Method for manufacturing a crankshaft

Abstract

Provided is a crankshaft having excellent fatigue strength. A crankshaft (10) has a journal portion (11), a pin portion (12), and a fillet portion (14), wherein the fillet portion (14) has a residual stress distribution in which the residual stress in a range from the surface to a depth position of at least 300 μm is a compressive residual stress, the maximum value of the compressive residual stress is 1000 MPa or more, and the surface roughness Rz is less than 3.00 μm.

Description

Technical Field

[0001] This invention relates to crankshafts and their manufacturing methods. Background Technology

[0002] Against the backdrop of global exhaust restrictions, efforts are being made to improve the fatigue strength of crankshafts. In high-frequency hardened crankshafts, increasing the carbon content increases hardness, thereby improving fatigue strength. However, increasing the carbon content can easily lead to quenching cracks.

[0003] International Publication No. 2013 / 15085 discloses a high-frequency quenching steel with excellent resistance to quenching cracks and a crankshaft made using the high-frequency quenching steel.

[0004] Japanese Patent No. 4000648 describes a method for manufacturing high-strength gears in which the gears are subjected to high-frequency quenching or carburizing quenching followed by two or more stages of shot peening hardening, and then only the tooth surface is ground.

[0005] Japanese Patent Application Publication No. 2000-337345 discloses a method for manufacturing a crankshaft as follows: after high-frequency heating of the fillet R portion at the joint between the crankpin and journal and the crank arm of the crankshaft, the fillet R portion is rapidly cooled and quenched, then the entire crankshaft is subjected to low-temperature tempering treatment, and then the fillet R portion is rolled.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2013 / 15085

[0009] Patent Document 2: Japanese Patent No. 4000648

[0010] Patent Document 3: Japanese Patent Application Publication No. 2000-337345 Summary of the Invention

[0011] The problem the invention aims to solve

[0012] Because quenching cracks are generated during high-frequency quenching, the carbon content cannot be significantly increased in crankshafts with complex shapes that are prone to quenching cracks. The maximum hardness achievable through high-frequency quenching depends on the carbon content, and there is a limit to the maximum hardness achievable through high-frequency quenching.

[0013] The objective of this invention is to provide a crankshaft with excellent fatigue strength and a method for manufacturing the same.

[0014] Solution for solving the problem

[0015] A crankshaft according to one embodiment of the present invention includes a journal, a pin portion and a fillet portion, wherein the fillet portion has a residual stress distribution of compressive residual stress ranging from the surface to a depth of at least 300 μm, the maximum value of the compressive residual stress being 1000 MPa or more, and the surface roughness Rz being less than 3.00 μm.

[0016] A method for manufacturing a crankshaft according to one embodiment of the present invention is a method for manufacturing the above-described crankshaft, wherein the method includes the following steps: preparing an intermediate product of the crankshaft; quenching the intermediate product; grinding the quenched intermediate product; and grinding the ground intermediate product at 550 kgf / mm. 2 The above Hertzian stress is used to roll the rounded corner.

[0017] The effects of the invention

[0018] According to the present invention, a crankshaft with excellent fatigue strength can be obtained. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a crankshaft according to one embodiment of the present invention.

[0020] Figure 2 This is a magnified view of a portion of the journal and pin.

[0021] Figure 3 This is a flowchart illustrating an example of a method for manufacturing a crankshaft according to an embodiment of the present invention.

[0022] Figure 4 This is a top view of the test axis. Detailed Implementation

[0023] The inventors of this application have studied methods for improving the fatigue strength of crankshafts and have obtained the following insights.

[0024] To improve the fatigue strength of crankshafts, it is effective to apply compressive residual stress to the surface layer of the fillet portion where the maximum stress is applied. On the other hand, to prevent internal failure, compressive residual stress needs to be pre-applied not only to the outermost layer of the fillet portion but also to a region extending from the surface of the fillet portion to a certain depth. Specifically, excellent fatigue strength can be obtained as long as the residual stress in the region from the surface to a depth of at least 300 μm is compressive residual stress and the maximum value of the compressive residual stress is 1000 MPa or more.

[0025] As a method to impart large compressive residual stress to the fillet portion, rolling with a surface pressure greater than a predetermined value (fillet rolling) after quenching the crankshaft is effective. This is believed to be because applying large surface pressure to the quenched microstructure (martensite) causes plastic deformation of the martensite.

