JP2012255551A - Method of manufacturing connecting rod - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a connecting rod, superior in strength of a rod body part, and suitable for weight reduction.SOLUTION: This method of manufacturing the connecting rod includes a preparing process S1 for preparing a workpiece formed of steel including carbon of 0.1-0.45 wt.%, a heat treatment process S3 and S4 of performing heat treatment including a plurality of times of carbonizing treatments under an atmosphere having carbon potential of 0.8% or more to the workpiece, and a surface smoothing process S5 of smoothing a surface of the workpiece so that the maximum trough depth in at least a part of a rod body part surface becomes smaller than a predetermined value, and the surface smoothing process S5 includes a barrel polishing process.

Description

  The present invention relates to a connecting rod and a manufacturing method thereof. The present invention also relates to an internal combustion engine and a transportation device provided with a connecting rod.

  In an internal combustion engine, a member called a connecting rod (connecting rod) is used to connect a piston and a crankshaft. The connecting rod includes a rod-shaped rod body (shaft), a small end provided at one end of the rod body, and a large end provided at the other end of the rod body. The small end is connected to the piston, while the large end is connected to the crankshaft.

  The connecting rod reciprocates inside the internal combustion engine. Therefore, when the connecting rod is reduced in weight, the internal combustion engine can be rotated more smoothly and vibration is reduced. Thus, the connecting rod is required to be lightweight.

  Further, the connecting rod is required to have sufficient strength because the explosive force generated in the combustion chamber is transmitted through the piston. In particular, in an internal combustion engine (for example, an internal combustion engine for a motorcycle) that is operated at a high rotational speed, the connecting rod is required to have higher strength, and further weight reduction is required to improve acceleration performance.

  As a technique for increasing the strength of steel connecting rods, carburizing treatment in which carbon penetrates the surface of the connecting rods is known. Since the carbon concentration in the vicinity of the surface of the connecting rod is increased by the carburizing treatment, the surface hardness is increased after quenching of the connecting rod. Therefore, the strength of the connecting rod is improved. Moreover, since sufficient strength can be secured even if the connecting rod is thin, the connecting rod can be reduced in weight. Even when carburizing is performed, the rod body is often subjected to carburizing prevention so that the surface hardness does not increase. This is because the rod body portion preferably has high toughness.

  Patent Document 1 proposes a high-concentration carburizing process as a technique for further increasing the surface hardness of the connecting rod. In the high-concentration carburizing treatment, carburizing is performed a plurality of times in an atmosphere having a carbon potential (CP) of 0.8% or more. As a result, 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 is reduced. Therefore, the surface hardness is remarkably increased.

  However, even in the high-concentration carburizing process, in order to ensure the toughness of the rod main body portion, the number of carburizing times is less than the other portions in the rod main body portion, or no carburizing is performed at all. In FIG. 8, the example of the manufacturing process of the connecting rod in the case of performing a high concentration carburizing process is shown.

  First, a steel material is formed into a connecting rod shape by forging (step S11). Next, masking is performed on the surface of the connecting rod with a carburizing agent or the like (step S12). Thereafter, machining is performed to form a piston pin hole at the small end and a crank pin hole at the large end (step S13).

  Next, first carburizing and cooling (or quenching) are performed (step S14). Carburization is performed in an atmosphere having a carbon potential of 0.8% or more. At this time, the carbon concentration in the vicinity of the surface is high in a portion not covered by masking such as the inner peripheral surface of the crankpin hole (that is, a portion where masking is removed by cutting during machining). On the other hand, the carbon concentration hardly changes in the portion covered by masking like the rod main body (that is, the portion subjected to the carburizing prevention treatment). Subsequently, the masking remaining on the surface of the connecting rod is removed (step S15).

  Thereafter, second carburizing, quenching, and tempering are sequentially performed (step S16). The second carburization is also performed in an atmosphere having a carbon potential of 0.8% or more. At this time, since the masking of the connecting rod surface has already been removed, the carbon concentration in the vicinity of the surface increases throughout the connecting rod. Finally, the inner peripheral surface of the small end and the inner peripheral surface of the large end are polished (step S17).

  According to the manufacturing method described above, carburization is performed twice on a portion that has not been subjected to carburization prevention treatment, whereas carburization is performed only once on a portion that has been subjected to carburization prevention treatment, such as a rod body. Is given. Therefore, the strength of the entire connecting rod can be greatly improved while ensuring the toughness of the rod body.

