High-wear-resistance and fatigue-resistance generator main shaft and preparation method thereof

Abstract

This invention relates to the field of metallurgical technology and discloses a high wear-resistant and fatigue-resistant generator spindle and its preparation method. The chemical composition of the spindle includes C, Si, Mn, Cr, Ni, Mo, Ce, Nb, B, etc., with Ti strictly limited to ≤0.003%, and no aluminum deoxidizer is added throughout the process. The preparation method includes: electric arc furnace carbon pre-deoxidation; LF refining to produce high-basicity slag; VD vacuum refining for vacuum carbon deoxidation, and sequential feeding of cerium-iron cored wire, niobium-iron, and boron-iron using a specific timing sequence. This invention uses the generation of spherical rare earth oxide sulfides as a core to induce the precipitation of niobium carbonitrides to form a core-shell structure to fix nitrogen and protect boron in a solid solution state. This technical solution eliminates large-size TiN and Al2O3 inclusions, effectively solves the problem of boron failure in titanium-free steel, and endows the spindle forging with excellent deep hardenability, extremely high fatigue strength ratio, and good transverse impact toughness.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a high wear-resistant and fatigue-resistant generator spindle and its preparation method. Background Technology

[0002] Currently, as generator sets are developing towards larger capacity and higher parameters, the generator main shaft, as a core component bearing torque and rotational inertia, is facing increasingly stringent service conditions. The main shaft endures enormous centrifugal force and alternating bending stress during high-speed rotation, requiring materials to possess not only high static strength but also excellent fatigue resistance and deep hardenability under large cross-sections to ensure no fracture failure occurs during its decades-long service life.

[0003] To address the stringent requirements for the core properties of large-section forgings, existing technologies often employ Cr-Ni-Mo low-alloy high-strength steel systems and introduce trace amounts of boron to utilize its significant grain boundary segregation effect to improve hardenability. To ensure boron exists in an effective solid solution state, the smelting process typically adds titanium or aluminum as a nitrogen-fixing agent. Titanium or aluminum preferentially combines with nitrogen atoms in molten steel to form thermodynamically more stable nitrides, thus preventing nitrogen from combining with boron to form boron nitride. This ensures boron's ability to suppress proeutectoid ferrite nucleation, allowing the forging core to acquire the desired bainitic or martensitic structure after quenching.

[0004] However, while the aforementioned traditional processes solved the hardenability problem, they also introduced side effects. Adding titanium effectively fixes nitrogen, but the resulting titanium nitride inclusions are highly hard and angular, making them impossible to deform during forging. These hard points easily form stress concentrations under alternating loads, becoming the main source of fatigue crack initiation and reducing the fatigue life of the spindle. If a strong aluminum deoxidation process is used instead, the resulting alumina inclusions tend to cluster, and the accompanying sulfides elongate into chains along the processing direction during forging deformation, leading to a deterioration in the material's transverse properties; the transverse impact toughness is far lower than the longitudinal toughness. Simply eliminating titanium and aluminum without adopting new nitrogen-fixing measures will cause free nitrogen to rapidly consume boron, resulting in a large amount of ferrite in the core of large-section forgings due to boron failure, rendering the strength and toughness unable to meet design standards.

[0005] Therefore, the present invention provides a high wear-resistant and fatigue-resistant generator spindle and its manufacturing method to overcome the shortcomings of the prior art. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a high wear-resistant and fatigue-resistant generator spindle and its preparation method, which solves the contradictory problem of adding titanium and aluminum to existing generator spindle steel to ensure hardenability, which introduces brittle inclusions and leads to deterioration of fatigue performance and transverse toughness, while simply removing titanium results in insufficient deep hardenability.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] In a first aspect, the present invention provides a high wear-resistant and fatigue-resistant generator spindle, which adopts the following technical solution:

[0009] A high wear-resistant and fatigue-resistant generator spindle is composed of the following chemical components by mass percentage: carbon 0.34%-0.38%, silicon 0.15%-0.30%, manganese 0.50%-0.70%, chromium 1.50%-1.65%, nickel 1.50%-1.65%, molybdenum 0.20%-0.30%, cerium 0.015%-0.025%, niobium 0.045%-0.065%, boron 0.0015%-0.0030%, acid-soluble aluminum 0.005%-0.009%, total oxygen 0.0020%-0.0025%, nitrogen 0.0030%-0.0050%, with limiting impurities: titanium ≤0.003%, sulfur ≤0.005%, phosphorus ≤0.009%, and the balance being iron and unavoidable impurities.

[0010] By adopting the above technical solution, the interruption effect of large-sized titanium nitride inclusions on the matrix continuity is eliminated by strictly limiting the titanium content, fundamentally removing the main crack source leading to high-cycle fatigue fracture. Simultaneously, nitrogen fixation is achieved through the synergistic effect of cerium and niobium, effectively capturing nitrogen under titanium-free conditions. The specific mechanism is as follows: by adding trace amounts of the rare earth element cerium, high-melting-point, spherical rare earth oxysulfide Ce₂O₂S is preferentially generated in the molten steel; this rare earth oxysulfide acts as a heterogeneous nucleation core, inducing the subsequent addition of niobium to precipitate on its surface, forming a niobium carbonitride Nb(C,N) coating layer. This composite precipitation mechanism efficiently consumes free nitrogen atoms in the matrix, preventing nitrogen from combining with boron to form stable boron nitride. Therefore, the added trace amount of boron can exist in solid solution form and segregate at the austenite grain boundaries during heat treatment, reducing the grain boundary energy and delaying the nucleation of proeutectoid ferrite, thereby improving the core hardenability of large cross-section forgings and ensuring that the core structure is transformed into a strong and tough bainite or martensite structure, thus achieving a balance between high fatigue life and deep hardenability.

[0011] Preferably, in the chemical composition, the mass ratio of cerium to sulfur is controlled between 3.0 and 6.0.

[0012] By adopting the above technical solution and controlling the cerium-sulfur ratio within this specific range, it is possible to ensure that Ce2O2S-type inclusions are preferentially formed thermodynamically, avoiding the formation of brittle Ce2O3 or low-melting-point CeS. Ce2O2S has good spherical morphology retention ability, does not deform during forging, has good isotropy, and can improve the transverse impact toughness of forgings.

