A high-contact fatigue gear steel for new energy vehicles, its production method and heat treatment process

Abstract

The present invention provides a high-contact fatigue gear steel for new energy vehicles, its production method, and heat treatment process. The steel comprises C 0.20-0.25%, Si 0.20-0.40%, Mn 1.10-1.30%, P ≤ 0.010%, S ≤ 0.010%, Cr 1.10-1.25%, Mo 0.50-0.70%, Nb 0.030-0.050%, Al 0.020-0.040%, Ti: 0.03-0.05%, Ni 0.4-0.6%, Cu ≤ 0.2%, [N] 30-50 ppm, TO ≤ 10 ppm, [H] ≤ 1.0 ppm, with the remainder being Fe and unavoidable impurities. Compared with the prior art, the present invention produces high-contact fatigue performance gears suitable for new energy vehicles through component design, production process optimization, carburizing process settings, and subsequent heat treatment.

Description

Technical Field

[0001] The present invention belongs to the field of gear steel for automobiles, and specifically relates to a gear steel for new energy vehicles with high contact fatigue resistance, a production method thereof, and a heat treatment process thereof. Background Art

[0002] As environmental protection becomes increasingly important, the increase in traditional fuel vehicles poses significant challenges to energy and environmental protection. At the same time, new energy vehicles are experiencing rapid development, with sales and market share reaching record highs. Gears, as core components for transmitting power in transmission systems, are subject to stringent technical requirements for their structure, strength, and fatigue performance.

[0003] Compared to traditional fuel-powered vehicles, new energy vehicles experience greater acceleration during startup, placing higher demands on the impact resistance and contact fatigue performance of gear components. Furthermore, the insufficient range of new energy vehicles has long been an international challenge. To increase their range, smaller and lighter vehicles are required to provide ample space for additional batteries while also conserving power.

[0004] In summary, the gears of new energy vehicles need to be durable, smaller in structure and higher in strength, so it is of great significance to develop high contact fatigue performance and heat treatment processes for new energy vehicles.

[0005] To solve the above problems, the patent with publication number CN 113278882A published on August 20, 2021, discloses a Chinese patent for a Nb microalloyed high contact fatigue performance carburized gear steel and its preparation method. The chemical composition of the steel is 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%~0.5 5%, Al: 0.020% ~ 0.040%, P: ≤0.010%, S: 0.005% ~ 0.035%, TO ≤ 20ppm, [H]: ≤ 2.0ppm, [N]: 80 ~ 120ppm, the rest are Fe and unavoidable impurity elements. After the carburizing process at 930℃, the grain size of the carburized layer is ≥ 8.0 grade, the grain size of the core is ≥ 9.0 grade, and the rated fatigue life L is 2000~2000 under the compressive stress of 4.0GPa. 10 ≥1.0×10 7 , median fatigue life L 50 ≥3.5×10 7 This patent adds S element. High Al and high S steels are very likely to cause nozzle blockage during the casting process. The number of continuous casting furnaces is reduced. Nb microalloying is used to refine the structure. The rated contact fatigue is only 1.0×10 7It is about 200 tons, which cannot meet the demand for gear steel for new energy vehicles.

[0006] To increase the surface hardness of workpieces, nitriding or carbonitriding is a viable process. Patent number CN 114000095 A, published on February 1, 2022, discloses a method for carbonitriding the surface of planetary shafts for tractor axles. After quenching, the surface hardness can reach 59 to 63 HRC. However, during nitriding or carbonitriding, the addition of nitrogen to the atmosphere imposes certain limitations on heat treatment. The treatment must be performed in equipment equipped for nitriding, and conventional carburizing furnaces lack the necessary conditions, making it difficult to complete the treatment. Summary of the Invention

[0007] The purpose of the present invention is to provide a high-contact fatigue gear steel for new energy vehicles, a production method and a heat treatment process thereof. By designing the chemical composition and optimizing the production process, and utilizing existing carburizing furnace equipment, through specific carburizing process settings and subsequent heat treatment condition settings, high-contact fatigue performance gears suitable for new energy vehicles can be produced.

[0008] The specific technical solutions of the present invention are as follows:

[0009] A high contact fatigue gear steel for new energy vehicles, comprising the following components in percentage by mass:

[0010] C: 0.20-0.25%, Si: 0.20-0.40%, Mn: 1.10-1.30%, P≤0.010%, S: ≤0.010%, Cr: 1.10-1.25%, Mo: 0.50-0.70%, Nb: 0.030-0.050%, Al: 0.020-0.040%, Ti: 0.03-0.05%, Ni: 0.4-0.6%, Cu≤0.2%, [N]: 30-50ppm, TO≤10ppm,

[0011] [H]≤1.0ppm, the rest are Fe and inevitable impurity elements.