JP 2000-313949 A

  However, in order to further reduce the weight of the connecting rod by further narrowing the rod body, further enhancement of the strength of the rod body is desired. For example, if the high-concentration carburizing process is performed without performing the carburizing prevention process on the rod body part, the surface hardness of the rod body part can be further increased.

  However, if the rod body is subjected to high-concentration carburizing treatment similar to other parts in this way, it is different from fatigue failure (generally referred to as “toughness is insufficient”). Breakage will occur. FIG. 9 shows a connecting rod 201 in which such a fracture has occurred. As shown in FIG. 9, the connecting rod 201 includes a rod main body portion 210, a small end portion 220, and a large end portion 230, and the vicinity of the small end portion 220 of the rod main body portion 210 (encircled by a dotted line in FIG. 9). The area is broken.

  FIGS. 10A and 10B show broken surfaces of the rod main body 210. FIG. As shown in FIG. 10 (a), the rod body 210 has an H-shaped cross-sectional shape in which the inner corner 10a and the outer corner 10b are defined. FIG. 10B shows an enlarged view of the vicinity of the inner corner 10a of the rod main body 210 (a region surrounded by a dotted line in FIG. 10A). As shown in FIG. 10B, the fracture occurs starting from the inner corner 10a. This fracture phenomenon is hereinafter referred to as “brittle fracture”.

  Thus, conventionally, when the rod body portion is subjected to high-concentration carburizing treatment at the same level as other portions, brittle fracture occurs in the rod body portion, so that it is difficult to further increase the strength of the rod body portion.

  This invention is made | formed in view of the said problem, The objective is to provide the connecting rod excellent in the intensity | strength of a rod main-body part, and suitable for weight reduction, and its manufacturing method.

  The connecting rod according to the first aspect of the present invention is a connecting rod formed of steel, comprising a rod body part, and a small end part and a large end part provided at one end and the other end of the rod body part, The carbon content at a depth of 0.1 mm from the surface of the rod body is 0.8 wt% or more, and the maximum valley depth of the surface of the rod body is 13 μm or less.

  A connecting rod according to a second aspect of the present invention is a connecting rod formed of steel, comprising a rod main body part, and a small end part and a large end part provided at one end and the other end of the rod main body part, The carbon content at a depth of 0.1 mm from the surface of the rod main body portion is 0.8 wt% or more, and the rod main body portion has an H shape in which a plurality of outer corner portions and a plurality of inner corner portions are defined. In the region having the smallest cross-sectional area of the rod main body portion, the maximum valley depth on the surface of the plurality of inner corner portions is 13 μm or less.

A connecting rod according to a third aspect of the present invention is a connecting rod formed of steel, comprising a rod main body, and a small end and a large end provided at one end and the other end of the rod main body, The carbon content at a depth of 0.1 mm from the surface of the rod body is 0.8 wt% or more, the maximum stress generated in the rod body is σ (MPa), and the maximum surface of the rod body is When the valley depth is a (m), the maximum stress σ and the maximum valley depth a satisfy the relationship of 10 ≧ 1.1σ (πa) ^1/2 .

The connecting rod according to the fourth aspect of the present invention is a connecting rod formed of steel, comprising a rod main body part, and a small end part and a large end part provided at one end and the other end of the rod main body part, The carbon content at a depth of 0.1 mm from the surface of the rod main body portion is 0.8 wt% or more, and the rod main body portion has an H shape in which a plurality of outer corner portions and a plurality of inner corner portions are defined. The maximum stress generated in the plurality of inner corners in the region having the smallest cross-sectional area of the rod main body portion is σ (MPa), and the maximum valley depth on the surface of the plurality of inner corner portions is When the thickness is a (m), the maximum stress σ and the maximum valley depth a satisfy the relationship of 10 ≧ 1.1σ (πa) ^1/2 .

  The internal combustion engine by this invention is provided with the connecting rod which has the said structure.

