A high surface hardness gear steel for new energy vehicles and a production method and heat treatment process thereof

Abstract

This invention provides a high surface hardness gear steel for new energy vehicles, its production method, and heat treatment process. The composition is: C 0.25-0.30%, Si 0.25-0.45%, Mn 1.15-1.35%, P ≤0.015%, S ≤0.015%, Cr 1.11-1.27%, Mo 0.7-1.00%, Al 0.025-0.035%, Ti 0.03-0.05%, Ni 1.4-1.6%, Cu ≤0.2%, [N] 30-50ppm, TO ≤15ppm, [H] ≤1.5ppm, with the remainder being Fe and unavoidable impurity elements. After isothermal normalizing pretreatment + carburizing gas quenching + low-temperature tempering, the carburized layer grain size is ≥10.0 grade, and the surface hardness is ≥800HV. 10 ≥3.0×10 7 L 50 ≥6.5×10 7 This meets the requirements of new energy vehicles for high contact fatigue performance.

Description

Technical Field

[0001] This invention belongs to the field of automotive gear steel, and relates to a high surface hardness gear steel for new energy vehicles, its production method and heat treatment process. Background Technology

[0002] With increasing emphasis on environmental protection, the rise of traditional gasoline-powered vehicles poses significant challenges to energy and environmental protection. Meanwhile, new energy vehicles are experiencing rapid development, with sales and market share reaching record highs. Gears, as core components in transmission systems, have extremely stringent technical requirements regarding their structure, strength, and fatigue performance.

[0003] Compared to traditional gasoline vehicles, new energy vehicles have greater acceleration upon startup, placing higher demands on the impact resistance and contact fatigue performance of gear components. Contact fatigue failure is mainly caused by surface defects and subsurface crack initiation, requiring higher standards for the microstructure, hardness, and inclusion content of the carburized layer. Surface hardness directly determines the contact fatigue performance and impact resistance of gears; higher surface hardness results in stronger impact resistance, meeting the instantaneous acceleration requirements of new energy vehicles. Currently, traditional carburized gears are carburized at around 910℃, which is relatively low, resulting in a slow carburizing rate and a surface hardness of only 600–700 HV after carburizing. This is insufficient to meet the surface hardness requirements of gear steel for new energy vehicles.

[0004] To address the aforementioned issues, the publication number CN was released on August 20, 2021. Patent 113278882A discloses a Chinese patent for a carburized gear steel with high contact fatigue performance using Nb microalloying and its preparation method. The chemical composition of the steel is as follows: C: 0.18%–0.22%, Si: 0.20%–0.30%, Mn: 0.80%–0.90%, Cr: 0.50%–0.70%, Mo: 0.15%–0.25%, Nb: 0.025%–0.060%, Ni: 0.40%–0.55%, Al: 0.020%–0.040%, P: ≤0.010%, S: 0.005%–0.035%, TO: ≤20ppm, [H]: ≤20ppm, [N]: 80–120ppm, with the remainder being Fe and unavoidable impurity elements. After carburizing at 930℃, the resulting gear steel has a carburized layer grain size ≥80 and a core grain size ≥90. However, this patent adds sulfur (S), and high-Al and high-S steels are prone to clogging the sprue during casting, reducing the number of consecutive castings. While Nb microalloying refines the microstructure, its rated contact fatigue is only 10 × 10⁻⁶. 7 The production efficiency is low, with a carburizing temperature of only 930℃. For these reasons, this patent cannot meet the demand for gear steel for new energy vehicles.

[0005] Nitriding or carbonitriding is a feasible process to improve the surface hardness of workpieces. Patent CN 114000095 A, published on February 1, 2022, discloses a method for carbonitriding the surface of a planetary shaft used in tractor axles, achieving a surface hardness of 59-63 HRC after quenching. However, the addition of nitrogen atmosphere during nitriding or carbonitriding limits the heat treatment process; this treatment must be carried out in equipment equipped for nitriding, which ordinary carburizing furnaces lack, making it difficult to complete. Summary of the Invention

[0006] The purpose of this invention is to provide a high surface hardness gear steel for new energy vehicles, its production method, and heat treatment process. Through chemical composition design and production process optimization, and by setting specific carburizing processes and subsequent heat treatment conditions, a high surface hardness gear steel meeting the requirements of new energy vehicles is obtained. This invention increases the carburizing temperature by 960℃ and employs gas quenching, which is environmentally friendly, improves production efficiency, increases rated contact life by more than three times, and enhances surface hardness, resulting in stronger impact resistance in the gears.

