Bearing steel for robot planetary screw roller with long service life and production method of bearing steel

Abstract

The invention relates to bearing steel for a long-service-life robot planetary lead screw roller and a production method of the bearing steel. The bearing steel comprises the following chemical components: 0.95-1.15% of C, 0.40-0.80% of Si, 0.80-1.25% of Mn, 0.80-1.30% of Cr, less than or equal to 0.0012% of Ti, less than or equal to 0.0006% of O, other element contents (such as Cu, Mo, Ni, Ca, Al, AS, Sn, Sb and Pb) which all meet the requirements of GB / T 18254-2016 High Carbon Chromium Bearing Steel, and the balance of Fe and inevitable impurities. The production process comprises the steps of electric arc furnace smelting, LF refining, VD furnace vacuum degassing, large-section continuous casting of a CCM large continuous casting billet, hot delivery of the continuous casting billet to an intermediate billet, rolling to form a material, slow cooling, spheroidizing annealing, polishing, surface and internal flaw detection, low-temperature tempering and packaging. And the quality requirement of the shaft steel for the long-service-life robot planetary lead screw roller is met.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and particularly relates to a bearing steel for a long-life robot planetary lead screw roller and a production method thereof. Background Art

[0002] Since industrial robots began to empower industrial production and with the increasing maturity of the industrial robot industry, robots with different forms and functions are bringing earth-shaking changes to economic and social activities. Therefore, humanoid robots have increasingly become an indispensable part of human social life and have developed vigorously.

[0003] Robots can be divided into six subsystems, namely a drive system, a mechanical structure system, a sensing system, an environment interaction system, a human-machine interaction system, and a control system. Among them, the transmission control system is the most core key component in the robot. According to the transmission principle, the robot relies on a precision reducer to achieve "rotary" transmission and relies on a planetary roller screw to achieve "linear" transmission. In order to better adapt to various real-life scenarios, the linear control system is more critical for humanoid robots.

[0004] The structure of the planetary roller screw is somewhat similar to that of the ball screw. The difference is that the load transmission element of the planetary roller screw is a threaded roller, which is a typical line contact, while the load transmission element of the ball screw is a ball, which is a point contact. The working principle of the planetary roller screw is as follows: Around the main threaded screw, 6 - 12 threaded roller screws are arranged in a planetary manner. Small threaded rollers are used inside the roller nut to mesh with the main screw. When the screw rotates and the circumferential direction of the nut is fixed, the rollers perform a planetary-like motion in the circumferential direction, and at the same time, the rotary motion of the screw is converted into a linear reciprocating motion of the nut through screw transmission. The ball screw is a mechanical form in which steel balls are installed in the screw shaft and the nut raceway for infinite rolling and circulation, thereby converting the rotary motion into an accurate linear positioning motion. Compared with the ball screw, the main advantages of the planetary roller screw are: 1. The planetary roller screw has a very strong load-bearing capacity through numerous line contacts of the rollers, and the bearing capacity per unit area is smaller, thereby reducing friction and extending the service life, and it is almost applicable to all occasions requiring precise linear motion control; 2. The balls of the ball screw move in different directions in the raceway, while the rollers run synchronously on the main screw, so the roller screw can withstand a higher input rotational speed to achieve a greater linear speed; 3. The rollers perform a circular motion synchronously with the main screw, so the roller screw has less vibration and noise and has the advantages of "high load-bearing capacity, high efficiency, high precision, and high reliability performance".

[0005] The planetary roller screw realizes the linear transmission function by relying on the thread meshing between parts. Therefore, the surface dimensional accuracy and straightness of the material are the key to improving the overall performance of the planetary roller screw. Secondly, when the planetary screw is working, the screw, roller and nut will be subjected to continuous, periodic and frequent friction, resulting in an increase in the temperature of the screw pair. Moreover, with the increase of the rotational speed and load, the temperature rise will increase significantly, ultimately leading to wear, thermal deformation, fatigue, etc. The rollers are usually made of high-carbon chromium bearing steel. After quenching heat treatment, this kind of material needs to have a high hardness, meet the requirements of high tissue uniformity and wear resistance, avoid wear fatigue, reduce the deformation problem of the material after heat treatment, and at the same time require the material to have a high purity to meet the long-life requirements.

