Ultra-high-strength steel for new energy automobile driving motor and production method of ultra-high-strength steel

Abstract

The invention relates to the technical field of steel production, in particular to ultrahigh-strength steel for a new energy automobile driving motor, which comprises the following chemical components in percentage by mass: less than or equal to 0.0020% of C, 3.0-4.0% of Si, 0.20-0.8% of Mn, 0.01-0.07% of P, less than or equal to 0.0010% of S, less than or equal to 0.0020% of N, less than or equal to 0.0010% of Ti, less than or equal to 0.0020% of V, 0.5-1.5% of Nb, 0.02-0.07% of Cr, 0.20-1.00% of Als and the balance of Fe and inevitable residual elements. By combining the incomplete recrystallization technology and the strengthening effect of niobium and chromium, the balance of high strength and excellent electromagnetic performance of the material is realized.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology, and specifically relates to a method for producing ultra-high strength steel for drive motors of new energy vehicles. Background Technology

[0002] Against the backdrop of the rapid development of the new energy vehicle industry, the performance requirements for drive motors are constantly increasing, especially the demand for higher efficiency, miniaturization, and higher performance. Permanent magnet synchronous motors, as the mainstream choice for drive motors in current new energy vehicles, are prone to deformation and even fatigue failure due to stress concentration in their rotors during high-speed rotation. Therefore, developing high-strength non-oriented electrical steel to meet these performance requirements has become particularly important. At the same time, the demand for high-speed motors is also increasing in high-end application fields such as aerospace, flywheel energy storage, and power tools, which also require non-oriented electrical steel with high strength and good magnetic properties.

[0003] In the continuous annealing process of materials, by precisely controlling the annealing temperature, time, and cooling rate, the material can primarily undergo recovery while minimizing recrystallization. The key to this technology lies in selecting appropriate annealing temperatures and times to repair internal defects and alleviate stress, while maintaining a relatively constant grain size. This technology is of great significance in the production of ultra-high-strength steel for new energy motors, effectively balancing the material's strength and electromagnetic properties, and providing a high-performance material solution for the manufacture of drive motors for new energy vehicles and other high-speed motors. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an ultra-high strength steel for drive motors of new energy vehicles, which achieves a balance between high strength and excellent electromagnetic properties by combining incomplete recrystallization technology with the strengthening effects of niobium and chromium.

[0005] To achieve the above objectives, the present invention employs the following technical solution: A type of ultra-high strength steel for drive motors of new energy vehicles has the following chemical composition by mass fraction: C≤0.0020%, Si: 3.0%~4.0%, Mn: 0.20%~0.8%, P: 0.01%~0.07%, S≤0.0010%, N≤0.0020%, Ti≤0.0010%, V≤0.0020%, Nb: 0.5%~1.5%, Cr: 0.02%~0.07%, Als: 0.20%~1.00%, with the remainder being Fe and unavoidable residual elements.

[0006] A type of ultra-high strength steel for drive motors of new energy vehicles, with the following mechanical properties: yield strength Rp0.2 ranging from 850 to 950 MPa, and magnetic induction intensity ≥1.50 T.

[0007] The chemical components function as follows: The carbon (C) and nitrogen (N) content is extremely low, controlled at ≤0.0020%, to avoid increased aging time due to excessive carbon content.

[0008] With a silicon (Si) content of 3.0%–4.0%, silicon is an important solid solution strengthening element that can significantly improve the strength and hardness of steel. Simultaneously, it reduces eddy current losses and optimizes electromagnetic properties by increasing resistivity. Furthermore, silicon can inhibit grain growth and refine grains, thereby enhancing the toughness and fatigue resistance of steel.

[0009] The manganese (Mn) content is 0.20% to 0.8%, and it is mainly used to improve the strength of steel while also improving its toughness.

[0010] The phosphorus (P) content is controlled between 0.01% and 0.07% to avoid adverse effects on the toughness and weldability of the steel.

[0011] The sulfur (S) content is strictly controlled to ≤0.0010% in order to reduce sulfide inclusions and improve the purity and electromagnetic properties of the steel.

[0012] Titanium (Ti) and vanadium (V) are present in trace amounts; titanium and vanadium are not used as strengthening elements in this invention.

[0013] Niobium (Nb) content is 0.5%–1.5%, and it is an important precipitation strengthening element in this invention. Niobium can combine with carbon and nitrogen in steel to form stable carbides and carbonitrides. These compounds not only refine the grains but also significantly improve the yield strength and tensile strength of the steel through dispersion strengthening. Furthermore, the addition of niobium can improve the toughness of the steel, lower the brittle transition temperature, and achieve better weldability and formability. In this invention, by controlling the niobium content and combining it with an incomplete recrystallization process, a balance between high strength and good electromagnetic properties is achieved.

