gear

Abstract

To provide a gear that has excellent seizure resistance and internal strength.SOLUTION: A gear is made of a steel that has a composition in which, in mass%, C: 0.31% to 0.35%, Si: 0.30% or less, Mn: 0.20% to 0.80%, P: 0.030% or less, S: 0.030% or less, Cu: 0.35% or less, Ni: 0.25% or less, Cr: 1.00% to 1.40%, Mo: 0.95% to 1.10%, and V: 0.25 to 0.30% are contained and the following formulas (1) and (2) are satisfied, with the balance being Fe and unavoidable impurities. The gear has a soft nitrided layer on the surface layer, and includes a compound layer containing mainly iron nitride and having a thickness of 5 μm or more. Formula (1): 3.36×[Cr]+13.7×[Mo]+5.59×[V]>15. Formula (2): 1.00×[C]-0.20×[Cr]+0.20×[Mo]+1.00×[V]-0.26>0.2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a gear, and more particularly to a gear having a soft-nitrided surface layer. [Background technology]

[0002] Gears used in automobile power transmission mechanisms, etc., are forged or machined into a specified shape from steel, and then subjected to surface hardening heat treatments such as carburizing and quenching, carbonitriding and nitriding (including nitriding and soft nitriding in the narrow sense) to improve mechanical properties such as wear resistance. In recent years, gears have come to be used in environments where seizure is more likely to occur than before, such as at higher speeds and with increased sliding speeds, and therefore particularly high seizure resistance is required. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-227675 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-125132 Summary of the Invention [Problem to be solved by the invention]

[0004] To improve seizure resistance, it is considered effective to form a compound layer by nitriding on the sliding surfaces of gears to prevent direct contact between metals (see, for example, Patent Documents 1 and 2). However, nitriding has a lower treatment temperature (approximately 500 to 650°C) than carburizing or carbonitriding, and the nitrogen diffusion depth is shallower. In addition, the parent phase deeper than the diffusion layer softens at the nitriding temperature. As a result, nitriding gears have the problem of being prone to case crushing (damage in which the surface hardened layer peels off over a wide area) and internal plastic deformation due to their low internal strength.

[0005] SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a gear that has excellent seizure resistance and internal strength. [Means for solving the problem]

[0006] The inventors conducted extensive research to solve the above problems and discovered the following: (a) Secondary hardening with Mo and V is effective in suppressing softening of the matrix during nitriding. (b) By taking into consideration the effect of each alloying element on the formation of a compound layer during nitriding and the effective hardened layer depth, and by appropriately balancing the amount of each alloying element added, it is possible to ensure excellent seizure resistance and internal strength. The present invention was made based on this finding.

[0007] The gist of the present invention is as follows.

[0008] [1] A gear made of steel containing, by mass%, C: 0.31% to 0.35%, Si: 0.30% or less, Mn: 0.20% to 0.80%, P: 0.030% or less, S: 0.030% or less, Cu: 0.35% or less, Ni: 0.25% or less, Cr: 1.00% to 1.40%, Mo: 0.95% to 1.10%, and V: 0.25 to 0.30%, and satisfying the following formulas (1) and (2), the balance being Fe and unavoidable impurities; the gear having a soft-nitrided surface layer and a compound layer having a thickness of 5 μm or more, which mainly contains iron nitrides: 3.36×[Cr]+13.7×[Mo]+5.59×[V]>15…Formula (1) 1.00×[C]-0.20×[Cr]+0.20×[Mo]+1.00×[V]-0.26>0.2 …Formula (2) (The brackets [ ] in formulas (1) and (2) indicate the mass % content of each element.)

