A method for short-process preparation of large ingots with high cleanliness

Abstract

This invention provides a short-process method for preparing large, high-purity ingots, belonging to the field of iron and steel smelting technology. The method involves feeding pretreated raw materials into an electric arc furnace for slag formation and refining to obtain primary molten steel; then transferring the primary molten steel to a hydrogen plasma refining furnace for hydrogen plasma refining; finally, transferring the refined molten steel to a vacuum casting device for vacuum casting to obtain large, high-purity ingots. This invention utilizes a three-stage, separate, short-process flow—electric arc furnace primary refining, hydrogen plasma refining, and vacuum casting—and leverages the synergistic effect of deep depurification by hydrogen plasma and simultaneous dehydrogenation during vacuum casting to achieve ultra-high purity (O≤10ppm, N≤30ppm, S≤15ppm, P≤20ppm, H≤3ppm) for 5t-10t large forging ingots, solving the industry problem of the difficulty in stable, large-scale production of hydrogen plasma processes.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and specifically relates to a short-process method for preparing large ingots with high cleanliness. Background Technology

[0002] With the rapid development of high-end equipment manufacturing, nuclear power engineering, aerospace, heavy machinery, and other fields, the transformation of base materials used in forging and plate production towards larger sizes has become an inevitable trend in the industry. Key components such as nuclear power pressure vessels, aero-engine rotors, large marine crankshafts, and heavy-duty press columns often use large ingots weighing 5 tons or even hundreds of tons as blanks, which are then forged and heat-treated. The cleanliness of the ingot directly determines the service life, operational safety, and long-term stability of the final equipment: oxygen in steel exists as oxide inclusions, which is the initiation source of fatigue cracks; nitrogen hardens as nitride precipitation and reduces toughness; sulfur forms MnS inclusions, leading to anisotropy and hot brittleness; phosphorus causes grain boundary segregation and cold brittleness sensitivity; and hydrogen is the root cause of white spots (cracking). Large ingots, due to slow cooling and long hydrogen diffusion paths, have particularly stringent requirements for hydrogen content. Therefore, related industries have set extremely high thresholds for the purity and overall mechanical properties of large ingots. High-cleanliness large ingots with ultra-low impurities, gas content, and no internal defects have become indispensable core raw materials.

[0003] The current mainstream preparation route for large ingots is electric arc furnace (EAF) (or converter), LF furnace slag refining, vacuum degassing furnace (VD) / vacuum oxygen blowing decarburization furnace (VOD) for vacuum degassing and casting. The LF furnace is responsible for slag formation, desulfurization, composition fine-tuning, and heat preservation, while VD / VOD is responsible for dehydrogenation, denitrification, and deoxidation. This route has the following inherent limitations: First, the LF furnace refining process requires the addition of large amounts of slag-forming materials such as lime, fluorite, and bauxite to form a high-alkalinity reducing slag, and the introduction of strong deoxidizers such as ferrosilicon, aluminum, or silicon-calcium alloys for precipitation deoxidation: 2[Al] + 3[O] = Al2O3, [Si] + 2[O] = SiO2, with the deoxidation products floating to the slag layer and being absorbed. For example, CN202011328483.X discloses a method for improving the cleanliness of molten steel through slag modification and magnesium and calcium treatment, and CN202510372148.6 discloses a smelting method for improving the cleanliness of austenitic stainless steel by using multiple reduction of FeSi and diffusion deoxidation with silicon-calcium powder. However, such methods heavily rely on top slag, resulting in long refining times and high costs for slag formation and deoxidation; in addition, the deoxidation products (especially fine and dispersed Al2O3) are difficult to float completely, forming non-metallic inclusions in the remaining steel. Even after modification treatment, these inclusions remain in the molten steel, making it impossible to achieve deep removal of impurities such as oxygen, nitrogen, and sulfur, and thus failing to meet the requirements of high-end large forgings for ultra-low O, N, and S content.

[0004] Second, even with the introduction of vacuum degassing (VD / VOD), the depth and efficiency of dehydrogenation remain bottlenecks in large ingot casting scenarios due to factors such as the limited exposed area of ​​molten steel in the vacuum chamber and the uneven bubble pumping effect. Furthermore, the separate steps of VD and casting result in a longer smelting cycle, a large temperature drop, and high energy consumption, which does not align with the industrial direction of short processes and low energy consumption.

