New energy automobile gear box gear steel and production method thereof

Abstract

The present application relates to a kind of new energy automobile gear box gear steel and its production method, belong to metallurgical technical field, C:0.25~0.31%, Si:0.2~0.4%, Mn:1.0~1.3%, Cr:1.0~1.3%, Ni:0.2~0.3%, Nb:0.010~0.020%, V:0.03~0.06%, Al:0.02~0.05%, N:0.0080~0.0110%, S≤0.005%, P≤0.020%, Cu≤0.20%, Mo≤0.10%, Ti≤0.002%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, the balance is Fe and inevitable impurities.The production process is initial refining→refining→vacuum degassing→continuous casting CCM→heating rolling→stacking cold→isothermal annealing→finishing→surface and internal flaw detection.

Description

Technical Field

[0001] This invention belongs to the field of iron-based alloy technology in metallurgy, and more specifically relates to steel for automotive gearboxes and its production method. Background Technology

[0002] As the automotive market continues to develop and improve, environmental protection issues have gradually gained global attention, leading to the rapid rise of the new energy vehicle industry, which boasts advantages such as low energy consumption and low pollution. To meet people's ever-increasing demands for vehicle performance, new energy vehicles face higher requirements in various aspects compared to traditional vehicles, particularly in their gearboxes: high efficiency, high wear resistance, and high stability are essential characteristics.

[0003] As a key component of new energy vehicles, the gearbox contains gears that are crucial parts of the gearbox transmission system. Gearbox gears withstand powerful alternating loads and impacts during high-speed operation. To ensure efficient, stable, and reliable operation of these gears, higher requirements are placed on the gear steel used in new energy vehicle gearboxes. Therefore, the material must possess properties such as high purity, high mechanical properties, and long service life. Currently, domestic steel mills mainly produce this type of gear steel using in-mold casting. However, this process suffers from significant disadvantages, including poor production continuity, inconsistent quality between batches, and high production costs. The "vacuum degassing + continuous casting" process, which offers better production continuity, stable quality, and more economical costs, is not yet mature for producing this type of gear steel. Furthermore, gear steel produced using the "vacuum degassing + continuous casting" process does not meet the requirements of this type of gear steel. Therefore, the research and development of the "vacuum degassing + continuous casting" process for producing this type of gear steel is imperative. Summary of the Invention

[0004] To obtain high-purity, high-mechanical-performance, and long-life gear steel for new energy vehicle gearboxes, this invention employs a "vacuum degassing + continuous casting" process that offers good production continuity, stable quality, and greater economy. This process reduces non-metallic inclusions, improves steel purity, and lowers oxygen content and residual harmful element content. Appropriate trace elements are added to enhance mechanical properties and wear resistance. Furthermore, a unique continuous casting and high-temperature rolling process is designed to improve the uniformity of the steel's microstructure. This invention develops a new production method for gear steel for new energy vehicle gearboxes that meets market demands.

[0005] To meet the performance requirements of high cleanliness, fine grain size, and high mechanical properties for gear steel used in gearboxes of new energy vehicles, this invention employs a unique and rational chemical composition design, as follows:

[0006] The composition by mass percentage is as follows: C: 0.25–0.31%, Si: 0.2–0.4%, Mn: 1.0–1.3%, Cr: 1.0–1.3%, Ni: 0.2–0.3%, Nb: 0.010–0.020%, V: 0.03–0.06%, Al: 0.02–0.05%, N: 0.0080–0.0110%, S≤0.005%, P≤0.020%, Cu≤0.20%, Mo≤0.10%, Ti≤0.002%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the balance being Fe and unavoidable impurities.

[0007] 1) Determination of C content

[0008] Carbon (C) is the most basic element in steel and also the most economical strengthening element. Among all elements, carbon has the greatest ability to increase the strength of steel. The strengthening effect of carbon on quenched and tempered steel is approximately 9 times that of chromium and 18 times that of manganese. Therefore, to ensure good strength and toughness in the core of gears after carburizing heat treatment, the steel needs to have a suitable carbon content. In this invention, the carbon content is determined to be in the range of 0.25% to 0.31%.

