-5 S032 Note that unavoidable impurities may include Al and/or Ca as a deoxidant.

[0019] These machine structural steels are generally produced through 1) oxidation refining of molten steel in an arc melting furnace or a converter furnace, 2) reduction reflning in a ladle refining furnace (LF), 3) rotary-flow vacuum degassing treatment by a rotary-flow vacuum degaseer (RH treatment), 4) 10 15 20 25 30 35 7 castíng of steel ingot by continuous casting or ingot casting and 5) plastic Working step including hot rolling or hot press forging and cold rolling or cold press forging of the steel ingot. The step for providing a shape as a steel material indicates the above steps, While the shape of the steel material indicates Shaped steel, steel bar, steel tube, wire rod, steel plate, and steel strip.

[0020] The steei material can be shaped to a desired material by subjecting it to plastic working, such as hot forging, semi-hot forging, Warm forging, cold forging, rolling forging, cold roliing, cold heading and drawing, in some cases, drawing and cold heading, and the cornbinations thereof, if necessary, heat treatment for softening or structure control, or turning. The step for providing a shape as a machine component in the present invention indicates the above steps.

[0021] The term "hot" in hot plastic working refers to recrystaliization temperature of the steel or higher. The term "Warm" in Warm plastic working refers to a temperature in the range of from room temperature to recrystallization temperature, while the term "cold" in cold plastic working refers to around room temperature.

[0022] Generaily, steel to which plastic working was conducted is then subjected to quenching and tempering for attaining a surface hardness of 58 HRC or more, such as entire hardening (through hardening), carburizing and quenching, carbonitriding and quenching, nitriding and quenching, and induction harclening, depending on the steel material and the intended use. The quenched and tempered steel is subjected to finishing treatment such as polishing and grinding, to produce a machine component that the present invention is to provide. The term "quenching and tempering treatment" used herein indicates the above treatments.

[0023] However, in order to attain the effects of the present invention, it is necessary to mandatorily conduct the step of closing the cavity existing at the interface between oxide inclusion and the matrix, at a stage before quenching and 10 15 20 25 30 35 8 tempering the machine component to attain a surface hardness of 58 HRC or more. The means for achieving this is preferred to be a method which is capable of applying a hydrostatic pressure of 80 MPa or more after heating to 780 °C. For example, such method may be hot isostatic press (HIP) method, hot press method, or hot forging method almost completely-sealed by die.

[0024] In hot forging, semi~hot forging, warm forging, cold forging, rolling forging, coid rolling, cold heading or drawing, in which the steel is not completely sealed in the die, it is impossible to attain the effects of the present invention because the hydrostatic pressure cannot be applied to the entire part of the steel or because the steel is continuousiy elongated in a certain direction.

[0025] The reasons for applying a hydrostatic pressure of 80 MPa or more after heating to 780 °C are as follows. That is, the steel is more deformable as the heating temperature of the steel materiai is higher. Therefore, the hydrostatic pressure necessary to close the cavity existing at the interface between oxicie-based non-metallic inciusions and the matrix can be lower as the heating temperature of the steel material is higher. As a result of the inventors' earnest investigation, the effects of the present invention can be attained when the steel is heated to a temperature of 780 °C or higher and a hydrostatic pressure of 80 MPa or more is applied to the steel. According to the first preferred embodiment, the above heating is conducted at 800 °C while the hydrostatic pressure is 100 MPa or more.

[0026] According to the second preferred embodiment, the machine structural steel to be subjected to the plastic working has been deoxidized by adding a deoxidization agent containing Si in addition to generally-used Al or by not adding a deoxidization agent containing Ai. Generally, machine structural steel is deoxidized with Al. Therefore, the oxide inclusions formed are mainly composed of Al203 base. Since Al203 is hard inclusions, which tend to aggiomerate after refinement to form the configuration of type B defined in ASTM 10 15 20 25 30 35 9 E 45, the conditional range for applying an Optimum hydrostatic pressure is iimited in order to completely close the cavity existing at the interface between the oxide inclusion and the matrix when appiying a hydrostatic pressure. Consequently, controlling the configuration of the oxide inclusions ieads to an increase in the effect of completely closing the cavity existing at the interface between the oxide inclusion and the matrix when applying a hydrostatic pressure. As means for achieving this, it is preferred that the oxide inclusion to be formed is softened to reduce the difference in deformability between the inciusion and the matrix, through deoxidization by adding a deoxidization agent containing Si in addition to generaliy-used Ai or by not adding a deoxidization agent containing Al.

