JP7568903B2 - Crankshaft and manufacturing method thereof - Google Patents

Description

The present invention relates to a crankshaft and a method for manufacturing the same.

Induction hardening is sometimes performed on crankshafts to improve their fatigue strength. Induction hardened crankshafts may have heat treatment deformation (hardening bending) caused by the induction hardening.

Patent Publication No. 4007033 discloses a crankshaft in which the induction hardened layer depth of the pin and the induction hardened layer depth of the journal at each of the arm critical cross sections in one throw of the crankshaft satisfy a specified relationship.

JP 2006-328497 A discloses a high-frequency heating method in which, when performing high-frequency hardening on a steel plate member, a conductive protective plate is placed to cover a part of the steel plate member and high-frequency heating is performed.

Patent No. 4007033 JP 2006-328497 A

The object of the present invention is to provide a crankshaft that is less susceptible to heat bending and a method for manufacturing the same.

A crankshaft according to one embodiment of the present invention is a crankshaft having a journal portion and a pin portion, each of which has a parallel portion having a constant outer diameter and a fillet portion formed continuously with the parallel portion, and at least one of the journal portion and the pin portion has a structure in which, over a length of at least 15.0% of the length of the parallel portion, the structure from the surface to a depth of at least 6.0% of the radius of the parallel portion contains 87.5% by volume or more of ferrite-pearlite, and the structure from the surface of the fillet portion to a depth of at least 6.0% of the radius of the parallel portion contains 87.5% by volume or more of martensite.

A method for manufacturing a crankshaft according to one embodiment of the present invention is the method for manufacturing the above-mentioned crankshaft, further comprising the step of quenching only the vicinity of the fillet portion from a temperature of Ac ₃ point or higher by a high-frequency induction heating device.

The present invention provides a crankshaft that is less susceptible to bending during quenching.

FIG. 1 is a schematic diagram of a crankshaft according to one embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view showing the vicinity of a pin portion of the crankshaft shown in FIG. FIG. 3 is a flow chart showing an example of a method for manufacturing the crankshaft of FIG. FIG. 4 is a perspective view of a three-dimensional analysis model used in the simulation. FIG. 5 is a front view of the three-dimensional analysis model of FIG. FIG. 6 is an enlarged view of the vicinity of the pin top of the three-dimensional analysis model of FIG. FIG. 7 is a cross-sectional view showing the arrangement of induction coils in the simulation. FIG. 8 is a cross-sectional view showing another arrangement of induction coils in the simulation. 9A is a contour map showing the tissue distribution of No. 1. 9B is a contour map showing the tissue distribution of No. 3. 9C is a contour map showing the tissue distribution of No. 5. 9D is a contour map showing the tissue distribution of No. 6.

The following describes in detail an embodiment of the present invention with reference to the drawings. The same or corresponding parts in the drawings are given the same reference numerals and their description will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[Crankshaft]
1 is a schematic diagram of a crankshaft 10 according to an embodiment of the present invention. The crankshaft 10 includes a journal portion 11, a pin portion 12, and an arm portion 13.

The crankshaft 10 is made of, for example, steel for mechanical construction. The crankshaft 10 can be made of, but is not limited to, carbon steel for mechanical construction according to JIS G 4051:2009, steel for mechanical construction with guaranteed hardenability according to JIS G 4052:2008 (H steel), alloy steel for mechanical construction according to JIS G4053:2008, etc. Among these steels, S45C, S50C, and S53C according to JIS G 4051:2009, SMn438 according to JIS G 4053:2008, and steels to which S has been added to improve machinability are preferable.

The chemical composition of the crankshaft 10 includes, in addition to Fe and impurities, by mass%, C: 0.30-0.60%, Si: 0.01-2.0%, Mn: 0.1-2.0%, Cr: 0.01-0.50%, Al: 0.001-0.06%, N: 0.001-0.02%, P: 0.03% or less, and S: 0.20% or less. The chemical composition of the crankshaft 10 may contain elements other than those listed above. The chemical composition of the crankshaft 10 may contain, for example, in mass%, Mo: 0-0.50%, Cu: 0-0.50%, Ni: 0-0.50%, Ti: 0-0.050%, Nb: 0-0.050%, Ca: 0-0.005%, Bi: 0-0.30%, and V: 0-0.20%.

The journal portion 11 is connected to a cylinder block (not shown). The pin portion 12 is connected to a connecting rod (not shown). The arm portion 13 connects the journal portion 11 and the pin portion 12.