[0012] The composition of the high contact fatigue gear steel for new energy vehicles also satisfies the following requirements: A value = (0.32×C%)×(1.2+1.12×Mn%)×(2+0.8×Si%)×(1+0.37×Ni%)×(1+1.4×Cr%)×

[0013] (6+2.7×Mo%)×(1+1.1×Nb%)×(2+Ti%+2.2×N%), 15.0≤A value≤28.0.

[0014] Since the hardenability of gear steel for new energy vehicles is required to be within a certain range, too high will easily cause deformation, and too low will easily cause the problem of incomplete hardenability caused by insufficient hardenability, the influence of alloying elements on hardenability is the most important. According to the influence of a large number of chemical components on hardenability, the parameter A value is introduced, 15.0≤A value≤28.0, the 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%)×(1+1.1×Nb%)×(2+Ti%+2.2×N%), at this time the hardenability can meet the hardenability requirements of J5: 42~48, J9: 38~45, J15: 37~43HRC, and J25: 32~38HRC;

[0015] The composition of the high contact fatigue gear steel for new energy vehicles also satisfies: 0.2≤Y value≤0.3;

[0016] Y=(0.8+3.2×Al%+2.9×Ti%) / (1000×N%). When this ratio is met, the precipitation of the secondary phase can be ensured to pin the grain boundaries and refine the grains without precipitating primary TiN.

[0017] When calculating with the above formula, the symbol of each element represents its mass percentage content × 100. Substitute it into the formula for the element with the unit of ppm, convert it to mass percentage content, and then multiply it by 100.

[0018] The present invention provides a method for producing high-contact fatigue gear steel for new energy vehicles, comprising the following process flow:

[0019] Electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into products;

[0020] The LF refining fully deoxidizes during the refining process to ensure a low oxygen content of <10ppm, and aluminum wire is added to adjust Al in the later stage of vacuum treatment, which can not only ensure the aluminum content but also prevent excessive inclusions in the steel.

[0021] The RH vacuum degassing: N needs to be soft-blown during the RH process, and the soft-blowing time must not be less than 30 minutes and not more than 60 minutes to ensure that TiN cannot be precipitated.

[0022] The rolling process includes heating, rolling and slow cooling;

[0023] Heating: The soaking temperature of the steel billet in the heating furnace is controlled at 1230-1250° C., and the total time of preheating, heating and soaking is controlled at 8.0 h to 12.0 h;

[0024] The rolling process includes a starting rolling temperature of 1120-1200°C and a finishing rolling temperature of 700-750°C.

[0025] The slow cooling: after rolling, it is cooled to 600-650℃ on a cooling bed and then put into a pit for slow cooling. The slow cooling time is ≥24h. After leaving the pit, it is ground and peeled to ensure that there is no decarburization and zero defects on the surface; the finished product is finished as needed.

[0026] The high contact fatigue gear steel for new energy vehicles provided by the present invention has an end hardenability that can meet J5: 42-48, J9: 38-45, J15: 37-43HRC, and J25: 32-38HRC.

[0027] The present invention provides a heat treatment process for high contact fatigue gear steel for new energy vehicles, comprising: an isothermal normalizing pretreatment process, followed by carburizing and gas quenching, and finally low-temperature tempering.

[0028] The isothermal normalizing pretreatment process includes: the normalizing high temperature section holding time t1 is (T1-800) / 120≤t1≤(T1-800) / 80, T1 is the normalizing high temperature section heating temperature, in degrees Celsius, which is 950-1050 degrees Celsius; t1 is the high temperature section holding time, in hours;

[0029] The isothermal normalizing pretreatment process includes: 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 °C, which is 650-720 °C; t2 is the holding time of the holding section, in hours;

[0030] Gear steel needs to undergo isothermal normalizing pretreatment during the processing to avoid the deterioration of banded structure. Therefore, high-temperature diffusion isothermal normalizing is carried out. At the same time, Mo is fully and evenly dissolved in the matrix, which can stabilize the retained austenite during subsequent carburizing.

[0031] The carburizing gas quenching is specifically as follows: heating in a carburizing furnace to 960±15° C. and then keeping the temperature for 5±0.5 hours, cooling to 860° C. and keeping the temperature for 2 hours, and then rapidly cooling to room temperature using nitrogen.