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

  A method for manufacturing a connecting rod according to the present invention is a method for manufacturing a connecting rod comprising a rod main body, and a small end and a large end provided at one end and the other end of the rod main body. A preparation step of preparing a workpiece formed of steel containing 0.45 wt% or less of carbon, and a plurality of carburizing treatments in an atmosphere having a carbon potential of 0.8% or more for the workpiece. A heat treatment step for performing heat treatment, and a surface smoothing step for smoothing the surface of the workpiece so that a maximum valley depth in at least a part of the surface of the rod body portion is smaller than a predetermined value. To do.

  In a preferred embodiment, the surface smoothing step is performed before the heat treatment step.

  In a preferred embodiment, the surface smoothing step is performed after the heat treatment step.

  In a preferred embodiment, the rod body has an H-shaped cross-sectional shape in which a plurality of outer corners and a plurality of inner corners are defined, and the surface smoothing step includes at least the plurality of inner corners. The maximum valley depth on the surface of the corner is set to be smaller than the predetermined value.

  In a preferred embodiment, the surface smoothing step is performed such that a maximum valley depth of the entire surface of the rod main body portion is smaller than the predetermined value.

  In the connecting rod according to the first aspect of the present invention, the carbon content at a depth of 0.1 mm from the surface of the rod main body is 0.8 wt% or more. That is, the rod main body is subjected to high-concentration carburizing treatment. Conventionally, when a high-concentration carburizing process similar to other parts is performed on the rod main body, brittle fracture may occur in the rod main body. On the other hand, in the connecting rod according to the first aspect of the present invention, the maximum valley depth of the surface of the rod main body portion is 13 μm or less, thereby suppressing the occurrence of brittle fracture in the rod main body portion. Therefore, the connecting rod according to the first aspect of the present invention is excellent in the strength of the rod body and is suitable for weight reduction.

  In the connecting rod according to the second aspect of the present invention, the rod body portion has an H-shaped cross-sectional shape in which a plurality of outer corner portions and a plurality of inner corner portions are defined, and is 0.1 mm from the surface of the rod body portion. The carbon content at a depth of 0.8 wt% or more. That is, the high-concentration carburizing process is performed on the rod main body portion having an H-shaped cross-sectional shape. In the connecting rod according to the second aspect of the present invention, the maximum valley depth at the surface of the plurality of inner corners is 13 μm or less in the region having the smallest cross-sectional area of the rod main body, whereby brittleness in the rod main body is achieved. The occurrence of destruction is suppressed. Therefore, the connecting rod according to the second aspect of the present invention is excellent in the strength of the rod main body and is suitable for weight reduction.

In the connecting rod according to the third aspect of the present invention, the carbon content at a depth of 0.1 mm from the surface of the rod main body is 0.8 wt% or more. That is, the rod main body is subjected to high-concentration carburizing treatment. In the connecting rod according to the third aspect of the present invention, the maximum stress σ (MPa) generated in the rod body and the maximum valley depth a (m) of the surface of the rod body are 10 ≧ 1.1σ (πa) ^{1 /} The relationship of ² is satisfied, whereby the occurrence of brittle fracture in the rod body is suppressed. Therefore, the connecting rod according to the third aspect of the present invention is excellent in the strength of the rod body and is suitable for weight reduction.

In the connecting rod according to the fourth aspect of the present invention, the rod body portion has an H-shaped cross-sectional shape in which a plurality of outer corner portions and a plurality of inner corner portions are defined, and is 0.1 mm from the surface of the rod body portion. The carbon content at a depth of 0.8 wt% or more. That is, the high-concentration carburizing process is performed on the rod main body portion having an H-shaped cross-sectional shape. In the connecting rod according to the fourth aspect of the present invention, the maximum stress σ (MPa) generated in the plurality of inner corners and the maximum in the surface of the plurality of inner corners in the region having the smallest cross-sectional area of the rod main body. The valley depth a (m) satisfies the relationship of 10 ≧ 1.1σ (πa) ^1/2 , thereby suppressing the occurrence of brittle fracture in the rod body. Therefore, the connecting rod according to the fourth aspect of the present invention is excellent in the strength of the rod body and is suitable for weight reduction.

  The connecting rod according to the present invention is suitably used for various internal combustion engines. Since the connecting rod according to the present invention is suitable for weight reduction, the responsiveness is improved in the internal combustion engine provided with the connecting rod according to the present invention. Further, since the weight of the connecting rod is reduced by reducing the weight of the connecting rod, the primary vibration (vibration generated once per cycle of the crankshaft due to the reciprocating motion of the reciprocating portion including the piston and connecting rod). ) Can be reduced. Furthermore, since the connecting rod according to the present invention is excellent in fatigue strength, the reliability of the internal combustion engine is also improved.