[0013] Secondly, the present invention provides a method for manufacturing a high wear-resistant and fatigue-resistant generator spindle, employing the following technical solution:

[0014] A method for manufacturing a high wear-resistant and fatigue-resistant generator spindle includes the following steps:

[0015] In the electric arc furnace primary smelting, the titanium content of the raw materials fed into the furnace is controlled. After melting, the steel is tapped. Carbon powder is added during the tapping process for pre-deoxidation. No aluminum deoxidizer is added throughout the process, and the initial oxygen activity of the molten steel after tapping is controlled.

[0016] LF refining involves creating high-basicity slag for diffusion deoxidation and desulfurization, and compensating for overheating of the molten steel before the refining process is completed.

[0017] VD vacuum refining involves a vacuum carbon deoxidation reaction under high vacuum conditions to reduce dissolved oxygen and nitrogen content. Then, cerium-iron cored wire is fed in under vacuum, allowed to stand and homogenize, and after breaking the vacuum, niobium-iron alloy and boron-iron alloy are added in sequence under a protective atmosphere.

[0018] Casting and hot working: The steel ingot is cast into a protective atmosphere and the solidification and cooling rate is controlled. The steel ingot is then heated and forged, and then subjected to quenching and tempering heat treatment.

[0019] By adopting the above technical solutions, a pure metallurgical environment with low oxygen, low nitrogen and no aluminum interference was constructed.

[0020] First, the aluminum deoxidation process is abandoned in the initial refining stage, and carbon powder pre-deoxidation is adopted to avoid the formation of clustered alumina inclusions and to retain a suitable initial oxygen environment for the subsequent formation of rare earth oxide sulfides.

[0021] Secondly, in the VD vacuum refining stage, the oxygen content is further reduced and nitrogen is removed by vacuum carbon deoxidation reaction.

[0022] This invention designs a specific microalloying sequence: Under vacuum high cleanliness and suitable oxygen and sulfur content, cerium is first added, and the settling time allows the cerium to fully react and generate dispersed micron-sized Ce₂O₂S cores; then niobium is added, where the existing Ce₂O₂S particles in the molten steel provide attachment sites for niobium precipitation, promoting the epitaxial growth of Nb(C,N) on the Ce₂O₂S surface; finally, boron is added, where the free nitrogen in the molten steel is effectively fixed by the niobium carbonitrides, and boron atoms are no longer consumed, thus ensuring the solid solution strengthening effect of boron. This process successfully solves the problem of boron protection in titanium-free steel.

[0023] Preferably, in the primary smelting step of the electric arc furnace, an eccentric bottom tapping method is adopted, the tapping temperature is controlled at 1630-1650℃, and the initial oxygen activity is controlled at 60-80ppm.

[0024] By adopting the above technical solution, the eccentric bottom tapping reduces the amount of slag. Combined with a suitable tapping temperature and initial oxygen activity, it provides a good semi-killed molten steel base for LF refining, which is conducive to the kinetics of subsequent slag-forming and desulfurization reactions. At the same time, an oxygen activity of 60-80 ppm is a necessary precursor condition for the subsequent formation of target inclusions.

[0025] Preferably, in the LF refining step, the binary basicity of the high basicity slag is controlled at 3.5-4.5, and the temperature of the molten steel at the end of refining is controlled at 75-90°C above the liquidus temperature.

[0026] By adopting the above technical solution, the high alkalinity slag system promotes the deep desulfurization reaction, reduces the sulfur content to an extremely low level, and prevents the formation of excessive sulfides; the high superheat compensates for the temperature drop during the long settling time of the subsequent VD treatment, ensuring that the inclusions have enough time to float and be modified.

[0027] Preferably, in the VD vacuum refining step, the vacuum degree of the vacuum carbon deoxidation reaction is controlled below 67 Pa, and the processing time is 15-20 minutes, until the total oxygen content in the molten steel drops to 20-25 ppm and the nitrogen content drops to 30-50 ppm.

[0028] By adopting the above technical solution, the nitrogen content is controlled below the critical safety value by using the CO bubbles, which are the product of the carbon-oxygen reaction, to remove nitrogen through the escape of CO bubbles. This reduces the consumption of nitrogen-fixing elements in the subsequent process, while also precisely controlling the oxygen content within the optimal thermodynamic window for the formation of rare earth oxides.

[0029] Preferably, in the VD vacuum refining step, the argon gas stirring intensity is controlled in stages: strong stirring is turned on during the vacuum carbon deoxidation stage, and the argon gas flow rate is 3-5 NL / (min·t); after feeding the cerium-iron cored wire, the argon gas is switched to soft blowing, and the argon gas flow rate is reduced to 0.5-1.5 NL / (min·t).

[0030] By adopting the above technical solution, the strong stirring in the early stage enhances the mass transfer process in the molten pool and accelerates the degassing reaction; the soft blowing of argon gas in the later stage avoids the exposed steel molten steel from absorbing gas and provides a stable flow field for the collision growth and floating removal of rare earth inclusions, which helps to retain small and dispersed nucleation cores.

[0031] Preferably, in the VD vacuum refining step, the settling and homogenization time is 6-8 minutes; the feeding sequence after breaking the vacuum is as follows: first add niobium-iron alloy and stir for 1-2 minutes, then add boron-iron alloy and stir for 1 minute, and then immediately tap the steel.

[0032] By adopting the above technical solution, a settling time of 6-8 minutes is the kinetic time required for the precipitation and stable distribution of rare earth oxide sulfide cores. The strict sequence of adding niobium first, then boron, combined with a specific stirring time, ensures that niobium has the opportunity to combine with nitrogen first, completing most of the nitrogen fixation process before the addition of boron. This prevents premature contact between boron and nitrogen, maximizing the effective utilization rate of boron.

[0033] Preferably, in the casting and hot working steps, bottom casting is used, and the average cooling rate of the steel ingot in the temperature range of 1350℃ to 1200℃ is controlled to be 1-5℃ / min; the forging adopts a multi-directional forging process, and the total forging ratio is ≥3.5.

[0034] By adopting the above technical solutions, controlling the solidification and cooling rate promotes the full precipitation and reasonable distribution of composite precipitates, avoiding segregation; the multi-directional forging with a large forging ratio breaks up the as-cast structure, welds the internal porosity, makes the spherical inclusions more uniformly distributed, and improves the density and mechanical properties of the material.