[0006] To meet the requirements of high surface dimensions, straightness and long life of the planetary screw rollers, foreign countries usually adopt the process of die casting + forging to produce bearing steel for planetary screw rollers. The steel produced by the die casting + forging process usually has the advantages of high tissue uniformity and high density, which can meet the requirement that the material still has high tissue uniformity after heat treatment and reduce the deformation problem of the material. However, this process is difficult to meet the requirements of ultra-high purity, significantly reduces the fatigue life, and has obvious disadvantages of low production capacity and high energy consumption. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a bearing steel for long-life robot planetary screw rollers and its manufacturing method in view of the above-mentioned prior art, so as to significantly reduce the production cost on the basis of meeting the quality requirements of the bearing steel for robot planetary screw rollers and make the product have strong market competitiveness.

[0008] The technical solution adopted by the present invention to solve the above problems is as follows: A bearing steel for long-life robot planetary screw rollers, the chemical composition of the steel by mass percentage includes C: 0.95 - 1.15%, Si: 0.40 - 0.80%, Mn: 0.80 - 1.25%, Cr: 0.80 - 1.30%, Ti ≤ 0.0012%, O ≤ 0.0006%, and the contents of the remaining elements (such as Cu, Mo, Ni, Ca, Al, AS, Sn, Sb and Pb) all meet the requirements of GB / T 18254-2016 "High-carbon chromium bearing steel", and the balance is Fe and unavoidable impurities. By reducing the content of Cr element and increasing the content of Si and Mn elements, the production cost is reduced, and at the same time, the hardening ability of the steel after heat treatment is improved; controlling the oxygen content in the steel and the non-metallic inclusions in the fine steel to ensure the purity requirements of the steel.

[0009] The design basis of the chemical composition of the present invention is as follows: 1) Determination of C content C is one of the most fundamental elements affecting the hardening ability of steel, and it is also the chemical element with the lowest cost. Increasing the C content can significantly improve the hardening ability of steel. Therefore, in order to meet the requirement of J27 ≥ 50HRC after product heat treatment, the C content of the method of the present invention is determined to be 0.95% - 1.15%.

[0010] 2) Determination of Si content and Mn content Si is a deoxidizing element in steel. Increasing the Si content can further improve the purity of molten steel and at the same time increase the strength of steel. However, too high Si content is likely to increase the decarburization tendency of steel. Therefore, the Si content of the present invention is adopted as 0.40% - 0.80%.

[0011] Mn can improve the hardening ability of steel. At the same time, Mn is a good desulfurizer. During the smelting process, it can form high-melting-point MnS with S, thereby weakening and eliminating the thermal brittleness caused by the S element. However, too high Mn content will increase the cracking sensitivity of steel. Therefore, the Mn content of the present invention is controlled at 0.80% - 1.25%.

[0012] 3) Determination of Cr content The Cr element is the most common alloying element in high-carbon chromium bearing steel. The Cr content is one of the elements that most affect the smelting cost of steel. The Cr element is prone to cause element segregation during continuous casting, reducing the tissue uniformity. Therefore, in order to reduce the production cost and ensure the hardening ability index of the present invention, the Cr content is determined to be 0.80% - 1.30%; 4) Determination of Ti content Ti is extremely easy to combine with N element or C element during the solidification of molten steel to form non-metallic inclusions such as TiN or Ti(C, N). Usually, they are blocky inclusions with "edges and corners" in shape, which are likely to cause stress concentration during the operation of planetary screws, resulting in cracking. Therefore, in order to meet the fatigue strength of planetary screws ≥ 1000MPa, the present invention requires Ti ≤ 0.0012% 5) Determination of O content Oxygen naturally enters the steel during the steelmaking process and remains in the steel in the later stage. Oxygen mainly exists in the form of non-metallic inclusions such as SiO2 and Al2O3 in steel. In particular, Al2O3 inclusions seriously affect the fatigue life of bearings. A large number of fatigue life tests have shown that reducing the oxygen content is significantly beneficial to improving the purity of steel, especially reducing the content and size of oxide brittle inclusions. Therefore, the present invention requires the oxygen content ≤ 0.0006%.

