JP2024040890A - Crank shaft and method for producing crank shaft - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crank shaft with superior fatigue strength.

SOLUTION: A crank shaft has a chemical composition including, in mass%, C: 0.36-0.45%, Si: 0.01-0.20%, Mn: 1.20-1.60%, P: 0.10% or less, S: 0.040-0.080%, Al: 0.005-0.060%, N: 0.001-0.020%, with the balance being Fe and impurities. The crankshaft has a quench-hardened layer on at least part of its surface, and the structure of the quench-hardened layer contains 1.00-5.00 vol.% of ε carbides.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2024,JPO&INPIT

Description

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

Crankshafts are sometimes subjected to surface hardening treatment such as induction hardening in order to improve fatigue strength and wear resistance.

Japanese Unexamined Patent Publication No. 2005-314753 describes a mechanical structural component using a steel material that is at least partially hardened. The hardened structure of this mechanical structural component contains prior austenite grains having a packet number of 3 or less in an area ratio of 30% or more.

Japanese Unexamined Patent Publication No. 2006-52459 describes a mechanical structural component using a steel material that is at least partially hardened. In the quenched structure of this mechanical structural component, the average grain size of prior austenite grains is 12 μm or less, and the maximum grain size is 4 times or less the average grain size.

Although not related to crankshafts, JP-A-2015-218361 describes a medium-high carbon steel material that has excellent wear resistance after induction hardening. The chemical composition of this medium-high carbon steel material satisfies (48/14)×[N]+10 ^{(−7000/T+2.75)} /[C]+0.001≦[Ti]≦0.1. Here, the contents of N, C, and Ti (unit: mass %) are substituted for [N], [C], and [Ti], and T is the heating temperature (unit: K) during induction hardening. Substituted.

Japanese Patent Application Publication No. 2005-314753 Japanese Patent Application Publication No. 2006-52459 Japanese Patent Application Publication No. 2015-218361 Japanese Patent Application Publication No. 2006-97109 Patent No. 5421029

For sliding parts such as crankshafts, it is important to suppress friction and wear on the sliding parts and to allow them to function in a state where damage such as mechanical damage and thermal cracks due to overload is suppressed. . Among these, in order to improve the wear resistance, it is effective to increase the C content of the steel material to improve the hardenability and harden the hardened layer. Also, from the viewpoint of improving fatigue strength, it is effective to increase the C content of the steel material to improve hardenability and harden the hardened layer. On the other hand, if the C content is increased, there are problems such as the machinability of the steel material being reduced and quench cracking becoming more likely to occur, and there is a limit to the improvement by increasing the C content.

An object of the present invention is to provide a crankshaft having excellent fatigue strength, slidability, and machinability.

The crankshaft according to an embodiment of the present invention has a chemical composition in mass %: C: 0.36 to 0.45%, Si: 0.01 to 0.20%, Mn: 1.20 to 1.60. %, P: 0.10% or less, S: 0.040 to 0.080%, Al: 0.005 to 0.060%, N: 0.001 to 0.020%, remainder: Fe and impurities. has a quench-hardened layer on at least a portion of its surface, and the structure of the quench-hardened layer contains ε carbide in a volume fraction of 1.00 to 5.00%.

The crankshaft according to an embodiment of the present invention has a chemical composition in mass %: C: 0.36 to 0.45%, Si: 0.01 to 0.20%, Mn: 1.20 to 1.60. %, P: 0.10% or less, S: 0.040 to 0.080%, Al: 0.005 to 0.060%, N: 0.001 to 0.020%, Cr: 0.25% or less , the remainder: Fe and impurities, and has a hardened layer on at least a portion of the surface, and the structure of the hardened layer includes ε carbide in a volume fraction of 1.00 to 5.00%.

A method for manufacturing a crankshaft according to an embodiment of the present invention is a method for manufacturing the above-mentioned crankshaft, and includes a step of hardening an intermediate product of the crankshaft, and heating the hardened intermediate product at 150 to 210°C. and a step of tempering at a temperature.

According to the present invention, a crankshaft having excellent fatigue strength, slidability, and machinability can be obtained.

FIG. 1 is a schematic diagram of an example of a crankshaft. FIG. 2 is a flow diagram of an example of a method for manufacturing a crankshaft.

The present inventors conducted an investigation focusing on the sliding properties of the crankshaft. As a result, a crankshaft with a predetermined chemical composition that has been induction hardened and then tempered at a lower temperature than normal, specifically at a temperature of 150 to 210 degrees Celsius, can omit tempering. It has been found that the new crankshaft has superior sliding properties compared to conventional crankshafts.

