Steel for high-power automatic driving agricultural machine gearbox shaft gear and manufacturing method thereof

Abstract

The invention relates to steel for a shaft gear of a gearbox of a high-power automatic driving agricultural machine and a manufacturing method of the steel, and belongs to the technical field of iron-based alloys. The hot-rolled steel comprises the following chemical components in percentage by weight: 0.28-0.34% of C, 0.25-0.45% of Si, 1.30-1.60% of Mn, less than or equal to 0.007% of P, 0.015-0.040% of S, 0.30-0.70% of Cr, 1.10-1.30% of Ni, 0.10-0.20% of Mo, 0.01-0.03% of Al, 0.007-0.020% of N, 0.02-0.04% of Nb, less than or equal to 0.0008% of O and the balance of Fe and inevitable impurity elements, the hot-rolled steel is delivered, the size of an NbN precipitated phase is 30-50nm, the distribution of the NbN precipitated phase is 3.5 * 10 < 4 >-4.5 * 10 < 4 > / mm, the austenite grain size of the hot-rolled steel is 15-25 the low-temperature impact energy at-50 DEG C is not less than 60J.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a steel for shaft gears and its manufacturing method. Background Technology

[0002] High-powered, autonomous agricultural machinery undertakes heavy workloads in agricultural production. As a core transmission component, the gearbox gears of high-powered agricultural machinery must withstand enormous torque, significant impact loads, and meshing friction during these demanding tasks. Therefore, the gearbox gears of high-powered agricultural machinery not only need high surface strength and hardness to resist wear and deformation, but also excellent toughness to prevent brittle fracture from impact, and outstanding fatigue resistance to cope with long-term, heavy workloads.

[0003] For the service conditions of components such as shafts and gears in high-power gearboxes, it is necessary to develop corresponding steels for machining, especially a shaft and gear steel with a shallow surface oxide decarburization layer. Summary of the Invention

[0004] This invention provides a steel for the gearbox shaft of a high-power autonomous agricultural machine and its manufacturing method.

[0005] To ensure the performance and fatigue life of the steel used in the gearbox shafts of high-power autonomous agricultural machinery, this invention employs a novel manufacturing method to produce steel with low oxygen and phosphorus content, small oxide size, reasonable NbN precipitate size and distribution, fine and stable austenite grains, and an extremely shallow oxide decarburized layer on the steel surface. The phosphorus content of the hot-rolled steel is ≤0.007%, the oxygen content is ≤0.0008%, the non-metallic inclusions (Bcoarse ≤20μm, Bfine ≤50μm) are controlled, the NbN precipitate size is controlled within 30-50nm, and the NbN distribution in the hot-rolled steel is controlled within 3.5×103.5 4 -4.5×10 4 The austenite grain size of hot-rolled steel is 15-25μm per mm³.

[0006] The strict control of steel surface decarburization in this invention stems from the special manufacturing process and performance requirements of the final part. This part requires high-temperature heating and forging during subsequent processing, and employs differentiated heat treatment: the gear portion undergoes carburizing and quenching to obtain high surface hardness and wear resistance, while the shaft portion is not carburized to maintain its high strength and toughness; the steel's impact energy at -50℃ is ≥60J.

[0007] This invention emphasizes the crucial importance of strictly controlling decarburization to ensure the performance of the final shaft gear component. During subsequent manufacturing, the gear portion of this component undergoes carburizing to achieve high hardness, while certain areas of the shaft are left uncarburized to maintain toughness. If excessive decarburization occurs during manufacturing, a softened layer forms on the surface. This layer cannot be removed from the uncarburized shaft areas by subsequent processes, directly creating weak points and significantly reducing the component's load-bearing capacity and fatigue life. Therefore, this invention controls the initial decarburized layer depth of the steel to ≤10μm through end-to-end process control, fundamentally preventing performance defects in the final product due to excessive decarburization.

[0008] The technical solution adopted in this invention is as follows: a steel for the gearbox shaft of a high-power automatic agricultural machine, with the following chemical composition by weight percentage: C: 0.28-0.34%, Si: 0.25-0.45%, Mn: 1.30-1.60%, P: ≤0.007%, S: 0.015-0.040%, Cr: 0.30-0.70%, Ni: 1.10-1.30%, Mo: 0.10-0.20%, Al: 0.01-0.03%, N: 0.007-0.020%, Nb: 0.02-0.04%, O: ≤0.0008%, with the balance being Fe and unavoidable impurity elements.