[Configuration of pin portion 12]
2 is an enlarged cross-sectional view of the pin portion 12 of the crankshaft 10. The pin portion 12 has a parallel portion 121 having a constant outer diameter and a fillet portion 122 formed continuously with the parallel portion 121. The thrust wall 14 and the pin top 15 are formed on the outside of the fillet portion 122. The parallel portion 121 is a portion that slides against a bearing of the connecting rod. The fillet portion 122 is a portion that connects the parallel portion 121 and the thrust wall 14, and is formed in a smooth shape to relieve stress concentration.

In the crankshaft 10 according to this embodiment, the surface structure of the parallel portion 121 is mainly composed of ferrite and pearlite over a certain length, and the surface structure of the fillet portion 122 is mainly composed of martensite. The respective structure distributions of the parallel portion 121 and the fillet portion 122 are described in detail below.

[Structure and hardness of parallel portion 121]
In this embodiment, the structure from the surface to a depth d1 within the range of length L1 of the parallel portion 121 (the structure of region 121a in FIG. 2) is a structure containing 87.5 volume % or more of ferrite-pearlite. Here, the length L1 is 15.0% or more of the length L of the parallel portion 121, and the depth d1 is 6.0% or more of the radius R of the parallel portion 121. Here, "ferrite-pearlite" includes full pearlite.

This configuration reduces the transformation expansion caused by martensitic transformation, and suppresses bending of the crankshaft 10.

Length L1 is 15.0% or more of length L of parallel portion 121. From the viewpoint of reducing bending during quenching, the longer length L1 is, the more preferable. The lower limit of length L1 is preferably 30.0% of length L, more preferably 50.0% of length L, even more preferably 70.0% of length L, and even more preferably 80.0% of length L. On the other hand, if length L1 is too large, it may be difficult to make the structure of the surface layer of fillet portion 122 a structure mainly composed of martensite. The upper limit of length L1 is preferably 95.0% of length L.

If the structure from the surface to the depth d1 of the parallel portion 121 within the length L1 contains 87.5 volume % or more of ferrite-pearlite, the effect of suppressing quench bending can be obtained. The structure at a position deeper than the depth d1 is arbitrary. The structure within the length L1 of the parallel portion 121 may be a structure mainly composed of ferrite-pearlite up to the core. The depth d1 is preferably 50.0% or more of the radius R, more preferably 80.0% or more of the radius R, and even more preferably 100.0% of the radius R. Here, the depth d1 being 100.0% of the radius R means that the structure within the length L1 of the parallel portion 121 is a structure mainly composed of ferrite-pearlite up to the core.

The hardness from the surface to depth d1 within the range of length L1 of parallel portion 121 (hardness of region 121a in FIG. 2) is preferably HV400 or less. The hardness of region 121a is measured by taking a sample with a cross section (longitudinal cross section) parallel to the axial direction of crankshaft 10 as the measurement surface, and measuring the measurement surface in accordance with JIS Z 2244 (2009). The test force is 300 gf (2.942 N). The hardness of region 121a is preferably HV350 or less, and more preferably HV320 or less. The lower limit of the hardness of region 121a is, for example, HV220.

[Structure and hardness of fillet portion 122]
In this embodiment, the structure from the surface of the fillet portion 122 to a depth d2 (the structure of the region 122a in FIG. 2) contains 87.5 volume % or more of martensite. Here, the depth d2 is 6.0% or more of the radius R of the parallel portion 121. The depth d2 may be the same as or different from the depth d1.

The fillet portion 122 is the part of the pin portion 12 that is subjected to the strongest stress. Furthermore, bending fatigue, which is a problem in the crankshaft 10, is predominantly influenced by the characteristics of the surface layer of the part. Therefore, by making the structure from the surface of the fillet portion 122 to a depth d2 mainly composed of martensite, a predetermined fatigue strength can be ensured.

The structure at a position deeper than the depth d2 is arbitrary. The structure of the fillet portion 122 may be a structure mainly composed of martensite up to the core. From the viewpoint of ensuring fatigue strength, the greater the depth d2, the more preferable it is. The lower limit of the depth d2 is preferably 8.0% of the radius R, more preferably 10.0% of the radius R, and even more preferably 12.0% of the radius R. On the other hand, if the hardening depth of the fillet portion 122 is too large, it may be difficult to ensure a structure mainly composed of ferrite and pearlite in the parallel portion 121. The upper limit of the depth d2 is preferably 30.0% of the radius R, more preferably 25.0% of the radius R, and even more preferably 20.0% of the radius R.

The hardness from the surface of the fillet portion 122 to a depth d2 (the hardness of the region 122a in FIG. 2) is preferably HV580 or more. The hardness of the region 122a is measured in accordance with JIS Z 2244 (2009) by taking a sample whose measurement surface is a cross section (longitudinal cross section) parallel to the axial direction of the crankshaft 10. The test force is 300 gf (2.942 N). The hardness of the region 122a is preferably HV600 or more, and more preferably HV620 or more. The upper limit of the hardness of the region 122a is, for example, HV800.