[0009] 2) Determination of Si content

[0010] Si dissolved in the ferrite phase has a strong solid solution strengthening effect, which can improve strength, elastic limit, and hardenability, but at the same time reduces the plasticity and toughness of ferrite. The Si content of the steel of this invention is determined to be in the range of 0.2% to 0.4%.

[0011] 3) Determination of Mn content

[0012] Mn, as a deoxidizing element in the steelmaking process, can improve the hardenability of steel. Mn can also fix the form of sulfur in steel and form MnS and (Fe,Mn)S, which are less harmful to steel properties, reducing or inhibiting the formation of FeS. Therefore, the presence of manganese in steel can improve its purity and properties. Simultaneously, manganese plays a role in solid solution strengthening and grain refinement in steel, thereby increasing strength and significantly improving hardenability; however, excessive manganese content has the disadvantage of promoting austenitizing grain growth. In this invention, the Mn content is determined to be in the range of 1.0% to 1.3%.

[0013] 4) Determination of Cr content

[0014] Cr increases the hardenability of steel and has a secondary hardening effect, thus strengthening the steel through solid solution and improving its hardenability. Additionally, Cr reduces the activity of C, decreasing the tendency of steel to overheat and the rate of surface decarburization. However, excessively high Cr content can lead to the formation of large carbides by combining with carbon in the steel. These insoluble carbides reduce the toughness of the steel and decrease gear life. Therefore, the Cr content in this invention is determined to be in the range of 1.0% to 1.3%.

[0015] 5) Determination of Ni content

[0016] Ni strengthens ferrite and refines pearlite in steel, improving its hardenability and being the most commonly used element to effectively enhance its wear resistance. The combined effect of Ni with Cr and residual P in steel further improves its corrosion and wear resistance. In this invention, the Ni content is defined as 0.2%–0.3%.

[0017] 6) Determination of Nb and V content

[0018] By adding Nb and V to steel, the CCT curve of the steel shifts to the lower right, increasing the stability of austenite. In steel, austenite exists mostly as carbides, nitrides, and carbonitride precipitates, which are typically nanoscale in size. These dispersed small precipitates pin the austenite grain boundaries, hindering austenite grain growth and refining the austenite grains of the steel. In this invention, the Nb content is designed to be 0.010%–0.020%, and the V content is designed to be 0.03%–0.06%.

[0019] 7) Determination of Al content

[0020] Al, added as a deoxidizing element in steel, not only reduces dissolved oxygen in molten steel, but also forms finely dispersed aluminum nitride with nitrogen, which refines the grain size. However, excessive Al content can easily lead to the formation of large-particle brittle inclusions such as Al2O3 during the steelmaking process, reducing the purity of the steel and affecting its service life. In this invention, the Al content is determined to be in the range of 0.02% to 0.05%.

[0021] 8) Determination of N content

[0022] Nitrogen (N) precipitates as nitrides along with Nb, Ti, and Al, significantly improving the stability of high-temperature austenite grains in gear steel and preventing high-temperature austenite grain growth. The nitrogen content range in this invention is defined as N: 0.0080–0.0110%.

[0023] 9) Determination of Ti content

[0024] During the solidification process of molten steel, Ti readily combines with N or C elements to form TiN or Ti(C, N) type non-metallic inclusions. These inclusions are typically angular, blocky structures with high hardness, severely affecting the uniformity of the microstructure. During operation, stress concentration can easily occur at the corners, significantly reducing the fatigue life of bearings. In this invention, the Ti content is defined as ≤0.002%.

[0025] 10) Determination of O content

[0026] Oxygen naturally enters the steel during the steelmaking process and remains there afterward. Oxygen in steel mainly exists as non-metallic inclusions such as SiO2 and Al2O3, with Al2O3 inclusions particularly affecting the fatigue life of bearings. Numerous fatigue life tests have shown that reducing oxygen content significantly improves steel purity, especially by reducing the content and size of brittle oxide inclusions. In this invention, the oxygen content is defined as ≤0.0009%.

[0027] 11) Determination of P and S content

[0028] Phosphorus (P) severely causes segregation during solidification in steel. P dissolves in ferrite, causing grain distortion and coarsening, and increasing cold brittleness. In this invention, the P content is defined as ≤0.020%. Sulfur (S) causes hot brittleness in steel, reducing its ductility and toughness. In this invention, the S content is defined as ≤0.005%.

