A high-nitrogen and high-aluminum gear steel for new energy vehicles and a manufacturing method thereof

Abstract

The application discloses a kind of high-nitrogen high-aluminum new energy automobile gear steel and manufacturing method thereof, steel chemical composition mass percentage: C 0.18-0.25%, Si 0.2-0.4%, Mn 0.6-0.9%, Cr 0.8-1.2%, Ni 0.3-0.6%, Mo≤0.10%, Al 0.05-0.09%, N 0.018-0.025%, P≤0.01%, S≤0.005%, the balance Fe and impurities.Preparation method: electric furnace smelting→LF refining→VD vacuum degassing nitrogen-increasing→continuous casting→grading temperature control rolling.The application forms dispersed AlN grain refinement by Al-N cooperation, strictly controls harmful element to improve purity, has high strength, high toughness, high wear resistance and high fatigue resistance, tensile strength≥1200MPa, adapts new energy automobile high speed high torque condition, process is stable and can be mass-produced.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a high-nitrogen, high-aluminum gear steel for new energy vehicles and its manufacturing method. Background Technology

[0002] With the rapid development of the new energy vehicle industry, the performance requirements for key components of new energy vehicles are increasing. Gears, as a core component of the transmission system in new energy vehicles, operate under complex conditions, needing to withstand high speeds, high torques, and frequent start-stop shocks. Traditional gear steels are gradually becoming insufficient to meet the development needs of new energy vehicles in terms of strength, toughness, wear resistance, and fatigue resistance.

[0003] Currently, common gear steels have some shortcomings. For example, while some steels may meet certain strength and hardness requirements, their toughness is poor, making them prone to fracture under impact loads. Conversely, some steels with good toughness have relatively low strength and wear resistance, leading to wear and deformation of gears during long-term use, thus affecting their service life and transmission efficiency. Furthermore, existing gear steels lack sufficient fatigue resistance when facing the high-frequency alternating stresses generated by the high speeds of new energy vehicle motors, making them prone to fatigue cracks and ultimately gear failure.

[0004] To improve gear performance, researchers have attempted to enhance the properties of steel by adding alloying elements. Nitrogen and aluminum, as important alloying elements, play unique roles in steel. Nitrogen can form nitrides with alloying elements in steel, refining the grain size and increasing strength and hardness. Aluminum not only acts as a deoxidizer to improve the purity of steel but also forms aluminum nitride (AlN) with nitrogen, further refining the grain size and improving the high-temperature strength and fatigue resistance of the steel. However, precisely controlling the content of nitrogen and aluminum in steel and their proportional relationship to achieve the best synergistic effect in improving the overall performance of steel remains a pressing issue. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a high-nitrogen, high-aluminum gear steel for new energy vehicles, addressing the shortcomings of the prior art. By optimizing the chemical composition design, the elements in the steel work together synergistically, significantly improving the strength, toughness, wear resistance, and fatigue resistance of the steel to meet the requirements of new energy vehicle gears under complex operating conditions. Simultaneously, this invention also provides a method for preparing this high-nitrogen, high-aluminum gear steel for new energy vehicles, ensuring the stable and efficient production of steel that meets performance requirements.

[0006] The technical solution adopted by the present invention to solve the above problems is as follows: a high-nitrogen, high-aluminum gear steel for new energy vehicles, the chemical composition of which, by mass percentage, includes: C: 0.18 - 0.25%, Si: 0.2 - 0.4%, Mn: 0.6 - 0.9%, Cr: 0.8 - 1.2%, Ni: 0.3 - 0.6%, Mo: ≤0.10%, Al: 0.05 - 0.09%, N: 0.018 - 0.025%, P ≤0.01%, S ≤0.005%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the roles and selection ranges of each element in the steel are based on the following: C (Carbon): Carbon is a key element affecting the strength and hardness of steel. In this invention, the carbon content is controlled between 0.18% and 0.25%. An appropriate amount of carbon can form carbides with other alloying elements, improving the strength and hardness of the steel. If the carbon content is too low, the strength and hardness of the steel will be insufficient; while if the carbon content is too high, the toughness of the steel will decrease.

[0008] Silicon (Si): Silicon primarily functions as a solid solution strengthener in steel, enhancing its strength and hardness. Simultaneously, silicon increases the steel's deoxidation capacity and improves its purity. This invention controls the silicon content to 0.2-0.4%. Within this range, silicon effectively exerts its strengthening effect without adversely affecting other properties of the steel.