[0027] According to the third preferred embodirnent, the machine structural steel to be subjected to the plastic working has been deoxidized by adding a deoxidization agent containing Ca in addition to generally-used Al. Generally, machine structural steei is deoxidized with using AI. Therefore, the oxide inclusions formed are mainly composed of AlzOg. Since Al203 constitutes a hard inclusion, which tends to agglornerate after refinement to form the configuration of type B defined in ASTM E 45, the conditional range for applying an Optimum hydrostatic pressure is limited in order to compieteiy close the cavity existing at the interface between the oxide inciusion and the matrix when applying a hydrostatic pressure. Consequently, controlling the configuration of the oxide inclusion leads to an increase in the effect of completely ciosing the cavity existing at the interface between the oxide inclusion and the matrix when applying a hydrostatic pressure. As means for achieving this, it is preferred that the oxide inciusion to be formed is controlled to have the configuration of type D (granular) defined in ASTM E 45, through deoxidization by adding a deoxidization agent containing Si in addition to generaily-used Al, so that a hydrostatic pressure can be evenly applied to the matrix at the surrounding of the non~meta||ic inclusion.

[0028] It is a matter of course that the first, second and/or 10 15 20 25 30 10 third preferred embodiments are arbitrarily combined to practice the present invention.

EXAMPLE 1

[0029] The conditions for practicing the first embodiment and results obtained thereby will be specifically explained. First of ail, the chemical composition of specimens is shown in Table 1.

The present specirnen is based on S032 steel, which satisfies the composition of JIS G 4805. Molten steel was subjected to an oxidation refining in an ark meiting furnace, a reduction refining in a ladle refining furnace (LF), a rotary-fiow degassing treatment in a rotary-flow vacuum degasser (RH treatment), and a continuous casting to cast a steel. The steel ingot was hot-rolled to produce a steel material. Then, the steel materia! was subjected to a spheroidized anneaiing at 800 °C.

[0030] [Table 1] Specimen Components (wt.°/o except that O refers to ppm) Steel C Si Mn S Cr Mo Al O SUJ2 1.04 0.22 0.32 0.007 1.44 0.03 0.011 6

[0031] Furthermore, the above spheroidized annealed steel material was processed in accordance with any one of conditions 1 to 3 as shown below.

Process Condition 1: The steel material was cut to the shape of a bearing washer, which is a member of a thrust~type rolling bearing.

Process Condition 2: The steel material heated to 600 °C, which is a warm condition in the range of from room temperature to recrystallization temperature, to be upset, and then cut to the shape of a bearing washer, which is a member of a thrust-type rolling bearing.

Process Condition 3: The steel material was coid-upset, and then cut to the shape of a bearing washer, which is a member of a thrust-type rolling bearing.

[0032] The washer-shaped products were respectively subjected to a hot isostatic press (HIP) treatment. The treatment conditions are shown in Table 2. Press conditions A and B satisfy the heating temperature condition and the 10 15 20 25 'li hydrostatic pressure condition of the present invention. Press conditions C to E fail to satisfy the heating temperature condition and the hydrostatic pressure condition of the present invention in that press conditions C and D does not satisfy the press condition of the present invention and press condition E does not involve the HIP treatment. The washer-shaped products according to these press conditions A and B were retained at 835 °C for 20 minutes, quenched by oil quenching, and then tempered at 170 °C for 90 minutes to attain a desired hardness of 58 HRC or higher. Furthermore, the products were polished to finish thrust-type roliing bearings, which were evaluated for roliing contact fatigue life. A commercially available ball for a thrust-type roliing hearing was used as the roliing element.

[0033] [Table 2] Press Conditions Condition Heating Press Remarks Temperature Pressure (°C) (MPH) A 800 100 Working B 1100 200 Examples C 700 100 Comparative D 800 50 Exampies E No press was conducted.

[0034] Thrust~type roliing fatigue test was conducted 10 times on each press condition as shown above, with the maximum Hertz stress Pmax being 5292MPa. From the results, the number of the cycle was measured until 10 % of the specimens cause flaking by counting from the short life side on the basis of Weibull distribution function, and named this number Lm life.

The surface hardness of these specimens after quenching and tempering and the Llo life calculated from the lives of the ten specimens of each condition to which thrust-type roliing fatigue test was conducted are shown in Table 3. For the specimen of each condition, the test was stopped at a time when reaching 1x108 cycles for test convenience, even if no fiaking occurred. 10 15 20 12

[0035] [Table 3] Process Press Surface Lm life Condition Condition Hardness (xlOs cycles.) (HRC) 1 Å 60.9 83 B 58.0 100* C 59.1 10.9 D 59.4 15.4 E 61.9 8.9 2 Å 58.4 81 B 61.3 100* C 58.1 11.7 D 60.3 10.1 E 61.4 9.5 3 Å 58.2 100* B 59.9 100* C 59.1 13.3 D 60.3 12.9 E 59.3 10.6 *: This symbol means that no flaking occurred in 1x108 cycles.

