[Title of Invention] CONNECTING ROD, INTERNAL COMBUSTION ENGINE, TRANSPORTATION APPARATUS, AND METHOD OF PRODUCING CONNECTING ROD [Technical Field]

[0001] The present invention relates to a connecting rod and a production method thereof. The present invention also relates to an internal combustion engine and a transportation apparatus including a connecting rod. [Background Art]

[0002] In an internal combustion engine, a member which is called a connecting rod (or con rod) is used for linking a crankshaft to a piston. A connecting rod includes: a bar- like rod main body (axis portion) ; a small end which is provided at one end of the rod main body; and a big end which is provided at another end of the rod main body. The small end is connected to a piston, whereas the big end is connected to a crankshaft.

[0003] The connecting rod reciprocates inside the internal combustion engine. Therefore, by reducing the weight of the connecting rod, the internal combustion engine is allowed to have smoother rotations and reduced vibrations. Thus, the connecting rod is expected to be light weight. [0004] Moreover, an explosion force occurring in the combustion chamber is transmitted to the connecting rod via the piston, and therefore the connecting rod is required to have sufficient mechanical strength. In particular, the connecting rod of an internal combustion engine which is operated at high revolutions (e.g., an internal combustion engine of a motorcycle) is required to have an even higher mechanical strength, and is expected to have a further reduced weight for improved acceleration performance.

[0005] One known technique of increasing the mechanical strength of a steel connecting rod is a carburization treatment, which allows carbon to permeate the surface of a connecting rod. A carburization treatment increases the carbon concentration in the surface vicinity of the connecting rod, and therefore an increased surface hardness is obtained after hardening of the connecting rod. Thus, the mechanical strength of the connecting rod is improved. Moreover, since a sufficient mechanical strength is ensured even if the connecting rod is made thin, the connecting rod can be reduced in weight. Note that, even when performing a carburization treatment, it is often the case that the rod main body is subjected to a treatment for preventing carburization, so as not to have a high surface hardness. The reason is that the rod main body should preferably be high in toughness.

[0006] Patent Literature 1 proposes a high-concentration carburization treatment as a technique of further enhancing the surface hardness of a connecting rod. In the high- concentration carburization treatment, carburization is performed a plural number of rounds in an ambient atmosphere having a carbon potential (CP) of 0.8% or more. As a result, carbide in minute particulate form deposits in the surface vicinity of the connecting rod, and also the crystal grain size of martensite structure in the surface vicinity is reduced. As a result, the surface hardness is remarkably- increased.

[0007] Also in the high-concentration carburization treatment, the rod main body is subjected to a fewer number of rounds of carburization than are the other portions, or subjected to no carburization at all, this being in order to ensure toughness of the rod main body. FIG. 8 shows exemplary production steps of a connecting rod in the case where a high-concentration carburization treatment is performed.

[0008] First, a raw material of steel is molded into the shape of a connecting rod through forging (step SIl) . Next, masking is applied to the surface of the connecting rod with an anti-carburization agent or the like (step S12) . Thereafter, a machining is performed to form a piston pin hole in the small end, a crankpin hole in the big end, and so on (step S13) .

[0009] Next, a first round of carburization and a cooling

(or hardening) step are performed (step S14) . The carburization is performed in an ambient atmosphere having a carbon potential of 0.8% or more. At this time, the carbon concentration in the surface vicinity is increased in any portion that is not covered by the masking pattern, e.g., the inner peripheral surface of the crankpin hole (that is, any portion from which the masking pattern has been removed through cutting at the time of machining) . On the other hand, in any portion that is covered by the masking pattern, e.g., the rod main body (that is, any portion that has been subjected to a carburization prevention treatment) , the carbon concentration is scarcely changed. Next, the remaining masking pattern on the surface of the connecting rod is removed (step S15) .

[0010] Thereafter, a second round of carburization, a hardening, and a tempering are seguentially performed (step S16) . The second round of carburization is also performed in an ambient atmosphere having a carbon potential of 0.8% or more. At this time, since the masking pattern on the connecting rod surface has already been removed, the carbon concentration in the surface vicinity is increased over the entire connecting rod. Finally, a polishing step for the inner peripheral surface of the small end and the inner peripheral surface of the big end is performed (step S17) .

[0011] According to the aforementioned production method, while two rounds of carburization are performed for the portions which have not been subjected to a carburization prevention treatment, only one round of carburization is performed for portions which have received a carburization prevention treatment, e.g., the rod main body. Therefore, while ensuring toughness of the rod main body, it is possible to greatly improve the mechanical strength of the entire connecting rod.

[0012] However, in order to further reduce the weight of the connecting rod by making the rod main body even thinner, a still higher mechanical strength of the rod main body is being desired. For example, performing a high-concentration carburization treatment without performing a carburization prevention treatment for the rod main body will further increase the surface hardness of the rod main body. In this case, however, the toughness of the rod main body will of course be impaired.