[0029] 12) Determination of the contents of As, Sn, Sb and Pb

[0030] Trace elements such as As, Sn, Sb, and Pb are all low-melting-point non-ferrous metals. Their presence in steel can cause soft spots and uneven hardness on the surface of parts. Therefore, they are considered harmful elements in steel. In this invention, the content range of these elements is determined as As≤0.04%, Sn≤0.03%, Sb≤0.005%, and Pb≤0.002%.

[0031] In order to obtain high-purity, high-mechanical-performance, fine-grained, and long-life gear steel for new energy vehicle gearboxes, this invention also explicitly defines the following main technical indicators for the steel:

[0032] The low-magnification microstructure of the steel of this invention is inspected and graded according to GB / T226 and GB / T 1979. It is required that the general porosity, central porosity and ingot segregation should not exceed grade 1.5, and shrinkage cavities, cracks and subcutaneous bubbles are not allowed.

[0033] The grain size of the steel in this invention is tested and rated according to GB / T6394 using the carburizing method, requiring a grain size ≥ 7.5.

[0034] The non-metallic inclusions in the steel of this invention are tested according to the K method of DIN 50602 standard, and the levels of various inclusions do not exceed the requirements of Table 1.

[0035] Table 1

[0036]

[0037] The macroscopic defects of the steel of this invention are inspected according to the water immersion high-frequency flaw detection method of SEP 1927 (method for determining the purity of forged and rolled steel bars by ultrasonic immersion testing). The length of a single inclusion does not exceed 1 mm, and the defect index does not exceed 5 mm / dm.3 .

[0038] The end hardenability of the steel of this invention is tested in accordance with GB / T225. The heat treatment regime is as follows: normalizing at 920±10℃, holding for 1 hour, and air cooling; end quenching at 900±5℃ and water cooling. The requirements are J5≥55HRC and J7≥50HRC.

[0039] The mechanical properties of the steel used in this invention are tested in accordance with GB / T228, and the specific requirements are shown in Table 2 below.

[0040] Table 2

[0041]

[0042] The steel used in this invention underwent a rotating bending fatigue life test according to GB / T 4337-2015. Under room temperature (20℃±5℃) and cyclic stress of 1000MPa, data were statistically analyzed according to GB / T 24176-2009. The final requirement is that the rotating bending fatigue life of the steel is ≥1×10⁻⁶. 7 Second-rate.

[0043] This invention relates to a production method for steel used in gearboxes of new energy vehicles. The process flow is as follows: primary refining in a converter or electric furnace → refining in a ladle refining furnace (LF furnace) → vacuum degassing in an RH furnace or VD furnace → continuous casting of CCM (large cross section) → heating and rolling → stacking cooling → isothermal annealing → finishing → surface and internal flaw detection → packaging.

[0044] The main steps are as follows:

[0045] (1) Steel smelting:

[0046] Converter or electric furnace primary refining: High-quality molten iron, low-sulfur scrap steel (containing S≤0.01%), and raw and auxiliary materials are added to the converter or electric furnace for primary refining. Oxygen is blown into the top of the furnace mouth for oxidation reaction, and argon is blown into the bottom for stirring. 35-40 cubic meters of oxygen and 5-6 cubic meters of argon are blown into each ton of steel, and active composite lime (CaO-SiO2-CaF2) is added. Under the combined action of oxygen and argon, harmful elements phosphorus (P≤0.020%) and titanium (Ti≤0.002%) are removed. The final carbon content at the end of the primary refining furnace is 0.15%-0.18%, and the tapping temperature is ≥1640℃. Slag is blocked during tapping, and some alloys are added during tapping (initial composition adjustment). After tapping, the steel is quickly hoisted to the refining LF furnace for smelting.