[0013] The main inspection items and technical indicators of the present invention are as follows: To meet the high organizational uniformity of the robot planetary screw roller and have a high hardening ability after heat treatment. The present invention requires that the steel has a Rockwell hardness of 50 HRC or more at a distance of 27 mm from the end after normalizing + quenching (normalizing at 870°C ± 10°C for 0.5 h and quenching at 850°C ± 5°C for 30 min).

[0014] To meet the ultra-high fatigue strength of the robot planetary screw roller, the steel is required to have a high purity. Therefore, the present invention puts forward strict requirements in terms of micro and macro non-metallic inclusions and fatigue strength.

[0015] Type A and Type C micro-plastic inclusions usually have high plasticity and are not likely to cause stress concentration during the operation of the planetary screw, thus generating fatigue cracks; Type B and Type D micro-brittle inclusions and macro inclusions generally have high hardness and are likely to cause stress concentration and generate fatigue cracks during the operation of the planetary screw; especially TiN inclusions. Compared with large-sized spherical inclusions, these TiN inclusions with obvious edges are extremely likely to generate fatigue cracks at the edge parts, resulting in early failure. The present invention requires the inspection of micro non-metallic inclusions according to Method A of GB / T 10561. TiN inclusions are classified into Type B, Type D, or DS according to their morphology for grading. The specific requirements for grading micro non-metallic inclusions are shown in Table 1 below. Secondly, the present invention needs to inspect macro non-metallic inclusions according to the method of SEP 1927 immersion ultrasonic high-frequency flaw detection, with a 5-level sensitivity detection, and the total detected volume ≥ 10 dm 3 , achieving zero defects.

[0016] Table 1

[0017] To meet the long life and high stability of the robot planetary screw roller, the finished steel is required to conduct a rotating bending fatigue test according to GB / T4337, and the fatigue strength of the steel is required to be ≥ 1000 MPa.