[Configuration of journal unit 11]
Although not shown, like the pin portion 12, the journal portion 11 also has a parallel portion having a constant outer diameter and a fillet portion formed continuously with the parallel portion.

In the journal portion 11, as in the case of the pin portion 12, the structure from the surface to a depth of at least 6.0% of the radius of the parallel portion over a length of at least 15.0% of the length of the parallel portion contains 87.5% or more by volume of ferrite-pearlite. Also, in the journal portion 11, as in the case of the pin portion 12, the structure from the surface of the fillet portion to a depth of at least 6.0% of the radius of the parallel portion contains 87.5% or more by volume of martensite.

With this configuration, as with the pin portion 12, it is possible to reduce the bending caused by annealing of the crankshaft 10 while ensuring a predetermined fatigue strength. The preferred size of the region in the parallel portion where the structure is mainly ferrite-pearlite and the preferred hardness range of the region, and the preferred size of the region in the fillet portion where the structure is mainly martensite and the preferred hardness range of the region are the same as those of the pin portion 12.

The structure and hardness of the parts other than the journal portion 11 and the pin portion 12 (e.g., the arm portion 13) are optional.

[Manufacturing method of crankshaft]
Next, a description will be given of an example of a method for manufacturing the crankshaft 10. The manufacturing method described below is merely an example and is not intended to limit the method for manufacturing the crankshaft 10.

Figure 3 is a flow diagram showing an example of a method for manufacturing the crankshaft 10. This manufacturing method includes a material preparation step (step S1), a hot forging step (step S2), a heat treatment step (step S3), a machining step (step S4), a quenching step (step S5), and a finishing step (step S6). Each step is described in detail below.

A raw material for the crankshaft 10 is prepared (step S1). The raw material is, for example, a steel bar. The raw material can be produced, for example, by continuous casting or blooming molten steel having a predetermined chemical composition.

The material is hot forged to produce the rough shape of the crankshaft (step S2). Hot forging may be performed in two stages: rough forging and finish forging.

The crankshaft rough product produced by hot forging is subjected to heat treatment such as normalizing as necessary (step S3). The heat treatment step (step S3) is an optional step, and this step may be omitted depending on the required characteristics of the crankshaft, etc.

The crankshaft rough product is machined (step S4). The machining includes cutting, grinding, drilling, etc. This process produces an intermediate product that has a shape close to that of the final product.

The intermediate crankshaft that has been machined is quenched (step S5). Specifically, it is heated to a predetermined heating temperature and then quenched. At this time, for example, a high-frequency induction heating device is used to heat only the vicinity of the fillet portion of the journal portion 11 and the vicinity of the fillet portion 122 of the pin portion 12, and the parallel portion of the journal portion 11 and the parallel portion 121 of the pin portion 12 are prevented from being heated as much as possible. The heating location can be adjusted by the shape of the induction coil, the distance between the induction coil and the target portion, the output frequency, etc. This quenching may be performed in multiple steps. The heating temperature is Ac ₃ or higher, preferably 900°C or higher. The quenching after heating is preferably water cooling. In order to suppress quench cracking, the intermediate product after quenching may be subjected to low-temperature tempering at a temperature that does not significantly affect the structure.

Finishing is performed on the intermediate product after quenching as necessary (step S6). For example, grinding or lapping is performed on the journal portion 11 and the pin portion 12 to adjust the surface shape.

The above describes the configuration of the crankshaft 10 according to one embodiment of the present invention and an example of a manufacturing method thereof. According to this embodiment, a crankshaft with reduced bending due to annealing is obtained.

In the above embodiment, the surface structure of the parallel portion in a range of a certain length or more in both the journal portion 11 and the pin portion 12 is mainly composed of ferrite-pearlite, and the surface structure of the fillet portion is mainly composed of martensite. However, the crankshaft 10 may have only one of the journal portion 11 and the pin portion 12 that has the above-mentioned structure distribution. If at least one of the journal portion 11 and the pin portion 12 has the above-mentioned structure distribution, the effect of reducing quench bending can be obtained.

When comparing the journal portion 11 and the pin portion 12, the pin portion 12 is more likely to bend due to annealing. This is because the thermal capacity of the pin top 15 (Figure 2) is smaller. Therefore, when only one of the journal portion 11 and the pin portion 12 is to have the above-mentioned texture distribution, it is preferable to have the pin portion 12 have the above-mentioned texture distribution. In this case, the pin top 15 can be made smaller, which also contributes to reducing the weight of the crankshaft 10. However, from the perspective of minimizing annealing bending, it is preferable to have both the journal portion 11 and the pin portion 12 have the above-mentioned texture distribution.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples.