[0009] The main functions and design basis of each chemical element in the steel of this invention are as follows: C: Carbon is the key element determining the strength and hardness of the steel in this invention. Carbon strengthens the shaft teeth by forming carbides through solid solution, thereby improving the surface strength and hardness. When the carbon content is below 0.28%, the strength and hardness of the shaft teeth are insufficient, and the shaft teeth are prone to wear and deformation during heavy operations. When the carbon content is above 0.34%, it leads to a decrease in the toughness of the shaft teeth, and the teeth are at great risk of impact fracture, making them unable to withstand the huge impact loads of high-powered, autonomous agricultural machinery. Therefore, the carbon content of the steel in this invention is selected in the range of 0.28% to 0.34%. Silicon (Si): Silicon, as a deoxidizing element, can effectively remove oxides from steel. Too low a silicon content hinders oxide removal, while too high a silicon content causes intergranular oxidation during the carburizing process of the shaft teeth, leading to reduced fatigue strength and lifespan. Therefore, the silicon content of the steel in this invention is selected in the range of 0.25% to 0.45%.

[0010] Mn: Manganese can significantly improve the strength and toughness of steel. It enhances the strength of ferrite through solid solution strengthening, while lowering the brittle transition temperature and improving the low-temperature toughness of the steel. However, excessive manganese content leads to coarse grain size, ultimately reducing the low-temperature impact performance of the steel. Therefore, the manganese content of the steel in this invention is selected in the range of 1.30–1.60%. P: Phosphorus is a harmful element in steel, easily accumulating at grain boundaries, leading to brittleness and significantly reducing the low-temperature impact performance of the steel. For high-power, automated agricultural machine gearbox shafts that need to withstand huge impact loads at low temperatures, the phosphorus content must be strictly controlled. This invention controls the phosphorus content to ≤0.007%, thereby avoiding the adverse effects of brittleness on the performance of the shafts. S: By adding an appropriate amount of sulfur to the steel of this invention, sulfur combines with manganese to form free-machining Class A non-metallic inclusions MnS, which significantly improves the machinability of the steel. Therefore, the sulfur content of the steel of this invention is selected in the range of 0.015% to 0.040%.

[0011] Cr: The purpose of adding a small amount of chromium to the steel of this invention is to improve the hardenability of the steel, while also enhancing its corrosion resistance and extending the service life of the shaft gears. Therefore, the chromium content of the steel of this invention is selected in the range of 0.30% to 0.70%. Ni: The addition of nickel to the steel of this invention can significantly improve the toughness of the steel, especially its low-temperature toughness. Simultaneously, it also enhances the strength of the steel. Adding sufficient nickel to the steel of this invention allows the steel to maintain excellent low-temperature toughness while possessing high strength, preventing brittle fracture of the shaft teeth under impact loads. Therefore, the nickel content of the steel of this invention is selected in the range of 1.10% to 1.30%. Mo: The addition of a small amount of molybdenum to the steel of this invention can refine the grain structure of the steel, improve its low-temperature stability, and ensure stable performance of the shaft gear during long-term operation in low-temperature environments. Therefore, the molybdenum content of the steel of this invention is selected in the range of 0.10% to 0.20%. Al: Aluminum in the steel of this invention is a strong deoxidizer, effectively removing oxygen and reducing oxide inclusions. If the aluminum content is too low, the deoxidation effect is not significant; if it is too high, excessive Class B non-metallic inclusions will form, reducing the fatigue life of the steel. Therefore, the aluminum content of the steel of this invention is selected in the range of 0.01–0.03%. Nb, N: Nitrogen in the steel reacts with niobium to form niobium nitride. Through a reasonable manufacturing method, the niobium nitride in the steel is made to be small and appropriately sized and evenly distributed. This small and evenly distributed niobium nitride can pin grain boundaries, resulting in fine and stable grains, inhibiting austenite grain growth, and thus improving the strength and low-temperature toughness of the steel. Therefore, the niobium content of the steel of this invention is selected in the range of 0.02–0.04%, and the nitrogen content is selected in the range of 0.007–0.020%.

[0012] Oxygen (O) in steel mainly exists as type B oxide inclusions, which disrupt the continuity of the steel, reduce its toughness and strength, and are extremely detrimental to the fatigue life of the gearbox shafts and teeth in high-power, automated agricultural machinery. This invention strictly controls the oxygen content to ≤0.0008% to reduce type B non-metallic inclusions in the steel, ensuring excellent fatigue resistance of the gearbox shafts and teeth, and meeting the demands of long-term, heavy-duty operation of high-power, automated agricultural machinery.

[0013] The manufacturing method of the steel for the gearbox shaft of the high-power autonomous agricultural machine includes the following steps: (1) The converter adopts a segmented dephosphorization and temperature control process. In the initial stage, CO2 gas is blown into the bottom of the converter molten pool, preferably with a gas flow rate of 0.3-0.5 m³ / min·t. The local temperature of the molten pool is controlled at 1320-1370℃ by utilizing the weak oxidizing and endothermic properties of CO2. At the same time, the lime dissolution is promoted, and the slag basicity is increased to 3.0-3.5, which focuses on removing silicon from the molten iron and creating basic conditions for the dephosphorization reaction. In the main dephosphorization stage, the oxygen supply intensity of the oxygen lance is gradually adjusted to 3.6-3.9 m³ / min·t, and the temperature of the molten pool is controlled at 1430-1480℃. The FeO content of the slag is maintained at 23%-25% to maintain high oxidizing properties and enhance the transfer reaction of phosphorus to the slag. Final stage: Switch the bottom blowing gas to Ar gas, control the flow rate to 0.10-0.15 m³ / min·t, and adjust the oxygen lance position to precisely control the molten pool temperature at 1600-1650℃; suppress phosphorus reversion by stabilizing slag composition, and keep the phosphorus content of molten steel stably below 0.007%.

