A super-high-strength torsion shaft steel and a method for manufacturing the same

Abstract

This invention relates to an ultra-high strength torsion shaft steel and its preparation method, belonging to the technical field of torsion shaft steel preparation. To address the problem of existing ultra-high strength torsion shaft steels failing to achieve a synergistic improvement in strength and toughness, this invention provides an ultra-high strength torsion shaft steel and its preparation method, comprising the following steps: electric / converter smelting → LF refining → VD vacuum degassing → continuous casting → slow cooling → electroslag remelting → hood cooling → large bar billet preparation → annealing → billet heating → small bar rolling → slow cooling / hot delivery → annealing → sample heat treatment. This invention optimizes the upper limit ratio of C, Si, and Mn, and matches an 850℃ quenching + 200℃ tempering process, resulting in a yield strength ≥1600MPa, tensile strength ≥2000MPa, and impact energy ≥35J for the torsion shaft steel, achieving a synergistic improvement in strength and toughness. Precise process control throughout the entire process improves the purity and uniformity of the steel, ensuring consistent and stable product quality.

Description

Technical Field

[0001] This invention belongs to the field of torsion shaft steel preparation technology, and particularly relates to an ultra-high strength torsion shaft steel and its preparation method. Background Technology

[0002] As a core load-bearing component of high-end equipment, ultra-high-strength torsion shafts must withstand higher alternating loads, impact loads, and complex stresses. Their performance directly determines the equipment's load-bearing capacity, service safety, and service life, placing more stringent requirements on the comprehensive mechanical properties of materials. Ultra-high-strength steel is generally defined as steel with a yield strength ≥1180MPa and a tensile strength ≥1380MPa, and it is the core material for manufacturing high-performance torsion shafts.

[0003] However, existing mature ultra-high strength steels generally face the technical bottleneck of the inversion of inherent strength and toughness. That is, while improving the strength of the material, it often leads to a significant decrease in toughness, which can easily cause failures such as cracks and fractures in the torsion shaft during service. This simply cannot meet the extreme requirements of the new generation of advanced equipment for material safety and reliability.

[0004] Research indicates that the key to solving the problem of the inversion of strength and toughness in ultra-high strength steel lies in the systematic optimization of the entire material preparation process. The core requires controlling three major material science elements: first, purity, to eliminate the hidden dangers of crack initiation from the source; second, uniformity, to effectively eliminate component segregation and microstructure gradient, and ensure the consistency and reliability of the overall mechanical properties of the material; and third, microstructure, to precisely control the multi-scale microstructure to achieve synergistic strengthening and toughness.

[0005] Currently, the chemical composition design and heat treatment regime of existing ultra-high strength torsion shaft steels still have room for optimization, failing to achieve synergistic control of the three major material science elements, resulting in difficulties in improving their comprehensive mechanical properties. Therefore, there is an urgent need to develop a new type of ultra-high strength torsion shaft steel that significantly improves toughness while maintaining ultra-high strength, to meet the pressing needs of high-end equipment for ultra-high strength torsion shaft steel. Summary of the Invention

[0006] To address the problem that existing ultra-high strength torsion shaft steels are difficult to improve in a synergistic way by enhancing both strength and toughness, this invention provides an ultra-high strength torsion shaft steel and its preparation method.

[0007] The technical solution of this invention:

[0008] A type of ultra-high strength torsion shaft steel, the chemical composition of which by weight percentage includes: C: 0.42~0.46wt%, Si: 0.17~0.37wt%, Mn: 0.50~0.80wt%, P≤0.015wt%, S≤0.015wt%, Cr: 0.80~1.10wt%, V: 0.10~0.20wt%, Mo: 0.20~0.30wt%, Ni: 1.39~1.80wt%, and Cu≤0.20wt%, with the remainder being Fe and unavoidable impurities.