[0005] With the development of hydrogen metallurgical steelmaking technology, hydrogen plasma smelting process, centered on high-temperature, highly chemically active hydrogen plasma, boasts significant advantages such as low carbon emissions, zero pollution, and highly efficient deep purification of molten steel, making it suitable for the green and low-carbon transformation and development of the steel industry. The active H· radicals in hydrogen plasma can react with impurities such as O, N, S, C, and P in molten steel in a gas-phase reaction to generate easily removable gaseous products such as H2O, NH3, H2S, and CH4. Theoretically, simultaneous deep removal of O, N, S, C, and P can be achieved, far superior to traditional LF slag-forming refining.

[0006] However, existing hydrogen plasma smelting processes still have significant shortcomings. For example, in the steel refining method disclosed in CN117500946A (corresponding to WO2022270226A1), the hydrogen plasma gun is embedded in an LF furnace, VOD / VAD furnace, or REDA vacuum degassing device, using hydrogen or hydrogen-containing gas as a gas source to generate hydrogen plasma for refining the molten steel. However, this scheme requires modification of the furnace cover structure to adapt to the installation and operation of the embedded plasma gun body, resulting in a large amount of equipment modification work and high implementation costs. Furthermore, the plasma gun and the original smelting electrodes are independent of each other, resulting in low system integration, which is not conducive to low-cost, large-scale promotion and application on existing steelmaking production lines. In the efficient nitrogen control method for steelmaking based on hydrogen-containing plasma blowing disclosed in patent CN116356119B, hollow graphite electrodes are used to generate hydrogen plasma blowing for refining. However, this method focuses on controlling the nitrogen content of molten steel and does not involve the synergistic removal of other impurity elements.

[0007] In summary, the following technical challenges still exist in this field: (1) The traditional LF+VD+casting refining route is difficult to achieve deep removal of impurities such as O, N, and S, which cannot meet the high cleanliness requirements of large ingots, and the process is lengthy and energy consumption is high; (2) Hydrogen plasma has the advantage of slag-free / low-slag refining, but the existing scheme has not formed a refining station configuration suitable for large ingots, making it difficult to achieve stable large-scale production; (3) The hydrogen plasma refining process inevitably leads to hydrogen increase in molten steel, and the existing scheme relies on depressurization to dehydrogenate, which is inefficient and has poor effect.

[0008] Therefore, there is a current need for a method that uses hydrogen plasma refining as the core, has a short process, is pollution-free, and can stably produce large-scale ingots with high purity in batches. Summary of the Invention

[0009] The purpose of this invention is to provide a method for the stable batch production of high-purity large ingots using hydrogen plasma as the core, which is short-process, pollution-free, and specifically addresses the following three problems: (1) Overcome the shortcomings of the traditional LF+VD route in that the inclusions are only modified but not removed, and achieve complete gas-phase removal of O, N, S, C and P; (2) Overcome the shortcomings of existing hydrogen plasma refining schemes, form a dedicated hydrogen plasma refining station for large casting ingots, achieve deep removal of impurity elements in molten steel, and meet the high cleanliness requirements of large casting ingots. (3) Through the coupling of parameters of the whole process of raw material pretreatment, electric arc furnace slag making primary refining, hydrogen plasma deep refining and vacuum casting, the ingots can achieve an ultra-high cleanliness level of O≤10ppm, N≤30ppm, S≤15ppm, P≤20ppm and H≤3ppm.

[0010] To address the aforementioned technical problems, this invention provides a short-process method for preparing large, high-purity ingots, comprising electric arc furnace melting, refining, and casting steps; employing a three-stage, separate short-process technology of electric arc furnace primary refining, hydrogen plasma refining, and vacuum casting, with the specific steps as follows: S1 Raw material pretreatment: Grinding to remove the oxide layer, oil and rust from the surface of the metal raw material, and high-temperature drying to remove moisture; S2 Electric Arc Furnace Primary Refining: The pretreated raw materials are fed into the electric arc furnace, and the power supply is 100kW / t-600kW / t of molten steel. After the temperature is raised to 1580℃-1620℃, a slag-forming agent is added. The power is kept constant for 10min-40min to carry out slag refining. Then the slag is discharged to obtain primary molten steel. S3 Hydrogen Plasma Refining: The primary molten steel is transferred to the hydrogen plasma refining furnace, and argon gas is blown from the bottom of the furnace to make the molten steel in a soft blowing state. Using hollow graphite electrodes as working electrodes, a hydrogen-argon mixture is introduced through the hollow channel. The hydrogen-argon mixture dissociates into hydrogen-containing plasma under the high temperature of the electric arc, which reacts with O, N, S, C and P impurities in the molten steel to generate gaseous products that are discharged with the flue gas. After refining for 10-60 minutes, the molten steel is sampled and analyzed when the temperature is >1600℃. Alloys are added to fine-tune the composition according to the target composition until the molten steel composition is qualified and the temperature is >1630℃. S4 Vacuum casting: The refined molten steel is transferred into a vacuum casting equipment, and the vacuum degree is evacuated to <67Pa. It is then cast at a uniform speed of 5kg / s-15kg / s. After casting, the vacuum pressure is maintained for 2-3 hours. After the furnace is slowly cooled to room temperature, the steel is demolded to obtain a large ingot with high cleanliness.