[0014] (2) During the refining process, the molten steel in the furnace is heated to 1580-1595℃. A wire feeder is used to feed Al-Si-Ca composite deoxidation line with a diameter of Φ12-15mm into the furnace at a speed of 0.5-0.8m / s for precipitation deoxidation. At the same time, 0.12-0.15kg / t of SiC powder with a diameter of Φ10-20μm is floated into the furnace surface for diffusion deoxidation. Then, 1.2-1.5kg / t of a new type of rare earth composite deoxidizer is added for deep deoxidation. The new type of rare earth composite deoxidizer has a mass percentage of 40% Mg-Al-Li alloy, 30% CeO2, 15% Y2O3, and 15% CaF2. It is used in conjunction with a large flow rate of bottom blowing argon. The flow rate of bottom blowing argon is 0.9-1.1L / min·t, and the stirring time is 12-15min. The above process reduces the oxygen content to below 8ppm.

[0015] (3) The refined molten steel is vacuum degassed, preferably in an RH furnace. Nitrogen is used as the RH booster gas to reduce the vacuum level in the furnace to below 63 Pa. The low vacuum environment is maintained for 15-20 min. Under high vacuum, NbFe alloy is added in three stages, with an interval of 4 min between each addition. The temperature is measured immediately after the NbFe alloy is added to control the temperature of the molten steel in the furnace to decrease slowly in steps. The temperature drop in the three stages is controlled at 8-12℃, so that the size of the NbN precipitate phase is controlled at 30-50 nm and the distribution of NbN is controlled at 2×10⁻⁶. 4 -6×10 4 pcs / mm³.

[0016] (4) The continuous casting crystallizer adopts a low-frequency alternating magnetic field with an alternating electromagnetic frequency of 2.5-3.5Hz and an alternating electromagnetic induction intensity of 0.32-0.38T. Furthermore, the continuous casting superheat is strictly controlled at 18-28℃, the continuous casting speed is strictly controlled at 0.5-0.6m / min, and the secondary cooling water ratio is strictly controlled at 0.35-0.45 L / kg. This promotes a more uniform distribution of NbN precipitates within the continuously cast billet, with the NbN distribution controlled at 3.5×10⁻⁶. 4 -4.5×10 4 The grain size is reduced to 80-100μm by the number of grains per mm³, resulting in a continuous casting billet with a size of 295mm*345mm.

[0017] (5) When the continuously cast billet is heated in the heating furnace, a segmented oxygen content control process is preferred: the preheating section temperature is 680-980℃, the preheating time is 50-60min, and the oxygen content in the furnace is controlled at a low level of 0.5-0.8%, with high oxidation sensitivity in the preheating temperature range; the heating section temperature is 980-1130℃, the heating time is 80-100min, and the oxygen content in the furnace is controlled at a high level of 2.3-2.8%, with lower oxidation sensitivity in the heating temperature range. The heating section temperature range must ensure sufficient CO combustion in the furnace to rapidly improve the casting efficiency. The billet temperature is 1130-1190℃, the soaking time is 20-30min, and the oxygen content in the furnace is controlled at a low level of 0.3-0.5%. The oxidation sensitivity in the soaking temperature range is extremely high. The final oxide layer thickness on the surface of the billet after heating is controlled to be 0.5-0.8mm. The oxide and decarburized layer on the surface of the billet is completely removed by high-pressure water descaling. It is preferred to use high-pressure water of 28-33MPa, with a single nozzle flow rate of 1.5-2.5L / s and an impact force of 60-80N / cm² when sprayed onto the surface of the billet.

[0018] (6) After descaling with high-pressure water, the billet is rough rolled at 1030-1070℃ in 3 passes. The deformation of a single pass is 18-23%, and the cumulative deformation is ≥56%. The billet grains are broken down to 50-70μm by utilizing the dynamic recrystallization effect of high-temperature austenite. The intermediate rolling temperature is controlled at 980-1020℃ and rolled in 4 passes. The deformation of a single pass is 10-13%, and the cumulative deformation is ≥45%. The billet grains are broken down to 30-40μm by utilizing the dynamic recrystallization effect of high-temperature austenite. The finishing rolling temperature is controlled at 920-960℃ and rolled in 5 passes. The deformation of a single pass is 5-8%, and the cumulative deformation is ≥30%. The billet grains are broken down to 15-25μm by utilizing the dynamic recrystallization effect of high-temperature austenite. The entire roughing, intermediate, and finishing rolling process relies on the high-density dispersion of NbN precipitates in the steel produced during continuous casting. These NbN precipitates pin the grain boundaries, effectively inhibiting dynamic grain growth during high-temperature rolling. The total time from roughing to finishing is controlled within 5-8 minutes, with a single-pass rolling interval of 12-18 seconds. Nitrogen gas is injected into the steel surface during rolling to form a continuous and stable gas curtain covering the surface without dead zones. The preferred nitrogen pressure is 0.6-0.8 MPa, the single-nozzle flow rate is 180-200 L / min, and the injection angle is 45°. After rolling, the steel is immediately placed in a slow-cooling furnace under a nitrogen protective atmosphere to slowly cool to room temperature. This prevents surface oxidation and decarburization of the steel during both the rolling and cooling processes.