[0007] The specific technical solution of this invention is as follows:

[0008] A high surface hardness gear steel for new energy vehicles comprises the following components by weight percentage:

[0009] C: 0.25-0.30%, Si: 0.25-0.45%, Mn: 1.15-1.35%, P≤0.015%, S: ≤0.015%, Cr: 1.11-1.27%, Mo: 0.7-1.00%, Al: 0.025-0.035%, Ti: 0.03-0.05%, Ni: 1.00-1.30%, Cu≤0.2%, [N]: 30-50ppm, TO≤15ppm, [H]≤1.5ppm, with the remainder being Fe and unavoidable impurity elements.

[0010] The composition of the high surface hardness gear steel for new energy vehicles also meets the following requirements: 25.0 ≤ A value ≤ 40.0;

[0011] A value = (0.32×C%)×(1.2+1.12×Mn%)×(2+0.8×Si%)×(1+0.37×Ni%)×(1+1.4×Cr%)×(6+2.7×Mo%)×(2+Ti%+2.2×N%);

[0012] The composition of the high surface hardness gear steel for new energy vehicles also satisfies: 0.2 ≤ Y value ≤ 0.3;

[0013] Y value = (0.8 + 3.2 × Al% + 2.9 × Ti%) / (1000 × N%);

[0014] When calculating using the above formula, each element symbol represents its mass percentage content × 100. Substitute the elements into the formula for calculation. For elements in ppm, convert them to mass percentage content and then multiply by 100.

[0015] This invention provides a method for producing high surface hardness gear steel for new energy vehicles, comprising the following process flow:

[0016] Electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into finished products;

[0017] The LF refining process involves adding Al for deoxidation. After reducing the oxygen content, aluminum wire is added during the RH process to adjust the Al content, ensuring that the molten steel is not oxidized again before continuous casting.

[0018] The RH vacuum degassing process requires soft blowing control of N during the RH process, with a soft blowing time of ≥10 min to ensure that TiN cannot be precipitated by liquid.

[0019] The rolled product includes heating, rolling, and slow cooling;

[0020] The heating process involves controlling the homogenization temperature of the steel billet in the heating furnace at 1230–1250°C, and controlling the total time for preheating, heating, and homogenization at 8.0–12.0 hours.

[0021] The rolling process is as follows: initial rolling temperature 1120~1200℃, final rolling temperature 700~750℃;

[0022] The slow cooling process involves cooling the rolled material to 600-650°C on a cooling bed before it is placed in a slow cooling pit for ≥24 hours. After removal from the pit, the material is ground and peeled to ensure that there is no decarburization and zero defects on the surface. The material is then finished into finished products as needed.

[0023] The present invention provides a high surface hardness gear steel for new energy vehicles, which is tested for end hardenability according to GB / T 225, J5: 43~48, J9: 42~47, J15: 39~43HRC, J25: 35~39HRC.

[0024] The present invention provides a heat treatment process for high surface hardness gear steel for new energy vehicles, comprising: isothermal normalizing pretreatment, followed by carburizing and gas quenching, and finally low-temperature tempering.

[0025] The isothermal normalizing pretreatment process includes: the holding time t1 in the high-temperature normalizing section is (T1-800) / 120≤t1≤(T1-800) / 80, where T1 is the heating temperature of the high-temperature normalizing section in °C, which is 950~1050 °C; and t1 is the holding time in the high-temperature section in h.

[0026] The isothermal normalizing pretreatment process includes: the holding time t2 of the normalizing heat preservation section is T2 / 160≤t2≤T2 / 100, where T2 is the heating temperature of the normalizing heat preservation section in °C, which is 650~720 °C; and t2 is the heat preservation time of the heat preservation section in h.

[0027] Gear steel needs to undergo isothermal normalizing pretreatment during processing to avoid the deterioration of banded structure. Therefore, high-temperature diffusion isothermal normalizing is performed, and at the same time, Mo is fully and uniformly dissolved in the matrix, which can play a role in stabilizing the residual austenite during subsequent carburizing.

[0028] The carburizing and gas quenching process specifically involves: slowly heating to 960-965℃ for a carburizing time of no less than 8 hours, then cooling down to 810℃ and holding for 2 hours, followed by nitrogen blowing for gas quenching, and finally air cooling after cooling to below 100℃.