[0009] A method for preparing ultra-high strength torsion shaft steel includes the following steps performed in sequence: electric / converter smelting → LF refining → VD vacuum degassing → continuous casting → slow cooling → shot blasting → electroslag remelting → hood cooling → bar billet opening → annealing → billet heating → bar rolling → slow cooling / hot delivery → annealing → straightening → grinding → inspection → sample heat treatment.

[0010] The electroslag remelting process is carried out under full argon protection and cold arc initiation of metal electrodes. The melting rate is controlled at 6~6.5 kg / min. Al powder is added for deoxidation in the last half stage of electrode ingot melting, and no Al powder is added in the first half stage.

[0011] Before the billet is opened, a cold heating process is adopted. First, it is kept at ≤700℃ for 1 hour, then heated to 850℃ at a heating rate of 30~90℃ / h, then heated to 1290℃ at a heating rate of 30~150℃ / h, and finally kept at 1290℃ for 2 hours.

[0012] The billet heating is controlled in three stages: stage 1 heating temperature ≤1120℃, stage 2 heating temperature 1160~1200℃, and soaking temperature 1140~1180℃. The billet exit temperature before small bar rolling is 1140~1180℃.

[0013] The heat treatment of the sample adopted a quenching and tempering process. The sample was quenched at 850℃ for 1 hour and then oil-cooled, and tempered at 200℃ for 3 hours and then air-cooled.

[0014] Furthermore, the tapping endpoint control of the electric / converter smelting is as follows: C is not less than 0.10wt%, P is ≤0.005wt%, and the tapping temperature is 1600~1640℃; Al ingots are added for pre-deoxidation during tapping, and 60Kg / t of quicklime and 30Kg / t of deoxidation and desulfurization refining slag are added to the ladle.

[0015] Furthermore, during the LF refining process, 20-30 kg / t of lime and 0-10 kg / t of deoxidation and desulfurization refining slag are added during each power supply, along with a mixed deoxidizer comprising 3 kg / t of aluminum granules, 2 kg / t of silicon carbide, and 2 kg / t of carbon powder; the total refining time is no less than 55 minutes, with the white slag retention time being no less than 20 minutes.

[0016] Furthermore, during the VD vacuum degassing process, the vacuum degree is controlled to be ≤67Pa, and the time under this vacuum condition is not less than 20 minutes; the silicon-calcium wire is fed into the casting furnace at a rate of 60~80m / furnace, and the silicon-calcium wire is fed into the continuous casting furnace at a rate of 40~60m / furnace.

[0017] Furthermore, the slow cooling time of the electrode billet obtained by continuous casting is not less than 48 hours, and the starting temperature is ≤200℃; the bending degree of the electrode billet is controlled to be ≤15mm / m, and the bending degree of the whole length does not exceed 20mm.

[0018] Furthermore, the electroslag remelting uses a 480 / 520mm crystallizer, with an arc ignition voltage of 60~85V, a slag-forming current of 0~5.0kA, and a refining current of 4.0~10.0kA. During a 20-minute current-raising transition period, the current is gradually increased from the slag-forming stage to 7.0±0.5kA, and then stabilized at a process current of 8.0±0.5~8.5±0.5kA during a 30-minute process current increase period. When the remaining ingot height is 300~500mm or the remaining electrode mass is 240kg, feeding is initiated: at a voltage of 54~82V, the initial feeding current of 7.0~9.0kA is gradually reduced to 4.5~6.5kA over 10~25 minutes, and finally reduced to 0kA over 15~25 minutes. The mold cooling time is 60 minutes in summer and 40 minutes in winter. After the electroslag ingot is demolded, it is placed in a cooling hood within 10 minutes, and the cooling time is not less than 72 hours.

[0019] Furthermore, the specific parameters for argon protection in the electroslag remelting process are as follows: the crystallizer is purged with argon for 15 minutes before slag formation, and the argon flow rate is 0.8 m³ / min. 3 / h; Argon flow rate maintained at 0.8m during slag formation period. 3 / h; the argon flow rate for the first 3 / 4 of the smelting process is 1.1m³ / h. 3 / h, the last quarter is 1.2m 3 / h.

