A carburized bearing steel with ultra-fine grain size for the eccentric shaft of a robot RV reducer and its production method

Abstract

The present invention relates to a carburized bearing steel with an ultrafine grain size for an eccentric shaft of a robot RV reducer. The steel has the following chemical composition: C: 0.10-0.20%, Si: 0.20-0.40%, Mn: 0.60-1.00%, P≤0.020%, S≤0.015%, Cr: 1.50-1.80%, Mo: 0.25-0.35%, Ni: 1.40-1.70%, Nb: 0.02-0.05%, W≤0.008%, Ti≤0.002%, Al: 0.020-0.050%, with the remainder being Fe and unavoidable impurities. The production process comprises: KR molten iron pretreatment - converter - LF refining - RH vacuum degassing - CCM continuous casting - slow cooling of continuous casting billet - forging continuous casting billet into intermediate billet - intermediate billet stack cooling - intermediate billet hot rolling into product - rolled product stack cooling - double quenching and tempering - finishing. The continuous casting billet should be slowly cooled in the pit, and the intermediate billet and rolled product should be pile cooled. The steel has high hardenability, high mechanical properties, fine grain size and high purity, and meets the requirements of carburized bearing steel for the eccentric shaft of the robot RV reducer.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and in particular relates to a carburized bearing steel with ultra-fine grain size for an eccentric shaft of a robot RV reducer and a production method thereof. Background Art

[0002] RV reducers are widely used in heavy-load applications such as the booms of industrial robots due to their high precision, large transmission ratio, high transmission efficiency, and high load-bearing capacity. The eccentric shaft, the power input component of an RV reducer, determines its service life based on its quality.

[0003] The eccentric shafts of RV reducers are mostly made of traditional high-carbon chromium bearing steel GCr15 material and are produced through hot forging + tempering treatment + turning. The eccentric shafts of RV reducers produced by this manufacturing process have high surface wear resistance and use high-carbon chromium bearing steel, which meets the long-life service conditions of the eccentric shafts. However, the eccentric shafts produced by this process can only withstand low stress and cannot meet the requirements of high-end fields such as automotive manufacturing, aerospace, and biomedicine for RV reducer eccentric shafts.

[0004] With the large-scale development of RV reducers, the input end needs to withstand more impact stress. Many domestic RV reducer manufacturers use carburized bearing steel as the raw material for the production of eccentric shafts. Usually, 20CrMoH high hardenability carburized bearing steel is used to replace traditional high carbon chromium bearing steel. The manufacturing process has also been gradually changed to a new production process of hot forging + high temperature quenching + low temperature tempering + carburizing heat treatment + turning. Carburized bearing steel not only ensures high surface wear resistance but also has a certain core impact toughness, which greatly extends the service life of the RV reducer and meets the extreme use conditions of the eccentric shaft of the RV reducer. The advantage of the new "hot forging" production process is that it does not put forward requirements for raw materials, but there is a bottleneck in the strength of the 20CrMoH material.

[0005] With the rapid development of heat treatment technology in my country, the traditional process of "hot forging" has gradually been replaced by the "cold forging" process. Although the cold forging process can greatly reduce material loss, it also places strict requirements on raw materials. The delivery state of the material usually requires spheroidizing annealing silver bright state, the structure needs to be spheroidized structure, the spheroidization rate ≥ 60%, the hardness ≤ 200HBW, the raw materials need to be spheroidized annealing treatment, and quenching + tempering heat treatment is required before carburizing heat treatment, resulting in a long production process and high processing energy consumption of this process, which does not have advantages.

