WO2012111527A1

WO2012111527A1 - Steel part, single-cylinder internal combustion engine, saddled vehicle, and process for manufacture of steel part

Info

Publication number: WO2012111527A1
Application number: PCT/JP2012/052990
Authority: WO
Inventors: 久保田　剛; 博文河田
Original assignee: ヤマハ発動機株式会社
Priority date: 2011-02-14
Filing date: 2012-02-09
Publication date: 2012-08-23
Also published as: JP5603889B2; JP2012184502A; MY185935A

Abstract

This steel part has a surface that is in contact with a rolling bearing. In the steel part, the amount of retained austenite is 50 vol% or more and the Vickers hardness (HV) is 710 or more at a depth of 0.1 mm from the surface. The present invention provides: a steel part in which the occurrence of flaking on a surface thereof that is in contact with a rolling bearing can be prevented and which has an excellent flaking life; and a process for manufacturing the steel part.

Description

Steel part, single-cylinder internal combustion engine, straddle-type vehicle, and method for manufacturing steel part

The present invention relates to a steel part and a manufacturing method thereof, and more particularly to a steel part having a surface in contact with a rolling bearing and a manufacturing method thereof. The present invention also relates to a single-cylinder internal combustion engine or a straddle-type vehicle provided with such steel parts.

In an internal combustion engine, a member called a connecting rod (sometimes abbreviated as “con'rod”) is used to connect a piston and a crankshaft. The connecting rod includes a rod-shaped rod main body, a small end provided at one end of the rod main body, and a large end provided at the other end of the rod main body. The small end is connected to the piston, while the large end is connected to the crankshaft. More specifically, the piston pin of the piston is inserted through the through hole formed in the small end. A crank pin of the crankshaft is inserted through a through hole formed at the large end. As a result, the connecting rod is connected to the piston and the crankshaft.

¡Connecting rods are roughly divided into a split type with the large end divided into two and an integrated type with the large end not split. The integral connecting rod is mainly used for a single cylinder internal combustion engine.

¡Rolling bearings such as needle bearings and ball bearings are arranged between the inner peripheral surface of the large end portion of the integrated connecting rod and the crank pin in order to reduce friction loss. The explosive force transmitted through the piston during operation of the internal combustion engine presses the connecting rod against the rolling bearing, so that a large stress is generated on the inner peripheral surface of the large end. When this stress is excessive, a fatigue fracture phenomenon called flaking occurs on the inner peripheral surface of the large end.

When flaking occurs in the connecting rod, smooth rotation of the internal combustion engine is impeded, and unpleasant sound and vibration are generated in the internal combustion engine and the vehicle, thereby impairing commerciality and comfort. Therefore, it is required that no flaking occurs in the connecting rod.

Conventionally, in order to suppress the occurrence of flaking and achieve a long life, it is generally performed to carburize a connecting rod formed from case-hardened steel (for example, JIS SCM420). By infiltrating carbon from the surface of the connecting rod by carburizing, the carbon concentration in the vicinity of the surface is increased. Therefore, the surface hardness is increased after quenching, thereby suppressing the occurrence of flaking.

Patent Document 1 proposes high-concentration carburizing treatment as a technique for further increasing the surface hardness of the connecting rod. In this technique, carburization is performed in an atmosphere having a carbon potential (CP) of 0.8% or more. Thereby, fine granular carbide precipitates in the vicinity of the surface of the connecting rod, and the crystal grain size of the martensite structure in the vicinity of the surface becomes small. Therefore, the surface hardness is remarkably increased, and the fatigue strength can be further improved. Patent Document 1 also mentions high-concentration carbonitriding as a technique for increasing the surface hardness of the connecting rod, as in high-concentration carburizing.

JP 2000-313949 A

However, in recent years, when the performance of an internal combustion engine has been improved, flaking is not possible in a connecting rod that has been subjected to general carburizing treatment, high-concentration carburizing treatment, or high-concentration carbonitriding treatment as disclosed in Patent Document 1. Has occurred in a relatively short time. For this reason, the flaking life hinders the improvement of the performance of the internal combustion engine, and in order to further improve the performance of the internal combustion engine, it is essential to increase the flaking life (that is, to extend the life of the connecting rod). Further, an increase in flaking life is demanded not only for connecting rods but also for other steel parts (for example, crank pins) having a surface in contact with a rolling bearing.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a steel part having excellent flaking life and a method for producing the same, in which the occurrence of flaking on the surface in contact with the rolling bearing is suppressed. It is in.

A steel part according to the present invention is a steel part having a surface in contact with a rolling bearing, and at a depth of 0.1 mm from the surface, the amount of retained austenite is 50 vol% or more, and the Vickers hardness HV is 710. That's it.

In a preferred embodiment, the carbon content at a depth of 0.1 mm from the surface is 1.1 wt% or more and less than 2.2 wt%.

In a preferred embodiment, the carbon content at a depth of 0.1 mm from the surface is 1.6 wt% or more and 2.0 wt% or less.

In a preferred embodiment, the crystal grain size of the steel structure at a depth of 0.1 mm from the surface is 9 μm or less.

In a preferred embodiment, the nitrogen content at a depth of 0.1 mm from the surface is 0.03 wt% or more and 0.19 wt% or less.

In a preferred embodiment, the steel part according to the present invention is carbonitrided or carburized and nitrided.

In a preferred embodiment, the carbide and carbonitride precipitated in the vicinity of the surface have a particle size of 10 μm or less.

In a preferred embodiment, the steel part according to the invention is a connecting rod.

In a preferred embodiment, the steel part according to the present invention, which is a connecting rod, is provided at the rod main body, a small end provided at one end of the rod main body, and the other end of the rod main body. A large end portion, and the inner peripheral surface of the large end portion is the surface in contact with the rolling bearing.

In a preferred embodiment, the steel part according to the invention is a crankpin.

An internal combustion engine according to the present invention includes a steel part having the above-described configuration.

In a preferred embodiment, the internal combustion engine according to the present invention further includes a rolling bearing provided in contact with the surface.

A motor vehicle according to the present invention includes an internal combustion engine having the above-described configuration.

A method for manufacturing a steel part according to the present invention is a method for manufacturing a steel part having a surface in contact with a rolling bearing, the step (A) of preparing a workpiece formed from steel, and the workpiece Carburizing treatment is performed in an atmosphere having a carbon potential of 1.1% or more, followed by nitriding treatment, or carbonitriding treatment in an atmosphere having a carbon potential of 1.1% or more with respect to the workpiece. and the applying step (B), after the step (B), using a quenching oil having a quenching intense size of less than 0.07 cm ^-1 or 0.11 cm ^-1, 5 kPa to ambient pressure on the surface of the quenching oil A step (C) of quenching the workpiece while controlling the pressure to 60 kPa or less.

In a preferred embodiment, the carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of less than 2.2%.

In a preferred embodiment, the carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of 1.6% or more and 2.0% or less.

In a preferred embodiment, the carburizing process or the carbonitriding process in the step (B) is performed under an atmosphere having a carbon potential of 1.4% or more, and the step (C) is performed by using the quenching oil. It is executed while controlling the atmospheric pressure on the surface to 50 kPa or more and 60 kPa or less.

In a preferred embodiment, the step (B) and the step (C) are performed in a carburizing furnace provided with a decompression mechanism capable of decompressing at least a space on the surface of the quenching oil.

In a preferred embodiment, the carburizing furnace further includes a heating chamber in which the step (B) is performed, and a cooling chamber in which the step (C) is performed. Is a vacuum carburizing furnace that can depressurize both the inside of the heating chamber and the inside of the cooling chamber.

