Medium carbon bearing steel for robot harmonic reducer flexspline and production method thereof

Abstract

The invention relates to a medium-carbon bearing steel for a flexible pulley of a robot harmonic reducer and a production method thereof. The chemical composition of the product is as follows: C: 0.36-0.43%, Si: 0.15-0.35%, Mn: 0.60-0.90%, Cr: 0.60-1.00%, S≤0.015%, P≤0.025%, Ni: 1.60-2.00%, Cu≤0.25%, Mo: 0.15-0.35%, Ca≤0.0010%, Ti≤0.003%, O≤0.0010%, As≤0.01%, Sn≤0.015%, Sb≤0.005%, and the balance is Fe and unavoidable impurities. The production process is molten iron pretreatment - converter smelting - LF refining - RH refining - large-section CCM continuous casting billet - slow cooling of continuous casting billet - continuous rolling - slow cooling of rolled product - softening annealing - finishing. The steel has high mechanical properties, high purity and high structural uniformity, and meets the requirements for flexible wheel steel for robot harmonic reducers.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and in particular relates to a medium-carbon bearing steel for a flexible pulley of a harmonic reducer of a robot and a production method thereof. Background Art

[0002] In recent years, rising labor costs and an aging population have put tremendous pressure on traditional manufacturing. Some companies have begun upgrading to intelligent manufacturing to replace traditional methods, improving production efficiency and reducing costs. The intelligent robotics industry has become a key development direction for the global manufacturing industry and has experienced rapid growth.

[0003] Intelligent robots have three core components: motion controllers, servo drive motors, and precision reducers. Precision reducers are the most critical, accounting for over 35% of manufacturing costs. Depending on the transmission method, precision reducers are generally divided into RV reducers and harmonic reducers. Harmonic reducers offer advantages such as compact structure, light weight, and small size, while also offering high transmission efficiency and precision. They are currently the most technologically advanced and competitive precision reducers for robotics.

[0004] A harmonic reducer consists of four basic components: a fixed internally toothed rigid wheel, a flexspline, a wave generator, and a flexible bearing. The wave generator, coupled with a flexible bearing, allows the flexspline to undergo controllable elastic deformation and mesh with the rigid wheel to transmit power. As the wave generator rotates, the meshing state between the flexspline and the rigid wheel constantly changes, requiring every part of the flexspline to withstand various forms of alternating stress for extended periods. This places extremely stringent demands on the flexspline's precision and performance. As a key component of a harmonic reducer, the fatigue life of the flexspline generally determines its service life. In summary, the flexspline components of a harmonic reducer need to be manufactured from special materials with high purity, high structural uniformity, and high fatigue life.

[0005] Currently, the material used for flexible pulleys in harmonic reducers is monopolized by foreign companies, leaving domestic research and development of this material largely unexplored. The long-term reliance on imported materials for core components of harmonic reducers has significantly increased the production cost of domestic intelligent robots, severely hindering the development of the industry.

[0006] Currently, the primary material used for the flexspline of harmonic reducers is steel produced through die casting. While die-cast steel offers certain advantages in terms of structural uniformity, it also suffers from significant disadvantages such as very low production efficiency, yield rate, and capacity, and very high energy consumption and production costs. Therefore, our company utilizes a vacuum degassing continuous casting and rolling process to improve key quality indicators such as steel purity, oxygen content, and residual harmful elements, thereby enhancing structural uniformity. This allows us to develop and produce world-leading steel for flexspline components in harmonic reducers, thereby promoting the localization of key materials for robotic reducers. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for manufacturing bearing steel with high mechanical properties, high structural uniformity, and high purity, in response to the above-mentioned existing technologies. By rationally designing the chemical composition and optimizing and controlling key processes based on current production methods for high-purity bearing steel, the present invention achieves the high mechanical properties, high structural uniformity, and high purity required for the performance of flexible splines used in harmonic robot reducers.

[0008] The present invention requires that the steel material has high strength, certain toughness and impact resistance. The specific requirements for mechanical properties are shown in Table 1. To ensure the high strength of the steel material, the present invention requires that the grain size be ≥ 6 and no mixed crystal structure be present.