[0006] For the new production process of eccentric shafts, the key lies in the control of the key quality of the carburized bearing steel raw materials to ensure high strength and impact toughness, as well as extremely fine high-temperature austenite grain size after carburizing treatment. Therefore, the present invention proposes a carburized bearing steel with ultra-fine grain size for the eccentric shaft of a robot RV reducer and a production method thereof. On the one hand, by combining the forging + rolling process, the forging process before the tempering treatment is reduced, the density and organizational uniformity of the raw materials are improved, and the production cost is reduced. On the other hand, by innovative proportioning of the chemical composition of the raw materials and innovative research on the double quenching + tempering process, the mechanical properties of the material are improved, and the austenite grain size after carburizing is refined. Secondly, the key smelting process is optimized, studied and controlled, and by improving the purity of the steel, a long-life, high-stability product is obtained, broadening the market application of RV reducers. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a method for producing ultra-fine-grained carburized bearing steel for the eccentric shaft of a robot RV reducer, in response to the above-mentioned prior art. By adopting a large-section continuous casting process and combining forging and rolling, the finished steel ensures its structural density. Through innovative research into a double quenching and tempering process, the product possesses a fine tempered bainite structure, with a high-temperature austenite grain size of ≥7. Currently, no product with similar performance has been reported in China. While ensuring the high surface wear resistance and core impact toughness of the eccentric shaft, the purity and structural uniformity of the steel are improved, making this product highly competitive in the market.

[0008] The technical solution adopted by the present invention to solve the above problems is: a carburized bearing steel with ultra-fine grain size for the eccentric shaft of the robot RV reducer, the chemical composition of which is as follows by mass percentage: C: 0.10-0.20%, Si: 0.20-0.40%, Mn: 0.60-1.00%, P≤0.020%, S≤0.015%, Cr: 1.50-1.80%, Mo: 0.25-0.35%, Ni: 1.40-1.70%, Nb: 0.02-0.05%, W≤0.008%, Ti≤0.002%, Al: 0.020-0.050%, and the balance is Fe and unavoidable impurities. The chemical composition design basis of the carburized bearing steel for the eccentric shaft of the robot RV reducer of the present invention is as follows:

[0009] (1) Determination of C content

[0010] Increasing the carbon content in steel can significantly improve its tensile strength, hardness, and wear resistance. It is also the most economical element for enhancing steel's hardenability. However, excessive carbon content can significantly reduce its plasticity and impact toughness. The carbon content of carburized bearing steel for RV reducer eccentric shafts is generally controlled at around 0.23%. To improve the product's impact toughness, the present invention sets the carbon content to 0.10% to 0.20%.

[0011] (2) Determination of Si content

[0012] The element Si plays a major role in reducing and deoxidizing during the steelmaking process. Increasing the Si content in steel helps reduce oxygen content during the steelmaking process. In killed steel, the Si content is generally 0.15% to 0.30% to ensure deoxidation. Increasing the Si content significantly improves the elastic limit, yield point, and tensile strength of the steel. However, excessive Si content significantly increases the depth of the decarburization layer in the steel. Therefore, the Si content in the present invention is 0.20% to 0.40%.

[0013] (3) Determination of Mn content

[0014] Mn is an excellent deoxidizing and desulfurizing element. The Mn content in killed steel is generally controlled between 0.30% and 0.50%. During the steelmaking process, Mn reacts with S to form high-melting-point MnS, thereby weakening and eliminating the hot brittleness caused by S. Mn also significantly improves the hardenability and strength of steel. Therefore, the Mn content in the present invention is controlled between 0.60% and 1.00%.

[0015] (4) Determination of P and S content

[0016] 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 ≤ 0.020%. The S element easily causes hot brittleness in steel, reducing the ductility and toughness of steel, but the formed sulfide improves the cutting performance. Therefore, S is determined to be ≤ 0.015%.

[0017] (5) Determination of Cr content

[0018] Increasing the Cr content significantly improves the hardenability of steel. Cr is also a strong carbide-forming element, easily precipitating fine, dispersed carbides during tempering, which can improve the strength and wear resistance of steel. However, excessive Cr content can lead to the formation of massive carbides, reducing the impact toughness of the steel and shortening its service life. Therefore, the Cr content in the present invention is set within a range of 1.50% to 1.80%.

[0019] (6) Determination of Mo content

[0020] Mo's primary role in carburized bearing steel is to increase hardenability and improve the steel's mechanical properties, particularly toughness. Therefore, based on the high hardenability requirements of steel for eccentric shafts in robot RV reducers, the Mo content in this invention is controlled within a range of 0.25% to 0.35%.