[0020] Furthermore, the electroslag remelting process uses 90 kg of pre-melted slag per 2.5 to 3.3 tons of electrode ingots + 0.9 kg of silica powder per 2.5 to 3.3 tons of electrode ingots. The silica powder is added in three batches at 10, 20, and 30 minutes after arc ignition.

[0021] Furthermore, the annealing temperature of the large bar after billet preparation is 680±10℃; the small bar is subjected to hot-feed annealing or slow-cooling annealing after rolling. The slow-cooling annealing involves placing the bar in a pit at 600~650℃ for slow cooling for at least 60 hours before annealing at a temperature of 740±10℃. When using a continuous annealing furnace, the roll speed is 6m / h. The beneficial effects of this invention are:

[0022] This invention optimizes the upper and middle limits of the core elements such as C, Si, and Mn, and matches them with the optimal heat treatment process of oil quenching at 850℃ for 1 hour followed by air tempering at 200℃ for 3 hours. This enables the mechanical properties of the steel used for torsion shafts to reach ultra-high strength standards with a yield strength ≥1600MPa and a tensile strength ≥2000MPa. At the same time, the impact energy is ≥35J, and the reduction of area and elongation remain stable and meet the requirements of premium-grade steel. While maintaining ultra-high strength, this invention effectively ensures the toughness and plasticity of the steel, achieving a synergistic improvement in strength and toughness. It solves the technical contradiction that the increase in steel strength leads to a decrease in toughness under traditional processes. This invention can meet the extreme requirements of safety, reliability, and impact resistance of torsion shaft steel for the next generation of high-end equipment under extreme service environments.

[0023] This invention establishes precise process parameter control standards for the entire process from steelmaking and smelting, electroslag remelting to rolling and heat treatment. It strictly regulates key aspects such as electrode billet preparation, electroslag remelting rate, annealing temperature, and flaw detection standards, effectively improving the purity, compositional uniformity, and microstructure of the steel. It eliminates problems such as compositional segregation, microstructure gradient, and surface and internal defects, ensuring the stability of the quality of torsion shaft steel products and batch-to-batch consistency, and providing a reliable material basis for the subsequent processing and use of finished shafts. Detailed Implementation

[0024] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0025] Example 1

[0026] This embodiment provides an ultra-high strength torsion shaft steel and its preparation method.

[0027] The chemical composition of the 45CrNiMoVA steel used for ultra-high strength torsion shafts in this embodiment, by weight percentage, includes: C: 0.44wt%, Si: 0.22wt%, Mn: 0.65wt%, P: 0.008wt%, S: 0.002wt%, Cr: 0.94wt%, V: 0.15wt%, Mo: 0.25wt%, Ni: 1.53wt%, and Cu: 0.03wt%, with the remainder being Fe and unavoidable impurities.

[0028] The preparation method of the steel for the torsion shaft in this embodiment has the following process flow: electric / converter smelting → LF refining → VD vacuum degassing → continuous casting → slow cooling → shot blasting → electroslag remelting → hood cooling → billet opening → annealing → billet heating → bar rolling → slow cooling / hot delivery → annealing → straightening → grinding → inspection → sample heat treatment.

[0029] Step 1: Smelting Process

[0030] This embodiment adopts a three-stage smelting process of electric / converter smelting + LF refining + VD vacuum refining. The specific process and parameter control are as follows:

[0031] (1) Electric / converter smelting:

[0032] Qualified scrap steel and molten iron are selected as raw materials and added to the furnace for melting and preliminary oxidation, decarburization, and dephosphorization. The final tapping temperature is controlled at 0.30 wt% C and 0.003 wt% P, with a tapping temperature of 1640℃. Strict control is maintained over slag during tapping, and Al ingots are added for pre-deoxidation according to the carbon content of the tapped steel. At the same time, 60 kg / t of quicklime and 30 kg / t of deoxidation and desulfurization refining slag are added to the ladle.