[0014] The chromium (Cr) content is 0.02% to 0.07% in this invention. Through the solid solution strengthening effect of chromium, the strength and hardness of the steel are further improved. In terms of electromagnetic properties, the addition of chromium can further increase the resistivity of the steel, thereby reducing eddy current losses and improving the efficiency of the motor.

[0015] The aluminum (Als) content is 0.20% to 1.00%. Aluminum can improve the strength of steel and also has a certain optimizing effect on magnetic properties.

[0016] A method for producing ultra-high strength steel for drive motors of new energy vehicles includes the following steps: converter smelting, continuous casting, hot rolling, cold rolling, and finished product annealing. Specifically, the method includes: (1) Hot rolling: During the hot rolling process, the heat soaking temperature is set at 1200-1280℃, the heat holding time is controlled at 220-280min, the furnace exit temperature is controlled at 1100-1180℃, the final rolling temperature is set at 900-950℃, and the coiling temperature is controlled between 700℃ and 780℃. (2) Cold rolling: Hot-rolled coils are directly produced by cold rolling, using a 5-7 pass cold rolling process, with the total reduction rate controlled at 80%-90%, and the reduction rate of each pass decreasing step by step. The reduction rate of the first two passes is 25%-35%, and the thickness tolerance of the steel plate is controlled within ±0.02mm; the rolling speed is controlled at 70-200m / min, the front tension is controlled at 10-25kN, and the back tension is controlled at 7-15kN; (3) Finished product annealing: The continuous annealing temperature is set at 730℃~800℃, the annealing time is controlled at 20~40s, and the cooling stage after annealing adopts a rapid cooling rate of 15℃ / s~30℃ / s. The converter smelting process described above employs RH vacuum refining, cyclic degassing and decarburization, controls the steelmaking endpoint hit rate to be greater than 90%, and maintains the reblowing rate to be less than 2%.

[0017] The continuous casting process involves electromagnetic stirring with a working current of 300-400A to ensure that the equiaxed crystal ratio of the billet is ≥55%. During production, the liquid level fluctuation in the crystallizer is controlled within ±3mm, and a slag baffle is used for steel tapping.

[0018] In the cold rolling process, the work roll changing cycle is for rolling strips weighing over 200t, and the intermediate roll changing cycle is for rolling strips weighing over 500t.

[0019] Compared with existing technologies, the beneficial effects of this invention are: This invention breaks through the bottleneck of traditional non-oriented silicon steel, which requires "high strength at the expense of magnetic induction," as detailed below: (1) Ultra-low carbon-high silicon-Nb-Cr microalloying C≤20ppm+N≤20ppm, eliminate grain boundary Cottrell gas clusters, reduce coercivity Hc, and avoid magnetic aging.

[0020] A Si content of 3.0–4.0% increases the resistivity ρ to over 0.3 μΩ·m, reduces eddy current losses, and simultaneously enhances strength through solid solution strengthening.

[0021] With a Nb content of 0.5–1.5%, Nb(C,N) nanoclusters (2–5 nm) are formed in the recovery annealing range of 730℃–800℃, which pin dislocations and subgrain boundaries and prevent recrystallization nucleation; at the same time, the strength is improved through precipitation strengthening.

[0022] A Cr content of 0.02–0.07% increases the matrix recrystallization temperature, increases stacking fault energy when dissolved in α-Fe, inhibits dynamic recovery, and synergistically forms “Cr-Nb co-clusters” with Nb, making the dislocation cell walls more stable during the recovery stage and enhancing the strengthening effect.

[0023] (2) Thermomechanical pathway of “non-normalization + incomplete recrystallization” Hot rolling at 1200℃~1280℃ for 220min~280min ensures 100% Nb solution; final rolling at 900~950℃ and coiling at 700~780℃ completes dynamic recrystallization and substatic recrystallization, resulting in 3~5μm fine grains and high dislocation density.

[0024] Cold rolling with a large reduction rate of 80-90% introduces shear bands and high-density dislocation cell structures to provide additional deformation energy storage during the recovery stage.

[0025] 730℃~800℃, 20~40s continuous annealing + 15~30℃ / s rapid cooling, only recovery occurs and no recrystallization occurs: dislocation density is reduced; cold rolling texture is retained to ensure that the magnetic induction is not deteriorated; at the same time, subgrain boundary + Nb(C,N) double pinning achieves ultra-high strength.