[0009] Table 1

[0010]

[0011] The present invention requires that the steel have a high degree of microstructure uniformity, and therefore has strict requirements on the macrostructure, requiring that the central porosity is ≤ level 1.0, the general porosity is ≤ level 1.0, the ingot segregation is ≤ level 1.0, and the central segregation is ≤ level 1.5. In addition, shrinkage cavities, bubbles, cracks, inclusions, delamination, peeling and white spots are not allowed to appear; secondly, in order to ensure the uniformity of the microstructure of the steel, the present invention requires that the banded structure rating does not exceed level 2.

[0012] In order to ensure that the flexible spline has a long service life, the steel material is required to have a high purity, so the present invention puts forward strict requirements on microscopic inclusions. Since Class B and Class D inclusions are brittle inclusions with high hardness, they will not deform under the action of stress. Therefore, during the use of the flexible spline, stress concentration is likely to occur near these brittle inclusions, thereby initiating cracks and causing premature failure of the flexible spline. Moreover, the larger the size of such inclusions, the greater the harm they cause. Since Class A and Class C inclusions are plastic inclusions with high ductility, they pose less harm to the service life of the flexible spline. The present invention requires that microscopic non-metallic inclusions be inspected according to the GB / T 10561A method. The specific requirements for the rating of microscopic non-metallic inclusions are shown in Table 2 below.

[0013] Table 2

[0014]

[0015] The technical solution adopted by the present invention to solve the above problems is: a bearing steel for a flexible pulley of a robot harmonic reducer, wherein the chemical composition is: C: 0.36-0.43%, Si: 0.15-0.35%, Mn: 0.60-0.90%, Cr: 0.60-1.00%, S≤0.015%, P≤0.025%, Ni: 1.60-2.00%, Cu≤0.25%, Mo: 0.15-0.35%, Ca≤0.0010%, Ti≤0.003%, O≤0.0010%, As≤0.01%, Sn≤0.015%, Sb≤0.005%, and the remainder is Fe and unavoidable impurities.

[0016] The chemical composition design basis of the bearing steel for the robot harmonic reducer flexspline of the present invention is as follows:

[0017] 1) Determination of C content

[0018] Carbon content is one of the most important elements affecting the hardness and strength of components. Increasing the carbon content can improve the hardness and strength of steel. However, excessive carbon content can reduce the steel's plasticity and toughness. The steel used in the flexspline of a harmonic robot reducer must possess both high strength and a certain degree of toughness and impact resistance. The carbon content in this invention is 0.36-0.43%.

[0019] 2) Determination of Si content

[0020] Silicon is a deoxidizing element in steel and increases its strength through solid solution strengthening. It reduces the diffusion rate of carbon in ferrite, preventing carbides from precipitating during tempering and increasing the steel's tempering stability. Silicon also reduces oxidation during frictional heating, thereby improving the steel's wear resistance. However, excessive silicon content can reduce the steel's toughness and increase its sensitivity to overheating, cracking, and decarburization. The Si content in this invention is preferably 0.15-0.35%.

[0021] 3) Determination of Mn content

[0022] Manganese strengthens steel through solid solution strengthening, increasing its strength. Manganese acts as a deoxidizing and desulfurizing element in steel. During the steelmaking process, manganese forms MnS inclusions with sulfur, making it easier to break during cutting and ensuring the steel's machinability. However, excessive manganese content can increase the steel's cracking susceptibility. The present invention controls the Mn content to 0.60-0.90%.

[0023] 4) Determination of Cr content

[0024] Chromium is a strong carbide-forming element that can improve the strength and wear resistance of steel. However, excessive chromium content can reduce the impact toughness of steel. The Cr content in the present invention is determined to be within the range of 0.60-1.00%.

[0025] 5) Determination of Ni content

[0026] Nickel can improve the hardenability of steel and significantly improve its impact toughness, but nickel is also a precious metal and too high a content will increase the cost. In the present invention, the Ni content is controlled at 1.60-2.00%.

[0027] 6) Determination of Mo content

[0028] Molybdenum's primary function is to enhance hardenability and improve the mechanical properties of steel, particularly toughness. It also lowers the ductile-brittle transition temperature of steel and inhibits temper brittleness. However, molybdenum is a precious metal, and adding too much increases manufacturing costs. Therefore, the Mo content in this invention is controlled within a range of 0.15-0.35%.

[0029] 7) Determination of Ca content

[0030] Calcium content increases the number and size of point-like oxides in steel. Furthermore, because these oxides are hard and have poor plasticity, they do not deform when the steel is deformed, and tend to form voids at the interface, degrading the steel's performance. The range of the Ca content in the present invention is determined to be ≤0.001%.