  When the internal combustion engine provided with the connecting rod according to the present invention is used for transportation equipment, since there is little primary vibration, unpleasant vibration felt by the occupant is reduced. In addition, since it is not necessary to take vibration countermeasures for the internal combustion engine and the vehicle body, a significant reduction in weight can be realized.

  In the method of manufacturing a connecting rod according to the present invention, in a heat treatment step, an atmosphere having a carbon potential of 0.8% or more with respect to a workpiece formed of steel containing 0.1 wt% or more and 0.45 wt% or less of carbon. The carburizing process is performed several times. That is, a high concentration carburizing process is performed on the workpiece. By this high-concentration carburizing treatment, the hardness of the entire connecting rod including the rod main body is remarkably increased. Furthermore, since the manufacturing method of the connecting rod according to the present invention includes a surface smoothing step of smoothing the surface of the workpiece, this surface smoothing step increases the maximum valley depth in at least a part of the surface of the rod body. It can be made smaller than a predetermined value. Therefore, the occurrence of brittle fracture in the rod body can be suppressed. Therefore, according to the connecting rod manufacturing method of the present invention, it is possible to manufacture a connecting rod that is excellent in strength of the rod main body and suitable for weight reduction.

  The surface smoothing step may be performed before the heat treatment step or after the heat treatment step. When the surface smoothing step is performed before the heat treatment step, the workpiece can be smoothed in a relatively soft state, so that the maximum valley depth can be easily reduced in a short time. On the other hand, if the surface smoothing step is performed after the heat treatment step, depending on the smoothing method (for example, smoothing by barrel polishing), compressive residual stress can be applied to the surface of the connecting rod, so that the strength is further increased. Can be improved.

  When the rod main body has an H-shaped cross-sectional shape in which a plurality of outer corners and a plurality of inner corners are defined, the surface smoothing step has a maximum valley depth on the surface of at least the plurality of inner corners. It is preferable to carry out so that it may become smaller than a predetermined value. This is because when the rod main body has an H-shaped cross-sectional shape, brittle fracture is likely to occur at the inner corner.

  From the viewpoint of more reliably preventing the occurrence of brittle fracture, the surface smoothing step is preferably performed so that the maximum valley depth of the entire surface of the rod main body portion is smaller than a predetermined value.

  ADVANTAGE OF THE INVENTION According to this invention, the connecting rod excellent in the intensity | strength of a rod main-body part and suitable for weight reduction and its manufacturing method are provided.

It is a figure which shows typically the connecting rod 1 in suitable embodiment of this invention, (a) is a top view, (b) is sectional drawing along the 1B-1B 'line in (a), (c) is It is sectional drawing along the 1C-1C 'line in (a). It is a flowchart which shows the manufacturing method of the connecting rod 1 in suitable embodiment of this invention. (A) is a figure which shows typically the metal structure formed in the surface vicinity of the connecting rod 1 by the 1st carburizing process, (b) is formed in the surface vicinity of the connecting rod 1 by the 2nd carburizing process. It is a figure which shows typically a metal structure. It is a flowchart which shows the manufacturing method of the connecting rod 1 in suitable embodiment of this invention. It is a graph which shows the result of having performed the real bending fatigue test about the connecting rod of Examples 1-4 and Comparative Examples 1-7. It is sectional drawing which shows typically the internal combustion engine 100 provided with the connecting rod 1 in suitable embodiment of this invention. It is a side view which shows typically the motorcycle provided with the internal combustion engine 100 shown in FIG. It is a flowchart which shows the manufacturing method of the conventional connecting rod. It is a photograph which shows the connecting rod 201 which the brittle fracture generate | occur | produced. (A) And (b) is a photograph which shows the torn surface of the rod main-body part of the connecting rod 201. FIG.

