A method for preparing a high-purity low-alloy ultra-high-strength steel rod

Abstract

The application provides a preparation method of high-purity low-alloy ultrahigh-strength steel rods, and adopts LF refining+RH circulation degassing+vacuum consumable smelting double synergistic smelting process, cooperates with accurate optimization of full-process chain parameters and multi-pass forging-heat treatment synergistic process, realizes synergistic control of gas removal and inclusion floating of the molten steel through design of the molten steel circulation parameters of RH degassing and the molten pool convection parameters of vacuum consumable remelting, refines the grains and optimizes the structure, and prepares the 300M steel rod with high purity and excellent strength-toughness matching, so that the tensile strength is maintained at 1930-2070 MPa, and the fracture toughness breaks through the technical bottleneck that the purity and the mechanical properties are difficult to be considered at the same time in the traditional process.

Description

Technical Field

[0001] This invention relates to the field of steel material preparation technology, specifically a method for preparing high-purity low-alloy ultra-high-strength steel bars. Background Technology

[0002] With the rapid development of high-end manufacturing industries such as aviation, aerospace, and defense, the comprehensive performance requirements for low-alloy ultra-high strength steel are becoming increasingly stringent. Not only are ultra-high strength and good plasticity and toughness required, but also extremely high standards are set for the purity of the steel. The content of gases (O, N, H) and non-metallic inclusions in the steel directly determines its fatigue strength, stress corrosion resistance and service safety.

[0003] 300M steel, as a typical representative of low-alloy ultra-high-strength steel, contains alloying elements such as C, Si, Mn, Cr, Ni, Mo, and V. After quenching and tempering, it can form tempered martensite or a mixture of lower bainite and martensite, which has excellent strength and toughness matching. It has become a key material for high-end components such as aircraft landing gear and core structural components of military equipment. Currently, the preparation of 300M steel mainly involves two processes: dual vacuum smelting (vacuum induction + vacuum consumable) and single vacuum smelting (electric furnace + LF + VD + vacuum consumable). While the dual vacuum process can ensure high purity, it suffers from high equipment investment, long production cycle, and high manufacturing cost, making it difficult to meet the needs of large-scale industrial production. On the other hand, existing single vacuum processes, such as the "electric furnace + LF refining + VD vacuum degassing + vacuum consumable remelting" process disclosed in CN117778661A, reduce production costs, but the core VD vacuum degassing process has limitations. After gas removal, inclusions are prone to secondary gas adsorption, and the inclusions lack sufficient buoyancy, making it difficult to achieve simultaneous and efficient removal of inclusions and gas.

[0004] Meanwhile, existing RH vacuum degassing processes, such as CN116716457A, are only designed for ordinary medium-carbon structural steel, with low energy consumption and fast speed as the core objectives. They do not take into account the alloy characteristics of 300M steel to design a synergistic process of degassing and inclusion flotation. After degassing, inclusions in the molten steel can still easily become nuclei for gas precipitation, which cannot meet the stringent purity requirements of 300M steel. In addition, the existing processes are mostly operated independently, lacking parameter coordination design across the entire process chain. The parameters of each process, such as smelting, degassing, remelting, and forging, are mismatched, making it difficult to balance the purity and mechanical properties of the steel. The rating of A and B type brittle inclusions in the steel is often higher than level 1, which can easily become fatigue crack initiation points, seriously affecting the operational safety of high-end equipment.

[0005] Although existing technologies attempt to improve the purity of 300M steel by optimizing deoxidation, desulfurization, or degassing processes, they still cannot overcome the technical bottleneck of synergistic control of inclusions and gases. Therefore, developing a 300M steel bar preparation process that balances high purity, excellent mechanical properties, and low-cost large-scale production, and achieving simultaneous and efficient removal of molten steel gases and inclusions, has become the key to promoting the upgrading of low-alloy ultra-high strength steel preparation technology. Summary of the Invention

