JP7343767B2 - Crankshaft and its manufacturing method - Google Patents

Description

The present invention relates to a crankshaft and a method for manufacturing the same, and more particularly to a crankshaft having a hardened layer on its surface and a method for manufacturing the same.

In order to improve wear resistance and fatigue strength, crankshafts are known in which a hardened layer is formed on the surface by induction hardening or nitrocarburizing.

Japanese Patent No. 4589885 discloses that C: 0.3 to 0.6% is contained, the thermal conductivity κ is 40 W/mK or more, and the surface hardness Hv after induction hardening is Hv>2.7× A crankshaft satisfying κ+420 is disclosed.

Japanese Patent No. 5499974 discloses a non-thermal nitrided crankshaft containing 0.25 to 0.60% C and having an HV hardness of 380 to 600 at a depth of 0.05 mm from the surface. There is.

JP 2017-110247 A discloses that the pin portion, journal portion, and fillet portion contain C: 0.35 to 0.60% and have a depth of 1 from the surface where a Vickers hardness of HV550 or more can be obtained. A crankshaft is disclosed that is greater than or equal to .0 mm.

Japanese Patent No. 6,354,455 discloses a crankshaft in which the outer circumferential surface of at least a portion of a plurality of pin portions and a plurality of journal portions serves as a rolling surface for a rolling bearing. This crankshaft contains C: 0.50 to 0.90%, the rolling surface is induction hardened, and the hardness within a range of at least 1.5 mm from the outermost surface is 650 HV or more.

Patent No. 4589885 Patent No. 5499974 JP 2017-110247 Publication Patent No. 6354455

In addition to fatigue strength and wear resistance, crankshafts are required to have seizure resistance. In recent years, with the aim of improving fuel efficiency, the viscosity of lubricating oil has been reduced and the sliding parts of crankshafts have become thinner, and crankshafts are required to have better anti-seizure properties.

An object of the present invention is to provide a crankshaft with excellent seizure resistance and a method for manufacturing the same.

A crankshaft according to an embodiment of the present invention is a crankshaft having a hardened layer having a volume fraction of martensite of 80% or more on its surface, the hardened layer having a depth of at least 2.0 mm from the surface layer at room temperature. The hardness to 0 mm is HV750 or higher, and the amount of solid solution carbon at room temperature is 0.46 to 0.65% by mass.

A method for manufacturing a crankshaft according to an embodiment of the present invention is the above-mentioned method for manufacturing a crankshaft, and includes the steps of preparing an intermediate product of the crankshaft, and induction hardening the intermediate product. Do not perform tempering after quenching.

According to the present invention, a crankshaft with excellent seizure resistance can be obtained.

FIG. 1 is a diagram showing the relationship between the surface hardness of a quench-hardened layer and the baking surface pressure. FIG. 2 is a diagram showing the relationship between the amount of solid solute carbon in the quenched hardened layer and the baking surface pressure at room temperature. FIG. 3 is a diagram showing the relationship between the amount of solid solute carbon in the quenched hardened layer after heating at 200° C. for 1 hour and the baking surface pressure. FIG. 4 is a diagram showing the relationship between the amount of solid solute carbon in the quenched hardened layer after heating at 300° C. for 1 hour and the baking surface pressure. FIG. 5A is a schematic diagram showing the sampling position of a test piece for hardness measurement. FIG. 5B is a schematic diagram of a test piece for hardness measurement. FIG. 6 is a flow diagram of analysis of electrolytic extraction residue. FIG. 7 is a flow diagram showing an example of a method for manufacturing a crankshaft. FIG. 8 shows a heat pattern of the test shaft produced in the example. FIG. 9 is a schematic diagram of the evaluation device used in the burn-in test. FIG. 10 is a schematic diagram of temporal changes in surface pressure applied to the test shaft.

It is said that in order to improve the seizure resistance, it is necessary to improve the surface hardness of the soft material of the sliding parts. There are also examples of crankshafts in which seizure resistance has been improved by improving the surface hardness of the soft bearing metal. On the other hand, the relationship between the surface hardness and seizure resistance of a crankshaft, which is a hard material, has not been systematically investigated so far.

FIG. 1 is a diagram showing the relationship between the surface hardness and seizure surface pressure of a test shaft simulating a crankshaft, obtained through research by the present inventors. It is generally believed that increasing the surface hardness improves wear resistance, which also improves seizure resistance. However, according to the research conducted by the present inventors, as shown in Figure 1, although there is a tendency for the seizure surface pressure to increase as the surface hardness increases, the correlation coefficient cannot be said to be that large; It was found that this cannot fully explain the seizure resistance.