[0035] Preferably, in the casting and hot working steps, the quenching and tempering heat treatment includes: heating the forging to 860-880℃ and holding it at that temperature before oil quenching, followed by high-temperature tempering at 600-620℃.

[0036] By adopting the above technical solution, the matrix structure is transformed into uniform tempered sorbite. Combined with the inhibitory effect of boron dissolved at the grain boundaries on ferrite, the best strength-toughness matching and fatigue resistance are obtained.

[0037] This invention provides a high wear-resistant and fatigue-resistant generator spindle and its manufacturing method. It has the following beneficial effects:

[0038] 1. This invention improves the fatigue resistance of materials by strictly limiting the titanium content and eliminating the aluminum deoxidation process. This approach eliminates large-sized, angular titanium nitride and clustered alumina inclusions introduced by titanium and aluminum, preventing these hard phases from becoming fatigue crack initiation sources under cyclic stress. Simultaneously, the synergistic effect of rare earth elements and niobium promotes the transformation of precipitates in the steel into fine, dispersed, and spherically distributed composite particles. These well-formed precipitates effectively reduce stress concentration in the micro-regions of the matrix, thereby improving the service life of the generator main shaft under high-cycle loads.

[0039] 2. This invention achieves high density and uniform performance in the deep microstructure of large-section forgings. Through a specific vacuum refining sequence and microalloying strategy, rare-earth oxysulfides induce niobium carbonitride precipitation to fix nitrogen atoms, ensuring that boron is not consumed and exists primarily in a solid solution state. These solid-solution boron atoms segregate at austenite grain boundaries during heat treatment, effectively suppressing the nucleation of proeutectoid ferrite at low cooling rates. This allows large forgings to transform into a strong and tough bainitic or martensitic structure even under slow core cooling conditions, solving the problem of insufficient core hardenability caused by boron nitride precipitation in traditional processes.

[0040] 3. This invention effectively solves the anisotropy problem of mechanical properties in large forgings and improves transverse impact toughness. By precisely controlling the ratio of rare earth elements to sulfur, the generated rare earth oxysulfides have high melting points and moderate hardness, maintaining a spherical shape without stretching even under large deformation during high-temperature forging. This morphological control avoids the elongation of traditional sulfides or oxides along the processing direction to form banded structures, eliminates the disruptive effect of inclusions on the transverse continuity of the matrix, and ensures that the spindle possesses excellent and consistent toughness reserves in different stress directions. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the dynamic evolution path of oxygen content in molten steel and the key window for rare earth modification in the entire smelting process of Embodiment 2 of the present invention.

[0042] Figure 2 This is a comparison chart of the differences in the solid solution boron ratio and the effective hardenability threshold between Example 2 and the comparative examples under different process systems. Detailed Implementation

[0043] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] Examples 1-3:

[0045] Example 1:

[0046] This embodiment provides a high wear-resistant and fatigue-resistant generator spindle and its manufacturing method. The chemical composition of the generator spindle steel, by mass percentage, includes:

[0047] Carbon 0.34%, silicon 0.15%, manganese 0.50%, chromium 1.50%, nickel 1.50%, molybdenum 0.20%, cerium 0.015%, niobium 0.045%, boron 0.0015%, titanium 0.002%, nitrogen 0.0030%, acid-soluble aluminum 0.005%, total oxygen 0.0020%, sulfur 0.003%, phosphorus 0.008%, with the balance being iron and unavoidable impurities.

[0048] The preparation method includes the following steps:

[0049] First, an electric arc furnace primary refining process is carried out, using high-quality scrap steel and pig iron as raw materials. The titanium content of the raw materials is controlled to be extremely low. After melting, eccentric bottom tapping is used, and the tapping temperature is controlled at 1630℃. During tapping, carbon powder and a small amount of ferrosilicon are added in slurry for pre-deoxidation. The addition of aluminum is strictly prohibited. The initial oxygen activity of the molten steel after tapping is controlled to be 60ppm. Subsequently, LF ladle refining is carried out, with lime and fluorite added to create a high-basicity reducing slag. The binary basicity is controlled at 3.5. Diffusion deoxidation is used to reduce the sulfur content to below 0.005%. At the end of refining, the molten steel is overheated and compensated, with the temperature set at 75℃ above the liquidus temperature.

[0050] Next, vacuum refining was performed, maintaining a vacuum level below 67 Pa. Argon gas was used for strong stirring at a flow rate of 4 NL / (min·t) for 15 minutes. Vacuum carbon deoxidation reduced the dissolved oxygen content to approximately 20 ppm, while simultaneously removing nitrogen to below 30 ppm. Then, cerium-iron cored wire was fed in while maintaining the vacuum. Immediately after feeding, soft blowing argon gas was switched to a flow rate of 1 NL / (min·t), and the mixture was allowed to stand for 6 minutes to homogenize, allowing the generated rare earth oxides to disperse. After standing, the vacuum was broken, and under argon curtain protection, niobium-iron alloy was added and stirred for 1 minute, followed by boron-iron alloy and stirred for another minute. The steel was then tapped.

[0051] Finally, casting and hot working are carried out. The steel ingot is cast into a steel ingot using the bottom casting method under argon protection. The average cooling rate of the steel ingot is controlled at 5℃ / min in the range of 1350℃ to 1200℃ to promote the formation of composite precipitates. The steel ingot is heated to 1200℃ for forging, with a total forging ratio of 3.5. After forging, it is subjected to quenching and tempering treatment, heated and held at 860℃ and then oil quenched, followed by high-temperature tempering at 600℃, finally obtaining a spindle forging with high hardenability and excellent fatigue performance.

[0052] Example 2:

[0053] This embodiment provides a high wear-resistant and fatigue-resistant generator spindle and its manufacturing method. The chemical composition of the generator spindle steel, by mass percentage, includes:

[0054] Carbon 0.36%, silicon 0.22%, manganese 0.60%, chromium 1.58%, nickel 1.58%, molybdenum 0.25%, cerium 0.020%, niobium 0.055%, boron 0.0022%, titanium 0.002%, nitrogen 0.0040%, acid-soluble aluminum 0.008%, total oxygen 0.0022%, sulfur 0.004%, phosphorus 0.007%, with the balance being iron and unavoidable impurities.

