A method for manufacturing a large-sized Cr-Ni-Co-Mo stainless steel

Abstract

The application provides a manufacturing method of large-size Cr-Ni-Co-Mo stainless steel, which adopts an electric furnace+VOD+vacuum consumable replacement of traditional methods, adopts an LF+VOD+LF+VD refining process to ensure high purity of the rod, and specifically, on one hand, special lime, fluorite powder and ferrosilicon are used as a reducing slag system in a specific proportion, and on the other hand, a special vacuum degassing process is designed, so that the oxygen content and the sulfur content of the base material are very low (O≤15ppm, S≤6ppm), and in the forging stage, the process of multiple upsetting and drawing, temperature reduction with each fire, low pressure and speed and low pressure forging makes the material full-section quality consistent, solves the problems of ultrapure smelting, high alloy segregation and low-cost industrial production of the new material, and prepares high-purity and low-segregation large-size rods, which provides a feasible scheme for industrial batch production.

Description

Technical Field

[0001] This invention relates to stainless steel smelting technology, and specifically proposes a method for manufacturing large-size Cr-Ni-Co-Mo stainless steel. Background Technology

[0002] Cryogenic Cr-Ni-Co-Mo stainless steel is a crucial material in the cryogenic systems of liquid rocket engines, accounting for over 30% of the engine's weight. Furthermore, to meet the requirements of lightweight structural design, performance consistency, and economic efficiency in heavy-duty rocket engines, cryogenic stainless steel must also satisfy engineering requirements for multi-process adaptability and low-cost manufacturing. This presents significant challenges to the alloy optimization and process optimization of cryogenic stainless steel.

[0003] As the demand in this field expands, users are using materials with increasingly larger specifications. The requirements for material microstructure and segregation are becoming increasingly stringent. Due to improper control of smelting, component segregation, and self-consumption processes, purity requirements are not met, leading to material failure during use.

[0004] Cr-Ni-Co-Mo stainless steel is currently typically produced using either a "vacuum induction + consumable waste metallurgy" or "vacuum induction + electroslag remelting" process, which is relatively expensive. If a single vacuum smelting process is considered, the following main problems exist: (1) Vacuum induction furnaces have high production costs and low capacity, and it is difficult to prepare large-size ingots; (2) Conventional electric arc furnace smelting is prone to gas absorption, and has high hydrogen, nitrogen and oxygen content, which easily leads to defects; (3) Elements such as cobalt, molybdenum, aluminum, and titanium suffer large burn-off, have large compositional fluctuations, and low recovery rates; (4) Large-sized ingots are prone to compositional segregation, inclusion aggregation, and uneven structure, resulting in a low pass rate for flaw detection. (5) The process is long and energy consumption is high, making it difficult to achieve low-cost large-scale production.

[0005] Therefore, developing a high-purity, low-cost method for manufacturing Cr-Ni-Co-Mo stainless steel suitable for large-scale ingot production has significant engineering value. Summary of the Invention

[0006] This invention proposes a method for manufacturing large-size Cr-Ni-Co-Mo stainless steel, which adopts a single vacuum smelting process of "electric furnace + VOD + vacuum self-consumption" to replace the smelting process of "vacuum induction + vacuum self-consumption" or "vacuum induction + electroslag" and significantly reduces production costs without changing product quality.

[0007] The technical solution of this invention is implemented as follows: This invention proposes a method for manufacturing large-size Cr-Ni-Co-Mo stainless steel, the steps of which include: S1. According to the target composition, the raw materials are prepared, and after melting and cleaning, they are oxidized, dephosphorized, and decarburized until C≤0.10% and P≤0.006%, and then transferred to the LF furnace; S2. Add 0.5-1.0% lime to the LF furnace to form slag, desulfurize to ≤0.002%, then raise the temperature to 1620-1650℃, remove all desulfurization slag, and transfer to the VOD furnace; S3. VOD furnace refining includes: S31. After evacuating to 10-15 kPa, oxygen blowing is used for decarburization, with main oxygen blowing for 10-15 minutes; S32. After stopping oxygen blowing, evacuate to ≤0.3 Torr and maintain for 15-20 minutes; S33. After adding pre-reducing slag into the furnace, vacuum slag for 15-20 minutes, remove the slag, and then transfer it to the LF furnace; The chemical composition of the reducing slag by mass ratio is CaO:CaF2:Al2O3:SiO2:FeSi = (14-18):(2:4):(1-3):(1-3):(2:4); S4.LF furnace refining: S41. Feed Al line, then use special lime and refining slag to re-form slag, adjust to slag white and maintain it; S42. Deoxidize the slag surface to ≤20ppm; S43. Adjust the target composition of the molten steel to the design range, and adjust the temperature to 1615-1635℃ to switch to a VD furnace; Argon gas is used to lift and stir in the S5.VD furnace. The total time under vacuum is 40-60 min, the vacuum degree is maintained below 0.1 Torr for 25-45 min, and the soft blowing time is 20-30 min. S6. Cast into a consumable electrode rod; S7. Vacuum arc remelting yields steel ingots; S8 steel ingots are subjected to high-temperature diffusion and forging to obtain the target large-size Cr-Ni-Co-Mo stainless steel.