In a preferred embodiment, the introduction of hydrocarbons into the heating chamber is stopped a plurality of times during the carburizing process or the carbonitriding process in the step (B).

In a preferred embodiment, introduction of hydrocarbons into the heating chamber is stopped four or five times during the carburizing process or the carbonitriding process in the step (B).

In a preferred embodiment, in the step (B), a refining step in which neither hydrocarbon nor ammonia is introduced into the heating chamber is performed between the carburizing treatment and the nitriding treatment.

In a preferred embodiment, the workpiece prepared in the step (A) includes 0.1 wt% or more and 0.4 wt% or less of carbon, 0.1 wt% or more and 0.5 wt% or less of silicon and 0.3 wt%. % To 1.2 wt% chromium.

Hereinafter, the operation of the present invention will be described.

In a steel part having a surface in contact with the rolling bearing, the stress is maximum at a depth of about 0.1 mm from the surface, not the outermost surface. Therefore, the material characteristics at that depth have a great influence on the flaking resistance. In the steel part according to the present invention, the amount of retained austenite and the Vickers hardness HV at a depth of 0.1 mm from the surface are set in a specific range. Specifically, in the steel part according to the present invention, the amount of retained austenite at a depth of 0.1 mm from the surface is 50 vol% or more, and the Vickers hardness HV at the depth is 710 or more. Thereby, the anti-flaking property is remarkably improved, and the occurrence of flaking can be prevented over a long period of time.

The carbon content at a depth of 0.1 mm from the surface is preferably 1.1 wt% or more and less than 2.2 wt%, and more preferably 1.6 wt% or more and 2.0 wt% or less. In steel parts manufactured under conditions such that the carbon content is less than 1.1 wt%, the amount of retained austenite may not be sufficiently increased. Moreover, the precipitation amount of a carbide | carbonized_material may decrease and hardness may fall. On the other hand, in steel parts manufactured under conditions such that the carbon content is 2.2% or more, the amount of precipitated carbide is excessively increased and the toughness may be reduced.

From the viewpoint of further prolonging the life, it is preferable that the crystal grain size of the steel structure at a depth of 0.1 mm from the surface is 9 μm or less.

From the viewpoint of further improving the flaking resistance, the nitrogen content at a depth of 0.1 mm from the surface is preferably 0.03 wt% or more and 0.19 wt% or less.

The steel part according to the present invention is typically carbonitrided or carburized and nitrided to improve fatigue strength. In that case, the particle sizes of the carbides and carbonitrides precipitated in the vicinity of the surface are preferably as small as possible, specifically 10 μm or less. When the particle size of the carbide and carbonitride exceeds 10 μm, the toughness may be lowered and sufficient strength may not be obtained.

The steel part according to the present invention is, for example, a connecting rod. The connecting rod includes a rod body part, a small end part provided at one end of the rod body part, and a large end part provided at the other end of the rod body part. In the connecting rod, the inner peripheral surface of the large end is in contact with the rolling bearing. According to the present invention, since the flaking life is improved, it is possible to apply a higher load to the steel part than before. Therefore, when the steel part according to the present invention is a connecting rod, it is possible to reduce the size of the large end and reduce the weight.

Of course, the steel part according to the present invention may be a part other than the connecting rod, for example, a crankpin. The outer peripheral surface of the crank pin is in contact with the rolling bearing.

The steel parts (for example, connecting rods and crank pins) according to the present invention are suitably used for internal combustion engines. In an internal combustion engine with a specification in which reduction of friction loss is important (for example, a single-cylinder internal combustion engine having one cylinder), generally the inner peripheral surface of the connecting rod and the outer peripheral surface of the crank pin Are provided with a rolling bearing (for example, a needle bearing or a ball bearing). When a rolling bearing is provided, stress is generated on the inner peripheral surface of the large end portion of the connecting rod and the outer peripheral surface of the crank pin when the connecting rod and the crank pin are pressed against the rolling bearing. If this stress is excessive, the occurrence of flaking is a concern. However, according to the present invention, since the anti-flaking property is improved, the occurrence of flaking is prevented for a long period of time.

The internal combustion engine provided with the steel parts according to the present invention is suitably used for various types of motor vehicles (for example, motorcycles). According to the invention, the flaking life is improved, which makes it possible to reduce the weight of the steel part (as already described for the case of the connecting rod). Therefore, since the internal combustion engine and the vehicle body of the motor vehicle can be reduced in weight, the traveling stability, ease of riding, ease of handling, etc. of the motor vehicle are improved, and the merchantability is improved.

In the method for manufacturing a steel part according to the present invention, a workpiece formed from steel is subjected to a carburizing process and a nitriding process, or a carbonitriding process is performed (step (B)). Carbonitriding (or carburizing and nitriding) increases the surface hardness of steel parts and improves fatigue strength. In the production method according to the present invention, the carbonitriding process and the carburizing process are performed in an atmosphere having a carbon potential (CP) of 1.1% or more. That is, a high concentration carbonitriding process or a high concentration carburizing process is performed on the workpiece. According to the high-concentration carbonitriding process or the high-concentration carburizing process, fine granular carbides and / or carbonitrides are precipitated in the vicinity of the surface of the workpiece, and the crystal grain size of the martensite structure in the vicinity of the surface is reduced. Therefore, the surface hardness is remarkably increased, and the effect of improving the fatigue strength is high.

Furthermore, in the manufacturing method according to the invention, after the above step (B), using a quenching oil having 0.07 cm ^-1 or 0.11 cm ^-1 of less than quenching intense degree (H value), on the surface of the quenching oil The step (C) of quenching the workpiece while controlling the atmospheric pressure at 5 kPa to 60 kPa is performed. When quenching is performed under reduced pressure using quenching oil having a H value of less than 0.11 cm ⁻¹ (so-called hot quench oil), the boiling point of the quenching oil is lowered, so that the workpiece becomes a vapor film of quenching oil at the initial stage of quenching. The time (vapor film stage) covered with (acting heat insulation) becomes longer. Therefore, the cooling rate at the initial stage of quenching becomes slow. However, on the other hand, since the cooling rate after the vapor film breakage is increased, the amount of retained austenite can be increased without affecting the martensitic transformation itself. In order to make the vapor film stage sufficiently long, the atmospheric pressure on the surface of the quenching oil is preferably 60 kPa or less. Hot quench oil has a low H value at normal pressure, but the convection stage start temperature decreases due to the reduced pressure, which improves hardenability (coolability in the convection stage) and increases the effective H value. can do. Therefore, the hardness in the vicinity of the surface of the steel part can be sufficiently increased. In contrast, a quenching oil (so-called cold quench oil) having a quenching intensity (H value) of 0.11 cm ⁻¹ or more generally contains an additive for shortening the vapor film stage. When such quenching oil (cold quench oil) is used, the vapor film stage cannot be made sufficiently long even if the pressure is reduced, and the amount of retained austenite cannot be increased. The quenching intensity (H value) of the quenching oil used in the step (C) is preferably 0.07 cm ⁻¹ or more. When the H value is less than 0.07 cm ⁻¹ , sufficient hardenability may not be obtained depending on the degree of decompression (the effective H value may not be sufficiently large). Furthermore, it is preferable to perform a process (C), controlling the atmospheric pressure on the surface of hardening oil to 5 kPa or more. When the atmospheric pressure is less than 5 kPa, the boiling point of the quenching oil becomes too low and the vapor film stage may become too long. Therefore, there is a possibility that hardenability is lowered and sufficiently high hardness cannot be obtained. Thus, according to the manufacturing method of the present invention, the amount of retained austenite structure that contributes to (good influence on) the anti-flaking property can be increased while maintaining the hardness of the steel part sufficiently high. it can. Therefore, the steel part manufactured by the manufacturing method according to the present invention has an excellent flaking life.