[0006] The technical problem this invention aims to solve is to overcome the shortcomings of existing low-alloy ultra-high strength steel (300M steel) preparation processes, specifically: ① Traditional double-vacuum smelting processes involve large equipment investment, long production cycles, and high manufacturing costs, making them difficult to adapt to the needs of large-scale industrial production. Furthermore, the process flow is complex, resulting in low efficiency in large-scale production. ② Existing single-vacuum processes employ a VD vacuum degassing step, which leads to secondary adsorption of residual gas by inclusions in the molten steel after degassing, making it impossible to achieve simultaneous removal of inclusions and gas, ultimately resulting in a high inclusion rating in the steel. ③ Existing ordinary RH vacuum degassing processes are only suitable for ordinary structural steel and do not take into account the alloy characteristics of 300M steel. The design incorporates a synergistic process for degassing and inclusion flotation. However, the poor circulation of molten steel during degassing results in insufficient buoyancy for inclusions, leading to low removal efficiency. Furthermore, the existing process suffers from mismatched parameters between vacuum consumable remelting and the preceding degassing process. This results in unstable convection in the molten pool during remelting, making it difficult for residual inclusions to float sufficiently. Consequently, the rating of brittle inclusions (Class A and B) is difficult to consistently control below 0.5, weakening the steel's toughness and fatigue performance. Finally, the existing process operates independently at each stage, lacking a synergistic design for inclusion control across the entire process chain. The parameters for each stage—smelting, refining, degassing, remelting, and forging—lack precise matching, making it difficult to balance steel purity and mechanical properties, resulting in significant performance fluctuations.

[0007] To address the above problems, this invention proposes a method for preparing high-purity low-alloy ultra-high-strength steel bars.

[0008] The technical solution of this invention is implemented as follows:

[0009] A method for preparing high-purity low-alloy ultra-high-strength steel bars, comprising the following steps: S1. Electric arc furnace smelting: Low-phosphorus, low-sulfur scrap steel and high-quality alloy materials are selected for electric arc furnace smelting. The steel is tapped after preliminary decarburization and impurity removal by oxygen blowing. S2.LF Refining: Lime and fluorite are added to the molten steel to create high-basicity white slag for deep desulfurization, with the refining endpoint S≤0.001%; ​​then silicon-manganese alloy is added for pre-deoxidation, followed by calcium carbide diffusion deoxidation, resulting in molten steel TO≤20ppm and Al≤0.01% after deoxidation; S3.RH circulation degassing: Under a vacuum of ≤10Pa, control the circulation flow rate of molten steel and introduce argon gas to assist in stirring and circulation degassing until the molten steel has H≤1.5ppm, N≤40ppm, and O≤15ppm; S4. Vacuum self-consumption smelting: Vacuum self-consumption remelting is carried out with the vacuum degree controlled at ≤0.1Pa until H≤1ppm, N≤25ppm, and O≤10ppm in the steel ingot; S5. Forging and Heat Treatment: S51. Heat to 1160-1200℃ before forging and hold for 2-4 hours; S52. The forging process is carried out by multiple upsetting and drawing, with an initial forging temperature of ≥1000℃ and a final forging temperature of ≥750℃. S53. Heat treatment: normalizing temperature 880-950℃, holding for 5-8 hours, air cooling to room temperature; tempering temperature 650-750℃, holding for 10-15 hours, air cooling to room temperature to obtain high purity low alloy ultra-high strength 300M steel bar.

[0010] Further, in step S1, the electric arc furnace melting temperature is controlled at 1580-1640℃, and the melting time is 35-45 min; and / or, the tapping temperature is 1580-1620℃.

[0011] Furthermore, in step S1, the oxygen blowing decarbonization is controlled to P≤0.020% and S≤0.003%.

[0012] Furthermore, in step S2, the white residue is kept for 20-30 minutes.

[0013] Furthermore, in step S3, during the cyclic degassing process, the steel liquid circulation flow rate is 80-90 t / h, the degassing time is 30-40 min; and / or, the argon flow rate is 0.7-0.9 L / min·t steel.

[0014] Furthermore, in step S3, the molten steel after circulation degassing is cast into an electrode with a diameter of 810 mm.

[0015] Further, in step S4, during the stable phase of the vacuum arc remelting process, the melting rate is controlled at 5-10 kg / min, and the droplet frequency is controlled at 6-10 Hz; and / or, the cooling water flow rate during the vacuum arc remelting process is ≥18 m / s. 3 / h.

[0016] Furthermore, in step S4, the current during the arc initiation stage of the vacuum self-consumable remelting process is 10-12kA and the voltage is 20-24V.

[0017] Furthermore, in step S4, the vacuum self-consuming remelting process enters the hot capping stage when the electrode has 1 / 4 to 1 / 3 of its total length remaining, and the parameters are gradually reduced to continuously replenish the electrode until the melting is completed.

[0018] Preferably, the control current during the thermal sealing stage is 6-8kA and the voltage is 18-22V.