[0032] The low temperature tempering is specifically: tempering at 180-220°C for 5-6 hours.

[0033] New energy vehicle high contact fatigue gear steel carburized layer grain ≥ 10.0 grade, surface hardness ≥ 750HV, retained austenite content < 20%, contact fatigue performance rated life L 10 ≥2.0×10 7 , median fatigue life L 50 ≥5.5×10 7 , inclusion DS≤1.0 level.

[0034] Contact fatigue failure in gear steel is primarily caused by surface defects and subsurface crack initiation, placing higher demands on the carburized layer's structure, hardness, and inclusion content. New energy vehicle gear steel manufactured using the above composition and process exhibits excellent microstructure uniformity and high contact fatigue performance.

[0035] The design ideas of the present invention are as follows:

[0036] C: C is the most basic and effective strengthening element in steel and the element that most effectively affects hardenability. It also has a low cost. In order to ensure that gear steel has sufficient strength and hardenability, it must contain a certain amount of carbon content. The present invention adopts a low carbon content. At the same time, to ensure that the core has sufficient strength and toughness, the carbon content is controlled at 0.20-0.25%.

[0037] Si: Si is a deoxidizer. It can also improve the hardness of steel through solid solution strengthening and the hardenability of gear steel. Si in the present invention mainly plays the role of solid solution strengthening. However, Si easily causes the carburized layer to be easily oxidized. Therefore, the carbon content is controlled at 0.20-0.40%.

[0038] Mn: Mn can expand the austenite phase and stabilize the austenite structure, improving the hardenability of the steel. However, excessive Mn can dissolve 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. The Mn content in this invention primarily serves to reduce the pearlite and ferrite transformation regions, increase the bainite transformation region, and improve hardenability. However, excessive Mn can reduce the plasticity of the steel and deteriorate its toughness during hot rolling. The Mn content is controlled within a range of 1.10-1.30%.

[0039] Cr: Cr improves the hardenability and strength of steel. Cr combines with carbon in the steel to form carbides. Since gear steel is tempered at low temperatures after quenching, no large carbides precipitate, but rather fine carbides. The precipitated carbides accumulate between the martensite laths, inhibiting their movement under stress. Dislocations in the martensite can be entangled, improving strength and fatigue resistance. However, excessive Cr content can form a carbide film, affecting the carburizing effect and reducing the performance of the carburized layer. The Cr content is controlled within a range of 1.10-1.25%.

[0040] Mo: Mo can significantly improve the hardenability of steel and prevent temper brittleness and overheating. Furthermore, the rational combination of Mo and Cr in the present invention can significantly improve hardenability and tempering resistance, and Mo can also refine grains. Mo also strengthens the retained austenite in the carburized layer. Mo atoms stabilize the austenite in the carburized layer, preventing it from undergoing phase transformation and increasing the hardness of the carburized layer. However, if the Mo content is too low, these effects are limited. If the Mo content is too high, it promotes the formation of ferrite films at the grain boundaries, which is detrimental to the steel's thermoplasticity, increases the steel's tendency to reheat cracking, and increases the cost. Therefore, the Mo content is controlled to 0.50-0.70%.

[0041] Nb: Nb has a strong affinity with carbon and oxygen, refining grain size and microstructure while also producing solid solution strengthening. This improves the strength and thermal sensitivity of the steel after heat treatment. Excessive Nb content has little effect on grain refinement or strength gain, but does increase costs. Therefore, the Nb content is controlled between 0.030% and 0.050%.

[0042] Ni: Nickel can significantly improve the toughness of the carburized layer. At the same time, Ni mainly plays the role of solid solution strengthening. After gas quenching, Ni is solid-dissolved in the carburized layer to increase the surface hardness while increasing the toughness of the carburized layer, significantly improving the contact fatigue performance of gear steel. However, Ni is expensive, and too high a Ni content reduces economic applicability, so the Ni content is controlled at 0.4-0.6%.

[0043] Cu: Copper has the effect of refining grains, but too much Cu will embrittle the grain boundaries and cause copper brittleness, so the copper content is controlled to ≤0.2%.

[0044] Ti: Ti has a strong affinity with carbon and oxygen, refining grain size and microstructure while also producing solid solution strengthening. This improves the strength and thermal sensitivity of steel after heat treatment. Excessive Ti content has little effect on grain refinement or strength gains, but does increase costs. Therefore, the Ti content is controlled within a range of 0.030-0.050%.