It is preferable that the carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of less than 2.2%. When the carbon potential is 2.2% or more, the amount of carbides precipitated becomes too large and the toughness may be lowered.

From the viewpoint of further prolonging the service life, it is preferable that the carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of 1.6% or more and 2.0% or less.

Further, from the viewpoint of further extending the life by reducing the crystal grain size of the steel structure in the vicinity of the surface (specifically to 9 μm or less), the carburizing treatment or carbonitriding treatment in the step (B) is 1 It is preferably performed in an atmosphere having a carbon potential of 4% or more, and the step (C) is performed while controlling the atmospheric pressure on the surface of the quenching oil to 50 kPa or more and 60 kPa or less.

Typically, step (B) and step (C) are performed in a carburizing furnace equipped with a pressure reducing mechanism capable of reducing the pressure on the surface of the quenching oil.

The carburizing furnace is, for example, a vacuum carburizing furnace. The vacuum carburizing furnace further includes a heating chamber in which the step (B) is executed inside, and a cooling chamber in which the step (C) is executed therein, and the inside of the heating chamber and the cooling chamber by the decompression mechanism. Both of the inside of the can be decompressed.

When using a vacuum carburizing furnace, it is preferable to stop introduction of hydrocarbons into the heating chamber a plurality of times during the carburizing process or the carbonitriding process in step (B). Thereby, the carbon potential can be adjusted with high accuracy, and the carbon content in the vicinity of the surface of the steel part can be controlled with high accuracy. For example, in order to accurately control the carbon content at a depth of 0.1 mm from the surface of the steel part to 1.6 wt% or more and 2.0 wt% or less, during the carburizing process or the carbonitriding process in the step (B) Furthermore, it is preferable to stop the introduction of hydrocarbons into the heating chamber four or five times.

In the step (B), when a refining step in which neither hydrocarbon nor ammonia is introduced into the inside of the heating chamber is performed between the carburizing treatment and the nitriding treatment, the particle size of the carbide precipitated near the surface is changed. It becomes possible to make it even smaller.

The workpiece prepared in step (A) is 0.1 wt% or more and 0.4 wt% or less of carbon, 0.1 wt% or more and 0.5 wt% or less of silicon, and 0.3 wt% or more and 1.2 wt% or less of chromium. It is preferable that it is formed from the steel containing. When the carbon content is 0.1 wt% or more and 0.4 wt% or less, the internal hardness (Vickers hardness HV) at a position deeper than 0.1 mm from the surface of the steel part after heat treatment (quenching and tempering) Since it can be 200 or more and 500 or less, the strength and toughness inside the steel part can be kept sufficiently high. Further, when the silicon content is increased, the anti-flaking property is improved, but the toughness may be lowered. When the silicon content is 0.1 wt% or more and 0.5 wt% or less, the flaking resistance can be sufficiently improved and sufficient toughness can be ensured. Moreover, when the chromium content is increased, the hardenability is improved. However, if the chromium content is excessively large, temper embrittlement may occur. When the chromium content is 0.3 wt% or more and 1.2 wt% or less, the occurrence of temper embrittlement can be prevented while obtaining appropriate hardenability.

According to the present invention, it is possible to provide a steel part excellent in flaking life and a method for producing the same, by suppressing the occurrence of flaking on the surface in contact with the rolling bearing.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically the connecting rod 1 in preferable embodiment of this invention, (a) is a top view, (b) is sectional drawing along the 1B-1B 'line in (a), (c) FIG. 4 is a cross-sectional view taken along line 1C-1C ′ in FIG. It is a figure which shows the stress distribution (stress distribution when a stress becomes the maximum at the time of operation | movement of an internal combustion engine) in the depth direction in the internal peripheral surface of the large end part of a general connecting rod. It is a flowchart which shows the manufacturing method of the connecting rod 1 in suitable embodiment of this invention. It is a figure which shows the example of the process conditions in the one part process of the manufacturing method shown by FIG. It is a figure which shows typically the carburizing furnace (vacuum carburizing furnace) 50. FIG. It is a figure which shows the example of the process conditions in the one part process of the conventional manufacturing method. It is a graph which shows distribution of the carbon concentration (carbon content) in the depth direction of the connecting rod. It is a figure which shows the example of the process conditions in the one part process of the manufacturing method shown by FIG. It is a graph which shows the hardness distribution in the depth direction of the connecting rod. It is a graph which shows the amount distribution of retained austenite in the depth direction of the connecting rod. It is the graph which plotted the result which changed the amount of retained austenites and Vickers hardness HV, and verified the influence on flaking life, taking Vickers hardness HV on the horizontal axis and L50 life on the vertical axis. It is the graph which plotted the result which changed the amount of retained austenite and the Vickers hardness HV, and verified the influence on the flaking life, taking the amount of retained austenite on the horizontal axis and the Vickers hardness HV on the vertical axis. It is a figure which shows the example of a change of the process conditions shown by FIG. It is a figure which shows the example of a change of the process conditions shown by FIG. It is sectional drawing which shows typically the internal combustion engine 100 provided with the connecting rod 1 in suitable embodiment of this invention. FIG. 16 is a side view schematically showing a motorcycle including the internal combustion engine 100 shown in FIG. 15.

The inventor of the present application examined in detail the reason why flaking occurs even in a connecting rod subjected to high-concentration carburizing treatment or high-concentration carbonitriding treatment, and as a result, obtained the knowledge described below.

The cause of flaking is that a large stress is transmitted from the rolling bearing such as a needle bearing or a ball bearing to the inner peripheral surface of the large end as described above. Therefore, it is considered that flaking can be prevented by increasing the surface hardness of the connecting rod by high-concentration carburizing treatment or high-concentration carbonitriding treatment. I can't. That is, even if the surface hardness of the connecting rod is simply increased, sufficient flaking resistance cannot be obtained.

Therefore, when the inventor of the present application analyzed the stress distribution in the depth direction of the connecting rod, it was found that the greatest stress acts at a certain depth from the surface, not the outermost surface. Furthermore, when the relationship between the material characteristics at the depth at which the maximum stress acts and the anti-flaking property was verified, the amount of retained austenite and Vickers hardness HV at the depth at which the maximum stress acts should be set within a specific range. Thus, it was found that the anti-flaking property was remarkably improved.

The present invention has been made based on the above findings found by the present inventors. Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following, a connecting rod will be described as an example. However, the present invention is not limited to the connecting rod, and is widely used for steel parts having a surface in contact with a rolling bearing.

1 (a) to 1 (c) show a connecting rod 1 according to this embodiment. FIG. 1A is a plan view schematically showing the connecting rod 1. 1B is a cross-sectional view taken along line 1B-1B ′ in FIG. 1A, and FIG. 1C is taken along line 1C-1C ′ in FIG. FIG.

As shown in FIGS. 1A and 1B, the connecting rod 1 is provided at the rod body 10, the small end 20 provided at one end of the rod body 10, and the other end of the rod body 10. The large end 30 is provided.

The rod body part (shaft part) 10 has a rod shape. The cross-sectional shape of the rod body 10 is typically H-shaped as shown in FIG.