[0031] 8) Determination of Ti content

[0032] Titanium and N elements can combine to form titanium nitride inclusions, which are also brittle inclusions. They are very hard and sharp, which has a great impact on the life of the flexible spline. Therefore, the present invention requires Ti≤0.0030%

[0033] 9) Determination of O content

[0034] Oxygen exists in steel as oxide inclusions. Under stress, the oxides do not deform, causing stress concentration near them and initiating cracks, which shortens the service life of the flexspline. The present invention requires that the oxygen content in the steel material cannot exceed 0.0010%.

[0035] 10) Determination of P and S content

[0036] The P element causes element segregation during the solidification of steel. It dissolves in ferrite, causing the grains to become distorted and coarse, and increasing cold brittleness. Therefore, P is determined to be ≤ 0.025%. The S element easily causes hot brittleness in steel, reduces the ductility and toughness of steel, and the formed sulfides also destroy the continuity of steel. Therefore, S is determined to be ≤ 0.015%.

[0037] 11) Determination of As, Sn, and Sb content

[0038] Trace elements such as As, Sn, and Sb are all low-melting-point non-ferrous metals. Their presence in steel causes soft spots on the surface of parts and uneven hardness. Therefore, they are considered harmful elements in steel. The content range of these elements in the present invention is determined to be As≤0.01%, Sn≤0.015%, and Sb≤0.005%.

[0039] Another object of the present application is to provide a method for producing bearing steel for a flexible pulley of a robot harmonic reducer, wherein the billet is smelted by continuous casting, and the production process is as follows: molten iron pretreatment - top and bottom blowing converter BOF - ladle refining furnace LF - vacuum circulating degassing furnace RH - large-section CCM continuous casting billet - continuous casting billet slow cooling - continuous rolling - rolled material slow cooling - soft annealing - finishing. The specific process steps are as follows:

[0040] (1) Molten steel smelting: The smelting raw materials are sequentially subjected to molten iron pretreatment KR, converter smelting, LF refining and RH vacuum degassing to obtain pure molten steel that meets the chemical composition. Molten steel smelting must undergo molten iron pretreatment to obtain clean molten iron and reduce the content of harmful elements P; when oxygen blowing smelting is carried out in the converter, the molten iron is converted into molten steel and the P content is further reduced. The end point C of the steel is controlled at 0.10-0.25%, and the end point P is controlled at ≤0.025%. The quality of scrap steel is strictly controlled, the content of residual element Cu is reduced, and Cu brittleness of steel is avoided. A slag control process is carried out before steel tapping to solve the problem of high content of harmful elements As, Sn, Pb and Sb in the existing technology; the converter tapping temperature is controlled at 1620℃~1700℃ to ensure The temperature entering the LF refining furnace is above 1500℃. During the LF refining process, low-Ti and low-Ca alloys and high-quality refractory materials are added to control the content of harmful elements Ti and Ca. High-performance synthetic slag slag making and deoxidation technology is used. The LF refining time is controlled at more than 30 minutes to ensure that the ladle maintains a long-term inclusion removal process and allows non-metallic inclusions to fully float. During RH vacuum degassing, ensure that the furnace reaches a sufficient vacuum level and maintains sufficient circulation processing time to remove the harmful gas content in the steel. By controlling the O content, non-metallic inclusions are further removed to ensure the purity of the steel.

[0041] (2) Continuous casting: Use large-section full-process protection casting to avoid secondary oxidation of molten steel; use large-section continuous casting billets with a size of 390mm×510mm and above to make the steel compression ratio greater than 20, thereby ensuring the material density and low-multiple structure uniformity; use the tundish induction heating technology, the continuous casting start temperature ≥1500℃, and low superheat casting, superheat ≤25℃, effectively improving the macro segregation of the continuous casting billet; use the light reduction process combined with electromagnetic stirring technology to effectively improve the micro segregation of the continuous casting billet caused by high alloy content, so that the material meets high uniformity, and this technology can effectively inhibit the growth of columnar crystal area, increase the central equiaxed crystal area, and make the structure grain finer; the continuous casting speed is controlled at 0.40-0.50m / min, so that the continuous casting process is stable and the liquid level fluctuation is small;

[0042] (3) Slow cooling of continuous casting billets: The continuous casting billets are slowly cooled in the pit, where the pit temperature is greater than 650°C and the slow cooling time is greater than 48 hours to avoid cracking of the steel due to surface and internal thermal stress and structural stress changes.