[0019] Furthermore, in step S5, the forging ratio of the forging process is controlled at 10-12; and / or, three-stage upsetting-drawing forging is adopted.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: The preparation method provided by this invention employs a dual synergistic smelting process of LF refining + RH circulating degassing + vacuum consumable remelting, combined with precise optimization of all process chain parameters and a multi-pass forging-heat treatment synergistic process. By designing the steel circulation parameters for RH degassing and the molten pool convection parameters for vacuum consumable remelting, the method achieves synergistic control of gas removal from the molten steel and inclusion flotation. Simultaneously, it refines the grains and optimizes the microstructure, producing high-purity 300M steel bars with excellent strength-toughness matching, ensuring tensile strength is maintained at 1930-2070 MPa and fracture toughness ≥55 MPa·m. 0.5 This breakthrough overcomes the technical bottleneck of traditional processes that struggle to balance purity and mechanical properties. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] This invention proposes a method for preparing high-purity low-alloy ultra-high-strength steel bars (300M), comprising the following steps:

[0023] 1. Electric furnace smelting

[0024] Low-phosphorus and low-sulfur scrap steel and high-quality alloy materials are selected for electric arc furnace smelting. The smelting temperature is controlled at 1580~1640℃ and the smelting time is 35~45min. Oxygen blowing decarburization is used to achieve preliminary impurity control. The P content of the steel tapped from the electric furnace is controlled at ≤0.020% and S at ≤0.003%, and the tapping temperature is 1580~1620℃, which lays a uniform steel liquid foundation for subsequent refining and degassing processes.

[0025] 2. LF Refining

[0026] After tapping, the molten steel is transferred to the LF refining furnace for deep refining, employing a synergistic process of white slag desulfurization and composite deoxidation. Specific parameters are as follows:

[0027] 1) Slag formation and desulfurization: Add lime and fluorite to form high-alkalinity white slag. The white slag is maintained for 20-30 minutes to achieve deep desulfurization. The final refining concentration is ≤0.001%.

[0028] 2) Composite deoxidation: First, add silicon manganese alloy for pre-deoxidation, then add calcium carbide for diffusion deoxidation. After deoxidation, the TO of the molten steel is ≤20ppm, and the Al of the molten steel is controlled to ≤0.01% to avoid the formation of high-hardness aluminum nitride inclusions.

[0029] 3. RH circulation degassing

[0030] A deep degassing process using an RH circulating degassing device is employed for molten steel. By optimizing the molten steel circulation flow rate and argon stirring parameters, synergistic gas removal and inclusion flotation are achieved. Specific parameters are as follows:

[0031] 1) Vacuum level: Controlled at ≤10Pa to provide a stable vacuum environment for rapid gas escape;

[0032] 2) Steel circulation and degassing time: The steel circulation flow rate is 85t / h and the degassing time is 30~40min, so that the steel forms a stable dynamic circulation, spraying the steel into fine droplets, increasing the gas-liquid contact area, and promoting the rapid escape of gas;

[0033] 3) Argon-assisted stirring: Argon gas is introduced during the degassing process at a flow rate of 0.8 L / min·t steel. This not only accelerates the diffusion of gas in the molten steel, but also provides the power for the inclusions to float to the surface, thus preventing the inclusions from adsorbing gas again.

[0034] 4) Degassing endpoint: After degassing, the molten steel has H≤1.5ppm, N≤40ppm, and O≤15ppm, and the initial flotation removal rate of inclusions is ≥60%.

[0035] (iv) Vacuum self-consumption smelting

[0036] The molten steel after RH cycle degassing was cast into Φ810mm electrodes and subjected to vacuum arc remelting. By optimizing the remelting parameters to form a stable molten pool convection, the removal of residual gas and the full flotation of inclusions were achieved synergistically. Specific parameters:

[0037] 1) Vacuum degree: The vacuum degree of the vacuum arc furnace is controlled at ≤0.1Pa to achieve the complete escape of residual gas in the molten steel;

[0038] 2) Remelting parameters: The electrode itself is consumed and melted. During the arc initiation stage, the current is 10~12KA and the voltage is 20~24V. During the stable stage, the melting rate is 5~10Kg / min and the droplet frequency is 6~10Hz, forming a stable high-temperature molten pool. The convection of the molten pool provides sufficient power for the floating of residual inclusions.

[0039] 3) Cooling control: Water-cooled copper crucibles are used, with a cooling water flow rate ≥18m³ / h, to ensure rapid and uniform solidification of steel ingots and avoid secondary gas precipitation and inclusion agglomeration.

[0040] 4) Hot sealing: When the electrode weighs 600 kg, the hot sealing stage begins. The current is 6-8 kA and the voltage is 18-22 V. The parameters are gradually reduced and the material is continuously fed until the melting is finished. After melting, the mold is cooled for 3.5 hours before demolding.

[0041] 5) Remelting endpoint: H≤1ppm, N≤25ppm, O≤10ppm in steel ingots, and inclusions of types A, B, C, and D are all controlled to level 0.5 or below.