[0011] Further, the slag-forming agent in step S2 has the following composition by mass percentage: CaO 40%-60%, SiO2 5%-15%, Al2O3 15%-30%, MgO 5%-15%; the amount of slag-forming agent added is 1%-10% of the total mass of the smelting raw materials.

[0012] Furthermore, in step S3, the hollow inner diameter of the hollow graphite electrode is 5%-15% of the outer diameter of the electrode, the electrode power supply is 100kW / t-500kW / t molten steel, the hydrogen content in the hydrogen-argon mixture is >10 vol.%, the blowing flow rate is 10NL / (min•t)-100NL / (min·t), and the blowing pressure is 0.5MPa-1MPa.

[0013] Furthermore, in step S3, the bottom-blowing argon flow rate is 1NL / (min•t)-10NL / (min·t), the argon blowing pressure is 0.6MPa-1MPa, and argon gas is introduced throughout the process.

[0014] Furthermore, in step S3, the hydrogen plasma refining time is 10 min to 40 min; in step S4, the casting speed is 5 kg / s to 15 kg / s, and the vacuum holding time is 2 h to 3 h.

[0015] Furthermore, the metal raw material mentioned in step S1 is one or more of industrial pure iron, alloy feedstock, and ingot return material, and the ingot corresponding to the raw material with a single weight ≥ 5t is an ingot for large forgings.

[0016] Furthermore, in step S3, the surface of the molten steel in the hydrogen plasma refining furnace is filled with a reducing atmosphere formed by hydrogen gas, which inhibits the oxidation of the molten steel.

[0017] Furthermore, the resulting large ingots contain oxygen content ≤10ppm, nitrogen content ≤30ppm, sulfur content ≤15ppm, phosphorus content ≤20ppm, and hydrogen content ≤3ppm.

[0018] The method provided by this invention produces high-purity large forging ingots with a single weight of 5t-10t. The ingots contain O≤10ppm, N≤30ppm, S≤15ppm, P≤20ppm, and H≤3ppm. The inclusions are spheroidized modified residues with no macroscopic segregation.

[0019] The present invention provides a short-process method for preparing large, high-purity ingots, which has the following beneficial effects: (1) Pre-treatment of raw materials can control impurities. Then, primary refining in an electric arc furnace can remove most of the S, P and macroscopic inclusions through slag formation. Next, hydrogen plasma refining can completely remove O, N, S, C and P in the gas phase (non-modified residues). Finally, vacuum casting can simultaneously remove dissolved hydrogen. The above process steps can solve the problem of traditional LF process only modifying but not removing impurities.

[0020] (2) The hydrogen plasma refining furnace used in the hydrogen plasma refining of the present invention emits hydrogen plasma with hollow graphite electrodes, which can achieve pollution-free depurification of molten steel and is easy to apply quickly on existing steelmaking production lines.

[0021] (3) This invention integrates the dehydrogenation process into the vacuum casting process (<67Pa≈0.5Torr, far below 10Torr), eliminating the VD / VOD dehydrogenation step in the traditional process, and shortening the smelting process to three stages: EAF melting, hydrogen plasma refining, and vacuum casting. Compared with the traditional process, the smelting cycle is shortened by about 15%-25%, while reducing the temperature drop of molten steel and energy consumption caused by process transitions, which can significantly improve production efficiency.