[0055] The preparation method includes the following steps:

[0056] First, the steel undergoes primary refining in an electric arc furnace, with strict screening of low-titanium scrap. The tapping temperature of the electric arc furnace is controlled at 1640℃. Carbon powder pre-deoxidation is used to create a semi-killed state, controlling the initial oxygen activity at 70ppm. Aluminum deoxidation is not used throughout the process. After entering the LF refining station, high-basicity white slag is produced for deep desulfurization, controlling the sulfur content to around 0.004%. Before leaving the station, the temperature of the molten steel is raised to 82℃ above the liquidus temperature to compensate for the temperature drop during the subsequent long settling time.

[0057] In the crucial VD vacuum treatment stage, the steel was vigorously stirred for 18 minutes under a vacuum of 65 Pa to precisely control the total oxygen content in the molten steel to 22 ppm through the carbon-oxygen reaction, while the nitrogen content was stably controlled at 40 ppm. Within this thermodynamic window, cerium-iron cored wire was fed in, where Ce preferentially combines with O and S to form Ce₂O₂S. Then, the mixture was gently stirred and allowed to stand for 7 minutes to encourage large inclusions to float to the surface, preserving the micron-sized rare earth cores. After breaking the vacuum, using the appropriate supercooling of the molten steel, niobium-iron alloy was first added to allow Nb to adsorb onto the surface of the rare earth cores. Two minutes later, boron-iron alloy was added to ensure that boron existed in a solid solution state.

[0058] In subsequent processes, the steel ingot is cast under a protective atmosphere, and the cooling rate in the temperature range of 1350℃ to 1200℃ is controlled at 3℃ / min, giving sufficient time for Nb(C,N) to grow epitaxially on the Ce2O2S surface; the initial forging temperature is 1215℃, and multi-directional forging is carried out using a 4000-ton hydraulic press, with a total forging ratio of 4.0, using high-temperature stable composite particles to pin the grain boundaries; finally, after quenching at 870℃ and tempering at 610℃, a finished spindle with uniform structure and fine grains is obtained.

[0059] Example 3:

[0060] This embodiment provides a high wear-resistant and fatigue-resistant generator spindle and its manufacturing method. The chemical composition of the generator spindle steel, by mass percentage, includes:

[0061] Carbon 0.38%, Silicon 0.30%, Manganese 0.70%, Chromium 1.65%, Nickel 1.65%, Molybdenum 0.30%, Cerium 0.025%, Niobium 0.065%, Boron 0.0030%, Titanium 0.003%, Nitrogen 0.0050%, Acid-soluble Aluminum 0.009%, Total Oxygen 0.0025%, Sulfur 0.005%, Phosphorus 0.009%, with the balance being iron and unavoidable impurities.

[0062] The preparation method includes the following steps:

[0063] First, an electric arc furnace primary refining process is carried out, ensuring an extremely low titanium introduction rate during the batching process. The tapping temperature is raised to 1650℃, and only carbon powder and a trace amount of ferrosilicon are used for pre-deoxidation, with the initial oxygen activity controlled at 80ppm. During LF refining, a high-basicity slag system (R=4.5) is used for deep purification, and the superheat of the molten steel is raised to 90℃ above the liquidus temperature to reserve enthalpy for subsequent deep vacuum treatment.

[0064] In the VD refining process, a high-intensity vacuum carbon deoxidation process is implemented, with the vacuum holding time extended to 20 minutes. Combined with strong argon stirring, this ensures that the finished product nitrogen content is reduced to below the critical safety line of 50 ppm, and the oxygen content is precisely controlled at 25 ppm, even with high alloy content and relatively high initial nitrogen content. Subsequently, the maximum amount of cerium-iron cored wire is fed in, and the soft-blowing settling time is extended to 8 minutes to ensure sufficient modification and homogenization of the high-concentration rare earth inclusions. After breaking the vacuum, the feeding sequence of niobium first, then boron is strictly followed; niobium-iron is added and stirred for 2 minutes before adding boron-iron.

[0065] During the casting stage, a fully enclosed argon gas protection system is used. In view of the high tendency of solidification segregation in high alloy steel ingots, the cooling rate is controlled at 1℃ / min in the range of 1350℃ to 1200℃. Slow cooling is used to promote the full growth and distribution of composite precipitates. The forging heating temperature is 1230℃, and large reduction forging is carried out, with a total forging ratio of 4.5 to ensure a dense core structure. Finally, the ingot is oil quenched at 880℃ and tempered at 620℃, which ensures extremely high strength while obtaining excellent deep hardenability and fatigue fracture resistance.

[0066] Comparative Examples 1-5:

[0067] Comparative Example 1:

[0068] This comparative example provides an existing titanium-containing generator spindle steel and its preparation method. Compared with Example 2, the difference is that 0.020% titanium (Ti) is actively added to the chemical composition, and a conventional strong aluminum deoxidation process is used in the LF refining stage to achieve an acid-soluble aluminum content of 0.025% and a total oxygen content of 8ppm in the finished steel. In the VD refining stage, only conventional vacuum degassing is performed, and a vacuum carbon deoxidation process is not executed. Furthermore, ferroniobium and ferroboron are added together with fertitanium, and a cerium-mediated sequential alloying step is not used. All other raw materials and process parameters are the same as in Example 2.

[0069] Comparative Example 2:

[0070] This comparative example provides a steel lacking rare earth components and its preparation method. Compared with Example 2, the difference is that cerium-iron cored wire is not fed in the VD refining stage, that is, it does not contain cerium (Ce) components. Only niobium iron and boron iron are added in the environment after vacuum carbon deoxidation. Rare earth oxysulfides are lacking as heterogeneous nucleation cores. The other raw materials and process parameters are the same as those in Example 2.