The small end portion 20 has a through hole (piston pin hole) 22 through which the piston pin passes. The small end 20 is connected to the piston via a piston pin. The inner peripheral surface 20a (the surface defining the outer edge of the piston pin hole 22) 20a typically contacts the piston pin without a bearing.

The large end portion 30 has a through hole (crank pin hole) 32 through which the crank pin passes. The large end 30 is connected to the crankshaft via a crankpin. Since a rolling bearing is typically disposed in the crankpin hole 32, the inner peripheral surface 30a (the surface defining the outer edge of the crankpin hole 32) 30a of the large end 30 is in contact with the rolling bearing. The connecting rod 1 is an integrated connecting rod in which the large end portion 30 is not divided into two.

The connecting rod 1 in this embodiment is made of steel (iron alloy). In addition, the connecting rod 1 is subjected to carbonitriding or carburizing and nitriding. The carbonitriding process (carburizing process) applied to the connecting rod 1 is a so-called high-concentration carbonitriding process (high-concentration carburizing process) performed in an atmosphere having a relatively high carbon potential (CP). According to the high-concentration carbonitriding process (high-concentration carburizing process), fine granular carbides and / or carbonitrides are precipitated in the vicinity of the surface of the connecting rod 1, and the crystal grain size of the martensite structure in the vicinity of the surface is small. Become. Therefore, the surface hardness is remarkably increased, and the effect of improving the fatigue strength is high. In general, carbonitriding (carburizing) when the carbon potential (CP) of the atmosphere is 0.8% or more is referred to as high-concentration carbonitriding (high-concentrating carburizing), which will be described later. For the reason, the carbon potential is preferably 1.1% or more.

Furthermore, in the connecting rod 1 in the present embodiment, the amount of retained austenite at a depth of 0.1 mm from the inner peripheral surface 30a of the large end portion 30 is 50 vol% or more, and the Vickers hardness HV at that depth is 710 or more. It is. This significantly improves the anti-flaking property.

FIG. 2 shows the result of calculating the stress distribution in the depth direction (stress distribution when the stress becomes maximum during operation of the internal combustion engine) on the inner peripheral surface of the large end portion of a general connecting rod. In FIG. 2, the depth from the inner peripheral surface is shown as a negative value. For example, a position at a depth of 0.15 mm is written as “−0.15”. Also, in FIG. 2, a plurality of curves indicating stress are numbered 1 to 22, and the larger the number, the greater the stress.

As can be seen from FIG. 2, the stress is not the largest on the outermost surface. Further, it can be seen from FIG. 2 that the stress becomes maximum at a depth of about 0.1 mm from the inner peripheral surface. Therefore, the inventor of the present application has examined in detail the relationship between the material characteristics and the anti-flaking property at a position of 0.1 mm depth, the amount of retained austenite and Vickers hardness HV at a position of depth 0.1 mm, It was found that the flaking resistance was greatly affected. Specifically, as will be described later along with the verification results, the amount of retained austenite at this depth is 50 vol% and the Vickers hardness HV is 710 or more, so that the effect of improving anti-flaking property is remarkably increased. I found it to be higher.

In the connecting rod 1 in the present embodiment, the amount of retained austenite at a depth of 0.1 mm from the inner peripheral surface 30a of the large end portion 30 is 50 vol% or more, and the Vickers hardness HV at the depth is 710 or more. Therefore, the anti-flaking property is remarkably improved, and the occurrence of flaking can be prevented over a long period of time. Therefore, the connecting rod 1 in the present embodiment is superior in flaking life compared to a connecting rod that has been simply subjected to high-concentration carbonitriding or high-concentration carburizing.

Residual austenite structure has high toughness and contributes to anti-flaking property (good effect) because it has the effect of reducing stress concentration inside due to fine irregularities near the surface. It is done. However, when the amount of retained austenite increases, the hardness decreases. Therefore, in a general connecting rod subjected to carburizing treatment, the amount of retained austenite in the vicinity of the surface is about 20 vol%, and even in a connecting rod subjected to high concentration carburizing treatment, the amount of retained austenite in the vicinity of the surface is from 30 to 40 vol. %. As can be seen from this fact, it has been difficult to sufficiently increase the amount of retained austenite while maintaining the hardness in the vicinity of the surface high in the conventionally proposed manufacturing method. For this reason, the amount of retained austenite could not be increased to 50 vol% or higher while maintaining the Vickers hardness HV in the vicinity of the surface at 710 or higher.

On the other hand, according to the manufacturing method described below, the amount of retained austenite can be sufficiently increased while maintaining the hardness near the surface of the connecting rod high. Hereinafter, the manufacturing method of the connecting rod 1 in this embodiment is demonstrated, referring FIG. 3 and FIG. FIG. 3 is a flowchart showing manufacturing steps of the connecting rod 1. FIG. 4 is a diagram illustrating an example of processing conditions in some steps.

First, a workpiece formed by forging from steel is prepared (step S1). The composition of the steel is not particularly limited, but the carbon (C) content of the steel is preferably 0.1 wt% or more and 0.4 wt% or less. When the carbon content is 0.1 wt% or more and 0.4 wt% or less, the internal hardness (Vickers hardness HV) of the connecting rod 1 after the heat treatment (quenching and tempering) can be 200 or more and 500 or less. Therefore, the strength and toughness inside the connecting rod 1 can be kept sufficiently high.

Further, the silicon (Si) content of the steel is preferably 0.1 wt% or more and 0.5 wt% or less. Increasing the silicon content improves anti-flaking properties but may reduce toughness. When the silicon content is 0.1 wt% or more and 0.5 wt% or less, the flaking resistance can be sufficiently improved and sufficient toughness can be ensured.

Furthermore, the chromium content is preferably 0.3 wt% or more and 1.2 wt% or less. Increasing chromium content improves hardenability (property of hardening by heat treatment), but excessive chromium content causes temper embrittlement (the iron alloy is kept in a predetermined temperature range for a long time). The embrittlement phenomenon that occurs when the When the chromium content is 0.3 wt% or more and 1.2 wt% or less, the occurrence of temper embrittlement can be prevented while obtaining appropriate hardenability.

As described above, steel that is a material of the workpiece (material of the connecting rod 1) is carbon of 0.1 wt% or more and 0.4 wt% or less, silicon of 0.1 wt% or more and 0.5 wt% or less of silicon, and 0.3 wt%. It is preferable to contain 1.2 wt% or less chromium. As an iron alloy having a carbon content, a silicon content, and a chromium content within the above ranges, for example, JIS SCM420 steel or JIS SCr420 steel can be used. SCM420 steel is made of 0.18 wt% or more and 0.23 wt% or less of carbon, 0.15 wt% or more and 0.35 wt% or less of silicon, 0.90 wt% or more and 1.2 wt% or less of chromium, 0.60 wt% or more and 0.002 wt% or less. 85% by weight or less of manganese and 0.15% by weight or more and 0.30% or less of molybdenum are included. The SCr420 steel is composed of 0.18 wt% or more and 0.23 wt% or less carbon, 0.15 wt% or more and 0.35 wt% or less silicon, 0.90 wt% or more and 1.2 wt% or less chromium, 0.60 wt% or more and 0. Contains 85 wt% or less manganese.

In addition, although forge was illustrated here, the shaping | molding method in the process of preparing a workpiece is not limited to this. The workpiece may be formed by, for example, sintering, casting, sintering forging, or the like.

