WO2009145168A1 - Manufacturing method for machine parts having superior rolling-contact fatigue life - Google Patents

Abstract

Provided is a manufacturing method for machine parts having a surface hardness of at least 58 HRC, in which a superior stable rolling-contact fatigue life is achieved even if there is no reduction in nonmetallic inclusions or decrease in size thereof when the steel is manufactured, compared to steel material for which there is a reduction in nonmetallic inclusions and a decrease in size thereof when the steel is manufactured. In addition to a process for imparting to steel for mechanical structures the steel shape or a process for imparting the subsequent machine part shape, the method includes a process in which the steel is worked in the previous process, the steel is heated to at least 780°C, and a hydrostatic pressure of at least 80 MPa is applied to create very close contact at the interface between the nonmetallic inclusions in the steel and the steel, which is the parent phase, after which some or all of the steel is quenched and tempered.

Description

Manufacturing method of machine parts with excellent rolling fatigue life Cross-reference of related applications

This application includes Japanese Patent Application No. 2008-138774 filed on May 27, 2008, Japanese Patent Application No. 2008-138775 filed on May 27, 2008, and May 27, 2008. It claims the priority of the Japanese Patent Application No. 2008-138776 filed, the entire disclosure content of which is incorporated herein by reference.

The present invention requires a rolling fatigue life in which non-metallic inclusions and cavities such as bearings, gears, hub units, continuously variable transmissions, constant velocity joints, piston pins, etc. are damaged, and has a surface hardness of 58 HRC or more. The present invention relates to a method for manufacturing a machine part made of a steel material that is used after being hardened.

In recent years, with the improvement in performance of various mechanical devices, the use environment of mechanical parts and devices that require a rolling fatigue life has become extremely severe, and there is a strong demand for improved life and improved reliability. In response to such demands, as countermeasures from the aspect of steel materials, optimization of steel components and reduction of impurity elements are performed.

Among the impurity elements of steel components, oxygen, nitrogen, and sulfur are known to be particularly harmful because they form non-metallic inclusions such as Al ₂ O ₃ , MnS, and TiN, respectively, and cause damage to steel parts. Yes. Furthermore, it is known that the rolling fatigue life of a steel part becomes shorter as the diameter of the nonmetallic inclusion is larger. For this reason, various types of high cleanliness steels having a small amount of nonmetallic inclusions, that is, high cleanliness of the steel and extremely large oxide nonmetallic inclusions having an inclusion diameter of 20 μm or more have been proposed (for example, special features (See Kaikai 2006-63402 and JP-A-6-192790).

Even if such a steel material made of high cleanliness steel is used, it is not possible to sufficiently prevent breakage with a short life. For this reason, development has been actively conducted to reduce non-metallic inclusions in steel materials and to further reduce the diameter of the non-metallic inclusions.

The present inventors have now eliminated the gap between the nonmetallic inclusions in the steel and the parent steel, without reducing the nonmetallic inclusions and reducing the diameter of the nonmetallic inclusions during the production of the steel. It was found that by using the steel material in the state, the surface hardness of the steel material is 58 HRC or more, and peeling can be suppressed and a machine part excellent in rolling fatigue life can be obtained.

That is, in order to improve the rolling fatigue life of bearings and other machine parts, it is important to reduce non-metallic inclusions from these machine part steels. Furthermore, if there are large non-metallic inclusions under the rolling surface of bearings and other machine parts, the machine parts will be peeled off, leading to damage. It is known that reducing non-metallic inclusions appearing at the site is particularly important for improving the life of bearings and other machine parts. Thus, many measures have been invented to reduce the diameter of nonmetallic inclusions in mass production processes, but it has been difficult to stably reduce the diameter of nonmetallic inclusions.