[0019] This invention addresses the stringent operating conditions of gearbox shafts and gears in high-power autonomous agricultural machinery by optimizing the chemical composition of the steel (optimizing the ratio of C, Si, Mn, Cr, Ni, Mo, and Al elements, reducing the content of harmful P and O elements through reasonable processing methods, and optimizing the ratio of Nb and N microalloying elements). A novel steelmaking process is employed, with a converter segmented dephosphorization and temperature control process to maintain a phosphorus content ≤0.007%. The refining process utilizes a three-stage deep deoxidation process using Al-Si-Ca, SiC powder, and a novel rare-earth composite deoxidizer, combined with high-flow-rate bottom-blowing argon to maintain an oxygen content ≤0.0008%. The RH furnace undergoes vacuum degassing and employs stepped temperature control, precisely controlling the timing of Nb addition to precisely control the size and distribution of the NbN precipitates. Continuous casting utilizes a low-frequency alternating magnetic field and optimized process parameters, resulting in a more uniform NbN distribution and refined grain structure in the continuously cast billet. The heating furnace employs staged control of heating time, temperature, and oxygen content to effectively control the depth of the decarburized layer on the billet surface, coupled with high-pressure water dephosphorization for efficient removal of the decarburized layer. The rolling process utilizes optimized multi-pass temperature control and deformation, along with a nitrogen protective gas curtain, to refine the steel grain structure while effectively inhibiting further oxidation and decarburization on the steel surface. The final high-power autonomous agricultural machine gearbox shaft gear steel produced has the following characteristics: phosphorus content ≤0.007%, oxygen content ≤0.0008%, non-metallic inclusions (B coarse ≤20μm, B fine ≤50μm), NbN precipitate size controlled at 30-50nm, and NbN distribution controlled at 3.5×10⁻⁶. 4 -4.5×10 4 With excellent properties such as austenitic grain size of 15-25μm, total depth of decarburized oxide layer on steel surface ≤10μm, and low-temperature impact energy ≥60J at -50℃, it can meet the requirements of high strength, high toughness, low-temperature impact resistance and fatigue resistance for gearbox shafts and gears of high-power autonomous agricultural machinery.

[0020] Compared with the prior art, the advantages of the present invention are as follows: (1) Innovatively adopted converter segmented dephosphorization temperature control process and refining furnace deep deoxidation technology: by using converter segmented bottom blowing gas CO2 and Ar gas and adjusting the oxygen lance position, the phosphorus content was controlled to be stable at ≤0.007%; in the refining stage, a three-stage deoxidation process of Al-Si-Ca composite deoxidation line, SiC powder diffusion deoxidation and new rare earth composite deoxidizer (Mg-Al-Li / CeO2 / Y2O3 / CaF2) was used, combined with high flow rate bottom blowing argon stirring, and the oxygen content was strictly controlled at ≤0.0008%, which significantly reduced the B-type oxide inclusions in the steel (B coarse ≤20μm, B fine ≤50μm), and greatly improved the fatigue resistance of the steel. (2) Precisely controlled the morphology and distribution of NbN precipitates in the steel: in the vacuum degassing process of RH furnace, NbFe alloy was added in stages by step temperature control, so that the size of NbN precipitates was refined to 30-50nm and the density was controlled at 2×10 4 -6×10 4 The uniformity of NbN distribution (3.5 × 10⁻⁶ mm³) was further controlled during the continuous casting stage by using a low-frequency alternating magnetic field (2.5-3.5 Hz, 0.32-0.38 T), combined with low superheat (18-28℃), low casting speed (0.5-0.6 m / min), and secondary cooling water control (0.35-0.45 L / kg). 4 -4.5×10 4 (3) Achieved grain refinement and shallow surface oxidation decarburization throughout the entire process: The heating furnace controls the heating temperature, heating time and low oxygen content in the furnace in stages, and uses high-pressure water dephosphorization to efficiently remove the oxidation decarburization layer; The rolling process uses NbN pinning effect to further refine the steel grains, and through multi-pass dynamic recrystallization (the cumulative deformation of roughing / medium / finishing rolling is ≥56% / 45% / 30%), the austenite grains of the steel are refined to 15-25μm. At the same time, nitrogen protective gas curtain and slow cooling furnace protection are used to ensure that the total depth of the oxidation decarburization layer on the steel surface is controlled to ≤10μm. (4) The steel produced in the end has excellent low temperature toughness (impact energy at -50℃ is ensured to be ≥60J). The steel of this invention fully meets the extreme working conditions requirements of high strength, high toughness and low temperature impact resistance of the gearbox shaft gear of high-power autonomous driving agricultural machine. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to embodiments. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Example 1 and Example 2: The two embodiments involve a manufacturing method for high-power, automated agricultural vehicle gearbox shaft gear steel: 150t converter smelting → 150t ladle refining → RH furnace vacuum degassing → continuous casting into square billets (295mm*345mm) → heating in a heating furnace → rough rolling → intermediate rolling → finish rolling → nitrogen protection → slow cooling in a slow cooling furnace to room temperature. Two batches of high-power, automated agricultural vehicle gearbox shaft gear steel were manufactured.

