A forging process for producing a solid sun gear shaft from a continuous casting billet

Abstract

This invention discloses a forging process for preparing solid sun gear shafts from continuously cast billets, belonging to the field of metal forging technology. The process includes: S1 billet preparation; S2 stepped heating and high-temperature diffusion; S3 temperature-division forging, sequentially performing low-temperature radial pre-upsetting, high-temperature axial upsetting and drawing, and medium-temperature radial rolling, forming the rim portion through radial rolling; S4 post-forging residual heat staged cooling; S5 low-temperature tempering; and S6 cold precision forging of the tooth profile. This invention, through a combination of material micro-alloying design and multi-process linkage forging in temperature-division zones, achieves differentiated control of the microstructure of the shaft and rim, solving technical problems such as tooth root bending fatigue, shaft torsional fatigue, and uneven cross-sectional hardness in sun gear shafts. The solid sun gear shaft produced by this invention has a tensile strength ≥1310MPa, a hardness difference between the rim and journal ≤2HRC, and a tooth root bending fatigue strength improvement of over 30%, showing broad industrial application prospects.

Description

Technical Field

[0001] This invention relates to the field of gear forging technology, and in particular to a forging process for preparing a solid sun gear shaft from a continuously cast billet. Background Technology

[0002] Solid sun gear shafts are core components of planetary transmission mechanisms, widely used in wind turbine gearboxes, construction machinery, and automotive transmissions. During operation, the shaft portion primarily bears torsional fatigue loads, while the rim teeth bear bending fatigue loads and contact wear loads. The mechanical performance requirements differ significantly between these different regions. Furthermore, due to the slender shaft portion and thick rim, the significant difference in cross-sectional dimensions between the shaft and rim easily leads to substantial uneven hardness during heat treatment and quenching. This results in a mismatch between the bending fatigue strength at the tooth root and the torsional fatigue life of the shaft, severely restricting the reliability and service life of the entire mechanism. Therefore, achieving differentiated performance control in different regions of the same component through forging processes has long been a challenging technical issue in this field.

[0003] Currently, the forging of solid sun gear shafts mainly uses die-cast ingots as raw materials. For example, Chinese Patent Publication No. CN117798313A discloses a processing technology for sun gear forgings, which uses an 18.3T large die-cast ingot for initial forging, performs two upsetting and drawing processes to pre-determine the forging ratio, and then uses three upsetting and three drawing processes to form the billet, followed by one ring rolling and one forging cycle. Although this process can meet certain performance requirements, the material utilization rate of the die-cast ingot is low (usually only about 75%), and the production process is long, energy-intensive, and costly. In addition, the die-cast ingot itself has metallurgical defects such as ingot segregation and shrinkage cavities, increasing the risk of the forged product failing flaw detection.

[0004] In recent years, there have been attempts to use continuously cast billets to replace die-cast ingots in the forging of shaft parts. For example, Chinese Patent Publication No. CN118744219A discloses a method for rapidly forming large solid shaft forgings using continuously cast billets, which forms the continuously cast bar billet into shaft extrusions through die forging. Chinese Patent Publication No. CN119016652A discloses a forging method for producing large-diameter solid forgings from continuously cast billets, which solves the problem of poor core compaction by using a cylindrical V-shaped anvil clamping jaw followed by bi-directional upsetting. However, the above-mentioned existing technologies still have the following shortcomings: the inherent metallurgical defects of continuously cast billets, such as central carbon segregation, core porosity, and surface columnar crystals, can only be improved to a limited extent under conventional unidirectional or multidirectional upsetting processes. In particular, the central carbon segregation zone is prone to become a crack source for torsional fatigue of the shaft during subsequent service, while the surface columnar crystals are not effectively utilized, making it difficult to further improve the wear resistance and fatigue strength of the rim teeth. More importantly, none of the existing processes mentioned above have proposed a forging scheme to control the microstructure and properties in a zoned manner, taking into account the structural characteristics of the large difference in thickness between the shaft and the rim of the sun gear shaft. The problem of uneven cross-sectional hardness remains prominent. Summary of the Invention

[0005] (a) Technical problems to be solved To address the shortcomings of existing technologies, the present invention aims to provide a forging process for preparing solid sun gear shafts using continuously cast billets. This process combines material micro-alloying design with multi-process linkage forging in temperature-controlled zones to fully utilize the advantages of continuously cast billet raw materials and specifically eliminate their inherent defects. This enables differentiated control of the microstructure of the shaft and rim of the solid sun gear shaft, solving specific technical problems such as tooth root bending fatigue, shaft torsional fatigue, tooth surface contact wear, and uneven cross-sectional hardness.