[0009] [2] A gear according to [1], characterized in that the effective hardened layer depth at which the Vickers hardness is 500HV is 0.25mm or more. [Brief explanation of the drawings]

[0010] [Figure 1] This figure shows the shape of the rollers used in the roller pitting test. (A) shows the test roller, and (B) shows the loaded roller. [Figure 2] FIG. 2A is an explanatory diagram showing a quenching and tempering treatment in an embodiment, and FIG. 2B is an explanatory diagram showing a gas soft nitriding treatment in an embodiment. [Figure 3] FIG. 1 is a schematic explanatory diagram of a roller pitching test. [Figure 4] FIG. 10 is an explanatory diagram of carburizing, quenching, and tempering treatments applied to Comparative Example 2. [Figure 5] FIG. 2 is a diagram showing the relationship between the index for the compound layer on the left side of formula (1) and the measured thickness of the compound layer. [Figure 6] FIG. 1 is a diagram showing the relationship between the index for the effective case depth on the left side of equation (2) and the measured effective case depth. [Figure 7] FIG. 10 is a diagram showing the relationship between the effective hardened layer depth and the depression depth. DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, a gear according to an embodiment of the present invention will be described in detail below. The gear of this embodiment has a soft-nitrided surface layer and a compound layer with a thickness of 5 μm or more. This gear is manufactured using steel containing, by mass%, C: 0.31% to 0.35%, Si: 0.30% or less, Mn: 0.20% to 0.80%, P: 0.030% or less, S: 0.030% or less, Cu: 0.35% or less, Ni: 0.25% or less, Cr: 1.00% to 1.40%, Mo: 0.95% to 1.10%, and V: 0.25 to 0.30%, and satisfying the above formulas (1) and (2), with the remainder being Fe and unavoidable impurities.

[0012] The reasons for limiting the chemical components of the steel used to manufacture the gear according to this embodiment are described in detail below. In the following description, "%" means "% by mass" unless otherwise specified.

[0013] C: 0.31% to 0.35% C is an element necessary to ensure the core hardness of gears. If the C content is less than 0.31%, the core strength becomes too low, resulting in a decrease in strength. On the other hand, if the C content exceeds 0.35%, the amount of carbides becomes too large, which deteriorates machinability and other workability.

[0014] Si:0.30% or less Silicon is added as a deoxidizer during melting. Because silicon reduces the hot workability and machinability of steel, the upper limit is set at 0.30%. However, silicon has the effect of increasing softening resistance, so 0.01% or more may be added.

[0015] Mn: 0.20% to 0.80% Mn is an element that is effective in improving the hardenability of steel. It also forms Mn-based sulfides, which improves machinability. To obtain these effects, the content is set to 0.20% or more. However, excessive addition increases the hardness of the steel and reduces its hot workability, so the upper limit is set to 0.80%. The preferred range of Mn content is 0.20% to 0.60%.

[0016] P:0.030% or less P is an impurity, so a low content is preferable. P segregates at grain boundaries and reduces the hot workability of steel, so the P content should be 0.030% or less. It is difficult to reduce the P content to zero, so the practical lower limit is 0.004%.

[0017] S: 0.030% or less S is an impurity, so a low S content is preferable. S tends to combine with Mn to form coarse MnS, which reduces the bending fatigue strength and hot workability of steel, so the S content should be 0.030% or less. It is difficult to reduce the S content to zero, so the practical lower limit is 0.001%.

[0018] Cu: 0.35% or less Cu suppresses the formation of carbides. Therefore, it is necessary to consider the balance with Cr, which promotes the formation of carbides. Furthermore, since excessive addition of Cu reduces hot forgeability and increases material costs, the Cu content is set to 0.35% or less. However, Cu may be added in an amount of 0.01% or more to improve gear strength as a solid solution strengthening element.

[0019] Ni: 0.25% or less Like Cu, Ni suppresses the formation of carbides. Therefore, it is necessary to consider the balance with Cr, which promotes the formation of carbides. Furthermore, since excessive addition of Ni reduces machinability and increases material costs, the Ni content is set to 0.25% or less. However, Ni may be added in an amount of 0.01% or more to improve gear strength as a solid solution strengthening element.