Next, machining is performed on the workpiece (step S2). By this machining, the outer diameter of the workpiece after forging is adjusted. For example, deburring, formation of the piston pin hole 22 and the crank pin hole 32, and end face processing of the small end portion 20 and the large end portion 30 are performed. Thus, cutting is mainly performed in this step.

Subsequently, a high-concentration carburizing process is performed on the workpiece (step S3). This step S3 (and steps S4 to S8 described later) is performed in a carburizing furnace. The inside of the carburizing furnace is set to a predetermined temperature, and hydrocarbons (in the form of gas) are introduced into the furnace so that the carbon potential is 1.1% or more, and carburizing is performed for a predetermined time. For example, as shown in FIG. 4, carburization is performed at 950 ° C. for 300 minutes. Further, during the high-concentration carburizing process, the period A for introducing hydrocarbons and the period B for stopping introduction of hydrocarbons are alternately repeated a plurality of times. That is, the introduction of hydrocarbons into the carburizing furnace is stopped a plurality of times during the high-concentration carburizing process.

Thereafter, gas cooling is performed (step S4). For example, cooling is performed by introducing nitrogen (N ₂ ) gas.

Next, nitriding is performed on the workpiece (step S5). While setting the inside of the carburizing furnace to a predetermined temperature, ammonia gas is introduced into the carburizing furnace and nitriding is performed for a predetermined time. For example, as shown in FIG. 4, nitridation is performed at 850 ° C. for 130 minutes.

Subsequently, the workpiece is quenched (oil-cooled) (step S6). This step S6 is performed using a quenching oil having a quenching intensity (referred to as “H value”) of 0.07 cm ⁻¹ or more and less than 0.11 cm ⁻¹ . Quenched oils having H values less than 0.11 cm ^-1 are commonly referred to as hot quench oils. In the present specification, the “H value” characterizing the quenching oil means an H value at almost normal pressure unless otherwise specified. More specifically, the oil temperature is determined by a test method based on JIS K 2242. It is an H value (cm ⁻¹ ) determined from a cooling curve when quenched from 120 ° C. and an atmospheric pressure of 100 kPa on the oil surface and from 800 ° C. without stirring. Moreover, this process S6 is performed, controlling the atmospheric pressure on the surface of hardening oil to 5 kPa or more and 60 kPa or less. That is, step S6 is performed at an atmospheric pressure lower than normal pressure (standard atmospheric pressure) (that is, under reduced pressure).

Next, tempering is performed (step S7). Tempering is performed at 190 ° C. for 120 minutes, for example, as shown in FIG. Thereafter, air cooling is performed (step S8).

Finally, machining is performed on the workpiece (step S9). For example, the inner peripheral surface 20a of the small end portion 20 and the inner peripheral surface 30a of the large end portion 30 are polished. Thus, polishing is mainly performed in this step. As described above, the connecting rod 1 is completed.

FIG. 5 shows an example of a carburizing furnace for performing steps S4 to S8. A carburizing furnace 50 shown in FIG. 5 includes a heating chamber 51 and a cooling chamber 52.

Inside the heating chamber 51, high-concentration carburizing treatment (step S3), nitriding treatment (step S5) and tempering (step S7) are performed on the workpiece WP. A heater 53 for heating is provided in the heating chamber 51. Although not shown here, a nozzle for introducing hydrocarbon gas or ammonia gas is also provided, and the hydrocarbon is introduced into the heating chamber 51 during the high-concentration carburizing process.

In the cooling chamber 52, gas cooling (step S4), quenching (step S6), and air cooling (step S8) are performed. An oil tank 54 in which quenching oil QO is stored is provided in the cooling chamber 52.

Doors

55a and 55b are provided between the heating chamber 51 and the cooling chamber 52, and between the cooling chamber 52 and the outside, respectively.

The carburizing furnace 50 further includes a decompression mechanism (for example, a vacuum pump) 56 that can decompress the inside of the heating chamber 51 and the inside of the cooling chamber 52. That is, the carburizing furnace 50 is a so-called vacuum carburizing furnace. The space on the surface of the quenching oil QO can be decompressed by the decompression mechanism 56.

According to the manufacturing method described above, it is possible to sufficiently increase the amount of retained austenite while maintaining the hardness near the surface of the connecting rod 1 high. Prior to explaining the reason, a conventionally proposed manufacturing method will be described with reference to FIG. FIG. 6 is a diagram showing processing conditions in some steps of the conventional manufacturing method.

Also in the conventional manufacturing method, first, a workpiece formed from steel is prepared, and then machining is performed on the workpiece. Subsequently, a high-concentration carburizing process is performed on the workpiece. For example, as shown in FIG. 6, carburization is performed at 950 ° C. for 300 minutes. At this time, however, hydrocarbons are continuously introduced into the carburizing furnace.

Next, gas cooling is performed, and then nitriding is performed on the workpiece. For example, as shown in FIG. 6, ammonia (ammonia gas) is introduced into a carburizing furnace and nitriding is performed at 850 ° C. for 130 minutes.

Thereafter, the workpiece is quenched (oil-cooled). In general, this step is performed using a quenching oil having a quenching intensity (H value) of 0.11 cm ⁻¹ or more. Quenched oils having an H value of 0.11 cm ^-1 or more are generally referred to as cold quench oils. In this step, the atmospheric pressure on the surface of the quenching oil is almost normal pressure (about 100 kPa).

Next, tempering is performed. Tempering is performed at 190 ° C. for 120 minutes, for example, as shown in FIG. Thereafter, air cooling is performed, and finally, machining is performed on the workpiece.

Thus, in the conventional manufacturing method, cold quench oil is used for quenching. This is because in the conventional manufacturing method, it is necessary to use quenching oil having high hardenability (that is, having a high H value). When carburizing (or carbonitriding) is performed in an atmosphere having a carbon potential of 0.8% or more, not only iron carbides are precipitated, but also chromium present in the steel is likely to precipitate as carbides. As a result, the chromium concentration in the steel base material decreases and the hardenability decreases. Therefore, it is necessary to use cold quench oil with high hardenability. In the conventional production method using cold quench oil, the amount of retained austenite cannot be sufficiently increased while maintaining the hardness near the surface of the connecting rod sufficiently high.

On the other hand, in the manufacturing method in the present embodiment, the step of quenching the workpiece (step S6 shown in FIG. 3) is a quenching oil having a quenching intensity (H value) of less than 0.11 cm ^−1. And the atmospheric pressure on the surface of the quenching oil is controlled to 60 kPa or less. When quenching is performed under reduced pressure using a quenching oil (hot quench oil) having an H value of less than 0.11 cm ⁻¹ , the boiling point of the quenching oil is lowered, so that the workpiece becomes a quenching oil vapor film ( The time (vapor film stage) covered with (insulating heat) becomes longer. Therefore, the cooling rate at the initial stage of quenching becomes slow. However, on the other hand, since the cooling rate after the vapor film breakage is increased, the amount of retained austenite can be increased without affecting the martensitic transformation itself. In order to make the vapor film stage sufficiently long, the atmospheric pressure on the surface of the quenching oil is preferably 60 kPa or less as in the present embodiment.

Hot quench oil has a low H value at normal pressure, but the convection stage start temperature decreases due to the reduced pressure, which improves hardenability (coolability in the convection stage) and increases the effective H value. can do. Therefore, the hardness in the vicinity of the surface of the connecting rod 1 (depth 0.1 mm) can be sufficiently increased. On the other hand, quenching oil (cold quench oil) having an H value of 0.11 cm ⁻¹ or more generally contains an additive for shortening the vapor film stage. Even if the pressure is reduced, the vapor film stage cannot be made sufficiently long, and the amount of retained austenite cannot be increased sufficiently.