[0022] The converter employs a segmented dephosphorization temperature control process. In the initial stage: CO2 gas is blown into the bottom of the converter's molten pool, with the gas flow rate controlled at 0.3 m³ / min·t (Example 1) and 0.5 m³ / min·t (Example 2). Utilizing the weak oxidizing and endothermic properties of CO2, the local temperature of the molten pool is controlled at 1321℃ (Example 1) and 1368℃ (Example 2). Simultaneously, lime dissolution is promoted, increasing the slag basicity to 3.0 (Example 1) and 3.5 (Example 2), primarily removing silicon from the molten iron and creating the necessary conditions for the dephosphorization reaction. In the main dephosphorization stage: the oxygen lance supply intensity is gradually adjusted to 3.6 m³ / min·t. 3 / min·t (Example 1), 3.9m 3 / min·t (Example 2), the molten pool temperature was adjusted to 1433℃ (Example 1) and 1479℃ (Example 2); the FeO content in the slag was maintained at 23% (Example 1) and 25% (Example 2) to maintain high oxidizing power and enhance the transfer reaction of phosphorus to the slag. Final stage: the bottom blowing gas was switched to Ar gas, and the flow rate was controlled at 0.10 m³ / min·t. 3 / min·t (Example 1), 0.15 m 3 / min·t (Example 2), in conjunction with adjusting the oxygen lance position, the molten pool temperature is precisely controlled at 1602℃ (Example 1) and 1650℃ (Example 2); by stabilizing the slag composition and suppressing phosphorus reversion, the phosphorus content of the molten steel is stably controlled below 0.007%.

[0023] In the refining process, the molten steel in the furnace was heated to 1580℃ (Example 1) and 1595℃ (Example 2). An Al-Si-Ca composite deoxidation wire with a diameter of Φ12mm (Example 1) and Φ15mm (Example 2) was fed in at a speed of 0.5m / s (Example 1) and 0.8m / s (Example 2) respectively for precipitation deoxidation. Simultaneously, 0.12kg / t (Example 1) and 0.15kg / t (Example 2) of SiC powder with a diameter of Φ10-20μm was floated into the furnace surface for diffusion deoxidation. Subsequently, 1.21kg / t (Example 2) of... Example 1) A novel rare earth composite deoxidizer with a mass percentage of 1.50 kg / t (Example 2) was used for deep deoxidation. The novel rare earth composite deoxidizer consisted of 40% Mg-Al-Li alloy, 30% CeO2, 15% Y2O3, and 15% CaF2, and was used in combination with a high flow rate of bottom-blown argon. The flow rate of bottom-blown argon was 0.9 L / min·t (Example 1) and 1.1 L / min·t (Example 2), and the stirring time was 12 min (Example 1) and 15 min (Example 2). The above process reduced the oxygen content to below 8 ppm.

[0024] The refined molten steel was degassed in an RH furnace under vacuum. Nitrogen was used as the booster gas in the RH furnace to lower the vacuum level to below 63 Pa. This low vacuum environment was maintained for 15 min (Example 1) and 20 min (Example 2). Then, NbFe alloy was added in three separate additions under high vacuum, with 4-min intervals between each addition. Temperature was measured immediately after each addition, and the temperature of the molten steel was controlled to decrease slowly and in steps. The temperature drop was controlled to be 8-12℃, resulting in NbN precipitate size of 30-50 nm and NbN distribution of 2 × 10⁻⁶ nm. 4 -6×10 4 pcs / mm³.

[0025] The continuous casting crystallizer employs a low-frequency alternating magnetic field, with alternating electromagnetic frequencies of 2.5Hz (Example 1) and 3.5Hz (Example 2), and alternating electromagnetic induction intensity of 0.32T (Example 1) and 0.38T (Example 2). Furthermore, the continuous casting superheat is strictly controlled at 18℃ (Example 1) and 28℃ (Example 2), the continuous casting speed is strictly controlled at 0.5m / min (Example 1) and 0.6m / min (Example 2), and the secondary cooling water volume is strictly controlled at 0.35L / kg (Example 1) and 0.45L / kg (Example 2). This promotes a more uniform distribution of NbN precipitates within the continuously cast billet, with the NbN distribution controlled at 3.5 × 10⁻⁶. 4 -4.5×10 4 The grain size is reduced to 80-100μm by the number of grains per mm³, resulting in a continuous casting billet with a size of 295mm*345mm.