[0071] Comparative Example 3:

[0072] This comparative example provides a steel and its preparation method using a conventional strong aluminum deoxidation process. Compared with Example 2, the difference is that aluminum wire is fed into the steel before the LF refining station for strong deoxidation, resulting in an acid-soluble aluminum content of 0.030% and a total oxygen content of 4 ppm in the finished steel. Due to the low oxygen content, the VD stage cannot carry out an effective carbon-oxygen reaction, and the added cerium mainly reacts with sulfur to form simple cerium sulfide or reacts with aluminum to form cerium aluminate, failing to generate the expected Ce2O2S nucleation particles. The other raw materials and process parameters are the same as in Example 2.

[0073] Comparative Example 4:

[0074] This comparative example provides a steel with a different feeding sequence and its preparation method. Compared with Example 2, the difference is that the alloying operation in the VD refining stage does not follow a step-by-step sequence. Instead, after vacuum treatment, cerium-iron cored wire, niobium-iron and boron-iron are added to the molten steel all at once, and the soft blowing and settling homogenization step after feeding cerium is omitted. Only simple stirring is performed before steel is poured, which results in the inability to form a core-shell structured composite precipitate. All other raw materials and process parameters are the same as in Example 2.

[0075] Comparative Example 5:

[0076] This comparative example provides a steel with excessive nitrogen content and its preparation method. Compared with Example 2, the difference is that the vacuum denitrification time in the VD refining stage is shortened and strong argon stirring is not used, resulting in poor denitrification effect. The nitrogen content in the finished steel is 0.0085% (85ppm). Under this high nitrogen environment, although the Ce-Nb synergistic process is adopted, the excess nitrogen still tends to consume boron to form BN. The other raw materials and process parameters are the same as in Example 2.

[0077] Test Examples 1-5:

[0078] Test Example 1: Verification of Precise Control of Oxygen Content and Evolution of Inclusion Types

[0079] Experimental steps:

[0080] In the entire preparation process described in Example 2, four melt samples were taken strictly according to the process time nodes. The sampling points were set sequentially as follows: Node A, the moment when the ladle arrived at the refining station after the electric arc furnace finished tapping steel; Node B, the moment when the ladle left the station after the LF refining was completed; Node C, the moment when the VD vacuum carbon deoxidation was completed and before the vacuum was broken but before the cerium-iron ladle core wire was fed; Node D, the finished steel after final casting (taken from 1 / 2 radius of the steel ingot).

[0081] The molten steel samples from the four nodes were processed into standard cylindrical specimens with a diameter of 5 mm and a length of 5 mm. The surface oxide scale was removed by mechanical grinding to eliminate external oxygen interference. The total oxygen (TO) content was determined using a LECOTCH-600 oxygen, nitrogen, and hydrogen analyzer. Three parallel samples were tested for each node, and the arithmetic mean was taken as the final recorded data.

[0082] Large-scale electrolysis of samples from each node was performed using the non-aqueous electrolytic extraction method (SPEED method). The electrolyte system consisted of a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution, and the constant current density was controlled at 50 mA / cm². 2 After electrolysis, the anode residue was collected, washed, and dried. The extracted residue powder was then subjected to phase analysis using an X-ray diffractometer (XRD), with a scanning range of 2... The range is 10°–90°, to determine the evolution of the main phase structure of the inclusions.

[0083] Experimental data:

[0084] Table 1. Oxygen content and evolution of precipitated phases at each process stage in Example 2

[0085]

[0086] Conclusion Analysis:

[0087] According to the data in Table 1, the oxygen content control of the generator spindle steel exhibits a clear stepwise decrease throughout the smelting cycle. At node A (after EAF tapping), due to the use of carbon powder pre-deoxidation instead of strong aluminum deoxidation, the initial oxygen content remains at a relatively high level of 75.2 ppm. At this point, the main inclusions in the steel are exogenous Al2O3 clusters. After diffusion deoxidation in the LF refining stage (node ​​B), the oxygen content drops to 39.4 ppm, but this concentration is still higher than the ideal thermodynamic equilibrium value required for precise rare earth modification, and the inclusions are still mainly alumina. At node C (after VD carbon deoxidation), through CO reaction under high vacuum, the total oxygen content is precisely reduced to 20.9 ppm, successfully entering the preset process window. The oxygen content of the final product (node ​​D) is stable at 21.5 ppm, and XRD phase analysis confirms that the inclusions have undergone a qualitative change at this point, with 88.7% of the precipitated phases transforming into Ce2O2S (rare earth oxygen sulfides), while the original Al2O3 content is reduced to 1.7%.

[0088] according to Figure 1 The curves in the figure clearly depict the dynamic evolution path of oxygen content throughout the entire process. The auxiliary dashed lines on the coordinate axes precisely calibrate the values ​​at each key process node: node A at 0 minutes corresponds to 75.2 ppm, and node B at 45 minutes corresponds to 39.4 ppm. The curve drops to 20.9 ppm at 75 minutes (node ​​C), a data point that falls precisely within the target process window (15-25 ppm) indicated by the gray background in the figure. Subsequently, the black solid line interval extending from node C (75 minutes) to node D (120 minutes) corresponds to the rare earth modification and inclusion transformation process stages of this invention. The oxygen variation characteristics in this range reveal the core timing logic of this invention: only when the oxygen content of the molten steel first enters a specific window of 15-25 ppm at node C, can the added cerium efficiently induce the transformation of inclusions from aluminum-based to rare earth oxysulfides during the settling and casting stage (i.e., the CD range), ultimately maintaining a stable oxygen content of 21.5 ppm at 120 minutes (node ​​D) and generating the expected Ce2O2S phase.

[0089] Based on Table 1 and Figure 1 The present invention discloses a VD vacuum carbon deoxidation process that successfully constructs the specific thermodynamic environment required for the generation of Ce2O2S cores. By strictly locking the oxygen content within the 15-25 ppm range shown in the gray area of ​​the figure, and coordinating specific operations within the time intervals indicated by the arrows, the originally harmful brittle alumina inclusions in the molten steel are transformed in situ into spherical, dispersed rare earth oxide sulfides. This precise phase control forms the physical metallurgical basis for subsequent induction of niobium carbonitride nucleation, achieving boron solid solution protection, and ultimately improving the fatigue performance of the material.