[0009] Mn (manganese): In the steelmaking process, manganese can act as a deoxidizer and desulfurizer, reducing the oxygen and sulfur content in steel and improving its quality. Furthermore, manganese can form alloy cementite with carbon, further enhancing the strength and hardness of the steel. In this invention, the manganese content is 0.6-0.9%, ensuring both good deoxidation and desulfurization effects while maintaining a certain level of toughness and improving steel strength.

[0010] Cr (chromium): Chromium can significantly improve the hardenability and wear resistance of steel, forming a dense oxide film on the steel surface, thus improving the corrosion resistance of the steel. In this invention, the chromium content is controlled at 0.8-1.2%, so that the steel has good hardenability and wear resistance while also possessing a certain degree of corrosion resistance.

[0011] Ni (Ni): Nickel can improve the strength and toughness of steel, and enhance its low-temperature performance. In this invention, the nickel content is 0.3-0.6%, which works synergistically with other alloying elements to give the steel good toughness and low-temperature impact resistance while ensuring strength.

[0012] Al (aluminum): Aluminum is a strong deoxidizer that can significantly improve the purity of steel. Simultaneously, the aluminum nitride (AlN) particles formed by aluminum and nitrogen can refine the grains, improving the strength, hardness, and fatigue resistance of the steel. In this invention, the aluminum content is 0.05-0.09%, ensuring that while providing excellent deoxidation, an appropriate amount of AlN is formed, thereby improving the overall performance of the steel.

[0013] Nitrogen (N): In steel, nitrogen forms nitrides with alloying elements, which refine the grain size and improve strength and hardness. In this invention, the nitrogen content is controlled at 0.018-0.025%, and it works synergistically with aluminum to form fine and dispersed AlN particles, further improving the performance of the steel.

[0014] P (phosphorus) and S (sulfur): Phosphorus and sulfur are harmful elements in steel. Phosphorus causes cold brittleness in steel, while sulfur causes hot brittleness, reducing the toughness and machinability of steel. Therefore, this invention strictly controls the phosphorus content to ≤0.01% and the sulfur content to ≤0.005% to reduce their adverse effects on the properties of steel.

[0015] In order to obtain high-nitrogen, high-aluminum gear steel for new energy vehicles, this invention also explicitly defines the following main technical indicators for the steel: The low-magnification microstructure of the steel used in this invention is inspected and graded according to GB / T 226. The requirements are that ingot segregation, central porosity, general porosity, and central segregation all not exceed grade 1.0, and that visible shrinkage cavities, bubbles, cracks, inclusions, delamination, flaking, and white spots are not permitted.

[0016] The grain size of the steel in this invention is determined by holding at 960±10℃ for 4 hours and water quenching according to GB / T6394. The austenitic grain size of the steel is tested and the qualified grade is ≥7.0.

[0017] The non-metallic inclusions in steel according to the GB / T10561 standard, method A, require the following grades: A fine ≤ 2.5; A coarse ≤ 2.0; B fine ≤ 1.5; B coarse ≤ 1.0; C fine ≤ 0; C coarse ≤ 0; D fine ≤ 1.0; D coarse ≤ 1.0; Ds ≤ 1.0.

[0018] The preparation method of the above-mentioned high-nitrogen, high-aluminum gear steel for new energy vehicles includes the following steps: Electric arc furnace smelting: Scrap steel, molten iron, and other raw materials are added to an electric arc furnace for smelting. Oxygen blowing is used for decarburization, controlling the final carbon content between 0.08% and 0.12%. Simultaneously, an appropriate amount of slag-forming agent is added, and the slag basicity is controlled between 2.5 and 3.5 to remove impurities such as phosphorus and sulfur from the molten steel. During tapping, slag-blocking operations are employed to prevent slag from entering the ladle.

[0019] LF Refining: Molten steel from electric arc furnace smelting is transferred to the LF refining furnace for heating, deoxidation, desulfurization, and composition fine-tuning. First, aluminum blocks are added for precipitation deoxidation, reducing the oxygen content in the molten steel to a low level. Then, diffusion deoxidation is performed by feeding in silicon-calcium wire, further reducing the inclusion content in the molten steel. During refining, argon gas is used for stirring to homogenize the composition and temperature of the molten steel. Simultaneously, based on the composition analysis results of the molten steel, appropriate amounts of alloying materials are added to adjust the alloy element content to achieve the target composition range. The refining time is controlled between 40 and 60 minutes, and the white slag retention time is no less than 30 minutes.