[0047] Ladle refining: During LF refining, SiC is added for diffusion deoxidation on the molten steel surface, while Al wire is fed in for precipitation deoxidation. A CaO-SiO2-MgO high-performance composite slagging agent is added for diffusion deoxidation and adsorption removal of harmful non-metallic inclusions on the molten steel surface. In the smelting process, SiC (50-100 kg) is first added to the molten steel, followed by Al wire (80-120 m), and then the CaO-SiO2-MgO high-performance composite slagging agent (300-350 kg). The ladle is then covered with a protective cover, and argon gas is introduced to the bottom of the ladle. Electrodes are then inserted into the slag for submerged arc energization. The power is stopped every 15 minutes to measure the temperature and take samples for analysis. The required main elements (V, Nb, etc.) are added according to the target requirements. The number of temperature measurements and sampling in the refining furnace is controlled at 3-4 times until the composition meets the product requirements. The refining time is controlled at over 50 minutes, and the soft blowing time of the molten steel is ≥15 minutes.

[0048] Vacuum degassing: During RH or VD vacuum degassing, the highest vacuum level in the vacuum furnace is ≤1mbar. The molten steel is kept in vacuum circulation for ≥15min to ensure that harmful gases in the steel are effectively removed. After vacuum treatment, argon is blown into the bottom of the ladle. The argon flow rate is controlled so that the molten steel is not exposed to the air. The soft blowing time of the molten steel is ≤15min. At the same time, 80-100m of silicon-calcium wire is fed in to convert Al2O3 or MgO·Al2O3 in the molten steel into calcium aluminates and composite inclusions with lower melting points, further removing harmful inclusions.

[0049] (2) Continuous casting: Argon gas is used for protection during the entire process to prevent secondary pollution and oxidation of molten steel; preferably, the continuous casting adopts a large cross section of 300mm×300mm or above, and low superheat is used for casting (superheat △T≤25℃); the amount of molten steel in the tundish is controlled at 25 to 30 tons, and light reduction control technology is adopted (reduction 13mm to 15mm), and the casting speed is 0.4 to 0.6m / min; an appropriate steel flow ratio of water volume (0.8 to 1.0L / kg) is adopted, and electromagnetic stirring is used in the tundish to break the bridging of columnar crystal zones during the solidification of the continuous casting billet; through the above control technologies, the center segregation of steel is further improved and the uniformity of steel structure is enhanced.

[0050] (3) Heating and rolling: The continuously cast billet is heated in a heating furnace with a neutral or weakly oxidizing atmosphere and then rolled into round bars. The specific rolling process is as follows: The continuously cast billet enters the walking beam heating furnace through the conveyor rollers. The steel is heated to 1220℃~1260℃ in the heating furnace. The total heating time is ≥3h. The initial rolling temperature is 1050℃~1150℃. The final rolling temperature is controlled at ≥950℃. A five-pass reciprocating large reduction technology is adopted (the steel is rotated 90° after each reduction). The reduction amounts of the five passes are 25%~30%, 15%~20%, 20%~25%, 15%~20%, and 10%~15%, respectively. This allows the core structure of the billet to undergo deformation and recrystallization preferentially during the deformation process, thereby making the core structure of the steel more uniform and dense under the action of large reduction. The steel is then rolled alternately by eight horizontal and vertical rolling mills, and finally rolled into round bars with a diameter of φ20mm to φ60mm, which are then cooled in the heat.

[0051] (4) Isothermal annealing: The round bars are cold-loaded and sent to a roller hearth heating furnace with a neutral or weak oxidizing atmosphere by a hoist. The steel is heated to 940℃~960℃ in the heating furnace and held at this temperature for 6-8 hours. After being taken out of the furnace, it is quickly (5-10 minutes) transferred to a furnace at 660℃~680℃ for furnace cooling to room temperature before being taken out of the furnace.

[0052] (5) Finishing: including straightening, chamfering and other finishing processes to ensure that the dimensions, curvature and other indicators meet the requirements.

[0053] (6) 100% non-destructive testing is carried out on the surface and inside. Only products that pass the inspection can be considered qualified products.

[0054] The advantages of this invention are:

[0055] 1. This invention is a high-fatigue-life carburizing steel with a new chemical composition. Compared with the traditional domestic die casting process, this invention has the advantages of good continuity, stable quality and more economical cost. The "vacuum degassing + continuous casting" process not only has high cleanliness, high microstructure uniformity and high hardenability, but also has high mechanical properties and high fatigue life that are not available in the traditional die casting process. This invention is suitable for gear steel for new energy vehicle gearboxes.