[0090] Test Example 2: Verification of Available Boron (Solidated Boron) Content and Nitrogen Capture Efficiency

[0091] Experimental steps:

[0092] Sample preparation:

[0093] From the same position (at 1 / 2 radius) of the final forgings of Example 2 (the recommended scheme of this invention), Comparative Example 1 (the conventional titanium-containing scheme), Comparative Example 2 (the cerium-free / single niobium scheme), and Comparative Example 5 (the high-nitrogen / process runaway scheme), cubic specimens with dimensions of 20mm × 20mm × 20mm were wire-cut. All specimens were polished, cleaned, and dried before use.

[0094] Total element determination:

[0095] The total nitrogen (TN) content of each group of samples was determined by inert gas melting and thermal conductivity method; the total boron (TB) content of the samples was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) as the calculation benchmark.

[0096] Phase separation and detection of boron in solid solution:

[0097] Phase separation was performed using a constant potential electrolysis method. The sample was used as the anode, stainless steel as the cathode, and the electrolyte was a 10% acetylacetone-1% tetramethylammonium chloride-methanol system. After electrolysis, vacuum filtration was performed using a 0.1 μm polycarbonate membrane. The filtrate contained effective boron dissolved from the matrix, while the filter residue contained precipitated boron nitride (BN) and other boron-containing phases. The filtrate was collected, and the boron concentration was accurately determined using ICP-MS (inductively coupled plasma mass spectrometry) to convert it into the effective boron content in the steel.

[0098] Data processing: According to the formula Calculate the effective boron utilization rate of each group of samples.

[0099] Experimental data:

[0100] Table 2. Analysis of nitrogen and boron content and dissolved boron ratio under different process systems

[0101]

[0102] Conclusion Analysis:

[0103] According to the quantitative detection data in Table 2, under the premise that the total boron content is basically the same (about 22 ppm), the dissolved boron content of each group of samples showed significant differences. In Example 2 (the scheme of this invention), without the addition of titanium, the dissolved boron content was measured to be 20.8 ppm, and the solid solution ratio was as high as 92.9%. This value is at the same level as, or even slightly better than, Comparative Example 1 (solid solution ratio 91.3%) which uses titanium protection. In contrast, although the nitrogen content of Comparative Example 2, which lacks cerium, is normally controlled (42 ppm), the dissolved boron ratio drops sharply to 42.5%, indicating that niobium alone cannot effectively inhibit the formation of BN under titanium-free conditions. As for Comparative Example 5, due to the excessive nitrogen content (86 ppm), even with Ce-Nb treatment, the dissolved boron ratio was only 26.5%, indicating that the nitrogen content had exceeded the control range of stoichiometry.

[0104] according to Figure 2 The bar chart visually illustrates the differences in the dissolved boron ratio under different process strategies. The dashed line running horizontally across the 80% mark of the Y-axis represents the effective hardenability guarantee threshold, which is the engineering baseline for meeting the deep hardenability requirements of the generator spindle.

[0105] The column heights of Example 2 (Ce-Nb synergy) and Comparative Example 1 (Ti-containing conventional) reached 92.9% and 91.3%, respectively, both significantly higher than the 80% threshold. This indicates that the titanium-free synergistic nitrogen fixation strategy of the present invention achieves the same boron protection effect as the conventional titanium-containing process.

[0106] Conversely, the column heights of Comparative Example 2 (no Ce / single Nb) and Comparative Example 5 (high nitrogen runaway) were 42.5% and 26.5%, respectively, far below this threshold. This difference indicates that in the absence of rare earth modification or when nitrogen content is runaway, boron mainly precipitates as ineffective BN compounds, resulting in insufficient effective dissolved boron and rendering it ineffective.

[0107] The X-axis labels at the bottom of the figure and the process descriptions in parentheses (such as Ce-Nb synergy, high nitrogen runaway, etc.) form a clear correspondence with the data above, confirming the decisive impact of specific process configurations on the final performance.

[0108] Based on Table 2 and Figure 2The content confirms that the Ce-Nb synergistic nitrogen fixation mechanism proposed in this invention is entirely feasible in engineering. Data from Example 2 demonstrates that, under suitable oxygen and nitrogen content windows, the preferential precipitation of niobium carbonitrides induced by rare-earth oxysulfides can efficiently consume free nitrogen in the matrix, similar to traditional titanium. This mechanism successfully avoids the combination of nitrogen and boron to form stable BN, thus ensuring that the vast majority of boron atoms (>90%) exist in a solid solution state. These solid-solution boron atoms will segregate at austenite grain boundaries during subsequent heat treatment, playing a crucial role in inhibiting ferrite nucleation and thus guaranteeing the deep hardenability required for the generator spindle.

[0109] Test Example 3: Hardenability and Deep Hardness Distribution Test

[0110] Experimental steps:

[0111] Sample preparation: From the forging bodies of Examples 1-3 and Comparative Examples 1-5, blanks were cut longitudinally, normalized, and then processed into standard end-quenched specimens (Jominy Bars) strictly according to ASTM A255 standards. The specimens were 25 mm in diameter and 100 mm in length, with a flange at one end for suspension.

[0112] Solution treatment: Place all samples in a box furnace and heat to 880°C, hold for 30 minutes to ensure uniform austenitization and prevent coarse grains.

[0113] End quenching: After the sample is taken out of the furnace, it is quickly (<5 seconds) transferred to the standard quenching table support, and the water valve below is opened to impact the bottom surface of the sample with a constant pressure of 20°C water column until the sample is completely cooled.

[0114] Hardness Testing: Two parallel test planes, 0.4 mm deep, were ground along the length of the sample. A Rockwell hardness tester (HRC scale) was used to measure the hardness values ​​at different locations from the quenched end. This test focused on recording the hardness at three key locations: J1.5 mm (representing the surface cooling rate of the forging), J15 mm (representing the cooling rate at half the radius), and J50 mm (representing the cooling rate of the core of the forging). The hardness difference between the surface and the core was calculated. .

[0115] Experimental data:

[0116] Table 3. Comparison of end-quench hardness distribution and hardenability differences among samples in each group.

[0117]

[0118] Conclusion Analysis:

[0119] According to the data in Table 3, Examples 1-3 maintained a high hardness level throughout the entire test distance. Taking Example 2 as an example, its surface hardness (J1.5mm) was 47.5HRC, while at J50mm, simulating the core cooling conditions of a large-section forging, the hardness remained at 45.8HRC, showing an overall hardness difference. It has a hardness of only 1.7 HRC. This extremely flat hardness distribution curve indicates that the trace amount of boron added to the steel is effectively protected, successfully suppressing the nucleation of proeutectoid ferrite at the austenite grain boundaries, allowing the material to completely transform into bainite / martensite structure even at low cooling rates, and possessing excellent deep hardenability.