Further investigation revealed that the quenched and hardened layer of the crankshaft thus manufactured had a structure containing a predetermined amount of ε carbide (Fe _2-3 C). It was found that the improvement in sliding properties is due to the fact that this ε carbide has a lower adhesion force than the base (matrix).

Note that when tempering is performed at a higher temperature, the ε carbide becomes a solid solution, and cementite (Fe ₃ C) is precipitated in place of the ε carbide. Cementite, like ε carbide, also has a lower adhesion force than the base (matrix), and is thought to contribute to improving sliding properties. On the other hand, when cementite precipitates, the hardness of the hardened layer decreases, making it difficult to ensure the necessary fatigue strength. Therefore, in order to achieve both fatigue strength and slidability, it is preferable to have a structure containing a predetermined amount of ε carbide (Fe _2-3 C).

The present invention was completed based on the above findings. Hereinafter, a crankshaft according to an embodiment of the present invention will be described in detail.

[Crankshaft]
[Chemical composition]
A crankshaft according to one embodiment of the invention has the chemical composition described below. In the following description, "%" in the content of an element means mass %.

C: 0.36-0.40%
Carbon (C) improves the hardenability of steel and contributes to improving the hardness and fatigue strength of the hardened layer. On the other hand, if the C content is too high, the quench cracking resistance and machinability will decrease. Therefore, the C content is 0.36-0.45%. The lower limit of the C content is preferably 0.38%, more preferably 0.40%, and even more preferably 0.41%. The upper limit of the C content is preferably 0.44%.

Si: 0.01~0.20%
Silicon (Si) has the effect of shifting the carbide formation temperature to a higher temperature side, and a high Si content makes it difficult for ε carbide to precipitate. Therefore, in this embodiment, the Si content is set to 0.20% or less. On the other hand, if the Si content is excessively reduced, the hardenability decreases and it becomes difficult to ensure the hardness of the hardened layer. Therefore, the Si content is 0.01 to 0.20%. The lower limit of the Si content is preferably 0.02%, more preferably 0.03%. The upper limit of the Si content is preferably 0.18%, more preferably 0.15%, even more preferably 0.12%, even more preferably 0.10%, and even more preferably 0. It is .08%.

Mn: 1.20-1.60%
Manganese (Mn) improves the hardenability of steel and contributes to improving the hardness and fatigue strength of the hardened layer. On the other hand, if the Mn content is too high, machinability will decrease. Therefore, the Mn content is 1.20-1.60%. The lower limit of the Mn content is preferably 1.30%, more preferably 1.40%. The upper limit of Mn content is preferably 1.55%.

P: 0.10% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces hot workability and toughness of steel. Therefore, the P content is 0.10% or less. The P content is preferably 0.03% or less, more preferably 0.02% or less. It is preferable that the P content is as low as possible.

S: 0.040-0.080%
Sulfur (S) forms MnS and improves the machinability of steel. On the other hand, if the S content is too high, the quench cracking resistance of the steel will decrease. Therefore, the S content is 0.040-0.080%. The lower limit of the S content is preferably 0.045%, more preferably 0.050%. The upper limit of the S content is preferably 0.075%, more preferably 0.070%.

Al: 0.005-0.060%
Aluminum (Al) deoxidizes steel. On the other hand, if the Al content is too high, the machinability of the steel will decrease. Therefore, the Al content is 0.005 to 0.060%. The lower limit of the Al content is preferably 0.010%. The upper limit of the Al content is preferably 0.050%, more preferably 0.040%, and still more preferably 0.030%.

N: 0.001-0.020%
Nitrogen (N) reduces hot workability of steel. On the other hand, excessively restricting N increases smelting costs. Therefore, the N content is 0.001-0.020%. The lower limit of the N content is preferably 0.005%. The upper limit of the N content is preferably 0.015%.

Cr: 0.25% or less Chromium (Cr) is an optional element. That is, the crankshaft according to this embodiment does not need to contain Cr. Cr improves the hardenability of steel. On the other hand, if the Cr content is too high, machinability decreases. Therefore, the Cr content is 0.25% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%. The upper limit of the Cr content is preferably 0.20%.

The remainder of the chemical composition of the crankshaft according to this embodiment is Fe and impurities. Impurities here refer to elements mixed in from ores and scrap used as raw materials for steel, or elements mixed in from the environment during the manufacturing process.