[0006] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: a forging process for preparing a solid sun gear shaft using continuously cast billets. This includes the following steps: A forging process for preparing a solid sun gear shaft from a continuously cast billet includes the following steps: S1 Billet Preparation: The continuously cast billet is sawn to obtain a cylindrical billet of the required specifications; S2 Step heating and high temperature diffusion: The billet is first heated to 650-700℃ and preheated, then heated to 1220-1250℃ for high temperature diffusion treatment, and then cooled to the forging temperature after diffusion is completed. S3 temperature zone forging: Low-temperature radial pre-upsetting, high-temperature axial upsetting and drawing, and medium-temperature radial rolling are performed sequentially; wherein, the low-temperature radial pre-upsetting temperature is 950~1000℃, the high-temperature axial upsetting temperature is 1150~1200℃, and the medium-temperature radial rolling temperature is 900~950℃, and the sun gear rim is formed by radial rolling to obtain the wheel blank preform; S4 Post-forging residual heat staged cooling: When the billet temperature after final forging is not lower than 850℃, it is cooled in three stages in sequence: air cooling, spray cooling, and furnace cooling. S5 Low-temperature tempering: Low-temperature tempering treatment is performed on the cooled billet; S6 Cold precision forging tooth profile: The involute tooth profile is formed by cold precision forging of the tempered wheel blank preform to obtain a solid sun gear shaft forging.

[0007] Further, in step S1, the chemical composition of the continuously cast billet, by mass fraction, includes: C 0.38–0.45%, Si 0.15–0.35%, Mn 0.80–1.20%, Cr 0.60–1.00%, Nb 0.02–0.05%, Ti 0.01–0.03%, P ≤ 0.015%, S ≤ 0.015%, with the balance being Fe and unavoidable impurities; wherein the mass ratio of Mn to Cr is (1.2–1.5):1. This composition system, through the rational proportioning of Mn and Cr and the micro-alloying design of Nb and Ti, can effectively suppress grain growth during high-temperature diffusion and temperature-division forging processes, while simultaneously improving the cold precision forging performance of the rim tooth section.

[0008] Further, in step S2, the temperature is raised to 650-700℃ at a heating rate of 80-100℃ / h, and then held for 2-3 hours to ensure uniform temperature inside and outside the billet and complete low-temperature stress relief. Then, the temperature is raised to 1220-1250℃ for high-temperature diffusion treatment, and the holding time is 4-6 hours to allow the alloying elements to diffuse fully and eliminate the original segregation and central porosity of the continuous casting billet. After diffusion is completed, the temperature is lowered to 1180-1220℃ to enter the forging process.

[0009] Furthermore, in step S3, the total deformation of low-temperature radial pre-upsetting is 10-15%, and the deformation force is applied symmetrically along the radial direction of the billet to initially break the as-cast structure and provide uniform initial rheological conditions for subsequent deformation; after pre-upsetting is completed, the billet is warmed back to the temperature range of high-temperature axial upsetting.

[0010] Furthermore, in step S3, the total deformation of the high-temperature axial upsetting is 40-45%. The forging process uses a centering fixture to keep the billet axis and the part axis coincide, thereby ensuring the center density and streamline continuity of the shaft body.

[0011] Furthermore, in step S3, the total deformation at the rim of the medium-temperature radial oscillating roll is 25-30%; the oscillating head tilt angle is 3°-5°, and the feed rate is 0.5-1.0 mm / s. This medium-temperature oscillating roll process can effectively promote the orderly radial flow of metal at the rim, forming a fiber distribution that conforms to the tooth profile.

[0012] Furthermore, in step S4, the air cooling stage cools to 780–800°C at a rate of 5–8°C / s; the spray cooling stage cools to 550–600°C at a rate of 15–20°C / s; and the furnace cooling slow cooling stage cools to 200°C before being removed from the furnace. This staged cooling system can avoid deformation and cracking caused by excessively rapid cooling, while also providing reasonable microstructure preparation for subsequent low-temperature tempering.

[0013] Furthermore, in step S5, the temperature of the low-temperature tempering is 200-240°C, the holding time is 3-5 hours, and after tempering, it is cooled to room temperature in the furnace to eliminate residual stress after forging and stabilize the microstructure.

[0014] (III) Beneficial Effects The present invention aims to provide a forging process for preparing solid sun gear shafts from continuously cast billets, which has the following beneficial effects: First, by using stepped heating and high-temperature diffusion treatment, alloying elements are fully diffused while avoiding thermal shock cracking of the billet. This effectively eliminates structural defects such as center segregation and porosity in the continuously cast billet, providing a high-quality core quality foundation for subsequent forging.

