JP6561816B2 - Crankshaft and manufacturing method thereof - Google Patents

Description

  The present invention relates to a crankshaft and a manufacturing method thereof.

  In transportation equipment represented by automobiles, trucks, and construction machines, there are increasing demands for improving fuel efficiency (increase in starting and stopping, reducing displacement) and complying with exhaust gas regulations (increasing high in-cylinder pressure). These requirements increase the load on the crankshaft. Therefore, the crankshaft is required to further improve the bending fatigue strength and seizure resistance.

  Surface hardening treatment is known as a technique for increasing the fatigue strength or seizure resistance of a crankshaft. As the surface hardening treatment, nitriding treatment, induction hardening, and fillet roll processing are well known. The nitriding treatment includes a nitriding treatment in a narrow sense that allows only nitrogen to permeate from the surface of the steel, and a soft nitriding treatment that simultaneously permeates nitrogen and carbon.

  The induction-hardened crankshaft has a higher bending fatigue strength than other crankshafts that have undergone surface hardening treatment. This is because the hardened layer formed on the crankshaft by induction hardening is deeper than the hardened layer formed by other surface hardening treatment, and the compressive residual stress introduced into the hardened layer is large.

  On the other hand, the crankshaft subjected to nitriding treatment has a low bending fatigue strength as compared with the case of induction hardening, but can maintain a high temperature strength at a high level and is excellent in seizure resistance. By nitriding, a compound layer mainly composed of iron nitride (ε and / or γ ′) is formed on the surface layer of the crankshaft, and a diffusion layer is formed between the compound layer and the base material. It is considered that excellent seizure resistance can be obtained by the compound layer and the diffusion layer.

  As described above, induction hardening treatment and nitriding treatment exhibit different effects. Therefore, composite heat treatment in which induction hardening is performed after nitriding is disclosed in JP-A-2015-59248 (Patent Document 1), JP-A-2014-1118070 (Patent Document 2), and JP-A-2007-77411 (Patent Document). Document 3).

  In the heat treatment method for steel disclosed in Patent Document 1, first, soft nitriding is performed on the steel. Next, the steel after the soft nitriding treatment is heated at 1000 to 1200 ° C. in the austenite region for 30 to 120 minutes, and then rapidly cooled (quenching treatment). In this case, nitrogen in the compound layer can be diffused into the steel during the quenching process. Therefore, it is described that the hardened layer can be formed deeper and the fatigue strength is increased.

  The manufacturing method of the crankshaft for ship propulsion devices disclosed in Patent Document 2 performs induction hardening after gas soft nitriding. As a result, the strength is increased, and corrosion resistance against seawater is maintained by the nitrided layer (compound layer and diffusion layer). In addition, the dimensional accuracy of a crankshaft falls by induction hardening. Therefore, it is described that the parts (pins, journals, etc.) that are accommodated in the crankcase and do not require corrosion resistance to seawater are ground to increase the dimensional accuracy. It is not described how much the surface layer is ground.

  In the method for manufacturing a mechanical structure component disclosed in Patent Document 3, a nitriding process is performed, the thickness of the compound layer is 5 μm or less, the depth of the nitride layer is 0.2 mm or more, and the depth from the surface is 0.05 mm. A nitride layer having a nitrogen concentration of 0.3 to 2.5% by weight is formed, and then induction hardening is performed under the condition that the nitride layer portion becomes austenite. This describes that excellent bending fatigue strength and wear characteristics can be obtained.

Japanese Patent Laying-Open No. 2015-59248 JP 2014-1118070 A JP 2007-77411 A WO2012 / 056785

  However, when the steel material disclosed in the above-mentioned patent document is a crankshaft, the bending fatigue strength may be low.

  An object of the present invention is to provide a crankshaft having excellent bending fatigue strength and seizure resistance and a method for producing the crankshaft.

The crankshaft according to the present embodiment includes a pin portion, a journal portion, and a fillet portion. This crankshaft is in mass%, C: 0.35% to 0.60%, Si: 0% to 1.50%, Mn: 0.50% to 2.0%, P: 0.025% or less , S: 0.010% to 0.10%, Cr: 0.05% to 2.0%, Al: 0% to 0.050%, N: 0.0030% to 0.020%, V: 0 It contains ˜0.20%, Ti: 0-0.050%, Cu: 0-0.20%, and Ni: 0-0.20%, and the balance has a chemical composition composed of Fe and impurities. In the pin part, the journal part, and the fillet part, the depth from the surface at which a Vickers hardness of HV550 or more is obtained is 1.0 mm or more. In the region from the surface to a depth of 50 μm, the average nitrogen concentration is 0.10% to 0.80%, and the matrix structure is composed of martensite with an area ratio of 80% or more and residual austenite with 0 to 20%, The number of iron nitrides having a circular equivalent diameter of 1 μm or more is less than 1 piece / 1000 μm ² . In the region from the surface to a depth of 20 μm, the number of oxides having a diameter in terms of a circle of 2 μm or more is less than 3/1000 μm ² .

  The crankshaft manufacturing method includes a step of hot forging a steel material having the above chemical composition to manufacture a crankshaft-shaped intermediate material, nitriding the intermediate material, and forming a thick layer on the surface of the intermediate material. A step of forming a compound layer having a thickness of 6 μm or more and a diffusion layer having a thickness of 50 μm or more, a step of subjecting the intermediate material after nitriding treatment to induction hardening treatment, and among the intermediate materials, a compound layer by induction hardening treatment And a step of removing an original compound layer containing an oxide and residual austenite formed under the original compound layer.

  The crankshaft according to the present embodiment has excellent bending fatigue strength and seizure resistance, and the manufacturing method according to the present embodiment can manufacture the crankshaft.

FIG. 1 is a schematic diagram illustrating an example of a main part of the crankshaft of the present embodiment. FIG. 2 is a schematic diagram of a high surface pressure tester used in a seizure test in the examples.

  The present inventors investigated and examined a crankshaft capable of achieving both excellent bending fatigue strength and excellent seizure resistance. As a result, the present inventors obtained the following knowledge.

  (1) In order to obtain an excellent bending fatigue strength in a crankshaft, it is effective to perform induction hardening to form a deep hardened layer and a large residual stress. As described above, the nitriding material is superior in high-temperature strength and seizure resistance as compared with an induction-quenched material. However, it is difficult to obtain a deep hardened layer only by nitriding. Therefore, forming a deep hardened layer by induction hardening is effective for bending fatigue strength.