The present inventors diligently examined the process leading to breakage in rolling fatigue, that is, peeling, by observing cracks using an artificial defect material. In the process of crack initiation and debonding from non-metallic inclusions, it undergoes an initial crack (hereinafter referred to as “opening-type initial crack”) process in which the crack is displaced by the stress concentration effect around the non-metallic inclusions. I found out. After that, it is a conventional knowledge that the crack is propagated by the propagation of the crack due to the shear stress. This means that if the opening-type initial crack found by the present inventors does not occur, subsequent crack propagation and breakage will not occur. In addition, the opening-type initial crack occurs on the premise that there is a physical gap, that is, a cavity, at the interface between the nonmetallic inclusion and the parent phase. If there is no physical gap, the opening-type crack It has also been verified by stress calculation (Iron and Steel, 94 (2008), p. 13 and 2008 Hyogo Prefectural University Doctoral Dissertation, Kazuhiko Hiraoka (2008 1 Month)).

Furthermore, the physical gap is any plastic processing that is always performed in the manufacturing process of steel materials and in the process of forming into members, that is, hot rolling, cold rolling, hot forging, warm forging, cold forging, rolling. It has also been found that it is caused by forging, cold rolling, cold header processing and drawing. FIG. 1 is a conceptual diagram showing an image obtained by observing the presence or absence of cavities around a nonmetallic inclusion 1 with a scanning electron microscope (FE-SEM) after being cut out from a hot rolled steel material and subjected to ion milling. In FIG. 1, reference numeral 2 is Al ₂ O ₃ and reference numeral 3 is a cavity. In particular, in machine structural steel, deoxidation with Al is usually performed. It has been confirmed that the Al ₂ O ₃ -based non-metallic inclusions generated at that time tend to generate cavities, particularly at the interface with the parent phase, due to the difference in deformability and shape from the parent material. The present invention has been made based on the above newly obtained knowledge.

Therefore, the object of the present invention is to improve the interface state between the non-metallic inclusions contained in the steel and the steel as the parent phase without reducing the non-metallic inclusions and reducing the diameter during the production of the steel. To provide a method of manufacturing a machine part that can stably obtain a superior rolling fatigue life compared to a steel material in which non-metallic inclusions are reduced and the diameter thereof is reduced when steel is manufactured. It is in.

According to the present invention, a method of manufacturing a machine part having a surface hardness of 58 HRC or more excellent in rolling fatigue life by quenching and tempering a part or the whole of a structural structural steel,
Machine structural steel is subjected to a step for imparting a steel material shape or a subsequent step for imparting a machine part shape, and the steel is subjected to plastic working in the step,
The steel subjected to the plastic working is heated to 780 ° C. or more to give a hydrostatic pressure of 80 MPa or more, thereby bringing the interface between the nonmetallic inclusions contained in the steel and the steel as the parent phase into close contact. ,afterwards,
There is provided a method comprising the step of subjecting part or all of the steel to a quenching and tempering treatment.

According to a first preferred embodiment of the present invention, there is provided the above production method, wherein the heating is performed at 800 ° C. or higher and the hydrostatic pressure is 100 MPa or higher.

According to a second preferred embodiment of the present invention, the mechanical structural steel subjected to the plastic working is added with a deoxidizer containing Si in addition to normal Al, or a deoxidizer composed of Al is added. The above production method is provided without deoxidization.

According to a third preferred aspect of the present invention, the mechanical structural steel subjected to the plastic working is deoxidized by adding a deoxidizing agent containing Ca in addition to normal Al. Is provided.

According to the method for manufacturing a machine part of the present invention, it is a non-metallic inclusion and a parent phase contained in steel by some plastic working without reducing non-metallic inclusions and reducing the diameter when manufacturing a steel material. If the physical gaps or cavities generated at the interface with the steel can be eliminated and the interface consisting of these can be adhered, peeling due to rolling fatigue starting from non-metallic inclusions can be avoided. Life expectancy is expected to improve.