[0018] Another object of the present application is to provide a production method for the steel used in the disc bearing of the above intelligent precision seeder, and the main steps are as follows: Another object of the present invention application is to provide a production method for bearing steel for long-life robot planetary screw rollers. The billet is smelted by vacuum degassing + continuous casting. The specific production process flow is electric arc furnace smelting - LF refining - VD furnace vacuum degassing - large-section continuous casting CCM large continuous casting billet - hot delivery of continuous casting billet to open intermediate billet - rolling into finished products - slow cooling - spheroidizing annealing - turning - surface and internal flaw detection - low-temperature tempering - packaging. The main production process characteristics are as follows: (1)Steel melting: First, primary melting is carried out in an electric furnace, using high-purity scrap steel, and ensuring that scrap steel / hot metal ≥ 50%. At the end of the tapping process, the end-point C is controlled ≥ 0.30%, the end-point P is controlled ≤ 0.020%, the end-point S is controlled ≤ 0.010%, and the tapping temperature is above 1600 °C. During tapping, according to the target composition of Si and Mn, ferrosilicon manganese, ferrosilicon, and ferrotitanium alloys are added for deoxidation, and at the same time, ferrochrome is added for preliminary composition adjustment. The tapping of the electric furnace uses a slag stopper to prevent slag and slag skimming and other control techniques to solve the problem of high content of harmful elements such as As, Sn, Pb, and Sb in bearing steel. After slag skimming, it is quickly lifted and transported to the LF refining furnace for smelting to prevent secondary oxidation of the molten steel; Precisely control the content of each element in the LF refining furnace. Pay attention to selecting low-Ti alloys and strictly control the Ti content in the molten steel so that the Ti content ≤ 0.0012%. In the initial stage of refining, silicon carbide and silicon-containing alloys are used for deoxidation, while preventing the silicon component from exceeding the standard (Si content ≤ 0.80%). During the refining process, aluminum pellets and high-performance refining synthetic slag are added to remove non-metallic inclusions. Argon stirring is used throughout the process. On the one hand, it can accelerate the mass transfer between the molten steel and the refining slag, which is beneficial to the desulfurization and deoxidation reactions. On the other hand, argon blowing can make the Al2O3 non-metallic inclusions float up and be removed sufficiently. High-quality refractory materials are used to control the amount of MgO in the molten steel and maintain a long LF refining time, LF time ≥ 1h. At the same time, the refining temperature is controlled at 1550 - 1580 °C to make the inclusions float up and be removed sufficiently; After LF refining, the molten steel must be subjected to vacuum degassing in the VD furnace to ensure sufficient vacuum in the furnace and maintain sufficient circulation treatment time to further remove harmful gases and non-metallic inclusions in the molten steel and improve the purity of the molten steel. The maximum vacuum degree ≤ 1.5 mbar, the high-vacuum time ≥ 25 min. After breaking the vacuum, argon soft blowing and stirring are used, and the soft blowing time ≥ 25 min. On the one hand, it ensures vacuum degassing, and on the other hand, it fully ensures the floating up and removal of inclusions; (2)Continuous casting: Square continuous casting billets with large cross-sectional dimensions are used, and the specifications of the continuous casting billets are 300 mm × 300 mm and above to ensure a large reduction ratio in the subsequent forging and rolling processes of the steel, with a reduction ratio ≥ 20, thus ensuring the density of the material. Before the continuous casting process, protective slag is added throughout the continuous casting process for protective casting to prevent the molten steel from being polluted by secondary oxidation. The intermediate ladle induction heating technology is used during the continuous casting process to maintain pouring with a high superheat, and the superheat is controlled ≤ 40 °C, which can make non-metallic inclusions easily rise to the surface and effectively remove inclusions. The soft reduction process is adopted, with the soft reduction amount controlled at 10 mm - 25 mm and the pouring casting speed at 0.40 - 0.85 m / min to reduce the composition segregation of the continuous casting billet and significantly improve the tissue uniformity of the continuous casting billet; (3)Continuous casting billet to bloom: After the continuous casting billet is hot-transported to a reheating furnace with a neutral or weakly oxidizing atmosphere for high-temperature diffusion heating, it is bloomed into a square bloom with a size of 150 mm×150 mm or above. The specific high-temperature diffusion process is as follows: the preheating section is at 800 - 950 °C, and the heating time is ≥45 min; the heating section temperature is controlled at 1100 - 1200 °C, and the heating time is ≥1.5 h; the soaking section temperature is controlled at 1200 - 1300 °C, and the heating time is ≥5 h, and the total heating time is ≥12 h. Strictly control the heating temperature and time to ensure that carbides are fully diffused and prevent the occurrence of liquid segregation-type carbides and microvoids. After the continuous casting billet exits the reheating furnace, high-pressure water is used to remove surface defects of the steel to perform pre-treatment for better control of surface quality in the subsequent process. Then, large reduction rolling is carried out by a roughing mill to make the structure from the edge to the center more dense, and the bloom is formed into a bloom through the roughing mill; (4)Bloom rolling into finished product: The bloom meeting the above dimensions is sent to a reheating furnace with a neutral or weakly oxidizing atmosphere for heating and then rolled into finished steel. It is necessary to carry out long-term sufficient heat preservation according to the size of the bloom, and the total heating time is ≥3 h. The heating temperature is controlled at 1050 °C - 1250 °C. The billet after being taken out of the furnace is rolled into bars with a specification of φ10 mm - 40 mm by a rolling mill group. Ensure that the finishing rolling temperature is ≥900 °C. After rolling is completed, it must be taken offline and sent to a slow cooling pit for slow cooling. The temperature requirement for entering the slow cooling pit is ≥450 °C to prevent the formation of coarse grains. The slow cooling time is ≥60 hours, and the temperature when exiting the slow cooling pit must be <200 °C; (5)Finishing process: To ensure that the straightness of the planetary screw is ≤0.3 mm / m, the material must undergo the processes of turning + straightening.