[0020] VD Vacuum Refining: The molten steel after LF refining is transferred to a VD vacuum refining furnace. Under a vacuum degree ≤133Pa, it is maintained for 15-25 minutes for vacuum degassing to remove gases such as hydrogen and nitrogen, as well as inclusions, from the molten steel. Before the end of the vacuum treatment, nitrogen is added by bottom blowing to bring the nitrogen content in the molten steel to the target range. After breaking the vacuum, calcium wire is fed in to treat the molten steel with calcium, improving the morphology and distribution of inclusions.

[0021] Continuous casting: The molten steel, after vacuum refining with VD (Vacuum Dioxide), is continuously cast using a continuous casting machine. Full-process protective casting is employed to prevent secondary oxidation of the molten steel. During continuous casting, the superheat of the molten steel is controlled at 15-25℃, the crystallizer vibration frequency is 180-220 times / minute, and the amplitude is ±3-±5mm. Electromagnetic stirring is used with a stirring current of 200-250A to improve the solidification structure of the billet. The billet pulling speed is adjusted appropriately according to the billet size and steel grade, generally controlled at 0.8-1.2m / min. After the billet exits the crystallizer, it undergoes water spray cooling in the secondary cooling zone. The cooling intensity is controlled according to the surface temperature of the billet to ensure uniform cooling and prevent defects such as cracks.

[0022] Rolling: The continuously cast billet is heated to 1150-1250℃ and held at that temperature for 2-3 hours, then rolled. The rolling process consists of roughing, intermediate rolling, and finishing rolling. The initial rolling temperature for roughing is not lower than 1100℃, and the rolling deformation rate is not less than 30%. The intermediate rolling temperature is controlled at 950-1050℃, and the rolling deformation rate is 20-30%. The finishing rolling temperature is controlled at 850-950℃, and the rolling deformation rate is 10-20%. Through multiple rolling passes, the microstructure of the steel is further refined, improving its overall performance. After rolling, the steel is cooled to room temperature by air cooling or controlled cooling.

[0023] Compared with the prior art, the present invention has the following beneficial effects: High strength and high hardness: This invention achieves high strength and hardness by rationally controlling the content of alloying elements such as carbon, silicon, manganese, chromium, and molybdenum, and by using aluminum nitride (AlN) particles formed by nitrogen and aluminum to refine the grains. This enables the steel to withstand the loads of gears in new energy vehicles under high speed and high torque conditions, thereby improving the service life of the gears.

[0024] Good toughness: The appropriate amount of nickel and the optimized alloy composition system significantly improve the toughness of the steel while ensuring its strength, making it less prone to fracture under impact loads and improving the reliability of the gears.

[0025] Excellent wear resistance: The addition of chromium and the refined grain structure form a hard wear-resistant layer on the surface of the steel, which effectively improves the wear resistance of the steel, reduces the wear of gears during long-term use, and ensures transmission accuracy.

[0026] High fatigue resistance: The dispersed distribution of AlN particles and optimized process control make the internal structure of the steel more uniform and reduce defects, thereby significantly improving the fatigue resistance of the steel. It can adapt to the high-frequency alternating stress brought about by the high speed of the motor of new energy vehicles and reduce the risk of gear fatigue failure.

[0027] Stable quality: The preparation method of this invention ensures the stability of steel composition and structure by precisely controlling each process parameter, so that the steel produced in each batch has uniform performance and reliable quality, meeting the needs of large-scale industrial production. Attached Figure Description

[0028] Figure 1 This is a metallographic image of the dispersed distribution of AlN particles in Example 1 of the present invention.

[0029] Figure 2 This is a metallographic image of the dispersed distribution of AlN particles in Example 2 of the present invention.

[0030] Figure 3 The image shows the grain size (grade 8) microstructure of the steel in Examples 1 and 2 of this invention.

[0031] Figure 4 This is a diagram of the mixed-grain microstructure of steel produced using existing processes. Detailed Implementation

[0032] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention.

[0033] The chemical composition (wt%) of the high-nitrogen, high-aluminum gear steel for new energy vehicles in various embodiments of the present invention is shown in Table 1.