[0026] This invention achieves a synergistic breakthrough in ultra-high strength, high magnetic induction, and low iron loss in ultra-thin motor steel through a combination of ultra-low carbon high silicon matrix, Nb-Cr nanoprecipitation, and incomplete recrystallization. The product can be directly applied to the rotor of new energy drive motors with speeds >15000rpm, greatly improving the operational safety of high-speed motors. Detailed Implementation

[0027] It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein in the specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of the application. The reference to "embodiment" herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0028] A type of ultra-high strength steel for drive motors of new energy vehicles has the following chemical composition by mass fraction: C≤0.0020%, Si: 3.0%~4.0%, Mn: 0.20%~0.8%, P: 0.01%~0.07%, S≤0.0010%, N≤0.0020%, Ti≤0.0010%, V≤0.0020%, Nb: 0.5%~1.5%, Cr: 0.02%~0.07%, Als: 0.20%~1.00%, with the remainder being Fe and unavoidable residual elements.

[0029] A method for producing ultra-high strength steel for drive motors of new energy vehicles includes: (1) Converter smelting: Advanced RH vacuum refining technology is used to deeply process molten steel. Through cyclic degassing and decarburization in a vacuum environment, the content of harmful gases such as hydrogen, nitrogen, and oxygen in the steel is significantly reduced, the formation of inclusions is reduced, and the purity of the molten steel is improved. During the RH vacuum refining process, the chemical composition of the molten steel is precisely controlled, especially by adding appropriate amounts of niobium (Nb) and chromium (Cr). During the smelting process, the steelmaking endpoint hit rate is achieved to be greater than 90% through precise control, and the reblowing rate is kept below 2%.

[0030] (2) Continuous casting: In the continuous casting process, advanced electromagnetic stirring technology is introduced. By increasing the working current of the electromagnetic stirring, the equiaxed crystal ratio of the billet is increased to ≥55%, which effectively improves the microstructure of the billet, increases the proportion of equiaxed crystals, and thus improves the uniformity and toughness of the billet. During the production process, the liquid level fluctuation in the crystallizer is controlled within ±3mm to further optimize the billet quality. When tapping the steel, a slag baffle is used to prevent secondary oxidation of the molten steel and the mixing of inclusions during the pouring process, thereby improving the purity of the billet.

[0031] (3) Hot rolling: During hot rolling, the soaking temperature is set at 1200-1280℃, and the holding time is controlled at 220-280min to ensure that alloying elements such as niobium and chromium are fully dissolved in the matrix, providing a basis for subsequent solid solution strengthening and precipitation strengthening. The furnace exit temperature is controlled at 1100-1180℃. The final rolling temperature is set at 900-950℃. Within this temperature range, it can avoid premature precipitation of precipitates caused by excessively low temperatures, and also ensure that the microstructure of the hot-rolled plate is most favorable for subsequent production. The coiling temperature is controlled between 700℃ and 780℃. Through precise temperature control, dynamic recrystallization and substatic recrystallization of the hot-rolled coil are promoted, forming a microstructure that is favorable for subsequent cold rolling and annealing processes, so that the product can provide stable magnetic properties under abnormal conditions.

[0032] (4) Normalization: In this invention, normalization is not performed, and the hot-rolled coil is directly cold-rolled.

[0033] (5) Cold rolling: A 5-7 pass cold rolling process is adopted, with the total reduction rate controlled at 80% to 90%. The reduction rate of each pass decreases gradually, with the first two passes having a higher reduction rate of about 25% to 35%, and the reduction rate of subsequent passes gradually decreasing to 5% to 10%. This reduction rate distribution method ensures that the strip can deform uniformly during the cold rolling process, avoids the problems of local stress concentration and uneven deformation, and at the same time ensures that the thickness tolerance is controlled within ±0.02mm.

[0034] The rolling speed is adjusted according to the strip thickness and material properties, and controlled between 70 and 200 m / min. The initial tension is controlled between 10 and 25 kN, and the subsequent tension is controlled between 7 and 15 kN. During cold rolling, the tension is dynamically adjusted according to the strip thickness and rolling passes to ensure the strip shape and stability, and to avoid defects such as waviness and warping.

[0035] The product of this invention has high hardness. To ensure the product's shape and surface quality, the roll changing cycle needs to be rationally arranged based on the reduction rate and target strip thickness during the rolling process. When rolling high-silicon, high-niobium, and high-chromium steel, the work rolls are replaced after rolling 200t of strip; the wear of the intermediate rolls is relatively small, and the roll changing cycle is extended to 500t.