  (2) When the nitrogen content of martensite formed by induction hardening is high, fine nitrides are formed against heat generated during sliding. Due to the precipitation strengthening of the nitride, a decrease in strength can be suppressed, and an excellent high temperature strength can be obtained. Therefore, if the structure of the outermost layer of the pin part, journal part and fillet part of the crankshaft is martensite having a high nitrogen content (hereinafter referred to as high nitrogen martensite), excellent bending fatigue strength and seizure resistance can be obtained. It is done. High-frequency martensite can be formed on the surface layer by subjecting the steel material after nitriding to induction hardening.

  (3) However, the present inventors have newly found that the following problems occur in a crankshaft that has been subjected to induction hardening after nitriding.

  When nitriding is performed, a compound layer is formed on the outermost layer of the steel material, and a diffusion layer is formed between the base material and the compound layer. The compound layer is mainly composed of iron nitride (ε and / or γ ′). When a steel material including such a compound layer and a diffusion layer is subjected to induction hardening, the compound layer is decomposed by high-temperature heating, and from porous martensite including iron nitride, oxide and voids, and residual austenite A mixed tissue is formed. Such a mixed structure is referred to as an original compound layer. Further, residual austenite is formed in a portion of the diffusion layer adjacent to the compound layer. The original austenite formed under the original compound layer and the original compound layer lowers the bending fatigue strength of the crankshaft.

  (4) Therefore, in this embodiment, the steel material after the induction hardening treatment is ground to remove the original compound layer and the retained austenite under the original compound layer. Thereby, the outstanding bending fatigue strength is obtained. Furthermore, high nitrogen martensite is formed in order to perform induction hardening after nitriding. Therefore, excellent seizure resistance is also obtained.

  (5) There may be a concept of removing the compound layer before the induction hardening treatment. However, induction hardening is usually performed in the atmosphere. Here, the case where the steel material from which the compound was removed before induction hardening processing is used is assumed. In this case, when the steel material is heated to the quenching temperature, oxygen in the atmosphere enters the steel, and coarse oxides are generated in the steel material. Oxides reduce the bending fatigue strength. If the compound layer is removed, nitrogen in the compound layer cannot be moved to the diffusion layer during the induction hardening process. As a result, the nitrogen concentration in the martensite constituting the hardened layer is lowered, and the seizure resistance is lowered.

  When the quenching process is performed using the steel material on which the compound layer is formed, the compound layer functions as a barrier that suppresses the entry of oxygen. Furthermore, the compound layer also functions as a supply source of nitrogen to the diffusion layer. Therefore, in the present embodiment, induction hardening is performed on the steel material on which the compound layer is formed, and then the original compound layer and the retained austenite are removed.

  (6) More specifically, induction hardening is performed on a steel material having a compound layer having a thickness of 6 μm or more and a diffusion layer having a thickness of 50 μm or more. In this case, since the original compound layer is removed from the crankshaft after the grinding treatment, the surface layer structure is mainly composed of martensite having a high nitrogen concentration. Further, iron nitride and oxide are suppressed in the surface layer. Therefore, the bending fatigue strength and seizure resistance of the manufactured crankshaft are increased.

The crankshaft according to the present embodiment completed by the above knowledge includes a pin portion, a journal portion, and a fillet portion. This crankshaft is in mass%, C: 0.35% to 0.60%, Si: 0% to 1.50%, Mn: 0.50% to 2.0%, P: 0.025% or less , S: 0.010% to 0.10%, Cr: 0.05% to 2.0%, Al: 0% to 0.050%, N: 0.0030% to 0.020%, V: 0 It contains ˜0.20%, Ti: 0-0.050%, Cu: 0-0.20%, and Ni: 0-0.20%, and the balance has a chemical composition composed of Fe and impurities. In the pin part, the journal part, and the fillet part, the depth from the surface at which a Vickers hardness of HV550 or more is obtained is 1.0 mm or more. In the region from the surface to a depth of 50 μm, the average nitrogen concentration is 0.10% to 0.80%, and the matrix structure is composed of martensite with an area ratio of 80% or more and residual austenite with 0 to 20%, The number of iron nitrides having a circular equivalent diameter of 1 μm or more is less than 1 piece / 1000 μm ² . In the region from the surface to a depth of 20 μm, the number of oxides having a diameter in terms of a circle of 2 μm or more is less than 3/1000 μm ² .

  The chemical composition of the crankshaft may contain one or more selected from the group consisting of V: 0.02 to 0.20% and Ti: 0.010 to 0.050%.

  The chemical composition of the crankshaft may include one or more selected from the group consisting of Cu: 0.02% to 0.20% and Ni: 0.02% to 0.20%.

  The crankshaft manufacturing method includes a step of hot forging a steel material having the above chemical composition to manufacture a crankshaft-shaped intermediate material, nitriding the intermediate material, and forming a thick layer on the surface of the intermediate material. A step of forming a compound layer having a thickness of 6 μm or more and a diffusion layer having a thickness of 50 μm or more, a step of subjecting the intermediate material after nitriding treatment to induction hardening treatment, and among the intermediate materials, a compound layer by induction hardening treatment And a step of removing an original compound layer containing an oxide and residual austenite formed under the original compound layer.

  Hereinafter, the crankshaft and the manufacturing method thereof according to the present embodiment will be described in detail. “%” Related to chemical components indicates “% by mass”.

[Configuration of crankshaft]
FIG. 1 is a schematic diagram illustrating an example of a main part of a crankshaft according to the present embodiment. The crankshaft 1 includes a pin part 2, a journal part 3, an arm part 4, and a fillet part 5. The journal portion 3 is disposed coaxially with the rotation axis of the crankshaft. The pin portion 2 is arranged so as to be shifted from the rotation axis of the crankshaft 1. The arm portion 4 is disposed between the pin portion 2 and the journal portion 3 and is connected to the pin portion 2 and the journal portion 3. The fillet portion 5 corresponds to a joint portion between the pin portion 2 and the arm portion 4 and a joint portion between the journal portion 3 and the arm portion 4. A connecting rod (not shown) is rotatably attached to the pin portion 2. The journal part 3 is rotatably supported by a bearing (not shown). The pin portion 2 rotates while contacting the connecting rod, and the journal portion 3 rotates while contacting the bearing. Therefore, the pin part 2 and the journal part 3 are required to have excellent seizure resistance, and the fillet part 5 where stress concentration occurs is required to have excellent bending fatigue strength. Hereinafter, the pin part 2 and the journal part 3 are also called a sliding part.