[0034] Table 1 Chemical Composition

[0035] Table 2 Non-metallic inclusions in steel from various embodiments

[0036] Table 3 Low-magnification data of steel in each embodiment

[0037] Table 4 Grain size data of steel samples Table 5. End-hardenability data of steels from each embodiment.

[0038] Table 6 Mechanical data of steel in each embodiment

[0039] This invention relates to a high-nitrogen, high-aluminum gear steel for new energy vehicles. The process flow is as follows: electric arc furnace (EAF) primary refining → ladle refining furnace (LF furnace) refining → VD furnace vacuum degassing → continuous casting CCM → heating and rolling → stacking cooling → finishing → surface and internal flaw detection → packaging.

[0040] Specifically, high-quality scrap steel and molten iron are added to an electric furnace for smelting. Oxygen blowing is used for decarburization, controlling the final carbon content between 0.05% and 0.12%. Simultaneously, an appropriate amount of slag-forming agent is added, with the slag basicity controlled at 2.5-3.5 to remove impurities such as phosphorus and sulfur from the molten steel. During tapping, slag-blocking is employed to prevent slag from entering the ladle. After tapping, the steel is quickly hoisted to the refining LF furnace. The molten steel from the electric furnace is then transferred to the LF refining furnace for heating, deoxidation, desulfurization, and composition fine-tuning. First, aluminum blocks are added for precipitation deoxidation, reducing the oxygen content in the molten steel to a low level. Then, diffusion deoxidation is performed by feeding silicon-calcium wire, further reducing the inclusion content in the molten steel. During refining, argon gas is used for stirring to homogenize the composition and temperature of the molten steel. Simultaneously, based on the composition analysis results of the molten steel, an appropriate amount of alloying material is added to adjust the alloy element content in the molten steel to achieve the target composition range. The refining time is controlled at 40-60 minutes, and the white slag holding time is no less than 30 minutes. The molten steel refined by LF is transferred to a VD vacuum refining furnace and held at a vacuum degree ≤133Pa for 15-25 minutes for vacuum degassing to remove hydrogen, nitrogen, and other gases, as well as inclusions, from the molten steel. Before the end of the vacuum treatment, nitrogen is added through bottom blowing to bring the nitrogen content in the molten steel to the target range. After breaking the vacuum, calcium wire is fed in to treat the molten steel with calcium, improving the morphology and distribution of inclusions. The VD vacuum-refined molten steel is then continuously cast using a continuous casting machine, employing full-process protective casting to prevent secondary oxidation of the molten steel. During continuous casting, the superheat of the molten steel is controlled at 15-25℃, the crystallizer vibration frequency is 180-220 times / minute, the amplitude is ±3-±5mm, and electromagnetic stirring is used with a stirring current of 200-250A to improve the solidification structure of the billet. The billet drawing speed is adjusted appropriately according to the billet size and steel grade, generally controlled between 0.8 and 1.2 m / min. After the billet is pulled out of the crystallizer, it undergoes water spray cooling in the secondary cooling zone. The cooling intensity is controlled according to the surface temperature of the billet to ensure uniform cooling and prevent defects such as cracks. The continuously cast billet is heated to 1150-1250℃ and held for 2-3 hours before rolling. The rolling process consists of roughing, intermediate rolling, and finishing rolling. The initial rolling temperature for roughing is not lower than 1100℃, and the rolling deformation rate is not less than 30%. The intermediate rolling temperature is controlled at 950-1050℃, and the rolling deformation rate is 20-30%. The finishing rolling temperature is controlled at 850-950℃, and the rolling deformation rate is 10-20%. Through multiple rolling passes, the microstructure of the steel is further refined, improving its overall performance. After rolling, the steel is cooled to room temperature by air cooling or controlled cooling. Then, the bar stock undergoes further flaw detection and finishing processes.

[0041] As can be seen from Tables 1, 2, 3, 4, 5, and 6, the high-nitrogen, high-aluminum gear steel for new energy vehicles in each embodiment of the present invention exhibits stable and excellent performance in various indicators. The low magnification mass, hardenability, inclusions, grain size, and mechanical properties of the present invention all meet the high-end quality requirements for gear steel for new energy vehicles.