[0042] (v) Forging and heat treatment

[0043] The steel ingots after vacuum arc remelting undergo a multi-pass forging-heat treatment process. Precise parameter matching refines the grain size and optimizes the microstructure, ensuring a good balance of strength and toughness in the steel. Specific parameters include:

[0044] 1) Pre-forging heating: Heating temperature 1160~1200℃, holding time 2~4h, to ensure uniform heating of steel ingots;

[0045] 2) Multi-pass forging: The forging ratio is controlled at 10~12, and three-pass upsetting-drawing forging is adopted. The initial forging temperature is ≥1000℃ and the final forging temperature is ≥750℃.

[0046] 3) Heat treatment: normalizing temperature 880~950℃, holding temperature for 5~8h, air cooling to room temperature; tempering temperature 650~750℃, holding temperature for 10~15h, air cooling to room temperature, to obtain a uniform and fine tempered martensite structure, thus obtaining a high-purity low-alloy ultra-high-strength steel bar.

[0047] The above preparation method has the following technical effects and advantages:

[0048] By employing a dual synergistic process of "LF deoxidation + RH degassing and flotation + vacuum consumable remelting for impurity removal", the simultaneous and efficient removal of gases and inclusions is achieved. The H content in the steel ingot is ≤1ppm, N content is ≤25ppm, and O content is ≤10ppm. Compared with the traditional single vacuum process, the gas content is reduced by ≥50%. The inclusions of types A, B, C, and D are all stably controlled at level 0.5 and below, effectively avoiding defects such as porosity and inclusion aggregation, and eliminating fatigue crack initiation points from the root.

[0049] Excellent mechanical properties and high stability: The prepared 300M steel bar possesses both ultra-high strength and good plasticity and toughness. The core mechanical properties are: tensile strength 1930-2070MPa, fracture toughness... Furthermore, its mechanical properties fluctuate within a range of ≤±3%, fully meeting the stringent requirements for materials in high-end fields such as aviation, aerospace, and national defense.

[0050] Low production cost and suitable for large-scale production: Using an electric furnace as the initial smelting equipment eliminates the vacuum induction melting step of the traditional double vacuum process, reducing the cost per ton of steel by 10,000 to 20,000 yuan compared to the double vacuum process, and is comparable to the cost of the traditional single vacuum process; at the same time, the parameters of the entire process chain are stable, the process is suitable for industrial continuous production, and the process qualification rate is increased to over 98%, solving the problem that the double vacuum process is difficult to scale up.

[0051] Service safety is greatly improved: There are no large and brittle inclusions in the steel, the gas content is controlled at an extremely low level, the risk of fatigue crack initiation is greatly reduced, the problem of component fracture caused by insufficient steel purity is effectively solved, the service life of steel used in high-end equipment is increased by more than 20%, and the service safety is significantly improved.

[0052] High process adaptability and high promotion value: The process design concept of this invention is based on the core requirement of gas-inclusion synergistic control of low alloy ultra-high strength steel. By fine-tuning the deoxidation, degassing and remelting parameters, it can be adapted to the preparation of other low alloy ultra-high strength steels such as 4340 and 4130. It provides a new technical path for the low-cost and high-purity preparation of high-end low alloy ultra-high strength steel and has broad industrial promotion value.

[0053] The design and principles related to the above preparation method are as follows:

[0054] 1) Synergistic process design of RH degassing and inclusion flotation: Innovative optimization of molten steel circulation flow rate and argon stirring parameters in RH circulating degassing, so that argon stirring not only accelerates gas diffusion and escape, but also provides continuous power for inclusion flotation, realizing the synergy of gas removal and initial inclusion flotation, and solving the technical problem of secondary gas adsorption by inclusions in traditional degassing processes.

[0055] 2) Parameter matching process between vacuum arc remelting and preceding degassing: Based on the state of the molten steel after RH degassing, the melting rate, droplet frequency and cooling parameters of vacuum arc remelting are optimized to form a stable high-temperature molten pool convection, which provides sufficient power for the floating of residual inclusions, and realizes the secondary synergy of residual gas removal and full floating of inclusions, thus breaking through the bottleneck of mismatch between the preceding degassing and subsequent remelting process parameters.

[0056] 3) Precise control process for deep deoxidation in LF refining: Design composite deoxidation parameters that match the alloy properties of 300M steel. Through the synergy of silicon-manganese alloy pre-deoxidation and calcium carbide diffusion deoxidation, control the TO content of molten steel to ≤20ppm and Al to ≤0.01%, avoid the formation of high-hardness aluminum nitride inclusions, lay the foundation for the flotation of inclusions during subsequent degassing and remelting processes, and solve the problem of incomplete deoxidation in traditional deoxidation processes.

