Alloy steel for pinion shafts and pinion shafts using the same

Abstract

To provide an alloy steal for a pinion shaft that does not require surface treatment such as carburization or nitriding, and has improved impression resistance, and a pinion shaft using the same.SOLUTION: Regarding the invention of alloy steel for pinion shafts, an alloy steel comprising,in weight percent, C: 1.10 to 1.50%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, Mo: 0.20 to 1.50%, V: 0.10 to 0.80%, and residual iron and unavoidable impurities is used. Further, in the invention of a pinion shaft using the alloy steel, an amount of retained austenite in a surface layer portion is 5 to 15 vol.%, and the surface hardness is 64HRC or more on the Rockwell C scale.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention mainly relates to alloy steel for pinion shafts incorporated in planetary gear devices, and to a pinion shaft using the same.

Background Art

[0002] Generally, a plurality of gears of various sizes are used in a planetary gear incorporated in a speed reducer, and a pinion shaft is a typical example of a shaft for such a gear. As a material for this pinion shaft, medium carbon steel and bearing steel disclosed in Patent Document 1 have been used hitherto.

[0003] As disclosed in Patent Documents 2 and 3, a pinion shaft is used in a harsh environment of high-speed rotation and high load, so its material has been required to have various properties such as appropriate wear resistance, static strength (indentation pressure resistance), and heat resistance. Therefore, medium carbon steel, which has been used as a material for pinion shafts, needed to have its surface hardness increased by subjecting the surface to carburizing treatment or carbonitriding treatment.

Prior Art Literature

Patent Literature

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problem to be Solved by the Invention

[0005] However, conventional bearing steels had the problem of reduced hardness and wear resistance when used in high-temperature environments around 300°C. Furthermore, creating a carburized or carbonitriding layer on the surface of medium-carbon steel required special equipment and special heat treatments such as carburizing and carbonitriding, which increased manufacturing costs.

[0006] Therefore, the object of the present invention is to provide an alloy steel for pinion shafts that does not require surface treatment such as carburizing or carbonitriding and has improved static strength (indentation resistance), and a pinion shaft using the same. [Means for solving the problem]

[0007] In this invention, the alloy steel for the pinion shaft has the following composition by weight percent: C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, Mo: 0.20-1.50%, V: 0.10-0.80%, and is an alloy steel containing residual iron and unavoidable impurities.

[0008] Furthermore, the invention of a pinion shaft using alloy steel for pinion shafts has a surface retained austenite content in the range of 5 to 15 volume percent, and a surface hardness of 64 HRC or higher on the Rockwell C scale. It is more preferable that the surface retained austenite content be in the range of 10 to 13 volume percent. [Effects of the Invention]

[0009] The invention of alloy steel for pinion shafts allows for the provision of alloy steel suitable for pinion shaft applications, with enhanced surface and internal hardness through conventional heat treatment (quenching and tempering) without the need for special surface treatments such as carburizing or nitriding.

[0010] Furthermore, the invention of a pinion shaft using alloy steel for pinion shafts has the effect of suppressing the decrease in hardness at high temperatures and improving heat resistance compared to conventional pinion shafts using bearing steel. It also has the effect of improving fatigue life by increasing indentation resistance (static load rating). [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram of the thrust life tester used in Example 2. [Modes for carrying out the invention]

[0012] The main chemical components of the alloy steel for pinion shafts, which is one embodiment of the present invention, are described below. The carbon (C) content in the alloy steel constituting the pinion shaft of the present invention is set to 1.10 to 1.50% by weight. Carbon plays a role in ensuring hardness in the alloy steel after quenching and tempering, and in ensuring a high rolling fatigue life. If the C content in the steel is less than 1.10%, the required surface hardness cannot be obtained. On the other hand, if the content exceeds 1.50%, the amount of carbides and retained austenite in the alloy steel increases, which reduces the fatigue life of the pinion shaft.

[0013] The silicon (Si) content should be between 0.70% and 2.50% by weight. Silicon plays a role in increasing the tempering softening resistance in alloy steel. If the Si content in the alloy steel falls below 0.70%, the necessary tempering softening resistance cannot be obtained, and if the Si content exceeds 2.50%, the hot forgeability of the pinion shaft will be significantly reduced.

[0014] The manganese (Mn) content should be 0.10 to 1.00% by weight. Manganese is effective in improving the hardenability of alloy steel and thus increasing fatigue life. If the Mn content in the alloy steel exceeds 1.00%, the hot forgeability of the pinion shaft will decrease significantly.