[0033] (2) LF Refining:

[0034] After the molten steel is transferred to the LF station, 30 kg / t of lime, 5 kg / t of deoxidizing and desulfurizing refining slag, 3 kg / t of aluminum granules, 2 kg / t of silicon carbide, and 2 kg / t of carbon powder are added during the first power supply. The target S in the first refining sample is ≤0.005 wt%, and the Al in the sample is ≤0.035 wt%, which is adjusted to 0.040 wt% by the Al line. The entire refining process takes 60 minutes, of which the white slag is maintained for 25 minutes, achieving deep desulfurization, removal of inclusions, and precise fine-tuning of the composition.

[0035] (3) VD vacuum degassing:

[0036] After LF refining, the ladle is sent to the VD station, where the vacuum degree is controlled to ≤67Pa and maintained under this vacuum condition for 25 minutes to fully remove harmful gases such as hydrogen and oxygen from the molten steel. The silicon-calcium wire is fed in at 80m / furnace for each start-up furnace and at 60m / furnace for each continuous furnace to improve the purity of the molten steel and avoid metallurgical defects.

[0037] Step 2: Continuous casting and slow cooling process:

[0038] The molten steel after VD vacuum treatment is continuously cast. The head and tail of the continuously cast square billet are cut off every 3 meters to remove end defects.

[0039] After continuous casting, the electrode billet is slowly cooled for no less than 48 hours, and the starting temperature is no less than 180℃. Within 7 days after the billet is slowly cooled and removed from the pit, electroslag remelting is carried out. The curvature of the electrode billet is controlled to be ≤15mm / m and the total length is no more than 20mm. Billets with excessive curvature are straightened after annealing to prevent them from affecting subsequent electroslag remelting.

[0040] Step 3: Shot blasting and electroslag remelting processes:

[0041] After shot blasting to remove surface oxide scale and impurities, the electrode blank is sent to the electroslag remelting station. Before use, the surface is cleaned with a steel brush to ensure that there is no rust or debris before it can be put into the furnace.

[0042] A Φ480 / 520mm water-cooled crystallizer is used, ensuring no water leakage. Before arc ignition, confirm that the bottom water tank and crystallizer surface are dry and the mold is in place. The furnace bottom humidity is measured to be ≤50% before arc ignition. A 20mm thick low-carbon steel base pad is used, with its surface polished by hand-held grinding wheel. The furnace opening is tightly covered with a cover plate, with a gap ≤5mm. Targeting an ingot weight of 3.0 tons, a Φ250mm×280mm, 3200kg metal electrode billet is used as the consumable electrode, and the following electroslag remelting process is executed:

[0043] Arc ignition and slag formation: The arc is initiated cold using an 85V metal electrode arc ignition voltage. The slag formation current is controlled between 0 and 4.0 kA, and the melting rate is controlled to a target of 6 kg / min. After the molten slag stabilizes, the refining stage begins.

[0044] Refining and flow ramping: The refining current is 4.0~5.0kA. During the 20-minute flow ramping transition period, the current is gradually increased from the slag-forming stage to 7.0±0.5kA. Then, during the 30-minute process current ramping period, the current is stabilized at 8.0±0.5kA. Refining continues for 25 minutes to ensure thorough purification of the molten steel. In the final half of the ingot smelting process, 350g of Al powder is added via a trolley for deoxidation. No Al powder is added in the first half of the process.

[0045] Slag control: Before slag formation, the crystallizer is purged with argon for 15 minutes at an argon flow rate of 0.8 m³ / min. 3 / h, during the slag-forming period, an argon gas protective hood is used, with a flow rate remaining at 0.8m³ / h. 3 / h, with a flow rate of 1.1m³ / h for the first 3 / 4 stages under full argon protection. 3 / h, the last quarter section is 1.2m 3 / h. The total slag volume is 96kg, including 90kg of pre-melted slag and 6kg of silica. After adding the slag, the mixture is stabilized for 20 minutes. Inclusions and harmful elements in the steel are removed by slag washing. The silica is precisely weighed using a small electronic scale. The slag-making process is carried out in three batches, with silica added at 10, 20, and 30 minutes after arc ignition.