[0120] Comparing the data from Example 2 and Comparative Example 2 (without Ce), it can be found that the surface hardness difference between the two is not significant (47.5 vs 46.5 HRC). However, at J50mm, the hardness of Comparative Example 2 drops sharply to 32.4 HRC, a hardness difference of 14.1 HRC. This is because, in the absence of cerium for synergistic nitrogen fixation, the niobium in the steel is insufficient to completely capture nitrogen atoms, leading to the combination of boron and nitrogen to form BN precipitation, thus losing the solid solution boron atoms that improve hardenability.

[0121] Similarly, comparing Example 2 with Comparative Example 5 (high N), it can be seen that when the nitrogen content exceeds the standard, the hardness at J50mm is only 30.5 HRC, and the hardenability is completely lost. This further confirms that the low oxygen, suitable nitrogen, and Ce-Nb synergistic process window described in this invention is the key to ensuring the uniformity of the cross-sectional performance of large-scale generator main shafts. It is worth noting that the hardenability index of Example 2 is comparable to that of Comparative Example 1 (J50mm = 45.2 HRC) using traditional titanium microalloying, proving that the titanium-free design of this invention lays the foundation for subsequent improvement of fatigue performance (elimination of TiN) without sacrificing hardenability.

[0122] Test Example 4: Impact Toughness and Anisotropy (Aspect Ratio) Test

[0123] Experimental steps:

[0124] Sampling direction definition: After the heat treatment of the forgings in Examples 1-3 and Comparative Examples 1-5 is completed, samples are taken at 1 / 2 radius of the forging body. The billets are cut along the forging flow line direction (longitudinal, L direction) and perpendicular to the forging flow line direction (transverse, T direction).

[0125] Sample preparation: The above-mentioned blank was precision-machined into a standard V-notch impact specimen according to GB / T229 (or ISO148-1) standard for Charpy impact testing of metallic materials. The specimen dimensions are 10mm×10mm×55mm, notch depth is 2mm, notch bottom curvature radius is 0.25mm, and notch angle is 45°.

[0126] Low-temperature impact test: Place the processed sample in a low-temperature constant temperature bath and keep it at -40℃ in a medium (alcohol) for 15 minutes, with the deviation controlled within ±1℃. After removing the sample, complete the impact test within 5 seconds and record the impact energy (Akv) absorbed by each sample. Test 3 parallel samples for each group and each direction, and take the arithmetic mean.

[0127] Data processing: According to the formula Calculate the degree of anisotropy of the mechanical properties of each group of materials. The closer the value is to 1, the better the isotropy.

[0128] Experimental data:

[0129] Table 4. Comparison of impact toughness and anisotropy coefficient at -40℃

[0130]

[0131] Conclusion Analysis:

[0132] According to the data in Table 4, Example 2 maintains extremely high longitudinal toughness (182J) while achieving a transverse toughness of up to 168J and an anisotropy coefficient of 0.92, indicating that the material exhibits excellent performance consistency in different stress directions.

[0133] Comparing Example 2 with Comparative Example 1 (containing Ti), it can be seen that although the longitudinal impact energy of the two is relatively close (182J vs 174J), the transverse impact energy of Comparative Example 1 drops to 108J, with an anisotropy coefficient of only 0.62. This is because traditional titanium-containing steel contains sharply angular TiN inclusions, which cannot deform during forging and are prone to becoming crack initiation points under transverse stress. Example 2, however, uses a titanium-free design, eliminating the cutting effect of TiN on the transverse matrix, thereby improving transverse toughness.

[0134] The differences are even more pronounced when comparing the data from Example 2 and Comparative Example 3 (strong aluminum deoxidation). Comparative Example 3 exhibits a transverse impact energy of only 92 J and an anisotropy coefficient as low as 0.56. This is because the aluminum deoxidation product Al2O3 tends to aggregate into clusters and elongate into chains (stringers) along the processing direction during forging, leading to deterioration of transverse properties. In contrast, the Ce2O2S inclusions generated by rare earth modification in this invention are spherically distributed with moderate hardness. They do not deform during high-temperature forging, maintaining their spherical shape and avoiding the formation of anisotropic banded structures.

[0135] In summary, this invention, through unique deoxidation and microalloying control, not only achieves high strength and toughness, but also fundamentally solves the problem of the shortcoming of transverse performance in large forgings, ensuring the safety of the generator main shaft under complex stress conditions.

[0136] Test Example 5: Rotary Bending Fatigue Limit Test

[0137] Experimental steps:

[0138] Specimen processing: In Examples 1-3 and Comparative Examples 1-5, blanks were longitudinally cut at half the radius of the forging body after quenching and tempering. First, standard tensile specimens were machined to determine basic mechanical properties. Subsequently, standard smooth rotary bending fatigue specimens were precision machined, with a working section diameter of 6 mm and a transition arc radius R30 mm. To avoid surface machining marks becoming fatigue crack initiations, the working section of all fatigue specimens was longitudinally polished to achieve a surface roughness Ra ≤ 0.2 μm.

[0139] Basic mechanical testing: Tensile tests were conducted at room temperature using a universal testing machine to determine the tensile strength of each group of specimens. ) and yield strength ( This serves as the basis for setting the stress level for fatigue testing.

[0140] Fatigue limit testing: A PQ-6 rotary bending fatigue testing machine was used. The loading method was pure bending, the stress ratio R = -1, and the rotational speed was 3000 r / min. The fatigue limit was determined using the lifting and lowering method, with a specified number of cycles of 10. 7 Next. If the sample is at 10 7 If the specimen does not fracture within one test, the stress level is increased by one step for the next specimen; if it fractures, the stress level is decreased by one step. The stress increment is set to 15-20 MPa, and each group of valid test specimens shall contain no fewer than 15 specimens.

[0141] Data calculation: Statistical rise and fall method data, calculate the average value of each group of materials at 10. 7 Conditional fatigue limit under cycle ( ). and according to the formula Evaluate the fatigue resistance efficiency of materials.