[0036] (6) Finished product annealing: In the continuous annealing process of this invention, by precisely controlling the process parameters, the material undergoes only full recovery and basically no recrystallization, thereby achieving a balance between the high strength and excellent electromagnetic properties of the ultra-high strength steel for new energy motors. The soaking temperature is set at 730℃~800℃, which is lower than the complete recrystallization temperature, ensuring that the material mainly undergoes the recovery process, the grain size is effectively controlled, and the average size of occasional point-like grains is below 5μm. At the same time, the soaking time is controlled at 20s~40s to promote the repair of internal defects and the relief of stress, but to avoid grain growth. In the cooling stage after annealing, a rapid cooling rate of 15℃ / s~30℃ / s is adopted to further suppress recrystallization, reduce residual stress, and improve dimensional stability. This fully recovered and basically non-recrystallized microstructure significantly improves the yield strength and tensile strength of the material, while meeting the requirements of new energy motors for high-efficiency electromagnetic performance. Furthermore, the rapid cooling process reduces the generation of residual stress and improves the dimensional stability of the material, enabling it to maintain good shape and dimensional accuracy during subsequent processing and use, thus reducing the risk of deformation and cracking. The microstructure after annealing is deformed ferrite that has undergone significant recovery but is still in a non-recrystallized state: the dislocation density is significantly reduced, forming clear dislocation cells and subgrain boundaries; the grain boundaries are wavy, and no new equiaxed grains appear.

[0037] To make the objectives, technical solutions, and technical effects of this invention clearer, the technical solutions in the embodiments of this invention are now described clearly and completely. However, the embodiments described below are only some embodiments of this invention, not all embodiments. All other embodiments obtained by those skilled in the art in conjunction with the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0038] The chemical composition of the examples is shown in Table 1; Table 1: Chemical Composition (%) Example 1: Production method of ultra-high strength steel for drive motors of new energy vehicles: a) Converter smelting RH vacuum refining technology is used to reduce the content of harmful gases and improve the purity of molten steel. The composition is adjusted.

[0039] b) Continuous casting During continuous casting, the electromagnetic stirring current is 370A, the equiaxed crystal ratio of the billet is 65%, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. Slag baffles are used during tapping to prevent secondary oxidation of the molten steel and the introduction of inclusions during the pouring process.

[0040] c) Hot-rolled Heat soaking temperature: 1275℃; holding time: 280min; furnace exit temperature: 1170℃; final rolling temperature: 950℃; coiling temperature: 770℃. Precise temperature control promotes dynamic and substatic recrystallization of the hot-rolled coil, enabling the product to provide stable magnetic properties under atypical conditions.

[0041] d) Cold rolling The process employs a 6-pass cold rolling process: raw material thickness 2.2mm, target thickness 0.35mm, total reduction rate 84%, reduction rate decreasing progressively in each pass, with the first two passes having reduction rates of 30% and 25% respectively, front tension 25kN, and rear tension 15kN, ensuring the strip shape and stability during the rolling process.

[0042] e) Finished product annealing Two-stage annealing: soaking temperature 800℃, soaking time 38s. Cooling rate after annealing 15℃ / s to ensure that the material mainly undergoes the recovery process, effectively controlling the grain size, with an average size of 4.7μm for occasional spot grains.

[0043] The performance of the 0.35mm specification product is shown in Table 2; Table 2: Example 2: Production method of ultra-high strength steel for drive motors of new energy vehicles: a) Converter smelting RH vacuum refining technology is used to remove impurities from steel and adjust its composition.

[0044] b) Continuous casting The continuous casting process is adopted, with an electromagnetic stirring current of 340A and an equiaxed crystal ratio of 62% for the billet. A slag baffle is used during tapping to control the slag thickness in the ladle to 90mm.

[0045] c) Hot-rolled Heat soaking temperature: 1230℃; holding time: 240min; furnace exit temperature: 1130℃; final rolling temperature: 930℃; coiling temperature: 740℃.

[0046] d) Cold rolling The process employs a 6-pass cold rolling process: raw material thickness 2.3mm, target thickness 0.30mm, total reduction rate 87%, with the reduction rate decreasing progressively in each pass. The reduction rates for the first two passes are 32% and 28%, respectively. The initial tension is 20kN, and the subsequent tension is 12kN. The strip shape and stability are ensured during the rolling process.