As a result of detailed investigation of the cause of the occurrence of brittle fracture in the rod body portion subjected to high-concentration carburizing treatment, the inventors of the present application have found that the surface roughness of the rod body portion has a great influence on the occurrence of brittle fracture. I found out. Specifically, when “maximum valley depth” which is one of the parameters representing the surface roughness is a (m) and the maximum stress during use of the connecting rod is σ (MPa), the following formula ( It was found that whether or not brittle fracture occurs can be predicted depending on the magnitude of the stress intensity factor KI represented by 1).
KI = 1.1σ (πa) ^1/2 (1)

  This invention is made | formed based on the said knowledge which this inventor discovered. Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment.

  The connecting rod 1 in this embodiment is shown to Fig.1 (a)-(c). FIG. 1A is a plan view schematically showing the connecting rod 1. 1B is a cross-sectional view taken along the line 1B-1B ′ in FIG. 1A, and FIG. 1C is taken along the 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. Large end 30. The connecting rod 1 is made of steel.

  The rod main body (shaft) 10 has a rod shape. As shown in FIG. 1C, the cross-sectional shape of the rod main body 10 is an H shape in which a plurality of inner corner portions 10a and a plurality of outer corner portions 10b are defined. Since the cross-sectional shape of the rod main body 10 is H-shaped, it is possible to reduce the weight while maintaining the strength of the rod main body 10.

  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 22a of the piston pin hole 22 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 is passed. The large end 30 is connected to the crankshaft via a crankpin. Since a rolling bearing such as a roller bearing is typically disposed in the crankpin hole 32, the inner peripheral surface 32a of the crankpin 32 is in contact with the rolling bearing.

  The connecting rod 1 is subjected to high-concentration carburizing treatment in its manufacturing process. Therefore, the carbon content (carbon concentration) at a depth of 0.1 mm from the surface of the rod main body 10 is 0.8 wt% or more. The distribution of the carbon content in the depth direction of the connecting rod 1 can be measured by, for example, an electron beam microanalyzer (EPMA).

As already described, the occurrence of brittle fracture can be predicted by the magnitude of the stress intensity factor KI (= 1.1σ (πa) ^1/2 ). Since the rod body that has not been subjected to high-concentration carburizing treatment has high toughness, even if the stress intensity factor KI is a large value exceeding 18 MPa · m ^1/2 , brittle fracture does not occur and fatigue fracture Will occur. On the other hand, in the rod main body subjected to the high-concentration carburizing treatment, when the stress intensity factor KI exceeds 10 MPa · m ^1/2 , brittle fracture occurs. For this reason, the rod main body is damaged even by a stress that does not cause fatigue failure.

In the connecting rod 1 in the present embodiment, the stress intensity factor KI is set to 10 or less. That is, the maximum stress σ (MPa) generated in the rod body 10 (assumed to occur during use of the connecting rod 1) and the maximum valley depth a (m) of the surface of the rod body 10 are expressed by the following formula (2 ) Satisfy the relationship.
10 ≧ 1.1σ (πa) ^1/2 (2)

  Therefore, the occurrence of brittle fracture in the rod body 10 is suppressed. Therefore, the connecting rod 1 in this embodiment is excellent in the strength of the rod body 10 and is suitable for weight reduction.

  Generally, in a steel material that has been strengthened by high-concentration carburizing treatment, fatigue failure occurs at a stress greater than 1400 MPa. Therefore, when high-concentration carburizing treatment is applied to a connecting rod, only a stress of 1400 MPa or less is generated. Design is done. Therefore, the maximum valley depth a may be set so that the above formula (2) is satisfied when the maximum stress σ is 1400 MPa. When such a maximum valley depth a is calculated from the equation (2), it is 13 μm or less. Therefore, if the maximum valley depth a on the surface of the rod body 10 is 13 μm or less, it can be said that the occurrence of brittle fracture in the rod body 10 is suppressed.

  Note that it is not always necessary that the relationship of the formula (2) is satisfied over the entire surface of the rod body 10 (or the maximum valley depth a is 13 μm or less). When the rod main body 10 has an H-shaped cross-sectional shape, brittle fracture is likely to occur at the inner corner 10a of the region having the smallest cross-sectional area of the rod main body 10. When the connecting rod 1 is manufactured by forging or casting, in consideration of the draft angle, the inner corner portion 10a tends to have a sharper shape than the outer corner portion 10b, and stress concentration is caused in the sharpened inner corner portion 10a. This is because it occurs. Therefore, in the inner corner portion 10a of the rod body 10 having the smallest cross-sectional area (typically in the vicinity of the small end portion 20), the relationship of Expression (2) is satisfied (or the maximum valley depth a is 13 μm). Therefore, the occurrence of brittle fracture in the rod body 10 can be sufficiently suppressed. Of course, from the viewpoint of more reliably preventing the occurrence of brittle fracture, it is preferable that the relationship of the formula (2) is satisfied over the entire surface of the rod body 10 (or the maximum valley depth a is 13 μm or less). .