[Quenched hardened layer]
FIG. 1 is a schematic diagram of a crankshaft 10, which is an example of a crankshaft. The crankshaft 10 includes a journal portion 11, a pin portion 12, and an arm portion 13. The journal portion 11 is connected to a bearing of a cylinder block (not shown). The pin portion 12 is connected to a bearing of a connecting rod (not shown). The arm portion 13 connects the journal portion 11 and the pin portion 12. The journal portion 11 and the pin portion 12 slide on bearings formed on the cylinder block and the connecting rod, respectively.

The crankshaft according to this embodiment has a hardened layer on at least a portion of the surface. Generally, in a crankshaft, induction hardening is selectively performed on the journal portion 11 and the pin portion 12, which are sliding portions, and a hardened layer is often formed on the surfaces of the journal portion 11 and the pin portion 12. The crankshaft according to this embodiment also preferably has a quenched hardened layer on the surfaces of the journal portion 11 and the pin portion 12. However, the position of the hardened layer according to this embodiment is not limited to these. For example, a hardened layer may be formed on the surface of only one of the journal portion 11 and the pin portion 12, or a hardened layer may be formed on a portion other than the journal portion 11 and the pin portion 12. .

In the crankshaft according to this embodiment, the structure of the quenched hardened layer contains ε carbide with a volume fraction of 1.00 to 5.00%.

The ε carbide has a lower adhesion force than the base (matrix) and contributes to improving sliding properties. On the other hand, if the volume fraction of ε carbides is too high, the hardness of the quenched hardened layer decreases, making it difficult to ensure the necessary fatigue strength. Therefore, the volume fraction of ε carbide contained in the structure of the quench-hardened layer is 1.00 to 5.00%. The lower limit of the volume fraction of ε carbide is preferably 1.50%, more preferably 2.00%, and still more preferably 2.50%. The upper limit of the volume fraction of ε carbide is preferably 4.50%, more preferably 4.00%, and even more preferably 3.50%.

The volume fraction of carbides other than ε carbide in the structure of the quench-hardened layer is arbitrary. In addition, in steel materials having a chemical composition in the above range, when tempering is performed in the temperature range of 150 to 210 ° C., mainly ε carbides are precipitated, and when tempered at a higher temperature, mainly cementite is precipitated. When cementite precipitates, the hardness of the steel decreases. Therefore, from the viewpoint of ensuring the necessary fatigue strength, it is preferable that the volume fraction of cementite is low. The volume fraction of cementite is preferably 0.50% or less, more preferably 0.10% or less. Most preferably, the structure of the quench-hardened layer does not contain cementite.

The remainder of the structure of the quench-hardened layer mainly consists of martensite (including tempered martensite; the same applies hereinafter). In the structure of the quenched hardened layer of the crankshaft according to this embodiment, the volume fraction of martensite is 90.0% or more, more preferably 95.0% or more. Note that when calculating the "volume fraction of martensite," the volume of carbides (including the volume of ε carbide and other carbides) is not included in the volume of martensite. That is, "volume of martensite" + "volume of carbide" + "volume of other structures" = "volume of quenched hardened layer".

The structure of the quenched hardened layer may include a structure other than carbide and martensite. Structures other than carbides and martensite include, for example, bainite and retained austenite. The volume fraction of structures other than carbides and martensite is preferably 5.0% or less, more preferably 2.0% or less, and even more preferably 1.0% or less.

In the crankshaft according to this embodiment, it is preferable that the hardened layer has a Vickers hardness of 550 HV or more. The Vickers hardness of the quenched hardened layer shall be measured at a depth of 0.25 mm from the surface. If the Vickers hardness of the quench-hardened layer is low, it will be difficult to ensure the necessary fatigue strength. The lower limit of the Vickers hardness of the quench-hardened layer is preferably 560 HV, more preferably 570 HV. The upper limit of the Vickers hardness of the quench-hardened layer is, for example, 600 HV, although it is not particularly limited.

The thickness of the hardened layer is preferably 1.0 mm or more. The lower limit of the thickness of the quenched hardened layer is more preferably 2.0 mm, and still more preferably 3.0 mm. The hardened layer preferably has a Vickers hardness of 550 HV or more from the surface to a depth of 1.0 mm or more. The depth at which the Vickers hardness is 550 HV or more is more preferably 2.0 mm or more, and still more preferably 3.0 mm or more. The upper limit of the thickness of the quench-hardened layer is not particularly limited. That is, the crankshaft according to this embodiment may have a hardened structure up to the core. The thickness of the hardened layer may be, for example, 5.0 mm or less.