[0013] Therefore, the inventors have attempted a fine- particle shot peening (Fine Particle Bombarding: FPB), as disclosed in Non-Patent Literature 1 and 2, for a connecting rod which has been subjected to a high-concentration carburization treatment. In a fine-particle shot peening, fine particles which are much finer than those used for traditional shot peening are used as the shot material (e.g., having diameters of 500 μ m or less) . The fine-particle shot peening is a technique of allowing such fine particles to rapidly collide against the surface of a metal part to modify the surface by utilizing various phenomena occurring as a result . [0014] By performing a fine-particle shot peening, nano- crystal structure having a high hardness and good toughness is formed on the surface of a metal part. Moreover, since a greater residual compressive stress can be conferred to the surface of the metal part than by the traditional shot peening, an improved fatigue strength is obtained. [0015] It is considered that, by performing a fine- particle shot peening for a connecting rod which has been subjected to a high-concentration carburization treatment, the mechanical strength and toughness of the rod main body are both increased. [Citation List] [Patent Literature]

[0016] [Patent Literature 1] Japanese Laid-Open Patent Publication No. 2000-313949 [Non-Patent Literature]

[0017] [Non-Patent Literature 1] Chuji Kagaya and one other, "Trends of Surface Origination Using High Velocity Impact Phenomena of Fine Particles", Denkiseiko (or "Electric Steelmaking") , January 2000, Vol. 71, No. 1, pp.51-58 [Non-Patent Literature 2] Shin-ichi TAKAGI and four others, "Surface Nonocrystallization of Carburized Steel JIS-SCr420 by Fine Particle Peening", Tetsu to Hagane (or "Iron and Steel"), 2006, Vol. 92, No. 5 [Summary of invention] [Technical Problem] [0018] However, it has been found through actual prototyping by the inventors that, when a fine-particle shot peening is performed for a connecting rod which has been subjected to a high-concentration carburization treatment, breakage may occur in the rod main body during use of the connecting rod due to fatigue cracks originating from the surface. The causes for such breakage will be described with reference to FIGS. 9 (a) and (b) . FIG. 9 (a) is a cross- sectional view schematically showing the surface vicinity of a connecting rod before being subjected to a fine-particle shot peening; FIG. 9 (b) is a cross-sectional view schematically showing the surface vicinity of the connecting rod after being subjected to a fine-particle shot peening.

[0019] As shown in FIG. 9 (a) , in the surface vicinity of the connecting rod which has been subjected to a high- concentration carburization treatment, carbide 3 and inclusions 4 are contained in a parent phase 2 of steel. When a fine-particle shot peening is performed, since the carbide 3 and the inclusions 4 are hard, mainly the parent phase 2 is ground off, with some plastic deformation. Thus, as shown in FIG. 9 (b) , the carbide 3 and the inclusions 4 will protrude from the surface 2a of the parent phase 2. Therefore, during use of the connecting rod, stress will concentrate at the roots of the protrusions of carbide 3 and inclusions 4, which may serve as beginning points of breakage.

[0020] The present invention has been made based on the above findings of the inventors, and an objective thereof is to provide a connecting rod whose rod main body has an excellent mechanical strength and which is suitable for weight reduction, as well as a production method thereof. [Solution to Problem]

[0021] The connecting rod according to the present invention is a connecting rod including: a rod main body; a small end provided at a first end of the rod main body; and a big end provided at a second end of the rod main body; wherein, the connecting rod is made of a steel; a residual compressive stress in at least a portion of a surface of the rod main body is 1000 MPa or more; and circumcircle diameters of carbide and circumcircle diameters of inclusions contained in a surface vicinity of the rod main body are each 10 μm or less .

[0022] In a preferred embodiment, the steel has an oxygen content of 10 wtppm or less and a sulfur content of 0.01wt% or less.

[0023] In a preferred embodiment, a carbon concentration of the rod main body at a depth of 0.1 mm from the surface thereof is no less than 0.9wt% and no more than 1.2wt%. [0024] An internal combustion engine according to the present invention includes a connecting rod having the above construction.

[0025] A transportation apparatus according to the present invention comprises an internal combustion engine having the above construction. [0026] A production method for a connecting rod according to the present invention is a production method for a connecting rod including a rod main body, a small end provided at a first end of the rod main body, and a big end provided at a second end of the rod main body, the method comprising the steps of: providing a workpiece made of a steel containing no less than 0.1wt% and no more than 0.45wt% of carbon, 10 wtppm or less of oxygen, and 0.01wt% or less of sulfur; performing a plural number of rounds of carburization treatment for the workpiece in an ambient atmosphere having a carbon potential of 0.9% or more, where a last round of carburization treatment among the plural number of rounds of carburization treatment is performed in an ambient atmosphere having a carbon potential of no less than 0.9% and no more than 1.2%; and performing a fine-particle shot peening for the workpiece after having been subjected to the carburization treatment.