[0046] The pre-melted slag used in this embodiment is a quaternary basic slag system of CaF2-Al2O3-CaO-MgO, with the core component ratio being CaF2: 43wt%, Al2O3: 32wt%, CaO: 25wt%, and MgO: 5wt%.

[0047] Feeding mechanism: Feeding is initiated when the remaining height of the steel ingot is 500mm: First, the current of 7.5kA is reduced to 6.0kA at 82V for 10 minutes, then to 4.5kA for 15 minutes, and finally to 0kA for 25 minutes to achieve dense feeding at the top of the steel ingot.

[0048] Cooling regime: After demolding, the mold cooling time is 60 minutes in summer and 40 minutes in winter, followed by cover cooling (pit cooling) for 72 minutes to avoid cracking of the steel ingot.

[0049] Within 10 minutes of demolding, the electroslag ingot should be placed in a cooling hood. The ground and insulation material at the cooling hood should be kept dry. The bottom of the electroslag ingot should be completely surrounded by insulation material to ensure a slow cooling effect.

[0050] Step 4: Billet preparation and rolling process:

[0051] The electroslag ingots are cold-charged into the furnace. They are first held at ≤700℃ for 1 hour, then the temperature is raised to 850℃ at a rate of 50℃ / h, and then raised to 1290℃ at a rate of ≤100℃ / h. Finally, they are held at 1290℃ for 2 hours to complete the heating and homogenization before billet making.

[0052] After heating, the billet is cut into blanks. The blanking yield is controlled at 88%, the blank size is controlled at 220 square blanks, and the length is controlled at 6.9~7m. During the blanking process, the head and tail are cut off, and the head and tail cuts are not less than 400mm. The steel billet must be marked with "H" steel stamp to correspond to the head and tail of the electroslag ingot.

[0053] After billet is opened, hot-feeding annealing is adopted, and the high-temperature steel billet is directly sent into the annealing process; if hot-feeding is not possible, the steel billet needs to be slowly cooled in the pit at 600~650℃, and annealing is carried out after at least 60 hours of slow cooling.

[0054] The annealing temperature after billet preparation is 680℃±10℃, and the holding time is 1.2 hours for the furnace load (tons).

[0055] The 220 square billet obtained from the initial billet rolling process enters the billet heating process, which is controlled in three stages: stage 1 heating ≤1120℃, stage 2 heating 1160~1200℃, and soaking stage heating 1140~1180℃, with a final exit temperature of 1140~1180℃. The total heating time is no less than 4.5 hours, and the heating time for stages 2 and 3 is no less than 3 hours.

[0056] After being hot-delivered, the steel billets are rolled into small bars to form steel bars for the torsion shaft of the target size. After rolling, they are rolled and annealed according to the process requirements.

[0057] Step 5: Annealing and Finishing Process

[0058] Annealing treatment: Continuous annealing furnace is used with an annealing temperature of 740±10℃ and a roller speed of 6m / h to ensure uniform structure and improve processing performance.

[0059] Straightening and grinding: After annealing, the steel bars are straightened by a straightening machine to ensure flatness, and then the surface is manually ground to remove surface scratches, burrs and other defects;

[0060] Automatic line flaw detection: Surface flaw detection is controlled at 0.2mm for surface defects, and internal flaw detection is carried out according to GB / T4162AA grade. Line 1 (ultrasonic + magnetic flux leakage) flaw detection is used. Each piece is supplemented with flaw detection within 100mm of the flaw detection blind zone end to ensure 100% flaw detection qualification before entering the inspection stage.