[0022] (4) The 5t-grade ingot prepared by the present invention contains O=7-9ppm, N=21-26ppm, S=11-14ppm, P=14-19ppm, and H=1.9-2.3ppm, and H≤3ppm does not require a separate dehydrogenation step, which illustrates the synergistic effect of hydrogen plasma deep depurification and vacuum casting synchronous dehydrogenation in the present invention.

[0023] (5) Applicable to mass production of large ingots: This invention can stably prepare forging ingots with a single weight of 5t-10t. Returned materials can be incorporated into raw materials, reducing raw material costs and improving economic benefits. Attached Figure Description

[0024] Figure 1 This is a flowchart of a short-process method for preparing large, high-purity ingots, provided as an embodiment of the present invention. Detailed Implementation

[0025] See Figure 1 The present invention provides a method for preparing large, high-purity ingots using a short-process method, comprising the following steps: S1 Raw material pretreatment: The metal raw materials are selected from one or more of the following: industrial pure iron, alloy materials, and ingot return materials; grinding is used to remove the surface oxide layer, oil stains, rust and other impurities, and high-temperature drying is used to remove the moisture from the raw materials, thus preventing the introduction of impurities from the source.

[0026] S2 Electric Arc Furnace Primary Refining: After pretreatment, the raw materials are fed into the electric arc furnace with a power supply of 100kW / t-600kW / t of molten steel. The furnace is powered to heat up and melt the steel. When the temperature of the molten steel reaches 1580℃-1620℃, a slag-forming agent is added. The slag-forming agent is composed of CaO 40%-60%, SiO 25%-15%, Al2O3 15%-30%, and MgO 5%-15% by mass, and the amount added is 1%-10% of the total mass of the raw materials. The power supply is kept constant, and slag refining is carried out for 10min-40min to remove some S, P and initial inclusions. After that, the slag is completely discharged to obtain primary molten steel with stable composition and low impurity content.

[0027] S3 Hydrogen Plasma Refining: Molten steel from the primary refining process is rapidly transferred to the hydrogen plasma refining furnace. A bottom-blowing element is installed at the furnace bottom to continuously introduce argon gas at a flow rate of 1 NL / (min•t)-10 NL / (min•t) and a pressure of 0.6 MPa-1 MPa. This ensures the molten steel is gently blown through the furnace, preventing violent turbulence and promoting mass transfer of hydrogen plasma within the molten pool. The working electrode is a hollow graphite electrode with an inner diameter 5%-15% of the outer diameter. The electrode power supply is 100 kW / t-500 kW / t of molten steel. A hydrogen-argon mixture is introduced through the hollow channel, with a hydrogen content >10 vol.%, a blowing flow rate of 10 NL / (min•t)-100 NL / (min•t), and a blowing pressure of 0.5 MPa-1 MPa. Under the high temperature of the electric arc (>3000℃), the hydrogen-argon mixture dissociates into H• and H2. + e - Highly chemically active hydrogen plasma reacts with O, N, S, C, and P on and near the surface of molten steel as follows: 4H• + [O] → 2H2O↑ 3H•+[N]→NH3↑ 2H•+[S]→H2S↑ 4H•+[C]→CH4↑ 3H•+[P]→PH3↑ The reaction products are all gaseous and are continuously discharged from the furnace through the exhaust system; at the same time, hydrogen forms a reducing atmosphere on the surface of the molten steel, inhibiting secondary oxidation. After refining for 10-60 minutes, when the temperature of the molten steel is >1600℃, samples are taken for composition analysis. Alloys are added to fine-tune the composition according to the target ingot composition. The samples are then heated for 5 minutes and resampled until the composition is qualified and the temperature of the molten steel is >1630℃.

[0028] It should be noted that the above "NL / (min·t)" means: the volume (in liters) of gas introduced per minute per ton of steel (or per ton of furnace charge) under standard conditions (0℃, 1 atmosphere).

[0029] S4 Vacuum Casting: After refining, the high-purity molten steel is transferred to a vacuum casting device. The vacuum level is evacuated to <67 Pa (absolute pressure), and the steel is cast at a steady, uniform rate of 5 kg / s-15 kg / s to avoid splashing and turbulence. Under vacuum, dissolved hydrogen in the steel further diffuses into the vacuum phase for removal, achieving simultaneous casting and hydrogen removal. After casting, vacuum pressure is maintained for 2-3 hours to allow the ingot to solidify slowly under vacuum and further dehydrogenate. Afterward, the ingot is slowly cooled to room temperature in the furnace before demolding, yielding a large, high-purity ingot.