[0015] Then, a temperature-divided deformation design was adopted, consisting of low-temperature radial pre-upsetting, high-temperature axial upsetting, and medium-temperature radial rolling. In the low-temperature stage, the billet underwent initial compression under surface hardening constraints, breaking up the coarse as-cast structure while introducing numerous dislocations and subgrain boundaries within the grains, providing ample nucleation sites for subsequent high-temperature dynamic recrystallization. In the high-temperature stage, high plasticity and low deformation resistance were used to complete the large deformation elongation of the shaft body. The substructure introduced by the low-temperature pre-deformation induced sufficient dynamic recrystallization in this stage. Combined with centering fixtures, this ensured shaft compactness and streamline continuity, effectively refining the shaft grains and eliminating the central segregation zone. In the medium-temperature stage, precise radial expansion of the rim region was achieved through rolling, forming a fiber distribution that matched the subsequent tooth forming. The synergistic effect of the three temperature zones resolved the contradiction that single-temperature forging could not simultaneously achieve shaft compactness and rim forming. The pinning effect of Nb and Ti microalloying elements at the grain boundaries further suppressed abnormal grain growth in the high-temperature stage, achieving a superimposed effect of fine-grain strengthening and precipitation strengthening.

[0016] Secondly, the three-stage cooling system ensures that the forgings do not crack, while achieving precise control over grain size and phase composition. Low-temperature tempering further eliminates residual stress, enabling the forgings to possess excellent comprehensive mechanical properties and dimensional stability in the wheel blank preform stage, thus creating conditions for subsequent cold precision forging of the tooth profile.

[0017] Finally, the involute tooth profile is formed by cold precision forging. On the one hand, the cold deformation strengthening effect is used to improve the hardness and fatigue resistance of the tooth surface. On the other hand, a high-precision tooth profile is obtained, which significantly reduces the subsequent machining allowance and improves material utilization and production efficiency. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0019] Example 1 This embodiment provides a forging process for preparing a solid sun gear shaft from a continuously cast billet, specifically used to prepare a solid sun gear shaft for a wind turbine gearbox. The finished product has a maximum diameter of Φ380mm, a shaft diameter of Φ180mm, and a total length of 1200mm. The specific steps include: S1 Billet Preparation: A continuously cast round billet with a diameter of Φ450mm was selected as the raw material and sawn into billets with a length of 1350mm. The chemical composition of this continuously cast billet, by mass fraction, is: C 0.41%, Si 0.28%, Mn 1.05%, Cr 0.78%, Nb 0.035%, Ti 0.020%, P 0.012%, S 0.008%, with the balance being Fe and unavoidable impurities. The calculated mass ratio of Mn to Cr is 1.35:1.

[0020] After cutting, the end face of the blank is inspected to confirm that there are no visible surface defects such as cracks or folds.

[0021] S2 Stepwise Heating and High-Temperature Diffusion: The sawn continuous casting billet is loaded into a bogie-type resistance heating furnace and heated from room temperature to 680°C at a heating rate of 90°C / h. After reaching the temperature, it is held for 2.5h to eliminate the internal stress of casting the continuous casting billet.

[0022] After the initial heat treatment, the temperature is increased to 1240℃ at a rate of 100℃ / h. Once the temperature is reached, the temperature is held for 5 hours for high-temperature diffusion treatment. The high-temperature diffusion treatment is carried out in a protective atmosphere furnace, with the oxygen content inside the furnace controlled at ≤100ppm to prevent severe oxidation of the billet surface.

[0023] After the high-temperature diffusion heat preservation is completed, the billet is cooled to 1200℃ in the furnace, ready for the forging process. Before tapping, an infrared thermometer is used to measure the surface temperature of the billet at multiple points to confirm that the surface temperature uniformity of the billet is within ±15℃.

[0024] S3 Temperature-Divided Forging Forming: This step involves three processes sequentially: low-temperature radial pre-upsetting, high-temperature axial upsetting and drawing, and medium-temperature radial rolling, as detailed below: S31 Low-Temperature Radial Pre-Upsetting: The billet, which has been diffused at high temperature and cooled to 1200℃, is taken out of the heating furnace. The billet temperature is monitored in real time using an infrared thermometer. When the surface temperature of the billet naturally cools to 980℃, it is immediately transferred to a 20000kN hydraulic high-speed forging machine for radial pre-upsetting.

[0025] Pre-upsetting uses upper and lower flat anvils, with deformation force applied symmetrically along the radial direction of the billet. The single reduction is controlled to be 8% of the original billet diameter, completed in two stages, with a total deformation of 14%. After pre-upsetting, the billet is short and thick, with the diameter increasing to approximately Φ485mm and the length shortening to approximately 1150mm.

[0026] After the pre-upsetting is completed, the billet is immediately returned to the furnace for heating until the billet temperature rises back to the forging temperature range.

[0027] S32 High-Temperature Axial Upsetting: After the billet temperature rises to 1180℃ and is held for 30 minutes to ensure uniform temperature, the billet is taken out and placed on the same high-speed forging machine for axial upsetting.

[0028] Axial upsetting is performed using both flat anvils, with the upsetting deformation controlled at around 20%, resulting in a shorter billet height and a larger diameter. The billet is then rotated 90° for drawing, using a full-anvil feed method with a single-pass reduction of 25%. This upsetting and drawing process is repeated twice, with a total deformation of 43%.