[0020] Cr: 1.00% to 1.40% Cr forms fine nitrides (CrN) in the hardened layer during soft nitriding, improving the hardness of the surface layer. To achieve this effect, Cr is contained in an amount of 1.00% or more. However, since excessive addition increases material costs, the upper limit is set at 1.40%. The preferred range of Cr content is 1.10% to 1.30%.

[0021] Mo: 0.95% to 1.10% Mo has the effect of suppressing the decrease in matrix hardness (internal hardness) during soft nitriding by precipitating carbides through secondary hardening, and is an effective element for ensuring effective hardened layer depth. To achieve this effect, Mo is contained in an amount of 0.95% or more. However, since excessive addition increases material costs, the upper limit is set to 1.10%. The preferred range of Mo content is 0.95% to 1.00%.

[0022] V: 0.25~0.30% Like Mo, V is an element that has the effect of suppressing the decrease in matrix hardness during soft nitriding by precipitating carbides through secondary hardening. To obtain this effect, V is contained in an amount of 0.25% or more. However, since excessive addition increases material costs, the upper limit is set at 0.30%.

[0023] 3.36×[Cr]+13.7×[Mo]+5.59×[V]>15…Formula (1) The left side of formula (1) is an index related to the thickness of the compound layer. Cr, Mo, and V are nitride-forming elements that are effective in stably forming a compound layer. By adjusting the components so that the value of the left side of formula (1) exceeds 15, a compound layer thickness of 5 μm or more can be ensured in the soft nitriding treatment.

[0024] 1.00×[C]-0.20×[Cr]+0.20×[Mo]+1.00×[V]-0.26>0.2 …Formula (2) The left side of equation (2) is an index related to the effective hardened layer depth. C, Mo, and V contribute to secondary hardening by precipitating carbides at the nitrocarburizing temperature (500°C to 650°C), thereby increasing the effective hardened layer depth. On the other hand, Cr suppresses the diffusion of nitrogen during nitrocarburizing, thereby reducing the effective hardened layer depth. By adjusting the composition so that the value of the left side of equation (2) exceeds 0.2, it is possible to ensure that the effective hardened layer depth of 0.25 mm or more, which corresponds to a Vickers hardness of 500 HV, is achieved in nitrocarburized gears.

[0025] Compound layer thickness: 5 μm or more The compound layer is a layer that mainly contains iron nitride. The compound layer formed on the surface of a gear can effectively suppress seizure on the sliding surface. The thickness of the compound layer on the gear is set to 5 μm or more, taking into account wear-related wear. A more preferred compound layer thickness is 10 μm or more. However, if the compound layer is too thick, it is likely to become the starting point for bending fatigue fracture, so the upper limit of the compound layer thickness is preferably set to 25 μm.

[0026] Effective hardened layer depth for Vickers hardness 500HV: 0.25mm or more When measuring the hardness distribution from the gear surface to the interior, the depth from the gear surface at which the Vickers hardness reaches 500 HV is called the effective hardened depth. The shallower this effective hardened depth is, the greater the likelihood of fracture occurring from the interior. According to the inventors' research, ensuring an effective hardened depth of 0.25 mm or more effectively suppresses damage originating from the interior (e.g., case crushing) compared to the soft nitriding layer, which includes the compound layer and diffusion layer.

[0027] The gear of this embodiment can be manufactured by processing (rough processing) steel having the above-described chemical composition into a predetermined shape, followed by quenching and tempering, finishing (fine processing) of the sliding surfaces, etc., and further gas soft nitriding.

[0028] In the quenching process, the intermediate part processed into a predetermined shape is held at a quenching temperature of 850 to 950°C. The holding time at the quenching temperature is not particularly limited, but is, for example, 30 to 60 minutes. After holding at the temperature for a predetermined time, the part is rapidly cooled to a temperature below the martensitic transformation start temperature Ms. The quenching medium is, for example, water or oil. The quenched intermediate part is then subjected to a well-known tempering process. The tempering temperature is, for example, 550 to 650°C. The holding time at the tempering temperature is, for example, 60 to 120 minutes.