[0008] Further, the Cr-Ni-Co-Mo stainless steel, by mass percentage, has the following composition: C≤0.03%, Si≤0.50%, Mn≤0.70%, S≤0.0012%, P≤0.008%, Cr: 10.50-12.00%, Ni: 7.50-9.00%, Mo: 1.80-2.20%, Co: 4.0-5.5%, V: ≤0.30%.

[0009] Furthermore, in step S41, during the Al wire feeding process, the Al wire quantity is calculated based on a target of 0.08-0.10%, ignoring burn loss; And / or, the mass ratio of the special lime to the refining slag is 1:2, and the refining slag is 55% Al2O3 and 45% SiO2.

[0010] Furthermore, in step S42, Al particles and ferrosilicon powder are added during the slag surface deoxidation process in a mass ratio of 5:6.

[0011] Furthermore, in step S6, protective slag is added during the casting process, and the casting temperature is 1520-1540℃; And / or, fill with argon gas 30 minutes before pouring to reduce the oxygen content in the mold to ≤50ppm.

[0012] Furthermore, in step S7, the vacuum self-consumption control vacuum degree is ≤0.3Pa, and the leakage rate is ≤0.4Pa / min; And / or, the current value of the arc ignition stage of the vacuum self-consuming process is 7000-8000A, the voltage value is 21-24V, and the arc ignition time is 60-90min; And / or, the melting rate of the vacuum self-consuming process is 4.0-6.0 kg / min, and the droplet melting time is 0.05-0.15 s per drop.

[0013] Furthermore, in step S8, the high-temperature diffusion conditions are 1180-1210℃, and the temperature is maintained for 25-40 hours.

[0014] Furthermore, the specifications of the large-size Cr-Ni-Co-Mo stainless steel are as follows: 90~600mm, the steel ingot mentioned in step S7 is 6-12t.

[0015] Preferably, in step S8, the forging process employs multiple upsetting and drawing, sequential cooling, low-pressure forging rate, and low-pressure forging, with a single-fire forging ratio ≥4, a total forging ratio ≥10, and a forging holding time of 3-5 hours, with an additional 0.5 hours for every 50 mm of diameter.

[0016] More preferably, the forging process is carried out at a low pressure rate of 70-120 mm / s, and / or the low pressure is 20-50 MN.

[0017] More preferably, the reheating temperature for each forging pass is 1030-1130℃, the temperature drop between adjacent passes is 10-50℃, and the reheating time for each pass is 2-3 hours; and / or, for For specifications of 90-280mm, the forging holding time is 3-6 hours. Specifications: 280-600mm, forging and holding time: 6-10h.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The preparation method described in this invention, through electric furnace + AOD / VOD + LF + VD + ingot casting, a reasonable combination of refining processes and appropriate slag components and deoxidation methods, ensures extremely low oxygen and sulfur content (O≤15ppm, S≤6ppm) in the base material. This solves the problems of ultra-pure smelting, high alloy segregation, and low-cost industrial production of this new material, and produces large-size bars with high purity and low segregation, providing a feasible solution for industrial mass production. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The result obtained in Embodiment 1 of the present invention Grain size observation results of 300mm rods under a microscope.

[0021] Figure 2 The result obtained in Embodiment 2 of the present invention Grain size observation results of 350mm rods under a microscope.

[0022] Figure 3 The result obtained in Embodiment 3 of the present invention Grain size observation results of 400mm rods under a microscope. Detailed Implementation

[0023] 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.