[0021] (7) Determination of Ni content

[0022] Nickel can significantly improve the yield strength and tensile strength of steel after quenching and tempering, and significantly improve impact toughness. To meet the use requirements of the eccentric shaft of the robot RV reducer, the Ni content of the present invention is controlled at 1.40% to 1.70%.

[0023] (8) Determination of Nb content

[0024] Nb is a strong carbide-forming element, existing in steel in the form of nano-scale carbonitrides. These carbonitrides often precipitate during high-temperature processes and pin austenite grain boundaries, significantly hindering the growth of austenite grains and refining the high-temperature austenite grain size of the steel. However, Nb also impairs the surface quality of the steel. Therefore, to balance the requirements for high-temperature grain size and surface quality of the eccentric shaft, the control range of the Nb content in the present invention is determined to be 0.02% to 0.05%.

[0025] (9) Determination of W content

[0026] W has the characteristics of high melting point and high density. It is a strong carbide-forming element and can promote the grain refinement of steel, especially at high temperature. W can also enhance the tempering stability of steel. However, as a precious metal element, W has a high cost of addition. Therefore, considering the ultra-fine high-temperature austenite grain size and production economy of the eccentric shaft, the W content of the present invention is ≤0.008%

[0027] (10) Determination of Ti content

[0028] Ti easily forms non-metallic inclusions such as TiN or Ti(C,N) in molten steel. These inclusions are extremely hard, have little plasticity, and are typically angular. During product processing, microcracks can easily form near these inclusions, severely reducing the fatigue life of the product. Therefore, the Ti content in this invention must not exceed 0.002%.

[0029] (11) Determination of Al content

[0030] Al is typically added as a primary deoxidizing element, reacting with dissolved oxygen in the molten steel to form high-melting-point Al2O3 non-metallic inclusions. These inclusions can be removed through specific methods. Any remaining Al in the molten steel can severely reduce its purity, thereby impacting the product's service life. Al also readily combines with nitrogen, forming stable nanoscale AlN precipitates at grain boundaries. These precipitates hinder austenite grain growth and significantly refine the austenite grains. Therefore, considering the high-temperature austenite grain size and ultra-long fatigue life requirements of the eccentric shaft, the Al content in the present invention is determined to be within the range of 0.020% to 0.050%.

[0031] (12) Determination of N content

[0032] The primary purpose of nitrogen in carburized bearing steel is to form more nanoscale AlN precipitates, thereby refining the austenite grain size. Since AlN precipitates at an atomic ratio of 1:1, or a weight ratio of 27:14, a higher nitrogen content favors AlN nucleation. However, this also leads to increased TiN formation. Therefore, to coordinate the compositional design with the Al content and reduce the precipitation of TiN inclusions, the nitrogen content in this invention must be controlled within 300 ppm.

[0033] (13) Determination of O content

[0034] The oxygen content represents the number of oxide inclusions in the steel. Extensive testing has shown that reducing the oxygen content significantly improves the purity of the steel and increases its service life. Therefore, to meet the high purity requirements for steel used in cycloid gears for robot RV reducers, the oxygen content in this invention is set within a range of 10 ppm.

[0035] The main technical indicators of the carburized bearing steel with ultra-fine grain size for the eccentric shaft of the robot RV reducer and the production method thereof of the present invention are as follows:

[0036] The present invention requires that the steel material first undergo normalizing at 900°C for 1 hour and quenching at 860°C for 1 hour, and then the terminal hardenability of the steel material is tested according to ISO 642. The specific hardenability requirements are shown in Table 1 below.

[0037] Table 1

[0038] End quenching distance, mm J5 J11 J25 J40 Hardness, HRC 39～48 36～47 31～43 29～41

[0039] Secondly, according to the use requirements of the eccentric shaft of the robot RV reducer, the steel is required to have high strength, as well as certain toughness and impact toughness. The specific mechanical property requirements are shown in Table 2 below.