[0026] The continuously cast billet is heated in a heating furnace. The preheating zone temperature is 680-980℃, and the preheating time is 50 min (Example 1) and 60 min (Example 2). The oxygen content in the furnace is controlled at a low level of 0.5% (Example 1) and 0.8% (Example 2). The oxidation sensitivity is high in the preheating zone temperature range. The heating zone temperature is 980-1130℃, and the heating time is 80 min (Example 1) and 100 min (Example 2). The oxygen content in the furnace is controlled at a high level of 2.3% (Example 1) and 2.8% (Example 2). The oxidation sensitivity is low in the heating zone temperature range. The heating zone temperature range needs to ensure complete CO combustion in the furnace to rapidly increase the billet temperature. The soaking zone temperature is 1131℃ (Example 1) and 1188℃ (Example 2). 2) The soaking time was 20 min (Example 1) and 30 min (Example 2). The oxygen content in the furnace was controlled to be low at 0.3% (Example 1) and 0.5% (Example 2). The oxidation sensitivity in the temperature range of the soaking section was extremely high. The final oxide layer thickness on the surface of the billet after heating was controlled to be 0.5 mm (Example 1) and 0.8 mm (Example 2). High-pressure water of 28 MPa (Example 1) and 33 MPa (Example 2) was used. The flow rate of the high-pressure water single nozzle was 1.5 L / s (Example 1) and 2.5 L / s (Example 2). The impact force of the high-pressure water sprayed onto the surface of the billet was 62 N / cm² (Example 1) and 79 N / cm² (Example 2). The oxide and decarburized layer on the surface of the billet was completely removed by high-pressure water dephosphorization.

[0027] After descaling with high-pressure water, the billet was rough-rolled at 1031℃ (Example 1) and 1070℃ (Example 2) in 3 passes. The deformation per pass was 18-23%, and the cumulative deformation was 56.5% (Example 1) and 66.3% (Example 2). The high-temperature austenite dynamic recrystallization effect was used to break the billet grains to 50-70μm. The intermediate rolling temperature was controlled at 982℃ (Example 1) and 1018℃ (Example 2) in 4 passes. The deformation per pass was 10-13%. The cumulative deformation during intermediate rolling was 45.8% (Example 1) and 51.9% (Example 2), utilizing the dynamic recrystallization effect of high-temperature austenite to break the slab grains to 30-40μm. The finishing rolling temperature was controlled at 923℃ (Example 1) and 959℃ (Example 2), employing 5 passes. The deformation per pass in finishing rolling was 5-8%, and the cumulative deformation during finishing rolling was 30.1% (Example 1) and 39.7% (Example 2), utilizing the dynamic recrystallization effect of high-temperature austenite to break the slab grains to 15-25μm. Throughout the roughing, intermediate, and finishing rolling processes, the high-density dispersion of NbN precipitates in the steel, generated during continuous casting, pinned the grain boundaries, effectively suppressing dynamic grain growth during high-temperature rolling. The total time from roughing to finishing rolling was controlled at 5 minutes (Example 1) and 8 minutes (Example 2), with a single-pass rolling interval of 12-18 seconds. During the rolling process, nitrogen gas was injected onto the steel surface at a pressure of 0.6 MPa (Example 1) and 0.8 MPa (Example 2), a single nozzle flow rate of 180 L / min (Example 1) and 200 L / min (Example 2), and an injection angle of 45°, forming a continuous and stable gas curtain covering the steel surface without dead angles. After rolling, the steel was immediately placed in a slow-cooling furnace under a nitrogen protective atmosphere to slowly cool to room temperature. This prevented surface oxidation and decarburization of the steel during the rolling and cooling processes.

[0028] The smelting composition of the steels obtained in Examples 1 and 2 is shown in Table 1.

[0029] Table 1. Smelting composition (wt%)

[0030] The dimensions of the coarse and fine non-metallic inclusions B in the hot-rolled steel prepared in Examples 1 and 2 are shown in Table 2.

[0031] Table 2

[0032] The size control of NbN precipitates and the distribution of NbN in the hot-rolled steels prepared in Examples 1 and 2 are shown in Table 3.

[0033] Table 3

[0034] The grain sizes of the hot-rolled steels obtained in Examples 1 and 2 are shown in Table 4.

[0035] Table 4

[0036] The total depth of the decarburized oxide layer on the surface of the hot-rolled steel obtained in Examples 1 and 2 is shown in Table 5.

[0037] Table 5

[0038] The impact energy of the hot-rolled steels prepared in Examples 1 and 2 at -50℃ is shown in Table 6.