[Chemical composition]
The chemical composition of the crankshaft of this embodiment described above contains the following elements.

C: 0.35-0.60%
Carbon (C) increases the strength of steel and increases the bending fatigue strength of the crankshaft. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the strength of the steel becomes too high, and the machinability decreases. Therefore, the C content is 0.35 to 0.60%. The minimum with preferable C content is 0.36%, More preferably, it is 0.37%. The upper limit with preferable C content is 0.55%, More preferably, it is 0.50%.

Si: 0 to 1.50%
Silicon (Si) may not be contained. When contained, Si increases the hardenability of steel and increases the temper softening resistance of martensite. However, if the Si content is too high, the strength of the steel becomes too high and the machinability deteriorates. Moreover, since heat conductivity will become low when Si becomes high, it will become difficult to dissipate heat | fever and will reduce seizure resistance. Therefore, the Si content is 0 to 1.50%. The minimum with preferable Si content for acquiring the said effect more effectively is 0.01%, More preferably, it is 0.05%. The upper limit with preferable Si content is 0.50%, More preferably, it is 0.30%.

Mn: 0.50 to 2.0%
Manganese (Mn) enhances the hardenability of the steel and makes the surface structure of the crankshaft martensite by induction hardening. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the strength of the steel becomes too high and the machinability deteriorates. Therefore, the Mn content is 0.50 to 2.0%. The minimum with preferable Mn content is 0.60%, More preferably, it is 0.70%. The upper limit with preferable Mn content is 1.7%, More preferably, it is 1.5%.

P: 0.025% or less Phosphorus (P) is an impurity. P embrittles the prior austenite grain boundaries in martensite. Therefore, the P content is 0.025% or less. The upper limit with preferable P content is 0.015%. The P content is preferably as low as possible.

S: 0.010 to 0.10%
Sulfur (S) mainly binds to Mn to form MnS and enhances the machinability of the steel. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, a large amount of coarse sulfide inclusions (including MnS) are generated, which becomes the starting point of the cracking. Therefore, the S content is 0.010 to 0.10%. The minimum with preferable S content is 0.012%, More preferably, it is 0.014%. The upper limit with preferable S content is 0.070%, More preferably, it is 0.065%.

Cr: 0.05-2.0%
Chromium (Cr) increases the nitrogen content in the diffusion layer during nitriding. Therefore, the nitrogen content in martensite obtained by induction hardening is increased. Thereby, the high temperature strength of martensite increases and the seizure resistance of the sliding portions (pin portion and journal portion) of the crankshaft increases. Cr further enhances the hardenability of the steel. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, the strength of the steel becomes too high and the machinability decreases. Therefore, the Cr content is 0.05 to 2.0%. The minimum with preferable Cr content is 0.07%, More preferably, it is 0.09%. The upper limit with preferable Cr content is 1.5%, More preferably, it is 1.2%.

Al: 0 to 0.050%
Aluminum (Al) is an optional element and may be contained for deoxidation. However, when other deoxidation elements such as Si are contained, they may not be contained. Moreover, when it contains, Al forms nitride and suppresses the coarsening of austenite grains during quenching heating as pinning particles. However, if the Al content is too high, coarse inclusions are generated and the toughness of the steel is reduced. Therefore, the Al content is 0 to 0.050%. The minimum with preferable Al content is 0.010%, More preferably, it is 0.015%, More preferably, it is 0.020%. The upper limit with preferable Al content is 0.045%, More preferably, it is 0.040%.

N: 0.0030% to 0.020%
Nitrogen (N) forms various nitrides and suppresses coarsening of austenite grains during quenching heating as pinning particles. However, if the N content is too high, the hot ductility of the steel decreases. Therefore, the N content is 0.0030 to 0.020%. The minimum with preferable N content for suppressing the coarsening of an austenite grain is 0.0060%. The upper limit with preferable N content is 0.016%.

  The balance of the chemical composition of the crankshaft according to the present embodiment consists of Fe and impurities. Here, impurities are mixed from ore, scrap, or production environment as raw materials when industrially producing steel for crankshaft, and have an adverse effect on the crankshaft of this embodiment. It means what is allowed in the range.

  The chemical composition of the crankshaft of this embodiment may further contain one or more selected from the group consisting of V and Ti instead of a part of Fe. All of these elements form fine carbonitrides, increase the strength of the steel, and suppress austenite coarsening during induction hardening.

V: 0 to 0.20%
Vanadium (V) is an optional element and may not be contained. When contained, V increases the strength of the steel by forming fine carbonitrides, and suppresses austenite coarsening during induction hardening. V further increases the nitrogen content in the diffusion layer during nitriding. Therefore, the nitrogen content in martensite obtained by induction hardening is increased. By increasing the nitrogen content, the high-temperature strength of martensite increases, and the seizure resistance of the sliding parts (pin part and journal part) of the crankshaft increases. However, if the V content is too high, the strength of the steel becomes too high and the machinability decreases. Therefore, the V content is 0 to 0.20%. The minimum with preferable V content for acquiring the said effect more effectively is 0.02%, More preferably, it is 0.05%. The upper limit with preferable V content is 0.15%, More preferably, it is 0.12%.

Ti: 0 to 0.050%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti increases the strength of the steel by forming fine carbonitrides, and suppresses austenite coarsening during induction hardening. However, if the Ti content is too high, the carbonitride becomes coarse and the steel becomes brittle. Therefore, the Ti content is 0 to 0.050%. The minimum with preferable Ti content for acquiring the said effect more effectively is 0.010%, More preferably, it is 0.015%. The upper limit with preferable Ti content is 0.045%, More preferably, it is 0.040%.

  The chemical composition of the crankshaft of this embodiment may further include one or more selected from the group consisting of Cu and Ni instead of a part of Fe. All of these elements increase the corrosion resistance of the steel.