[0029] Throughout the high-temperature axial upsetting process, a centering fixture is used for positioning. This centering fixture is a V-shaped centering device mounted on the lower anvil, which, in conjunction with the clamping arm of the manipulator, ensures that the axis of the billet coincides with the axis of the final part, with the deviation controlled within ±3mm. After the high-temperature axial upsetting is completed, the billet is approximately cylindrical with a diameter of Φ420mm.

[0030] S33 Medium Temperature Radial Rolling: The billet that has been axially uptaken at high temperature is returned to the furnace. When the billet temperature naturally cools to 930℃, it is taken out and immediately transferred to the rolling mill for radial rolling to form the sun wheel rim.

[0031] The oscillating roll mill uses a conical oscillating head with an inclination angle set at 4°. The billet is placed in the lower die of the oscillating roll mill, which has a cavity that matches the shape of the sun gear rim. The oscillating roll mill is started, and the oscillating head performs a planetary oscillating motion around the central axis of the billet, while simultaneously applying axial pressure to the rim of the billet. The feed rate of the oscillating head is controlled at 0.7 mm / s, causing the metal in the rim to flow radially and fill the cavity.

[0032] During the rolling process, the total deformation at the rim is controlled at 28%. Through rolling, the rim of the billet obtains a pre-formed shape that closely resembles the shape of the finished rim, thus producing a pre-formed wheel blank. After rolling, the metal flow lines at the rim are continuously distributed along the circumference.

[0033] S4 Post-Forging Residual Heat Staged Cooling: After the final forging process, i.e., after medium-temperature radial rolling, the surface temperature of the billet is immediately measured to ensure it is not lower than 850℃. In this embodiment, the final forging temperature is 862℃. The billet is then rapidly transferred to an online controlled cooling system for three-stage cooling: S41 Air Cooling Stage: The billet is placed in air for natural cooling, with a cooling rate controlled at 6℃ / s, cooling to 790℃. The cooling rate is controlled by adjusting the airflow speed around the billet in this stage.

[0034] S42 Spray Cooling Stage: When the surface temperature of the billet drops to 790℃, the spray cooling device is activated. This device consists of an array of ring nozzles arranged around the billet, spraying atomized cooling water. The spray cooling rate is controlled at 18℃ / s, cooling the billet to 580℃. During the spray cooling process, the spray pressure and nozzle opening are adjusted to ensure uniform circumferential cooling of the billet, with the temperature difference controlled within ±20℃.

[0035] S43 Furnace Cooling and Slow Cooling Stage: When the surface temperature of the billet drops to 580℃, the billet is immediately transferred to a slow cooling furnace preheated to 250℃. The furnace door is closed, and the billet is allowed to cool naturally in the furnace to 200℃ before being removed. The entire furnace cooling and slow cooling stage takes approximately 4 hours.

[0036] S5 Low-Temperature Tempering: The billets that have undergone the above-mentioned staged cooling and are removed from the furnace are immediately transferred to a tempering furnace for low-temperature tempering. The tempering temperature is 220℃, and the holding time is 4 hours. After the holding time is completed, the heating power of the tempering furnace is turned off, and the billets are removed from the furnace after cooling to room temperature with the furnace.

[0037] S6 Cold Forged Tooth Profile: Preparation work before cold forging of the tempered wheel blank preform: First, shot blasting is performed on the surface of the wheel blank to remove the surface oxide scale; then a lubricating coating is applied to the surface of the wheel blank, specifically a molybdenum disulfide-based lubricant, to reduce the frictional resistance during the cold forging process.

[0038] The surface-treated wheel blank preform is placed in a cold forging die and cold-forged using an 80,000 kN cold forging press at room temperature. A 0.4 mm machining allowance is left on the wheel blank rim before cold forging, resulting in an involute tooth profile after the process. The tooth surface deformation during cold forging is 22%.

[0039] After cold precision forging, a solid sun gear shaft forging is obtained. Testing shows that the tooth profile accuracy reaches IT7 grade, and the surface roughness Ra≤3.2μm.

[0040] Example 2 This embodiment provides a forging process for preparing a solid sun gear shaft from a continuously cast billet. The main differences between this process and Embodiment 1 lie in the process parameters and product specifications, as detailed below: S1 billet preparation A continuously cast round billet with a diameter of Φ550mm was selected as the raw material and sawn into a billet with a length of 1600mm. This billet was used to manufacture the solid sun gear shaft in the mining machinery transmission. The maximum diameter of the finished product is Φ480mm, and the shaft diameter is Φ220mm.

[0041] The chemical composition of the continuously cast billet, by mass fraction, is: C 0.43%, Si 0.32%, Mn 1.15%, Cr 0.85%, Nb 0.045%, Ti 0.028%, P 0.010%, S 0.006%, with the balance being Fe and unavoidable impurities. The Mn / Cr mass ratio is 1.35:1.