[0029] The low-temperature tempering specifically involves holding the temperature at 180-200℃ for 4 to 8 hours.

[0030] After carburizing, the high surface hardness gear steel for new energy vehicles, tested according to GB / T 6394, has a carburized layer grain size ≥10.0 grade, a surface hardness ≥800HV, a carburized layer microstructure mainly of acicular martensite, and a core microstructure mainly of bainite. Simultaneously, the rated contact fatigue life L... 10 ≥3.0×10 7 L 50 ≥6.5×10 7 .

[0031] The design concept of this invention is as follows:

[0032] C: C is the most basic and effective strengthening element in steel, and it is the most effective element affecting hardenability. Moreover, it is relatively inexpensive. In order to ensure that gear steel has sufficient strength and hardenability, it must contain a certain amount of carbon. This invention uses a low carbon content, and at the same time, in order to ensure that the core has sufficient strength and toughness, the carbon content is controlled at 0.25-0.30%.

[0033] Si: Si is a deoxidizer, and it can also improve the hardness of steel through solid solution strengthening, and improve the hardenability of gear steel. In this invention, Si mainly plays the role of solid solution strengthening, but Si is prone to causing the carburized layer to oxidize easily. The carbon content should be controlled at 0.25-0.45%.

[0034] Mn: Mn can expand the austenite phase region and stabilize the austenite structure, improving the hardenability of steel. However, excessive Mn can dissolve in ferrite, increasing the hardness and strength of ferrite and austenite in the steel. Simultaneously, Mn can improve the stability of the austenite structure, significantly enhancing the hardenability of the steel. In this invention, Mn is mainly used to reduce the pearlite and ferrite transformation regions, increase the bainite transformation region, and improve hardenability. However, excessive Mn will reduce the plasticity of the steel, causing a deterioration in toughness during hot rolling. The Mn content is controlled at 1.15-1.35%.

[0035] Cr: Cr can improve the hardenability and strength of steel. Cr combines with carbon in steel to form carbides. Because gear steel is tempered at low temperatures after quenching, no large carbides precipitate; instead, fine carbides precipitate. These precipitated carbides accumulate between martensite laths, inhibiting lath movement under stress. Dislocations in martensite can become entangled, improving strength and fatigue resistance. However, excessive Cr can form a carbide film, affecting the carburizing effect and reducing the performance of the carburized layer. The Cr content should be controlled between 1.11-1.27%.

[0036] Mo (Mo) significantly improves the hardenability of steel and prevents temper brittleness and overheating tendency. Furthermore, the appropriate combination of Mo and Cr elements in this invention significantly improves hardenability and tempering resistance, and Mo refines grain size. Simultaneously, Mo strengthens the retained austenite in the carburized layer; Mo atoms stabilize austenite within the carburized layer, preventing phase transformation and increasing the hardness of the carburized layer. However, too low a Mo content limits these effects, while too high a Mo content promotes the formation of grain boundary ferrite films, which is detrimental to the hot plasticity of steel, increases the tendency for reheat cracking, and increases costs. Therefore, the Mo content is controlled at 0.70-1.00%.

[0037] Ni: Nickel can significantly improve the toughness of the carburized layer. At the same time, Ni mainly plays a role in solid solution strengthening. However, while ensuring the high hardness of the carburized layer, it will reduce the toughness of the carburized layer. Adding a certain amount of Ni can stabilize the structure of the carburized layer and ensure that the carburized layer has a certain toughness. However, too much Ni will increase the content of residual austenite. Therefore, the Ni content should be controlled at 1.00-1.30%.

[0038] Ti: Ti has a strong affinity for carbon and oxygen, which can refine grains and microstructure, and also produce solid solution strengthening. After heat treatment, it improves the strength and heat sensitivity of steel. However, excessively high Ti content does not significantly improve grain refinement or strength, but increases costs. Therefore, the Ti content should be controlled at 0.030-0.050%.

[0039] 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 obvious, and when it is above 0.040%, it easily forms coarse inclusions, which deteriorates the steel's properties. Another role of Al in this invention is to reduce the decrease in austenite coarsening temperature caused by the addition of B. Therefore, the timing of Al addition during the steelmaking process needs to be carefully adjusted to ensure that the Al content is controlled between 0.025% and 0.035%.

[0040] P and S: Sulfur readily combines with manganese in steel to form MnS inclusions, causing hot brittleness; P is an element with a strong tendency to segregate, increasing cold brittleness, reducing plasticity, and detrimental to the uniformity of product microstructure and properties. Control P ≤ 0.015%, S ≤ 0.015%.