The H value of the quenching oil used in the quenching step is preferably 0.07 cm ⁻¹ or more. When the H value is less than 0.07 cm ⁻¹ , sufficient hardenability may not be obtained depending on the degree of decompression (the effective H value may not be sufficiently large).

Further, the step of quenching is preferably performed while controlling the atmospheric pressure on the surface of the quenching oil to 5 kPa or more. When the atmospheric pressure is less than 5 kPa, the boiling point of the quenching oil becomes too low and the vapor film stage may become too long. Therefore, there is a possibility that hardenability is lowered and sufficiently high hardness cannot be obtained.

Thus, according to the manufacturing method in the present embodiment, the amount of the retained austenite structure that contributes to (good influence on) the anti-flaking property is increased while maintaining the hardness of the connecting rod 1 sufficiently high. Can do. Therefore, the connecting rod 1 manufactured by the manufacturing method in this embodiment is excellent in flaking life. Moreover, it becomes possible to apply a higher load to the connecting rod 1 in the present embodiment with an improved flaking life than in the prior art. Therefore, the size of the large end portion 30 can be reduced and the weight can be reduced.

In the present embodiment, the case where the high-concentration carburizing process and the nitriding process are separately (sequentially) performed on the workpiece has been described. However, the nitriding process may be performed simultaneously when the high-concentration carburizing process is performed. That is, you may perform a high concentration carbonitriding process with respect to a workpiece.

The high-concentration carburization treatment (high-concentration carbonitriding treatment) is preferably performed in an atmosphere having a carbon potential of 1.1% or more as in this embodiment. When the carbon potential is less than 1.1%, the amount of retained austenite may not be sufficiently increased. Moreover, the precipitation amount of a carbide | carbonized_material may decrease and hardness may fall.

Moreover, it is preferable that the high-concentration carburizing treatment (high-concentration carbonitriding treatment) is performed in an atmosphere having a carbon potential of less than 2.2%. When the carbon potential is 2.2% or more, the amount of carbides precipitated becomes too large and the toughness may be lowered.

The carbon potential of the atmosphere in the high-concentration carburizing process (high-concentration carbonitriding process) almost corresponds to the carbon content at a depth of 0.1 mm from the surface. Accordingly, the carbon content at a depth of 0.1 mm from the surface of the connecting rod 1 is preferably 1.1 wt% or more and less than 2.2 wt%. Further, as will be described later, the carbon content at a depth of 0.1 mm from the surface of the connecting rod 1 is 1.6 wt% or more and less than 2.0 wt% from the viewpoint of further extending the life. Therefore, the high-concentration carburizing treatment (high-concentration carbonitriding treatment) is preferably performed in an atmosphere having a carbon potential of 1.6% or more and 2.0% or less.

Furthermore, from the viewpoint of further prolonging the life, it is also preferable that the crystal grain size of the steel structure at a depth of 0.1 mm from the surface is 9 μm or less. For example, high-concentration carburization (high-concentration carbonitriding) is performed in an atmosphere having a carbon potential of 1.4% or more, and quenching is controlled so that the atmospheric pressure on the surface of the quenching oil is 50 to 60 kPa. However, the grain size of the steel structure can be reduced to 9 μm or less.

In addition, from the viewpoint of increasing toughness, the particle size of carbides and carbonitrides precipitated in the vicinity of the surface is preferably as small as possible, specifically 10 μm or less.

Furthermore, from the viewpoint of further improving the flaking resistance, the nitrogen content at a depth of 0.1 mm from the surface is preferably 0.03 wt% or more and 0.19 wt% or less, and 0.04 wt%. It is more preferably 0.18 wt% or less, and further preferably 0.05 wt% or more and 0.15 wt% or less.

When using a vacuum carburizing furnace as in the present embodiment, it is preferable to stop introduction of hydrocarbons into the heating chamber 51 a plurality of times during high-concentration carburizing treatment (or high-concentration carbonitriding treatment). Thereby, the carbon potential can be adjusted with high accuracy, and the carbon content in the vicinity of the surface of the connecting rod 1 can be controlled with high accuracy. Although FIG. 4 shows an example in which the introduction of hydrocarbons is stopped twice, the introduction of hydrocarbons may be stopped three or more times.

Note that the vacuum carburizing furnace is not necessarily used. The decompression mechanism 56 only needs to be able to decompress at least the space on the surface of the quenching oil (that is, the inside of the cooling chamber 52). Therefore, you may use the gas carburizing furnace comprised so that the inside of the cooling chamber 52 could be pressure-reduced. When the gas carburizing furnace is used, the carburizing process can be performed as an equilibrium reaction, so that the carbon potential can be more reliably controlled within the above-described preferable range (1.1% or more and less than 2.2%).

FIG. 7 shows a distribution of carbon concentration (carbon content) in the depth direction of the connecting rod 1 actually manufactured by the manufacturing method according to the present embodiment. As the steel that is the material of the connecting rod 1, case-hardened steel (JIS SCM420) was used. Processing conditions such as high-concentration carburizing processing are as shown in FIG. The high-concentration carburizing treatment was performed at 950 ° C. for 300 minutes. At that time, the period A for introducing hydrocarbons (specifically acetylene) was set to 1 hour and the period B for stopping introduction of hydrocarbons was set to 40 minutes, and these were repeated three times alternately. That is, introduction of hydrocarbons into the heating chamber 51 was stopped three times. The nitriding treatment was performed at 850 ° C. for 130 minutes. Quenching was performed using a quenching oil having an H value of 0.10 cm ⁻¹ while controlling the atmospheric pressure on the surface of the quenching oil to 15 kPa. Tempering was performed at 190 ° C. for 120 minutes.

In the example shown in FIG. 7, the carbon concentration at a depth of 0.1 mm from the surface is about 1.4 wt%. The carbon concentration distribution as shown in FIG. 7 can be measured by, for example, an electron beam microanalyzer (EPMA).

FIG. 9 shows the hardness distribution in the depth direction of the connecting rod 1, and FIG. 10 shows the residual austenite amount distribution in the depth direction. As shown in FIG. 9, the Vickers hardness HV at a depth of 0.1 mm from the surface is 740. Further, as shown in FIG. 10, the amount of retained austenite at a depth of 0.1 mm from the surface is 58%. Thus, according to the manufacturing method in the present embodiment, the amount of retained austenite at the depth can be 50 vol% or more while maintaining the Vickers hardness HV at a depth of 0.1 mm from the surface at 710 or more. it can. The connecting rod 1 was measured for flaking life (having a cumulative failure probability of 50%, referred to as “L50 life”) and was a high value of 1.91 × 10 ⁶ cycles.

Next, the results of verifying the influence on the flaking life by changing the amount of retained austenite at a depth of 0.1 mm from the surface and the Vickers hardness HV will be described.

In the following Table 1, in Examples 1 to 10 hardening is applied to the work piece in the following ambient pressure 60kPa using a quenching oil H value 0.10 cm ^-1, H values are 0.11 cm ^-1 The verification results are shown for Comparative Examples 1 to 11 in which the workpiece was quenched at an atmospheric pressure of 100 kPa using the above quenching oil. The amount of retained austenite can be measured by an analyzer using an X-ray diffraction technique (for example, an X-ray residual stress measuring machine). The results shown in Table 1 are plotted with the Vickers hardness HV on the horizontal axis and the L50 life on the vertical axis. FIG. 11 shows the amount of retained austenite on the horizontal axis and the Vickers hardness on the vertical axis. FIG. 12 is a plot of HV. 11 and 12, ex1 to 10 correspond to Examples 1 to 10, and ce1 to 11 correspond to Comparative Examples 1 to 11. Moreover, the numerical value attached | subjected to the point plotted in FIG. 11 has shown the amount of retained austenite.

