Steel for large-size high static torsional strength output gears, manufacturing method thereof, and carburizing method

Abstract

The present invention provides a large-scale, high-static-torsional-strength steel for output gears, a manufacturing method thereof, and a carburizing method thereof. The steel comprises the following components: C: 0.20-0.25%, Si: 0.15-0.30%, Mn: 1.00-1.10%, Cr: 1.08-1.18%, Ni: 1.00-1.10%, Al: 0.030-0.050%, P: ≤0.015%, B: 0.0007%-0.0030%, S: 0.020-0.035%, TO: ≤15ppm, [N]: 80-120ppm, with the remainder being Fe and unavoidable impurities. The steel has an end hardenability of 42-47HRC, J15: 41-45HRC, and J25: 38-43HRC. After carburizing, the steel exhibits a rotating bending fatigue strength of ≥920MPa, a torsional fatigue strength of ≥620MPa, and a torque of ≥309N·m.

Description

Technical Field

[0001] The invention belongs to the field of gear steel, and relates to a large-size high-static torsional strength output gear steel with a diameter of more than 90 mm, a manufacturing method thereof, and a carburizing method thereof, which is suitable for manufacturing high-static torsional strength automobile speed change output gears. Background Art

[0002] With the introduction and implementation of the dual carbon goals, lightweighting and improving vehicle efficiency have become key development directions for the automotive industry. As a power transmission device in vehicles, differentials place higher demands on torque. Traditional gear steels are manufactured by adding microalloying elements such as Nb and V, or alloying elements such as Ni. This significantly increases material costs, severely restricting the application and promotion of high-torque automotive gear steels.

[0003] Hardenability is a key indicator for improving a material's torsional strength. Currently, 17CrNiMo6 gear steel is commonly used in the new energy gear market, with a hardenability range of 37-47 HRC for J9, 34-46 HRC for J15, and 31-43 HRC for J25. However, this material contains approximately 1.0 wt% Ni, which accounts for over 40% of the total alloy cost. Therefore, it is necessary to develop a low-cost, large-scale, high-hardenability gear material to meet the automotive industry's demand for low-cost, high-output torque, and high-performance gear steel.

[0004] A Chinese patent, publication number CN 111286671 A, published on June 16, 2020, discloses an ultra-pure, high-temperature, fine-grained gear steel, a manufacturing method, and its application. By controlling the reasonable ratio of Al, B, and N elements, the invention can ensure that the grain size remains above level 6 after high-temperature treatment above 960°C. However, it does not impose strict requirements on hardenability, and therefore cannot meet the demand for low-cost, high-output torque, and high-performance gear steel.

[0005] A Chinese patent, publication number CN 112981271 A, published on June 18, 2021, discloses a method for manufacturing steel for electric vehicle reducer gears. By increasing the concentrations of elements such as C, Mn, and Cr within a relatively high range, the invention effectively improves the material's hardenability, achieving J9: 37-49 HRC, J15: 31-43 HRC, and J25: 26-38 HRC. While this further improvement in hardenability is achieved, it still cannot meet the demand for low-cost, high-output torque, and high-performance gear steel.

[0006] A Chinese patent, publication number CN107604253A, published on January 19, 2018, discloses a high-hardenability Mn-Cr carburizing steel. By increasing the concentrations of elements such as C, Mn, and Cr within a relatively high range, the invention effectively improves the material's hardenability, achieving hardenability of 40-46 HRC for J9, 34-40 HRC for J15, and 30.5-33.5 HRC for J25. Controlled Al and N content prevents significant austenite grain growth during carburization. Despite this improved hardenability, it still fails to meet the demand for high-performance, low-cost gear steel with high torque output.

[0007] Currently, CrNiMo-based gear steels possess excellent strength and toughness, and while certain technical developments have been achieved, they still fall short of achieving both improved hardenability and high output torque, comparable to 20Cr2Ni4, while also meeting the demand for significantly reduced costs. Furthermore, the trend toward lightweighting and increased acceleration in passenger and commercial vehicles places higher demands on the static torsional strength of gear steels. However, limited research has been conducted on the impact of static torsional strength, leading to an urgent need for new technologies to improve hardenability, toughness, high output torque, and low-cost high-performance gear steels to support the continued development of the automotive industry. Furthermore, banded structure is a common problem in large-diameter gear steel bars with a diameter of 90 mm or more, and once formed, it is difficult to eliminate. To ensure the processing and performance of gear steel, the banded structure grade is generally required to be no higher than 3.0, and at higher levels, no higher than 2.0. Summary of the Invention

[0008] The object of the present invention is to provide large-scale high-static torsional strength output gear steel and a manufacturing method thereof. Through component design and matching manufacturing methods, the terminal hardenability of the obtained gear steel is J9: 42-47HRC, J15: 41-45HRC, J25: 38-43HRC, and the banded structure level of the gear steel is controlled below level 2.0, without adding precious alloys and at low cost.