[0042] S2 Stepwise Heating and High-Temperature Diffusion The temperature is increased to 700℃ at a heating rate of 85℃ / h and held for 3 hours; then increased to 1250℃ at a heating rate of 100℃ / h and held for 6 hours for high-temperature diffusion treatment; after diffusion, the temperature is reduced to 1190℃ in the furnace in preparation for forging.

[0043] S3 temperature zone forging S31 Low-temperature radial pre-upsetting: Radial pre-upsetting is performed when the billet is cooled to 960℃, and is completed in two stages, with a total deformation of 15%.

[0044] S32 High-Temperature Axial Upsetting: The billet is heated to 1180℃ for axial upsetting, using three upsetting steps + three drawing steps, with a total deformation of 45%. Centering fixtures are used to ensure that the billet axis coincides with the part axis, with the deviation controlled within ±2mm.

[0045] S33 Medium Temperature Radial Rolling: The billet is cooled to 940℃ and then radially rolled. The tilt angle of the roller head is 5°, the feed rate is 0.6mm / s, and the total deformation at the rim is 30%.

[0046] S4 post-forging residual heat staged cooling: The final forging temperature is 858℃. The air cooling stage cools to 785℃ at 7℃ / s; the spray cooling stage cools to 560℃ at 16℃ / s; and the furnace cooling stage slowly cools to 200℃ before unloading.

[0047] S5 Low-temperature tempering: The tempering temperature is 230℃, the holding time is 5h, and the furnace is cooled to room temperature.

[0048] S6 Cold Forged Tooth Profile: The cold forged front wheel blank has a machining allowance of 0.3mm and a tooth surface deformation of 25%.

[0049] Example 3 This embodiment provides a forging process for preparing a solid sun gear shaft from a continuously cast billet. The main difference between this process and Embodiment 1 is that the Mn / Cr ratio in the chemical composition of the continuously cast billet is near the upper and lower limits of the specified range, in order to verify its hardenability control effect. The details are as follows: S1 Billet Preparation: A continuously cast round billet with a diameter of Φ400mm was selected as the raw material and sawn into billets with a length of 1100mm. The chemical composition of the continuously cast billet, by mass fraction, is: C 0.39%, Si 0.20%, Mn 0.85%, Cr 0.62%, Nb 0.025%, Ti 0.015%, P 0.013%, S 0.009%, with the balance being Fe and unavoidable impurities. The Mn / Cr mass ratio is 1.37:1.

[0050] The remaining steps S2 to S6 are the same as in Example 1.

[0051] Example 4 This embodiment provides a forging process for preparing a solid sun gear shaft from a continuously cast billet. The difference between this process and Embodiment 1 lies in that the process parameters take different values ​​within the scope of the claims, as detailed below: S1 billet preparation: Same as in Example 1.

[0052] S2 Step heating and high-temperature diffusion: Heat to 650℃ at a heating rate of 80℃ / h and hold for 3 hours; then heat to 1220℃ and hold for 4 hours for high-temperature diffusion treatment; after diffusion, cool down to 1180℃ to prepare for forging.

[0053] S3 temperature zone forging: low temperature radial pre-upsetting temperature is 950℃, total deformation is 10%; high temperature axial upsetting temperature is 1150℃, total deformation is 40%; medium temperature radial rolling temperature is 900℃, rolling head angle is 3°, feed rate is 1.0mm / s, and total deformation at the rim is 25%.

[0054] S4 post-forging residual heat staged cooling: The final forging temperature is 852℃. The air cooling stage cools to 780℃ at 5℃ / s; the spray cooling stage cools to 550℃ at 15℃ / s; and the furnace cooling stage slowly cools to 200℃ before unloading.

[0055] S5 Low-temperature tempering: Tempering temperature is 200℃, and holding time is 5h.

[0056] S6 Cold Forged Tooth Profile: The cold forged front wheel blank has a machining allowance of 0.5mm and a tooth surface deformation of 20%.

[0057] Comparative Example 1 This comparative example uses a traditional ingot casting process to prepare a solid sun gear shaft, for comparison with the embodiments of the present invention. A 450 mm diameter ingot was selected as the raw material, with the same chemical composition as in Example 1. The following process was employed: The billet is heated and forged at 1200℃, using two unidirectional upsetting processes, resulting in a total deformation of 45%. It is then cooled, sawn, and reheated to 1200℃ for final forging, again using unidirectional upsetting, with a total deformation of 30%. After forging, it is air-cooled, normalized, rough-machined, and quenched and tempered. The tempering process involves quenching at 860℃ followed by high-temperature tempering at 560℃. Finally, it is finish-machined and has its teeth cut to obtain the finished product. The material utilization rate of the ingot is approximately 75%.