[0047] e) Finished product annealing Two-stage annealing: soaking temperature 730℃, soaking time 30s. Cooling rate after annealing 25℃ / s, ensuring that the material mainly undergoes the recovery process, effectively controlling the grain size, with an average size of 2.4μm for occasional spot grains.

[0048] The performance of the 0.30mm specification product is shown in Table 3; Table 3: Example 3: Production method of ultra-high strength steel for drive motors of new energy vehicles: a) Converter smelting The composition is adjusted using RH vacuum refining technology to improve the purity of the materials.

[0049] b) Continuous casting During continuous casting, the electromagnetic stirring current is 250A, the equiaxed crystal ratio of the billet is 56%, and the liquid level fluctuation in the crystallizer is controlled within ±3mm. A slag baffle is used during tapping to control the slag thickness in the ladle to 87mm.

[0050] c) Hot-rolled Heat soaking temperature: 1210℃; holding time: 220min; furnace exit temperature: 1110℃; final rolling temperature: 910℃; coiling temperature: 705℃.

[0051] d) Cold rolling The process employs a 6-pass cold rolling process: raw material thickness 2.5mm, target thickness 0.25mm, total reduction rate 90%, reduction rate decreasing progressively in each pass, with reduction rates of 33% and 26% in the first two passes, initial tension 13kN, and final tension 7kN, ensuring the strip shape and stability during the rolling process.

[0052] e) Finished product annealing Two-stage annealing: soaking temperature 765℃, soaking time 23s. Cooling rate after annealing 30℃ / s to ensure that the material mainly undergoes the recovery process, and the grain size is effectively controlled.

[0053] The performance of the 0.25mm specification product is shown in Table 4; Table 4: Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A type of ultra-high strength steel for drive motors of new energy vehicles, characterized in that, The chemical composition by mass fraction is as follows: C≤0.0020%, Si: 3.0%~4.0%, Mn: 0.20%~0.8%, P: 0.01%~0.07%, S≤0.0010%, N≤0.0020%, Ti≤0.0010%, V≤0.0020%, Nb: 0.5%~1.5%, Cr: 0.02%~0.07%, Als: 0.20%~1.00%, with the remainder being Fe and unavoidable residual elements.

2. The ultra-high strength steel for a new energy vehicle drive motor according to claim 1, characterized in that, Mechanical properties: Yield strength Rp0.2 ranges from 850 to 950 MPa, magnetic induction intensity ≥1.50 T.

3. The method for producing ultra-high strength steel for drive motors of new energy vehicles according to claim 1, characterized in that, The process steps are converter smelting, continuous casting, hot rolling, cold rolling, and finished product annealing, specifically including: (1) Hot rolling: During the hot rolling process, the heat soaking temperature is set at 1200-1280℃, the heat holding time is controlled at 220-280min, the furnace exit temperature is controlled at 1100-1180℃, the final rolling temperature is set at 900-950℃, and the coiling temperature is controlled between 700℃ and 780℃. (2) Cold rolling: Hot-rolled coils are directly produced by cold rolling, using a 5-7 pass cold rolling process, with the total reduction rate controlled at 80%-90%, and the reduction rate of each pass decreasing step by step. The reduction rate of the first two passes is 25%-35%, and the thickness tolerance of the steel plate is controlled within ±0.02mm; the rolling speed is controlled at 70-200m / min, the front tension is controlled at 10-25kN, and the back tension is controlled at 7-15kN; (3) Finished product annealing: The continuous annealing temperature is set at 730℃~800℃, the annealing time is controlled at 20~40s, and the cooling stage after annealing adopts a rapid cooling rate of 15℃ / s~30℃ / s.

4. The method for producing ultra-high strength steel for drive motors of new energy vehicles according to claim 3, characterized in that, The converter smelting process described above employs RH vacuum refining, cyclic degassing and decarburization, controls the steelmaking endpoint hit rate to be greater than 90%, and maintains the reblowing rate to be less than 2%.

5. A method for producing ultra-high strength steel for a new energy vehicle drive motor according to claim 3, characterized in that, The continuous casting process involves electromagnetic stirring with a working current of 300-400A to ensure that the equiaxed crystal ratio of the billet is ≥55%. During production, the liquid level fluctuation in the crystallizer is controlled within ±3mm, and a slag baffle is used for steel tapping.

6. The method for producing ultra-high strength steel for drive motors of new energy vehicles according to claim 3, characterized in that, In the cold rolling process, the work roll changing cycle is for rolling strips weighing over 200t, and the intermediate roll changing cycle is for rolling strips weighing over 500t.

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