[0057] 4) Precise and coordinated control system for all process chain parameters: The parameters of each link, including electric furnace smelting, LF refining, RH circulating degassing, vacuum self-consuming remelting, forging, and heat treatment, are precisely matched and coordinated. Each process is not operated independently, realizing closed-loop control of "impurity control-deoxidation-degassing-impurity removal-toughening", taking into account the high purity and excellent mechanical properties of steel, and solving the performance fluctuation problem caused by the disconnection of each link in the traditional process.

[0058] Compared with CN117778661A: This invention replaces VD vacuum degassing with RH circulating degassing. By optimizing the steel liquid circulation and argon stirring parameters of RH degassing, it achieves synergy between gas removal and inclusion flotation, avoiding the problem of secondary gas adsorption by inclusions after VD degassing. At the same time, it optimizes the LF refining deoxidation parameters, and precisely matches the vacuum self-consumable remelting with the preceding degassing process parameters. The inclusion rating is stably controlled at 0.5 level or below, and the synergistic design of the entire process chain improves the stability of steel performance.

[0059] Compared with CN116716457A: This invention targets the alloy characteristics of 300M steel and adds a dynamic design for inclusion flotation in the RH degassing process, rather than solely aiming at low-energy, fast-paced degassing; at the same time, it combines deep deoxidation of LF refining and deep impurity removal of vacuum consumable remelting to form a "deoxidation-degassing-floating" full-process chain inclusion and gas synergistic control system, rather than only focusing on the gas removal efficiency of ordinary structural steel.

[0060] Example 1

[0061] 1) Electric arc furnace smelting: Low-phosphorus and low-sulfur scrap steel and high-quality alloy materials are selected for electric arc furnace smelting. The smelting temperature is 1610℃ and the smelting time is 40min. Initial impurity control is achieved by oxygen blowing for decarburization. The steel tapped from the electric arc furnace has P=0.015% and S=0.002% at a tapping temperature of 1600℃, which lays a uniform steel liquid foundation for subsequent refining and degassing processes.

[0062] 2) LF Refining: After tapping, the molten steel is transferred to the LF refining furnace for deep refining, using a synergistic process of white slag desulfurization and composite deoxidation. Slag desulfurization: 8.5 kg / ton of lime and 0.6 kg / ton of fluorite are added to create high-alkalinity white slag, which is maintained for 25 minutes to achieve deep desulfurization. The refining endpoint S=0.0009%. Composite deoxidation: 1.1 kg / ton of silicon manganese alloy is added for pre-deoxidation, followed by 0.38 kg / ton of calcium carbide for diffusion deoxidation. After deoxidation, the TO of the molten steel is 17 ppm, and the Al of the molten steel is controlled at 0.008% to avoid the formation of high-hardness aluminum nitride inclusions.

[0063] 3) RH circulation degassing: The RH circulation degassing device is used to perform deep degassing of the molten steel at a vacuum of 8 Pa; the circulation flow rate of the molten steel is 85 t / h, and the degassing time is 35 min, so that the molten steel forms a stable dynamic circulation, spraying the molten steel into fine droplets, increasing the gas-liquid contact area, and promoting the rapid escape of gas; Argon gas is introduced during the degassing process at a flow rate of 0.8 L / min·t steel, which not only accelerates the diffusion of gas in the molten steel, but also provides power for the inclusions to float to the surface, avoiding the inclusions from adsorbing gas again; Degassing endpoint: After degassing, the molten steel has H=1.3 ppm, N=35 ppm, and O=12 ppm.

[0064] 4) Vacuum self-consumption smelting: The molten steel after RH cycle degassing is cast into Φ810mm electrodes for vacuum self-consumption remelting; the vacuum degree of the vacuum self-consumption furnace is controlled at 0.08Pa to ensure the complete escape of residual gases in the molten steel; remelting parameters: arc initiation stage current 10~11kA, voltage 21~22V, stabilization stage melting rate 7.5Kg / min, droplet frequency 8Hz, forming a stable high-temperature molten pool, utilizing the convection of the molten pool to provide sufficient power for the floating of residual inclusions; cooling control: a water-cooled copper crucible is used, with a cooling water flow rate of 20m / min. 3 / h, to ensure rapid and uniform solidification of the steel ingot, avoiding secondary gas precipitation and inclusion agglomeration; hot capping: when the electrode has 600kg remaining, the hot capping stage begins, with a current of 6~7kA and a voltage of 19~20V, gradually reducing the parameters and continuously feeding until the melting ends, and demolding after 3.5h of mold cooling after melting; remelting endpoint: H=0.7ppm, N=20ppm, O=8ppm in the steel ingot, and inclusions of types A, B, C, and D are all grade 0.5.