[0029] In the gas nitrocarburizing process, the intermediate part is heated to a temperature below the A1 transformation point in an atmosphere containing NH3, causing nitrogen and carbon to penetrate the surface, hardening the surface layer through the formation of a solid solution of nitrogen or the precipitation of fine carbonitrides. As a result, a nitrocarburized layer is formed on the surface of the intermediate part, consisting of a compound layer containing mainly iron nitrides and a diffusion layer formed directly below that where nitrogen is diffused into the matrix.

[0030] In gas soft nitriding, the treatment is carried out using a mixed gas of (NH3 + CO2 + N2). The nitriding conditions can be adjusted as appropriate. If the nitriding temperature is too low, the diffusion rate of nitrogen decreases, resulting in a long treatment time. On the other hand, if the nitriding temperature is too high, softening of the matrix progresses, resulting in a decrease in internal hardness. The nitriding temperature can be exemplified as 500 to 650°C. The nitriding treatment time can be determined in conjunction with the nitriding temperature so that the desired compound layer thickness and effective hardened layer depth can be obtained. The nitriding treatment time can be, for example, 2 to 5 hours.

[0031] After gas nitrocarburizing, the gear may be subjected to a lubricating coating treatment or the like, if necessary.

[0032] (Example) The effects of one embodiment of the present invention will now be described in more detail with reference to examples. Test pieces (test rollers and loaded rollers) manufactured through the steps of melting and casting → rolling → rough machining → quenching and tempering → precision machining → gas soft nitriding were used to evaluate the compound layer thickness, effective hardened layer depth, seizure resistance, and internal strength for the 24 examples and 5 comparative examples shown in Table 1 below.

[0033] (Production of test specimens) A 150 kg steel ingot with the specified chemical composition was melted in a vacuum induction melting furnace and cast into an ingot. This ingot was then rolled into a bar, which was then cut into a test roller 10 shown in FIG. 1(A) and a load roller 20 shown in FIG. 1(B) by rough machining. The B dimension of the test roller 10 shown in FIG. 1(A) is 28 mm, and 12 in the figure is a through hole that engages with a separate shaft. The G dimension of the load roller 20 shown in FIG. 1(B) is 18 mm, and the R dimension is 700 mm, and 22 in the figure is a through hole that engages with a separate shaft.

[0034] [Table 1]

[0035] Next, the test roller 10 and the load roller 20 were quenched and tempered using the heat pattern shown in Fig. 2(A). After that, the rolling surface 10a of the roller 10 was subjected to lapping so that the surface roughness Ra was 0.1±0.05 µm, and the rolling surface 20a of the roller 20 was subjected to lapping so that the surface roughness Rz was 1.6±0.5 µm.

[0036] (Gas soft nitriding treatment) The test roller 10 and the load roller 20 manufactured as described above were subjected to gas soft-nitriding treatment to form a soft-nitrided layer on the surface of each. The gas soft-nitriding treatment was carried out using a gas soft-nitriding furnace with a mixed gas of (NH3 + CO2 + N2) and the heat pattern shown in Figure 2(B). The specific treatment temperature and treatment time are as shown in Table 2 below. However, in Comparative Example 2, vacuum carburizing was performed instead of gas soft nitriding, and the heat pattern at that time is shown in FIG.

[0037] (Compound layer thickness measurement) A portion of the test roller 10 including the rolling surface 10a was cut, and the cross section perpendicular to the rolling surface 10a was polished and etched, then observed under an optical microscope to measure the thickness of the compound layer. Etching was performed using a 3% nital solution for 20 to 30 seconds. The compound layer was observed as a white, uncorroded layer. For five fields of view in structural photographs taken with an optical microscope at 1000x magnification, three compound layer locations were extracted at 20 μm intervals, and their thicknesses were measured. The compound layer thickness was calculated as the average of the measurements from a total of 15 locations, and the results are shown in Table 2 below.