[0042] In addition to the above embodiments, the present invention can also adjust parameters such as billet thickness and continuous casting process according to the production requirements of converters and electric furnaces of different tonnages. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A high-nitrogen, high-aluminum gear steel for new energy vehicles, characterized in that, The chemical composition by mass percentage is as follows: C: 0.18-0.25%, Si: 0.2-0.4%, Mn: 0.6-0.9%, Cr: 0.8-1.2%, Ni: 0.3-0.6%, Mo: ≤0.10%, Al: 0.05-0.09%, N: 0.018-0.025%, P≤0.01%, S≤0.005%, with the balance being Fe and unavoidable impurities.

2. The high-nitrogen, high-aluminum gear steel for new energy vehicles according to claim 1, characterized in that, The low-magnification microstructure of the steel conforms to the GB / T 226 standard: ingot segregation, central porosity, general porosity, and central segregation are all ≤1.0 grade, and there are no visible shrinkage cavities, bubbles, cracks, inclusions, delamination, flaking, or white spots.

3. The high-nitrogen, high-aluminum gear steel for new energy vehicles according to claim 1, characterized in that, The austenitic grain size conforms to GB / T 6394 standard: after holding at 960±10℃ for 4 hours and water quenching, the grain size is ≥7.0 grade.

4. The high-nitrogen, high-aluminum gear steel for new energy vehicles according to claim 1, characterized in that, Non-metallic inclusions conform to GB / T 10561 Method A: A fine ≤ 2.5 grade, A coarse ≤ 2.0 grade, B fine ≤ 1.5 grade, B coarse ≤ 1.0 grade, C fine / C coarse = 0 grade, D fine / D coarse ≤ 1.0 grade, Ds ≤ 1.0 grade.

5. The high-nitrogen, high-aluminum gear steel for new energy vehicles according to claim 1, characterized in that, Mechanical properties of φ15mm sample after heat treatment: oil quenching at 860±10℃ + air cooling at 200±10℃, tensile strength ≥1200MPa, yield strength ≥1000MPa, elongation ≥15%, impact energy KU2 ≥50J, hardness HRC38-43.

6. A method for manufacturing high-nitrogen, high-aluminum gear steel for new energy vehicles as described in any one of claims 1-5, characterized in that, Includes the following steps: 1) Electric furnace smelting: 60-75% scrap steel + 25-40% molten iron, final carbon 0.08-0.12%, slag basicity 2.5-3.5, slag blocking and tapping steel; 2) LF refining: Deoxidation and alloying at 1570-1590℃, staged stirring with argon gas, refining for 40-60 minutes, maintaining white slag for ≥30 minutes; 3) VD vacuum refining: vacuum degree ≤133Pa, pressure held for 15-25min, bottom blowing nitrogen to precisely increase nitrogen to 0.018-0.025%, calcium treatment after breaking the vacuum; 4) Full-process protective continuous casting: molten steel superheat 15-25℃, crystallizer vibration 180-220 times / min, electromagnetic stirring 200-250A, casting speed 0.8-1.2m / min; 5) Graded temperature-controlled rolling: Hold the billet at 1150-1250℃ for 2-3 hours, rough rolling at ≥1100℃ with a deformation rate of ≥30%, intermediate rolling at 950-1050℃ with a deformation rate of 20-30%, and finish rolling at 850-950℃ with a deformation rate of 10-20%, and then cool to room temperature.

7. The method for manufacturing a high-nitrogen, high-aluminum gear steel for new energy vehicles according to claim 6, characterized in that, Step 3) The nitrogen flow rate during bottom blowing is 30-60 L / min, and the blowing time is 5-6 min; the calcium wire feed rate is 0.3-0.35 kg / t molten steel.

8. A method for manufacturing a high-nitrogen, high-aluminum gear for new energy vehicles according to claim 6, characterized in that, In step 4), the cooling intensity of the secondary cooling zone in continuous casting is 0.8-1.0 L / kg molten steel, and a weak or medium cooling process is adopted.

9. A method for manufacturing a high-nitrogen, high-aluminum gear for new energy vehicles according to claim 6, characterized in that, In step 5), after rolling, air cooling or controlled cooling is used, with a controlled cooling rate of 5℃ / s.

10. A method for manufacturing a high-nitrogen, high-aluminum gear for new energy vehicles according to claim 6, characterized in that, In step 2), the amount of aluminum blocks added is 0.8-1.0 kg / t of molten steel, and the argon gas stirring speed is 60-90 L / min in the early stage and 30-50 L / min in the later stage.

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