JP6802501B2 - Pinion shaft and its manufacturing method - Google Patents

Description

The present invention mainly relates to a pinion shaft incorporated in a planetary gear device and a method for manufacturing the pinion shaft.

Generally, a plurality of large and small gears are used for planetary gears (planetary gears) built in a speed reducer, and a pinion shaft is a typical shaft of the gears. As a material for this pinion shaft, medium carbon steel and bearing steel disclosed in Patent Document 1 have been used so far.

Since the pinion shaft is used in a harsh environment of high speed rotation and high load as disclosed in Patent Documents 2 and 3, the material is required to have various characteristics such as appropriate wear resistance and fatigue resistance. It had been. Therefore, the surface hardness of medium carbon steel, which has been widely used as a material for pinion shafts, needs to be increased by carburizing or nitriding the surface thereof.

Special Fair 7-47795 Gazette Japanese Patent No. 5168898 JP-A-2015-105435

However, the planetary gears on which the pinion shaft is used reach a high temperature atmosphere of about 300 ° C. after long-term use, and as a result of the surface hardness being lowered as compared with the operating environment at room temperature, the durability is also significantly lowered.

In addition, in recent years, the tendency to reduce the amount of lubricating oil in the speed reducer has become remarkable for the purpose of protecting the natural environment, and it is said that the heat resistance required for the material of the pinion shaft is insufficient for use in a high temperature atmosphere close to 300 ° C. There was a problem.

Therefore, it is an object of the present invention to provide a pinion shaft capable of maintaining sufficient heat resistance even when used in a high temperature atmosphere and a method for manufacturing the pinion shaft.

In order to solve the above-mentioned problems, the pinion shaft of the present invention contains at least 0.10 to 0.50% carbon, 0.40 to 1.00% silicon, and 0.25 to 1 manganese in mass%. In a pinion shaft made of an iron-based alloy containing .00%, carbon at 1.50 to 4.00%, and molybdenum at a ratio of 0.10 to 1.00%, with the balance being iron and unavoidable impurities. A surface layer portion containing carbon and nitrogen is provided, and the total content of carbon and nitrogen in the surface layer portion is 2.14% or more in mass% and the carbon content is 1.78% or more in mass% .

Further, the hardness at a depth of 0.1 mm from the outermost surface of the surface layer portion may be 750 HV or more in Vickers hardness, and the residual austenite amount of the surface layer portion may be 20% by volume or more. Further, the area ratio of the carbides and carbonitrides present in the surface layer portion in the matrix structure is in the range of 10 to 25%, and the number of carbides and carbonitrides having a particle size of 0.5 μm or more is 300 or more per 1000 μm ^2. It can also be.

Regarding the invention of the method for producing a pinion shaft, at least in mass%, carbon is 0.10 to 0.50%, silicon is 0.40 to 1.00%, manganese is 0.25 to 1.00%, and chromium is Carburizing and nitriding treatment is performed on a pinion shaft material made of an iron-based alloy, which contains 1.50 to 4.00% and molybdenum at 0.10 to 1.00%, and the balance is iron and unavoidable impurities. As a result, a surface layer portion containing carbon and nitrogen is formed in the pinion shaft material, the total amount of carbon and nitrogen contained in the surface layer portion is 2.14% or more in mass%, and the carbon content is 1.78 in mass%. The manufacturing method includes each step of the first step of increasing the percentage to or more, and the second step of tempering the pinion shaft material in an atmosphere of 230 ° C. or lower after the first step .

The pinion shaft of the present invention is a so-called medium carbon steel pinion shaft having a carbon content in the range of 0.10 to 0.50% by mass, to which a predetermined alloy element is added, and carbon in the surface layer portion thereof. And the total nitrogen content was specified within the prescribed range. As a result, the pinion shaft of the present invention has the effect of being able to maintain sufficient heat resistance for use in a high temperature atmosphere (near 300 ° C.).

It is a block diagram of the testing machine used in the rolling fatigue test of Example 1. It is a Weibull distribution map which is the rolling fatigue test result of Example 1.

The chemical composition and microstructure of the pinion shaft of the present invention will be described below. First, regarding the chemical composition of the pinion shaft of the present invention, carbon is 0.10 to 0.50%, silicon is 0.40 to 1.00%, manganese is 0.25 to 1.00%, and chromium is used in mass%. An iron-based alloy (medium carbon steel) containing 1.50 to 4.00% and molybdenum at a ratio of 0.10 to 1.00%, with the balance being iron and unavoidable impurities. Further, nickel contained in the steel may be 0.20% or less, and copper may be 0.15% or less.