[0026] As described in Japanese Patent No. 4000648, surface hardness in gears and the like is increased by shot peening after high-frequency quenching. However, compared to the hardened layer thickness of a few millimeters introduced by high-frequency quenching, the hardened layer thickness introduced by shot peening is only tens of micrometers, and the improvement in fatigue strength is insufficient compared to the additional process. In contrast, roll forming can impart compressive residual stress to areas on the order of hundreds of micrometers.

[0027] Because the notch sensitivity of the structure after quenching is high, it is necessary to reduce surface roughness to improve fatigue strength. Although it also depends on the degree of surface roughness before machining, the surface roughness increases if roll forming is performed. Therefore, in order to reduce the surface roughness after roll forming, it is necessary to sufficiently reduce the surface roughness before roll forming.

[0028] This invention is based on the above insights. Embodiments of the invention will now be described in detail with reference to the accompanying drawings. Identical or equivalent parts in the drawings are labeled with the same reference numerals, and their descriptions are not repeated. The dimensional ratios between the constituent components shown in the figures do not necessarily represent actual dimensional ratios.

[0029] [crankshaft]

[0030] Figure 1 This is a schematic diagram of a crankshaft 10 according to an embodiment of the present invention. The crankshaft 10 includes a journal 11, a pin portion 12, and an arm portion 13.

[0031] The crankshaft 10 is made of, for example, steel for mechanical construction. However, the crankshaft 10 is not limited to this; it can be made of carbon steel for mechanical construction according to JIS G 4051:2016, alloy steel for mechanical construction according to JIS G 4053:2016, etc. Among these steels, S45C and S50C of JIS G 4051:2016 and SMn438 of JIS G 4053:2016 are preferred. Steels with added sulfur (S) are particularly preferred to improve machinability.

[0032] The chemical composition of crankshaft 10, in mass percent, is, for example, C: 0.10%–0.55%, Si: 0.05%–1.00%, Mn: 0.80%–1.50%, P: less than 0.030%, S: 0.005–0.100%, Cr: 0.05–0.30%, Al: 0.005–0.050%, N: 0.0050–0.0200%, Ni: 0–0.20%, Mo: 0–0.45%, V: 0–0.20%, with the remainder being Fe and impurities.

[0033] Furthermore, the lower limit of the C content, expressed in mass percent, is preferably 0.30%, more preferably 0.35%, further preferably 0.37%, and even more preferably 0.40%.

[0034] Journal 11 is supported on the cylinder block (not shown). Pin 12 is connected to the connecting rod (not shown). Arm 13 connects journal 11 and pin 12. Journal 11 and pin 12 slide relative to bearings formed in the cylinder block and connecting rod, respectively.

[0035] Figure 2 This is a partial enlarged view of the journal 11 and pin 12 of the crankshaft 10. The crankshaft 10 also has rounded corner portions 14 formed at the junction of the journal 11 and the arm 13, and at the junction of the pin 12 and the arm 13. In this specification, the rounded corner portion adjacent to the journal 11 and the rounded corner portion adjacent to the pin 12 are not distinguished, and both are referred to as rounded corner portion 14.

[0036] The fillet portion 14 is the part of the crankshaft 10 where the strongest stress is applied. The fatigue characteristics of the crankshaft 10 are greatly affected by the fatigue strength of the fillet portion 14.

[0037] The rounded corner portion 14 of the crankshaft 10 in this embodiment has a residual stress distribution in which the residual stress is compressive residual stress in the range from the surface to a depth of at least 300 μm, and the maximum value of the compressive residual stress is 1000 MPa or more.

[0038] By applying compressive residual stress to the surface layer of the fillet portion 14, the occurrence of crack initiation, which can become the starting point of cracks, can be suppressed, thereby increasing fatigue strength. Cracks usually occur on the surface where the maximum stress is applied. However, depending on the residual stress distribution and hardness distribution of the fillet portion 14, there is a possibility of failure originating from the inside (internal failure). In order to suppress internal failure, compressive residual stress needs to be pre-applied not only to the outermost region of the fillet portion 14, but also to the region extending from the surface of the fillet portion 14 to a certain depth. Specifically, excellent fatigue strength can be obtained as long as the fillet portion 14 has a residual stress distribution in which the residual stress is compressive residual stress from the surface to a depth of at least 300 μm, and the maximum value of the compressive residual stress is 1000 MPa or more.