[0019] First, the material must be pre-treated (spheroidizing annealing heat treatment). The rolled round bar is loaded into a spheroidizing annealing furnace, and the round bar is slowly heated to 800 °C - 890 °C with the furnace, and the heating rate is controlled within 10 °C / min to avoid surface stress cracks caused by too fast heating rate. And it is held at this temperature for more than 3.5 h to fully austenitize the steel; Subsequently, the round bar is quickly transferred to another furnace with a furnace temperature of 680 °C - 750 °C within 15 min for heat preservation for 5 h ± 15 min to completely transform the lamellar pearlite into fine and dispersed granular pearlite, significantly improving the structure spheroidization rate; Finally, the steel is slowly cooled to 500 °C in the furnace and taken out of the furnace, and the cooling rate is 3 °C / min - 5 °C / min, and then it is naturally cooled in the air.

[0020] After spheroidizing annealing, the microstructure of the material is granular pearlite, which has a relatively low hardness. Therefore, the material must be skived to meet the surface dimensional accuracy. Specifically, the skiving allowance on each side is 0.3 mm - 0.8 mm to ensure that the surface dimensional accuracy is within the range of ±0.2 mm. Subsequently, it undergoes at least 2 low-speed straightening processes. If the straightening speed is too fast, it is easy to cause the temperature rise of the bar surface and the linear guide surface, resulting in defects such as stuttering nodules and burns. The straightening speed is set at 20 - 40 m / min to ensure that the straightness is ≤0.3 mm / m.

[0021] For surface and internal flaw detection, 100% non-destructive testing is adopted, and only the qualified ones can become qualified products. After the flaw detection is qualified, the material needs to undergo low-temperature aging treatment. The temperature of low-temperature tempering is 200°C - 300°C, and the holding time is within 1.5 h to avoid aggravating the straightness due to thermal stress.

[0022] The bearing steel for long-life robot planetary screw rollers produced by the present invention meets the following index requirements: (1) After normalizing + quenching heat treatment of the steel, J27 ≥ 50 HRC.

[0023] (2) Microscopic inclusions are inspected according to Method A of GB / T 10561, meeting A-class fine series ≤ 2.0, A-class coarse series ≤ 1.5, B-class fine series ≤ 1.0, B-class coarse series ≤ 0.5, C-class fine series = 0, C-class coarse series = 0, D-class fine series ≤ 1.0, D-class coarse series ≤ 0.5, DS-class ≤ 1.0.

[0024] (3) Macroscopic defects are inspected according to the SEP 1927 immersion high-frequency flaw detection method, detected with a 5-level sensitivity, and the total detected volume ≥ 10 dm 3 , and the macroscopic defects reach zero defects.

[0025] (4) According to GB / T 4337, a rotating bending fatigue test is carried out, meeting the fatigue strength ≥ 1000 MPa.

[0026] Compared with the prior art, the advantages of the present invention are as follows: Based on the current vacuum degassing + continuous rolling production of bearing steel, the present invention optimizes and improves key parameters such as chemical composition, vacuum degassing, and continuous casting process, ensuring the high purity and high hardness requirements of the steel for planetary screw rollers, fully meeting the requirements of ultra-high fatigue strength and hardening ability after heat treatment; and through the optimization and improvement of the pre-treatment process before skiving, the straightness of the product is further improved, making the product highly competitive in the market. Specific embodiments

[0027] The technical solution of the present invention will be described in more detail in combination with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred embodiments of the present invention and cannot impose any limitation on the scope of the present invention.

[0028] The manufacturing process of the bearing steel for long-life robot planetary lead screw rollers in the embodiments of the present invention is as follows: electric arc furnace smelting - LF refining - VD furnace vacuum degassing - large cross-section continuous casting CCM large continuous casting billet - hot delivery of continuous casting billet to open intermediate billet - rolling into finished products - slow cooling - spheroidizing annealing - turning - surface and internal flaw detection - low-temperature tempering - packaging.