[0065] 5) Forging and heat treatment: The steel ingots after vacuum consumable remelting are subjected to a multi-pass forging-heat treatment synergistic process; Pre-forging heating: heating temperature 1180℃, holding time 3h, to ensure uniform heating of the steel ingot; Multi-pass forging: the forging ratio is controlled at 1:1, and three-pass upsetting-drawing forging is adopted, with an initial forging temperature of 1050℃ and a final forging temperature of 800℃; Heat treatment: normalizing temperature 915℃, holding time 6.5h, air cooling to room temperature; tempering temperature 700℃, holding time 12.5h, air cooling to room temperature, to obtain a uniform and fine tempered martensite structure.

[0066] Example 2

[0067] 1) Electric arc furnace smelting: Low-phosphorus and low-sulfur scrap steel and high-quality alloy materials are selected for electric arc furnace smelting. The smelting temperature is 1580℃ and the smelting time is 35min. Initial impurity control is achieved by oxygen blowing for decarburization. The P=0.018% and S=0.0028% of the steel produced by the electric arc furnace, and the tapping temperature is 1580℃.

[0068] 2) LF Refining: After tapping, the molten steel is transferred to the LF refining furnace for deep refining. Slag formation and desulfurization: 8.0 kg / ton of lime and 0.5 kg / ton of fluorite are added to create high-alkalinity white slag. The white slag is maintained for 20 minutes, and the refining endpoint S=0.0010%; Composite deoxidation: 1.0 kg / ton of silicon-manganese alloy is added for pre-deoxidation, and then 0.35 kg / ton of calcium carbide is added for diffusion deoxidation. After deoxidation, the TO of the molten steel is 19 ppm, and the Al of the molten steel is controlled at 0.009%.

[0069] 3) RH circulation degassing: vacuum degree 5Pa; steel liquid circulation flow rate 85t / h, degassing time 30min; argon flow rate 0.8L / min·t steel; degassing endpoint: H=1.1ppm, N=32ppm, O=11ppm.

[0070] 4) Vacuum consumable metallurgy: Φ810mm electrode; vacuum degree 0.05Pa; arc initiation stage current 9~10KA, voltage 19~20V; stabilization stage melting rate 5kg / min, droplet frequency 6Hz; cooling water flow rate 18m / min 3 / h; heat sealing current 5~6KA, voltage 17~18V; remelting endpoint: H=0.6ppm, N=18ppm, O=7ppm, inclusions of types A, B, C, and D are all grade 0.5.

[0071] 5) Forging and heat treatment: Pre-forging heating temperature 1160℃, holding time 2h; forging ratio 10, initial forging temperature 1000℃, final forging temperature 750℃; normalizing temperature 880℃, holding time 5h; tempering temperature 650℃, holding time 10h, air cooling to room temperature.

[0072] Example 3

[0073] 1) Electric arc furnace smelting: Low-phosphorus and low-sulfur scrap steel and high-quality alloy materials are selected for electric arc furnace smelting. The smelting temperature is 1640℃ and the smelting time is 45min. Initial impurity control is achieved by oxygen blowing for decarburization. The steel produced by electric arc furnace has P=0.012% and S=0.0015%, and the steel is produced at a temperature of 1620℃.

[0074] 2) LF refining: Slag formation and desulfurization: Add 9.0 kg / ton of lime and 0.7 kg / ton of fluorite to form high-alkalinity white slag. The white slag is maintained for 30 min. The refining endpoint S=0.0008%; Composite deoxidation: First, add 1.2 kg / ton of silicon-manganese alloy for pre-deoxidation, then add 0.40 kg / ton of calcium carbide for diffusion deoxidation. After deoxidation, the TO of the molten steel is 15 ppm, and the Al of the molten steel is controlled at 0.007%.

[0075] 3) RH circulation degassing: vacuum degree 10Pa; steel liquid circulation flow rate 85t / h, degassing time 40min; argon flow rate 0.8L / min·t steel; degassing endpoint: H=1.4ppm, N=38ppm, O=14ppm.

[0076] 4) Vacuum consumable metallurgy: Φ810mm electrode; vacuum degree 0.1Pa; arc initiation stage current 11~12kA, voltage 23~24V; stabilization stage melting rate 10kg / min, droplet frequency 10Hz; cooling water flow rate 22m / min 3 / h; heat sealing current 7~8kA, voltage 21~22V; remelting endpoint: H=0.8ppm, N=22ppm, O=9ppm, inclusions of types A, B, C, and D are all grade 0.5.