[0039] The residual stress distribution of the rounded corner 14 can be calculated using X-ray diffraction (XRD) via the cosα method. Specifically, the surface of the rounded corner 14 is dissolved to the desired measurement depth by electrolytic polishing, and X-rays are irradiated onto the resulting surface to calculate the residual stress at that depth. Furthermore, the residual stress at the outermost layer can be calculated by directly irradiating the surface of the rounded corner 14 with X-rays.

[0040] The residual stress distribution of the rounded corner 14 is preferably compressive residual stress in the range from the surface to a depth of at least 400 μm. More preferably, the residual stress in the range from the surface to a depth of at least 500 μm is compressive residual stress; even more preferably, the residual stress in the range from the surface to a depth of at least 600 μm is compressive residual stress; and even more preferably, the residual stress in the range from the surface to a depth of at least 650 μm is compressive residual stress.

[0041] The maximum value of the compressive residual stress of the rounded corner portion 14 is preferably 1200 MPa or more, more preferably 1300 MPa or more, even more preferably 1500 MPa or more, even more preferably 1700 MPa or more, and even more preferably 1900 MPa or more. The upper limit of the maximum value of the compressive residual stress of the rounded corner portion 14 is not particularly limited, but is, for example, 2300 MPa.

[0042] The deeper the compressive residual stress is imparted, the better it suppresses internal failure and the better it improves fatigue strength. The location of the maximum compressive residual stress is preferably at a depth of 250 μm or more from the surface of the fillet portion 14. More preferably, the location of the maximum compressive residual stress is at a depth of 300 μm or more from the surface; even more preferably, it is at a depth of 350 μm or more from the surface; and still more preferably, it is at a depth of 400 μm or more from the surface.

[0043] Furthermore, in the crankshaft 10 of this embodiment, the surface roughness Rz of the fillet portion 14 is less than 3.00 μm. Even if the fillet portion 14 has the above-mentioned residual stress distribution, if the surface roughness of the fillet portion 14 is inappropriate, sufficient fatigue strength may not always be obtained.

[0044] In this embodiment, the maximum height roughness Rz is used instead of the arithmetic mean roughness Ra to evaluate surface roughness. It is assumed that fatigue failure occurs from the weakest point. In this case, the weakest point is presumably the location where the roughness valley penetrates the deepest. While the arithmetic mean roughness Ra represents the average roughness over the evaluation length, Rz represents the maximum height (Rp) + maximum depth (Rv) over the evaluation length. In evaluating fatigue failure, Rz is considered more readily expressive than Ra.

[0045] Surface roughness Rz (maximum height roughness Rz) is the surface roughness defined in JIS B 0601:2013. The reference length for measuring surface roughness Rz is set to 1.25 mm, the cutoff value λs is set to 2.5 μm, and the cutoff value λc is set to 0.25 mm. The measurement direction is set to the axial direction of crankshaft 10.

[0046] The surface roughness Rz of the rounded corner portion 14 is preferably 2.80 μm or less, more preferably 2.50 μm or less, even more preferably 2.00 μm or less, even more preferably 1.50 μm or less, and even more preferably 1.20 μm or less.

[0047] In this embodiment, the crankshaft 10 preferably has a hardness of HV650 or higher at a depth of 250 μm from the surface of the rounded corner portion 14. The lower limit of the hardness at this depth is preferably HV680, more preferably HV750. The upper limit of the hardness at this depth is not particularly limited, but may be, for example, HV900.

[0048] Regarding the hardness of the fillet portion 14, a sample with a cross-section (longitudinal cross-section) parallel to the axial direction of the crankshaft 10 as the measurement surface can be collected and the measurement surface can be measured according to JIS Z 2244:2020. The test force is set to 100 gf (0.981 N).

[0049] The microstructure of the surface portion of the rounded corner portion 14 is preferably a martensite-containing microstructure. More specifically, the microstructure of the surface portion of the rounded corner portion 14 is preferably a microstructure containing 80% by volume or more of martensite. The volume percentage of martensite in the surface portion of the rounded corner portion 14 is more preferably 90% by volume or more, and even more preferably 95% by volume or more. The rounded corner portion 14 may have a microstructure containing martensite only in the surface portion, or it may have a microstructure containing martensite up to the core portion. The thickness of the region containing martensite is preferably 1.0 mm or more, more preferably 2.0 mm or more, and even more preferably 3.0 mm or more.