[0038] (Measurement of effective hardened layer depth) Vickers hardness (HV) was measured according to the Vickers hardness test method specified in "JIS Z2244," using a micro-Vickers hardness tester. A diamond square pyramid indenter with a facing angle of 136°, specified in "JIS B7725," was used as the indenter, and an indentation was made on the mirror-polished designated surface of each test piece with a test load that was not destructive. The Vickers hardness was calculated using the following formula from the diagonal length d [mm] of the indentation and the test load F [N]. HV=0.189×(F / d^2)

[0039] The effective case depth was determined by measuring the hardness at 100 μm intervals in the depth direction under a load of 300 g, starting from a position 100 μm deep from the surface (rolling surface surface) on a cut surface perpendicular to the rolling surface 10a of the test roller 10. Then, from the approximation curve showing the relationship between the depth from the surface and Vickers hardness, the depth from the surface corresponding to 500 HV was determined as the effective case depth (ECD), and the results are shown in Table 2.

[0040] (Seizure resistance evaluation) The test roller 10 and the load roller 20 prepared above were attached to a roller pitting tester, and a roller pitting test was carried out to evaluate the seizure resistance. Figure 3 is a schematic diagram of the roller pitting test. As shown in the figure, the outer periphery 20a of the load roller 20 was pressed against the outer periphery 10a of the test roller 10 under a load P, and the two were rotated while supplying lubricant to the contact area to check for the occurrence of seizure. In the figure, reference numeral 16 denotes a torque measuring instrument, and 18 denotes a thermocouple for measuring the temperature on the rolling surface. The specific test conditions are as follows: Load P:2kN Test roller rotation speed: 4000 rpm Load roller rotation speed: 400 rpm Lubricant temperature: 90℃

[0041] Under the above test conditions, test roller 10 is 1 x 10 5 The test roller 10 was rotated cyclically to check for the presence or absence of seizure. Specifically, the coefficient of friction was calculated from the measured torque of the test roller 10 using the following formula, and a coefficient of friction exceeding 0.08 was determined to be the occurrence of seizure. Friction coefficient = (measured torque) / ((test load) x (roller radius)) For the judgement, 1 x 10 5 The case where no seizure occurred up to the end of the cycle was marked with "Good", and the case where seizure occurred was marked with "Poor". The results are shown in Table 2 below.

[0042] (Internal strength evaluation) As in the evaluation of seizure resistance, the test roller 10 and the loaded roller 20 were attached to a roller pitting tester, and a roller pitting test was carried out to evaluate the internal strength. The test conditions were as follows: Load P:18.8kN Test roller rotation speed: 1500 rpm Load roller rotation speed: 1480 rpm Lubricant temperature: 90℃

[0043] Under the above test conditions, test roller 10 is 1 x 10 5 After the cycle of rotation, the deformation amount (depression depth) in the depth direction of the rolling surface 10a was investigated. The shape profile of the rolling surface 10a (unpeeled portion) was measured before and after the test, and the depth from the initial surface was defined as the depression depth. The shape profile in the axial direction of the rolling surface 10a was measured using a surface roughness measuring device (Tokyo Seimitsu Co., Ltd., SURFACOM 1500SD-13). In this case, the measurement length was 21 mm and the cutoff wavelength was 0.8 mm. The internal strength was judged as "good" when the depression depth was less than 10 μm, and as "poor" when the depression depth was 10 μm or more. The results are shown in Table 2 below.