[0027] In a preferred embodiment, the step of performing a fine-particle shot peening is performed so that a residual compressive stress in at least a portion of a surface of the rod main body is 1000 MPa or more.

[0028] In a connecting rod according to the present invention, a residual compressive stress in at least a portion of the surface of the rod main body is 1000 MPa or more. That is, a residual compressive stress which is greater than what would be conferred by a traditional shot peening is conferred to the connecting rod according to the present invention. Specifically, a residual compressive stress of 1000 MPa or more can be conferred through a fine- particle shot peening. Conventionally, when a fine-particle shot peening is performed for a connecting rod which has been subjected to a high-concentration carburization treatment, carbide and inclusions protruding from the surface of the parent phase would become causes of breakage. On the other hand, in the connecting rod according to the present invention, circumcircle diameters of carbide and inclusions contained in the surface vicinity of the rod main body are as small as 10 β m or less, so that stress concentration at the surface is prevented, and breakage originating from the surface is unlikely to occur. Thus, since a residual compressive stress as large as 1000 MPa or more is conferred to the surface and also breakage originating from the surface is unlikely to occur, the connecting rod according to the present invention has an excellent fatigue strength. Moreover, since the connecting rod according to the present invention is unlikely to experience breakage originating from the surface even when subjected to a fine-particle shot peening, it can be suitably produced by using a high- concentration carburization treatment and a fine-particle shot peening in conjunction. Therefore, the connecting rod according to the present invention has a connecting rod main body with an excellent mechanical strength, and is suitable for weight reduction.

[0029] In a steel which is used as the material of the connecting rod, inclusions whose circumcircle diameters have the possibility of growing to 10 μ m or more are oxide and sulfide. By setting the oxygen content in the steel to 10 wtppm or less, the circumcircle diameters of the oxide are ensured to be 10 μ m or less. By setting the sulfur content in the steel to 0.01wt% or less, the circumcircle diameters of the sulfide are ensured to be 10 μ m or less. Thus, by- setting the oxygen content in the steel to 10 wtppm or less and setting the sulfur content in the steel to 0.01wt% or less, it can be ensured that the circumcircle diameters of the inclusions are 10 μm or less.

[0030] By setting the carbon potential of the ambient atmosphere when performing a high-concentration carburization treatment (more specifically, when performing the last round of carburization treatment) to 1.2% or less, the circumcircle diameters of the carbide are ensured to be 10 μ m or less. By setting the carbon potential to 0.9% or more, carbide is allowed to deposit in the surface vicinity with an increased certainty, thus making it possible to sufficiently obtain the effects of the high-concentration carburization treatment. The carbon potential of the ambient atmosphere when performing the last round of carburization treatment is substantially equal to the carbon concentration at a depth of 0.1 mm from the surface of the rod main body of the completed connecting rod. Thus, it can be said that, when the carbon concentration at a depth of 0.1 mm from the surface of the rod main body is no less than 0.9wt% and no more than 1.2wt%, the circumcircle diameter of the carbide is ensured to be 10 μ m or less, while sufficiently obtaining the effects of the high-concentration carburization treatment. [0031] The connecting rod according to the present invention is suitably used for various types of internal combustion engines. Since the connecting rod according to the present invention is suitable for weight reduction, an internal combustion engine having the connecting rod according to the present invention has an improved response. Since the weight of the connecting rod is reduced, the weight of a reciprocating portion in the internal combustion engine is reduced, so that primary vibration (vibration which occurs due to reciprocation of the reciprocating portion including the piston and the connecting rod, with a cycle of one per rotation of the crankshaft) can be reduced. Furthermore, the connecting rod according to the present invention has an excellent fatigue strength, and therefore the reliability of the internal combustion engine is also improved. [0032] When an internal combustion engine having the connecting rod according to the present invention is used in a transportation apparatus, there is little primary vibration thus providing a much smoother ride for a passenger. Moreover, since there is no need to provide countermeasures against vibration for the vehicle body, a significant reduction in weight can be realized.

[0033] In a production method for the connecting rod according to the present invention, a workpiece which is made of a steel containing no less than 0.1wt% and no more than 0.45wt% of carbon is subjected to a plural number of rounds of carburization treatment in an ambient atmosphere having a carbon potential of 0.9% or more. That is, a high- concentration carburization treatment is performed. Furthermore, a fine-particle shot peening is performed for the workpiece which has been subjected to the high- concentration carburization treatment. Through this fine- particle shot peening, a large residual compressive stress can be conferred to the surface of the workpiece. In the production method for the connecting rod according to the present invention, at the step of providing a workpiece, a workpiece which is made of a steel containing 10 wtppm or less of oxygen and 0.01wt% or less of sulfur is provided. Therefore, the circumcircle diameters of oxide and sulfide in the surface vicinity of the completed connecting rod can be ensured to be 10 μ m or less. Moreover, since the carbon potential of the ambient atmosphere when performing a last round of carburization treatment among the plural number of rounds of carburization treatment is 1.2% or less, the circumcircle diameter of carbide in the surface vicinity of the completed connecting rod is ensured to be 10 μm or less. Thus, since the circumcircle diameters of carbide and inclusions (oxide and sulfide) are as small as 10 μ m or less, even if a fine-particle shot peening is performed after the high-concentration carburization treatment, breakage originating from the surface is unlikely to occur. Therefore, with the production method for the connecting rod according to the present invention, a connecting rod having an excellent fatigue strength is produced. Moreover, since a fine-particle shot peening is performed in addition to a high-concentration carburization treatment in the production method for the connecting rod according to the present invention, there is provided a connecting rod whose connecting rod main body has an excellent mechanical strength and which is suitable for weight reduction.