[0039] Table 6

[0040] This invention addresses the stringent operating conditions of gearbox shafts and gears in high-power autonomous agricultural machinery by optimizing the chemical composition of the steel (optimizing the ratio of C, Si, Mn, Cr, Ni, Mo, and Al elements, reducing the content of harmful P and O elements through reasonable processing methods, and optimizing the ratio of Nb and N microalloying elements). A novel steelmaking process is employed, with a converter segmented dephosphorization and temperature control process to maintain a phosphorus content ≤0.007%. The refining process utilizes a three-stage deep deoxidation process using Al-Si-Ca, SiC powder, and a novel rare-earth composite deoxidizer, combined with high-flow-rate bottom-blowing argon to maintain an oxygen content ≤0.0008%. The RH furnace undergoes vacuum degassing and employs stepped temperature control, precisely controlling the timing of Nb addition to precisely control the size and distribution of the NbN precipitates. Continuous casting utilizes a low-frequency alternating magnetic field and optimized process parameters, resulting in a more uniform NbN distribution and refined grain structure in the continuously cast billet. The heating furnace employs staged control of heating time, temperature, and oxygen content to effectively control the depth of the decarburized layer on the billet surface, coupled with high-pressure water dephosphorization for efficient removal of the decarburized layer. The rolling process utilizes optimized multi-pass temperature control and deformation, along with a nitrogen protective gas curtain, to refine the steel grain structure while effectively inhibiting further oxidation and decarburization on the steel surface. The final high-power autonomous agricultural machine gearbox shaft gear steel produced has the following characteristics: phosphorus content ≤0.007%, oxygen content ≤0.0008%, non-metallic inclusions (B coarse ≤20μm, B fine ≤50μm), NbN precipitate size controlled at 30-50nm, and NbN distribution controlled at 3.5×10⁻⁶. 4 -4.5×10 4 The steel exhibits excellent properties, including austenitic grain size of 15-25 μm in hot-rolled steel, a total depth of ≤10 μm for the decarburized oxide layer on the steel surface, and a low-temperature impact energy of ≥60 J at -50℃. This invention fills a gap in the domestic market for high-power, automated driving agricultural machinery gearbox shaft gears.

Claims

1. A type of steel for the gearbox shaft gears of a high-power, automated agricultural machine, characterized in that: The chemical composition by weight percentage is: C: 0.28–0.34%, Si: 0.25–0.45%, Mn: 1.30–1.60%, P: ≤0.007%, S: 0.015–0.040%, Cr: 0.30–0.70%, Ni: 1.10–1.30%, Mo: 0.10–0.20%, Al: 0.01–0.03%, N: 0.007–0.020%, Nb: 0.02–0.04%, O: ≤0.0008%, with the balance being Fe and unavoidable impurities. It is delivered in the hot-rolled state. The NbN precipitate size is 30–50 nm, and the NbN precipitate distribution is 3.5 × 10⁻⁶. 4 -4.5×10 4 The austenite grain size of hot-rolled steel is 15-25μm, and the total depth of the oxide decarburized layer on the steel surface is ≤10μm.

2. The steel for the gearbox shaft of a high-power automatic agricultural machine according to claim 1, characterized in that: Non-metallic inclusions in hot-rolled steel: coarse B ≤ 20 μm, fine B ≤ 50 μm.

3. The steel for the gearbox shaft of a high-power automatic agricultural machine according to claim 1, characterized in that: The impact energy of steel at -50℃ is ≥60J.

4. A method for manufacturing steel for the gearbox shaft of a high-power automatic agricultural machine as described in claim 1, characterized in that: The steps include, (1) Initial smelting: A segmented dephosphorization and temperature control process is adopted. In the initial stage, CO2 gas is blown into the bottom of the converter molten pool, and the gas flow rate is controlled at 0.3-0.5 m³ / min·t. The local temperature of the molten pool is controlled at 1320-1370℃, and the slag basicity is increased to 3.0-3.