[0041] TO and [H]: TO forms oxide inclusions in steel, so TO should be controlled to ≤15ppm; [H] forms white spots in steel, which seriously affects product performance, so [H] should be controlled to ≤1.5ppm.

[0042] [N]: It can form compounds with Nb, B and Al, which can refine grains. A reasonable Al / [N] ratio has a significant effect on grain refinement, while excessive [N] can form continuous casting defects such as bubbles. However, due to the addition of Ti, in order to avoid the formation of TiN and reduce fatigue performance, the [N] content should be controlled at 30-50 ppm.

[0043] The hardenability of steel mainly depends on the stability of supercooled austenite. The more stable the supercooled austenite, the lower the critical cooling rate of the steel, and the greater its hardenability. Factors affecting the stability of supercooled austenite mainly include the chemical composition of the steel, austenite uniformity, austenite grain size, and austenitization state. Studies have found that alloying elements such as Cr, Mn, and Mo can increase the hardenability of materials; however, due to their scarcity and high cost, it is desirable to avoid using alloying elements such as Cr and Mo. Generally, adding Al can act as a deoxidizer, and AlN also has a grain boundary pinning effect, which can also prevent grain coarsening. Increasing the Si content can increase hardenability while strengthening through solid solution. This invention patent controls the N element content to avoid the formation of TiN, while Al and Ti compete to form N compounds. This composite refinement of grains further avoids the formation of TiN. At the same time, the chemical composition ratio requirements of this invention patent must be met to manufacture gear steel with high contact fatigue strength and high surface hardness.

[0044] Compared with existing technologies, this invention, through the design of chemical composition and optimization of production processes, and by utilizing existing carburizing furnace equipment and setting specific carburizing process and subsequent heat treatment conditions, can produce high surface hardness gears suitable for new energy vehicles. According to GB / T 225, the end-hardening performance is tested as follows: J5: 43~48HRC, J9: 42~47HRC, J15: 39~43HRC, J25: 35~39HRC. According to GB / T 6394, the carburized layer grain size is ≥10.0 grade, the surface hardness is ≥800HV, and the rated contact fatigue life L... 10 ≥3.0×10 7 Median fatigue life L 50 ≥6.5×10 7 This invention not only fundamentally solves the requirement of high contact fatigue performance for new energy vehicles, but also significantly shortens the carburizing time at 960℃ compared to conventional 930℃ carburizing, thus improving production efficiency, while maintaining the same carburized layer depth. Attached Figure Description

[0045] Figure 1 The grain size after carburizing in Example 1;

[0046] Figure 2 This is the core structure after carburizing in Example 1;

[0047] Figure 3 The grain size after carburizing in Example 2;

[0048] Figure 4 This is the core structure after carburizing in Example 2. Detailed Implementation

[0049] The present invention will be further described below with reference to embodiments and comparative examples.

[0050] Examples 1-5

[0051] A high surface hardness gear steel for new energy vehicles comprises the following composition by mass percentage: as shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities.

[0052] The production methods for high surface hardness gear steel for new energy vehicles in each embodiment include the following process flow:

[0053] Electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into finished products;

[0054] After being heated and held at 1230–1250℃ for ≥5 hours, the continuously cast billet is rolled into round steel. The total time for preheating, heating, and homogenization is controlled at 8.0–12.0 hours. The initial rolling temperature is 1120–1200℃, and the final rolling temperature is 700–750℃. After rolling, the billet is cooled to 600–650℃ on a cooling bed and then slowly cooled in a pit for ≥24 hours. After exiting the pit, the billet is ground and peeled to ensure that there is no decarburization and zero defects on the surface. The production process parameters for each embodiment are shown in Table 2.

[0055] Comparative Example 1 - Comparative Example 2

[0056] Two heats of microalloyed SAE 8620 steel (centerline) were produced as a control steel, with the composition shown in Table 1. The balance not shown in Table 1 is Fe and unavoidable impurities.

[0057] The process involves electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, and rolling (finishing). The continuously cast billets are heated to 1200–1230℃ and held for ≥4 hours before being rolled into round bars. The initial rolling temperature is 1100–1150℃, and the final rolling temperature is 750–850℃. After rolling, the bars are cooled to 600–650℃ on a cooling bed and then slowly cooled in a pit for ≥24 hours. After exiting the pit, the bars are ground and peeled to ensure complete decarburization of the surface. The production process parameters for each embodiment are shown in Table 2.