[0040] Table 2

[0041]

[0042] The carburized bearing steel for the eccentric shaft of the robot RV reducer of the present invention has an ultra-fine high-temperature austenite grain size. The steel is required to be carburized at 980°C for 5 hours, air-cooled, and the austenite grain size is inspected. The grain size should be finer than level 7, and mixed crystal structure is not allowed to exist.

[0043] In order to meet the requirement of long fatigue life of the eccentric shaft of the robot RV reducer, the steel is required to have ultra-high macro and micro purity. First, to ensure macro purity, the present invention requires high-frequency flaw detection of the steel in accordance with SEP 1927, requiring defects not exceeding 2mm / dm 3 , a single defect does not exceed 5mm. Secondly, to ensure microscopic purity, 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 non-metallic inclusions are shown in Table 3 below.

[0044] Table 3

[0045]

[0046] The production process for the ultra-fine-grained carburized bearing steel for the eccentric shaft of the robot RV reducer is as follows: KR hot metal pretreatment - converter - LF refining - RH vacuum degassing - CCM continuous casting - slow cooling of the continuous casting billet - forging the continuous casting billet into an intermediate billet - intermediate billet stack cooling - intermediate billet hot rolling into a finished product - rolled product stack cooling - double quenching and tempering - and finishing. The main production process features are as follows:

[0047] (1) KR hot metal pretreatment is used to reduce the content of harmful element S in the hot metal. The S content in the hot metal after deoxidation is ≤0.03%. In order to improve the deoxidation performance of the hot metal, the Si content in the hot metal is guaranteed to be ≥0.15%; secondly, the hot metal after KR pretreatment is initially smelted in the converter to reduce the content of C and P elements. The end point C of the converter is ≤0.2%, and the end point P is ≤0.02%. The converter tapping temperature is ≥1600℃. When the converter taps 3-5 tons of steel, 3-5 bags of SiC are manually added to further increase the Si content in the steel, so that the steel has good deoxidation conditions when it arrives at the LF furnace. At the end of the converter tapping, ≥250kg of aluminum iron is added from the high-level silo for pre-deoxidation; secondly, the temperature from the converter to the LF furnace is ≥1520℃, and W iron, Nb iron, Ni iron, Mo iron and low Ti chromium iron alloy are added in sequence. , strictly follow the alloy ratio to ensure the hardenability of the material, while preventing the formation of large-sized TiN, and use imported refractory materials to prevent the formation of large-sized Al-Mg oxides, ensure that the material passes the high-frequency flaw detection test, use high-performance refined synthetic slag, maintain a long refining and slagging process, LF process ≥50min, so that non-metallic inclusions can be fully floated; when the RH furnace is vacuum degassing, the furnace reaches a sufficient vacuum degree, the vacuum degree needs to be maintained between 100-150Pa, and the high vacuum time is ≥20min to further remove harmful gases and non-metallic inclusions in the molten steel. After breaking the air, the nitrogen content of the molten steel is increased by manual argon connection, and the nitrogen flow rate is ≤300L / min to ensure that the nitrogen content of the molten steel is within 300ppm, further making the inclusions fully float;

[0048] (2) Continuous casting: Use three machines and three streams for large cross-section continuous casting billets, with specifications of 350mm×500mm and above, to ensure high compression ratio rolling of steel, thereby ensuring the density of the material; use argon protection casting and add protective slag throughout the process to protect the molten steel from secondary oxidation contamination; use soft reduction technology, with a light reduction amount of ≤18mm, to effectively improve low-magnification defects such as central porosity and shrinkage cavities; use high superheat casting, with the superheat controlled at ≥30℃, to reduce the number of inclusions in the billet; the pulling speed of the continuous casting billet is ≤0.8m / min, the remaining steel volume in the ladle is not less than 5 tons, the continuous casting billet should be slowly cooled at a temperature above 600℃ to prevent cracking of the continuous casting billet, the slow cooling time should be greater than 48 hours, and the exit temperature should be ≥200℃;