[0043] (4) Continuous rolling: Using a high-temperature heating process, the continuous casting billet is heated to 1210℃-1260℃ in a heating furnace with a neutral or weakly oxidizing atmosphere. The billet is fully kept warm according to the size of the continuous casting billet. The total heating time is more than 3 hours to ensure that the material is fully austenitized and recrystallized, so that the steel has a high and uniform grain size and high uniformity of the material structure. After the billet is removed from the furnace, it is first dephosphorized with high-pressure water and then rolled at a temperature of 1000-1150℃. The final rolling temperature is ≥950℃. After three processes of rough rolling, intermediate rolling and finish rolling, it is rolled into round bars. The diameter of the finished bar ranges from 20 to 100mm, and the total compression ratio is ≥20, of which the compression ratio of rough rolling is ≥8, the compression ratio of intermediate rolling is ≥5, and the finish rolling ensures the dimensional accuracy of the finished steel.

[0044] (5) Slow cooling of rolled products: The rolled bars are subjected to slow cooling at a temperature of 480°C or above. The slow cooling time is greater than 60 hours and the exit temperature is less than 200°C. This is to prevent coarsening of grains. Slow cooling is also beneficial to eliminating residual stress on the steel surface.

[0045] (6) Softening annealing: The rolled bar should be softened and annealed within 24 hours after pitting. The softening annealing temperature is 650-750℃, and the holding time is ≥7h. Then it is cooled to 500℃ in the furnace and then air-cooled to room temperature. The purpose of softening annealing is to meet the needs of users for machining the finished steel, thereby reducing the hardness of the finished steel and ensuring that the hardness of the steel after softening annealing is ≤269HBW;

[0046] (7) Finishing: including straightening, chamfering and non-destructive testing and other finishing processes. All products must undergo 100% non-destructive testing to ensure the surface and internal quality of the products.

[0047] Compared with the prior art, the advantages of the present invention are:

[0048] (1) Steelmaking requires pretreatment of iron in molten iron to control the content of harmful element P;

[0049] (2) Add clean scrap steel to converter smelting, strictly control the scrap steel quality, reduce the content of residual element Cu, and avoid Cu brittleness caused by excessive Cu content; perform slag control process before converter tapping to solve the problem of high content of harmful elements As, Sn, Pb, and Sb in existing technology;

[0050] (3) The LF refining process uses low-Ti, low-Ca alloys and high-quality refractory materials to reduce the content of harmful elements Ti and Ca in molten steel; adopts high-performance synthetic slag slag deoxidation technology to ensure that the LF time is ≥30 minutes, ensuring that inclusions are fully floated and removed with the slag, and effectively controlling the number and size of inclusions;

[0051] (4) RH vacuum cycle degassing technology is used to reduce the O and H contents in the steel to extremely low levels, effectively controlling the gas element content in the steel, thereby further reducing the number of inclusions and ensuring the high purity requirements of the steel. Testing has shown that the number and size of inclusions and O content of the product of this invention have reached world-leading levels;

[0052] (5) Continuous casting adopts tundish induction heating technology, the start pouring temperature of continuous casting is ≥1500℃, and low superheat pouring is adopted, the superheat is ≤25℃, which effectively improves the macro segregation of continuous casting billets; large cross-section continuous casting billets with a size of 390mm×510mm and above are used to make the steel compression ratio greater than 20, thereby ensuring the density of the material and the uniformity of the macrostructure; the continuous casting speed is controlled at 0.40-0.50m / min, so that the continuous casting process is stable and the liquid level fluctuation is small;

[0053] (6) In order to meet the high mechanical properties of the harmonic reducer flexible wheel, the medium carbon bearing steel of the present invention reduces the carbon content and increases the content of Cr, Ni and Mo alloy elements. The high alloy element content will cause serious micro segregation to the continuous casting billet. The present invention adopts a light reduction process combined with electromagnetic stirring technology during the continuous casting process, which effectively improves the micro segregation caused by the high alloy content to the continuous casting billet, so that the material meets the high uniformity. In addition, this technology can effectively inhibit the growth of the columnar crystal area, increase the central equiaxed crystal area, and make the microstructure grains finer.