[0034] From the standpoint of increasing the fatigue strength of the connecting rod, the step of performing a fine-particle shot peening is preferably performed in such a manner that the residual compressive stress at the surface of the portion corresponding to the rod main body of the workpiece is as large as possible, or specifically, so that the residual compressive stress in at least a portion of the surface of the rod main body is 1000 MPa or more.

[Advantageous Effect of Invention]

[0035] According to the present invention, there is provided a connecting rod whose rod main body has a good mechanical strength and which is suitable for weight reduction, as well as a production method thereof. [Brief Description of Drawings]

[0036] [FIG. 1] A diagram schematically showing a connecting rod 1 according to a preferred embodiment of the present invention, where: (a) is a plan view; (b) is a cross- sectional view taken along line IB-IB ¹ in (a) ; and (c) is a cross-sectional view taken along line 1C-1C in (a). [FIG. 2] A cross-sectional view schematically showing a surface vicinity of the connecting rod 1 according to a preferred embodiment of the present invention. [FIG. 3] A flowchart showing a production method for the connecting rod 1 according to a preferred embodiment of the present invention.

[FIG. 4] (a) is a diagram schematically showing metallographical structure which is formed in the surface vicinity of the connecting rod 1 through a first round of carburization treatment, and (b) is a diagram schematically showing metallographical structure which is formed in the surface vicinity of the connecting rod 1 through a second round of carburization treatment.

[FIG. 5] A graph showing results of performing a bending fatigue test for actual parts, with respect to Examples 1 to 4, Comparative Examples 1 to 5, and conventional connecting rods .

[FIG. 6] A cross-sectional view schematically showing an internal combustion engine 100 having the connecting rod 1 according to a preferred embodiment of the present invention. [FIG. 7] A side view schematically showing a motorcycle having the internal combustion engine 100 shown in FIG. 6. [FIG. 8] A flowchart showing a production method for a conventional connecting rod.

[FIG. 9] (a) is a cross-sectional view schematically showing the surface vicinity of a connecting rod before being subjected to a fine-particle shot peening; and (b) is a cross-sectional view schematically showing the surface vicinity of the connecting rod after being subjected to a fine-particle shot peening. [Description of Embodiments]

[0037] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiment .

[0038] FIGS. 1 (a) to (c) show a connecting rod 1 according to the present embodiment. FIG. 1 (a) is a plan view schematically showing the connecting rod 1. FIG. 1 (b) is a cross-sectional view taken along line IB-IB ¹ in FIG. 1 (a) . FIG. 1 (c) is a cross-sectional view taken along line 1C-1C in FIG. l(a) .

[0039] As shown in FIGS. 1 (a) and (b) , the connecting rod 1 includes: a rod main body 10; a small end 20 which is provided at one end (first end) of the rod main body 10; and a big end 30 which is provided at another end (second end) of the rod main body 10. The connecting rod 1 is made of steel. [0040] The rod main body (axis portion) 10 has a bar shape. A cross-sectional shape of the rod main body 10 is typically the letter "H", as shown in FIG. 1 (c) . [0041] The small end 20 has a throughhole (piston pin hole) 22 through which a piston pin is to be inserted. The small end 20 is connected to a piston via the piston pin. An inner peripheral surface 22a of the piston pin hole 22 is in contact with the piston pin, typically not via any bearing. [0042] The big end 30 has a throughhole (crankpin hole) 32 through which a crankpin is to be inserted. The big end 30 is connected to a crankshaft via the crankpin. A rolling bearing such as a roller bearing is typically placed in the crankpin hole 32, and so an inner peripheral surface 32a of the crankpin hole 32 is in contact with the rolling bearing. [0043] During its production steps, the connecting rod 1 has been subjected to a high-concentration carburization treatment. Therefore, as shown in FIG. 2, carbide 3 has deposited in the surface vicinity of the connecting rod 1. Specifically, the carbide 3 may be Fe ₃ C or Cr ₃ C, for example. Moreover, inclusions 4 such as oxide and/or sulfide also exist in the surface vicinity of the connecting rod 1. Specifically, the oxide may be Al ₂ O ₃ or SiO ₂ , and the sulfide may be FeS or MnS, for example.