[0061] Step Six: Inspection and Sample Heat Treatment

[0062] Steel bars that pass the flaw detection undergo comprehensive inspection for chemical composition, dimensions, and surface quality. Once the inspection is passed, they are submitted for future use.

[0063] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0064] Example 2

[0065] The only difference between this embodiment and Embodiment 1 is that the chemical composition of the 45CrNiMoVA steel used for the ultra-high strength torsion shaft in this embodiment, by weight percentage, includes: C: 0.47wt%, Si: 0.29wt%, Mn: 0.75wt%, P: 0.009wt%, S: 0.002wt%, Cr: 0.99wt%, V: 0.13wt%, Mo: 0.25wt%, Ni: 1.53wt%, and Cu: 0.03wt%, with the remainder being Fe and unavoidable impurities.

[0066] The mechanical properties of the heat-treated samples in Examples 1 and 3 were tested respectively, and the results are shown in Table 1.

[0067] Table 1

[0068]

[0069] Table 1 shows that, at the same tempering temperature, increasing the content of C, Si, and Mn elements significantly increases the tensile strength and yield strength of the steel, while the reduction of area, elongation, and impact energy do not change significantly. The impact energy of all these components is ≥35J, meeting the impact energy requirements for premium-grade steel. After this heat treatment, the steel obtained in Example 2 has a yield strength ≥1600MPa, tensile strength ≥2000MPa, and impact energy ≥35J, exhibiting the best overall mechanical properties.

[0070] Example 3

[0071] The difference between this embodiment and Embodiment 1 is that the chemical composition of the 45CrNiMoVA steel used for the ultra-high strength torsion shaft in this embodiment, by weight percentage, includes: C: 0.45wt%, Si: 0.23wt%, Mn: 0.65wt%, P: 0.008wt%, S: 0.002wt%, Cr: 0.93wt%, V: 0.15wt%, Mo: 0.26wt%, Ni: 1.54wt%, and Cu: 0.03wt%, with the remainder being Fe and unavoidable impurities.

[0072] Step six of this embodiment, inspection and sample heat treatment process:

[0073] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 850℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0074] Example 4

[0075] The only difference between this embodiment and Embodiment 3 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0076] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0077] Example 5

[0078] The only difference between this embodiment and Embodiment 3 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0079] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 890℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0080] Example 6

[0081] The only difference between this embodiment and Embodiment 3 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0082] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 900℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0083] Example 7

[0084] The only difference between this embodiment and Embodiment 3 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0085] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 910℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0086] The mechanical properties of the heat-treated samples in Examples 3-7 were tested respectively, and the results are shown in Table 2.

[0087] Table 2

[0088]

[0089] Table 2 shows that, with the tempering process unchanged, the tensile strength Rm generally increases with increasing quenching temperature, reaching its highest point at quenching temperatures of 890-900℃; however, the yield strength decreases significantly, with the highest yield strength at quenching temperature of 850℃. With the tempering process unchanged, the elongation after fracture A and reduction of area Z generally show a stable fluctuation trend with increasing quenching temperature, without significant changes.

[0090] Example 8

[0091] The difference between this embodiment and Embodiment 1 is that the chemical composition of the 45CrNiMoVA steel used for the ultra-high strength torsion shaft in this embodiment, by weight percentage, includes: C: 0.47wt%, Si: 0.29wt%, Mn: 0.74wt%, P: 0.009wt%, S: 0.002wt%, Cr: 0.98wt%, V: 0.13wt%, Mo: 0.26wt%, Ni: 1.52wt%, and Cu: 0.03wt%, with the remainder being Fe and unavoidable impurities.

[0092] Step six of this embodiment, inspection and sample heat treatment process:

[0093] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 200℃, and after holding for 3 hours, it is air-cooled.

[0094] Example 9

[0095] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0096] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 230℃, and after holding for 3 hours, it is air-cooled.

[0097] Example 10

[0098] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0099] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 250℃, and after holding for 3 hours, it is air-cooled.