Further, from the viewpoint of more reliably preventing the occurrence of brittle fracture, taking into account manufacturing variations in the surface smoothing process, the stress intensity factor KI is more preferably 9 MPa · m ^1/2 or less, The valley depth a is more preferably 10 μm or less. Note that setting the maximum valley depth a to less than 0.2 μm may have disadvantages in the manufacturing process (for example, a longer time for the surface smoothing process described later), so from the viewpoint of simplifying the manufacturing process. The maximum valley depth a is preferably 0.2 μm or more.

  Then, the manufacturing method of the connecting rod 1 in this embodiment is demonstrated, referring FIG. FIG. 2 is a flowchart showing the manufacturing process of the connecting rod 1.

  First, a workpiece formed by forging from steel containing 0.1 wt% or more and 0.45 wt% or less of carbon is prepared (preparation step: step S1). As the steel containing 0.1 wt% or more and 0.45 wt% or less of carbon, for example, SCM420 which is chromium molybdenum steel can be used. The SCM 420 contains 0.18 to 0.23 wt% carbon, 0.90 to 1.20 wt% chromium and 0.15 to 0.30 wt% molybdenum. In addition to the SCM 420 described above, SCr420, SCM435, SCM440, and the like can be used as the steel that is the material of the workpiece. In addition, although forging 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.

  Subsequently, a heat treatment including a plurality of carburizing processes is performed on the workpiece (heat treatment process). Specifically, first, carburizing and quenching (or furnace cooling) is performed on the workpiece (step S3). The first carburizing process is performed in an atmosphere having a carbon potential of 0.8% or more. Note that masking is not performed on the workpiece prior to performing the carburizing process. The temperature of the first carburizing treatment is set to be equal to or higher than the A1 transformation point (eutectoid transformation temperature of steel). The surface of the steel is excessively carburized by the first carburizing treatment. FIG. 3A schematically shows a metal structure formed near the surface of the connecting rod 1 by the first carburizing process. As shown in FIG. 3A, network-like carbides 3 are precipitated between relatively large martensite crystal grains 2 '.

  Next, the second carburizing, quenching and tempering are performed on the workpiece (step S4). The second carburizing process is also performed in an atmosphere having a carbon potential of 0.8% or more. The temperature of the second carburizing treatment is set to not less than the A1 transformation point and not more than the Acm transformation point (transformation temperature at which cementite precipitates from the austenite of the steel). By the second carburizing treatment, the carbon of the surface layer that has been excessively carburized diffuses inside. FIG. 3B schematically shows a metal structure formed in the vicinity of the surface of the connecting rod 1 by the second carburizing process. As shown in FIG. 3 (b), fine granular carbides 3 are precipitated between relatively small martensite crystal grains 2 '.

  Subsequently, the surface of the workpiece is smoothed so that the maximum valley depth a in at least a part of the surface of the rod main body 10 is smaller than a predetermined value (surface smoothing step: step S5). This surface smoothing step can be performed, for example, by wet barrel polishing using a centrifugal barrel polishing machine. Further, in order to make the maximum valley depth a smaller, buffing may be performed after barrel polishing. Note that the method of performing the surface smoothing step is not limited to the one exemplified here. Smoothing may be performed by paper wrapping, pickling, electrolytic polishing, or the like.

  Thereafter, final polishing is performed on the workpiece (step S6). For example, the inner peripheral surface 22a of the piston pin hole 22 and the inner peripheral surface 32a of the crank pin hole 32 are polished. In this way, the connecting rod 1 is completed.