[0044] Furthermore, during its production steps, the connecting rod 1 has been subjected to a fine-particle shot peening, whereby a residual compressive stress is conferred thereto. Moreover, as shown in FIG. 2, since the parent phase 2 has been ground off through the fine-particle shot peening, the carbide 3 and inclusions 4 protrude from the surface 2a of the parent phase 2, at the surface of the connecting rod 1.

[0045] In the present embodiment, a residual compressive stress of 1000 MPa or more is conferred to at least a portion of the surface of the rod main body 10. A traditional shot peening can only confer a residual compressive stress of about 700 MPa to about 800 MPa at the most. In other words, a residual compressive stress which is greater than what would be conferred by a traditional shot peening is conferred to the connecting rod 1 of the present embodiment. Note that the residual compressive stress can be measured with an x-ray- residual stress measurement apparatus, for example. [0046] Conventionally, when a fine-particle shot peening is performed for a connecting rod which has been subjected to a high-concentration carburization treatment, the carbide and inclusions protruding from the surface of the parent phase become causes of breakage. As a result of a detailed study of the relationship between occurrence of breakage originating from the surface of the rod main body and microstructures in the surface vicinity of the rod main body, the inventors have found that there is a correlation between likeliness of breakage and the sizes of the carbide and inclusions .

[0047] In the connecting rod 1 of the present embodiment, circumcircle diameters Dl and D2 of the carbide 3 and inclusions 4 contained in the surface vicinity (specifically, a region down to a depth 0.05 mm from the surface) of the rod main body 10 (see FIG. 2) are as small as 10 μ m or less each, so that stress concentration at the surface of the rod main body 10 is prevented, whereby breakage originating from the surface is suppressed. As used herein, the circumcircle diameter Dl of the carbide 3 and the circumcircle diameter D2 of the inclusions 4 are, literally, diameters of imaginary circles circumscribed around the carbide 3 and inclusions 4, as can be seen from FIG. 2. The sizes of the carbide 3 and inclusions 4 (e.g., the aforementioned circumcircle diameters) can be measured with a metallurgical microscope

(optical microscope) or a scanning electron microscope.

[0048] Thus, since a residual compressive stress as large as 1000 MPa or more is conferred to the surface and also breakage originating from the surface is unlikely to occur, the connecting rod 1 of the present embodiment has an excellent fatigue strength. Moreover, since the connecting rod of the present embodiment has been subjected to a high- concentration carburization treatment and a fine-particle shot peening during its production steps, the connecting rod main body 10 has an excellent mechanical strength, and thus the connecting rod is suitable for weight reduction. When a conventional connecting rod is subjected to a fine-particle shot peening in addition to a high-concentration carburization treatment, breakage originating from the surface may occur. On the other hand, as has already been described, the connecting rod 1 of the present embodiment is unlikely to experience breakage originating from the surface, so that it can be suitably produced by a production method which involves both a high-concentration carburization treatment and a fine-particle shot peening.

[0049] Next, with reference to FIG. 3, a production method for the connecting rod 1 of the present embodiment will be described. FIG. 3 is a flowchart showing production steps of the connecting rod 1.

[0050] First, a workpiece which has been molded via forging from a steel containing no less than 0.1wt% and no more than 0.45wt% of carbon is provided (step Sl) . As a steel containing no less than 0.1wt% and no more than 0.45wt% of carbon, SCM420 (which is chrome molybdenum steel) can be used, for example. SCM420 contains 0.18wt% to 0.23wt% carbon, 0.90wt% to 1.20wt% chromium and 0.15wt% to 0.30wt% molybdenum. Other than the aforementioned SCM420, it is also possible to use SCr420, SCM435, SCM440, or the like for the steel which is the workpiece material.

[0051] Moreover, the steel used in the present embodiment has an oxygen content of 10 wtppm or less, and a sulfur content of 0.01wt% or less. A steel which is commonly-used as the material of the connecting rod has an oxygen content of 15 wtppm or more, and a sulfur content of about 0.020wt%. In other words, according to the present embodiment, a steel having a smaller oxygen content and a smaller sulfur content (i.e., highly clean steel) than those of commonly-used steels is used. Although forging is illustrated as an example, the molding technigue in the step of providing a workpiece is not limited thereto. The workpiece may be molded by sintering, casting, sinter forging, or the like, for example. [0052] Next, the workpiece is machined (step S2) . Through this machining, the outer dimensions of the workpiece after forging are adjusted. For example, removal of burrs, formation of the piston pin hole 22 and the crankpin hole 32, end-facing of the small end 20 and the big end 30, and the like are performed.

[0053] Then, the workpiece is subjected to a first round of carburization and a hardening (or furnace cooling) (step S3) . This carburization treatment is performed in an ambient atmosphere having a carbon potential of 0.9% or more. Note that no masking is applied to the workpiece before performing the carburization treatment. The temperature of the first round of carburization treatment is set equal to or greater than the Al transformation point (i.e., a eutectic transformation temperature of the steel) . Through the first round of carburization treatment, the surface of the steel is excessively carburized. FIG. 4 (a) schematically shows metallographical structure which is formed in the surface vicinity of the connecting rod 1 through the first round of carburization treatment. As shown in FIG. 4 (a) , mesh-like carbide 3 has deposited in between relatively large crystal grains 2' of martensite.