[0142] Experimental data:

[0143] Table 5. Results of tensile strength, yield strength and rotational bending fatigue limit tests

[0144]

[0145] Conclusion Analysis:

[0146] According to the data in Table 5, Examples 1-3 exhibit excellent strength-toughness balance and fatigue resistance. Taking Example 2 as an example, its tensile strength reaches 985 MPa, yield strength 895 MPa, and fatigue limit as high as 565 MPa, with a fatigue strength ratio stable at 0.57. This value is significantly higher than the average level of 0.50-0.52 for typical high-strength alloy steels, indicating that the material is extremely pure internally and that the microstructure has a very strong ability to impede cyclic stress.

[0147] Comparing Example 2 with Comparative Example 1 (containing Ti), it can be seen that although the static tensile strengths of the two are very close (985 MPa vs. 980 MPa), the fatigue limit of Comparative Example 1 is only 510 MPa, and the fatigue strength ratio drops to 0.52. This confirms that removing titanium eliminates the negative effect of large-sized angular TiN inclusions as fatigue crack initiation sources. In Example 2, the fine, dispersed spherical rare earth oxides are tightly bonded to the matrix, making them less prone to stress concentration and thus improving fatigue life.

[0148] By comparing the data from Example 2 and Comparative Example 4 (with incorrect timing), the decisive influence of process timing on the microstructure is revealed. Although Comparative Example 4 added the same amount of Ce and Nb, it failed to form the expected composite precipitation structure with Ce2O2S as the core and Nb(C,N) as the shell because it did not follow the timing sequence of Ce deoxidation to control S- followed by Nb microalloying. The lack of this composite structure prevented Nb(C,N) from effectively refining the grains through the pinning effect, ultimately leading to grain coarsening in Comparative Example 4, with a fatigue limit of only 465 MPa, which was 100 MPa lower than that of Example 2. This demonstrates that specific precipitate morphology control is crucial for improving the high-cycle fatigue performance of the generator spindle.

Claims

1. A high wear-resistant and fatigue-resistant generator spindle, characterized in that, It consists of chemical components comprising the following percentages by mass: Carbon content: 0.34%-0.38%; Silicon 0.15%-0.30%; Manganese 0.50%-0.70%; Chromium 1.50%-1.65%; Nickel 1.50%-1.65%; Molybdenum 0.20%-0.30%; Cerium 0.015%-0.025%; Niobium 0.045%-0.065%; Boron 0.0015%-0.0030%; Acid-soluble aluminum content: 0.005%-0.009%; Total oxygen: 0.0020%-0.0025%; Nitrogen 0.0030%-0.0050%; Limiting impurities: Titanium ≤ 0.003%; Sulfur content ≤ 0.005%; Phosphorus ≤ 0.009%; The balance is iron and unavoidable impurities, and the sum of the mass percentages of the above components is 100%.

2. The high wear-resistant and fatigue-resistant generator spindle according to claim 1, characterized in that, In the chemical composition, the mass ratio of cerium to sulfur is controlled between 3.0 and 6.

0.

3. A method for preparing a high wear-resistant and fatigue-resistant generator spindle, characterized in that, The method for preparing the high wear-resistant and fatigue-resistant generator spindle according to any one of claims 1-2 includes the following steps: The titanium content of the raw materials fed into the furnace is controlled. After primary refining and melting in an electric arc furnace, the steel is tapped. Carbon powder is added during the tapping process for pre-deoxidation. No aluminum deoxidizer is added throughout the process. The initial oxygen activity of the molten steel after tapping is controlled to obtain primary refined molten steel. The primary steel liquid is subjected to LF refining to produce high-basicity slag for diffusion deoxidation and desulfurization. Before the end of refining, the steel liquid is overheated to obtain refined steel liquid. The refined steel liquid is subjected to VD vacuum refining, and a vacuum carbon deoxidation reaction is carried out under high vacuum conditions to reduce the dissolved oxygen and nitrogen content. Then, cerium iron cored wire is fed in under vacuum, and the mixture is allowed to stand and homogenize. After breaking the vacuum, niobium iron alloy and boron iron alloy are added in sequence under a protective atmosphere to obtain the steel liquid to be cast. The molten steel to be poured is cast into steel ingots under a protective atmosphere, the solidification and cooling rate is controlled, and then the steel ingots are heated and forged, followed by tempering heat treatment.

4. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the primary refining step of the electric arc furnace, an eccentric bottom tapping method is adopted, the tapping temperature is controlled at 1630-1650℃, and the initial oxygen activity is controlled at 60-80ppm.

5. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the LF refining step, the binary basicity of the high basicity slag is controlled at 3.5-4.5, and the temperature of the molten steel at the end of refining is controlled at 75-90°C above the liquidus temperature.

6. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the VD vacuum refining step, the vacuum degree of the vacuum carbon deoxidation reaction is controlled below 67 Pa, and the processing time is 15-20 minutes, until the total oxygen content in the molten steel drops to 20-25 ppm and the nitrogen content drops to 30-50 ppm.

7. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the VD vacuum refining step, the argon gas stirring intensity is controlled in stages: During the vacuum carbon deoxidation stage, strong stirring is started, and the argon flow rate is 3-5 NL / (min·t); After feeding in the cerium-iron cored wire, switch to soft-blown argon gas, with the argon flow rate reduced to 0.5-1.5 NL / (min·t).

8. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the VD vacuum refining step, the settling and homogenization time is 6-8 minutes; The feeding sequence after breaking the void is as follows: first add niobium-iron alloy and stir for 1-2 minutes, then add boron-iron alloy and stir for 1 minute before immediately tapping out the steel.

9. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the casting and hot working steps, the bottom pouring method is used to control the average cooling rate of the steel ingot in the temperature range of 1350℃ to 1200℃ to be 1-5℃ / min; The forging process employs a multi-directional forging technique, with a total forging ratio ≥3.

5.

10. The method for preparing a high wear-resistant and fatigue-resistant generator spindle according to claim 3, characterized in that, In the casting and hot working steps, the quenching and tempering heat treatment includes: The forgings are heated to 860-880℃ and held for a period of time before being oil quenched, followed by high-temperature tempering at 600-620℃.

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