[0045] Al: Al is an effective deoxidizer and can form AlN grains for finer 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 properties of the steel. Another function of the Al element in the present 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 must be carefully adjusted to ensure that the Al content is controlled within the range of 0.020-0.040%.

[0046] P and S: Sulfur easily forms MnS inclusions with manganese in steel, causing hot brittleness. P is an element with a strong tendency to segregate, increasing cold brittleness and reducing plasticity, which is detrimental to the uniformity of product structure and performance. Control P ≤ 0.010%, and S ≤ 0.010%.

[0047] TO and [H]: TO forms oxide inclusions in steel, so control TO≤10ppm; [H] forms white spots in steel, which seriously affects product performance, so control [H]≤1.0ppm.

[0048] [N]: It can form compounds with Nb, B and Al, and refine grains. A reasonable Al / [N] ratio has a significant effect on grain refinement, while excessively high [N] will cause 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 30-50ppm.

[0049] The hardenability of steel depends primarily 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, the uniformity of the austenite, the size of the austenite grains, and the austenitization state. Studies have found that alloying elements such as Cr, Mn, and Mo can increase the hardenability of the material, reducing the content of residual elements such as P and Sn that embrittle grain boundaries, and repeated quenching to refine the crystal grains also help improve the hardenability of the material. However, due to resource scarcity and high prices, it is desirable to avoid the use of alloying elements such as Cr and Mo. Generally, Al can be added as a deoxidizer, and AlN also has a grain boundary pinning effect and can also be added to prevent grain coarsening. Previous studies have also shown that free Al in austenite can delay the transformation of austenite to ferrite, and the reason is believed to be related to the distribution of Al near the ferrite-austenite transformation interface. High Si content improves solid solution strengthening while also increasing hardenability, promoting bainite transformation, and reducing surface retained austenite. This patented method controls the nitrogen content to avoid TiN formation while also meeting the chemical composition ratio requirements of this patented method to produce gear steel with high contact fatigue strength.

[0050] Compared with the prior art, the present invention, through chemical composition design, tests the end-hardenability performance according to GB / T 225, with J5: 42-48, J9: 38-45, J15: 37-43 HRC, and J25: 32-38 HRC. Furthermore, the pinning effect of Al, Ti, and Nb on grain boundaries refines the grains, and according to GB / T 6394, the grain size of the carburized layer after carburization is ≥ 10.0. Furthermore, due to the low nitrogen content, the solid solution Ti content is increased, which stabilizes the hardenability of the gear steel. Furthermore, the high Mo and Ni elements increase the toughness of the carburized layer under high surface hardness, which plays a role in rebound under contact stress. Through the optimization of the production process, the gear steel produced undergoes preliminary heat treatment, further enhancing the role of the solid solution elements. Furthermore, by utilizing existing carburizing furnace equipment, through specific carburizing process settings and subsequent heat treatment conditions, high-contact fatigue performance gears suitable for new energy vehicles can be produced. The hardness of the surface carburized layer is ≥750HV, and the rated life of the contact fatigue performance is L 10 ≥2.0×10 7 , L 50 ≥5.5×10 7 The present invention not only fundamentally solves the demand for high contact fatigue performance of new energy vehicles, but also shortens the carburizing time by more than half under the same carburizing layer depth, thereby improving production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 is the grain size of the gear steel after carburizing of the present invention;

[0052] Figure 2 It is the worst visual field inclusion in the gear steel of the present invention. DETAILED DESCRIPTION

[0053] The present invention is further described below with reference to Examples and Comparative Examples.

[0054] Example 1-Example 5

[0055] A high contact fatigue gear steel for new energy vehicles includes the following mass percentage components: as shown in Table 1, and the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0056] The production method of high contact fatigue gear steel for new energy vehicles in each embodiment includes the following process flow:

[0057] Electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into products;

[0058] LF refining fully deoxidizes during the refining process to ensure a low oxygen content of <10ppm, and adds aluminum wire to adjust Al in the later stage of vacuum treatment. This can not only ensure the aluminum content but also prevent excessive inclusions in the steel.

[0059] The RH vacuum degassing: N needs to be soft-blown during the RH process, and the soft-blowing time must not be less than 30 minutes and not more than 60 minutes to ensure that TiN cannot be precipitated.