The crankshaft according to the present embodiment preferably satisfies the following formula, where [S] is the S content of the chemical composition, and D is the prior austenite grain size of the quench-hardened layer.
1.00≦[S]×D≦2.80
In the above formula, the unit of [S] is mass %, and the unit of D is μm.

If [S]×D is too large, quench cracking is likely to occur. On the other hand, if [S]×D is too small, the machinability of the steel will decrease. The lower limit of [S]×D is more preferably 1.10. The upper limit of [S]×D is more preferably 2.50, still more preferably 2.00, and still more preferably 1.50.

The prior austenite grain size D may be any size as long as [S]×D is within the above range. The prior austenite grain size D is preferably 10.00 to 50.00 μm. The lower limit of the prior austenite grain size is more preferably 15.00 μm. The upper limit of the prior austenite grain size is more preferably 40.00 μm, and still more preferably 30.00 μm.

[Crankshaft manufacturing method]
Next, an example of a method for manufacturing a crankshaft according to this embodiment will be described. The manufacturing method described below is merely an example, and does not limit the manufacturing method of the crankshaft according to this embodiment.

FIG. 2 is a flow diagram of an example of a method for manufacturing a crankshaft. This manufacturing method includes a step of preparing an intermediate crankshaft (step S1), a step of hardening the intermediate crankshaft (step S2), and a step of tempering the hardened intermediate crankshaft (step S2). Step S3).

An intermediate crankshaft product is prepared (step S1). Intermediate products for crankshafts are manufactured, for example, by hot forging a material with the chemical composition described above to form the rough shape of the crankshaft, subjecting it to heat treatment such as normalizing as necessary, and then machining it. be able to.

The intermediate part of the crankshaft is hardened (step S2). Specifically, for example, an intermediate part of the crankshaft is heated to a predetermined temperature by high-frequency induction heating, and then rapidly cooled. Heating may be performed using a heating furnace instead of high frequency induction heating. Hardening of the intermediate product may be performed only on specific parts such as the pin portion and the journal portion, or may be performed on the entire intermediate product. As a result, a hardened layer is formed on at least a portion of the surface of the intermediate product.

The higher the heating temperature during quenching, the higher the Vickers hardness of the quenched hardened layer tends to be. On the other hand, if the heating temperature during quenching is too high, the prior austenite grain size D of the quench-hardened layer increases, making quench cracking more likely to occur. The heating temperature during hardening is preferably Ac ₃ points or higher. The lower limit of the heating temperature during quenching is preferably 800°C, more preferably 850°C. The upper limit of the heating temperature during quenching is preferably 1100°C, more preferably 1050°C, even more preferably 1000°C, and even more preferably 950°C. Cooling after heating is preferably water cooling.

The intermediate part of the hardened crankshaft is tempered at a temperature of 150 to 210°C. If the tempering temperature is too low, ε carbide will not precipitate sufficiently. On the other hand, if the tempering temperature is too high, cementite will precipitate and the Vickers hardness of the hardened layer will decrease, making it difficult to ensure the necessary fatigue strength. The lower limit of the tempering temperature is preferably 160°C, more preferably 180°C. The upper limit of the tempering temperature is preferably 205°C.

The preferred tempering time is, for example, 0.5 to 90 minutes, although it also depends on the tempering temperature. The lower limit of the tempering time is more preferably 1 minute, and even more preferably 5 minutes. The upper limit of the tempering time is more preferably 60 minutes, still more preferably 30 minutes. Cooling after tempering is, but is not limited to, water cooling or air cooling, for example.

An example of a crankshaft and a method for manufacturing the same according to an embodiment of the present invention has been described above. According to this embodiment, a crankshaft having excellent fatigue strength, slidability, and machinability can be obtained.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The invention is not limited to these examples.

An intermediate crankshaft made of steel having the chemical composition shown in Table 1 was prepared. The pin portion and journal portion of the intermediate crankshaft were subjected to induction hardening and tempering under the conditions shown in Table 2. The holding time for tempering was 30 minutes.

The structure and Vickers hardness of the hardened layer were measured on the hardened layer formed on the pin portion.