[0015] The content of Cr (chromium) is 1.00 to 4.00% by mass percent. Chromium improves the hardenability of alloy steel, thermally stabilizes cementite, and functions to inhibit the solid solution of cementite into the matrix in a high temperature range. When the Cr content in the alloy steel is less than 1.00% by mass, the hardenability of the steel deteriorates; when the Cr content exceeds 4.00% by mass, coarse carbides are generated in the alloy steel, which reduces the fatigue life of the pinion shaft.

[0016] The content of Mo (molybdenum) is 0.20 to 1.50% by mass percent. Molybdenum forms carbides in steel and contributes to securing hardness. In addition, when the Mo content in the alloy steel exceeds 1.50% by mass, coarse carbides are generated, which reduces the fatigue life of the pinion shaft.

[0017] The content of V (vanadium) is 0.10 to 0.80% by mass percent. V functions to increase tempering softening resistance when added in combination with silicon in alloy steel. In addition, when the content of V in the alloy steel exceeds 0.80% by mass, coarse carbides are generated, which reduces the fatigue life of the pinion shaft.

[0018] It should be noted that similar effects can also be obtained by containing Nb (niobium) as an alternative element for V. In this case, the Nb content is preferably in the range of 0.05 to 0.40% by mass percent. In addition, W (tungsten) can also be contained in the range of 0.40 to 3.00% by mass percent in terms of W equivalent (W+2Mo).

[0019] In this case, similar to molybdenum, tungsten forms carbides in alloy steel and contributes to securing hardness. When the W equivalent in steel is less than 0.40% by mass, the required tempering hardness and softening resistance cannot be obtained. On the other hand, when the W equivalent exceeds 3.00% by mass, coarse carbides are generated, which reduces the fatigue life of the pinion shaft.

[0020] Next, the alloy steel pinion shaft according to one embodiment of the present invention will be described. The pinion shaft of the present invention can be manufactured by quenching the alloy steel having the aforementioned chemical composition in a temperature range of 840 to 880°C, then performing tempering treatment in a temperature range of 150 to 180°C. By performing the specified heat treatment, the amount of retained austenite in the surface layer of the pinion shaft can be adjusted to a range of 5 to 15% by volume. In this case, the surface hardness of the pinion shaft is set to 64 HRC or more on the Rockwell C scale.

Examples

[0021] (Example 1) A steel ball pressing test (hereinafter referred to as the present test) was conducted using two types of alloy steel: the alloy steel for pinion shafts of the present invention (hereinafter referred to as the invention product) and conventional bearing steel (hereinafter referred to as the comparative product). The test results will be described below. Table 1 shows the chemical components (unit: % by weight) of the invention products (two levels with different chemical components) and the comparative product (bearing steel: SUJ2) used in the present test, and Table 2 shows the surface hardness (unit: HRC) and the amount of retained austenite (unit: % by volume), respectively. The amount of retained austenite of the invention products and the comparative product was calculated by dedicated software from the ratio of integrated intensities of the diffracted X-ray intensity distributions of the austenite phase and martensite phase measured by an X-ray diffraction analyzer.

[0022] The present test is a test for objectively comparing and measuring the degree of indentation on a material surface by intensively applying a static load to a specific location, and the indentation resistance can be evaluated based on the indentation amount (depth) measured after pressurization. In the present test, a steel ball with a diameter of 9.525 mm was pressed against the sample surface at a pressure of 4500 MPa for 10 seconds (with a loading speed of 0.1 mm / min), and then the depth of the indentation mark left by the steel ball on the sample surface was measured with a laser microscope.

[0023] Invention materials 1 and 2 used in the present test were quenched at 850°C for 120 minutes, then pre-tempered at 160°C for 120 minutes; the comparative material was quenched at 850°C for 40 minutes, then pre-heat treated by tempering at 190°C for 90 minutes.

[0024] [Table 1]

[0025] [Table 2]

[0026] The depth of the depressions formed on the surface of the samples was measured, and the result was 0.220 μm for Inventive Material 1 and 0.210 μm for Inventive Material 2. In contrast, the comparative material was 0.330 μm. From the above test results, it can be concluded that Inventive Materials 1 and 2, which are alloy steels of the present invention, have a high amount of retained austenite on the material surface. 5 By setting the surface hardness to 64 HRC or higher on the Rockwell C scale within a range of ~15 volume%, the material surface is less prone to denting even when a load is applied to a specific point from the outside, compared to the comparative material (bearing steel SUJ2). Therefore, it has excellent indentation resistance and is suitable for bearing components in areas with large dynamic or static loads.