[0036] The symboi "*" for Llo life in Table 3 means that no flaking occurred in 1x108 cycles. In Table 3, press conditions A and B satisfying the heating temperature condition and the hydrostatic pressure condition of the present invention result in a surface hardness of 58 HRC or more. Press conditions C to E not satisfying the heating temperature condition and the hydrostatic pressure condition of the present invention also result in a surface hardness of 58 HRC or more. l-iowever, working exampies under conditions A and B result in a significantly improved roiling contact fatigue life, compared with comparative examples under press conditions C to E, regardless of whether the finishing process is hot plastic working, Warm plastic working, or cold plastic working.

Example 2

[0037] The conditions for practicing the second ernbodiment and results obtained thereby will be specifically explained.

First of all, the chemical compositions of specirnens are shown in Table 4. Steels A and B of these specimens are both based on SUJZ steei, which satisfies the chemical cornposition of JIS G 10 15 20 25 30 13 4805. Molten steel was subjected to an oxidation refining in an ark melting furnace, a reduction refining in a ladle refining furnace (LF), a rotary~f|ow degassing treatment in a rotary-flow vacuum clegasser (RH treatment), and a continuous casting to cast a steel. The steel ingot was hot-rolled to produce a steel material. Steel A of the specimen has been deoxidized with Si and Mn without adding Al during deoxidation, and therefore 0.003 % of AI shown in Table 4 is contained as unavoidable impurities. Steel B has been deoxidized with Al as conventionally conducted. The steel material obtained through the hot rolling was subjected to a spheroidized annealing at 800 °C.

[0038] [Table 4] Speclmen Components (wt.% except that O refers to ppm) Steel C Si Mn S Cr Mo Al O A 1.00 0.26 0.30 0.009 1.41 0.03 0.003 10 B 1.04 0.22 0.32 0.007 1.44 0.03 0.011 6

[0039] Furthermore, the above spheroidized annealed steel material was processed in accordance with any one of conditions 1 to 3 as shown below.

Process Condition 1: The steel material was cut to the shape of a bearing washer, which is a member of a thrust~type rolling bearing.

Process Condition 2: The steel material was heated to 600 °C, “ which is a Warm condition in the range of from room temperature to recrystallization temperature, to be upset, and then cut to the shape of a bearing washer, which is a member of a thrust~type rolling bearing.

Process Condition 3: The steel material was coId-upset, and then cut to the shape of a bearing washer, which is a member of a thrust-type roliing bearing.

[0040] The washer-shaped products subjected to a hot isostatic press (HIP) treatment or a hot press treatment. The treatment conditions are shown in Table 5.

Press condition (1) relates to a hot press treatment, while press conditions (2) to (4) relates to a HIP treatment. Press were respectively 10 15 20 25 14 conditions (1) to (3) are working examples satisfying the heating temperature condition and the hydrostatic pressure condition of the present invention. In contrast, press condition (4) is a comparative exarnpie in that the heating temperature is 700 °C, which is lower than that of the HIP treatment of the present invention, failing to satisfy the conditions of the present invention. Furthermore, press condition (5) is a comparative example in which no press is conducted. The washer-shaped products were retained at 835 °C for 20 minutes, quenched by oil quenching, and then tempered at 170 °C for 90 minutes to attain a desired hardness of 58 HRC or higher. Furthermore, the products were polished to finish thrust-type roliing bearings, which were evaluated for roliing contact fatigue life. A commercially availabie bail for a thrust-type rolling bearlng was used as the roliing element.

[0041] [Table 5] Press Conditions Condition Heating Press Remarks Temperature Pressure (°C) (MP8) (1) 780 80 Working (2) 800 100 Examples (3) 1100 150 (4) 700 80 Comparative (5) No press was conducted. Examples

[0042] Thrust-»type roiling fatigue test was conducted 10 times on each press condition as shown above, with the maximum Hertz stress Prnax being 5292MPa. From the results, the number of cycie was measured until 10 % of the specimens cause flaking by counting from the short life side on the basis of Weibull distribution function, and named this number L1o iife.

The surface hardness of these specimens after quenching and tempering and the Lm life calculated from the iives of the ten specirnens of each condition to which thrust~type roliing fatigue test was conducted are shown in Table 6 for steel A and Table 7 for steel B. For each specimen, the test was stopped at a time when reaching lx108 cycles for test convenience, even if no 15 flaking occurred.