The measurement of the volume fraction of ε carbide in the quench-hardened layer is described in Shinya Teramoto et al., "Effect of Fe carbide on mechanical properties of high Si-containing medium carbon martensitic steel", Tetsu to Hagane, Vol. 106 (2020), No. 3, pp. This was carried out according to the method using small angle X-ray scattering (SAXS) described in 165-173. The sample for SAXS measurement was made into a thin film with a thickness of 100 μm or less by mechanical polishing.

The scattering intensity from second phase particles such as carbide depends on the difference in scattering length density Δρ ² (Δρ=ρ _matrix −ρ _carbide ) between the parent phase (matrix) and the second phase particles in the following equation (1). do. Whereas ε carbide has a large Δρ ^{2 and} can be detected even in trace amounts, cementite has a small Δρ ² and cannot be detected. The scattering profile of a sample tempered at 550 °C, in which all carbide is thought to have been replaced by cementite, was used as a background reference, and by subtracting it from the scattering profile of each sample, only the scattering from ε carbide was extracted. By fitting the extracted scattering profile with the following equation (1), a size distribution curve of ε carbide was derived. All carbides (second phase particles) were assumed to be ε carbides, the shape of the ε carbides was disk-like or spherical, and the size distribution function was assumed to be a lognormal distribution. The volume fraction was calculated by integrating this size distribution curve.

Here, q is the scattering vector (q = 4πsinθ/λ (2θ: scattering angle, λ: X-ray wavelength)), I(q) is the scattering intensity, and N(r) is the number density of second phase particles with radius r. , V(r) is the volume of the second phase particles, f(r) is the size distribution function of the second phase particles, and F(q, r) is the shape factor of the second phase particles.

The scattering length density ρ is defined by the following equation (2).

Here, d _N is the atomic density, b is the scattering length (amplitude of scattered waves), and c is the atomic fraction (concentration of each element).

The volume fraction of cementite was measured by cross-sectional observation and image processing using a scanning electron microscope (SEM). Specifically, the observation surface was corroded with nital and observed at a magnification of 5000 times. An image of a field of view of approximately 5 μm×approximately 4 μm was photographed, and cementite and other regions were color-coded (binarized) by image processing to determine the area ratio of cementite. The volume fraction of cementite was considered to be equal to the area fraction.

The prior austenite grain size D of the quench-hardened layer was measured. Specifically, a sample was cut out from the quenched hardened layer so that the surface perpendicular to the axial direction of the crankshaft was the observation surface, polished, and placed in a 50°C hot bath filled with a corrosive solution based on a saturated aqueous solution of picric acid for about 10 minutes. Soaked for minutes. Thereafter, it was neutralized with a 1% NaOH aqueous solution, washed with water and dried, and the prior austenite grains were visualized. For this sample, a 0.5 mm x 0.5 mm area centered at a depth of 0.25 mm from the surface was observed using an optical microscope, and the number of prior austenite grains within the field of view was counted. Finally, the area of the visual field was divided by the number of particles to obtain the area per grain, and the diameter was calculated assuming that the grain was a perfect circle.

The Vickers hardness of the quenched hardened layer was measured in accordance with JIS Z 2244 (2009). The test force was 300gf (2.942N). The hardness at a depth of 0.25 mm from the surface was defined as the Vickers hardness of the quenched hardened layer.

One throw of each crankshaft was cut and subjected to a torsional fatigue test. Specifically, the journal portions at both ends of one throw were fixed to a testing machine, and a predetermined torque was repeatedly applied to the journal portions, thereby repeatedly applying shear force to the pin portions. The test was continued until fatigue fracture was observed or the number of repetitions reached 1.0 x 10 ⁷ times, and the fatigue strength (fatigue limit) was determined.

A sliding test was conducted to evaluate sliding properties. Specifically, a test piece with a diameter of 20 mm and a thickness of 3 mm was taken from the surface of a plate-shaped test material that had been subjected to the same heat treatment as the above-mentioned crankshaft. The surface of the test piece was mirror finished. The sliding test was performed using a ball-on-disc friction and wear tester. The ball was made of alumina and had a diameter of 6 mm. The load was 10 N, the sliding speed was 10 mm/sec, and no lubrication was used. After the sliding test, the width of the sliding marks was measured to evaluate the sliding properties.

To evaluate machinability, the amount of wear on the outer circumference of the drill was measured. Specifically, a guide hole (diameter 5mm 0 mm), an oil hole was further formed using a long drill. After repeating this series of drilling 600 times, the amount of wear on the margin portion of the long drill (on one side) was measured using a laser microscope, and this was taken as the amount of wear on the outer periphery of the drill.