[0009] The present invention also provides a carburizing method for large-scale, high-static-torsional-strength output gear steel. Through carburizing process control, the large-scale, high-static-torsional-strength output gear steel has a rotational bending fatigue strength of ≥920 MPa, a torsional fatigue strength of ≥620 MPa, and a torque of ≥309 N·m after carburizing.

[0010] The specific technical solutions of the present invention are as follows:

[0011] Large size high static torsional strength output gear steel, including the following mass percentage composition:

[0012] C: 0.20-0.25%, Si: 0.15-0.30%, Mn: 1.00-1.10%, Cr: 1.08-1.18%, Ni: 1.00-1.10%, Al: 0.030-0.050%, P: ≤0.015%, B: 0.0007%-0.0030%, S: 0.020-0.035%, TO: ≤15ppm, [N]: 80-120ppm, the rest are Fe and unavoidable impurity elements.

[0013] The composition of the steel for large-size, high static torsional strength output gears also satisfies the following requirements: 0.01% ≤ Alf = Al-1.52×[N] ≤ 0.030%, 2.5 ≤ Al / N ≤ 5, wherein 2.5 ≤ Al / N ≤ 5 ensures that a sufficient amount of austenite AlN second phase can precipitate to ensure fine and uniform grain size. Excessive Al can easily cause uneven AlN distribution and result in mixed crystals. Free Alf can significantly improve hardenability, but too high a content can reduce hardenability. The coefficient of [N] has been refitted and revised, and the critical value for free Alf to improve hardenability is 0.01%-0.030%. Too low a content has little effect, while too high a content can reduce hardenability.

[0014] The terminal hardenability of the steel for the large-size high static torsional strength output gear is J9: 42-47HRC, J15: 41-45HRC, and J25: 38-43HRC.

[0015] The large-size high static torsional strength output gear has a gear diameter of more than 90 mm.

[0016] The present invention provides a method for manufacturing large-scale, high-static-torsional-strength output gear steel, comprising the following process flow:

[0017] Smelting - LF refining - RH vacuum treatment - continuous casting - heating - rolling - slow cooling;

[0018] The smelting is carried out by using an electric arc furnace or a converter;

[0019] In the LF refining, the steel is fully deoxidized during the refining process to ensure that the oxygen content is less than 12ppm, and aluminum wire is added to adjust Al to ensure that the free aluminum control is met.

[0020] Heating: Higher temperatures and longer heating times lead to more uniform composition, but also more severe oxidation. For large gear steel bar rolling, the optimal heating temperature is 1200-1250°C. The soaking temperature of the steel in the heating furnace is controlled at 1200-1250°C, and the total preheating, heating, and soaking time is controlled at 5.0-10.0 hours.

[0021] The rolling process includes a starting rolling temperature of 1130-1180°C and a finishing rolling temperature of 780-820°C.

[0022] The slow cooling: after rolling, the steel is quickly moved to a slow cooling pit through a cooling bed, the temperature entering the pit is 600-650°C, and the slow cooling time is ≥24h.

[0023] After slow cooling out of the pit, the steel is ground and peeled to ensure that there is no decarburization and zero defects on the surface.

[0024] The carburizing method of large-scale high static torsional strength output gear steel provided by the present invention comprises the following steps:

[0025] 1) The gear material is first normalized and kept warm;

[0026] 2) then carburizing treatment is performed;

[0027] 3) After carburizing, lower the temperature and keep warm;

[0028] 4) Oil cooling quenching is then performed;

[0029] 5) Low temperature tempering.