Next, the microstructure of the pinion shaft of the present invention will be described. The surface layer portion (surface side) of the pinion shaft is coated with a carburized layer and a nitrided layer or a carburized nitrided layer. The total content of carbon and nitrogen contained in the surface layer portion shall be 1.80% or more in mass%. Further, the carbon content can be contained in a mass% of more than 0.50% and 4.00% or less, and the nitrogen content can be contained in a mass% of 0.10% or more and 0.5% or less. In this case, the carbon content in the surface layer portion of the pinion shaft is at least 1.00% or more in mass%.

The carbide or carbonitride present on the surface layer has a higher hardness than the surrounding matrix structure and is effective for the wear resistance of the pinion shaft. However, if their particle size is coarse, cracks may occur from the carbides and carbonitrides existing in the structure of the pinion shaft under a high load environment, and eventually the pinion shaft may be damaged. ..

Therefore, regarding the carbides and carbonitrides existing on the surface layer of the pinion shaft, the area ratio is set to 10% or more and 25% or less, and the number of carbides and carbonitrides having a particle size of 0.5 μm is 300 per 1000 μm ^2. That is all.

An evaluation test (rolling fatigue test) of rolling fatigue characteristics as a "rolling material" was conducted using three types of comparative products having different chemical components from the invention product, and the test results will be described. Table 1 shows the chemical components (unit: mass%) of Invention 1 and 3 types of Comparative Products 1 to 3 used in the rolling fatigue test.

The invention products 1 and the three types of comparative products 1 to 3 shown in Table 1 are all manufactured under the same conditions and then subjected to carburizing nitriding treatment to provide a carburized nitriding layer on the surface. The amount of C contained in the carburized nitrided layer in Invention 1 and Comparative Products 1 to 3 was 1.0 to 2.5% by mass, and the amount of N was in the range of 0.1 to 0.3% by mass.

The testing machine shown in FIG. 1 was used for the rolling fatigue test. First, the lubricating oil 5 mixed with a predetermined amount of foreign matter is injected into an oil tank to which a disk-shaped test piece 3 having a diameter of φD is attached, and the table 4 is pushed up. After that, the steel ball 2 supported by the cage is received by the thrust bearing 1 to load a predetermined surface pressure P. In that state, an evaluation test is performed by rotating a shaft 10 that transmits power from a motor (not shown). Test conditions were performed 4-9 times of the test under the following conditions, the cumulative failure rate of the Weibull distribution is compared to evaluate lifetime seeking 10% become L ₁₀ life.
-Test piece dimensions: diameter (φD) 61 mm x thickness 6 mm
-Test surface pressure (P): 4.5 GPa
・ Rotation speed: 1800 rpm
・ Lubricating oil: Naphthenic acid mineral oil ・ Total amount of foreign matter: 20mg

The results of this test are shown in FIG. 2 as a Weibull distribution map. In FIG. 2, the horizontal axis represents the number of repetitions (unit: times), and the vertical axis represents the cumulative damage rate (unit:%). Repeating cumulative failure rate at 10% was 4.1 × 10 ⁶ times inventions 1. In contrast, all the comparative products 1 to 3 the number of repetition is less than or equal 2.3 × 10 ⁶ times. From the above test results, the chemical composition of the invention product 1 was superior to the chemical components of the three types of comparative products 1 to 3 in rolling fatigue characteristics. That is, it was confirmed that the pinion shaft, which is an invention product, has an improved rolling life as compared with the pinion shaft having other chemical components.

Next, the change in hardness in a high temperature atmosphere due to the amount of C and N of the carburized nitrogenized layer applied to the surface of the pinion shaft was measured, and the results will be described. The chemical composition of the pinion shaft material used in this test was the same as that of the invention of Example 1. Then, a plurality of test pieces (test pieces of a total of 5 levels of invention products 11 to 14 and comparative products 11) in which the C amount and N amount of the carburized nitride layer were changed were used.

In this test, the hardness of all test pieces is measured at room temperature (25 ° C). Then, the test piece was held at 300 ° C. for 3 hours and then slowly cooled, and the hardness was measured again in the same manner as described above. The hardness was measured at a depth of 0.1 mm from the outermost surface of the test piece. Table 2 shows the C amount and N amount of each test piece, their total amount (C + N amount), the hardness of the test piece at 5 levels at room temperature, and the change in hardness after cooling at 300 ° C.