[0077] 5) Forging and heat treatment: Pre-forging heating temperature 1200℃, holding time 4h; forging ratio 12, initial forging temperature 1100℃, final forging temperature 850℃; normalizing temperature 950℃, holding time 8h; tempering temperature 750℃, holding time 15h, air cooling to room temperature.

[0078] Comparative Example 1

[0079] The difference from Example 1 is that the argon flow rate in the RH cycle degassing is reduced to 0.2 L / min·t steel (only to meet the basic stirring of the molten steel, without additional floating force of inclusions), while the other process parameters are completely consistent with Example 1.

[0080] Comparative Example 2

[0081] The difference from Example 1 is that the melting rate during the stabilization stage of vacuum self-consuming remelting is increased to 12 kg / min (which exceeds the given parameter range, causing turbulent convection in the molten pool and making it impossible for residual inclusions to float fully). The other process parameters are completely consistent with those of Example 1.

[0082] Comparative Example 3

[0083] The difference from Example 1 is that LF refining only uses aluminum particles of 0.8 kg / ton steel for single deoxidation (without using the synergistic process of silicon manganese alloy pre-deoxidation + calcium carbide diffusion deoxidation). After deoxidation, the molten steel has TO=32ppm and Al=0.018% (exceeding the given parameter range), while the other process parameters are completely consistent with those of Example 1.

[0084] Comparative Example 4

[0085] The difference from Example 1 is that VD vacuum degassing is used instead of RH cycle degassing (VD vacuum degree 67Pa, degassing time 25min, argon blowing flow rate 0.5L / min·t steel, and the synergistic parameters of RH cycle degassing of the present invention are not used), while the other process parameters are completely consistent with those of Example 1.

[0086] Performance Testing and Comparison

[0087] The gas content, inclusion rating, and mechanical properties of the 300M steel bars prepared in the above three examples and four comparative examples were tested. The production cost per ton of steel was also calculated. The testing methods are as follows:

[0088] 1. Gas content (H, N, O): Detected using an oxygen, nitrogen, and hydrogen analyzer, in accordance with GB / T 20124 standard;

[0089] 2. Inclusion rating: Observed using a metallographic microscope, in accordance with GB / T 10561 standard;

[0090] 3. Mechanical properties: Tensile testing shall be performed in accordance with GB / T 228.1 standard, and fracture toughness testing shall be performed in accordance with GB / T 4161 standard;

[0091] 4. Cost per ton of steel: This includes the comprehensive cost of equipment, raw materials, energy consumption, etc., at each stage of the process.

[0092] The specific test results are shown in Table 1.

[0093] Inspection items Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Nitrogen content (ppm) 20 18 22 30 25 28 22 O content (ppm) 8 7 9 16 14 25 18 H content (ppm) 0.7 0.6 0.8 1.2 1.0 1.1 1.0 Class A inclusions (grade) 0.5 0.5 0.5 1.0 1.0 1.5 1.0 Class B inclusions (grade) 0 0 0 1.5 0.5 2.0 0 Class C inclusions (grade) 0.5 0.5 0.5 0.5 0.5 1.0 0.5 Class D inclusions (grade) 0.5 0.5 0.5 0.5 1.0 1.5 1.0 Tensile strength (MPa) 1992 1985 1995 1973 1908 1951 1952 <![CDATA[Fracture toughness (Mpa·m 0.5 ).]]> 79 78 80 68 65 58 70 Cost per ton of steel (ten thousand yuan / ton) 3.5 3.4 3.6 3.5 3.7 3.4 3.5

[0094] As shown in Table 1 above, the steel bars prepared in the three examples all meet the design requirements in terms of performance. The gas content, inclusion rating, and mechanical properties are all superior to those of the four comparative examples. The specific analysis is as follows:

[0095] (1) The test results of the three examples 1-3 show that the H in the steel ingot is ≤1ppm, N is ≤25ppm, O is ≤10ppm, and inclusions of types A, B, C, and D are all stably controlled at grade 0.5 and below. The tensile strength is 1985-1995MPa and the fracture toughness is 78-80MPa·m. 0.5 The cost per ton of steel is 34,000-36,000 yuan, which proves that the process parameters of this invention are reasonable and stable, and can achieve high purity, low cost and large-scale production.

[0096] (2) Comparative Example 1: Since there is no floating force for inclusions during RH degassing, the inclusions cannot float effectively, resulting in excessive gas content and inclusion rating, and decreased mechanical properties. This proves that the synergistic process of RH degassing and inclusion flotation is the key innovation point for achieving high purity.

[0097] (3) Comparative Example 2: Due to the mismatch between vacuum self-consumption remelting and the preceding degassing parameters, the convection of the molten pool is disordered, the residual inclusions cannot float up sufficiently, and the performance indicators decrease. This proves that the process of matching the parameters of vacuum self-consumption remelting and the preceding degassing can effectively improve the purity and mechanical properties of steel.