[0056] 2. Special smelting process:

[0057] (1) Converter or electric furnace: Add high-quality molten iron, raw materials and low-sulfur scrap steel (containing S≤0.01%) for primary refining. Blow 35-40 cubic meters of oxygen and 5-6 cubic meters of argon per ton of steel, and add active composite lime (CaO-SiO2-CaF2). Under the combined action of oxygen and argon, remove harmful elements phosphorus (P≤0.020%) and titanium (Ti≤0.002%).

[0058] (2) Refining furnace and vacuum degassing furnace: In the LF refining furnace, SiC is added to the surface of the molten steel for diffusion deoxidation, while Al wire is fed in to the molten steel for precipitation deoxidation. A high-performance CaO-SiO2-MgO composite slagging agent is added to the surface of the molten steel for diffusion deoxidation and adsorption removal of harmful non-metallic inclusions. In the RH or VD vacuum degassing furnace, the highest vacuum level is ≤1 mbar. Non-metallic inclusions and harmful gases in the steel are removed in a vacuum environment. After vacuum treatment, the soft blowing time of the molten steel is ≤15 min. After soft blowing, 80-100 m of silicon-calcium wire is fed in to convert Al2O3 or MgO·Al2O3 in the molten steel into lower melting point calcium aluminates and composite inclusions, further removing harmful inclusions. This makes the molten steel cleaner, meeting the requirements for long fatigue life gearbox steel for new energy vehicles.

[0059] (3) Argon gas is used for continuous casting to protect the molten steel from secondary oxidation and contamination. The continuous casting adopts a large cross section of 300mm×300mm and above, and low superheat casting is adopted (superheat △T≤25℃). The amount of molten steel in the tundish is controlled at 25 to 30 tons, and light reduction control technology is adopted (reduction 13mm to 15mm), and the casting speed is 0.4 to 0.6m / min. An appropriate steel flow ratio (0.8 to 1.0L / kg) is adopted to further improve the center of the steel and the segregation of the ingot shape, and improve the uniformity of the steel structure.

[0060] 3. Special rolling process:

[0061] A five-pass reciprocating large reduction technique is employed (the steel is rotated 90° after each reduction pass). The reduction amounts for the five passes are 25%–30%, 15%–20%, 20%–25%, 15%–20%, and 10%–15%, respectively. This allows the core structure of the cast billet to preferentially deform and recrystallize during the deformation process, resulting in a more uniform and dense core structure under the large reduction. The steel is then alternately rolled through eight horizontal and vertical rolling mills to finally produce round bars with a diameter of Φ20mm–Φ60mm.

[0062] 4. Special isothermal annealing

[0063] The steel is heated to 940℃~960℃ in a heating furnace and held at this temperature for 6-8 hours. After being taken out of the furnace, it is quickly transferred (5-10 minutes) to a furnace at 660℃~680℃ for furnace cooling to room temperature before being taken out. The purpose of isothermal annealing is to make the microstructure more uniform, and after annealing, a uniform ferrite and pearlite microstructure is obtained. Detailed Implementation

[0064] The present invention will be further described in detail below with reference to the embodiments. The embodiments are exemplary and intended to explain the present invention, but should not be construed as limiting the present invention.

[0065] The chemical composition (wt%) of the steel used in the gearboxes of new energy vehicles in various embodiments of the present invention is shown in Tables 3 and 4.

[0066] Table 3

[0067]

[0068] Table 4

[0069]

[0070] Table 5 Non-metallic inclusions in steel from various embodiments

[0071]

[0072] Table 6. Low-magnification data of steel in each embodiment.

[0073]

[0074] Table 7 Grain size data of steel samples

[0075]

[0076] Table 8. Water immersion high-frequency flaw detection data of steel in each embodiment.

[0077]

[0078] Table 9 Mechanical property data of steel in each embodiment

[0079]

[0080] Table 10. End-hardenability data of steels in each embodiment.

[0081]

[0082] Table 11 Rotational bending fatigue life data of steel in each embodiment

[0083]

[0084] This invention relates to a production method for steel used in gearboxes of new energy vehicles. The process flow is as follows: primary refining in a converter or electric furnace → refining in a ladle refining furnace (LF furnace) → vacuum degassing in an RH furnace or VD furnace → continuous casting of CCM (large cross section) → heating and rolling → stacking cooling → isothermal annealing → finishing → surface and internal flaw detection → packaging.