[0049] (3) Forging intermediate billets for continuous casting: The continuous casting billets are sent to a heating furnace in a neutral or weakly oxidizing atmosphere for heating. The heating temperature is controlled at 1050-1250°C, and the heating time of the billets is ≥5h to ensure that the material is fully heated. Subsequently, the billets are forged into square intermediate billets of 200mm×200mm-300mm×300mm in a precision forging machine. After studying the thermoplasticity of the carburized bearing steel of the present invention, it is found that the forging temperature is around 950°C, and the material has good high-temperature plasticity. Therefore, low-temperature forging and three-pass forging processes are adopted during the forging process, and the forging rate is strictly controlled. Specifically, the starting forging temperature is 1000±50°C, the initial forging rate is 7.0m / min, the second pass forging rate is 5.0m / min, the last pass forging rate is 2.0m / min, the interval between each pass is ≤5min, the final forging temperature is controlled at ≥850°C, and after forging, the intermediate billet is pile-cooled for a cooling time of ≥60h. The novel low-temperature forging process of the present invention reduces the initial forging temperature, adopts a three-step forging process and strictly controls the forging rate and interval time to ensure the final forging temperature, making the forging process more suitable for the thermoplasticity of the carburized bearing steel of the present invention;

[0050] (4) Rolling the intermediate billet into finished products: The intermediate billet is sent to the heating furnace for heating and then rolled into finished steel products. A high-temperature heating process is used, specifically: the temperature of the preheating section is controlled at 650-900℃, the temperature of the heating section is controlled at 1100-1250℃, the temperature of the soaking section is controlled at 1200-1280℃, the compression ratio of the first rolling process is 2.3-3.0, the temperature of the second rolling process is 980℃-1100℃, the compression ratio is controlled at above 2.8, the temperature of the third final rolling process is controlled at above 920℃, the compression ratio is controlled at above 2.5, the total compression ratio of the three rolling processes is ≥20, and the temperature of the rolled material pile out of the cold pit is ≤200℃;

[0051] (5) Double quenching + tempering: In order to ensure that the eccentric shaft of the RV reducer has an extremely small grain size after carburizing, it is necessary to perform a quenching and tempering preheating treatment so that the structure can obtain a finer and more uniform tempered martensite structure. The new double quenching + tempering quenching and tempering process in this invention is as follows: the rolled round bar is loaded into an induction heating quenching furnace, and the round bar is heated with the furnace (heating rate 5℃ / min) to 860℃-900℃, and kept at this temperature for 3.5h-4.0h, and then taken out of the furnace for oil cooling and placed for 1~2h, and then the round bar is quickly transferred to another furnace at a temperature of 770℃-810℃ and kept at this temperature for 2.0h-3.0h, and then taken out of the furnace for oil cooling. The two quenching heat treatments further refine the austenite grains. After cooling, the round bar is tempered at a tempering temperature of 150℃-200℃ and kept at this temperature for 1.5h-2.5h. Finally, the steel is taken out of the furnace and cooled naturally in the air.

[0052] (6) 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.

[0053] Compared with the prior art, the advantages of the present invention are:

[0054] (1) The hardenability of steel meets J5 (39-48HRC), J11 (36-47HRC), J25 (31-43HRC), J40 (29-41HRC).

[0055] (2) The yield strength of the steel shall be ≥905MPa, the tensile strength shall be ≥1200Mpa, the elongation after fracture shall be ≥11%, the cross-sectional shrinkage shall be ≥35%, and the impact energy at room temperature shall be ≥50J.

[0056] (3) After high-temperature carburizing, the steel is required to have a grain size finer than level 7 and no mixed crystal structure.