[0058] Comparative Example 2 This comparative example uses continuously cast billets as raw materials, but only employs the conventional unidirectional upsetting forging process, without using the temperature-zone linkage forging scheme of this invention. Continuously cast billets of the same specifications and chemical composition as in Example 1 are selected, and the following process is used: sawing the continuously cast billet, heating to 1200℃, unidirectional upsetting with a total deformation of 45%, air cooling after forging, reheating to 860℃ for quenching followed by tempering at 560℃, rough turning, finish turning, and machining of the teeth to obtain the finished product.

[0059] Comparative Example 3 This comparative example uses the same temperature zone forging scheme as the present invention, namely steps S1 to S3, but does not perform post-forging residual heat staged cooling. Instead, it adopts the traditional post-forging air cooling plus reheating and tempering treatment.

[0060] A continuously cast billet of the same specifications and chemical composition as in Example 1 was selected, and steps S1 to S3 were the same as in Example 1. After forging, the billet was allowed to cool naturally to room temperature in air, then reheated to 860°C for oil quenching, and then tempered at 560°C. The subsequent S6 cold precision forging of the tooth profile was the same as in Example 1.

[0061] Comparative Example 4 This comparative example uses the same process steps and parameters as the present invention, but does not add Nb and Ti microalloying elements to the chemical composition of the continuously cast billet.

[0062] The chemical composition of the continuously cast billet, by mass fraction, is: C 0.41%, Si 0.28%, Mn 1.05%, Cr 0.78%, P 0.012%, S 0.008%, excluding Nb and Ti, with the balance being Fe and unavoidable impurities. The remaining steps S1 to S6 are exactly the same as in Example 1.

[0063] Comparative Example 5 This comparative example uses the same chemical composition and process steps as the continuous casting billet of the present invention, but in step S3, the low-temperature radial pre-upsetting process is not performed. Instead, it directly enters the high-temperature axial upsetting process after high-temperature diffusion.

[0064] Specifically: S1 and S2 are the same as in Example 1. S3 only involves high-temperature axial upsetting at 1180°C with a total deformation of 43%, and medium-temperature radial rolling at 930°C with a flange deformation of 28%. The low-temperature radial pre-upsetting process is omitted. S4 to S6 are the same as in Example 1.

[0065] Performance testing and effect comparison: The solid sun gear shaft forgings prepared in the above embodiments and comparative examples were subjected to performance testing. The testing methods and results are as follows.

[0066] Mechanical property testing: Tensile properties were tested in accordance with GB / T 228.1 "Metallic materials - Tensile testing - Part 1: Test method at room temperature"; impact toughness was tested in accordance with GB / T 229 "Metallic materials - Charpy impact test method".

[0067] Hardness testing: The Brinell hardness of the shaft is tested at the core, surface and tooth root of the rim in accordance with GB / T 231.1 "Metallic materials - Brinell hardness test - Part 1: Test method".

[0068] Grain size testing: The grain size was determined using the intercept method according to GB / T 6394 "Method for determination of average grain size of metals".

[0069] Central segregation rating: Central segregation rating of transverse low magnification specimens was performed in accordance with GB / T 1979 "Rating Chart of Microstructural Defects in Structural Steel".

[0070] Inclusion detection: Inclusions are classified into A, B, C, and D categories according to GB / T 10561 "Standard rating chart microscopic examination method for determination of non-metallic inclusion content in steel".

[0071] Tooth root bending fatigue strength: The pulsating loading fatigue test was carried out in accordance with GB / T 14230 "Test Method for Bending Fatigue Strength of Gears".

[0072] Metal flow lines: The longitudinal section of the gear teeth was subjected to hot acid etching test according to GB / T 226 "Acid Etching Test Method for Low Magnification Structure and Defects of Steel" to observe the distribution of flow lines.

[0073] Test results: Table 1 Comparison of mechanical properties and hardness uniformity of each embodiment and comparative example Table 2 Comparison of microstructure and defect ratings for each embodiment and comparative example Table 3 Comparison of tooth root bending fatigue strength and streamline state in various embodiments and comparative examples Table 4. Comparison of material utilization and energy consumption for each embodiment and comparative example. Results analysis: Comparing Example 1 and Comparative Example 1, the overall performance of each embodiment of the present invention is significantly superior to that of the traditional ingot casting process. The tensile strength of Example 1 is 1328 MPa, a 7.5% increase compared to 1235 MPa in Comparative Example 1. The yield strength is 1068 MPa, a 10.8% increase compared to 964 MPa in Comparative Example 1. The room temperature impact energy is 162 J, a 25.6% increase compared to 129 J in Comparative Example 1. The tooth root bending fatigue strength is 485 MPa, a 34.7% increase compared to 360 MPa in Comparative Example 1. The hardness difference between the rim and shaft is significantly reduced from 4.5 HRC in Comparative Example 1 to 1.6 HRC, resulting in a significant improvement in cross-sectional uniformity. Furthermore, the material utilization rate of the present invention is over 98%, a significant improvement over the 75% of the ingot casting process, and the total production energy consumption is reduced by approximately 28% compared to the ingot casting process.