[0085] Specifically, high-quality molten iron, raw materials, and low-sulfur scrap steel (containing S≤0.01%) are added together to a converter or electric furnace for primary refining. Oxygen is blown into the top of the furnace mouth for oxidation, and argon is blown into the bottom for stirring. 35–40 cubic meters of oxygen and 5–6 cubic meters of argon are blown into each ton of steel, along with 2–3 tons of active composite lime (CaO-SiO2-CaF2). Smelting is carried out under the combined action of oxygen and argon. The final carbon content at the tapping point of the primary refining furnace is 0.15%–0.18%, the tapping temperature is 1645℃–1650℃, the final P content is controlled at ≤0.020%, and the final Ti content is ≤0.002%. Slag is blocked during tapping, and some alloys are added during tapping (initial composition adjustment). After tapping, the steel is quickly hoisted to the refining LF furnace. In the LF refining furnace, SiC (50–100 kg) is first added to the molten steel, followed by feeding Al wire (80–120 kg). Add CaO-SiO2-MgO high-performance composite slag-forming agent (300-350 kg) again, cover with a steel ladle protective cover, connect argon gas to the bottom of the steel ladle, and then insert the electrode into the slag for submerged arc energization. Every 15 minutes, stop the power to measure the temperature of the molten steel and take samples for analysis. Add the required main elements (V, Nb, etc.) according to the target requirements. The number of temperature measurement and sampling in the refining furnace is controlled at 3-4 times until the composition meets the product requirements. Refining time should be controlled at over 50 minutes, and soft blowing time of molten steel should be ≥15 minutes. During RH or VD vacuum degassing, the highest vacuum degree in the vacuum furnace should be ≤1 mbar, and the vacuum circulation treatment time of molten steel should be maintained at ≥15 minutes to ensure effective removal of harmful gases from the steel. After vacuum treatment, argon gas should be blown into the bottom of the ladle, and the argon gas flow rate should be controlled so that the molten steel is not exposed to air. The soft blowing time of molten steel should be ≤15 minutes, and 80-100m of silicon-calcium wire should be fed in simultaneously. Continuous casting should use a large cross-section of 300mm×300mm or larger, and low superheat casting should be used (superheat ΔT≤25℃). The amount of molten steel in the tundish should be controlled at 25-30 tons, and light steel should be used. The reduction control technology (reduction amount 13mm~15mm), casting speed 0.4~0.6m / min; adopting an appropriate steel flow ratio and water volume (0.8~1.0L / kg); the continuously cast billet enters the walking beam furnace through the conveyor roller conveyor, the steel is heated to 1220℃~1260℃ in the furnace, the total heating time is ≥3h, the initial rolling temperature is 1050℃~1150℃, the final rolling temperature is controlled ≥950℃, and a five-pass reciprocating large reduction technology is adopted (the steel is rotated 90° after each reduction), the reduction amounts of the five passes are 25%~30%, 15%~20%, 20%~25%, 15%~20%, and 10%~15%, respectively.The steel is then rolled alternately by eight horizontal and vertical rolling mills to finally produce round bars with a diameter of Φ20mm to Φ60mm. After being removed from the line, the bars are stacked and cooled to room temperature. The round bars are then cold-loaded by a hoist and sent to a roller hearth furnace with a neutral or weakly oxidizing atmosphere. The steel is heated to 940℃ to 960℃ in the furnace and held at this temperature for 6-8 hours. After being removed from the furnace, the bars are quickly transferred (5-10 minutes) to a furnace at 660℃ to 680℃ for in-furnace cooling to room temperature. After being stacked and cooled to room temperature, the bars are then subjected to subsequent flaw detection and finishing processes.

[0086] As shown in Tables 3, 4, 5, 6, 7, 8, 9, 10, and 11, the gear steel for new energy vehicle gearboxes in the various embodiments of the present invention achieves the internationally advanced level of control over harmful elements such as phosphorus, sulfur, oxygen, titanium, and non-metallic inclusions. From the results of low magnification, mechanical properties, end hardenability, microstructure grain size, and rotational bending fatigue life, the low magnification mass, hardenability, microstructure density, mechanical properties, and rotational bending fatigue life of the present invention all meet the requirements for gear steel for new energy vehicle gearboxes.