[0030] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to the following embodiments.

[0031] Example 1: 5t large forging ingot (industrial pure iron + alloy materials) S1 Raw material pretreatment: Using industrial pure iron + alloy ingredients (Ni, Cr, Mo master alloy) as raw material, the surface oxide scale, oil and rust are removed by grinding, and the raw material is dried at 200℃ for 2 hours. The total weight of the raw material is about 5.8t (considering burn loss).

[0032] S2 Electric Arc Furnace Primary Refining: The electric arc furnace is loaded with a power supply of 2000kW (approximately 400kW / t of molten steel) and the temperature is raised. When the molten steel temperature reaches 1600℃, 120kg of slagging agent is added. The composition of the slagging agent (mass%) is: CaO 50%, SiO2 10%, Al2O3 23%, MgO 12%, and the balance being impurities. The refining process is carried out for 20 minutes while maintaining the power. During this period, the slag color is observed to change from black to grayish-white, indicating that desulfurization is in progress. The slag is discharged, and a primary refining sample is taken: S decreases from 0.025% in the raw material to 0.008%, and P decreases from 0.030% to 0.012%.

[0033] S3 Hydrogen Plasma Refining: Molten steel from the primary refining process is transferred to a hydrogen plasma refining furnace (capacity 6t) via a ladle. Argon is blown from the bottom of the furnace at a flow rate of 15 NL / min (approximately 3 NL / (min·t)) and a pressure of 0.8 MPa, resulting in a soft, micro-fluctuating blowing pattern. The hollow graphite electrode has an outer diameter of 400 mm and an inner diameter of 30 mm (inner diameter ratio 7.5%), and a power supply of 1500 kW. A hydrogen-argon mixture is injected through the hollow channel, with hydrogen comprising 30 vol.% of the gas, a flow rate of 50 NL / min (approximately 10 NL / (min·t)), and a pressure of 0.6 MPa. After arc ignition, the arc zone temperature exceeds 3200℃, hydrogen dissociates into hydrogen plasma, and a pale blue glow is visible on the surface of the molten steel. H2O and trace amounts of NH3 are detected in the exhaust pipe. After 30 minutes of refining, the molten steel temperature reaches 1640℃. Samples were taken showing: O = 13 ppm, N = 38 ppm, S = 0.005%, and C = 0.22%. According to the target composition (for producing low alloy steel forgings), Fe-Mn and Fe-Si were added to slightly adjust C and Mn. The sample was heated for 5 minutes for replication: O=9ppm, N=25ppm, S=11ppm, C=0.18%, temperature 1650℃, which is qualified.

[0034] S4 Vacuum Casting: The ladle is transferred to the vacuum casting chamber, and the vacuum level is evacuated to 45 Pa. The ladle is then poured into a 5t ingot mold at a uniform rate of 12 kg / s. After casting, the pressure is maintained for 3 hours, during which the vacuum level is kept <50 Pa. The ladle is then slowly cooled to room temperature and demolded. Testing results: O=9ppm, N=23ppm, S=11ppm, P=14ppm, H=2.3ppm; the microstructure is dense and free of white spots.

[0035] Example 2: Ingot for large forgings weighing 5t each (including returned material) S1: Industrial pure iron + alloy ingredients + ingot return material (return material accounts for about 25%), grinding and drying are the same as in Example 1.

[0036] S2: Electric arc furnace with power supply of 1800kW, 150kg of slag-forming agent (CaO 52%, SiO2 12%, Al2O3 27%, MgO 12%) added at 1600℃, slag discharged after refining for 15min.

[0037] S3: Bottom blowing of Ar at 25 NL / min and 0.9 MPa in hydrogen plasma refining furnace; hollow electrode φ400×40mm (inner diameter ratio 10%), power 1800kW; hydrogen-argon mixture at 60 NL / min, hydrogen at 40 vol.% and 0.5 MPa, refining for 20 min; molten steel at 1660℃ after composition adjustment, and the sample passed the retest.

[0038] S4: Vacuum casting, vacuum degree 55Pa, casting speed 13kg / s, holding pressure for 2h, slow cooling and demolding.

[0039] Test results: O=7ppm, N=21ppm, S=13ppm, P=19ppm, H=1.9ppm. The introduction of returned material did not lead to a deterioration in cleanliness, proving the gas-phase purification capability of hydrogen plasma for the inclusions / coatings inherent in the returned material.