FIG. 5 is a conceptual diagram showing an image obtained by observing the presence or absence of cavities around non-metallic inclusions with a scanning electron microscope (FE-SEM) after cutting out a sample from hot-rolled steel and performing ion milling.

The steel for machine structure in the present invention widely includes steels required for machine parts such as bearings, gears, hub units, continuously variable transmissions, constant velocity joints, piston pins and the like. Specifically, as such machine structural steel, high carbon chromium bearing steel generally defined in JIS G 4805, carbon steel for mechanical structure defined in JIS G 4051, JIS G Steel steel for structural use (H steel) that guarantees the hardenability specified in 4052, alloy steel for machine structure specified in JIS G 4053, alloy steel pipe for machine structure specified in JIS G 3441, Carbon steel pipe for machine structure specified in JIS G 3445, Carbon steel for cold heading specified in JIS G 3507-1-Part 1: Wire, Cold specified in JIS G 3507-2 Carbon steel for forging-Part 2: Wire, alloy steel for cold forging specified in JIS G 3509-1-Part 1: Wire, specified in JIS G 3509-2 Alloy steels for cold heading-Part 2: Wires and their related foreign standard steels, as well as their respective component-similar steels and component-modified steels, are described in this specification by reference. Embedded in the book. The steel for machine structure in the present invention includes steel materials that satisfy the chemical components described in the JIS standard.

The numerical range (mass%) of the preferable composition of the steel for machine structure in the present invention is as follows.
Preferred range More preferred range More preferred range
C 0.10 to 1.10 0.95 to 1.10 0.95 to 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 to 1. 60 1.30 to 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
Remaining Fe and unavoidable impurities Fe and unavoidable impurities Fe and unavoidable impurities <br/> Remarks JIS G 4805 JIS G 4805
SUJ1 ~ 5 SUJ2
Inevitable impurities may also include Al and Ca as deoxidizers.

Such mechanical structural steels generally have 1) oxidation refining of molten steel by an arc melting furnace or converter, 2) reductive refining by a ladle refining furnace (LF), and 3) a reflux-type vacuum degassing apparatus (RH). ) Refluxing vacuum degassing treatment (RH treatment) by 4) Casting of steel ingot by continuous casting or general ingot and 5) Hot rolling or hot forging of steel ingot and cold rolling or cold rolling A steel material is manufactured through a plastic working process by pressure forging. The process for obtaining the steel material shape in the present invention refers to the above-described process, and the steel material shape refers to a shape steel, a steel bar, a pipe, a wire, a steel plate, and a steel strip.

The steel is then hot forged, sub-hot forged, warm forged, cold forged, rolling forged, cold rolled, cold header processed and drawn, sometimes drawn and cold header processed, as described above. A desired plastic member is formed by performing combined plastic working and, if necessary, heat treatment or turning for the purpose of softening or microstructure adjustment. The process for obtaining the machine part shape in the present invention refers to the above process.

In the present invention, the hot of the hot plastic working refers to a temperature higher than the recrystallization temperature of the steel, the warm of the hot plastic working refers to a temperature higher than the room temperature and lower than the recrystallization temperature, the cold of the cold plastic working. Refers to room temperature and its vicinity.

Generally, the steel subjected to plastic working is then subjected to overall quenching (sub-quenching), carburizing quenching, carbonitriding quenching, nitriding quenching, carburizing and nitrogen quenching, induction quenching, etc. to obtain a surface hardness of 58 HRC or more. The quenching and tempering treatment is applied according to the steel material and the application, and after finishing treatment such as polishing and grinding, the machine part targeted by the present invention is manufactured. The quenching and tempering treatment in the present invention refers to the above treatment.

However, in order to obtain the effect of the present invention, the mechanical parts are forcibly tempered at the interface between the non-metallic oxide inclusions and the matrix phase before obtaining a surface hardness of 58 HRC or higher by quenching and tempering the machine parts. It is necessary to go through a process for eliminating the existing cavities. As the means, a method that can apply a hydrostatic pressure of 80 MPa or higher after heating to 780 ° C. or higher is preferable. For example, a hot isostatic pressing (HIP) method, a hot pressing method, or a hot forging method with complete closure or complete sealing is preferable.