[0054] Next, the workpiece is subjected to a second round of carburization, a hardening, and a tempering (step S4) . This carburization treatment is performed in an ambient atmosphere having a carbon potential of no less than 0.9% and no more than 1.2%. The temperature of the second round of carburization treatment is set equal to or greater than the Al transformation point but equal to or less than the Acm transformation point (i.e., a transformation temperature at which cementite deposits from austenite of the steel) . Through the second round of carburization treatment, the carbon within the excessively-carburized surface layer diffuses into the interior. FIG. 4 (b) schematically shows metallographical structure which is formed in the surface vicinity of the connecting rod 1 through the second round of carburization treatment. As shown in FIG. 4 (b) , carbide 3 in minute particulate form has deposited in between relatively small crystal grains 2' of martensite.

[0055] Next, a fine-particle shot peening is performed for the workpiece which has been subjected to the plural number of rounds of carburization treatment (step S5) . This step is carried out to ensure that there is a residual compressive stress of 1000 MPa or more (and more preferably 1200 MPa or more) in at least a portion of the surface of the rod main body 10. Note that, as can be seen from FIG. 1, the rod main body 10 becomes thinner from the big end 30 toward the small end 20. Therefore, regarding the surface of the rod main body 10, it is preferable that there is a residual compressive stress of 1000 MPa or more in at least the vicinity of the small end 20. Of course, it is more preferable that there is a residual compressive stress of 1000 MPa or more over the entire surface of the rod main body 10. As the shot material, fine metal particles or fine non- metal particles having a diameter of 500 μ. m or less

(typically about 20 μm to about 200 μm) are used. The shot material is accelerated to about 200 m/s, and allowed to collide against the surface of the workpiece. Through adjustment of the shot pressure, the velocity of the shot material is controlled as appropriate.

[0056] Thereafter, the workpiece is polished (step S6) . For example, the inner peripheral surface 22a of the piston pin hole 22 and the inner peripheral surface 32a of the crankpin hole 32 are polished. In this manner, the connecting rod 1 is completed.

[0057] In the production method of the present embodiment, a workpiece which is made of a steel containing no less than 0.1wt% and no more than 0.45wt% of carbon is subjected to a plural number of rounds of carburization treatment, in an ambient atmosphere having a carbon potential of 0.9% or more (steps S3 and S4) . In other words, a high-concentration carburization treatment is performed. Furthermore, a fine- particle shot peening is performed for the workpiece which has been subjected to the high-concentration carburization treatment (step S5) . Through this fine-particle shot peening, a residual compressive stress as large as 1000 MPa or more can be conferred to the surface of the workpiece. [0058] Moreover, in the production method according to the present embodiment, in the step of providing a workpiece, a workpiece which is made of a steel containing 10 wtppm or less of oxygen and 0.01wt% or less of sulfur is provided. In the steel which is used as the material of the connecting rod 1, the inclusions 4 whose circumcircle diameters have the possibility of growing to 10 μ m or more are oxide and sulfide. As in the present embodiment, by setting the oxygen content and sulfur content in the steel so as to be smaller than their commonly-used values (respectively, 10 wtppm or less, and 0.01wt% or less), the circumcircle diameters of the oxide and sulfide in the surface vicinity of the completed connecting rod 1 are ensured to be 10 μm or less. Namely, it is ensured that no inclusions 4 whose circumcircle diameters exceed 10 μm exist in the surface vicinity of the rod main body 10.

[0059] Furthermore, in the production method according to the present embodiment, the carbon potential of the ambient atmosphere when performing a second round of carburization treatment (i.e., the last round of carburization treatment among the plural number of rounds of carburization treatment) is 1.2% or less, so that the circumcircle diameter of the carbide 3 in the surface vicinity of the completed connecting rod 1 is ensured to be 10 μ m or less. Note that, by ensuring a carbon potential of 0.9% or more, carbide 3 is allowed to deposit in the surface vicinity with an increased certainty, thus making it possible to sufficiently obtain the effects of the high-concentration carburization treatment. The carbon potential of the ambient atmosphere when performing the second round of carburization treatment (the last round of carburization treatment) can be regarded as substantially equal to the carbon concentration at a depth of 0.1 mm from the surface of the completed connecting rod 1. Therefore, since the carbon concentration at the depth of 0.1 mm from the surface of the rod main body 10 is no less than 0.9wt% and no more than 1.2wt%, it is possible to ensure that the circumcircle diameter of the carbide 3 is 10 μ m or less, while sufficiently obtaining the effects of the high- concentration carburization treatment.