[0087] Meanwhile, each embodiment was subjected to water immersion high-frequency flaw detection according to the SEP 1927 method, and the macroscopic inclusions achieved zero defects.

[0088] In summary, the new energy vehicle gearbox steel in the various embodiments of the present invention adopts a "vacuum degassing + continuous casting" process route with good continuity, stable quality and more economical cost. Through unique chemical composition design, special smelting process, rolling process and heat treatment process, a steel with high cleanliness, high microstructure uniformity, high hardenability, high mechanical properties and high fatigue life is obtained, resulting in a new type of steel with high efficiency, large capacity, low cost and high quality production mode.

[0089] In addition to the above embodiments, the present invention also includes other embodiments. 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 method for producing steel for gearboxes in new energy vehicles, characterized in that: The elemental composition of the steel, by mass percentage, is: C: 0.25–0.31%, Si: 0.2–0.4%, Mn: 1.0–1.3%, Cr: 1.0–1.3%, Ni: 0.2–0.3%, Nb: 0.010–0.020%, V: 0.03–0.06%, Al: 0.02–0.05%, N: 0.0080–0.0110%, S≤0.005%, P≤0.020%, Cu≤0.20%, Mo≤0.10%, Ti≤0.002%, O≤0.0009%, As≤0.04%, Sn≤0.03%, Sb≤0.005%, Pb≤0.002%, with the balance being Fe and unavoidable impurities; (Steps) include, I. Steelmaking: This includes primary refining, refining, and vacuum degassing. Primary refining involves refining high-quality molten iron, scrap steel with S ≤ 0.01%, and raw and auxiliary materials together. Oxygen is blown into the top of the furnace mouth for oxidation, while argon is blown into the bottom for stirring. Active composite lime (CaO-SiO2-CaF2) is added to remove harmful elements such as phosphorus and titanium. The final carbon content at the tapping point of the primary refining furnace is 0.15% to 0.18%, and the tapping temperature is ≥ 1640℃. Slag is blocked during tapping, and some alloying elements are added during tapping. After tapping, the steel is quickly hoisted to the refining furnace. Steel ladle refining furnace refining: SiC is added to the surface of molten steel for diffusion deoxidation, while Al wire is fed in to the molten steel for precipitation deoxidation. CaO-SiO2-MgO high-performance composite slag-forming agent is added to the surface of molten steel for diffusion deoxidation and adsorption removal of harmful non-metallic inclusions. In the smelting process, SiC is first added to the molten steel, then Al wire is fed in, and then CaO-SiO2-MgO is added again. The steel ladle is covered with a protective cover, and argon gas is connected to the bottom of the steel ladle. Then, electrodes are inserted into the slag for submerged arc energization. The power is stopped every 15 minutes to measure the temperature of the molten steel and take samples for analysis. The required alloying elements are added according to the target requirements. The number of temperature measurements and sampling in the refining furnace is controlled at 3 to 4 times until the composition meets the product requirements. Vacuum degassing: The highest vacuum degree in the vacuum furnace is ≤1mbar. The molten steel is kept in vacuum circulation for ≥15min. After the vacuum treatment is completed, argon gas is blown into the bottom of the ladle. The argon gas flow rate is controlled so that the molten steel is not exposed to the air. The soft blowing time of the molten steel is ≤15min. At the same time, silicon-calcium wire is fed in to convert Al2O3 or MgO·Al2O3 in the molten steel into calcium aluminates and composite inclusions with lower melting points. II. Continuous Casting: Argon gas is used for the entire continuous casting process to form continuous casting billets. Low superheat casting is used: superheat △T≤25℃; the amount of molten steel in the tundish is controlled at 25-30 tons, and light reduction control technology is used: reduction 13mm-15mm, casting speed: 0.4-0.6m / min; an appropriate steel flow ratio of 0.8-1.0L / kg is used, and electromagnetic stirring is used in the tundish to break the bridging of columnar crystal zones during the solidification of the continuous casting billet; III. Heating and Rolling: The continuously cast billet is heated in a heating furnace with a neutral or weakly oxidizing atmosphere and then rolled into round bars. The rolling is carried out in the austenitic phase region. The continuously cast billet enters the walking beam heating furnace through the conveyor rollers. The steel is heated to 1220℃~1260℃ in the heating furnace. The total heating time is ≥3h. The initial rolling temperature is 1050℃~1150℃. The final rolling temperature is controlled at ≥950℃. The rolling method is a five-pass reciprocating large reduction rolling method: after each reduction, the steel is rotated 90°. The reduction amounts for the five passes are 25%~30%, 15%~20%, 20%~25%, 15%~20%, and 10%~15%, respectively. Then, it is rolled alternately by 8 horizontal and vertical rolling mills. Finally, it is rolled into round bars with a diameter of φ20mm~φ60mm and then cooled off the line. IV. Isothermal Annealing: The round steel is heated in a heating furnace to 940℃~960℃ and held at this temperature for 6-8 hours. After being taken out of the furnace, it is quickly transferred to a furnace at 660℃~680℃ within 5-10 minutes for furnace cooling to room temperature before being taken out of the furnace. The end-hardenability of the steel is tested according to GB / T225. The heat treatment process is as follows: normalizing temperature 920±10℃, holding for 1 hour, air cooling; end-quenching temperature 900±5℃, water cooling, J5≥55HRC, J7≥50HRC. Mechanical properties are tested according to GB / T228. The heat treatment process is as follows: first quenching: oil quenching at 880±20℃; second quenching: oil quenching at 790±20℃; tempering at 170~200℃, air cooling. Mechanical properties must meet the following requirements: tensile strength R... m ≥1450MPa, elongation A≥20%, reduction of shrinkage Z≥70%, impact absorbed energy KU2≥150J; Rotational bending fatigue life test was conducted according to GB / T 4337-2015, under room temperature: 20℃±5℃ and 1000MPa cyclic stress conditions. Data statistics and analysis were performed according to GB / T 24176-2009, and the rotational bending fatigue life of the steel was ≥1×10⁻⁶. 7 Second-rate.