As a result of further investigation, it was found that seizure resistance has a strong correlation with the amount of solid solute carbon. FIG. 2 is a diagram showing the relationship between the amount of solid solute carbon and the seizure surface pressure. From FIG. 2, it can be seen that the seizure resistance can be well explained by the amount of solid solute carbon.

Furthermore, since the surface temperature of the crankshaft when seizure occurs is between 200° C. and 300° C., it is thought that characteristics in this temperature range have a greater influence on seizure resistance. FIGS. 3 and 4 are diagrams showing the relationship between the amount of solid solution carbon and the seizure surface pressure after heating at 200° C. and 300° C. for 1 hour, respectively. From FIGS. 3 and 4, it can be seen that the seizure resistance can be better explained by the amount of solid solute carbon after heating at 200° C. and 300° C. for 1 hour.

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]
The crankshaft according to this embodiment has a hardened layer on its surface. The quench-hardened layer may be formed on the entire crankshaft, or may be formed only on a portion of the crankshaft (for example, the pin and/or journal).

In this embodiment, the quench-hardened layer means a layer in which the volume fraction of martensite is 80% or more. The thickness of the hardened layer of the crankshaft according to this embodiment is preferably 2.0 mm or more, more preferably 4.0 mm or more.

The crankshaft according to this embodiment may be hardened not only to the surface layer but also to the core portion. However, from the viewpoint of productivity, it is preferable that the core has an unhardened structure (specifically, ferrite, pearlite, or ferrite/pearlite).

The hardened layer of the crankshaft according to the present embodiment has a hardness of HV750 or more from the surface layer to a depth of at least 2.0 mm at room temperature, and has a solid solution carbon content at room temperature (hereinafter referred to as "room temperature solid solution carbon content"). ) is 0.46 to 0.65% by mass.

If the hardness from the surface layer to a depth of at least 2.0 mm at room temperature is HV750 or more and the amount of solid solution carbon at room temperature is 0.46% by mass or more, excellent seizure resistance can be obtained. The seizure resistance has a strong correlation with the amount of solid solute carbon in the quenched hardened layer, and the higher the amount of solid solute carbon in the hardened layer, the better the seizure resistance. The lower limit of the amount of solid solute carbon at room temperature is preferably 0.47% by mass.

On the other hand, if the amount of solid solution carbon at room temperature exceeds 0.65% by mass, machinability decreases, making machining difficult, making it difficult to finish into a smooth surface shape, and forming retained austenite. This causes problems such as increased quenching and quench cracking. In particular, regarding quench cracking, when a crack occurs on a sliding surface, the seizure surface pressure is significantly reduced due to the opening of the crack itself and burrs generated around the opening. Therefore, the upper limit of the amount of solid solution carbon at room temperature is 0.65% by mass. The upper limit of the amount of carbon in solid solution at room temperature is preferably 0.60% by mass, more preferably 0.55% by mass, and even more preferably 0.52% by mass.

Although there is no particular upper limit to the surface hardness at room temperature, it is preferably HV850, more preferably HV800, and even more preferably HV780. Note that the surface hardness is defined as the hardness at a depth of 100 μm from the surface.

The quenched hardened layer of the crankshaft according to the present embodiment preferably has a solid solution carbon content after heating at 200°C for 1 hour (hereinafter referred to as "200°C solid solution carbon content") of 0.42% by mass or more, The amount of solid solution carbon after heating at 300°C for 1 hour (hereinafter referred to as "300°C solid solution carbon amount") is 0.34% by mass or more.

The hardened layer of the crankshaft according to this embodiment preferably has a hardness of HV470 or higher from the surface layer to a depth of at least 2.0 mm after heating at 200°C for 1 hour, and preferably has a hardness of HV470 or higher from the surface layer to a depth of at least 2.0 mm after heating at 300°C for 1 hour. The hardness is HV400 or higher up to a depth of at least 2.0 mm.

The hardness of the hardened layer at room temperature is measured as follows. As shown in FIGS. 5A and 5B, a test piece SMP including the surface SF is cut out from the crankshaft. The measurement plane MF is a plane parallel to the axial direction of the crankshaft. The measurement surface MF and its back surface are polished parallel to each other. A plurality of points in the depth direction are rolled down with a test force of 300 gf, and the Vickers hardness at each depth point is measured. The Vickers hardness at each depth point is measured at five locations parallel to the axial direction of the crankshaft, and the average value is taken as the value.