[0029] Specifically, before smelting, the molten steel is smelted in an electric furnace, using clean scrap steel and high-quality raw and auxiliary materials, with scrap steel / iron melt = 65%. The end-point C of tapping is controlled to be ≥0.30%, the end-point P is controlled to be ≤0.020% or less, the end-point S is controlled to be ≤0.010% or less, and the tapping temperature is about 1680°C; when performing LF refining, low-Ti, low-Ca alloys, high-quality refractory materials, and high-performance refining synthetic slag are selected; during the vacuum degassing process, sufficient vacuum degree and sufficient circulation treatment time are maintained; during the continuous casting process, protective casting is adopted throughout, and tundish induction heating, soft reduction, and electromagnetic stirring technologies are used. To effectively remove non-metallic inclusions, casting is carried out with a high superheat degree, and the superheat degree is controlled to be ≤40°C; The intermediate billet of 200mm * 200mm is rolled. The preheating section is at 900°C, and the heating time is 50min; the temperature in the heating section is controlled at 1150°C, and the heating time is 2h; the temperature in the soaking section is controlled at 1280°C, and the heating time is 6h; subsequently, the intermediate billet is rolled into a bar of the target size. After rolling, the bar must be taken off the production line for slow cooling at a temperature above 450°C, and the slow cooling time is ≥60 hours. The temperature when leaving the slow cooling pit must be <200°C; The rolled round bar is loaded into a spheroidizing annealing furnace. The round bar is slowly heated to 880°C with the furnace, and the heating rate is controlled within 10°C / min to avoid surface stress cracks caused by too fast heating rate and to fully austenitize the steel; subsequently, the round bar is quickly transferred to another furnace with a temperature of 720°C within 15min, so that the lamellar pearlite is completely transformed into fine and dispersed granular pearlite, significantly improving the spheroidization rate of the structure; finally, the steel is slowly cooled to 500°C in the furnace and then taken out for air cooling.

[0030] After spheroidizing annealing, the material must undergo a turning process with a unilateral turning allowance of 0.5mm to ensure that the surface dimension accuracy is within the range of ±0.2mm, and then undergo 2 low-speed straightening processes. After straightening, the bar must undergo low-temperature tempering, surface and internal quality flaw detection, and the qualified target bar finished product is obtained after passing the inspection.

[0031] The chemical compositions (wt%) of the steel for flexible bearings of robot harmonic reducers in the embodiments of the present invention and the conventional materials used in the current market (for comparison) are shown in Tables 2 and 3.

[0032] Table 2 Chemical Compositions of Each Embodiment

[0033] From the comparison of chemical components, it can be seen that in each embodiment of the present invention, the content of alloying element Cr is reduced, and the content of alloying element Mn is increased, which plays a role in reducing costs.

[0034] Table 3 Non-metallic inclusions in the steel of each embodiment

[0035] From the inspection results of non-metallic inclusions, due to the strict control of the oxygen content in the present invention and the relatively low control of the Al content during the smelting process, the control of B-class, D-class, and DS-class inclusions is better than that of traditional materials. The steel of the present invention fully meets the high-purity requirements of bearing steel for long-life robot planetary screw rollers.

[0036] Table 4 Immersion high-frequency flaw detection data of the steel of each embodiment

[0037] According to the SEP 1927 method, immersion high-frequency flaw detection is carried out, and 5-level sensitivity detection is used. The total detection volume is 10 dm 3 , and no macroscopic defects are found in each embodiment of the present invention.

[0038] Table 5 Hardening energy and fatigue strength data of the steel of each embodiment

[0039] The present invention significantly improves the hardening energy of the steel by increasing the C content and Mn content and reducing the Cr content, meeting J27≥50HRC, and further improves the fatigue strength by controlling the purity. It is more competitive in terms of production efficiency, production cost, and product quality stability.