[0060] The continuous casting slab is heated and held at 1230-1250°C for ≥5 hours. The total preheating, heating, and soaking time is controlled at 8.0-12.0 hours. Round steel rolling is then performed, with a starting rolling temperature of 1120-1200°C and a finishing rolling temperature of 700-750°C. After rolling, the slab is cooled on a cooling bed to 600-650°C and then slowly cooled in a pit for ≥24 hours. After exiting the pit, the slab is ground and peeled to ensure a decarburized and defect-free surface. The production process parameters for each embodiment are shown in Table 2.

[0061] Comparative Example 1-Comparative Example 2

[0062] As required, 2 heats of microalloyed SAE 8620 steel (center line) were produced as control steel. The composition is shown in Table 1. The remainder not shown in Table 1 is Fe and unavoidable impurities. The steel was smelted in an electric arc furnace, refined by LF, vacuum treated by RH, continuously cast, and rolled (finished). 1200～1230℃ After heating and holding for ≥4h, round steel rolling is carried out, and the rolling temperature is: 1100 ～1150℃ , final rolling temperature 750～850 After rolling, the steel is cooled to 600-650℃ on a cooling bed and then put into a pit for slow cooling. The slow cooling time is ≥24h. After leaving the pit, it is ground and peeled to ensure that there is no decarburization and zero defects on the surface.

[0063] Comparative Example 3

[0064] A gear steel for high contact fatigue for new energy vehicles and a production method thereof include the following components in mass percentage: as shown in Table 1, the remainder not shown in Table 1 is Fe and unavoidable impurities.

[0065] The production method of high contact fatigue gear steel for new energy vehicles in each embodiment includes the following process flow:

[0066] Electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into products;

[0067] The continuous casting slab is heated and held at 1230-1250°C for ≥5 hours. The total preheating, heating, and soaking time is controlled at 8.0-12.0 hours. Round steel rolling is then performed, with a starting rolling temperature of 1120-1200°C and a finishing rolling temperature of 700-750°C. After rolling, the slab is cooled on a cooling bed to 600-650°C and then slowly cooled in a pit for ≥24 hours. After exiting the pit, the slab is ground and peeled to ensure a decarburized and defect-free surface. The production process parameters for each embodiment are shown in Table 2.

[0068] Table 1 Chemical compositions of the present invention and comparative examples (units: [N], TO, [H] are in ppm, others are in wt%)

[0069]

[0070]

[0071] Table 2 Steel rolling production process parameters of each embodiment and comparative example

[0072] Example Soaking temperature / ℃ Total heating time / h Rolling temperature / ℃ Finish rolling temperature / ℃ Pit entry temperature / ℃ Slow cooling time / h Example 1 1232 8.5 1125 710 639 30 Example 2 1235 9.2 1132 730 633 29 Example 3 1235 9.5 1126 725 636 29 Example 4 1240 10.0 1123 730 641 27 Example 5 1245 10.5 1127 727 639 30 Comparative Example 1 1230 10.5 1125 762 638 27 Comparative Example 2 1230 9.5 1128 800 639 32 Comparative Example 3 1225 8.0 1128 740 640 30

[0073] The above examples and comparative examples were tested for end hardenability according to GB / T 225. The results are shown in Table 3.

[0074] Table 3 End hardenability values ​​(HRC) of gear steels in various embodiments and comparative examples

[0075]

[0076]

[0077] It can be seen from Table 3 that the hardenability control J9, J15, and J25 values ​​of the gear steels described in Examples 1 to 5 of the present invention are all within the range required for new energy gear steels.

[0078] The heat treatment process of the high contact fatigue gear steel for new energy vehicles in the above embodiments and comparative examples includes: an isothermal normalizing pretreatment process, followed by carburizing and gas quenching, and finally low-temperature tempering.

[0079] The isothermal normalizing pretreatment process includes: the normalizing high temperature section holding time t1 is (T1-800) / 120≤t1≤(T1-800) / 80, T1 is the normalizing high temperature section heating temperature, in degrees Celsius, which is 950-1050 degrees Celsius; t1 is the high temperature section holding time, in hours;

[0080] The isothermal normalizing pretreatment process includes: 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 °C, which is 650-720 °C; t2 is the holding time of the holding section, in hours;

[0081] Gear steel needs to undergo isothermal normalizing pretreatment during the processing to avoid the deterioration of banded structure. Therefore, high-temperature diffusion isothermal normalizing is carried out. At the same time, Mo is fully and evenly dissolved in the matrix, which can stabilize the retained austenite during subsequent carburizing.

[0082] The carburizing gas quenching is specifically as follows: heating in a carburizing furnace to 960±15° C. and then keeping the temperature for 5±0.5 hours, cooling to 860° C. and keeping the temperature for 2 hours, and then rapidly cooling to room temperature using nitrogen.