[0050] [Crankshaft manufacturing method]

[0051] The following is an example of a crankshaft manufacturing method. Figure 3 This is a flowchart illustrating an example of a crankshaft manufacturing method. The manufacturing method includes a raw material preparation process (step S1), a hot forging process (step S2), an oxide scale removal process (step S3), a rough machining process (step S4), a quenching process (step S5), a fine grinding process (step S6), and a rolling process (step S7).

[0052] Prepare the raw materials for the crankshaft (step S1). The raw materials can be manufactured, for example, by continuous casting or primary rolling of molten steel with a predetermined chemical composition.

[0053] The raw material is hot forged to form the rough shape of the crankshaft (step S2). Hot forging can also be carried out in two ways: rough forging and finish forging.

[0054] After hot forging, shot peening and other processes are performed to remove the oxide scale formed during hot forging (step S3).

[0055] The rough crankshaft is rough-machined (step S4). Rough machining includes cutting, grinding, and drilling. This process produces an intermediate product with a shape close to the final product.

[0056] The intermediate crankshaft product, after rough machining, is quenched (step S5). Specifically, it is rapidly cooled after being heated to a predetermined heating temperature. This can be achieved either by localized heating using a high-frequency induction heating device or by heating the entire intermediate product using a heat treatment furnace. High-frequency quenching is more preferred. The heating temperature is preferably at or above point Ac3. Specifically, the heating temperature is preferably 850°C or higher, more preferably 900°C or higher, and even more preferably 950°C or higher.

[0057] The quenched intermediate product is precision ground (step S6). During precision grinding, the surface roughness Rz of the fillet portion 14 before roll forming is reduced to less than 1.00 μm. Since the surface roughness Rz can increase after the subsequent roll forming process (step S7), it is necessary to reduce the surface roughness Rz of the fillet portion 14 beforehand compared to the case where roll forming is not performed. If the surface roughness Rz of the fillet portion 14 after precision grinding is 1.00 μm or more, it becomes difficult to reduce the surface roughness Rz of the fillet portion 14 after the roll forming process (step S7) to less than 3.00 μm.

[0058] The fine grinding process (step S6) includes polishing grinding (step S6-2) and grinding using a grinding stone (step S6-1). Grinding using a grinding stone can be performed using, for example, white corundum grinding stones, SG grinding stones, CBN grinding stones, etc. Polishing grinding is performed multiple times as needed. Specifically, the first polishing is performed using a film with alumina abrasive grains, and if Rz is not small enough, subsequent polishing is performed using a film with diamond abrasive grains.

[0059] The intermediate product that has undergone fine grinding is subjected to a rolling process (corner rolling) in which the crankshaft 10 is rotated while the roller contacts the fillet 14 (step S7). At this time, the contact surface pressure (Hertz stress) between the fillet 14 and the roller is set to 550 kgf / mm. 2The pressure at the contact surface between the rounded corner 14 and the roller is less than 550 kgf / mm. 2 In such cases, it becomes difficult to achieve a maximum compressive residual stress of 1000 MPa or more. The lower limit of the contact surface pressure between the rounded corner 14 and the roller is preferably 600 kgf / mm². 2 There is no particular limit to the upper limit of the contact surface pressure between the rounded corner 14 and the roller, but it is, for example, 900 kgf / mm. 2 .

[0060] Thus, by performing a composite surface treatment using a quenching process (step S5) and a rolling process (step S7), a rounded corner portion 14 is obtained having a residual stress distribution with residual stress ranging from the surface to a depth of at least 300 μm, wherein the residual stress is compressive residual stress and the maximum value of the compressive residual stress is 1000 MPa or more.

[0061] The above description illustrates an example of the structure of a crankshaft according to an embodiment of the present invention and its manufacturing method. According to this embodiment, a crankshaft with excellent fatigue strength can be obtained.

[0062] Example

[0063] The present invention will be described in more detail below through examples. The present invention is not limited to these examples.