[0054] (7) The continuous rolling process adopts a high-temperature heating process, the heating temperature is controlled at 1210℃-1260℃, and the total heating time is more than 3 hours to ensure that the material is fully austenitized and recrystallized, so that the steel has a high and uniform grain size and ensures high uniformity of the material structure.

[0055] (8) The bearing steel for the flexible pulley of the robot harmonic reducer produced by the present invention meets the following index requirements:

[0056] Microscopic inclusions are tested according to GB / T 10561A method: B fine series ≤ Grade 1.0, B coarse series ≤ Grade 0.5, D fine series ≤ Grade 1.0, D coarse series ≤ Grade 0.5, and DS series ≤ Grade 1.0. Macrostructure requirements are: central porosity ≤ Grade 1.0, general porosity ≤ Grade 1.0, ingot-shaped segregation ≤ Grade 1.0, and central segregation ≤ Grade 1.5. Shrinkage cavities, bubbles, cracks, inclusions, delamination, peeling, and white spots are not permitted. After annealing, the metallographic structure must not contain banded structures that extend through the field of view. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 This is the annealed structure of the product of Example 1 (1 / 2R);

[0058] Figure 2 This is the annealed structure of the product of Example 2 (1 / 2R);

[0059] Figure 3 This is the annealed structure (1 / 2R) of the comparative product (produced using the die casting process). DETAILED DESCRIPTION

[0060] The present invention will be further described below with reference to examples.

[0061] The manufacturing process of the medium-carbon bearing steel for the flexible wheel of the robot harmonic reducer in the embodiment of the present invention is to produce steel by adopting a forming process of molten iron pretreatment - top and bottom blowing converter BOF - ladle refining furnace LF - vacuum circulation degassing furnace RH - large-section CCM continuous casting billet - slow cooling of continuous casting billet - continuous rolling - slow cooling of rolled material - softening annealing - finishing.

[0062] Specifically, high-quality molten iron, scrap steel and raw and auxiliary materials are selected during smelting, and high-quality deoxidizers and refractory materials are selected. The end point C of the converter steel tapping is controlled at 0.10-0.25%, the end point P is controlled at ≤0.025%, and the tapping temperature is 1620℃-1700℃. The pouring temperature of continuous casting is ≥1500℃, the superheat is controlled within ≤25℃, and the pulling speed is controlled at 0.40-0.50m / min. The produced continuous casting billet is slowly cooled in the pit, wherein the pit temperature is greater than 650℃, the pit slow cooling time is greater than 48 hours, and the pit outlet temperature is less than 300℃. After the slow cooling is completed, the above-mentioned continuous casting billet is sent to a heating furnace with a neutral or weakly oxidizing atmosphere for heating and rolling into products. The steel rolling heating, rolling and cooling processes are shown in Table 3 below. The rolled product needs to be slowly cooled in a pit, with an entry temperature of ≥480°C and a starting temperature of ≤300°C for ≥60 hours. Within 24 hours of exiting the pit, it undergoes a soft annealing treatment at 650-750°C for ≥5 hours. The product is then cooled in the furnace to 400°C and then air-cooled to room temperature. The bar is then straightened and inspected for flaws to produce the desired finished product.

[0063] Table 3 Steel rolling heating, rolling and cooling process

[0064]

[0065] The chemical compositions (wt %) of the flexible wheel materials of various embodiments of the present invention and those currently used in the market for die-cast harmonic reducers (for comparison) are shown in Table 4.

[0066] Table 4

[0067] C Si Mn P S Cr Cu Ni Example 1 0.39 0.24 0.77 0.021 0.006 0.8 0.02 1.71 Example 2 0.39 0.28 0.79 0.018 0.007 0.8 0.02 1.72 Comparative Example 0.41 0.23 0.81 0.019 0.008 0.8 0.18 1.61

[0068] Table 4

[0069] Mo As Sn Sb Ca Ti O Example 1 0.23 0.0038 0.0015 0.0013 0.0002 0.0012 0.00041 Example 2 0.23 0.0038 0.0017 0.0013 0.0004 0.0012 0.00045 Comparative Example 0.16 0.0040 0.0087 0.0013 0.0003 0.0018 0.00133