A simulation of the heat bending of the pin part of a crankshaft was carried out using the finite element method. Figures 4 to 6 show the three-dimensional analysis model used in this simulation. Figure 4 is a perspective view of this model, Figure 5 is a front view, and Figure 6 is an enlarged view of the area near the pin top. As shown in Figures 5 and 6, the thrust distance is 21.0 mm, the parallel part length L is 15.0 mm, the parallel part radius R is 24.0 mm, and the fillet part curvature radius is 3.0 mm, and only the half area on the pin top side was modeled to reduce the calculation load. The material constants of S45C were used for various calculations.

The structure and hardness distribution, as well as the amount of displacement, were calculated when this crankshaft was induction hardened. Figures 7 and 8 are cross-sectional views showing the arrangement of induction coils in this simulation. In the arrangement of Figure 7, induction coils are arranged only near the fillet portion. In the arrangement of Figure 8, induction coils are arranged not only near the fillet portion but also near the parallel portion. Using these induction coils, induction heating was performed with an output frequency of 10 kHz and a current application time of 22 seconds, and after heating, the crankshaft was cooled to 20°C with a heat transfer coefficient of 40 kW/(m ² K) simulating water cooling. The results are shown in Table 1.

In Table 1, the "Coil Arrangement" column with "Fillet" indicates data simulating induction hardening with the coil arrangement in Figure 7, while the "Fillet + Parallel" column indicates data simulating induction hardening with the coil arrangement in Figure 8. The values in the "Current" column are the current values passed through the induction coil, and are listed as relative values with the current value of No. 6 set to 100%.

The value in the "Pin top x-direction displacement" column is the amount of displacement in a direction parallel to the axial direction of the crankshaft at the position of point P2 in Figure 6, and is listed as a percentage of the parallel section length (15.0 mm). The value in the "Pin top y-direction displacement" column is the amount of displacement in a direction perpendicular to the axial direction of the crankshaft at the position of point P2 in Figure 6, and is listed as a percentage of the parallel section radius (24.0 mm). The value in the "Parallel section y-direction displacement" column is the amount of displacement in a direction perpendicular to the axial direction of the crankshaft at the position of point P1 in Figure 5, and is listed as a percentage of the parallel section radius (24.0 mm).

Figures 9A to 9D are contour maps showing the microstructure distribution of No. 1, No. 3, No. 5, and No. 6, respectively. In these figures, "M ≧ 87.5%" indicates the region where the volume fraction of martensite is 87.5% or more, and "α + P ≧ 87.5%" indicates the region where the volume fraction of ferrite-pearlite is 87.5% or more.

As shown in Table 1, No. 1 to No. 4, in which L1/L was 15.0% or more, showed less bending due to annealing, and in particular, the amount of displacement of the pin top in the x direction was smaller, compared to No. 5 and No. 6, in which L1/L was less than 15.0%.

Although one embodiment of the present invention has been described above, the above-mentioned embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-mentioned embodiment, and it is possible to implement the above-mentioned embodiment by appropriately modifying it within the scope that does not deviate from the spirit of the present invention.

10 Crankshaft 11 Journal portion 12 Pin portion 121 Parallel portion 122 Fillet portion 13 Arm portion 14 Thrust wall 15 Pin top

Claims (3)

A crankshaft including a journal portion and a pin portion,
each of the journal portion and the pin portion has a parallel portion having a constant outer diameter and a fillet portion formed continuously with the parallel portion;
The pin portion further includes a thrust wall and a pin top.
In the pin portion,
the parallel portion has a structure including 87.5% by volume or more of ferrite-pearlite in a length of at least 70.0 % of the length of the parallel portion from the surface to a depth of at least 80.0 % of the radius of the parallel portion;
a structure from the surface of said fillet portion to a depth of at least 12.0 % of the radius of said parallel portion, and a structure of said thrust wall and said pin top, each of said crankshafts is a structure containing 87.5% by volume or more of martensite. 2. The crankshaft of claim 1,
In the pin portion,
the parallel portion has a hardness of HV400 or less along a length of at least 70.0 % of the length of the parallel portion from a surface to a depth of at least 80.0 % of the radius of the parallel portion;
The hardness of the crankshaft, from the surface of the fillet portion to a depth of at least 12.0 % of the radius of the parallel portion, is HV580 or more. A method for manufacturing a crankshaft according to claim 1 or 2, comprising the steps of:
A method for manufacturing a crankshaft, comprising a step of hardening only the vicinity of the fillet portion from a temperature of Ac ₃ or higher using a high-frequency induction heating device.

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