[0027] (Example 2) Using the inventive materials 1 and 2 and a comparative material used in Example 1, test specimens of predetermined dimensions were prepared, and rolling fatigue characteristics were evaluated (thrust life test). The test results are described below. The chemical composition (unit: weight %) of the inventive materials 1 and 2 and the comparative material used in the thrust life test is shown in Table 1 of Example 1. A schematic diagram of the test equipment (thrust life tester) used in this example is shown in Figure 1.

[0028] In this test, as shown in Figure 1, a disc-shaped test piece 3 with a diameter φD is attached to an oil tank into which lubricating oil 5 is injected, and the table 4 is pushed up. Then, a predetermined surface pressure P is applied by receiving the steel ball 2, which is supported by a retainer, with a thrust bearing 1. In this state, the shaft 10 that transmits power from a motor (not shown) is rotated at a predetermined rotational speed to perform the evaluation test. The test is continued until the test piece breaks, and the total number of rotations at the time of breakage is recorded, at which point the test is terminated.

[0029] Furthermore, even if the test specimen is not damaged, the total number of rotations is 1 × 10⁻⁶.8 The test was terminated when the number of trials reached a certain point. The test conditions were repeated five times under the following conditions, and a Weibull distribution graph was created. The L10 lifetime at which the cumulative failure rate reached 10% was read from the graph, and the evaluated lifetimes of each test specimen were compared and evaluated. • Test specimen dimensions: Diameter (φD) 61mm x Thickness 6mm • Test surface pressure (P): 4900 MPa • Rotation speed: 1000 rpm • Test temperature: Room temperature (approximately 23°C) • Lubricant: ENEOS Turbine Oil 68

[0030] Based on the results of this test, the number of cycles at which the cumulative failure rate reaches 10% is 1 × 10 for both Invention 1 and Invention 2. 8 The number of cycles was 4.11 to 5.17 × 10⁶ for the comparative material test specimens. 7 The lifespan was shorter compared to the results for inventions 1 and 2. From the above test results, it was found that the chemical components of invention materials 1 and 2 have superior rolling fatigue characteristics compared to the chemical components of comparative materials 1 and 2.

[0031] (Example 3) Test specimens were prepared using the inventive materials 1 and 2 used in Example 1, as well as a comparative material, and high-temperature hardness tests were conducted. The test results are described below. The prepared test specimens (thrust plate specimens: diameter 61 mm x thickness 6 mm) were held for 1 hour at a total of eight temperatures: 100°C, 120°C, 140°C, 160°C, 180°C, 200°C, 250°C, and 300°C. The surface hardness of the test specimens at each temperature (unit: Rockwell C scale) was then measured. The measurement results of the surface hardness of each test specimen at each temperature (8 levels) are shown in Table 3.

[0032] [Table 3]

[0033] As shown in Table 3, the comparative material's hardness dropped to 62 HRC or less above 180°C up to 200°C, to less than 60 HRC above 200°C, and to approximately 56 HRC at 300°C. On the other hand, both inventive materials 1 and 2 maintained a hardness of 62 HRC or higher even when the holding temperature exceeded 200°C, indicating superior surface hardness at high temperatures compared to the comparative material.

Claims

1. An alloy steel for pinion shafts characterized by having, by weight percent, C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, Mo: 0.20-1.50%, and V: 0.10-0.80%, and consisting of residual iron and unavoidable impurities.

2. An alloy steel for pinion shafts characterized by having, by weight percent, C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, Mo: 0.20-1.50%, and Nb: 0.05-0.40%, and consisting of residual iron and unavoidable impurities.

3. An alloy steel for pinion shafts characterized by having, by weight percent, C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, W+2Mo: 0.40-3.00%, and V: 0.10-0.80%, with residual iron and unavoidable impurities.

4. An alloy steel for pinion shafts characterized by having, by weight percent, C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, W+2Mo: 0.40-3.00%, and Nb: 0.05-0.40%, consisting of residual iron and unavoidable impurities.

5. Using the alloy steel for pinion shafts described in any one of claims 1 to 4, A pinion shaft characterized by having a surface layer containing 5 to 15 volume percent of retained austenite and a surface hardness of 64 HRC or higher on the Rockwell C scale.

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