Cu: 0 to 0.20%
Copper (Cu) is an optional element and may not be contained. When contained, Cu increases the corrosion resistance of the steel. However, if the Cu content is too high, the hot ductility of the steel decreases, and the slab productivity during continuous casting decreases. Therefore, the Cu content is 0 to 0.20%. The minimum with preferable Cu content for acquiring the said effect more effectively is 0.02%, More preferably, it is 0.05%. The upper limit with preferable Cu content is 0.15%, More preferably, it is 0.12%.

Ni: 0 to 0.20%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the corrosion resistance and toughness of the steel. However, since Ni is expensive, if the Ni content is too high, the manufacturing cost increases. Therefore, the Ni content is 0 to 0.20%. The preferable lower limit of the Ni content for obtaining the above effect more effectively is 0.02%, and more preferably 0.05%. The upper limit with preferable Ni content is 0.15%, More preferably, it is 0.12%.

[Vickers hardness D _HV550 at pin, journal and fillet]
In the pin part, the journal part, and the fillet part of the crankshaft of the present embodiment, the depth from the surface at which the Vickers hardness D _{HV550 of} HV550 or more is obtained is 1.0 mm or more.

In the crankshaft after induction hardening, the bending fatigue strength increases as the depth of the hardened layer formed on the surface layer increases. In the pin part, the journal part, and the fillet part, when the depth D _HV550 from the surface at which a Vickers hardness of HV550 or higher is obtained is less than 1.0 mm, sufficient bending fatigue strength cannot be obtained. Therefore, in the pin part, the journal part, and the fillet part, the depth D _HV550 from the surface at which the Vickers hardness of HV550 or more is obtained is 1.0 mm or more. A preferable lower limit of the depth D _HV550 is 1.2 mm, and more preferably 1.5 mm.

Vickers hardness is measured by the following method. A sample is taken so that the cross section perpendicular to the surface of the pin part, journal part and fillet part becomes the test surface. After polishing the test surface of the sample, a Vickers hardness test in accordance with JIS Z2244 (2009) is performed at a pitch of 0.1 mm from the surface in the depth direction. The test force is 2.94N. A hardness profile near the surface layer is created by continuously connecting the obtained hardness at each position. Based on the created hardness profile, the depth from the surface at which a Vickers hardness of HV550 or higher is obtained is defined as D _HV550 (mm).

[Average nitrogen concentration in the region from the surface of the pin part, journal part and fillet part to a depth of 50 μm]
In the present embodiment, the surface layer structure in the pin portion, journal portion, and fillet portion is mainly composed of high nitrogen martensite. Therefore, the average nitrogen concentration C _{Nave in} the region from the surface to the depth of 50 μm in the pin portion, journal portion, and fillet portion is 0.10 to 0.80%. If the average nitrogen concentration C _Nave is 0.10% or more, martensite contains a sufficient amount of nitrogen, so that excellent seizure resistance can be obtained. On the other hand, if the average nitrogen concentration C _Nave exceeds 0.80%, the amount of retained austenite increases excessively. Residual austenite is soft and therefore decreases bending fatigue strength. Therefore, the average nitrogen concentration C _Nave is 0.10 to 0.80%. A preferable lower limit of the average nitrogen concentration C _Nave is 0.12%, and a preferable upper limit is 0.60%.

The average nitrogen concentration C _Nave is measured by the following method. The nitrogen concentration in a region from the surface to a depth of 50 μm at any 10 locations of the pin portion, the journal portion, and the fillet portion is determined by EPMA (electron beam microanalyzer) line analysis, and the average value is determined. The average of the 10 average values obtained is defined as the average nitrogen concentration C _Nave (%).

[Matrix structure of pin part, journal part and fillet part]
The matrix structure in the region from the surface of the pin part, journal part and fillet part to a depth of 50 μm is mainly composed of martensite. “Mainly” means that the total area ratio of martensite in the matrix is 80% or more. “Martensite” here also includes tempered martensite.

  The matrix structure may contain 20% or less of retained austenite by area ratio. That is, the matrix structure in the above region contains 80% martensite and 0 to 20% retained austenite by area ratio. The total area ratio of martensite in the matrix may be 100%.

  Since the strength of martensite is high, the bending fatigue strength of the crankshaft is increased. On the other hand, retained austenite has lower strength than martensite, and therefore lowers bending fatigue strength. If the matrix structure of the surface layer contains martensite with an area ratio of 80% and the area ratio of retained austenite is suppressed to 20% or less, excellent bending fatigue strength can be obtained. A preferable upper limit of the area ratio of retained austenite is 15%.

  An original compound layer and retained austenite are formed on the steel material after the nitriding treatment. Since these lower the bending fatigue strength, they are removed by a grinding process. Therefore, in the region from the surface of the pin portion, journal portion, and fillet portion to a depth of 50 μm, the area ratio of retained austenite is kept low, and excellent bending fatigue strength is obtained.

  The area ratio of martensite and the area ratio of retained austenite in the region from the surface of the pin part, journal part and fillet part to a depth of 50 μm are obtained by the following method. A crankshaft is cut | disconnected from the surface of a pin part, a journal part, and a fillet part to a depth direction, and the sample containing the cut surface of the surface vicinity is extract | collected. The sample is buried in resin and mirror-polished so that the region from the surface to a depth of 50 μm becomes the observation surface. After polishing, the observation surface is etched with nital. Any five visual fields (field area = 150 μm × 50 μm depth) of the etched observation surface are observed with an optical microscope (observation magnification 1000 times). Martensite and retained austenite differ in contrast (brightness) on the observation surface. Therefore, martensite and retained austenite are specified according to the contrast.

The total area of martensite (μm ² ) specified in all five fields of view and the total area of retained austenite (μm ² ) are determined. Based on the total visual field area (150 μm × 50 μm depth × 5), the area ratio (%) of martensite and the area ratio (%) of retained austenite are obtained. The total area of martensite and tempered martensite and the total area of retained austenite can be determined by, for example, known image processing.

[Coarse iron nitride at pin, journal and fillet]
In the region 50 μm deep from the surface of the pin portion, journal portion, and fillet portion of the crankshaft of this embodiment, the number TN _Fe of iron nitrides (hereinafter referred to as coarse iron nitrides) having a diameter of 1 μm or more in terms of a circle is 1 Pieces / 1000 μm ^{2 or} less. Here, the iron nitride in the present specification means ε (Fe _2-3 N) and γ ′ (Fe ₄ N).