The hardness of the quenched hardened layer after heating at 200°C and 300°C for 1 hour was determined by holding a test piece prepared in the same way as the test piece used for measuring hardness at room temperature at 200°C and 300°C for 1 hour, respectively. After air cooling to room temperature, the hardness is measured in the same manner as the hardness measurement at room temperature.

The room temperature solid solute carbon content of the quenched hardened layer is measured as follows. First, the proportion (mass %) of carbon in a non-solid solution state in the quenched hardened layer is determined by electrolytic extraction residue measurement. FIG. 6 is a flow diagram of analysis of electrolytic extraction residue.

A sample for electrolytic extraction residue measurement is collected from the location where the quenched hardened layer is formed. For example, if the steel material to be measured has a cylindrical shape with a diameter of 47.9 mm, the sample to be collected can be a part of the steel material cut out to have a cylindrical shape with a height of 12.0 mm and a diameter of 47.9 mm. The cross section of this sample is masked and only the surface layer is dissolved. Specifically, by a constant current electrolysis method using a 10 mass% acetylacetone-1 mass% tetramethylammonium chloride/methanol solution (AA solution) as an electrolyte, a region up to 40 μm from the surface was electrolyzed at a current density of 20 mA/cm ² . is anodically dissolved (AA electrolysis). The solution is filtered through a glass filter with a pore size of 0.2 μm, and the residue is collected.

The collected residue is analyzed by high frequency combustion infrared absorption method to determine the carbon content (mass%) in the residue. The high-frequency combustion infrared absorption method is performed in accordance with 8.2 (quantitative operation) b) of JIS G 1211-3:2018 (Iron and steel - Carbon quantitative method -). Specifically, put the glass filter in a furnace and add a combustion improver (a mixture of tungsten and tin (tungsten: tin = 9:1). However, if high frequency is difficult to apply, follow JIS Z 2615: 2015 8.13. Add pure iron.) and burn it to turn the carbon on the glass filter into CO and _CO2 . The amount of carbon detected is determined by integrating the detected amounts of CO and _CO2 and comparing the integrated amount with a calibration curve obtained in advance.

The difference between the carbon content (mass %) in the chemical composition of the crankshaft and the carbon content (mass %) in the residue is defined as the room temperature solid solution carbon content (mass %).

The amount of solid solute carbon at 200°C and the amount of solute carbon at 300°C in the quenched hardened layer were determined by holding test pieces prepared in the same way as the test pieces used for measuring the amount of solid solute carbon at room temperature at 200°C and 300°C for 1 hour, respectively. After air cooling to room temperature, the measurement is carried out in the same manner as the measurement of the amount of solid solute carbon at room temperature.

The crankshaft according to the present embodiment is made of, but not limited to, carbon steel for machine structures according to JIS G 4051:2009, steel for machine structures with guaranteed hardenability (H steel) according to JIS G 4052:2008, and JIS G4053. :2008 alloy steel for machine structures, etc. can be used. Among these steel materials, S50C, S55C, S60C, etc. of JIS G 4051:2009 are preferred, and steel materials to which S is added to improve machinability are particularly preferred.

The chemical composition of the crankshaft is preferably C: 0.48 to 0.62%, Si: 0.01 to 2.0%, Mn: 0.1 to 2.0%, Cr: 0 in mass %. .01 to 0.50%, Al: 0.001 to 0.06%, N: 0.001 to 0.020%, P: 0.03% or less, S: 0.20% or less, balance Fe and impurities It is.

[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 method of manufacturing the crankshaft according to this embodiment.

FIG. 7 is a flow diagram showing an example of a method for manufacturing a crankshaft. This manufacturing method includes a step of preparing a material (step S1), a hot forging step (step S2), a heat treatment step (step S3), a machining step (step S4), a quenching step (step S5), and a finishing step. (Step S6). Each step will be explained in detail below.

A crankshaft material is prepared (step S1). The material of the crankshaft is not limited to this, but for example, the chemical composition in mass% is C: 0.48 to 0.62%, Si: 0.01 to 2.0%, Mn: 0.1 to 2. .0%, Cr: 0.01-0.50%, Al: 0.001-0.06%, N: 0.001-0.020%, P: 0.03% or less, S: 0.20 % or less can be used. The material of the crankshaft may contain elements other than those mentioned above (for example, V, Nb, etc.).

The material is, for example, steel bar. The material can be manufactured, for example, by continuous casting or blooming rolling of molten steel having the above chemical composition.