[0030] Normalizing in step 1), normalizing temperature: 910-930°C, holding time 1±0.5h;

[0031] Carburizing treatment in step 2): carburizing at 910-930° C. for 6-8 hours;

[0032] In step 3), the temperature is lowered to 800-880°C and kept at this temperature for 30-40 minutes;

[0033] In step 4), the quenching oil is cooled to room temperature;

[0034] In step 5), low-temperature tempering is performed at 160-180° C. for not less than 2 hours.

[0035] After nitriding treatment, the gear grain size is 8.5 or above, the rotational bending fatigue strength is ≥920MPa, the torsional fatigue strength is ≥620MPa, the torque is ≥309N·m; and the surface hardness is ≥715HV.

[0036] The design idea of ​​the present invention is as follows: the quenching oil temperature here refers to the temperature range, and there is no need to express the time

[0037] C: C is the most basic, effective and economical strengthening element in steel, and is the most effective element affecting hardenability. In order to ensure that gear steel has sufficient strength and hardenability, the C content needs to be increased, and the increase in C content helps to increase the hardness of the core matrix, so that the carbon content in the martensite laths in the matrix increases, and the increase in the carbon content in the core helps to increase the content of retained austenite in the core. Retained austenite can increase the coordinated deformation ability of the matrix, so that it can deform in a coordinated manner under high torque and is not easy to crack. The increase in martensite hardness is much greater than the decrease in hardness brought about by the increase in retained austenite. The increase in core hardness can increase its static torsional strength, so the carbon content is not less than 0.20, but too high a carbon content will cause greater damage to toughness, so the carbon content is determined to be 0.20-0.25%.

[0038] Si: Si is a deoxidizer. It can also improve the hardness of steel through solid solution strengthening and the hardenability of gear steel. The Si content cannot be lower than 0.15%. However, excessive silicon increases the activity of C, promotes the decarburization and graphitization tendency of steel during rolling and heat treatment, and makes the carburized layer easily oxidized. Therefore, the Si content is controlled at 0.15%-0.30%.

[0039] Mn: Mn expands the austenite phase and stabilizes the austenite structure, improving the hardenability of steel. However, Mn is soluble in ferrite, increasing the hardness and strength of both ferrite and austenite in the steel. Mn also improves the stability of the austenite structure, significantly improving the hardenability of the steel. However, excessive Mn reduces the steel's plasticity and deteriorates its toughness during hot rolling. The Mn content should be controlled within a range of 1.00-1.10%.

[0040] Cr: This element improves the hardenability of steel and contributes to increased strength. When the carbon content is low, adding an appropriate amount of Cr can ensure that the steel achieves the desired hardenability and strength. Therefore, the present invention controls the Cr content to 1.08-1.18%.

[0041] Ni: Ni can effectively increase the core toughness of steel, lower the ductile-brittle transition temperature, improve low-temperature impact resistance, and enhance the fatigue strength of steel. Another function of Ni in the present invention is to increase the stacking fault energy, improve the barrier for dislocations to cross, and enhance torsional resistance. However, Ni is relatively expensive, and excessive Ni content can reduce machinability after hot working. Therefore, the Ni content is controlled within 1.00-1.10%.

[0042] Boron (B) segregates at grain boundaries, improving grain boundary strength, high-temperature plasticity, and hardenability. Below 0.0007%, this effect is insignificant, while above 0.0030%, its effect is negligible and may even produce ferrite, impacting performance. Additionally, B improves the toughness of the carburized layer in case-hardened steel and enhances the material's resistance to crack growth. For high-torque output gears, it enhances wear resistance in the teeth and bending resistance at the tooth root, preventing tooth surface wear and gear breakage. Therefore, the B content should be controlled between 0.0007% and 0.0030%.

[0043] Al: Al is an effective deoxidizer and can form AlN to refine grains. When the Al content is below 0.030%, its effect is not significant. When it exceeds 0.040%, it tends to form coarse inclusions, deteriorating the steel's properties. Therefore, the timing of Al addition during the steelmaking process must be carefully adjusted to ensure that the Al content is controlled within the range of 0.030-0.050%.

[0044] P and S: Sulfur easily forms MnS inclusions with manganese in steel, causing hot brittleness. However, adding a small amount of S can significantly improve the cutting performance of gear steel without affecting product performance. MnS also has the effect of refining the grain size. P is an element with a strong tendency to segregate, increasing cold brittleness and reducing plasticity, which is detrimental to the uniformity of the product's structure and properties. Control P to ≤ 0.015% and S to 0.020-0.035%.