In hot forging, sub-hot forging, warm forging, cold forging, rolling forging, cold rolling, cold header processing, and drawing processing that are not completely sealed in the mold, all steel parts are subjected to hydrostatic pressure. Can not be imparted, or the material is continuously stretched in a certain direction, so the effects of the present invention cannot be obtained.

The reason for applying a hydrostatic pressure of 80 MPa or higher after heating to 780 ° C. or higher is as follows. That is, the higher the heating temperature of the steel material, the easier it is for the steel material to deform. Therefore, the higher the heating temperature of the steel material, the lower the hydrostatic pressure required to eliminate gaps or cavities existing at the interface between the oxide-based nonmetallic inclusions and the parent phase. As a result of intensive studies by the present inventors, the effect of the present invention can be obtained if heating is performed at 780 ° C. or higher and a hydrostatic pressure of 80 MPa or higher can be applied. In addition, according to the 1st preferable aspect of this invention, it is preferable that the said heating is performed at 800 degreeC or more, and the said hydrostatic pressure is 100 Mpa or more.

According to the second preferred embodiment of the present invention, the mechanical structural steel subjected to plastic working is added with a deoxidizer containing Si in addition to normal Al, or a deoxidizer composed of Al is added. Without deoxidation. Generally, mechanical structural steel is deoxidized with Al. For this purpose, the oxide-based non-metallic inclusions produced are mainly Al ₂ O ₃ -based. The Al ₂ O ₃ system is a hard inclusion, aggregates after refining, and easily takes the shape of type B defined in ASTM E 45. In order to completely eliminate the cavity existing at the interface between the system nonmetallic inclusion and the parent phase, the condition range at the time of applying the optimum hydrostatic pressure is limited. Therefore, if the form of the oxide-based nonmetallic inclusions is controlled, the effect of completely eliminating the cavities existing at the interface between the oxide-based nonmetallic inclusions and the parent phase when hydrostatic pressure is applied is Increase. As the means, oxide-based nonmetallic inclusions produced by adding a deoxidizing agent containing Si in addition to normal Al or by deoxidizing without adding a deoxidizing material made of Al. It is preferable to reduce the difference in deformability from the parent phase by softening.

According to the third preferred embodiment of the present invention, the steel for mechanical structure subjected to plastic working is deoxidized by adding a deoxidizer containing Ca in addition to normal Al. Generally, mechanical structural steel is deoxidized with Al. For this reason, the oxide-based nonmetallic inclusions produced are mainly Al ₂ O ₃ -based. The Al ₂ O ₃ system is a hard inclusion, aggregates after refining, and easily takes the shape of type B defined in ASTM E 45. In order to completely eliminate the cavity existing at the interface between the system nonmetallic inclusion and the parent phase, the condition range when applying the optimum hydrostatic pressure is limited. Therefore, if the form of oxide-based nonmetallic inclusions is controlled, the effect of completely eliminating cavities existing at the interface between the oxide-based nonmetallic inclusions and the parent phase when hydrostatic pressure is applied is Increase. As a means for this, by adding a deoxidizer containing Ca in addition to ordinary Al to deoxidize, the oxide-based non-metallic inclusions to be generated are shaped into the type D shape (granularity) defined in ASTM E45. It is preferable that a uniform hydrostatic pressure can be applied around the nonmetallic inclusions.

Needless to say, the present invention may be implemented by arbitrarily combining the first, second to third preferred embodiments described above.