[0064] Test shafts No.1 to No.11 were made using steel with the chemical composition shown in Table 1 as raw material.

[0065] [Table 1]

[0066]

[0067] Specifically, a 120mm diameter steel billet was heated to 1100°C and hot-forged into a 70mm diameter cylinder. Then, a shaft with a diameter of 15mm and a length of 120mm was machined from one-quarter of the diameter. The shaft underwent heat treatment for microstructure homogenization. Specifically, it was heated to 900°C, oil-quenched, cooled to 500°C, and then removed from the oil and air-cooled. It was then machined to produce a test shaft. Subsequently, it underwent high-frequency quenching, heating to a temperature above Ac3 using a high-frequency induction heating device followed by water cooling.

[0068] Figure 4 This is a top view of the test shaft 20. Regarding the test shaft 20, the outer diameter D0 is 12 mm, the length L is 116 mm, the length L1 of the gripping part is 37 mm, the diameter D of the parallel part is 8 mm, and the radius of curvature R of the connection between the parallel part and the gripping part is 24 mm. The test shaft 20 has an annular groove 21 simulating a rounded corner. The depth of the groove 21 is 0.5 mm, and the radius of curvature is 1.5 mm.

[0069] After high-frequency quenching, all test shafts except No. 9 and No. 10 were precision ground. The precision grinding included polishing and grinding using a grinding stone. Grinding using a grinding stone employed white corundum grinding stones with grit sizes #100 to #220. For polishing, the first pass used a film with alumina abrasive particles, and subsequent passes used a film with diamond abrasive particles if the surface roughness Rz was not sufficiently reduced.

[0070] Other test pieces prepared under the same conditions were used, and the surface roughness Rz of the groove 21 after the fine grinding process was measured using a contact roughness testing machine (Mitutoyo SJ-412, Mitutoyo Corporation). The surface roughness Rz of the groove 21 after high-frequency quenching was measured for No.9 and No.10, which did not undergo the fine grinding process. The measured surface roughness Rz is shown in Table 1.

[0071] After precision grinding of test shafts No.1 to No.4 and No.9 to No.11 (No.9 and No.10 were after high-frequency quenching), the groove 21 of the test shafts was subjected to a simulated fillet rolling process. Specifically, the test shaft was rotated while the groove 21 and the roller were brought into contact with the contact surface pressure in the column of "Rolling Hertz Stress" in Table 1.

[0072] Test shaft No. 5 was not subjected to roll forming but only to fine grinding.

[0073] Shot peening was used instead of rolling for test shafts No. 6 and No. 7. During shot peening, iron balls (HV800 or higher) with a diameter of 0.3 mm were used as the projectile material and were vertically projected onto the bottom of groove 21 while the test shaft was rotated. The projection time was 15 seconds, the arc height was set to 0.227 mmA, and the coverage was set to 500%.

[0074] Hammer hardening was used instead of roll forming for test shaft No. 8. Hammer hardening employed a method called Ultrasonic Impact Treatment (UIT). UIT involves placing a metal pin between a tool subjected to ultrasonic vibration and an object, and using the impact of the ultrasonic waves to strike the object, causing plastic deformation. UIT was applied to the groove 21 of the test shaft. The UIT conditions were: pin material SUJ2 (HRC62 or higher), pin diameter 0.5 mm, pin tip curvature radius 0.5 mm, and ultrasonic frequency 27 kHz.

[0075] The surface treatments performed on each test shaft are summarized in the "Composite Surface Treatment Methods" column of Table 1. In this column, "IH" means high frequency quenching, "finishing" means fine grinding, "rolling" means rolling, "SP" means shot peening, and "HP" means hammer hardening.

[0076] The surface roughness Rz, residual stress distribution, and hardness at a depth of 250 μm from the surface were measured for each fabricated test shaft in the groove 21. In the residual stress distribution measurement, a CrKα line with a collimation diameter of 0.3 mm was used to measure the bottom of the groove 21.

[0077] Fatigue tests were conducted on each test shaft. Specifically, the rotary bending fatigue test as described in JIS Z 2274 was performed using an Ono-type rotary bending fatigue testing machine. The rotational speed of the test shaft was set to 3600 rpm. The SN curves of each test shaft were obtained through this fatigue test, and the test was repeated 1.0 × 10⁻⁶ times. 7 The maximum bending stress value that did not break during the test is set as the fatigue strength of the test shaft.