[0045] TO: TO forms oxide inclusions in steel, and TO should be controlled to be ≤15ppm.

[0046] [N]: It can form compounds with Nb, B, and Al, refining grains. A reasonable Al / [N] ratio significantly improves grain refinement, while excessively high [N] ratios can cause continuous casting defects such as bubbles. A value of 2.5 ≤ Al / [N] ≤ 5 ensures fine and uniform grains and a certain proportion of free Al. Therefore, the [N] content should be controlled between 80-120 ppm.

[0047] Alf=Al-1.52×[N] is the free aluminum content in steel. When the free aluminum is higher than 0.010%, it can improve the hardenability of the material. However, when the free aluminum content exceeds 0.030%, the effect on improving the hardenability of the steel is not obvious. Therefore, it should be controlled within 0.01%-0.030%.

[0048] The hardenability of steel primarily depends on the stability of supercooled austenite. The more stable the supercooled austenite, the slower the critical cooling rate, and the greater its hardenability. Factors influencing the stability of supercooled austenite include the steel's chemical composition, austenite homogeneity, austenite grain size, and the austenitization state. Research has found that alloying elements such as Cr, Ni, and Mo can increase the material's hardenability. Reducing the content of residual elements that embrittle grain boundaries, such as P and Sn, and refining the crystal grains through repeated quenching also contribute to improving the material's hardenability. However, due to their scarcity and high cost, it is desirable to avoid the use of alloying elements such as Mo. Extreme reductions in P and Sn require rigorous screening of scrap steel. Furthermore, while repeated quenching is an effective method for grain refinement, it requires increased heat treatment time and additional equipment. Al is generally added as a deoxidizer, while AlN also has a grain boundary pinning effect and can be added to prevent grain coarsening. Previous studies have shown that free aluminum in austenite can delay the transformation from austenite to ferrite, believed to be due to the distribution of aluminum near the ferrite-austenite transformation interface. However, reports on aluminum improving hardenability are rare. While free aluminum can improve hardenability to a certain extent, its practical application requires coordinated steelmaking and rolling processes.

[0049] The present invention is a large-scale output gear. The difficulty in controlling large-scale output gears with a diameter of more than 90 mm lies in the large problem of banded structure level. It is necessary to reduce the banded structure by means of rolling process and reducing the degree of segregation of alloy elements. The present invention adopts reasonable rolling process measures, which mainly include the heating process of the ingot, the final rolling temperature and the post-rolling cooling process measures. The higher the heating temperature and the longer the heating time of the ingot, the more complete the composition homogenization, but the more serious the oxidation. For large gear gear steel bar rolling ingots, the optimal heating temperature is preferably 1200-1250℃. In addition, large-scale gear steel has higher requirements for hardenability, so the alloy element B is added in the present invention to improve the hardenability.

[0050] Compared with existing technologies, this invention primarily increases carbon content, reduces the high-cost nickel content, and eliminates the addition of precious metals Nb and Mo. The carbon content increases core hardness by 1 to 2 HRC, significantly improving torque. This results in torque and torsional fatigue resistance comparable to comparable steels, while significantly reducing costs. While maintaining a certain nickel content, toughness is not compromised. Furthermore, the use of free aluminum improves hardenability to a certain extent, improving overall hardenability. This reduces alloy cost, improves hardenability, increases core hardness, and enhances static torsional strength. Furthermore, the invention utilizes a direct quenching process after carburizing, reducing heat treatment costs compared to the secondary quenching process used in other patents.

[0051] Large-scale, high-torsional-strength steel for output gears produced using the above composition and rational manufacturing methods exhibits an end-hardenability of 42-47 HRC, J15: 41-45 HRC, and J25: 38-43 HRC. After carburizing, the gear steel exhibits a rotating bending fatigue strength of ≥920 MPa, a torsional fatigue strength of ≥620 MPa, and a torque of ≥309 N·m. Furthermore, the present invention utilizes a combination of composition control (reducing segregated alloying elements) and rolling process control to provide a method for controlling the banded structure of large-scale gear steel bars (90 mm and larger), achieving a banded structure within Grade 2. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a metallographic photograph of the gear steel structure after carburizing in Example 1;

[0053] Figure 2 This is a metallographic photograph of the gear steel structure after carburizing in Example 2;