The implementation conditions and the obtained effects of the first aspect of the present invention will be specifically described. First, Table 1 shows the component composition of the test materials. This sample material is based on SUJ2 steel, which is a steel satisfying the composition of JIS G 4805. Oxidation refining of molten steel in an arc melting furnace, reduction refining in a ladle refining furnace (LF), recirculation vacuum degassing treatment (RH treatment) in a recirculation type vacuum degassing device (RH), and continuous casting A steel ingot was cast, and the steel ingot was hot-rolled to produce a steel material. Next, spheroidizing annealing was performed at 800 ° C.

Furthermore, the above spheroidized steel was processed according to any one of the following process conditions 1 to 3.
Process condition 1: A steel material was cut into a washer shape which is a member of a thrust type rolling bearing.
Process condition 2: The steel material was heated to 600 ° C. at a temperature not lower than the room temperature and not higher than the recrystallization temperature, and after that, the steel material was cut into a bearing disc shape which is a member of a thrust type rolling bearing.
Process condition 3: After the steel material was cold upset, the steel material was cut into a bearing disc shape which is a member of a thrust type rolling bearing.

Each of the obtained washer-shaped products was subjected to hot isostatic pressing (HIP) treatment. Table 2 shows the processing conditions. Press condition A and press condition B satisfy the heating temperature condition and the hydrostatic pressure condition of the present invention. The pressing condition C and the pressing condition D are pressing conditions, and the pressing condition E does not perform the HIP treatment, and these do not satisfy the heating temperature condition and the hydrostatic pressure condition of the present invention. These press condition A and press condition B washer-shaped products were held at 835 ° C. for 20 minutes, then quenched by oil cooling, and then subjected to tempering at 170 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher. It was. Furthermore, it was ground and finished into a thrust type rolling bearing, and the rolling fatigue life was evaluated. The rolling element used was a commercially available thrust type rolling bearing ball.

The thrust type rolling fatigue test was performed at a maximum hertz stress Pmax of 5292 MPa, and was performed 10 times for each of the above pressing conditions. From the result, based on the Weibull distribution function, the total number of revolutions until the 10% specimen was peeled from the short life side was obtained, and this was defined as the L ₁₀ life. Table 3 shows the L ₁₀ life evaluated from the surface hardness after quenching and tempering and the life of 10 test pieces under each condition in which the thrust type rolling fatigue test was conducted. In addition, the test piece of each condition was stopped when it reached 1 × 10 ⁸ cycles for the convenience of the test even if it did not come off.

Incidentally, → is the L ₁₀ life of Table 3 at 1 × 10 ⁸ cycle, meaning that did not peel off. In Table 3, press conditions A and B satisfying the heating temperature condition and the hydrostatic pressure condition of the present invention have a surface hardness of 58 HRC or more. The pressing conditions C to E that do not satisfy the heating temperature condition and the hydrostatic pressure condition of the present invention have a surface hardness of 58 HRC or more. However, the invention examples of the press condition A and the press condition B according to the present invention are less in rolling fatigue than the comparative examples of the press conditions C to E regardless of whether the final is hot plastic working, warm plastic working, or cold plastic working. Life is much better.

The implementation conditions and the obtained effects of the second aspect of the present invention will be specifically described. First, Table 4 shows the component composition of the test material. Steel types A and B which are the test materials are all based on SUJ2 steel which is a steel satisfying the composition of JIS G 4805. Oxidation refining of molten steel in an arc melting furnace, reduction refining in a ladle refining furnace (LF), recirculation vacuum degassing treatment (RH treatment) in a recirculation type vacuum degassing device (RH), and continuous casting A steel ingot was cast, and the steel ingot was hot-rolled to produce a steel material. Steel type A of the test material was deoxidized with Si and Mn without adding Al during deoxidation, and 0.003% of Al shown in Table 4 is contained as an inevitable impurity. It is. Steel type B was generally deoxidized with Al. The steel material obtained by hot rolling was subjected to spheroidizing annealing at 800 ° C.