[0040] Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various changes and modifications can be made to the present invention for those skilled in the art. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A bearing steel for a long-life robot planetary screw roller, characterized in that: The chemical composition of the steel is, by mass percentage, C: 0.95-1.15%, Si: 0.40-0.80%, Mn: 0.80-1.25%, Cr: 0.80-1.30%, Ti≤0.0012%, O≤0.0006%, and the remainder is Fe and unavoidable impurities.

2. The bearing steel for long-life robot planetary screw roller according to claim 1, characterized in that: After the normalizing + quenching heat treatment, the bearing steel is J27≥50HRC, normalized at 870℃±10℃ for 0.5h, and quenched at 850℃±5℃ for 0.5h.

3. The bearing steel for long-life robot planetary screw roller according to claim 1, characterized in that: The microscopic inclusions of the bearing steel are tested according to GB / T 10561 A method, and meet the requirements of 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, Class C coarse system = 0, Class D fine system ≤1.0, Class D coarse system ≤0.5, DS class ≤1.

0. Macroscopic defects are tested according to SEP 1927 water immersion high frequency flaw detection method, using 5-level sensitivity detection, and the total detection volume is ≥10dm 3 , macro defects reach zero defects, and the rotary bending fatigue test is carried out in accordance with GB / T 4337, meeting the fatigue strength ≥1000MPa.

4. A method for producing bearing steel for long-life robot planetary screw rollers as claimed in claim 1, characterized in that: The method adopts vacuum degassing + continuous casting to smelt the billet, and the specific production process is arc furnace smelting - LF refining - VD furnace vacuum degassing - large-section continuous casting CCM large continuous casting billet - hot delivery of continuous casting billet to open intermediate billet - rolling into products - slow cooling - spheroidizing annealing - lathing - surface and internal flaw detection - low-temperature tempering - packaging.

5. The method for producing bearing steel for long-life robot planetary screw roller according to claim 4, characterized in that: The method mainly comprises the following steps: (1) Molten steel smelting: First, the electric furnace is used for primary smelting, using high-clean scrap steel, and at the same time ensuring that the scrap steel / molten iron ratio is ≥50%, the endpoint C of the steel-making process is controlled to be ≥0.30%, the endpoint P is controlled to be ≤0.020%, the endpoint S is controlled to be ≤0.010%, and the steel-making temperature is above 1600℃; when tapping, silicon manganese, ferrosilicon and aluminum-iron alloy are added for deoxidation according to the target composition of Si and Mn, and ferrochrome is added for initial composition adjustment. The electric furnace uses slag stoppers to block slag, and the slag is removed after the furnace. After the slag is removed, it is quickly hoisted to the LF refining furnace for smelting to prevent secondary oxidation of the molten steel; In the LF refining furnace, the content of each element is precisely controlled, low Ti alloy is selected, the Ti content in the molten steel is controlled to make the Ti content ≤ 0.0012%, high-quality refractory materials are used, the amount of MgO in the molten steel is controlled, and the LF refining time is maintained for a long time, LF time ≥ 1h, and the refining temperature is controlled at 1550~1580℃, so that the inclusions can be fully floated and removed; After LF refining, the molten steel is vacuum degassed in the VD furnace, with the highest vacuum degree ≤1.5mbar and the high vacuum time ≥25min. After breaking the air, argon soft blowing is used for stirring, and the soft blowing time is ≥25min. On the one hand, vacuum degassing is ensured, and on the other hand, inclusions are fully floated and removed; (2) Continuous casting The square continuous casting billet with large cross-section size is used, and the specification of the continuous casting billet is 300mm×300mm and above, to ensure the large compression ratio of the steel in the subsequent forging and rolling process, the compression ratio is ≥20. Before the continuous casting process, protective slag is added throughout the continuous casting process for protective pouring. The continuous casting process adopts the induction heating technology of the tundish to maintain high superheat pouring, and the superheat is controlled at ≤40℃. The light pressure reduction process is adopted, and the light pressure reduction is controlled at 10mm~25mm, and the casting speed is 0.40~0.85m / min, which reduces the component segregation of the continuous casting billet and significantly improves the uniformity of the organization of the continuous casting billet; (3) Opening the intermediate billet of continuous casting billet The continuous casting billet is hot sent to a heating furnace in a neutral or weakly oxidizing atmosphere for high-temperature diffusion heating, and then cut into square intermediate billets of 150 mm × 150 mm or more; (4) Intermediate billet is rolled into finished product The intermediate billet meeting the above dimensions is sent to a heating furnace in a neutral or weakly oxidizing atmosphere for heating and then rolled into finished steel products. After rolling, it must be taken off the line and enter a slow cooling pit for slow cooling. The temperature entering the slow cooling pit is required to be ≥450°C to prevent coarse grains. The slow cooling time is ≥60 hours, and the temperature out of the slow cooling pit must be <200°C; (5) Finishing process First, the material must be subjected to spheroidizing annealing heat treatment, and then the round bar is quickly transferred to another furnace within 15 minutes and kept at 680℃-750℃ for 5h±15min, so that the lamellar pearlite is completely transformed into fine, dispersed granular pearlite, significantly improving the spheroidization rate of the organization; finally, the steel is slowly cooled to 500℃ in the furnace and taken out of the furnace, with a cooling rate of 3℃ / min-5℃ / min, and then naturally cooled in the air. After spheroidizing annealing, the material passes through the car skin to meet the surface dimensional accuracy, and finally the straightening process is carried out. The straightening speed is set at 20~40m / min to ensure that the curvature is ≤0.3mm / m.