[0054] Figure 3 This is a metallographic photograph of the gear steel structure after carburizing in Example 3;

[0055] Figure 4 This is a metallographic photograph of the gear steel structure after carburizing in Comparative Example 1;

[0056] Figure 5 This is a metallographic photograph of the gear steel structure after carburizing in comparative example 2. DETAILED DESCRIPTION

[0057] Example 1-Example 5

[0058] Large size high static torsional strength output gear steel, including the following mass percentage composition:

[0059] C: 0.20-0.25%, Si: 0.15-0.30%, Mn: 1.00-1.10%, Cr: 1.08-1.18%, Ni: 1.00-1.10%, Al: 0.030-0.050%, P: ≤0.015%, B: 0.0007%-0.0030%, S: 0.020-0.035%, TO: ≤15ppm, [N]: 80-120ppm, 0.01%≤Alf=Al-1.52×[N]≤0.030%, 2.5≤Al / N≤5, the remainder being Fe and inevitable impurity elements. Five heats of steel were produced. The steel compositions of the various examples are shown in Table 1. The remainder not shown in Table 1 is Fe and inevitable impurities.

[0060] Comparative Example 1-Comparative Example 2

[0061] The gear steel comprises the following mass percentage components as shown in Table 1. The remainder not shown in Table 1 is Fe and unavoidable impurities. Comparative Examples 1 and 2 are two heats of 18CrNiMo7-6 steel produced in accordance with the requirements of EN10084 as comparative steel.

[0062] Table 1 Chemical compositions of the present invention and comparative examples (unit: [N] is ppm, others are wt%)

[0063]

[0064] Examples 1-5 and Comparative Examples 1-2 were produced using an electric arc furnace smelting-refining-vacuum treatment-continuous casting-heating-rolling-slow cooling-(finishing) process. The continuously cast billets were heated and held at 1200-1250°C for ≥5 hours, with the total preheating, heating, and soaking time controlled at 5.0-10.0 hours. Round steel rolling was then performed, with a starting rolling temperature of 1130-1180°C and a finishing rolling temperature of 780-820°C. After rolling, the billets were cooled on a cooling bed to 600-650°C and then slow-cooled in a pit for 48 hours.

[0065] Comparative Examples 3, 4, and 5 respectively used the compositions of Examples 3, 4, and 5 above, but did not use the rolling processes of Examples 1-5 and Comparative Examples 1-2 above. That is, Comparative Example 3 used the compositions of Example 3, Comparative Example 4 used the compositions of Example 4, and Comparative Example 5 used the compositions of Example 5. The only difference between the rolling processes of Comparative Examples 3, 4, and 5 and those of the above Examples and Comparative Examples was that the continuous casting billets were heated and held at 1130-1200°C for ≥5 hours, with the total preheating, heating, and soaking time controlled at 5.0-10.0 hours before being rolled into round steel. The starting rolling temperature was 1030-1080°C, the finishing rolling temperature was 680-780°C, and after rolling, the billets were cooled on a cooling bed to 600-650°C and then slowly cooled in a pit for 48 hours. The production process parameters and corresponding banded structure levels of Examples 1-5 and Comparative Examples 1-5 are shown in Table 2. As can be seen from the table, the banded structure level of Examples 1-5 was controlled at level 2 or below.

[0066] Table 2 Steel rolling production process parameters of the embodiments of the present invention and the comparative examples

[0067]

[0068] Table 3 shows the terminal hardenability values ​​of the embodiments of the present invention and the comparative examples. It can be seen from Table 3 that the hardenability control J9, J15, and J25 values ​​of the gear steels described in Examples 1-5 of the present invention are all within the range required for steel used in high-torque automobile transmission output gears. Compared with the comparative examples, the hardenability is equivalent, but the cost is lower.