  When induction hardening is performed after nitriding, the compound layer formed by nitriding is decomposed by high-temperature heating in the induction hardening. As a result, the compound layer changes to an original compound layer having a porous mixed structure including iron nitride, oxide, and voids. Since the mechanical properties of the original compound layer are low, the bending fatigue strength of the crankshaft is reduced.

In the present embodiment, the original compound layer is removed in the manufacturing process. Therefore, the region of the crankshaft of the present embodiment does not substantially contain coarse iron nitride. Specifically, the number of coarse iron nitrides TN _Fe is less than 1 piece / 1000 μm ² . Therefore, excellent bending fatigue strength can be obtained. Preferably, there is no iron nitride (including those having a diameter in terms of a circle less than 1 μm) in a region having a depth of 50 μm from the surface of the pin portion, journal portion, and fillet portion.

[Coarse oxides in pin, journal and fillet]
As described above, in this embodiment, induction hardening is performed after nitriding. At this time, the compound layer changes to a porous original compound layer containing iron nitride and an oxide. Oxides are formed by oxygen in the atmosphere entering and diffusing from the surface into the steel during heating in the induction hardening process, and bonding with internal alloying elements (Fe, Mn, Si, Cr, etc.). A coarse oxide having a circle-equivalent diameter of 2 μm or more (hereinafter referred to as a coarse oxide) reduces the bending fatigue strength.

Pin, the journal portion and fillet portion, if 20μm the number TN _O is less than 3/1000 .mu.m ² of coarse oxides in the region up to a depth from the surface, excellent bending fatigue strength is obtained. The upper limit of the number of coarse oxides TN 2 _O is preferably less than 2 pieces / 1000 μm ² , most preferably 0 pieces / 1000 μm ² .

  The number of coarse iron nitrides and coarse oxides in the pin part, journal part and fillet part is measured by the following method. The crankshaft is cut along a plane (cross section) including the central axis of the journal portion, and a sample including cross sections of the pin portion, journal portion, and fillet portion is collected. The sample is buried in resin and mirror-polished so that the cross section from the surface to a depth of 50 μm becomes the observation surface. After polishing, the observation surface is etched with nital. Observed at 1000 times SEM in any 20 fields of view of the etched viewing surface (field of view = 1000 μm × 50 μm for specific case of coarse iron nitride, field of view = 1000 μm × 20 μm for specific case of coarse oxide) Based on the contrast of the reflected electron image, the iron nitride and the oxide are identified.

  In the reflected electron image, the visual field is displayed as a gray scale image. The contrast of the matrix (base material), oxide, iron nitride, and other precipitates (carbonitride, etc.) in the reflected electron image is different. Numerical ranges of brightness (multiple gradations) indicating iron nitride and oxide are determined in advance by SEM and EDS (energy dispersive X-ray microanalyzer), respectively. In each field of view, iron nitride and oxide are specified based on the above numerical range.

  Calculate the circle equivalent diameter of the identified iron nitride and oxide. Specifically, the area of each of iron nitride and oxide is obtained, and the diameter in terms of a circle (μm) is obtained from the obtained area. Among the iron nitrides in the region having a depth of 50 μm from the surface, an iron nitride having a circle-converted diameter of 1 μm or more is specified as coarse iron nitride. Similarly, among oxides in a region 20 μm deep from the surface, an oxide having a circle-equivalent diameter of 2 μm or more is specified as a coarse oxide.

The number of the specified coarse iron nitride and coarse oxide is counted. Based on the counted number, the number of coarse iron nitrides TN _Fe (pieces / 1000 μm ² ) and the number of coarse oxides TN ₀ (pieces / 1000 μm ² ) are obtained.

  The crankshaft of the present embodiment having the above chemical composition and structure has excellent bending fatigue strength and seizure resistance.

[Production method]
An example of a method for manufacturing the crankshaft described above will be described.

  The crankshaft manufacturing method of the present embodiment includes a step of preparing a crankshaft steel material (preparation step), a step of hot forging the crankshaft steel material to manufacture an intermediate material (forging step), and an intermediate material A step of performing nitriding treatment (nitriding treatment step), a step of performing induction hardening treatment on at least the pin portion, journal portion and fillet portion of the intermediate material after nitriding treatment (induction hardening treatment step), and high frequency A step of grinding at least the pin portion, the journal portion, and the fillet portion of the intermediate material after the quenching treatment to remove the original compound layer and the retained austenite (grinding treatment step). Hereinafter, each step will be described.

[Preparation process]
A crankshaft steel material having the above-described chemical composition is prepared. The steel material for crankshaft is manufactured by the following method, for example.

  A molten steel having the above chemical composition is produced. The molten steel is made into a slab by a continuous casting method. You may make molten steel into an ingot (steel ingot) by the ingot-making method. The slab or ingot may be hot-worked into a steel slab or steel bar. The crankshaft steel material is manufactured by the above process.

[Forging process]
The intermediate material which has the shape of a crankshaft is manufactured by hot forging the steel material for crankshafts. The produced intermediate material is cooled (for example, allowed to cool in the atmosphere). Shot blasting is performed on the cooled intermediate material to remove oxide scale generated during hot forging. If necessary, the intermediate material after cooling or the intermediate material from which the oxide scale has been removed is machined to grind the crankshaft shape closer to the product.

[Nitriding process]
Nitriding is performed on the intermediate material after the forging process. As described above, the nitriding process of the present embodiment includes not only a strict nitriding process but also a soft nitriding process.

  The nitriding treatment and soft nitriding treatment in a narrow sense are performed by a known method. For example, in the case of soft nitriding, a nitriding atmosphere in which RX gas and ammonia gas are mixed 1: 1 is used, the nitriding temperature is 500 to 650 ° C., and the holding time at the nitriding temperature is 0.5 to 8 hours. To do. The intermediate material after nitriding is oil-cooled, for example. The nitriding conditions are not limited to the above, and are appropriately set so that the compound layer and the diffusion layer have the thickness described later.

  The following compound layer and diffusion layer are generated by the above nitriding treatment.

[Thickness of compound layer of intermediate material after nitriding treatment: 6 μm or more]
A compound layer is formed on the outermost surface layer of the intermediate material after the nitriding treatment, and a diffusion layer is formed under the compound layer (between the compound layer and the base material). That is, the base material, the diffusion layer, and the compound layer are arranged in this order from the inside of the intermediate material toward the surface.