6. The method for producing bearing steel for long-life robot planetary screw roller according to claim 5, characterized in that: In the initial stage of LF refining in step (1), silicon carbide and silicon-containing alloys are used for deoxidation, while preventing the silicon content from exceeding the standard, and the Si content is ≤0.80%. During the refining process, aluminum particles and high-performance refined synthetic slag are added to remove non-metallic inclusions. Argon is used for stirring throughout the process. On the one hand, it can accelerate the transfer of substances between the molten steel and the refining slag, which is beneficial to the desulfurization and deoxygenation reactions. On the other hand, argon blowing can fully float and remove Al2O3 non-metallic inclusions.

7. The method for producing bearing steel for long-life robot planetary screw roller according to claim 5, characterized in that: The high temperature diffusion process in step (3) is specifically as follows: the preheating section is 800-950°C, and the heating time is ≥45min; the temperature of the heating section is controlled at 1100-1200°C, and the heating time is ≥1.5h; the temperature of the soaking section is controlled at 1200-1300°C, and the heating time is ≥5h, and the total heating time is ≥12h. After the continuous casting billet is taken out of the heating furnace, high-pressure water is used to remove surface defects of the steel material, and then the billet is rolled with a large reduction mill to make the structure from the edge to the center more dense, and the billet is opened into an intermediate billet by the roughing mill.

8. The method for producing bearing steel for long-life robot planetary screw roller according to claim 5, characterized in that: In step (4), the intermediate billet is kept warm for a long time according to the size of the intermediate billet, the total heating time is ≥3h, the heating temperature is controlled at 1050℃-1250℃, and the billet after being taken out of the furnace is rolled into bars with a specification of φ10mm-40mm by the rolling mill to ensure that the final rolling temperature is ≥900℃.

9. The method for producing bearing steel for long-life robot planetary screw roller according to claim 5, characterized in that: In step (5), the spheroidizing annealing heat treatment is to load the rolled round bars into a spheroidizing annealing furnace, slowly heat the round bars to 800°C-890°C in the furnace, and control the heating rate within 10°C / min to avoid surface stress cracks caused by excessive heating rate, and keep the temperature at this temperature for more than 3.5 hours to fully austenitize the steel.

10. The method for producing bearing steel for long-life robot planetary screw roller according to claim 5, characterized in that: The car skin process in step (5) is specifically as follows: the single-side car skin allowance is 0.3mm-0.8mm, ensuring that the surface dimensional accuracy is within the range of ±0.2mm, and then undergoing ≥2 low-speed straightening processes.

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