[0057] (4) Microscopic inclusions are tested according to GB / T 10561A method. The following requirements are met: Class A fine system ≤ 2.0, Class A coarse system ≤ 1.5, Class B fine system ≤ 1.0, Class B coarse system ≤ 0.5, Class C fine system ≤ 0.5, Class C coarse system ≤ 0.5, Class D fine system ≤ 1.0, Class D coarse system ≤ 0.5, Class DS ≤ 1.0. Macroscopic inclusions are tested according to SEP1927 high frequency flaw detection method. The defect does not exceed 2mm / dm. 3 , a single defect does not exceed 5mm. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 is the grain size of the products of Examples 1 and 2 of the present invention;

[0059] Figure 2 The grain size of the products of Comparative Examples 1 and 2 of the present invention;

[0060] Figure 3 The metallographic structures of the products of various embodiments of the present invention under the “double quenching + tempering” heat treatment. DETAILED DESCRIPTION

[0061] The technical solution of the present invention will be described in more detail with reference to the preferred embodiments of the present invention. However, these embodiments are merely descriptions of preferred implementations of the present invention and do not limit the scope of the present invention in any way.

[0062] The chemical compositions of the embodiments of the present invention and the comparative example (carburized bearing steel for conventional eccentric shafts) are shown in Table 4.

[0063] Table 4

[0064]

[0065] The test data of the terminal hardenability of various embodiments of the present invention and comparative examples are shown in Table 5.

[0066] Table 5

[0067] Serial number J5 J11 J25 J40 Example 1 45 44.5 40 35 Example 2 43.5 43.0 40.5 37 Comparative Example 1 45.0 43.5 35 29 Comparative Example 2 44 42.5 32.5 28

[0068] The mechanical property results of the embodiments of the present invention after double quenching + tempering and the comparative examples after conventional quenching and tempering are shown in Table 6.

[0069] Table 6

[0070]

[0071] The examples of the present invention and the comparative examples were subjected to carburizing heat treatment at 980°C for 5 hours and then air-cooled. The austenite grain size was tested and shown in Table 7. Figure 1 and 2 .

[0072] Table 7

[0073] Serial number Grain size / grade Example 1 9.0 Example 2 9.0 Comparative Example 1 7.0 Comparative Example 2 mixed crystal

[0074] The microscopic non-metallic inclusion ratings and high-frequency flaw detection results of the embodiments of the present invention and the comparative examples are shown in Table 8.

[0075] Table 8

[0076]

[0077]

[0078] Compared with the chemical composition, hardenability, mechanical properties, grain size and purity of conventional RV reducer eccentric shaft products 1 and 2, the embodiments of the present invention reduce the C content and Mn content, increase the Cr and Mo content, ensure that the hardenability of the material meets the requirements, and improve the Rockwell hardness of J25 and J40; to refine the high-temperature grain size, the present invention adds Nb and W elements; to improve the strength of this product, the present invention increases the Ni content; because the molten steel smelting adopts a specific vacuum degassing technology, the oxygen content tested on the finished steel of each embodiment of the present invention is relatively low, and the inclusion and high-frequency flaw detection tests are better than the comparative example.

[0079] The technical solution of the present invention is described in more detail below in conjunction with the preferred embodiments of the present invention.

[0080] Specifically, the molten iron first undergoes KR pretreatment, achieving a converter (BOF) end-point C ≤ 0.2% and P ≤ 0.02%, with a BOF tapping temperature ≥ 1600°C. During LF refining, W-iron, Nb-iron, Ni-iron, Mo-iron, and a low-Ti chromium-iron alloy are sequentially added. Imported refractory materials are used to prevent the formation of large-sized Al-Mg oxides, and high-performance synthetic slag is employed to maintain a long refining and slagging process. During vacuum degassing in the RH furnace, a high vacuum time of ≥ 20 minutes is maintained to further remove harmful gases and non-metallic inclusions from the molten steel. After degassing, manual argon injection is used to increase the nitrogen content of the molten steel to within 300 ppm and further promote the buoyancy of inclusions. The continuous casting process utilizes guarded pouring throughout, induction heating in the tundish, soft reduction, and electromagnetic stirring. Superheat is controlled at ≥ 30°C, and the cross-sectional dimensions of the continuously cast billets are 390mm*510mm.