[0074] Comparing Example 1 and Comparative Example 2, both used the same raw material, continuously cast billets. However, Comparative Example 2 employed conventional unidirectional upsetting and drawing with independent quenching and tempering. The tensile strength of Example 1 was increased by 6.4%, the yield strength by 8.8%, and the impact energy by 16.5% compared to Comparative Example 2. Particularly noteworthy is that the center segregation rating of Comparative Example 2 was 2.5, while that of Example 1 decreased to 1.0, indicating that the high-temperature diffusion-assisted forging process of this invention has a significant effect on eliminating center segregation in continuously cast billets. Furthermore, the dynamic recrystallization ratio of Comparative Example 2 was only 40%, while that of Example 1 was as high as 88%, indicating that the dislocations and subgrain boundaries introduced by low-temperature pre-deformation provided sufficient nucleation sites for dynamic recrystallization, resulting in a significant grain refinement effect.

[0075] Comparing Example 1 and Comparative Example 3, Comparative Example 3 adopted the same temperature-zone forging scheme as Example 1, but after forging, it used traditional air cooling followed by reheating and tempering treatment (860℃ oil quenching + 560℃ high-temperature tempering). Although the mechanical properties of Comparative Example 3 were better than those of Comparative Example 1 and Comparative Example 2, they were still lower than those of Example 1. In particular, the hardness difference between the rim and the shaft was 3.5 HRC, which was much higher than the 1.6 HRC of Example 1. It should be noted that although the high-temperature tempering (560℃) of Comparative Example 3 could eliminate stress, it would lead to the widening of martensite laths and coarsening of carbides. Furthermore, due to re-austenitization, the grain refinement effect established by the original forging was partially lost. More importantly, the root cause of the uneven cross-sectional hardness lies in the difference in cooling rate between thick and thin sections during the quenching cooling stage, rather than during the tempering stage. The reheating and quenching of Comparative Example 3 could not solve this problem. However, this invention utilizes the residual heat after forging for direct staged cooling, combined with a hardenability design optimized for the Mn / Cr ratio, achieving a more uniform martensitic transformation during the quenching stage. This solves the problem of uneven cross-sectional hardness at its source, which is the fundamental reason why the hardness difference of Example 1 is significantly better than that of Comparative Example 3. At the same time, online quenching using the residual heat of forging eliminates the reheating process, and the energy consumption of Comparative Example 3 is approximately 1.16 times that of Example 1.

[0076] Comparing Example 1 and Comparative Example 4, the continuously cast billet of Comparative Example 4 did not contain niobium and titanium microalloying elements, but adopted the exact same process steps and parameters as Example 1. The grain size of Comparative Example 4 was grade 6.5, lower than grade 7.5 of Example 1. The dynamic recrystallization ratio was 75%, lower than 88% of Example 1. This indicates a significant synergistic effect between niobium-titanium microalloying and the temperature-controlled forging process. Titanium nitrides pin grain boundaries at low temperatures, while niobium carbonitrides induce strain precipitation and promote recrystallization nucleation during intermediate-temperature deformation. The synergistic effect of these two factors is the key to grain refinement and increased recrystallization ratio. The tooth root bending fatigue strength of Comparative Example 4 was 430 MPa, 11.3% lower than 485 MPa of Example 1, further demonstrating the positive contribution of grain refinement to fatigue performance.

[0077] Comparing Example 1 and Comparative Example 5, Comparative Example 5 omitted the low-temperature radial pre-upsetting process but retained all other steps and parameters. The dynamic recrystallization ratio of Comparative Example 5 was only 65%, far lower than the 88% of Example 1. The grain size was grade 6.5, lower than the 7.5 of Example 1. This indicates that although the deformation amount of low-temperature radial pre-upsetting is only 10% to 15%, the large number of dislocations and subgrain boundaries introduced in the temperature range of 950 to 1000°C provide a large number of dynamic recrystallization nucleation cores for the subsequent high-temperature axial upsetting and drawing at 1150 to 1200°C, making the recrystallization more complete and uniform. This is an indispensable key link in the temperature-zone progressive linkage system of this invention.

[0078] Comparing the various embodiments, Examples 1 to 4 are all within the scope of the claims of this invention and all achieve better overall results than the comparative examples. In Example 2, the parameter combination is within the middle of the preferred range, resulting in the best mechanical properties and the most uniform microstructure, indicating that further optimization of the parameters within the preferred range can achieve even better results. In Example 3, the manganese-chromium ratio is 1.37:1, which is in the middle of the range, resulting in good hardenability stability. The hardness difference between the rim and shaft is 1.9 HRC, still significantly better than the comparative examples, demonstrating that the manganese-chromium ratio range defined by this invention has good process adaptability. Example 4 uses a combination of lower limit values ​​for each parameter, such as 10% for pre-upsetting deformation, 40% for axial upsetting deformation, and 25% for oscillating deformation, and its overall performance is still significantly better than the comparative examples, proving that the parameter range defined by the claims of this invention is reasonable and universal.