The results are shown in Table 2.

As shown in Table 2, the crankshafts of test numbers 3 to 5, 16 to 18, and 23 had excellent fatigue strength, slidability, and machinability. Specifically, these crankshafts had a fatigue strength of 800 MPa or more, a wear scar width of 160 μm or less, and a drill outer circumferential wear amount of less than 90 μm.

The crankshafts of test numbers 9 and 20 also had excellent fatigue strength, slidability, and machinability. However, these crankshafts have a value of [S]×D exceeding 2.80, and may be susceptible to quench cracking. The reason why the value of [S]×D exceeded 2.80 is considered to be because the heating temperature during quenching was too high.

The crankshaft of Test No. 1 had a fatigue strength of less than 800 MPa. This is considered to be because the C content of this crankshaft was too low.

The crankshafts of test numbers 2, 7, and 15 had wear scar widths exceeding 160 μm. This is considered to be because the volume fraction of ε carbide in the structure of the quenched hardened layer was low. The reason why the volume fraction of ε carbides was low is considered to be because no tempering was performed or because the tempering temperature was too low.

The crankshafts of test numbers 6, 8, and 19 had fatigue strengths of less than 800 MPa. In these crankshafts, the volume fraction of ε carbide in the structure of the hardened layer was not within an appropriate range. It is thought that the fatigue strength of these crankshafts was low because the tempering temperature was too high, causing cementite to precipitate and the hardness of the hardened layer to decrease.

The crankshafts of test numbers 10 and 21 had a drill outer circumferential wear amount of 90 μm or more. This is considered to be because the S content of these crankshafts was too low.

The crankshaft of test number 11 had a fatigue strength of less than 800 MPa. This is considered to be because the Si content of this crankshaft was too low.

The crankshaft of test number 12 had a wear scar width of more than 160 μm. This is considered to be because the volume fraction of ε carbide in the structure of the quenched hardened layer was low. The reason why the volume fraction of ε carbides was low is considered to be because the Si content of this crankshaft was too high.

The crankshaft of test number 13 had a fatigue strength of less than 800 MPa. This is considered to be because the Mn content of this crankshaft was too low.

The crankshaft of test number 14 had a drill outer circumferential wear amount of 90 μm or more. This is considered to be because the Mn content of this crankshaft was too high.

The crankshaft of test number 22 had a drill outer circumferential wear amount of 90 μm or more. This is considered to be because the C content of this crankshaft was too high.

Although one embodiment of the present invention has been described above, the embodiment described above is merely an example for implementing the present invention. Therefore, the present invention is not limited to the embodiments described above, and can be implemented by appropriately modifying the embodiments described above without departing from the spirit thereof.

Claims (5)

The chemical composition is in mass%,
C: 0.36-0.45%,
Si: 0.01-0.20%,
Mn: 1.20-1.60%,
P: 0.10% or less,
S: 0.040-0.080%,
Al: 0.005-0.060%,
N: 0.001 to 0.020%,
The remainder: Fe and impurities,
having a quenched hardened layer on at least a part of the surface;
A crankshaft in which the structure of the quenched hardened layer contains ε carbide at a volume fraction of 1.00 to 5.00%. The chemical composition is in mass%,
C: 0.36-0.45%,
Si: 0.01-0.20%,
Mn: 1.20-1.60%,
P: 0.10% or less,
S: 0.040-0.080%,
Al: 0.005-0.060%,
N: 0.001 to 0.020%,
Cr: 0.25% or less,
The remainder: Fe and impurities,
having a quenched hardened layer on at least a part of the surface;
A crankshaft in which the structure of the quenched hardened layer contains ε carbide at a volume fraction of 1.00 to 5.00%. The crankshaft according to claim 1 or 2,
A crankshaft, wherein the hardened layer has a Vickers hardness of 550 HV or more. The crankshaft according to claim 1 or 2,
A crankshaft that satisfies the following formula, where the S content of the chemical composition is [S] and the prior austenite grain size of the quench-hardened layer is D.
1.00≦[S]×D≦2.80
In the above formula, the unit of [S] is mass %, and the unit of D is μm. A method for manufacturing the crankshaft according to claim 1 or 2, comprising:
A process of hardening the intermediate product of the crankshaft,
A method for manufacturing a crankshaft, comprising the step of tempering the quenched intermediate product at a temperature of 150 to 210°C.

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