2. The method according to claim 1, characterized in that: In the initial refining stage of step one, 35-40 cubic meters of oxygen and 5-6 cubic meters of argon are blown into each ton of steel to reduce the P content in the molten iron to P≤0.020% and the titanium content to Ti≤0.002%. During the refining process, 50-100 kg of SiC is first added to the molten steel, followed by 80-120 m of Al wire, and then 300-350 kg of CaO-SiO2-MgO high-performance composite slagging agent is added. The refining time is controlled at more than 50 minutes, and the soft blowing time of the molten steel is ≥15 minutes. After vacuum degassing, 80-100 m of silicon-calcium wire is fed into the molten steel.

3. The method according to claim 1, characterized in that: Step four: The round bars are cold-loaded by a hoist and sent to a roller hearth furnace with a neutral or weakly oxidizing atmosphere for isothermal annealing.

4. The method according to claim 1, characterized in that: Step 2: Use continuous casting process to cast molten steel into continuously cast square billets with a cross section of 300mm×300mm or larger.

5. The method according to claim 1, characterized in that: After isothermal annealing in step four, the finishing of the round steel also includes straightening and chamfering to meet dimensional and curvature requirements.

6. The method according to claim 1, characterized in that: The low-magnification microstructure of the steel is inspected and graded according to GB / T226 and GB / T 1979, meeting the requirements that general porosity, central porosity, and ingot segregation do not exceed grade 1.5, and that shrinkage cavities, cracks, and subcutaneous bubbles are not present. Non-metallic inclusions are inspected according to the K method of DIN 50602 standard, and the grades of various inclusions meet the following requirements: K1(S)≤1.0, K1(O)≤2.0, K3(S)≤1.0, K3(O)≤1.

0. Macroscopic defects are inspected according to the water immersion high-frequency flaw detection method of SEP 1927 "Water Immersion Ultrasonic Determination Method for Purity of Forged and Rolled Steel Bars", with the length of a single inclusion not exceeding 1 mm and the defect index not exceeding 5 mm / dm. 3 .

7. The method according to claim 1, characterized in that: The steel is tested and rated according to GB / T6394 using the carburizing method, with a grain size ≥7.5.

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