[0098] (4) Comparative Example 3: Due to incomplete deoxidation in LF refining, the oxygen and Al content in the molten steel exceeded the standard, generating a large amount of aluminum nitride inclusions, which led to a significant decrease in performance. This proves that the precise control process of deep deoxidation in LF refining is the foundation for ensuring the quality of steel.

[0099] (5) Comparative Example 4: Replacing RH degassing with VD degassing cannot achieve simultaneous removal of gas and inclusions. Its performance is better than other comparative examples but worse than the example, proving that RH cycle degassing has significant advantages over traditional VD degassing.

[0100] In summary, this invention achieves simultaneous and efficient removal of gas and inclusions from 300M steel bars through a dual synergistic process of "LF deoxidation + RH degassing and flotation + vacuum consumable remelting for impurity removal" and precise matching of parameters across the entire process chain. While ensuring high purity and excellent mechanical properties of the steel, it reduces production costs and is suitable for large-scale industrial production, demonstrating significant economic benefits and technological advantages.

[0101] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-purity low-alloy ultra-high-strength steel bars, characterized in that the steps include... include: S1. Electric arc furnace smelting: Low-phosphorus, low-sulfur scrap steel and high-quality alloy materials are selected for electric arc furnace smelting. The steel is tapped after preliminary decarburization and impurity removal by oxygen blowing. S2.LF Refining: Lime and fluorite are added to the molten steel to create high-basicity white slag for deep desulfurization, with the refining endpoint S≤0.001%; ​​then silicon-manganese alloy is added for pre-deoxidation, followed by calcium carbide diffusion deoxidation, resulting in molten steel TO≤20ppm and Al≤0.01% after deoxidation; S3.RH circulation degassing: Under a vacuum of ≤10Pa, control the circulation flow rate of molten steel and introduce argon gas to assist in stirring and circulation degassing until the molten steel has H≤1.5ppm, N≤40ppm, and O≤15ppm; S4. Vacuum self-consumption smelting: Vacuum self-consumption remelting is carried out with the vacuum degree controlled at ≤0.1Pa until H≤1ppm, N≤25ppm, and O≤10ppm in the steel ingot; S5. Forging and Heat Treatment: S51. Heat to 1160-1200℃ before forging and hold for 2-4 hours; S52. The forging process is carried out by multiple upsetting and drawing, with an initial forging temperature of ≥1000℃ and a final forging temperature of ≥750℃. S53. Heat treatment: normalizing temperature 880-950℃, holding for 5-8 hours, air cooling to room temperature; tempering temperature 650-750℃, holding for 10-15 hours, air cooling to room temperature to obtain high purity low alloy ultra-high strength 300M steel bar.

2. The manufacturing method as described in claim 1, characterized in that, In step S1, the electric arc furnace melting temperature is controlled at 1580-1640℃, and the melting time is 35-45 min; and / or, the tapping temperature is 1580-1620℃.

3. The manufacturing method as described in claim 1, characterized in that, In step S1, the oxygen blowing decarbonization is controlled to P≤0.020% and S≤0.003%.

4. The manufacturing method as described in claim 1, characterized in that, In step S2, the white residue is kept for 20-30 minutes.

5. The manufacturing method as described in claim 1, characterized in that, In step S3, during the cyclic degassing process, the steel molten flow rate is 80-90 t / h, the degassing time is 30-40 min; and / or, the argon flow rate is 0.7-0.9 L / min·t steel.

6. The preparation method according to claim 1, characterized in that, In step S4, during the stable phase of the vacuum arc remelting process, the melting rate is controlled at 5-10 kg / min, and the droplet frequency is controlled at 6-10 Hz; and / or, the cooling water flow rate during the vacuum arc remelting process is ≥18 m / s. 3 / h.

7. The preparation method according to claim 1, characterized in that, In step S4, the current during the arc initiation stage of the vacuum self-consuming remelting process is 10-12kA and the voltage is 20-24V.

8. The manufacturing method as described in claim 1 or 7, characterized in that, In step S4, the vacuum self-consuming remelting process enters the hot capping stage when the electrode has 1 / 4 to 1 / 3 of its total length remaining, and the parameters are gradually reduced to continuously replenish the electrode until the melting is completed.

9. The preparation method according to claim 8, characterized in that, The hot sealing stage controls the current at 6-8kA and the voltage at 18-22V.

10. The preparation method according to claim 1, characterized in that, In step S5, the forging ratio is controlled at 10-12 during the forging process; and / or, three-stage upsetting-drawing forging is adopted.

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