  As described above, the compound layer is mainly composed of iron nitride (ε and / or γ ′). The compound layer is a porous layer and is removed after induction hardening as described below. However, the compound layer serves as a supply source of nitrogen to the inside during induction hardening, and further functions as a barrier that blocks oxygen from entering the inside of the compound layer during induction hardening.

  If the thickness of the compound layer is less than 6 μm, the amount of nitrogen supplied to the diffusion layer is insufficient at the time of induction hardening, and further, it does not function effectively as an oxygen barrier. Therefore, in this embodiment, a compound layer having a thickness of 6 μm or more is formed by nitriding. A preferable thickness of the compound layer is 8 μm or more. The upper limit of the thickness of the compound layer is not particularly limited, but is 80 μm, for example, considering the nitriding time.

  The thickness of the compound layer can be measured by the following method. The intermediate material after nitriding is cut along a plane including the axial direction, and a sample including a cross section of the surface layer portion is collected. Mirror polishing is performed so that the cross section of the surface layer portion becomes the observation surface. After polishing, the observation surface is etched with nital. On the etched observation surface, the compound layer can be observed as a white layer. The thickness of the compound layer is measured at any 10 locations on the etched observation surface, and the average of the 10 thicknesses is defined as the thickness (μm) of the compound layer of the intermediate material.

[Diffusion layer thickness of intermediate material after nitriding treatment: 50 μm or more]
The diffusion layer formed after the nitriding treatment is a layer in which nitrogen is dissolved in the base material, or a layer in which a solid solution of nitrogen and fine nitrides are dispersed and deposited. This diffusion layer becomes high nitrogen martensite by the induction hardening process of the next step. High nitrogen martensite is excellent in bending fatigue strength. Further, even if heat is generated by sliding, fine nitrides are deposited to maintain the strength, so that the seizure resistance is also excellent.

  If the diffusion layer is too thin, the bending fatigue strength and seizure resistance are reduced. Specifically, if the thickness of the diffusion layer is less than 50 μm, a high nitrogen martensite having a sufficient thickness cannot be formed, and sufficient bending fatigue strength and seizure resistance cannot be obtained. Therefore, the thickness of the diffusion layer is 50 μm or more. A preferable thickness of the diffusion layer is 100 μm or more. The upper limit of the thickness of the diffusion layer is not particularly limited, but is 1000 μm, for example, considering the nitriding time.

  The thickness of the diffusion layer can be measured by the following method. The intermediate material after nitriding is cut along a plane including the axial direction, and a sample including a cross section of the surface layer portion is collected. Mirror polishing is performed so that the cross section of the surface layer portion becomes the observation surface. After polishing, a line analysis is performed in the depth direction by EPMA (X-ray microanalyzer) on the observation surface, and the thickness of the diffusion layer is measured. The thickness of the region where nitrogen is increased by 10% or more from the nitrogen concentration of the matrix (base material) is defined as the thickness (μm) of the diffusion layer.

[Induction hardening process]
A known induction hardening is performed on the intermediate material after the nitriding treatment. The quenching temperature of induction hardening is equal to or higher than the Ac ₃ transformation point. In induction hardening, the quenching temperature is adjusted according to the desired hardened layer depth. The quenching temperature is, for example, 950 ° C. or higher.

  By the induction hardening process, the compound layer becomes an original compound layer that is a porous mixed structure including iron nitride, oxide, and voids.

  On the other hand, the diffusion layer becomes a hardened layer containing high nitrogen martensite and retained austenite by induction hardening. Residual austenite is generated at the boundary between the diffusion layer and the compound layer.

  The induction hardening process may be performed at least on portions corresponding to the pin portion, the journal portion, and the fillet portion of the crankshaft-shaped intermediate material. In order to reliably induction-harden the fillet part, the vicinity of the fillet part of the arm part may be subjected to induction hardening at the same time. Moreover, you may implement an induction hardening process with respect to the whole intermediate material.

[Grinding process]
Of the intermediate material after induction hardening, the original compound layer and the retained austenite decrease the bending fatigue strength. Therefore, a grinding process is performed on the surface layer of the intermediate material after induction hardening to remove the original compound layer and the retained austenite.

  Through the above steps, a crankshaft having the above-described chemical composition and structure is manufactured.

  If necessary, the intermediate material after induction hardening may be tempered at a temperature of less than 200 ° C. In this case, part or all of the martensite in the pin portion, journal portion, and fillet portion becomes tempered martensite.

The test material which assumed the crankshaft was manufactured with the following manufacturing process.
[Preparation process]
Molten steel having the chemical composition shown in Table 1 was produced by a converter.

  Slabs were produced by continuous casting using molten steel of each steel number. The slab was subjected to ingot rolling to produce a billet having a cross section of 162 mm × 162 mm. The billet was heated to 1000-1280 ° C. and rolled to produce a steel bar having a diameter of 100 mm.

[Forging process]
After heating the steel bar to a temperature of about 1000 to 1280 ° C., hot forging (forging) was performed to produce a round bar with a diameter of 65 mm. The round bar was allowed to cool in the air and cooled to room temperature.

[Collecting specimens]
A plurality of Ono-type rotary bending fatigue test pieces were produced by machining from R / 2 part of a round bar having a diameter of 65 mm. The R / 2 portion is a portion that bisects the center and the outer periphery of the round bar cross section (circular shape). The diameter of the parallel part of the Ono-type rotating bending fatigue test piece was 9 mm, and a notch extending in the circumferential direction was formed at the longitudinal center of the parallel part. The cross-sectional shape of the notch was an arc. The shape of the notch bottom was determined by taking into account the planned amount of grinding so that the diameter of the notch bottom was 6.72 mm and the curvature radius of the notch bottom was 1.14 mm after the grinding process described later. Shaped. Further, a plurality of columnar seizure test pieces having a length of 282 mm were produced by turning a round bar having a diameter of 65 mm. The diameter of the test surface of the seizure test piece was set to a value in consideration of the planned amount of grinding so that the diameter would be 48 mm after the grinding treatment.

[Heat treatment and grinding conditions of specimen]
With respect to the produced Ono-type rotary bending fatigue test piece and seizure test piece (hereinafter collectively referred to simply as test pieces), the manufacturing steps shown in Table 2 and Table 3 (nitriding treatment, induction hardening treatment, and Grinding). As described above, in each test number, a plurality of Ono-type rotating bending fatigue test pieces and seizure test pieces were produced.