[0083] The low temperature tempering is specifically: tempering at 180-220°C for 5-6 hours.

[0084] The gear steels produced in the above embodiments and comparative examples were heat treated, and the specific heat treatment process parameters are shown in Table 4 below.

[0085] Table 4 Main parameters of heat treatment of each embodiment and comparative example

[0086]

[0087] The gear steels produced in the above examples and comparative examples were carburized and tested according to GB / T 6394. The surface hardness and contact fatigue properties of the carburized layer grains were tested according to GB / T 230 and GB / T 10622. The results are shown in Table 5.

[0088] Table 5 Grain size, surface hardness and contact fatigue performance of carburized steel according to the present invention

[0089]

[0090]

[0091] The underlined data do not meet the requirements of the present invention.

[0092] The present invention adopts a fixed chemical composition ratio to obtain a good hardenability gear steel, and at the same time utilizes a high temperature diffusion isothermal normalizing process, uniformly dissolves Mo elements, adopts high Mo to stabilize the retained austenite in the carburized layer, improves the retained austenite hardness, and improves the carburizing hardness so that the contact fatigue performance is improved. At the same time, microalloying is used to increase the carburizing temperature, fix carbon atoms in the martensite matrix, and the hardness of the needle-shaped martensite and retained austenite is improved at the same time. And after grain refinement, the toughness of the carburized layer is improved, so the contact fatigue performance is greatly improved. The composition of Comparative Example 3 meets the requirements of the present invention, but the isothermal normalizing preheat treatment process is not adopted, and the Y value does not meet the requirements of the present invention. Although some chemical composition processes meet the technical conditions of the patent, the surface hardness is slightly improved, but the contact fatigue performance is still insufficient.

Claims

1. A high contact fatigue gear steel for new energy vehicles, characterized in that: The high contact fatigue gear steel for new energy vehicles includes the following components in percentage by mass: C: 0.20-0.25%, Si: 0.20-0.40%, Mn: 1.10-1.30%, P≤0.010%, S: ≤0.010%, Cr: 1.10-1.25%, Mo: 0.50-0.70%, Nb: 0.030-0.050%, Al: 0.020-0.040%, Ti: 0.03-0.05%, Ni: 0.4-0.6%, Cu≤0.2%, [N]: 30-50ppm, TO≤10ppm, [H]≤1.0ppm, the rest are Fe and unavoidable impurity elements; The composition of the high contact fatigue gear steel for new energy vehicles also satisfies: 15.0≤A value≤28.0, wherein 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%)×(1+1.1×Nb%)×(2+Ti%+2.2×N%); 0.2≤Y value≤0.3; Y=(0.8+3.2×Al%+2.9×Ti%) / 1000×N%.

2. A method for producing high contact fatigue gear steel for new energy vehicles according to claim 1, characterized in that: The production method includes the following process flow: Electric arc furnace smelting - LF refining - RH vacuum treatment - continuous casting - rolling into products.

3. The production method according to claim 2, characterized in that The rolling process includes heating, rolling and slow cooling; The heating process includes controlling the soaking temperature of the steel billet in the heating furnace to be between 1230° C. and 1250° C., and controlling the total time of preheating, heating and soaking to be between 8.0 h and 12.0 h.

4. The production method according to claim 3, characterized in that The rolling process includes a starting rolling temperature of 1120-1200° C. and a finishing rolling temperature of 700-750° C.

5. The production method according to claim 3, characterized in that The slow cooling: after rolling, the steel is cooled on a cooling bed to 600-650°C and then put into a pit for slow cooling, and the slow cooling time is ≥24h.

6. A heat treatment process for the high contact fatigue gear steel for new energy vehicles according to claim 1, characterized in that: The heat treatment process includes: isothermal normalizing pretreatment process, carburizing gas quenching, and finally low-temperature tempering.

7. The heat treatment process according to claim 6, characterized in that: The isothermal normalizing pretreatment process includes: 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, the unit is ° C, and is 950-1050 ° C; t1 is the holding time of the high-temperature section, the unit is 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, the unit is ° C, and is 650-720 ° C; t2 is the holding time of the holding section, the unit is h.

8. A heat treatment process according to claim 6 or 7, characterized in that: The high contact fatigue gear steel for new energy vehicles has a carburized layer with a grain size of ≥10.0 and a surface hardness of ≥750HV. The rated life of the contact fatigue performance is L 10 ≥2.0×10 7 , L 50 ≥5.5×10 7 .

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