[0044] [Table 2]

[0045] The evaluation results in Table 2 reveal the following: Comparative Example 1 was a material equivalent to SCR420H, which has traditionally been used as a steel for soft-nitrided parts, that was subjected to soft-nitriding. The Mo and V contents of Comparative Steel 1 were below the lower limits of the ranges specified in the present invention, and did not satisfy the requirements of formulas (1) and (2). As a result, the compound layer was not formed to the target thickness (5 μm or more), and the evaluation result for galling resistance was "×." Furthermore, the effective case depth (ECD) was shallow at 0.1 mm, and the evaluation result for internal strength was "×."

[0046] Comparative Example 2 uses a material equivalent to SCR420H, as in Comparative Example 1, but is subjected to vacuum carburizing instead of soft nitriding. The effective case depth (ECD) is as deep as 0.7 mm, and the internal strength was evaluated as "Good", but no compound layer was formed, and the seizure resistance was evaluated as "Poor".

[0047] In Comparative Example 3, the C content was below the lower limit of the range specified in the present invention and did not satisfy the requirement of formula (2). As a result, the effective case depth (ECD) was shallow at 0.18 mm, and the internal strength was evaluated as "poor."

[0048] In Comparative Example 4, the Mo content was below the lower limit of the range specified in the present invention, and did not satisfy the requirements of formulas (1) and (2). As a result, the compound layer was not formed to the target thickness, and the evaluation result for seizure resistance was "×". In addition, the effective hardening depth was shallow at 0.22 mm, and the evaluation result for internal strength was "×".

[0049] In Comparative Example 5, the amount of V was below the lower limit of the range specified in the present invention, and did not satisfy the requirement of formula (2). As a result, the effective hardening depth was shallow at 0.2 mm, and the internal strength was evaluated as "×".

[0050] Thus, in Comparative Examples 1 to 5, the evaluation results for seizure resistance or internal strength were "Bad."

[0051] In contrast to this, in Examples 1 to 24 in which the amount of each element added and the compound layer thickness fell within the ranges specified in the present invention, the evaluation results for both seizure resistance and internal strength were "good." Therefore, it is estimated that a gear made of steel having the composition of this example and provided with a compound layer having a thickness of 5 μm or more by soft nitriding will have excellent seizure resistance and will be able to effectively suppress damage such as case crushing that results from low internal strength.

[0052] 5 shows the relationship between the index for the compound layer on the left side of formula (1) and the measured thickness of the compound layer. This figure indicates that if the value of the left side of formula (1) exceeds 15, a compound layer with a thickness of 5 μm or more can be ensured. Figure 6 shows the relationship between the index for the effective case depth on the left side of formula (2) and the measured effective case depth. It can be seen that if the value of the left side of formula (2) exceeds 0.2, an effective case depth (500 HV) of 0.25 mm or more can be ensured. Next, Figure 7 shows the relationship between the effective case depth and the depression depth. As long as the effective case depth is 0.25 mm or more, the depression depth can be made less than 10 μm. In other words, even if the test piece is nitrided, as long as the steel used is adjusted to satisfy formula (2), it is possible to obtain an internal strength equivalent to that of the carburized Comparative Example 2.

Claims

1. By mass% C: 0.31% to 0.35%, Si: 0.30% or less, Mn: 0.20% to 0.80%, P: 0.030% or less, S: 0.030% or less, Cu: 0.35% or less, Ni: 0.25% or less, Cr: 1.00% to 1.40%, Mo: 0.95% to 1.10%, V: 0.25-0.30%, and a steel having a composition that satisfies the following formulas (1) and (2), with the balance being Fe and unavoidable impurities; the gear having a soft-nitrided surface layer and a compound layer that contains mainly iron nitrides and has a thickness of 5 μm or more: 3.36×[Cr]+13.7×[Mo]+5.59×[V]>15…Formula (1) 1.00×[C]-0.20×[Cr]+0.20×[Mo]+1.00×[V]-0.26>0.2...Formula (2) (The brackets in formulas (1) and (2) indicate the mass % content of each element.)

2. 2. The gear according to claim 1, wherein the effective hardened layer depth at which the Vickers hardness is 500 HV is 0.25 mm or more.

Images