  In the manufacturing method in the present embodiment, a plurality of carburizing processes are performed in an atmosphere having a carbon potential of 0.8% or more on a workpiece formed from steel containing 0.1 wt% or more and 0.45 wt% or less of carbon. Performed once (steps S3 and S4). That is, a high concentration carburizing process is performed. Thereby, the hardness of the whole connecting rod 1 containing the rod main-body part 10 becomes remarkably high. Furthermore, since the manufacturing method in the present embodiment includes a surface smoothing step of smoothing the surface of the workpiece, the maximum valley depth in at least a part of the surface of the rod body 10 is obtained by the surface smoothing step. Can be made smaller than a predetermined value. Therefore, the occurrence of brittle fracture in the rod body 10 can be suppressed. Therefore, according to the manufacturing method in the present embodiment, it is possible to manufacture the connecting rod 1 that is excellent in strength of the rod main body 10 and is suitable for weight reduction.

  2 illustrates the case where the surface smoothing step is performed after the heat treatment step, the surface smoothing step may be performed before the heat treatment step. In FIG. 4, the other example of the manufacturing method of the connecting rod 1 in this embodiment is shown. In the example shown in FIG. 4, a surface smoothing process (process S2) is performed after the preparation process (process S1) by forging. Thereafter, a machining step (step S3), a heat treatment step (steps S4 and S5), and a polishing step (step S6) are sequentially performed.

  When the surface smoothing step is performed before the heat treatment step, the workpiece can be smoothed in a relatively soft state, so that the maximum valley depth a can be easily reduced in a short time. On the other hand, if the surface smoothing step is performed after the heat treatment step, a compressive residual stress can be applied to the surface of the connecting rod 1 depending on the smoothing method (for example, smoothing by barrel polishing). Can be improved.

  In FIG. 5, the result of having measured fatigue strength about the connecting rod 1 (Examples 1-4) actually manufactured with the manufacturing method of this embodiment is shown. FIG. 5 is a graph showing the relationship between the number of bending repetitions N until breakage and the maximum stress σ when a solid bending fatigue test is performed on the connecting rods 1 of Examples 1 to 4. In manufacturing the connecting rod 1 of Examples 1 to 4, surface smoothening was performed by performing wet barrel polishing with a centrifugal barrel polishing machine for 20 minutes using an abrasive-containing baking medium (rice ball type) having a particle diameter of 6 mm. It was. For Examples 3 and 4, buffing using alumina powder was further performed after barrel polishing.

  Moreover, in FIG. 5, the result of having measured the fatigue strength about the connecting rod (Comparative Examples 1-3) manufactured with the manufacturing method similar to this embodiment except the point which does not include a surface smoothing process is shown collectively. Further, FIG. 5 also shows the results for the connecting rods (Comparative Examples 4 to 7) manufactured by the conventional manufacturing method described with reference to FIG. 8 (including the masking step for the rod main body). Table 1 shows the stress intensity factor KI and the maximum valley depth a of the rod body surface of the connecting rods of Examples 1 to 3 and Comparative Examples 1 to 7. The measurement of the maximum valley depth a was performed with a roughness shape measuring instrument.

As can be seen from Table 1, in Examples 1 to 4, the maximum valley depth a is 13 μm or less, and the stress intensity factor KI is 10 MPa · m ^1/2 or less. On the other hand, in Comparative Examples 1 to 7, the maximum valley depth a exceeds 13 μm, and as a result, the stress intensity factor KI exceeds 10 MPa · m ^1/2 . In Comparative Examples 1 to 7, the maximum valley depth “a” is increased due to the process of forming the workpiece. For example, the surface of the workpiece becomes rough during forging, and the maximum valley depth a becomes large. Therefore, when the surface smoothing step is not performed as in the present embodiment, the maximum valley depth a exceeds 13 μm.

As shown in FIG. 5, in Comparative Examples 1 to 7, breakage occurs with a smaller number of repetitions N than in Examples 1 to 4. That is, in Examples 1 to 4 in which the maximum valley depth a is 13 μm or less (the stress intensity factor KI is 10 MPa · m ^1/2 or less), the maximum valley depth a exceeds 13 μm (the stress intensity factor KI is Fatigue strength is improved as compared with Comparative Examples 1-7 (over 10 MPa · m ^1/2 ). In the example shown in FIG. 5, the fatigue strengths of Examples 1 to 4 are improved by about 30% at maximum as compared with the fatigue strengths of Comparative Examples 1 to 7.