  Referring to Table 2, in this test, soft nitriding treatment was adopted as nitriding treatment. The treatment temperature of the soft nitriding treatment was 600 ° C., and the holding time at the treatment temperature was 2 hours. The soft nitriding treatment was performed in a nitriding atmosphere in which RX gas and ammonia gas were mixed at a ratio of 1: 1. The test piece after holding was immersed in an oil bath at 90 ° C. to cool the oil, and then cooled to room temperature.

  Two types of conditions for induction hardening were prepared. In condition B1 in Table 2, the treatment temperature was 1000 ° C., and the holding time at the treatment temperature was 30 seconds. In condition B2, the treatment temperature was 860 ° C. Conditions other than the treatment temperature in Condition B2 were the same as those in Condition B1. The test piece after holding was water-cooled.

  Five types of conditions C1 to C5 were prepared as conditions for the grinding treatment. Under any of the conditions, the Ono-type rotating bending fatigue test was performed using a grindstone having a radius of curvature of 1.14 mm, grinding the notches, and performing final polishing. The diameter of the notch bottom after finish polishing was 6.72 mm, and the radius of curvature of the notch bottom was 1.14 mm. About the seizure test piece, finish polishing was performed on the peripheral surface which is the test surface. The diameter of the test surface of the baked specimen after finish polishing was 48 mm. As shown in Table 2, the grinding amount in each grinding treatment was set to 20 to 300 μm. The average roughness (Ra) of the surface of each test piece after grinding was set within 0.2 μm, and the maximum height (Rmax) was set within 2 μm. The average roughness and the maximum roughness were determined according to JIS B0601 (2001).

  With respect to the test piece of each test number, the first to third steps in Table 3 were performed to complete the test piece. In Table 3, “-” in the first to third steps means that the corresponding step was not performed. For example, in test number 8, it means that the nitriding process was not performed, the induction hardening process of test number B1 was performed, and then the grinding process of condition C2 was performed.

[Thickness of compound layer and diffusion layer]
The thickness of the compound layer (μm) and the thickness of the diffusion layer (μm) were determined by the above-described method for the test pieces of each test number after nitriding and before induction hardening. In test numbers 16 and 17, the thicknesses of the compound layer and the diffusion layer of the test piece after the grinding treatment were obtained. In Test No. 8, since soft nitriding was not performed, the compound layer and the diffusion layer were not measured. Further, in test numbers 9 to 11, since the induction hardening was not performed after the soft nitriding treatment, the compound layer and the diffusion layer were not measured. The obtained results are shown in Table 3.

[Microstructure observation test]
Microstructure observation was performed using the test piece after 3rd process implementation. The test piece was cut, and a sample having a cross section including the vicinity of the surface as an observation surface was collected. Using the sample, the structure (martensite, retained austenite) was specified by the above-described method, and the area ratio of each phase was calculated. The obtained results are shown in Table 3.

[Average nitrogen concentration measurement test]
Using the sample used in the microstructure observation test, the average nitrogen concentration C _Nave (%) in the region from the surface to a depth of 50 μm was determined by the above method. The obtained results are shown in Table 3.

[Coarse iron nitride number TN _Fe and coarse oxide number TN _O measurement test]
Using the sample used in the microstructure observation test, the number of coarse nitrides TN _Fe (pieces / 1000 μm ² ) in the region from the surface to a depth of 50 μm and the region from the surface to a depth of 20 μm by the above-described method. The number of coarse oxides TN _O (pieces / 1000 μm ² ) was determined. The obtained results are shown in Table 3.

[Hardness test]
In the hardness test, a hardness profile of the surface layer was created by the above-described method using the same specimen as that for microstructural observation. Based on the obtained hardness profile, a depth D _HV550 (mm) from the surface at which a Vickers hardness of HV550 or higher was obtained was obtained.

[Ono type rotating bending fatigue test]
Using the Ono-type rotating bending fatigue strength test piece manufactured under the manufacturing conditions shown in Table 3, a fatigue test was performed at room temperature (25 ° C.) and in an air atmosphere under the conditions of both swings of 3000 rpm, and after 10 ⁷ cycles. The highest stress that did not break was defined as bending fatigue strength (MPa). In this example, when the bending fatigue strength was 550 MPa or more, it was determined that the bending fatigue strength was excellent.

[Seizure test]
The seizure test was conducted using the high surface pressure tester shown in FIG. 2 under the following conditions. Referring to FIG. 2, the high surface pressure tester includes three housings 50 and a plurality of plain bearings 10 and 20. Each housing was formed with a cylindrical through hole, and the three housings were arranged coaxially. A pair of slide bearings 10 is fitted in the through holes of the housings at both ends, and the slide bearing 20 for seizure test is disposed in the central housing 50. A commercially available slide bearing 20 was used. The plain bearing was formed by forming an Al alloy layer having a thickness of 0.3 mm on steel having a width of 15.5 mm and a thickness of 1.2 mm.

  The seizure test piece 40 was inserted into the slide bearings 10 and 20, and the test was started. The gap between the seizure test piece 40 and the slide bearing 20 was set to 50 μm. Lubricating oil was supplied from the supply hole 30 to the sliding bearing 20 during the test. As the lubricating oil, commercially available SAE standard 0W-20 was used. The oil supply temperature was 150 ° C., and the oil supply amount was 0.8 L / min. The rotational speed of the seizure test piece 40 was 6000 rpm.

  A load was applied in the radial direction of the seizure test piece 40 by applying a load in the radial direction of the central housing 50. The load applied to the seizure test piece 40 was increased stepwise. Specifically, during the test, the rotational torque was measured, and the operation was performed for 3 minutes after reaching the target load. After 3 minutes, if there is no sudden increase in rotational torque, further increase the 2kN load and measure the rotational torque. After reaching the target load, operate for 3 minutes and measure the rotational torque during operation. Was repeated. The surface pressure when the rotational torque increased rapidly was defined as the surface pressure at which seizure occurred (seizure surface pressure, unit: MPa). In this example, it was determined that the seizure resistance was excellent when the seizure surface pressure was 80 MPa or more.