As shown in Table 2, the test results of the four levels of the invention products (invention products 11 to 14) showed that the decrease in hardness was about 3 HV to 6 HV, all of which were within the range of less than 10 Hv. However, the comparative product 11 had a hardness of 25 HV, and the difference between the hardness after holding at 300 ° C. and the hardness at room temperature exceeded 10 HV. That is, the hardness of the comparative product 11 in which the total amount of C and N contained in the carburized nitrided layer was less than 1.80% was significantly lower than that of the other invention products 11-14. From the above measurement results, it was possible to suppress the decrease in hardness even in a high temperature atmosphere by setting the total amount of C and N of the carburized nitride layer provided on the surface of the pinion shaft material to a predetermined amount or more.

Next, changes in the internal hardness of the carburized nitrided layer and the amount of retained austenite (γR amount: unit is volume%) due to the tempering temperature of the pinion shaft material were confirmed. The test piece used in this test has the chemical composition of the product of the present invention used in Example 1, and the total amount of C and N contained in the carburized nitrided layer is an amount corresponding to the product 12 used in Example 2. And said. For the tempering temperature in this test, any 7 levels were selected in the range of 160 ° C to 250 ° C. Table 3 shows the measurement results of the amount of γR in the tempering temperature range of 160 ° C to 250 ° C.

As shown in Table 3, the internal hardness of the carburized nitride layer was around 800 HV (797 HV to 819 HV) in all the test pieces at a total of 7 levels in which the tempering temperature was changed. However, the amount of γR varied considerably depending on the tempering temperature. Specifically, at a tempering temperature of 160 ° C, the amount of γR = 36.5%, at 200 to 230 ° C, the amount of γR = 21.8 to 27.8%, and at 240 to 250 ° C, the amount of γR = around 13%. Met.

The test piece having a tempering temperature of 230 ° C. or less and having a γR amount of 20% by volume or more is composed of a tough matrix structure and high hardness carbonitride, and is excellent in wear resistance and fatigue strength. On the other hand, the test piece having a tempering temperature of 240 ° C. or higher and having a γR amount of less than 20% by volume has low toughness of the matrix structure, and fine carbonitride is precipitated in the matrix structure.

When these fine carbonitrides are precipitated in an amount exceeding a certain amount and the amount of γR is less than 20% by volume, the sensitivity to cracks becomes high and damage such as flaking is likely to occur. That is, it is considered that the amount of γR in the matrix structure is defined to be a certain amount or more and the high hardness carbonitride is precipitated to contribute to the improvement of wear resistance and fatigue strength. By setting the amount of γR other than the carburized nitrided layer to less than 10%, it is considered that the dimensional change due to the transformation of retained austenite in a high temperature environment is small and it can contribute to the suppression of bending as a shaft material.

Claims (3)

At least in mass%, carbon is 0.10 to 0.50%, silicon is 0.40 to 1.00%, manganese is 0.25 to 1.00%, chromium is 1.50 to 4.00%, A pinion shaft made of an iron-based alloy containing molybdenum at a ratio of 0.10 to 1.00% and the balance being iron and unavoidable impurities. The pinion shaft has a surface layer portion containing carbon and nitrogen. the has, characterized in that the surface layer portion sum of carbon and nitrogen contained in the I der 2.14% or more by mass%, and the carbon content is in weight percent 1.78% or more Pinion shaft. The claim is characterized in that the hardness at a depth of 0.1 mm from the outermost surface of the surface layer portion is 750 Hv or more in Vickers hardness, and the residual austenite amount of the surface layer portion is 20% by volume or more. The pinion shaft according to 1. At least in mass%, carbon is 0.10 to 0.50%, silicon is 0.40 to 1.00%, manganese is 0.25 to 1.00%, chromium is 1.50 to 4.00%, and molybdenum. Is contained in each ratio of 0.10 to 1.00%, and carbon and nitrogen are added to the pinion shaft material by carburizing and nitriding the pinion shaft material made of an iron-based alloy whose balance is iron and unavoidable impurities. The first step of forming a surface layer portion containing the above-mentioned material so that the total amount of carbon and nitrogen contained in the surface layer portion is 2.14% or more in mass% and the carbon content is 1.78% or more in mass%. A method for producing a pinion shaft, which comprises a second step of tempering the pinion shaft material in an atmosphere of 230 ° C. or lower after the first step .

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