[0043] [Tabie 6] Process Press Surface Lw life Condition Condition l-iardness (x106 cycles) (HRC) 1 (1) 58.2 100* (2) 59.1 100* (3) 60.3 100* (4) 59.9 37.9 (5) 61.3 23.1 2 (1) 59.6 100* (2) 60.4 100* (3) 58.0 100* (4) 59.8 28.4 (5) 60.4 11.3 3 (1) 59.7 100* (2) 62.4 100* (3) 60.9 100* (4) 58.2 30.1 (5) 60.6 10.9 *z This symbol means that no flaking occurred in 1x108 cycles.

[0044] [Table 7] Process Press Surface Lm life Condition Condition Hardness (x106 cycles) (HRC) 1 (1) 59.7 16.4 (2) 60.1 80.4 (3) 58.3 100* (4) 61.4 10.9 (5) 60.3 7.3 2 (1) 58.0 19.3 (2) 59.1 65.3 (3) 59.9 100* (4) 58.3 10.4 (5) 61.2 6.2 3 (1) 61.4 10.9 (2) 59.8 77.9 (3) 58.1 100* (4) 60.9 9.9 (5) 62.1 5.4 *z This symbol means that no flaking occurred in 1x1O8 cycles.

[0045] Steel A in Table 6 and steel B in Table 7 satisfying the configuration of the present invention result in a surface 10 15 20 25 30 16 hardness of 58 HRC or more. Press conditions (1) to (3) satisfying the heating temperature condition and the hycirostatic pressure condition of the present invention are superior in roiiing contact fatigue iife, compared with press conditions (4) and (5), which are comparative examples not satisfying the conditions of the present invention. Furthermore, steels A and B are superior to steel C in that it is possible to expand the ranges of the conditions when applying an optimum hydrostatic pressure under press conditions (1) to (3), which satisfy the conditions of the present invention.

Exampie 3

[0046] The conditions for practicing the third embodiment and results obtained thereby will be specificaily explained. First of all, compositions of specimens are shown in Tabie 8. Steels A and B of this specimen are both based on SUJZ steel, which satisfies the composition of JIS G 4805. Moiten steel was subjected to an oxidation refining in an ark melting furnace, a reduction refining in a iadie refining furnace (LF), a rotary-flow degassing treatment in a rotary~flow vacuum degasser (RH treatment), and a continuous casting to cast a steel. The steel ingot was hot-roiled to produce a steel material. Steel A of the specimen has been basically deoxidized with Al, and added with Ca after the LF compieted. Steei B has been deoxidized with Al as conventionaliy conducted. The steel material obtained through the hot roiling was subjected to a spheroidized annealing at 800 °C.

[0047] [Table 8] Specimen Components (wtP/n except that Ca and O refer to ppm) Steel C Si Mn S Cr Mo Ai Ca 0 A 1.01 0.22 0.35 0.008 1.42 0.03 0.015 9 8 B 1.04 0.22 0.32 0.007 1.44 0.03 0.011 - 6

[0048] Furthermore, the above spheroidized annealed steel material was processed in accordance with any one of conditions 1 to 3 as shown below.

Process Condition 1: The steel material was cut to the shape of a bearing washer, which is a member of a thrust-type roiling 10 15 20 25 17 beaflng.

Process Condition 2: The steel material was heated to 600 °C, which is a warm condition in the range of from room temperature to recrystallization temperature, to be upset, and then cut to the shape of a bearing washer, which is a member of a thrust-type rolling bearing.

Process Condition 3: The steei material was cold-upset, and then cut to the shape of a hearing washer, which is a member of a thrust~type rolling hearing.

[0049] The washer~shaped products respectively subjected to a hot isostatic press (HIP) treatment or a hot press treatment. The treatment conditions are shown in Table 9.

Press condition (1) relates to a hot press treatment, while press conditions (2) to (4) reiates to a HIP treatment. Press conditions (1) to (3) are working examples satisfying the heating temperature condition and the hydrostatic pressure condition of the present invehtion. In contrast, press condition (4) is a comparative example in that the heating temperature is 700 °C, which is lower than that of the 'HIP treatment of the present invention, faiiing to satisfy the conditions of the present invention. Furthermore, press condition (5) is a comparative example in which no press is conducted. The washer~shaped products were retained at 835 °C for 20 minutes, quenched by oil quenching, and then tempered at 170 °C for 90 minutes to attain a desired hardness of 58 HRC or higher. Furthermore, the products were polished to finish thrust-type rolling bearings, which were evaiuated for roliing contact fatigue iife. A commercially avaiiable ball for a thrust-type roliing hearing was used as the rolling eiement.