[0058] Table 1 Chemical composition of embodiments and comparative examples of the present invention (unit: [N], TO, [H] are ppm, others are wt%)

[0059] Example C Si Mn P S Cr Mo Ni Ti Al [N] [H] TO Cu A Y Example 1 0.25 0.25 1.16 0.008 0.011 1.11 0.71 1.15 0.035 0.025 34 1.3 13 0.12 25.9 0.29 Example 2 0.25 0.25 1.16 0.010 0.007 1.12 0.72 1.14 0.037 0.027 37 1.4 13 0.14 26.1 0.27 Example 3 0.27 0.25 1.20 0.011 0.008 1.2 0.8 1.19 0.04 0.03 45 1.4 14 0.11 31.2 0.22 Example 4 0.26 0.26 1.3 0.009 0.010 1.22 0.85 1.23 0.042 0.035 46 1.3 12 0.13 32.7 0.22 Example 5 0.28 0.32 1.32 0.012 0.007 1.26 0.87 1.25 0.045 0.035 49 1.5 11 0.14 37.6 0.21 Comparative Example 1 0.20 0.21 0.76 0.008 0.006 1.12 0.31 0.031 / 0.033 94 1.5 15 0.11 10.2 0.096 Comparative Example 2 0.22 0.28 0.94 0.006 0.008 1.21 0.40 0.033 0.035 0.034 104 1.5 12 0.13 14.0 0.097

[0060] Table 2. Steel rolling production process parameters for each embodiment and comparative example.

[0061]

[0062]

[0063] The end-hardenability performance of the above embodiments and comparative examples was tested according to GB / T 225, J5: 43~48, J9: 42~47, J15: 39~43HRC, J25: 35~39HRC, and the results are shown in Table 3.

[0064] Table 3. End-hardenability values ​​(HRC) of the embodiments and comparative examples of the present invention.

[0065] Example J5 J9 J15 J25 Require 43～48 42～47 39～43 35～39 Example 1 44.2 43.4 40.1 35.7 Example 2 45.3 43.8 40.5 36.8 Example 3 45.7 44.1 41.2 36.9 Example 4 46.4 44.5 41.5 37.2 Example 5 46.7 45.5 41.7 37.9 Comparative Example 1 42.5 39.5 32.0 32.0 Comparative Example 2 43.1 41.0 33.2 32.9

[0066] As can be seen from Table 3, the hardenability control values ​​J9, J15, and J25 of the gear steel described in Examples 1 to 5 of this invention are all within the range required for gear steel used in new energy applications. J5: 43-48, J9: 42-47, J15: 39-43 HRC, J25: 35-39 HRC.

[0067] The heat treatment processes in the above embodiments and comparative examples include: isothermal normalizing pretreatment, followed by carburizing and gas quenching, and finally low-temperature tempering.

[0068] The isothermal normalizing pretreatment process includes: the holding time t1 in the high-temperature normalizing section is (T1-800) / 120≤t1≤(T1-800) / 80, where T1 is the heating temperature of the high-temperature normalizing section in °C, which is 950~1050 °C; and t1 is the holding time in the high-temperature section in h.

[0069] The isothermal normalizing pretreatment process includes: the holding time t2 of the normalizing heat preservation section is T2 / 160≤t2≤T2 / 100, where T2 is the heating temperature of the normalizing heat preservation section in °C, which is 650~720 °C; and t2 is the heat preservation time of the heat preservation section in h.

[0070] Gear steel needs to undergo isothermal normalizing pretreatment during processing to avoid the deterioration of banded structure. Therefore, high-temperature diffusion isothermal normalizing is performed, and at the same time, Mo is fully and uniformly dissolved in the matrix, which can play a role in stabilizing the residual austenite during subsequent carburizing.

[0071] The carburizing and gas quenching process specifically involves: slowly heating to 960-965℃ for a carburizing time of no less than 8 hours, then cooling down to 810℃ and holding for 2 hours, followed by nitrogen blowing for gas quenching, and finally air cooling after cooling to below 100℃.

[0072] The low-temperature tempering specifically involves holding the temperature at 180-200℃ for 4 to 8 hours.