[0060] As described above, with the production method according to the present embodiment, it is possible to produce a connecting rod 1 in which the carbide 3 and inclusions 4 contained in the surface vicinity have circumcircle diameters of 10 μ m or less. As has already been described, breakage originating from the surface is suppressed by ensuring that the circumcircle diameters of the carbide 3 and inclusions 4 contained in the surface vicinity of the rod main body 10 are 10 μ m or less. Therefore, with the production method according to the present embodiment, a connecting rod having an excellent fatigue strength can be produced. Moreover, with the production method according to the present embodiment, a fine-particle shot peening is performed in addition to a high-concentration carburization treatment, so that a connecting rod 1 whose connecting rod main body 10 has an excellent mechanical strength and which is suitable for weight reduction is produced.

[0061] Table 1 shows, regarding connecting rods 1 (Examples 1 to 4) which were actually produced by the production method according to the present embodiment, a relationship between the carbon potential (CP) of the ambient atmosphere in the second round of carburization treatment, the oxygen content and sulfur content in the steel, the circumcircle diameter Dl of the carbide 3 and the circumcircle diameters D2 of the inclusions 4 (oxide and sulfide) . Note that, in the production of Examples 1 to 4, molding in the forging step was performed by a hot forging at 1200 ⁰ C, using a steel containing 0.25wt% or less of carbon (step Sl) . Moreover, the first round of carburization treatment was performed at 910 °C to 930 °C for 300 minutes, and the second round of carburization treatment was performed at 850 °C to 920 °C for 150 minutes. The fine-particle shot peening was performed at a shot pressure of 0.5 MPa for 30 seconds, where fine particles made of high speed steel SKH59 having an average diameter of 45 μm were used as the shot material. [0062] [Table 1 ]

[0063] Table 1 also shows similar data of connecting rods (Comparative Examples 1 to 5) which were produced by production methods differing from the present embodiment with respect to at least one of the carbon potential, the oxygen content, and the sulfur content.

[0064] As can be seen from Table 1, in Examples 1 to 4, the carbon potential of the carburization ambient atmosphere is 1.2% or less, and the circumcircle diameter Dl of the carbide 3 is 10 μ m or less. On the other hand, in Comparative Examples 1 and 5, the carbon potential of the carburization ambient atmosphere exceeds 1.2%, so that the circumcircle diameter Dl of the carbide 3 exceeds 10 μm. [0065] Moreover, as can be seen from Table 1, in Examples 1 to 4, the oxygen content in the steel is 10 wtppm or less and the sulfur content in the steel is 0.01wt% or less, so that the circumcircle diameters D2 of the oxide and sulfide (inclusions 4) are 10 μ m or less. On the other hand, in Comparative Examples 1 to 4, the oxygen content in the steel exceeds 10 wtppm and the sulfur content in the steel exceeds 0.01wt%, so that the circumcircle diameters D2 of the oxide and sulfide (inclusions 4) exceed 10 μm.

[0066] FIG. 5 shows results of measuring fatigue strength with respect to Examples 1 to 4 and Comparative Examples 1 to 5. FIG. 5 is a graph, when performing a bending fatigue test for actual parts with respect to Examples 1 to 4 and Comparative Examples 1 to 5, showing a relationship between a number N of repetitions of bending until reaching breakage and a stress amplitude σ a. FIG. 5 also shows results of measuring fatigue strength with respect to connecting rods which were produced by the conventional production method described with reference to FIG. 8.

[0067] As shown in FIG. 5, in Comparative Examples 1 to 5, breakage occurs with smaller numbers N of repetitions than in Examples 1 to 4. In other words, as compared to Comparative Examples 1 to 5 in which the circumcircle diameters Dl and D2 of the carbide 3 and inclusions 4 exceed 10 μ m, the fatigue strength is improved in Examples 1 to 4 in which the circumcircle diameters Dl and D2 of the carbide 3 and inclusions 4 are 10 μm or less. Moreover, as shown in FIG. 5, the conventional connecting rods which have not been subjected to a fine-particle shot peening experienced breakage with numbers N of repetitions which were even smaller than those in Comparative Examples 1 to 5. In the example shown in FIG. 5, the fatigue strengths of Examples 1 to 4 are improved by about 40% as compared to the fatigue strengths of the conventional connecting rods. [0068] As described above, according to the present invention, a connecting rod 1 is obtained whose rod main body 10 has an excellent mechanical strength and which is suitable for weight reduction. Specifically, the weight of the connecting rod 1 can be reduced by about 20% according to the present invention. Although FIG. 1 illustrates a connecting rod 1 of a so-called integral type as an example, the present invention is not limited thereto. The present invention is also suitably used for a connecting rod of a so-called split- type, in which the big end is split into two. Although the present embodiment illustrates an exemplary case where the carburization treatment is performed twice, the carburization treatment may be performed any plural number of times; as described in Patent Literature 1, the carburization treatment may be performed three times or more. For example, in the case of vacuum carburization, it is preferable to perform three or more rounds of carburization treatment; in that case, the carbon potential in the last round of carburization treatment is set to no less than 0.9% and no more than 1.2%.