[0069] Table 3 End Hardenability Values ​​(HRC) of Examples of the Invention and Comparative Examples

[0070] Example J9 J15 J25 Require 42～47 41～45 38～43 Example 1 43.2 42.3 40.3 Example 2 43.4 42.8 40.5 Example 3 43.5 42.7 41.3 Example 4 43.6 42.9 41.5 Example 5 43.9 43.0 41.6 Comparative Example 1 43.6 42.5 40.1 Comparative Example 2 43.0 42.9 40.6

[0071] The gear steels of each embodiment and comparative example were subjected to a carburizing process (samples were taken from bars with a diameter of 120 mm for Examples 1, 2, 3, 4, and 5, and from bars with a diameter of 50 mm for Comparative Examples 1 and 2): the gear material was first normalized at 910-930°C for a holding time of 1±0.5 h, then carburized at 910-930°C for 6-8 h. After carburizing, the temperature was lowered to 840-880°C for a holding time of 30-40 min, followed by oil quenching at a quenching oil temperature of 120-200°C. After quenching, the gear steels were subjected to a low-temperature tempering process at 160-180°C for a tempering time of not less than 2 h. The specific parameters of the carburizing process for each embodiment and comparative example after the carburizing process are shown in Table 4, and the bending fatigue strength, torsional fatigue strength, and static torsional strength are also shown in Table 5.

[0072] Table 4 Carburizing process of the embodiments of the present invention and the comparative examples

[0073]

[0074] Table 5 Grain size, fatigue strength and static torsional strength after carburizing of the embodiments of the present invention and the comparative examples

[0075]

[0076] The present invention improves the core hardness of the gear steel by increasing the carbon content and microalloying B, thereby improving its static torsional strength. In addition, a reasonable Al / [N] ratio has a significant effect on grain refinement. Finally, the present invention adopts a specific composition and a reasonable preparation method to produce a large-scale high static torsional strength output gear steel, a manufacturing method thereof, and a carburizing method. After carburizing, the gear steel has a surface hardness of ≥715 HV, a rotational bending fatigue strength of ≥924 MPa, a torsional fatigue strength of ≥624 MPa, and a torque of ≥309 N·m, which are comparable to the performance of the comparative example, but with significantly reduced costs.

Claims

1. Large-size high static torsional strength output gear steel, characterized by: The large-size high static torsional strength output gear steel includes the following mass percentage components: C: 0.20-0.25%, Si: 0.15-0.30%, Mn: 1.00-1.10%, Cr: 1.08-1.18%, Ni: 1.00-1.10%, Al: 0.030-0.050%, P: ≤0.015%, B: 0.0007%-0.0030%, S: 0.020-0.035%, TO: ≤15ppm, [N]: 80-120ppm, the rest are Fe and unavoidable impurity elements; The large-size high static torsional strength output gear has a gear diameter of more than 90mm; The banded structure level of the large-size high static torsional strength output gear is controlled below level 2.

0.

2. The large-size high static torsional strength output gear steel according to claim 1 is characterized in that: The composition of the steel for large-size and high static torsional strength output gears satisfies the following: 0.01%≤Alf=Al-1.52×[N]≤0.030%.

3. The large-size high static torsional strength output gear steel according to claim 1 or 2, characterized in that: The composition of the steel for the large-size high static torsional strength output gear satisfies the following: 2.5≤Al / N≤5.

4. The large-size high static torsional strength output gear steel according to claim 1, characterized in that: The terminal hardenability of the steel for the large-size high static torsional strength output gear is J9: 42-47HRC, J15: 41-45HRC, and J25: 38-43HRC.

5. A method for manufacturing large-size high static torsional strength output gear steel according to any one of claims 1 to 4, characterized in that: The production method of the large-size high static torsional strength output gear steel comprises: heating, rolling and slow cooling, wherein the heating: the soaking temperature of the steel billet in the heating furnace is controlled at 1200-1250° C., and the total time of preheating, heating and soaking is controlled at 5.0 h to 10.0 h; The rolling process includes a starting temperature of 1130-1180°C and a finishing temperature of 780-820°C. The slow cooling: after rolling, the steel is quickly moved to a slow cooling pit through a cooling bed, the temperature entering the pit is 600-650°C, and the slow cooling time is ≥24h.

6. A carburizing method for large-size high static torsional strength output gear steel according to any one of claims 1 to 4, characterized in that: The carburizing method comprises the following steps: 1) The gear material is first normalized and kept warm; 2) then carburizing treatment is performed; 3) After carburizing, lower the temperature and keep warm; 4) Oil cooling quenching is then performed; 5) Low temperature tempering.

7. The carburizing method according to claim 6, characterized in that After nitriding treatment, the gear grain size is 8.5 or above, the rotation bending fatigue strength is ≥920MPa, the torsional fatigue strength is ≥620MPa, and the torque is ≥ 309N·m; surface hardness ≥715HV.

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