[0073] The gear steels produced in the above embodiments and comparative examples were subjected to heat treatment, and the specific heat treatment process parameters are shown in Table 4 below. Through this normalizing process, Mo in the element is fully dissolved in the carburized layer and the matrix, stabilizing the residual austenite in the carburized layer.

[0074] Table 4. Main heat treatment parameters for each embodiment and comparative example.

[0075]

[0076]

[0077] The gear steels produced in the above embodiments and comparative examples were carburized and tested according to GB / T 6394. The grain size of the carburized layer was measured, and the surface hardness and contact fatigue properties were tested according to GB / T230.1 and GB / T 10622. The results are shown in Table 5.

[0078] Table 5 Grain size, surface hardness, and contact fatigue properties of carburized steel in the embodiments of the present invention.

[0079]

[0080] The data underlined above do not meet the requirements of this invention.

[0081] This invention employs a fixed chemical composition ratio to obtain gear steel with good hardenability. Simultaneously, it utilizes a high-temperature diffusion isothermal normalizing process to uniformly dissolve Mo. High Mo is used to stabilize the retained austenite in the carburized layer, increasing its hardness. This increased carburized hardness improves contact fatigue performance. Furthermore, microalloying is employed to raise the carburizing temperature, fixing carbon atoms within the martensitic matrix. This simultaneously increases the hardness of both acicular martensite and retained austenite. Moreover, grain refinement enhances the toughness of the carburized layer, thus significantly improving contact fatigue performance.

Claims

1. A high surface hardness gear steel for new energy vehicles, characterized by, The new energy automobile high surface hardness gear steel comprises the following components in percentage by mass: C: 0.25-0.30%, Si: 0.25-0.45%, Mn: 1.15-1.35%, P≤0.015%, S:≤0.015%, Cr: 1.11-1.27%, Mo: 0.7-1.00%, Al: 0.025-0.035%, Ti: 0.03-0.05%, Ni: 1.00-1.30%, Cu≤0.2%, [N]: 30-50ppm, T.O≤15ppm, [H]≤1.5ppm, the rest being Fe and inevitable impurity elements; The components of the new energy automobile high surface hardness gear steel also satisfy 25.0≤A value≤40.0; A value=(0.32×C%)×(1.2+1.12×Mn%)×(2+0.8×Si%)×(1+0.37×Ni%)×(1+1.4×Cr%)×(6+2.7×Mo%)×(2+Ti%+2.2×N%); The components of the new energy automobile high surface hardness gear steel also satisfy 0.2≤Y value≤0.3; Y value=(0.8+3.2×Al%+2.9×Ti%) / (1000×N%).

2. The production method of the high surface hardness gear steel for new energy vehicles according to claim 1, characterized in that, The production method comprises the following technological process: Electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-rolling into finished products.

3. The production method according to claim 2, characterized by, The rolling into finished products comprises heating, rolling and slow cooling; the heating: the soaking temperature of the billet in the heating furnace is controlled at 1230-1250℃, the total time of preheating, heating and soaking is controlled at 8.0h-12.0h.

4. The production method according to claim 3, characterized by, The rolling: the open rolling temperature is 1120-1200℃, and the finish rolling temperature is 700-750℃.

5. The production method according to claim 3, characterized by, The slow cooling: after rolling, the workpiece is cooled to 600-650℃ on the cooling bed and then is slowly cooled in the pit, and the slow cooling time is≥24h.

6. The heat treatment process of the high surface hardness gear steel for new energy vehicles according to claim 1, characterized in that, The heat treatment process comprises an isothermal normalizing pretreatment process, then carburizing gas quenching, and finally low-temperature tempering.

7. The heat treatment process of claim 6, wherein, The isothermal normalizing pretreatment process comprises: the holding time t1 of the normalizing high-temperature section is (T1-800) / 120≤t1≤(T1-800) / 80, T1 is the heating temperature of the normalizing high-temperature section, in ℃, and is 950-1050℃; t1 is the holding time of the high-temperature section, in h; the holding time t2 of the normalizing holding section is T2 / 160≤t2≤T2 / 100, T2 is the heating temperature of the normalizing holding section, in ℃, and is 650-720℃; t2 is the holding time of the holding section, in h.

8. Heat treatment process according to claim 6 or 7, characterized in that The high surface hardness gear steel for new energy vehicles after carburizing has a carburized layer grain of ≥10.0 grade, a surface hardness of ≥800HV, and a contact fatigue performance rated life L 10 ≥3.0×10 7 , L 50 ≥6.5×10 7 .

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