The raw material is hot forged into a rough shape of a crankshaft (step S2). Hot forging may be performed separately into rough forging and finish forging.

Heat treatment such as normalizing is performed as necessary on the rough-shaped crankshaft manufactured by hot forging (step S3). This heat treatment step (step S3) is an optional step, and may be omitted depending on the required characteristics of the crankshaft.

The rough-shaped crankshaft is machined (step S4). Machining includes cutting, grinding, drilling, and the like. Through this process, an intermediate crankshaft product having a shape close to the final product is manufactured.

Quenching intermediate parts of machined crankshafts. As a result, a hardened layer is formed on the surface. Specifically, after heating to a predetermined heating temperature, it is rapidly cooled. The quenching is preferably oil cooling or water cooling, more preferably water cooling. At this time, the intermediate product may be heated locally using a high-frequency induction heating device, or the entire intermediate product may be heated using a heat treatment furnace. The heating temperature is preferably Ac ₃ points or higher, more preferably 900°C or higher.

In this example, the intermediate crankshaft is not tempered after being hardened. This is because when tempering is performed, carbides such as cementite precipitate and the amount of solid solution carbon decreases.

The intermediate product on which the quench-hardened layer has been formed is subjected to finishing processing as necessary (step S6). For example, the surface shape of crankshaft journals and pins is adjusted by grinding or lapping. A crankshaft can be manufactured through the above steps.

The hardness at room temperature, the hardness at 200° C., and the hardness at 300° C. of the quench-hardened layer can be adjusted by, for example, the chemical composition of the material, the cooling rate during quenching, and the like. Specifically, when the carbon content is increased or when the cooling rate during quenching is increased, the values of the hardness at room temperature, hardness at 200°C, and hardness at 300°C of the quenched hardened layer become higher.

The room temperature solid solution carbon content, the 200°C solid solution carbon content, and the 300°C solid solution carbon content (hereinafter these may be simply referred to as "solid solution carbon content" without distinction) of the quenched hardened layer are, for example, It can be adjusted by the chemical composition of the material, heat treatment after forging, etc. Specifically, when the carbon content of the material is increased, the amount of solid solution carbon in the quenched hardened layer increases.

In order to increase the value of the amount of solid solute carbon, it is preferable not to hold the steel material for a long time in a temperature range where carbides precipitate. Further, it is preferable to reduce the content of elements (such as Cr) that form carbides.

The crankshaft and the manufacturing method thereof according to one embodiment of the present invention have been described above. According to this embodiment, a crankshaft with excellent seizure resistance 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.

A test shaft for the seizure test was prepared using steel having the chemical composition shown in Table 1. FIG. 8 shows the heat pattern during test shaft production.

Specifically, the material was heated at 1250°C for 1 hour, then hot forged at 1100 to 850°C, and after the forging was completed, it was allowed to cool to room temperature. Thereafter, normalization was performed by heating at 1250° C. for 20 minutes and cooling in air, and then the outer diameter was made 47.9 mm by machining (cutting). Thereafter, induction hardening was performed to form a hardened layer. In each of these test shafts, a quenched hardened layer having a volume fraction of martensite of 80% or more was formed with a thickness of 3.0 mm or more.

After hardening, the test shaft was ground and lapped as finishing processing so that the arithmetic mean height Pa of the cross-sectional curve was 0.07 to 0.09 μm. The outer diameter of the test shaft was adjusted so that the clearance with the bearing used in the seizure test described below was approximately 0.090 mm (numbers 1 to 4 and 9 to 12 in Tables 2 and 3 below).

As a comparative example, test shafts were prepared which were tempered for 90 minutes at 210°C (numbers 5 to 6 in Table 2 below) and 420°C (numbers 7 and 8 in Table 2 below) after quenching. Except for tempering, the manufacturing conditions for the test shafts numbered 5 to 9 in Table 2 below are the same as those for numbers 1 to 4 and 9 to 12 in Table 2 below.

The hardness of the quenched layer of each test shaft was measured according to the method described in the embodiment. Specifically, the hardness was measured at a point at a depth of 100 μm from the surface and at a point at a depth of 2.0 mm from the surface layer. In addition, the hardness after heating at 600°C for one hour was also measured in the same manner as in the case of 200°C and 300°C.

According to the method described in the embodiment, the amount of solid solute carbon at room temperature, the amount of solid solute carbon at 200° C., and the amount of solid solute carbon at 300° C. in the quenched hardened layer of each test shaft were measured.