From Table 1 and FIG. 11, it can be seen that the flaking life of Examples 1 to 10 is longer than the flaking life of Comparative Examples 1 to 11. Moreover, from Table 1 and FIG. 12, in the conventional manufacturing method using cold quench oil for quenching, when the amount of retained austenite is 50 vol% or more, Vickers hardness HV of 710 or more cannot be maintained, whereas hot quench oil for quenching is maintained. It can be seen that, in the manufacturing method according to the present embodiment using V, a Vickers hardness HV of 710 or more can be maintained even when the amount of retained austenite is 50 vol% or more.

Thus, the flaking life can be made longer than before by setting the amount of retained austenite at a depth of 0.1 mm from the surface to 50 vol% or more and the Vickers hardness HV at the depth to 710 or more. I understood it.

Table 2 shows the crystal grain size of the steel structure at a depth of 0.1 mm from the surface, the number of repetitions of periods A and B, and the depth of 0.1 mm from the surface for Examples 1 to 10 and Comparative Examples 1 to 11. The carbon content is shown. The crystal grain size of the steel structure was measured by determining the average diameter of the crystal grains according to “Evaluation Method by Counting Method” in “JIS-G-0551“ Steel—Microscopic Test Method for Grain Size ””. The carbon content was measured using an electron beam microanalyzer (EPMA). For Comparative Examples 3 to 10, some values are not shown.

From a comparison between Examples 4 and 8 and Examples 1 to 3, 7, 9, and 10, it can be seen that a longer life can be achieved by making the grain size of the steel structure 9 μm or less.

Also, from Tables 1 and 2, in order to make the grain size of the steel structure 9 μm or less, the carbon content at a depth of 0.1 mm from the surface is 1.4 wt% or more (that is, high-concentration carburizing treatment or It is understood that the high concentration carbonitriding treatment is performed in an atmosphere having a carbon potential of 1.4% or more), and the atmospheric pressure on the surface of the quenching oil during quenching is preferably 50 kPa or more.

As can be seen from Examples 1 to 3 and 10, when the atmospheric pressure is less than 50 kPa (both are 40 kPa or less in this case), the crystal grain size of the steel structure is sufficiently refined even if the carbon content is sufficiently large. I can't. Further, as can be seen from Comparative Examples 1, 2, and 11, even if the atmospheric pressure is 50 kPa or more, if the carbon content is small, the crystal grain size of the steel structure cannot be sufficiently refined.

As already described, when the atmospheric pressure exceeds 60 kPa, the vapor film stage cannot be made sufficiently long, and the amount of retained austenite cannot be increased sufficiently. Therefore, the atmospheric pressure is preferably 50 kPa or more and 60 kPa or less. I can say that.

Further, from the comparison between Examples 5 and 6 and Examples 1 to 3, 7, 9, and 10, it is possible to further extend the service life by setting the carbon content to 1.6 wt% or more and 2.0 wt% or less. It is understood that is possible. Therefore, the carbon content is preferably 1.6 wt% or more and 2.0 wt% or less.

In order to accurately control the carbon content at a depth of 0.1 mm from the surface to 1.6 wt% or more and 2.0 wt% or less, the periods A and B during the high concentration carburizing treatment (high concentration carbonitriding treatment) Is preferably repeated 4 or 5 times. That is, it is preferable to stop introduction of hydrocarbons into the heating chamber 51 four or five times.

Table 3, ambient pressure of 15kPa with a quenching oil H value Example 11 quenching is performed at ambient pressure of 15kPa with a quenching oil is 0.07 cm ^-1, H values are 0.06 cm ^-1 Comparative Example 12 that was quenched in Example 10, Comparative Example 13 that was quenched at an atmospheric pressure of 3 kPa or less using a quenching oil that had an H value of 0.10 cm ^{−1, and} a quenching oil that had an H value of 0.10 cm ⁻¹ Vickers hardness HV and amount of retained austenite are shown for Comparative Example 14 in which quenching was performed at an atmospheric pressure of 65 kPa.

In Example 11, the Vickers hardness HV is 710 or more, whereas in Comparative Examples 12 and 13, the Vickers hardness HV is less than 710. As described above, from the comparison between Example 11 and Comparative Example 12 and from the comparison between Example 11 and Comparative Example 13, the H value of the quenching oil is 0. It is preferable that the pressure is 07 cm ⁻¹ or more, and the atmospheric pressure on the surface of the quenching oil is preferably 5 kPa or more. In Example 11, the amount of retained austenite is 50 vol% or more, whereas in Comparative Example 14, the amount of retained austenite is less than 50 vol%. Thus, from the comparison between Example 11 and Comparative Example 14, it can be seen that the atmospheric pressure on the surface of the quenching oil is preferably 60 kPa or less from the viewpoint of sufficiently increasing the amount of retained austenite.

It is also preferable to execute a refining process in which neither hydrocarbon nor ammonia is introduced into the heating chamber 51 between the high-concentration carburizing process and the nitriding process. By the refining process, it is possible to further reduce the particle size of the carbide precipitated in the vicinity of the surface.

For example, the processing conditions shown in FIGS. 4 and 8 can be modified as shown in FIGS. 13 and 14, respectively. Under the processing conditions shown in FIG. 13 and FIG. 14, introduction of hydrocarbons and ammonia is performed between the high-concentration carburizing process and the nitriding process at 850 ° C. for 60 minutes (not limited to this temperature and time of course). A refining step that is not performed is performed, and then gas cooling is performed. By such a refining process, the particle size of the carbide precipitated in the vicinity of the surface can be further reduced. For example, under the processing conditions shown in FIG. 8, the particle size of the carbide was 7-9 μm, whereas under the processing conditions shown in FIG. 14, the particle size of the carbide was 4-6 μm.

The connecting rod 1 in this embodiment is suitably used for an internal combustion engine. FIG. 15 shows an example of the internal combustion engine 100 provided with the connecting rod 1 in the present embodiment. The internal combustion engine 100 includes a crankcase 110, a cylinder block 120, and a cylinder head 130.

A crankshaft 111 is accommodated in the crankcase 110. The crankshaft 111 has a crankpin 112 and a crank web 113. The crank pin 112 and the crank web 113 are formed separately. That is, the crankshaft 111 is an assembly-type crankshaft.

A cylinder block 120 is provided on the crankcase 110. The cylinder block 120 is fitted with a cylindrical cylinder sleeve 121, and the piston 122 is provided so as to reciprocate within the cylinder sleeve 121.

A cylinder head 130 is provided on the cylinder block 120. The cylinder head 130 forms a combustion chamber 131 together with the piston 122 and the cylinder sleeve 121 of the cylinder block 120. The cylinder head 130 has an intake port 132 and an exhaust port 133. An intake valve 134 for supplying air-fuel mixture into the combustion chamber 131 is provided in the intake port 132, and an exhaust valve 135 for exhausting the combustion chamber 131 is provided in the exhaust port 133. Yes.

The piston 122 and the crankshaft 111 are connected by the connecting rod 1. Specifically, the piston pin 123 of the piston 122 is inserted into the piston pin hole formed in the small end portion 20 of the connecting rod 1, and the crankshaft 111 of the crankshaft 111 is inserted into the crankpin hole formed in the large end portion 30. The crank pin 112 is inserted, whereby the piston 122 and the crank shaft 111 are connected.