Furthermore, the above spheroidized steel was processed according to any one of the following process conditions 1 to 3.
Process condition 1: A steel material was cut into a washer shape which is a member of a thrust type rolling bearing.
Process condition 2: The steel material was heated to 600 ° C. at a temperature not lower than the room temperature and not higher than the recrystallization temperature, and then the steel material was cut into a bearing disk shape which is a member of a thrust type rolling bearing.
Process condition 3: After the steel material was cold upset, it was cut into the shape of a washer which is a member of a thrust type rolling bearing.

Each of the obtained washer-shaped products was subjected to hot isostatic pressing (HIP) processing or hot pressing processing. Table 5 shows the processing conditions. The pressing condition (1) is based on hot pressing, and the pressing conditions (2) to (4) are based on HIP processing. The pressing conditions (1) to (3) are examples of the present invention that satisfy the heating temperature condition and the hydrostatic pressure condition of the present invention. On the other hand, press condition (4) is a comparative example in which the heating temperature is 700 ° C. which is lower than the heating condition of the HIP treatment of the present invention and does not satisfy the conditions of the present invention. Furthermore, the pressing condition (5) is a comparative example without pressing. The washer-shaped product was held at 835 ° C. for 20 minutes, then quenched by oil cooling, and then tempered at 170 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher. Furthermore, it was ground and finished into a thrust type rolling bearing, and the rolling fatigue life was evaluated. The rolling element used was a commercially available thrust type rolling bearing ball.

The thrust type rolling fatigue test was performed at a maximum hertz stress Pmax of 5292 MPa, and was performed 10 times for each of the above pressing conditions. From the result, based on the Weibull distribution function, the total number of revolutions until the 10% specimen was peeled from the short life side was obtained, and this was defined as the L ₁₀ life. Further, Table ₁₀ shows the L ₁₀ life and Table 7 shows the steel ₁₀ for the L ₁₀ life evaluated based on the surface hardness after quenching and tempering and the life of 10 test pieces under the conditions of the thrust type rolling fatigue test. Show. Each test piece was stopped when it reached 1 × 10 ⁸ cycles for the sake of the test, even if it did not come off.

Steel type A which is a steel satisfying the constitution of the present invention in Table 6 and Steel type B in Table 7 have a surface hardness of 58 HRC or more, and press conditions (1) which satisfy the heating temperature condition and hydrostatic pressure condition of the present invention. (3) to (3) are excellent in rolling fatigue life as compared with conditions (4) and (5) which are comparative examples not satisfying the conditions of the present invention. Furthermore, steel type A and steel type B, compared with steel type C, can expand the range of conditions for applying the optimum hydrostatic pressure in conditions (1) to (3) of the press conditions that satisfy the conditions of the present invention. Are better.

The implementation conditions and the obtained effects of the third aspect of the present invention will be specifically described. First, Table 8 shows the component composition of the test material. Steel types A and B which are the test materials are all based on SUJ2 steel which is a steel satisfying the composition of JIS G 4805. Oxidation refining of molten steel in an arc melting furnace, reduction refining in a ladle refining furnace (LF), recirculation vacuum degassing treatment (RH treatment) in a recirculation type vacuum degassing device (RH), and continuous casting A steel ingot was cast, and the steel ingot was hot-rolled to produce a steel material. In addition, the steel type A of the test material was based on Al deoxidation, and Ca was added after the end of LF. Steel type B was generally deoxidized with Al. The steel material obtained by hot rolling was subjected to spheroidizing annealing at 800 ° C.

Furthermore, the above spheroidized steel was processed according to any one of the following process conditions 1 to 3.
Process condition 1: A steel material was cut into a washer shape which is a member of a thrust type rolling bearing.
Process condition 2: The steel material was heated to 600 ° C. at a temperature not lower than the room temperature and not higher than the recrystallization temperature, and then the steel material was cut into a bearing disk shape which is a member of a thrust type rolling bearing.
Process condition 3: After the steel material was cold upset, it was cut into the shape of a washer which is a member of a thrust type rolling bearing.