[0040] Example 3: Ingot for large forgings weighing 10t each S1: Industrial pure iron + alloy ingredients + recycled materials (recycled materials account for 30%), polished and dried.

[0041] S2: Electric arc furnace with power supply of 3200kW, 200kg of slag-forming agent (CaO 53%, SiO2 11%, Al2O3 28%, MgO 12%) added at 1590℃, slag discharged after refining for 40min.

[0042] S3: Bottom-blown Ar 20NL / min, 0.9MPa; hollow electrode φ400×40mm, power 2500kW; hydrogen-argon mixture 150NL / min, hydrogen 50 vol.%, 0.5MPa, refining for 40min; molten steel qualified at 1645℃ after composition adjustment.

[0043] S4: Vacuum casting, vacuum degree 58Pa, casting speed 16kg / s, holding pressure for 3h, slow cooling and demolding.

[0044] Test results: O=8ppm, N=26ppm, S=14ppm, P=18ppm, H=2.1ppm. Process stability was verified by 10t scale-up.

[0045] Comparative Example 1 (Simulating CN117500946A route: LF embedded plasma + separate depressurized dehydrogenation) The raw materials and EAF primary refining are the same as in Example 1, except for S3: the hydrogen plasma gun is embedded in the LF furnace, the graphite electrode is not hollow, and a separate plasma gun is used to blow hydrogen; after hydrogen plasma treatment, the H of the molten steel rises to 5.8 ppm, and it needs to be transferred to RH vacuum degassing, and dehydrogenated for 8 minutes below 10 Torr (separate step), and then cast under atmospheric conditions.

[0046] Results: O=11ppm, N=28ppm, S=12ppm, H=3.1ppm (still too high after dehydrogenation, and the dehydrogenation step alone makes the total smelting cycle 35min longer than in Example 1. After the 10t scale-up, the temperature drop needs to be compensated by LF temperature increase, which increases energy consumption).

[0047] Comparative Example 2 (Simulating the traditional EAF+LF+VD route) Using the same raw materials, the process employs EAF primary refining, LF slag formation and silicon-calcium diffusion deoxidation, VD vacuum degassing and casting.

[0048] Results: O=18ppm (impurities modified but not removed), N=42ppm, S=15ppm, H=2.8ppm. O and N are significantly higher than in the embodiments of the present invention, illustrating the limitations of traditional LF which only modifies but does not remove impurities.

[0049] This invention provides a short-process method for preparing large, high-purity ingots, the ranges of which were determined through repeated experiments: Electric arc furnace power: 100kW / t - 600kW / t: If too low, the material will melt slowly and the temperature will drop greatly; if too high, the electric arc will erode the furnace lining and increase nitrogen.

[0050] The slag-forming agent CaO 40%-60% is the main slag system to ensure the desulfurization and adsorption of Al2O3 inclusions; MgO 5%-15% protects the furnace lining; Al2O3 15%-30% adjusts the slag viscosity; SiO2 is controlled at a low level to reduce silicon reversion.

[0051] The inner diameter of the hollow electrode should be 5%-15% of the outer diameter: if it is too low, the resistance to hydrogen flow will be large and it will be easy to clog; if it is too high, the electrode strength will decrease and it will be easy to break.

[0052] Hydrogen blowing pressure 0.5MPa - 1MPa: to ensure the stability of the plasma arc. If the pressure is less than 0.5, the plasma volume will be insufficient. If the pressure is greater than 1, the solubility of hydrogen in molten steel will increase abnormally (although vacuum casting can remove it, it is wasteful).

[0053] Bottom blowing Ar 1NL / (min•t)-10NL / (min·t), 0.6MPa -1MPa: soft blowing range. Too low a pressure will result in poor mass transfer, while too high a pressure will result in slag entrainment.

[0054] Vacuum degree <67Pa: equivalent to <0.5Torr, which can ensure that hydrogen continues to diffuse into the vacuum phase during the casting process.

[0055] Pouring speed 5kg / s - 15kg / s: 5t ingots correspond to about 5min-17min to complete the pouring. If it is too fast, it will splash and entrap air; if it is too slow, the temperature will drop too much and a crust will form.