Were 10 15 18

[0050] [Table 9] Press Conditions Condition Heating Press Rernarks Temperature Pressure (°C) (MPa) (1) 780 80 Working (2) 800 100 Examples (3) 1100 150 (4) 700 80 Comparative (5) No press was conducted. Examples

[0051] Thrust~type roiling fatigue test was conducted 10 times on each press condition as shown above, with the maximum Hertz stress Pmax being 5292MPa. From the results, the number of cycle was measured until 10 % of the specimens cause fiaking by counting from the short life side on the basis of Weibull distribution function, and nameci this number Lm life.

The surface hardness of these specimens after quenching and tempering and the Lw life calculated from the lives of the ten specimens of each condition to which thrust-type roiling fatigue test was conducted are shown in Table 10 for steei A and Table 11 for steel B. For each specimen, the test was stopped at a time when reaching 1x1O8 cycles for test convenience, even if no fiaking occurred.

[0052] [Table 10] 19 Process Press Surface Lw iife Condition Condition Hardness (x106 cycles) (HRC) 1 (1) 58.9 100* (2) 58.1 100* (3) 60.9 100* (4) 58.0 29.1 (5) 62.1 18.6 2 (1) 58.0 100* (2) 61.3 100* (3) 58.9 100* (4) 61.8 22.3 (5) 60.1 9.6 3 (1) 60.4 100* (2) 61.3 100* (3) 58.1 100* (4) 59.9 34.3 (5) 62.4 14.3 *: This symbol means that no fiaking occurred in [oo53][Tabæ 11] 1x108 cycles.

Process Press Surface Lm iife Condition Condition Hardness (x106 cycles) (HRC) 1 (1) 59.7 16.4 (2) 60.1 80.4 (3) 58.3 100* (4) 61.4 10.9 (5) 60.3 7.3 2 (1) 58.0 19.3 (2) 59.1 65.3 (3) 59.9 100* (4) 58.3 10.4 (5) 61.2 6.2 3 (i) 61.4 10.9 (2) 59.8 77.9 (3) 58.1 100* (4) 60.9 9.9 (5) 62.1 5.4 *z This symboi means that no fiakâng occurred in 1x108 cycles.

[0054] In Table 10, steel A satisfying the configuration of the present invention resuitsin a surface harciness of 58 HRC or more. Press conditions (1) to (3) satisfying the heating 20 temperature condition and the hydrostatic pressure condition of the present invention are superior in rolling contact fatigue life, compared with press conditions (4) and (5), which are comparative examples not satisfying the conditions of the present invention. Furthermore, steel A is superior to steel B shown in Table 11 in that it is possible to expand the ranges of the conditions when applying an Optimum hydrostatic pressure under press conditions (1) to (3), which satisfy the conditions of the present invention.

Claims (7)

21 CLAIMS

1. A method for producing a machine component having a surface hardness of 58 HRC or more and a superior rolling contact fatigue life by subjecting a part or the whole of a machine structural steel to a quenching and tempering treatment, the method comprisâng the steps of: subjecting the machine structural steei to a step for providing a shape as a steel material or a subsequent step for providing a shape as a machine component, wherein the steel is subjected to a plastic working; heating the steel, to which the plastic working has been conducted, to 780 °C or higher to apply a hydrostatic pressure of 80 MPa or more, whereby bringing the non-metallic inclusion contained in the steel and the steel as a matrix into close contact with each other in an interface; and thereafter subjecting a part or the whole of the steel to a quenching and tempering treatment.

2. The method according to claim 1, wherein the heating is conducted at 800 °C or higher, and wherein the hydrostatic pressure is 100 MPa or more.

3. The method according to claim 1 or 2, wherein the machine structural steei to be subjected to the plastic working has been deoxidized by adding a deoxidization agent comtaining Si in addition to generally-used Al or by not adding a deoxidization agent containing Al.

4. The method according to claim 1 or 2, wherein the machine structural steel to be subjected to the plastic working has been deoxidized by adding a deoxidizatíon agent containing Ca in addition to generally-used Al.

5. The method according to any one of clairns 1 to 4, wherein the plastic working was conducted a piurality of times, 22 wherein the last piastic working in the piuraiity of times is a hot piastic working.

6. The method according to any one of ciaims 1 to 4, wherein the piastic working was conducted a pluraiity of times, wherein the iast piastic working in the piurality of times is a Warm piastic working.

7. The method according to any one of claims 1 to 4, wherein the piastic working was conducted a plurality of times, wherein the last piastic working in the piurality of times is a coid piastic working.