[0081] The continuous casting billet is sent to a heating furnace in a neutral or weakly oxidizing atmosphere for heating. The heating temperature is controlled at around 1150°C for 6 hours. The billet is then forged into a 230mm×230mm square intermediate billet. After forging, it is pile-cooled for 72 hours. Subsequently, the intermediate billet is sent to a heating furnace for high-temperature heating. The temperature in the soaking section is controlled at around 1250°C. It is then rolled into a φ60mm rolled product. The temperature of the rolled product out of the pile-cooling pit is ≤200°C.

[0082] A double quenching and tempering treatment is performed: the rolled round bars are placed in an induction quenching furnace, heated to 880°C, held at this temperature for 4.0 hours, removed from the furnace, oil-cooled, and left for 1.2 hours. The bars are then quickly transferred to another furnace and held at 790°C for 2.5 hours. The double quenching treatment further refines the austenite grains. After cooling, the bars are tempered at 180°C for 2 hours. The steel is then removed from the furnace and allowed to cool naturally in air. Finally, the steel undergoes finishing processes such as straightening, chamfering, and non-destructive testing to produce the finished bars.

[0083] In summary, the present invention relates to a carburized bearing steel for an eccentric shaft of a robot RV reducer and a production method thereof. In terms of purity, key parameters of each process of molten iron pretreatment, refining, and vacuum degassing are optimized and controlled to effectively remove harmful non-metallic inclusions; in terms of high-temperature grain size, the austenite grain size of the steel is controlled by adding a certain amount of Al, N, Nb, and W elements to the molten steel, and adopting a high-temperature heating process during the rolling process. The Ni content is added to improve the mechanical properties of the steel; the forging + rolling process is adopted to ensure the density and uniformity of the steel. Thus, on the basis of meeting the hardenability and mechanical properties of the steel, the steel obtains a higher purity and a higher grain size, which significantly enhances the competitiveness of the product in terms of production efficiency, production cost, and product quality stability.

[0084] 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 carburized bearing steel for an eccentric shaft of a robot RV reducer, characterized in that: The chemical composition of the steel is as follows by mass percentage: C: 0.10-0.20%, Si: 0.20-0.40%, Mn: 0.60-1.00%, P≤0.020%, S≤0.015%, Cr: 1.50-1.80%, Mo: 0.25-0.35%, Ni: 1.40-1.70%, Nb: 0.02-0.05%, 0<W≤0.008%, Ti≤0.002%, Al: 0.020-0.050%, and the balance is Fe and unavoidable impurities. The method specifically comprises the following steps: (1) Molten steel smelting: KR molten iron pretreatment is used to reduce the content of harmful element S in the molten iron, and the S content is ≤0.03%. In order to improve the deoxidation performance of the molten iron, the Si content in the molten iron is guaranteed to be ≥0.15%; secondly, the converter is used for primary smelting, the converter end point C ≤0.2%, the end point P ≤0.02%, and the converter tapping temperature ≥1600℃; the temperature from the converter to the LF furnace is ≥1520℃, and the LF furnace is added with low Ti alloy to prevent the formation of large-sized TiN, and refractory materials and high-performance refined synthetic slag are used to prevent the formation of large-sized Al-Mg oxides. The refining and slagging process is maintained for a long time, and the LF process is ≥50min, so that non-metallic inclusions can be fully floated; vacuum degassing is carried out in the RH furnace, and the high vacuum time is ≥20min. After breaking the air, the nitrogen content of the molten steel is increased by manual argon connection to ensure that the nitrogen content of the molten steel is within 300ppm, further allowing the inclusions to fully float; (2) Large-section CCM continuous casting billet: Use large-section continuous casting billet with a specification of 350mm×500mm or more to ensure a large compression ratio of steel, thereby ensuring the density of the material; use argon protection casting and add protective slag throughout the process to protect the molten steel from secondary oxidation contamination; use soft pressure reduction technology and high superheat casting to reduce the number of inclusions in the billet; the continuous casting billet is slowly cooled at a temperature above 600℃ to prevent cracking of the continuous casting billet, the slow cooling time is ≥48h, and the exit temperature is ≥200℃; (3) Forging the continuous casting billet into an intermediate billet: The continuous casting billet is sent to a heating furnace in a neutral or weakly oxidizing atmosphere for heating. The heating temperature is controlled at 1050-1250℃. The heating time of the billet is ≥5h to ensure that the material is fully heated. Low temperature forging and three-pass forging process are adopted. The forging rate is strictly controlled to forge the billet into a square intermediate billet of 200mm×200mm-300mm×300mm. After forging, the intermediate billet is pile-cooled. The pile-cooling time is ≥60h. The starting forging temperature is 1000±50℃, the initial forging rate is 7.0m / min, the second pass forging rate is 5.0m / min, the last pass forging rate is 2.0m / min, the interval time between each pass is ≤5min, and the final forging temperature is controlled to be ≥850℃. (4) Rolling the intermediate billet into finished products: The intermediate billet is sent to the heating furnace for heating and then rolled into finished steel products. The temperature of the rolled product out of the cold pit is ≤200℃; (5) "Double quenching + tempering" quenching and tempering treatment: the rolled round bar is loaded into an induction heating quenching furnace, and the round bar is heated to 860℃-900℃ with the furnace, and kept at this temperature for 3.5h-4.0h, and then taken out of the furnace for oil cooling and placed for 1~2h, and then quickly transferred to another furnace at a temperature of 770℃-810℃ for 2.0h-3.0h, and then taken out of the furnace for oil cooling. The two quenching heat treatments further refine the austenite grains. After cooling, the round bar is tempered at a tempering temperature of 150℃-200℃ and kept at this temperature for 1.5h-2.5h. Finally, the steel is taken out of the furnace and cooled naturally in the air; (6) Finishing: including straightening, chamfering and non-destructive testing finishing processes. All products must undergo 100% non-destructive testing to ensure the surface and internal quality of the products.

2. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: The steel is first subjected to a normalizing heat treatment of 900°C*1h and a quenching heat treatment of 860°C*1h, and then the terminal hardenability of the steel is tested according to ISO 642. Specifically, the hardenability is J5: 39-48HRC, J11: 36-47HRC, J25: 31-43HRC, and J40: 29-41HRC.

3. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: The steel has a yield strength of ≥905 MPa, a tensile strength of ≥1200 MPa, an elongation after fracture of ≥11%, a cross-sectional shrinkage of ≥35%, an impact energy at room temperature of ≥50 J, and a grain size finer than grade 7.

4. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: The steel is tested using GB / T 10561 A method for microscopic non-metallic inclusions: A fine series ≤ 2.0, A coarse series ≤ 1.5, B fine series ≤ 1.0, B coarse series ≤ 0.5, C fine series ≤ 0.5, C coarse series ≤ 0.5, D fine series ≤ 1.0, D coarse series ≤ 0.5, DS ≤ 1.0; macroscopic inclusions are tested using the SEP1927 high-frequency flaw detection method, with defects not exceeding 2mm / dm 3 , a single defect does not exceed 5mm.

5. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: Step (1) When the converter is tapping 3-5 tons of steel, manually add 3-5 bags of SiC. When the tapping is completed, add ≥250 kg of aluminum iron from the high-level silo for pre-deoxidation.

6. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: In step (2), a three-machine three-stream large-section continuous casting billet is used, the casting speed of the continuous casting billet is ≤0.8m / min, and the remaining steel amount in the ladle is not less than 5 tons.

7. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: In step (4), a high-temperature heating process is adopted, specifically: the temperature of the preheating section is controlled at 650-900°C, the temperature of the heating section is controlled at 1100-1250°C, the temperature of the soaking section is controlled at 1200-1280°C, the compression ratio of the first rolling process is 2.3-3.0, the temperature of the second rolling process is 980°C-1100°C, the compression ratio is controlled at above 2.8, the temperature of the third final rolling process is controlled at above 920°C, the compression ratio is controlled at above 2.5, and the total compression ratio of the three rolling processes is ≥20.

8. The method for manufacturing carburized bearing steel for an eccentric shaft of a robot RV reducer according to claim 1, characterized in that: The heating rate of the first quenching in step (3) is 5°C / min.

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