Each of the obtained washer-shaped products was subjected to hot isostatic pressing (HIP) processing or hot pressing processing. Table 9 shows the processing conditions. The pressing condition (1) is based on hot pressing, and the pressing conditions (2) to (4) are based on HIP processing. The pressing conditions (1) to (3) are examples of the present invention that satisfy the heating temperature condition and the hydrostatic pressure condition in the present invention. On the other hand, press condition (4) is a comparative example in which the heating temperature is 700 ° C. which is lower than the heating condition of the HIP treatment of the present invention and does not satisfy the conditions of the present invention. Furthermore, the pressing condition (5) is a comparative example without pressing. The washer-shaped product was held at 835 ° C. for 20 minutes, then quenched by oil cooling, and then tempered at 170 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher. Furthermore, it was ground and finished into a thrust type rolling bearing, and the rolling fatigue life was evaluated. The rolling element used was a commercially available thrust type rolling bearing ball.

The thrust type rolling fatigue test was performed at a maximum Hertz stress Pmax of 5292 MPa, and was performed 10 times for each of the above pressing conditions. From the result, based on the Weibull distribution function, the total number of revolutions until the 10% specimen was peeled from the short life side was obtained, and this was defined as the L ₁₀ life. Further, Table 10 shows steel L and Table 11 shows L ₁₀ life evaluated based on the surface hardness after quenching and tempering and the life of 10 test pieces under the respective conditions of the thrust type rolling fatigue test. Show. Each test piece was stopped when it reached 1 × 10 ⁸ cycles for the convenience of the test even if it did not come off.

In Table 10, steel type A, which is a steel that satisfies the constitution of the present invention, has a surface hardness of 58 HRC or more, and pressing conditions (1) to (3) that satisfy the heating temperature condition and hydrostatic pressure condition of the present invention are as follows. Compared with press conditions (4) and (5) which are comparative examples not satisfying the conditions of the present invention, the rolling fatigue life is excellent. Furthermore, compared with steel type B in Table 11, steel type A can expand the range of conditions for applying the optimum hydrostatic pressure in the pressing conditions (1) to (3) that satisfy the conditions of the present invention. Is excellent.

Claims (7)

1. A method of manufacturing a machine part having a surface hardness of 58 HRC or more excellent in rolling fatigue life by quenching and tempering a part or the whole of a machine structural steel,
   Machine structural steel is subjected to a step for imparting a steel material shape or a subsequent step for imparting a machine part shape, and the steel is subjected to plastic working in the step,
   The steel subjected to the plastic working is heated to 780 ° C. or more to give a hydrostatic pressure of 80 MPa or more, thereby bringing the interface between the nonmetallic inclusions contained in the steel and the steel as the parent phase into close contact. ,afterwards,
   A method comprising a step of subjecting part or all of the steel to a quenching and tempering treatment.
2. The method according to claim 1, wherein the heating is performed at 800 ° C or more and the hydrostatic pressure is 100 MPa or more.
3. The steel for mechanical structure subjected to the plastic working is deoxidized by adding a deoxidizer containing Si in addition to normal Al or without adding a deoxidizer composed of Al. The method according to claim 1 or 2.
4. The method according to claim 1 or 2, wherein the mechanical structural steel subjected to the plastic working is deoxidized by adding a deoxidizer containing Ca in addition to normal Al.
5. The method according to any one of claims 1 to 4, wherein the plastic working is performed a plurality of times, and the last plastic working in the plurality of times is a hot plastic working.
6. The method according to any one of claims 1 to 4, wherein the plastic working is performed a plurality of times, and the last plastic working in the plurality of times is a warm plastic working.
7. The method according to any one of claims 1 to 4, wherein the plastic working is performed a plurality of times, and the last plastic working in the plurality of times is a cold plastic working.

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