[Test results]
The test results are shown in Table 3. The chemical compositions of the steels with test numbers 1 to 7 were appropriate. Furthermore, the manufacturing process was also appropriate, and the thickness of the compound layer and the thickness of the diffusion layer after the soft nitriding treatment were appropriate. Therefore, the average nitrogen concentration C _Nave of the surface layer of the test piece, the depth D _HV550 from the surface at which a Vickers hardness of HV550 or higher is obtained, the martensite area ratio, the residual austenite area ratio, the number of coarse iron nitrides TN _Fe , and number TN _O of coarse oxide was neither appropriate. As a result, in the test pieces of test numbers 1 to 7, the bending fatigue strength was 550 MPa or more, and the seizing surface pressure was 80 MPa or more.

On the other hand, in test number 8, soft nitriding was not performed. Therefore, the average nitrogen concentration C _Nave of the surface layer was as low as less than 0.100%, and high nitrogen martensite was not formed. Therefore, the seizure surface pressure was less than 80 MPa.

In test number 9, only nitriding was performed, and induction hardening and grinding were not performed. As a result, the depth D _HV550 was less than 1.0 mm. Furthermore, the number of coarse iron nitrides TN _Fe was 1/1000 μm ² or more. As a result, the bending fatigue strength was less than 550 MPa.

In test numbers 10 and 11, although soft nitriding and grinding were performed, induction hardening was not performed. Therefore, the depth D _HV550 was less than 1.0 mm, and the bending fatigue strength was less than 550 MPa.

In test number 12, the heating temperature of induction hardening was too low. Therefore, the effective hardened layer depth D _HV550 was less than 1.0 mm, and the bending fatigue strength was less than 550 MPa.

In test number 13, the grinding process was not performed. Therefore, the average nitrogen concentration C _Nave was too high, and the number of coarse iron nitrides TN _Fe was 1/1000 μm ² or more. Furthermore, the area ratio of retained austenite exceeded 20%. As a result, the bending fatigue strength was less than 550 MPa. As a result of the omission of the grinding step, the original compound layer containing a high nitrogen concentration remained. As a result, the average nitriding concentration C _Nave was high, and the number of coarse iron nitrides TN _Fe was 1/1000 μm ² or more.

In test number 14, as a result of the grinding amount being too small, the number of coarse iron nitride TN _Fe was 1/1000 μm ² or more. Furthermore, the average nitrogen concentration C _Nave was too high, and the area ratio of retained austenite exceeded 20%. As a result, the bending fatigue strength was less than 550 MPa.

In test number 15, the grinding amount was too much. Therefore, high nitrogen martensite in the surface layer was removed, and the average nitrogen concentration C _Nave was too low. As a result, the seizing surface pressure was less than 80 MPa.

In test number 16, grinding treatment was performed after the soft nitriding treatment, and then induction hardening was performed. As a result, the number of coarse iron nitrides TN _Fe was 1/1000 μm ² or more, and the number of coarse oxides TN _O was 3/1000 μm ² or more. Furthermore, the area ratio of retained austenite exceeded 20%. Therefore, the bending fatigue strength was low. Since the compound layer just before the induction hardening process was thin, it is considered that oxygen entered from the outside during the induction hardening process and excessively large oxides were generated. Furthermore, since grinding was not performed after induction hardening, it was considered that the original compound layer remained, and as a result, the number of coarse iron nitrides TN _Fe and residual austenite could not be suppressed.

In test number 17, a grinding process was performed after the soft nitriding process, and then a high frequency process was performed. In test number 17, the thickness of the compound layer of the test piece immediately before induction hardening was 0 μm. Therefore, the average nitrogen concentration C _Nave was too low, and excessive coarse oxide remained. As a result, the bending fatigue strength was less than 550 MPa, and the seizure surface pressure was less than 80 MPa. Since there was no compound layer during induction hardening, it was thought that the supply of nitrogen to the diffusion layer was insufficient, and as a result, the nitrogen concentration in martensite was lowered. In addition, it was considered that excessive oxide was generated because there was no compound layer that would block oxygen from entering during induction hardening.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

1 Crankshaft 2 Pin 3 Journal 5 Fillet

Claims (4)

A crankshaft comprising a pin part, a journal part and a fillet part,
% By mass
C: 0.35% to 0.60%,
Si: 0% to 1.50%,
Mn: 0.50% to 2.0%,
P: 0.025% or less,
S: 0.010% to 0.10%,
Cr: 0.05% to 2.0%
Al: 0% to 0.050%,
N: 0.0030% to 0.020%,
V: 0 to 0.20%,
Ti: 0 to 0.050%,
Cu: 0 to 0.20%, and
Ni: 0 to 0.20% is contained, the balance has a chemical composition consisting of Fe and impurities,
In the pin part, journal part, and fillet part, the depth from the surface at which a Vickers hardness of HV550 or higher is obtained is 1.0 mm or more,
In the region from the surface to a depth of 50 μm, the average nitrogen concentration is 0.10% to 0.80%, the matrix structure is composed of martensite with an area ratio of 80% or more and residual austenite with 0 to 20%, The number of iron nitrides with a diameter in terms of a circle of 1 μm or more is less than 1/1000 μm ²
A crankshaft characterized in that in the region from the surface to a depth of 20 μm, the number of oxides having a circular equivalent diameter of 2 μm or more is less than 3/1000 μm ² . The crankshaft according to claim 1,
The chemical composition is
V: 0.02 to 0.20%, and
A crankshaft containing at least one selected from the group consisting of Ti: 0.010 to 0.050%. The crankshaft according to claim 1 or 2,
The chemical composition is
Cu: 0.02% to 0.20%, and
Ni: One or more types selected from the group consisting of 0.02% to 0.20% are included, and the crankshaft is characterized in that: A step of hot forging the steel material having the chemical composition according to any one of claims 1 to 3 to produce a crankshaft-shaped intermediate material;
Nitriding the intermediate material to form a compound layer having a thickness of 6 μm or more and a diffusion layer having a thickness of 50 μm or more on a surface layer of the intermediate material;
A step of performing induction hardening on the intermediate material after the nitriding treatment;
Of the intermediate material, the compound layer is changed by the induction hardening process, and includes a step of removing an original compound layer containing an oxide and residual austenite formed under the original compound layer. The method for manufacturing a crankshaft according to any one of claims 1 to 3.

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