[0100] Example 11

[0101] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0102] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 270℃, and after holding for 3 hours, it is air-cooled.

[0103] Example 12

[0104] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0105] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 300℃, and after holding for 3 hours, it is air-cooled.

[0106] Example 13

[0107] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0108] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 350℃, and after holding for 3 hours, it is air-cooled.

[0109] Example 14

[0110] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0111] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 380℃, and after holding for 3 hours, it is air-cooled.

[0112] Example 15

[0113] The only difference between this embodiment and Embodiment 8 is that step six, inspection and sample heat treatment, is different in this embodiment.

[0114] Sample heat treatment: The qualified torsion shaft is processed into finished products / samples from steel and subjected to quenching and tempering treatment. In order to solve the tempering brittleness and achieve the optimal strength and toughness, the quenching temperature is determined to be 870℃, and after holding for 1 hour, it is oil-cooled; the tempering temperature is 420℃, and after holding for 3 hours, it is air-cooled.

[0115] The mechanical properties of the heat-treated samples in Examples 8-15 were tested respectively, and the results are shown in Table 3.

[0116] Table 3

[0117]

[0118] Table 3 shows that, with the quenching process unchanged, the tensile strength Rm generally increases and then decreases with increasing tempering temperature: the strength increases slowly in the tempering temperature range of 200~250℃, and decreases rapidly in the tempering temperature range of 270~420℃. The yield strength Rp0.2 generally increases and then decreases with increasing tempering temperature, reaching its highest value at 250℃. The strength increases slowly in the tempering temperature range of 200~350℃, and decreases rapidly in the tempering temperature range of 270~420℃.

[0119] With the quenching process unchanged, as the tempering temperature increases, the elongation after fracture (A) generally shows a stable fluctuation trend with no significant change; while the reduction of area (Z) generally shows an upward trend: the reduction of area fluctuates steadily in the tempering temperature range of 200~250℃, increases rapidly in the range of 270~350℃, and fluctuates steadily in the range of 350~420℃.

Claims

1. A type of ultra-high strength torsion shaft steel, characterized in that, The chemical composition of the steel for ultra-high strength torsion shafts, by weight percentage, includes: C: 0.42~0.46wt%, Si: 0.17~0.37wt%, Mn: 0.50~0.80wt%, P≤0.015wt%, S≤0.015wt%, Cr: 0.80~1.10wt%, V: 0.10~0.20wt%, Mo: 0.20~0.30wt%, Ni: 1.39~1.80wt%, and Cu≤0.20wt%, with the remainder being Fe and unavoidable impurities.

2. The method for preparing ultra-high strength torsion shaft steel according to claim 1, characterized in that, The process includes the following steps in sequence: electric / converter smelting → LF refining → VD vacuum degassing → continuous casting → slow cooling → shot blasting → electroslag remelting → hood cooling → bar rolling → annealing → billet heating → bar rolling → slow cooling / hot delivery → annealing → straightening → grinding → inspection → sample heat treatment. The electroslag remelting process employs full argon protection and cold arc initiation of the metal electrode, controlling the melting rate at 6~6.5 kg / min. Al powder is added for deoxidation in the last half stage of electrode ingot melting, while no Al powder is added in the first half stage. Before the billet is opened, a cold heating process is adopted. First, it is kept at ≤700℃ for 1 hour, then heated to 850℃ at a heating rate of 30~90℃ / h, then heated to 1290℃ at a heating rate of 30~150℃ / h, and finally kept at 1290℃ for 2 hours. The billet heating is controlled in three stages: stage 1 heating temperature ≤1120℃, stage 2 heating temperature 1160~1200℃, and soaking temperature 1140~1180℃. The billet exit temperature before small bar rolling is 1140~1180℃. The heat treatment of the sample adopted a quenching and tempering process. The sample was quenched at 850℃ for 1 hour and then oil-cooled, and tempered at 200℃ for 3 hours and then air-cooled.