  As described above, according to the present invention, the connecting rod 1 which is excellent in the strength of the rod body portion 10 and suitable for weight reduction can be obtained. Specifically, according to the present invention, the connecting rod 1 can be reduced in weight by about 20%. In addition, although the connecting rod 1 of the type called an integrated type is illustrated in FIG. 1, this invention is not limited to this. The present invention is also suitably used for a connecting rod of a type called a split type in which a large end portion is divided into two. Moreover, although the case where the carburizing process is performed twice is illustrated in the present embodiment, the carburizing process may be performed a plurality of times, and the carburizing process may be performed three or more times as described in Patent Document 1. For example, in the case of vacuum carburizing, it is preferable to perform the carburizing process three times or more.

  The connecting rod 1 in this embodiment is suitably used for an internal combustion engine for various transportation equipment such as passenger cars, motorcycles, buses, trucks, tractors, airplanes, motor boats, and civil engineering vehicles. FIG. 6 shows an example of an internal combustion engine 100 including 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.

  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.

  Piston 122 and crankshaft 111 are connected by 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 crank is inserted into the crankpin hole formed in the large end portion 30. A pin 112 is inserted, whereby the piston 122 and the crankshaft 111 are connected. A roller bearing (rolling bearing) 114 is provided between the inner peripheral surface of the crankpin hole and the crankpin 112.

  Since the connecting rod 1 in this embodiment is suitable for weight reduction, in the internal combustion engine 100 provided with the connecting rod 1 in this embodiment, the responsiveness is improved. Further, since the weight of the reciprocating portion in the internal combustion engine 100 is reduced by reducing the weight of the connecting rod 1, primary vibration (one rotation per one rotation of the crankshaft 111 by the reciprocating motion of the reciprocating portion including the piston 120 and the connecting rod 1). (Vibration generated in a cycle) can be reduced. Furthermore, since the connecting rod 1 in this embodiment is excellent in fatigue strength, the reliability of the internal combustion engine 100 is also improved.

  FIG. 7 shows a motorcycle including the internal combustion engine 100 shown in FIG. In the motorcycle shown in FIG. 7, 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.

  A rear arm 309 extending rearward 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. 6 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 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.

  The motorcycle shown in FIG. 7 uses the internal combustion engine 100 provided with the connecting rod 1 in the present embodiment, so that there is little primary vibration and unpleasant vibration felt by the occupant is reduced. In addition, since it is not necessary to take measures against vibration on the vehicle body, a significant reduction in weight can be realized.

  ADVANTAGE OF THE INVENTION According to this invention, the connecting rod excellent in the intensity | strength of a rod main-body part and suitable for weight reduction and its manufacturing method are provided.

  The connecting rod according to the present invention is widely used in internal combustion engines for various transportation equipment (for example, internal combustion engines for motorcycles).

1 Connecting rod 10 Rod body (shaft)
10a Inner corner 10b Outer corner 20 Small end 22 Piston pin hole 30 Large end 32 Crank pin hole

Claims (6)

A method for manufacturing a connecting rod comprising a rod main body, and a small end and a large end provided at one end and the other end of the rod main body,
A preparation step of preparing a workpiece formed of steel containing carbon of 0.1 wt% or more and 0.45 wt% or less;
A heat treatment step for performing heat treatment including a plurality of carburizing treatments in an atmosphere having a carbon potential of 0.8% or more on the workpiece;
A surface smoothing step of smoothing the surface of the workpiece so that the maximum valley depth in at least a part of the surface of the rod main body is smaller than a predetermined value,
The surface smoothing step is a connecting rod manufacturing method including a barrel polishing step.   The connecting rod manufacturing method according to claim 1, wherein the surface smoothing step is performed before the heat treatment step.   The said surface smoothing process is a manufacturing method of the connecting rod of Claim 2 including the buff grinding | polishing process performed after a barrel grinding | polishing process.   The connecting rod manufacturing method according to claim 1, wherein the surface smoothing step is performed after the heat treatment step. The rod main body has an H-shaped cross-sectional shape in which a plurality of outer corners and a plurality of inner corners are defined,
5. The connecting rod manufacturing method according to claim 1, wherein the surface smoothing step is performed such that a maximum valley depth at least on the surfaces of the plurality of inner corners is smaller than the predetermined value.   The said surface smoothing process is a manufacturing method of the connecting rod in any one of Claim 1 to 4 performed so that the maximum valley depth of the whole surface of the said rod main-body part may become smaller than the said predetermined value.

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