In an internal combustion engine having a specification in which reduction of friction loss is important (for example, a single-cylinder internal combustion engine having one cylinder), as shown in FIG. A needle bearing 114 is provided between the peripheral surface 30 a and the crank pin 112. When the needle bearing 114 is provided, stress is generated on the inner peripheral surface 30 a of the large end portion 30 by pressing the connecting rod 1 against the needle bearing 114. If this stress is excessive, the occurrence of flaking is a concern. However, since the connecting rod 1 in this embodiment is excellent in anti-flaking property, the occurrence of flaking is prevented for a long period of time longer than that required for a product.

In FIG. 15, the needle bearing 114 is exemplified as the rolling bearing, but the rolling bearing is not limited to a roller bearing such as a needle bearing, and may be a ball bearing (ball bearing).

FIG. 16 shows a motorcycle including the internal combustion engine 100 shown in FIG. In the motorcycle shown in FIG. 16, a head pipe 302 is provided at the front end of the main body frame 301. A front fork 303 is attached to the head pipe 302 so as to be able to swing in the left-right direction of the vehicle. A front wheel 304 is rotatably supported at the lower end of the front fork 303.

A seat rail 306 is attached so as to extend rearward from the upper rear end of the main body frame 301. A fuel tank 307 is provided on the main body frame 301, and a main seat 308 a and a tandem seat 308 b are provided on the seat rail 306.

Also, a rear arm 309 extending backward is attached to the rear end of the main body frame 301. A rear wheel 310 is rotatably supported at the rear end of the rear arm 309.

The internal combustion engine 100 shown in FIG. 13 is held at the center of the main body frame 301. The internal combustion engine 100 includes the connecting rod 1 in the present embodiment. A radiator 311 is provided in front of the internal combustion engine 100. An exhaust pipe 312 is connected to the exhaust port of the internal combustion engine 100, and a muffler 313 is attached to the rear end of the exhaust pipe 312.

A transmission 315 is connected to the internal combustion engine 100. A drive sprocket 317 is attached to the output shaft 316 of the transmission 315. The drive sprocket 317 is connected to the rear wheel sprocket 319 of the rear wheel 310 via a chain 318. The transmission 315 and the chain 318 function as a transmission mechanism that transmits the power generated by the internal combustion engine 100 to the drive wheels.

In the motorcycle shown in FIG. 16, the internal combustion engine 100 including the connecting rod 1 according to the present embodiment is used, so that flaking is prevented from occurring for a long period of time longer than a period necessary for a product. Moreover, the connecting rod 1 in the present embodiment is also suitable for reducing the size and weight. This is because it is possible to apply a high load to the connecting rod 1 by extending the life. By reducing the size and weight of the connecting rod 1, the internal combustion engine 100 and the vehicle body are also lightened, and the running stability, ease of riding, and ease of handling of the motorcycle are improved, and the merchantability is improved.

In addition, the internal combustion engine 100 provided with the connecting rod 1 in the present embodiment is not limited to a motorcycle, but is preferably used for all motor vehicles and particularly preferably for saddle riding type vehicles on which riders ride. Examples of the straddle-type vehicle include an ATV such as a buggy in addition to the illustrated motorcycle.

Further, the connecting rod 1 in the present embodiment can also be used for a small internal combustion engine used in a generator, an agricultural machine, or the like.

In this embodiment, the connecting rod has been described as an example. However, the present invention is not limited to the connecting rod and the manufacturing method thereof. The steel part according to the present invention may be a part other than the connecting rod, for example, a crankpin. The outer peripheral surface of the crank pin is in contact with the rolling bearing.

The steel parts according to the present invention are widely used in various internal combustion engines for motor vehicles (for example, internal combustion engines for motorcycles).

DESCRIPTION OF SYMBOLS 1 Connecting rod 10 Rod main-body part 20 Small end part 20a Inner peripheral surface of a small end part 22 Piston pin hole 30 Large end part 30a Inner peripheral surface of a large end part 32 Crank pin hole 100 Internal combustion engine

Claims

A steel part having a surface in contact with a rolling bearing,
A steel part having a residual austenite amount of 50 vol% or more and a Vickers hardness HV of 710 or more at a depth of 0.1 mm from the surface.
The steel part according to claim 1, wherein a carbon content at a depth of 0.1 mm from the surface is 1.1 wt% or more and less than 2.2 wt%.
The steel part according to claim 2, wherein the carbon content at a depth of 0.1 mm from the surface is 1.6 wt% or more and 2.0 wt% or less.
The steel part according to any one of claims 1 to 3, wherein a crystal grain size of a steel structure at a depth of 0.1 mm from the surface is 9 µm or less.
The steel part according to any one of claims 1 to 4, which is a connecting rod.
The rod body,
A small end provided at one end of the rod body,
A large end provided at the other end of the rod body,
The steel part according to claim 5, wherein the inner peripheral surface of the large end is the surface in contact with the rolling bearing.
The steel part according to any one of claims 1 to 4, which is a crankpin.
An internal combustion engine comprising the steel part according to any one of claims 1 to 7.
The internal combustion engine according to claim 8, further comprising a rolling bearing provided so as to be in contact with the surface.
An automatic vehicle comprising the internal combustion engine according to claim 8 or 9.
A method of manufacturing a steel part having a surface in contact with a rolling bearing,
Preparing a workpiece formed from steel (A);
Carburizing treatment is performed in an atmosphere having a carbon potential of 1.1% or more with respect to the workpiece, followed by nitriding treatment, or having a carbon potential of 1.1% or more with respect to the workpiece. Performing carbonitriding in an atmosphere (B),
After the step (B), using a quenching oil having a quenching intensity of 0.07 cm −1 or more and less than 0.11 cm −1, while controlling the atmospheric pressure on the surface of the quenching oil to 5 kPa or more and 60 kPa or less, A method of manufacturing a steel part including the step (C) of quenching the workpiece.
The method of manufacturing a steel part according to claim 11, wherein the carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of less than 2.2%.
The method of manufacturing a steel part according to claim 12, wherein the carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of 1.6% or more and 2.0% or less.
The carburizing process or the carbonitriding process in the step (B) is performed in an atmosphere having a carbon potential of 1.4% or more,
The method of manufacturing a steel part according to any one of claims 11 to 13, wherein the step (C) is performed while controlling an atmospheric pressure on the surface of the quenching oil to 50 kPa or more and 60 kPa or less.
The said process (B) and the said process (C) are performed in the carburizing furnace provided with the decompression mechanism which can decompress the space on the surface of the said quenching oil at least. Of manufacturing steel parts.
The carburizing furnace further includes a heating chamber in which the step (B) is performed, and a cooling chamber in which the step (C) is performed. The method of manufacturing a steel part according to claim 15, which is a vacuum carburizing furnace capable of reducing the pressure inside both of the cooling chambers.
The method for manufacturing a steel part according to claim 16, wherein introduction of hydrocarbons into the heating chamber is stopped a plurality of times during the carburizing process or the carbonitriding process in the step (B).
The method for manufacturing a steel part according to claim 17, wherein the introduction of hydrocarbons into the heating chamber is stopped four times or five times during the carburizing process or the carbonitriding process in the step (B).
The refining process in which neither hydrocarbon nor ammonia is introduced into the heating chamber is performed between the carburizing process and the nitriding process in the step (B). The manufacturing method of the steel components as described in 2.