[0070] From the perspective of composition, the present invention has optimized the chemical composition of conventional harmonic reducer flexible wheel materials to a certain extent, retained the plasticity of the finished steel by reducing the C content, and improved the strength and impact toughness of the steel by increasing the Ni and Mo contents. Through converter smelting and strict control of the quality of scrap steel, the content of residual element Cu is reduced to avoid the Cu brittle phenomenon caused by excessive Cu content. The slag control process is carried out before the converter is tapped to effectively reduce the Sn content. The Cu and Sn contents of the steel of the present invention are much lower than those of die castings. Since LF refining adopts low Ti, low Ca alloys, the Ti content of the present invention is much lower than that of die castings. LF refining adopts high-performance synthetic slag slag making and deoxidation technology, RH vacuum cycle degassing and continuous casting full-process protection pouring technology. The O content of the present invention is significantly better than that of die castings, and the actual quality has reached the international leading level.

[0071] The mechanical property data of the embodiments of the present invention and the comparative examples are shown in Table 5.

[0072] Table 5

[0073]

[0074] In terms of mechanical properties, the steel of the present invention has strength and plasticity comparable to those of die-cast steel, meeting the requirements for use in harmonic reducer flexsplines. This demonstrates that reducing the carbon content and increasing the Ni and Mo contents does not significantly reduce strength and plasticity. However, the impact energy of the steel of the present invention is significantly higher than that of die-cast steel, indicating that the addition of Ni and Mo improves impact toughness. The use of soft reduction and electromagnetic stirring in the continuous casting process, as well as high-temperature heating in the continuous rolling process, effectively achieves uniform microstructure and grain refinement. Due to the soft annealing treatment employed in the present invention, the post-annealing hardness is significantly lower than that of die-cast steel, making it more user-friendly for machining.

[0075] The grain size rating results of the steel materials of various embodiments of the present invention and comparative examples are shown in Table 6.

[0076] Table 6

[0077] Grain size / grade Example 1 8.5 Example 2 8 Comparative Example 7

[0078] Judging from the grain size results, since the continuous casting process of the present invention adopts light reduction combined with electromagnetic stirring technology and the continuous rolling adopts high-temperature heating technology, the grain size rating results of the steel of the present invention are significantly higher than those of the die-cast material, and have reached the international advanced level.

[0079] The low-magnification data of the steel materials of various embodiments of the present invention and comparative examples are shown in Table 7.

[0080] Table 7

[0081] Loose center Generally loose Ingot segregation Central segregation Example 1 1.0 1.0 1.0 0 Example 2 1.0 1.0 1.0 0 Comparative Example 1.0 1.0 1.0 0

[0082] The steel material of the present invention has a low-density mass equivalent to that of a die-cast material.

[0083] The microscopic non-metallic inclusion rating results of the steels of the embodiments of the present invention and the comparative examples are shown in Table 8.

[0084] Table 8

[0085]

[0086] Because the LF refining of the present invention utilizes high-performance synthetic slag slagging and deoxidation technology, RH vacuum cycle degassing, and continuous casting full-process protective pouring technology, the oxygen content of the steel of the present invention is much lower than that of the die-cast steel. This further results in the number and size of inclusions of the present invention being much smaller than those of the die-cast steel. In particular, the level of inclusion control of the B-fine and DS-type brittle inclusions, which may cause premature failure of the flexspline material, has reached the international advanced level.

[0087] The annealing structure of the product of Example 1 of the present invention at 1 / 2R is shown in FIG. Figure 1 , the annealing structure of the product of Example 2 at 1 / 2R is shown in Figure 2 , see the annealing structure view of the comparative product at 1 / 2R Figure 3 .

[0088] Since the continuous casting process of the present invention adopts a light pressure combined with an electromagnetic stirring process, the micro-segregation of the material can be significantly improved. After annealing, the banded structure of the material is evenly distributed and the band width is narrow, which is significantly smaller than that of the die-cast material. In addition, no banded structure is found to penetrate the field of view in the steel of the present invention, and the banded structure is superior to that of the die-cast material.

[0089] In summary, the present invention relates to a bearing steel for a flexspline of a harmonic reducer and its production method. By improving the purity of the steel, the invention employs a high-efficiency, high-capacity, low-cost process involving vacuum degassing, continuous casting, and rolling. Key processes are optimized and controlled, resulting in a high purity and uniformity of the steel while maintaining the required mechanical properties. This significantly enhances the competitiveness of Chinese products in terms of production efficiency, production costs, and product quality stability.