[0069] The connecting rod 1 according to the present embodiment is suitably used for the internal combustion engines of various transportation apparatuses, such as a car, a motorbike, a bus, a truck, a tractor, an airplane, a motorboat, and a vehicle for civil engineering use. FIG. 6 shows an exemplary internal combustion engine 100 having the connecting rod 1 according to the present embodiment. The internal combustion engine 100 includes a crankcase 110, a cylinder block 120, and a cylinder head 130.

[0070] A crankshaft 111 is accommodated in the crankcase 110. The crankshaft 111 has a crankpin 112 and a crankweb 113.

[0071] A cylinder block 120 is provided above the crankcase 110. A cylinder sleeve 121 having a cylindrical shape is fitted within the cylinder block 120, such that a piston 122 is allowed to reciprocate inside the cylinder sleeve 121.

[0072] A cylinder head 130 is provided above the cylinder block 120. In conjunction with the piston 122 and the cylinder sleeve 121 in the cylinder block 120, the cylinder head 130 define a combustion chamber 131. The cylinder head 130 has an intake port 132 and an exhaust port 133. An intake valve 134 for supplying vapor to the interior of the combustion chamber 131 is provided within the intake port 132, and an exhaust valve 135 for enabling evacuation of the combustion chamber 131 is provided within the exhaust port 133.

[0073] The piston 122 and the crankshaft 111 are linked via the connecting rod 1. Specifically, a piston pin 123 of the piston 122 is inserted in a piston pin hole which is formed in the small end 20 of the connecting rod 1, and the crankpin 112 of the crankshaft 111 is inserted in a crankpin hole which is formed in the big end 30, thus linking the piston 122 and the crankshaft 111. A roller bearing (rolling bearing) 114 is provided between the inner peripheral surface of the crankpin hole and the crankpin 112.

[0074] Since the connecting rod 1 according to the present embodiment is suitable for weight reduction, the internal combustion engine 100 having the connecting rod 1 according to the present embodiment has an improved response. Since the weight of the connecting rod 1 is reduced, the weight of a reciprocating portion in the internal combustion engine 100 is reduced, so that primary vibration (vibration which occurs due to reciprocation of the reciprocating portion including the piston 120 and the connecting rod 1, with a cycle of one per rotation of the crankshaft 111) can be reduced. Furthermore, the connecting rod 1 according to the present embodiment has an excellent fatigue strength, and therefore the reliability of the internal combustion engine 100 is also improved .

[0075] FIG. 7 shows a motorcycle having the internal combustion engine 100 shown in FIG. 6. In the motorcycle shown in FIG. 7, a head pipe 302 is provided at the front end of a body frame 301. To the head pipe 302, front forks 303 are attached so as to be capable of swinging in the right- left direction of the vehicle. At the lower end of the front forks 303, a front wheel 304 is supported so as to be capable of rotating.

[0076] A seat rail 306 is attached at an upper portion of the rear end of the body frame 301 so as to extend in the rear direction. A fuel tank 307 is provided on the body frame 301, and a main seat 308a and a tandem seat 308b are provided on the seat rail 306.

[0077] Rear arms 309 extending in the rear direction are attached to the rear end of the body frame 301. At the rear end of the rear arms 309, a rear wheel 310 is supported so as to be capable of rotating.

[0078] At the central portion of the body frame 301, the internal combustion engine 100 shown in FIG. 6 is held. The internal combustion engine 100 includes the connecting rod 1 according to the present embodiment. A radiator 311 is provided in front of the internal combustion engine 100. An exhaust pipe 312 is connected to an exhaust port of the engine 100, and a muffler 313 is attached to the rear end of the exhaust pipe 312. [0079] A transmission 315 is linked to the internal combustion engine 100. Driving sprockets 317 are attached on an output axis 316 of the transmission 315. Via a chain 318, the driving sprockets 317 are linked to rear wheel sprockets 319 of the rear wheel 310. The transmission 315 and the chain 318 function as a transmitting mechanism for transmitting the motive power generated in the internal combustion engine 100 to the driving wheel.

[0080] Since the internal combustion engine 100 including the connecting rod 1 according to the present embodiment is used in the motorcycle shown in FIG. 7, there is little primary vibration, whereby the rider experiences a much smoother ride. Moreover, since there is no need to provide countermeasures against vibration for the vehicle body, a significant reduction in weight can be realized. [Industrial Applicability]

[0081] According to the present invention, there is provided a connecting rod whose rod main body has a good mechanical strength and which is suitable for weight reduction, as well as a production method thereof. The connecting rod according to the present invention is suitably used for various types of internal combustion engines (e.g., an engine for a transportation apparatus) . [Reference Signs List]

1 connecting rod

2 parent phase surface of parent phase carbide inclusions rod main body (axis portion) small end piston pin hole big end crankpin hole