5. The silicon element in the molten iron is mainly removed, which creates the basic conditions for the dephosphorization reaction. In the main dephosphorization stage, the oxygen supply intensity of the oxygen lance is gradually adjusted to 3.6-3.9 m³ / min·t, and the temperature of the molten pool is controlled at 1430-1480℃. The FeO content of the slag is maintained at 23%-25% to maintain high oxidizing properties. In the final stage, the bottom blowing gas is switched to Ar gas, and the flow rate is controlled at 0.10-0.15 m³ / min·t. The molten pool temperature is controlled at 1600-1650℃ in conjunction with the adjustment of the oxygen lance position to suppress phosphorus reversion and keep the phosphorus content of the molten steel stable below 0.007%. (2) Refining: Heat the molten steel to 1580-1595℃, feed it into the Al-Si-Ca composite deoxidation line for precipitation deoxidation, and at the same time float SiC powder onto the surface of the molten steel for diffusion deoxidation. Then add rare earth composite deoxidizer for deep deoxidation and use a large flow rate of bottom blowing argon. The above process reduces the oxygen content to below 8ppm. (3) Vacuum degassing: The refined molten steel is degassed in an RH furnace under vacuum. Nitrogen is used as the RH booster gas. NbFe alloy is added in three stages under high vacuum, with a 4-minute interval between each addition. The temperature is measured immediately after each addition of NbFe alloy to control the temperature of the molten steel in the furnace to decrease slowly in steps. The temperature drop is controlled at 8-12℃ for the three stages, so that the size of the NbN precipitate phase is controlled at 30-50nm and the distribution of NbN is controlled at 2×10 4 -6×10 4 pcs / mm³; (4) Continuous casting: The continuous casting crystallizer uses a low-frequency alternating magnetic field to promote uniform NbN distribution; (5) Heating: The continuously cast billet is heated in a heating furnace in stages. The oxygen content in the heating furnace is controlled in each stage. Finally, the oxide layer thickness on the surface of the billet after leaving the heating furnace is controlled to be 0.5-0.8 mm. High-pressure water descaling is used to remove the oxide and decarburized layer on the surface of the billet. (6) Rolling: Rough rolling is carried out at 1030-1070℃ in 3 passes, with a single pass deformation of 18-23% and a cumulative deformation of ≥56%, utilizing the dynamic recrystallization effect of high-temperature austenite to break the slab grains to 50-70μm; Intermediate rolling is carried out at 980-1020℃ in 4 passes, with a single pass deformation of 10-13% and a cumulative deformation of ≥45%, utilizing the dynamic recrystallization effect of high-temperature austenite to break the slab grains to 30-40μm; Finish rolling is carried out at 980-1020℃ in 4 passes, with a single pass deformation of 10-13% and a cumulative deformation of ≥45%, utilizing the dynamic recrystallization effect of high-temperature austenite to break the slab grains to 30-40μm; Finish rolling is carried out at 980-1020℃ in 4 passes. The temperature is 920-960℃, and 5-pass rolling is adopted. The deformation of a single pass in the finishing rolling is 5-8%, and the cumulative deformation of the finishing rolling is ≥30%. The high-temperature austenite dynamic recrystallization effect is used to break the grains of the billet to 15-25μm. The total time from rough rolling to finishing rolling is controlled at 5-8min, and the interval time between single passes is controlled at 12-18s. Nitrogen gas is sprayed onto the surface of the steel during the rolling process to form a continuous and stable gas curtain covering the surface of the steel without dead angles. After the rolling is completed, the steel is immediately placed in a slow cooling furnace under nitrogen protection atmosphere to slowly cool to room temperature.

5. The method according to claim 4, characterized in that: In step (2), during the refining stage, 1.2-1.5 kg / t of rare earth composite deoxidizer is added to the molten steel for deep deoxidation. The rare earth composite deoxidizer is composed of 40% Mg-Al-Li alloy, 30% CeO2, 15% Y2O3, and 15% CaF2 by mass percentage. During the refining and deoxidation stage, the flow rate of bottom-blown argon is 0.9-1.1 L / min·t, and the stirring time is 12-15 min.

6. The method according to claim 4, characterized in that: Step (3) Vacuum degassing stage: reduce the vacuum level in the furnace to below 63 Pa and maintain it in a low vacuum environment for 15-20 minutes.

7. The method according to claim 4, characterized in that: Step (4): The alternating electromagnetic frequency of the continuous casting mold is 2.5-3.5Hz, the alternating electromagnetic induction intensity is 0.32-0.38T, the continuous casting superheat is 18-28℃, the continuous casting speed is 0.5-0.6m / min, and the secondary cooling water volume is 0.35-0.45 L / kg. This promotes a more uniform distribution of NbN precipitates within the continuously cast billet, and the NbN distribution is controlled at 3.5×10⁻⁶. 4 -4.5×10 4 The grain size is controlled to be refined to 80-100μm within the continuously cast billet, and finally a continuously cast billet with a size of 295mm*345mm is produced.

8. The method according to claim 4, characterized in that: Step (5) Heating stage: The preheating temperature is 680-980℃, the preheating time is 50-60min, the oxygen content in the furnace is controlled at a low level of 0.5-0.8%, and the oxidation sensitivity in the preheating temperature range is high; The heating temperature is 980-1130℃, the heating time is 80-100min, the oxygen content in the furnace is controlled at a high level of 2.3-2.8%, and the oxidation sensitivity in the heating temperature range is low. The heating temperature range needs to ensure that CO combustion in the furnace is complete and the billet temperature is rapidly increased; The soaking temperature is 1130-1190℃, the soaking time is 20-30min, the oxygen content in the furnace is controlled at a low level of 0.3-0.5%, and the oxidation sensitivity in the soaking temperature range is extremely high. High pressure water of 28-33MPa is used, the flow rate of a single high pressure water nozzle is 1.5-2.5L / s, and the impact force of the high pressure water sprayed onto the billet surface is 60-80N / cm².

9. The method according to claim 4, characterized in that: In step (6) the rolling stage, the nitrogen pressure is 0.6-0.8MPa, the single nozzle flow rate is 180-200L / min, and the injection angle is 45°.

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