[0090] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing medium carbon bearing steel for a flexible pulley of a robot harmonic reducer, characterized by: The chemical composition of the steel is: C: 0.36-0.43%, Si: 0.15-0.35%, Mn: 0.60-0.90%, Cr: 0.60-1.00%, S≤0.015%, P≤0.025%, Ni: 1.60-2.00%, Cu≤0.25%, Mo: 0.15-0.35%, Ca≤0.0010%, Ti≤0.003%, O≤0.0010%, As≤0.01%, Sn≤0.015%, Sb≤0.005%, and the balance is Fe and unavoidable impurities. The method comprises the following steps: (1) Molten steel smelting The smelting raw materials are sequentially subjected to hot metal pretreatment (KR), converter smelting, LF refining, and RH vacuum degassing. The hot metal pretreatment is used during molten steel smelting to reduce the content of harmful element P. During oxygen blowing smelting in the converter, the tapping endpoint C is controlled at 0.10-0.25%, and the endpoint P is controlled below 0.025%. The quality of scrap steel is strictly controlled, and slag control is performed before tapping. The converter tapping temperature is controlled at 1620-1700°C to ensure that the temperature entering the LF refining furnace is above 1500°C. Low-Ti, low-Ca alloys and high-quality refractory materials are added during the LF refining process, and high-performance synthetic slag slag deoxidation technology is used. The LF refining time is controlled at more than 30 minutes. During RH vacuum degassing, sufficient vacuum is ensured in the furnace and sufficient circulation time is maintained to remove harmful gas content in the steel. (2) Large-section CCM continuous casting billet Large-section continuous casting billets with a size of 390mm×510mm or larger are used to achieve a steel compression ratio greater than 20. Tundish induction heating technology is used, with a continuous casting start temperature of ≥1500°C and low superheat pouring, with a superheat of ≤25°C. A soft reduction process is used in conjunction with electromagnetic stirring technology, and the continuous casting speed is controlled at 0.40-0.50m / min. (3) Slow cooling of continuous casting billets, where the pit temperature is greater than 650 ° C, and the slow cooling time in the pit is ensured to be greater than 48 hours; (4) Continuous rolling The continuous casting billet is heated to 1210-1260℃ in a heating furnace with a neutral or weakly oxidizing atmosphere. The total heating time is more than 3 hours. After the billet is removed from the furnace, it is first dephosphorized with high-pressure water and then rolled at a temperature of 1000-1150℃. The final rolling temperature is ≥950℃. After three processes of rough rolling, intermediate rolling and finish rolling, it is rolled into round bars. The diameter of the finished bars ranges from 20 to 100mm, and the total compression ratio is ≥20, of which the compression ratio of rough rolling is ≥8, the compression ratio of intermediate rolling is ≥5, and the finish rolling ensures the dimensional accuracy of the finished steel. (5) Slow cooling of rolled products: The rolled bars are subjected to slow cooling at a temperature of 480°C or above, with a slow cooling time of more than 60 hours and a temperature of less than 200°C. The bars are subjected to softening annealing treatment again within 24 hours after the pit is opened; (6) Softening annealing: Softening annealing temperature is 650-750℃, holding time is ≥7h, then cooled to 500℃ in the furnace and air-cooled to room temperature; (7) Finishing, including straightening, chamfering and non-destructive testing and other finishing processes. All products must undergo 100% non-destructive testing to ensure the surface and internal quality of the products.

2. The medium carbon bearing steel for a flexible pulley of a robot harmonic reducer according to claim 1, characterized in that: The steel grain size is ≥ 6, and there is no mixed crystal structure. The microscopic non-metallic inclusions are tested using the gold GB / T 10561 A method, and the B fine series is ≤ 1.5, B coarse series ≤ 1.0, D fine series ≤ 1.0, D coarse series ≤ 1.0, and DS series ≤ 1.

0. The macrostructure of the steel is graded using GB / T 1979, and the requirements are central porosity ≤ 1.5, general porosity ≤ 1.0, ingot segregation ≤ 1.0, and central segregation ≤ 1.

5. The yield strength requirement is ≥ 980 MPa, the tensile strength requirement is ≥ 980 MPa, the elongation is ≥ 12%, the cross-sectional reduction rate is ≥ 55%, the room temperature impact resistance AKV ≥ 78 J, the annealing hardness is ≤ 269 HBW, and the metallographic structure after annealing does not have banded structure that runs through the field of view.

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