[0056] The above parameter combinations in this invention are not isolated choices, but rather represent the overall requirements of the synergistic "hydrogen plasma enrichment and simultaneous removal by vacuum casting": if the vacuum level is relaxed to >67Pa (such as ordinary VD at 100Pa-500Pa level), the dehydrogenation rate during casting is insufficient, and H is likely to remain at >3ppm; if the hydrogen blowing pressure is <0.5MPa, the plasma arc is unstable, and the removal rates of O and N decrease.

[0057] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing high-purity large ingots using a short-process method, comprising electric arc furnace melting, refining, and casting steps; characterized in that, The process employs a three-stage, separate process: electric arc furnace primary refining, hydrogen plasma refining, and vacuum casting. The specific steps are as follows: S1 Raw material pretreatment: Grinding to remove the oxide layer, oil and rust from the surface of the metal raw material, and high-temperature drying to remove moisture; S2 Electric Arc Furnace Primary Refining: The pretreated raw materials are fed into the electric arc furnace, and the power supply is 100kW / t-600kW / t of molten steel. After the temperature is raised to 1580℃-1620℃, a slag-forming agent is added. The power is kept constant for 10min-40min to carry out slag refining. Then the slag is discharged to obtain primary molten steel. S3 Hydrogen Plasma Refining: The primary molten steel is transferred to the hydrogen plasma refining furnace, and argon gas is blown from the bottom of the furnace to make the molten steel in a soft blowing state. Using hollow graphite electrodes as working electrodes, a hydrogen-argon mixture is introduced through the hollow channel. The hydrogen-argon mixture dissociates into hydrogen-containing plasma under the high temperature of the electric arc, which reacts with O, N, S, C and P impurities in the molten steel to generate gaseous products that are discharged with the flue gas. After refining for 10-60 minutes, the molten steel is sampled and analyzed when the temperature is >1600℃. Alloys are added to fine-tune the composition according to the target composition until the molten steel composition is qualified and the temperature is >1630℃. S4 Vacuum casting: The refined molten steel is transferred into a vacuum casting equipment, and the vacuum degree is evacuated to <67Pa. It is then cast at a uniform speed of 5kg / s-15kg / s. After casting, the vacuum pressure is maintained for 2-3 hours. After the furnace is slowly cooled to room temperature, the steel is demolded to obtain a large ingot with high cleanliness.

2. The method according to claim 1, characterized in that, The slag-forming agent mentioned in step S2 has the following composition by mass percentage: CaO 40%-60%, SiO2 5%-15%, Al2O3 15%-30%, MgO 5%-15%; the amount of slag-forming agent added is 1%-10% of the total mass of the smelting raw materials.

3. The method according to claim 1, characterized in that, The hollow inner diameter of the hollow graphite electrode mentioned in step S3 is 5%-15% of the outer diameter of the electrode, the electrode power supply is 100kW / t-500kW / t molten steel; the hydrogen content in the hydrogen-argon mixture is >10 vol.%, the blowing flow rate is 10NL / (min•t)-100NL / (min·t), and the blowing pressure is 0.5MPa-1MPa.

4. The method according to claim 1, characterized in that, In step S3, the bottom-blowing argon flow rate is 1NL / (min•t)-10NL / (min·t), the argon blowing pressure is 0.6MPa-1MPa, and argon gas is introduced throughout the process.

5. The method according to claim 1, characterized in that, In step S3, the hydrogen plasma refining time is 10 min to 40 min; in step S4, the casting speed is 5 kg / s to 15 kg / s, and the vacuum holding time is 2 h to 3 h.

6. The method according to claim 1, characterized in that, The metal raw material mentioned in step S1 is one or more of industrial pure iron, alloy feedstock, and ingot return material. The ingot corresponding to the raw material with a single weight ≥ 5t is an ingot for large forgings.

7. The method according to claim 1, characterized in that, In step S3, hydrogen gas forms a reducing atmosphere on the surface of the molten steel in the hydrogen plasma refining furnace, which inhibits the oxidation of the molten steel.

8. The method according to claim 1, characterized in that, The resulting large ingots contain oxygen content ≤10ppm, nitrogen content ≤30ppm, sulfur content ≤15ppm, phosphorus content ≤20ppm, and hydrogen content ≤3ppm.

9. A high-purity large forging ingot prepared by the method according to any one of claims 1-8, with a single weight of 5t-10t, characterized in that, The ingot contains O≤10ppm, N≤30ppm, S≤15ppm, P≤20ppm, and H≤3ppm. The inclusions are spheroidized modified residues with no macroscopic segregation.

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