[0078] The results are shown in Table 2.

[0079] [Table 2]

[0080]

[0081] The surface roughness Rz of the groove 21 of each test shaft is shown in the "Rz" column of Table 2. The maximum value of the compressive residual stress of the groove 21 is shown in the "Maximum value of compressive residual stress" column. The location (depth from the surface) of the groove 21 with the maximum compressive residual stress is shown in the "Depth at which the maximum value of the compressive residual stress is shown" column. The maximum depth at which the residual stress of the groove 21 is equal to the compressive residual stress is shown in the "Maximum depth at which the residual stress is equal to the compressive stress" column. The hardness at a depth of 250 μm from the surface of the groove 21 is shown in the "Hardness at a depth of 250 μm from the surface" column.

[0082] The fatigue strength of each test shaft is shown in the "Fatigue Strength" column of Table 2. Furthermore, the improvement ratio based on the fatigue strength of test shaft No. 5 is shown in the "Improvement Ratio from Baseline" column.

[0083] As shown in Table 2, for test shafts No.1 to No.3, the surface roughness Rz of the groove 21 is less than 3.00 μm, the maximum compressive residual stress of the groove 21 is 1000 MPa or more, and the residual stress distribution is defined as the range from the surface to a depth of at least 300 μm. Compared with test shaft No.5, the fatigue strength of the above-mentioned test shafts is improved by more than 30%.

[0084] Test shafts No. 6 to No. 8 were subjected to shot peening or hammer hardening instead of roll forming. Although the maximum compressive residual stress of these test shafts exceeded 1000 MPa, the range of compressive residual stress was relatively shallow. Therefore, the improvement in fatigue strength compared to test shafts No. 1 to No. 3 was not sufficient.

[0085] Regarding test shafts No. 9 and No. 10, the maximum compressive residual stress in the groove 21 is over 1000 MPa, and there is a residual stress distribution with compressive residual stress extending from the surface to a depth of at least 300 μm. However, the surface roughness Rz of the groove 21 is over 3.00 μm. Therefore, the improvement in fatigue strength compared to test shafts No. 1 to No. 3 is not sufficient.

[0086] Regarding test shafts No. 4 and No. 11, the maximum value of the compressive residual stress in groove 21 is less than 1000 MPa. Therefore, the improvement in fatigue strength compared to test shafts No. 1 to No. 3 is not sufficient. The reason why the maximum value of the compressive residual stress is less than 1000 MPa is believed to be due to the insufficient contact surface pressure during the rolling process.

[0087] The present invention has been described above as an embodiment, but the above embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above embodiment, and can be implemented by appropriate modifications without departing from its spirit.

[0088] Explanation of reference numerals in the attached figures

[0089] 10. Crankshaft; 11. Journal; 12. Pin; 13. Arm; 14. Rounded corner.

Claims

1. A method of manufacturing a crankshaft, which is a method of manufacturing a crankshaft, the crankshaft having a journal portion, a pin portion, and a fillet portion, wherein the fillet portion has a residual stress distribution in which a residual stress in a range from a surface to a depth position of at least 600 μm is a compressive residual stress, a maximum value of the compressive residual stress is 1000 MPa or more, and a surface roughness Rz is less than 3.00 μm, a position of the maximum compressive residual stress is a depth position at a distance of 250 μm or more from a surface of the fillet portion, the surface roughness Rz is a maximum height roughness, the method of manufacturing the crankshaft includes the following steps: preparing an intermediate product of the crankshaft; quenching the intermediate product; grinding the intermediate product subjected to the quenching; and the surface roughness Rz of the fillet portion after the step of grinding and before the step of roll forming is less than the surface roughness Rz of the fillet portion after the step of roll forming. The intermediate product subjected to the grinding was rolled at a Hertz stress of 550 kgf / mm 2 The above Hertz stress was applied to the rounded portion to perform the rolling process, 2. The method of manufacturing the crankshaft according to claim 1, wherein a hardness at a depth position at a distance of 250 μm from the surface of the fillet portion is HV 650 or more.

3. The method of manufacturing the crankshaft according to claim 1, wherein the surface roughness Rz of the fillet portion after the step of grinding and before the step of roll forming is less than 1.00 μm. ​

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