[0079] From the perspective of streamline analysis, in Examples 1 to 4 using the cold precision forging tooth profile scheme of this invention, the metal streamlines at the rim and tooth root are continuously distributed along the tooth profile, and the streamlines are intact at the tooth root without any cutting off. In contrast, Comparative Examples 1 and 2, which use machining to cut the teeth, show that the metal streamlines at the tooth root are significantly cut off. The integrity of the streamlines is one of the key factors affecting the bending fatigue strength of the tooth root. This invention, through a combination of warm forging pre-forming and cold precision forging final forming process, allows the streamlines to be naturally distributed with the metal flow during the final tooth forming process, avoiding damage to the streamlines caused by machining. This is a significant reason for the more than 30% improvement in tooth root bending fatigue strength.

[0080] In summary, the embodiments of the present invention, through the combination of material micro-alloying design and multi-stage forging process with temperature zones, effectively eliminate inherent defects such as center segregation and core porosity in continuously cast billets, achieving differentiated control of the microstructure of the sun gear shaft and rim. This results in excellent comprehensive mechanical properties, microstructure uniformity, and tooth root bending fatigue performance, while maintaining a material utilization rate of over 98% and significantly reducing production energy consumption. The data from the comparative examples further verify the synergistic effect and inventive contribution among the various technical features of the present invention.

[0081] 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 alterations 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 forging process for preparing a solid sun gear shaft from a continuously cast billet, characterized in that, Specifically, the steps include the following: S1 billet preparation: The continuously cast billet is sawn into blanks; S2 stepped heating and high-temperature diffusion: The billet is first heated to 650-700℃ and preheated, then heated to 1220-1250℃ for high-temperature diffusion treatment, and then cooled to the forging temperature after diffusion is completed. S3 temperature zone forging: Low-temperature radial pre-upsetting, high-temperature axial upsetting and drawing, and medium-temperature radial rolling are performed sequentially; the low-temperature radial pre-upsetting temperature is 950-1000℃, the high-temperature axial upsetting temperature is 1150-1200℃, and the medium-temperature radial rolling temperature is 900-950℃. The sun gear rim is formed by radial rolling to obtain the wheel blank preform. S4 post-forging residual heat staged cooling: When the billet temperature after final forging is not lower than 850℃, it is cooled in three stages in sequence: air cooling, spray cooling, and furnace cooling slow cooling. S5 Low-temperature tempering: Low-temperature tempering treatment is performed on the cooled billet; S6 Cold Precision Forging Tooth Profile: The involute tooth profile is formed by cold precision forging of the tempered wheel blank preform to obtain a solid sun gear shaft forging.

2. The forging process for preparing a solid sun gear shaft from a continuously cast billet according to claim 1, characterized in that, In step S1, the chemical composition of the continuously cast billet, by mass fraction, includes: C 0.38–0.45%, Si 0.15–0.35%, Mn 0.80–1.20%, Cr 0.60–1.00%, Nb 0.02–0.05%, Ti 0.01–0.03%, P ≤ 0.015%, S ≤ 0.015%, with the balance being Fe and unavoidable impurities; wherein the mass ratio of Mn to Cr is 1.2:1 to 1.5:

1.

3. The forging process according to claim 1, characterized in that, In step S2, the temperature is raised to 650-700℃ at a heating rate of 80-100℃ / h and held for 2-3 hours; the holding time for high-temperature diffusion treatment is 4-6 hours; after diffusion, the temperature is lowered to 1180-1220℃ to enter the forging process.

4. The forging process according to claim 1, characterized in that, In step S3, the total deformation of low-temperature radial pre-upsetting is 10-15%, and the deformation force is applied symmetrically along the radial direction of the billet; after the pre-upsetting is completed, the billet is warmed back to the temperature range of high-temperature axial upsetting.

5. The forging process according to claim 1, characterized in that, In step S3, the total deformation of the high-temperature axial upsetting is 40-45%; the forging process uses a centering fixture to keep the billet axis and the part axis aligned.

6. The forging process according to claim 1, characterized in that, In step S3, the total deformation at the rim of the medium-temperature radial roller is 25-30%; the tilt angle of the roller head is 3°-5°, and the feed rate is 0.5-1.0 mm / s.

7. The forging process according to claim 1, characterized in that, In step S4, the air cooling stage cools the temperature to 780-800°C at a rate of 5-8°C / s; the spray cooling stage cools the temperature to 550-600°C at a rate of 15-20°C / s; and the furnace cooling slow cooling stage cools the temperature to 200°C before the product is removed from the furnace.

8. The forging process according to claim 1, characterized in that, In step S5, the temperature of the low-temperature tempering is 200-240℃, the holding time is 3-5h, and after tempering, it is cooled to room temperature with the furnace.

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