A seizure test was conducted using the prepared test shaft. The seizure test was conducted using a crank metal wear resistance seizure evaluation device manufactured by Shinko Zoki Co., Ltd. A schematic diagram of the evaluation device 20 is shown in FIG. The test shaft TP was inserted into the plurality of bearings 21, and the test shaft TP was rotated at 8000 rpm by a motor (not shown) while the bearings 21 were oiled. The metal of the bearing was an Al alloy with an HV of 40 to 50. The lubricating oil was 0W-20, the oil supply temperature was 140°C, and the oil pressure was 0.8 MPa.

In this state, one of the bearings 21 was pulled down to gradually increase the surface pressure applied to the test shaft TP, and the test shaft was operated until seizure occurred. FIG. 10 schematically shows the temporal change in the surface pressure applied to the test shaft TP. The holding time at the same surface pressure was 3 minutes, the width of increase in surface pressure per step was 4.35 MPa, and the time taken to increase the surface pressure was 15 seconds. It was determined that seizure occurred when the surface temperature of the test shaft TP became 280° C. or higher or the torque applied to the test shaft became 25 Nm or higher.

The results are shown in Tables 2 and 3. It should be noted that the test shaft numbered 12 suffered from quench cracking during the finishing process and could not be subjected to the seizing test.

As shown in Tables 2 and 3, the test shafts numbered 1 to 4 have a hardness of 750 HV or more from the surface layer to a depth of 2.0 mm at room temperature, and a room temperature solid solute carbon content of 0.46 to 4. It was 0.65% by mass. These test shafts exhibited a seizure pressure of 79 MPa or more and had superior seizure resistance compared to test shafts numbered 5 to 11.

In the test shaft of test number 12, quench cracking occurred during finishing processing. This is considered to be because the amount of solid solution carbon in the test shaft of test number 12 was too high.

FIG. 1 is a diagram showing the relationship between hardness and seizure pressure at a point 100 μm from the surface layer at room temperature, created from the results in Table 2. In FIG. 1, the white marks represent the results of test axes numbered 1 to 4, and the solid marks represent the results of test axes numbered 5 to 11. The same applies to FIGS. 2 to 4. It is generally believed that increasing the surface hardness improves wear resistance, which also improves seizure resistance. However, as shown in Figure 1, although there is a tendency for the seizure pressure to increase as the surface hardness increases, the correlation coefficient is not that large, and surface hardness alone cannot fully explain seizure resistance. I understand that.

FIG. 2 is a diagram showing the relationship between the amount of solid solute carbon at room temperature and the seizure pressure, which was created from the results in Table 3. FIGS. 3 and 4 are diagrams showing the relationship between the amount of solid solute carbon at 200° C., the amount of solid solute carbon at 300° C., and the seizure surface pressure, respectively. From FIGS. 2 to 4, it can be seen that the seizure resistance can be well explained by the amount of solid solute carbon.

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)

A crankshaft having a quenched hardened layer on its surface having a volume fraction of martensite of 80% or more,
The quenched hardened layer is
The hardness from the surface layer to a depth of at least 2.0 mm at room temperature is HV750 or more,
A crankshaft having a solid solute carbon content of 0.46 to 0.65% by mass at room temperature. The crankshaft according to claim 1,
The quenched hardened layer is
A crankshaft having a solid solute carbon content of 0.42 mass% or more after heating at 200°C for 1 hour, and a solid solution carbon content of 0.34 mass% or more after heating at 300°C for 1 hour. The crankshaft according to claim 1 or 2,
The quenched hardened layer is
After heating at 200°C for 1 hour, the hardness from the surface layer to a depth of at least 2.0mm is HV470 or more, and after heating at 300°C for 1 hour, the hardness from the surface layer to at least a depth of 2.0mm is HV400 or more. Yes, there's a crankshaft. The crankshaft according to any one of claims 1 to 3,
The chemical composition is in mass%,
C: 0.48-0.62%,
Si: 0.01-2.0%,
Mn: 0.1 to 2.0%,
Cr: 0.01-0.50%,
Al: 0.001-0.06%,
N: 0.001 to 0.020%,
P: 0.03% or less,
S: 0.20% or less,
The remainder: Fe and impurities, the crankshaft. A method for manufacturing a crankshaft according to any one of claims 1 to 4, comprising:
A step of preparing an intermediate product for the crankshaft;
and a step of induction hardening the intermediate product,
A method for manufacturing a crankshaft, in which no tempering is performed after the induction hardening.

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