3. The method for preparing ultra-high strength torsion shaft steel according to claim 2, characterized in that, The tapping endpoint control for the electric / converter smelting is as follows: C not less than 0.10wt%, P ≤ 0.005wt%, and tapping temperature is 1600~1640℃; Al ingots are added for pre-deoxidation during tapping, and 60Kg / t of quicklime and 30Kg / t of deoxidation and desulfurization refining slag are added to the ladle.

4. A method for preparing ultra-high strength torsion shaft steel according to claim 2 or 3, characterized in that, During the LF refining process, 20-30 kg / t of lime and 0-10 kg / t of deoxidation and desulfurization refining slag are added during a single power supply, along with a mixed deoxidizer comprising 3 kg / t of aluminum granules, 2 kg / t of silicon carbide, and 2 kg / t of carbon powder. The total refining time is no less than 55 minutes, with the white slag retention time being no less than 20 minutes.

5. The method for preparing ultra-high strength torsion shaft steel according to claim 4, characterized in that, During the VD vacuum degassing process, the vacuum degree is controlled to be ≤67Pa, and the time under this vacuum condition is not less than 20 minutes; the silicon-calcium wire is fed into the furnace at a rate of 60~80m / furnace for each initial casting furnace, and at a rate of 40~60m / furnace for each continuous casting furnace.

6. The method for preparing ultra-high strength torsion shaft steel according to claim 5, characterized in that, The slow cooling time of the electrode billet obtained by continuous casting shall not be less than 48 hours, and the starting temperature shall be ≤200℃; the bending degree of the electrode billet shall be controlled to be ≤15mm / m, and the bending degree of the whole length shall not exceed 20mm.

7. The method for preparing ultra-high strength torsion shaft steel according to claim 6, characterized in that, The electroslag remelting process uses a 480 / 520mm crystallizer with an arc ignition voltage of 60~85V, a slag-forming current of 0~5.0kA, and a refining current of 4.0~10.0kA. During a 20-minute current-raising transition period, the current is gradually increased from the slag-forming stage to 7.0±0.5kA, and then stabilized at a process current of 8.0±0.5~8.5±0.5kA during a 30-minute process current increase period. When the remaining ingot height is 300~500mm or the remaining electrode mass is 240kg, feeding is initiated: at a voltage of 54~82V, the initial feeding current of 7.0~9.0kA is gradually reduced to 4.5~6.5kA over 10~25 minutes, and finally reduced to 0kA over 15~25 minutes. The mold cooling time is 60 minutes in summer and 40 minutes in winter. After the electroslag ingot is demolded, it is placed in a cooling hood within 10 minutes, and the cooling time is not less than 72 hours.

8. The method for preparing ultra-high strength torsion shaft steel according to claim 7, characterized in that, The specific parameters for argon protection in the electroslag remelting process are as follows: the crystallizer is purged with argon for 15 minutes before slag formation, and the argon flow rate is 0.8 m³ / min. 3 / h; Argon flow rate maintained at 0.8m during slag formation period. 3 / h; the argon flow rate for the first 3 / 4 of the smelting process is 1.1m³ / h. 3 / h, the last quarter is 1.2m 3 / h.

9. The method for preparing ultra-high strength torsion shaft steel according to claim 8, characterized in that, The electroslag remelting process uses 90 kg of pre-melted slag per 2.5 to 3.3 tons of electrode ingots and 0.9 kg of silica powder per 2.5 to 3.3 tons of electrode ingots. The silica powder is added in three batches at 10, 20, and 30 minutes after arc ignition.

10. The method for preparing ultra-high strength torsion shaft steel according to claim 9, characterized in that, The annealing temperature of the large bar after billeting is 680±10℃; the small bar is annealed by hot conveying or annealing after slow cooling after rolling. The annealing after slow cooling is carried out by slow cooling in the pit at 600~650℃ for no less than 60 hours before annealing. The annealing temperature is 740±10℃. When using a continuous annealing furnace, the roll speed is 6m / h.

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