[0024] This invention proposes a method for manufacturing large-size Cr-Ni-Co-Mo stainless steel. It adopts a single vacuum smelting process of "electric furnace + VOD + vacuum consumable charge" to replace the smelting process of "vacuum induction + vacuum consumable charge" or "vacuum induction + electroslag remelting". Through the process concept of "electric arc furnace + ladle refining (LF + VOD + LF + VD) smelting + vacuum consumable charge remelting", it solves the problem of large-size, low-cost, industrial-scale ultra-pure smelting of Cr-Ni-Co-Mo stainless steel, and produces large-size bars that meet the application indicators (room temperature tensile strength Rm≥1250MPa, yield strength Rp0.2≥1150MPa, impact toughness KU2≥108J).

[0025] The basic process of the manufacturing method described in this invention includes: steelmaking → ingot casting → annealing → peeling or turning → welding → vacuum arc remelting → annealing → finishing → high-temperature homogenization → forging billet and forging finished product → inspection → finishing, flaw detection, packaging and warehousing.

[0026] The specific steps are as follows: 1. Raw Material Batching: A reasonable material composition is used, selecting scrap steel with low phosphorus and low sulfur content (P≤0.025%, S≤0.005%), nickel plates (purity ≥99.95%), cobalt plates (purity ≥99.9%), low-phosphorus ferrochrome (P≤0.025%), ferromolybdenum, and cobalt plates, etc., strictly prohibiting the mixing of low-quality scrap. Surface moisture and oil contamination are strictly controlled to minimize the introduction of H and O gases during smelting. The charge ratio is strictly designed according to the target composition (percentages as shown in Table 1) to ensure subsequent composition adjustment margins and smelting process stability.

[0027] 2. Electric arc furnace (electric furnace + alloy melting furnace + LF + VOD + LF + VD) smelting

[0028] (1) Melting stage: A 50-ton closed electric arc furnace and a 20-ton alloy melting furnace are used to melt the material. Argon gas is introduced into the electric arc furnace to form a slight positive pressure to suppress hydrogen and nitrogen absorption. After melting and cleaning, oxidation, decarburization and dephosphorization are carried out until C≤0.10% and P≤0.006%; the ladle is hoisted to the LF furnace for pre-adjustment.

[0029] (2) Pre-adjustment of LF furnace

[0030] LF refining furnace adds 0.5-1.0% lime to form slag, creating a high-alkalinity, strongly reducing slag system for deoxidation, desulfurization, and alloying.

[0031] Desulfurize to S: 0.0015%, heat to 1620-1650℃ and change pit, remove all desulfurization slag and transfer to VOD furnace.

[0032] (3) VOD Refining

[0033] ① Close the lid and evacuate the vacuum. After the vacuum reaches 10-15 kPa, slowly blow oxygen through the oxygen gun. After 5 minutes, start the main oxygen blowing for 10-15 minutes.

[0034] ② After stopping oxygen blowing, immediately switch to vacuum carbon deoxidation (carbon and oxygen react under vacuum), evacuate to the limit: ≤0.3 Torr, and hold for 15-20 minutes.

[0035] ③ Break the vacuum, add 1000-1100 kg of pre-reduction slag (special lime, fluorite powder and ferrosilicon). The chemical composition of the pre-reduction slag is CaO:CaF2:Al2O3:SiO2:FeSi = (14-18):(2:4):(1-3):(1-3):(2:4). Close the lid and evacuate the vacuum for slag removal for 15-20 minutes. Break the vacuum, take a sample, and remove all the slag from the ladle. After removing the slag, transfer the ladle to the LF furnace.

[0036] (4) LF furnace refining

[0037] ① After entering the pit, feed Al line (without considering burn-off, target 0.08-0.10%), use 250kg of special lime SiO2 and 500kg of refining slag (55% Al2O3: 45% SiO2) to re-form slag, adjust to white slag, and maintain white slag during the refining process.

[0038] ② Use 250 kg of Al particles and 300 kg of ferrosilicon powder (FeSi) to deoxidize the slag surface (O content ≤ 20 ppm) for 25-35 minutes.

[0039] ③ Adjust the steel composition according to the target composition of the process. Ensure that elements such as C, Cr, Ni, Mo, and Co are within the design range. Perform vacuum degassing to ensure extremely low hydrogen, oxygen, and nitrogen content.

[0040] ④Turning to VD at temperatures of 1615℃-1635℃.

[0041] (5) VD furnace

[0042] Argon gas is used for stirring during the vacuum process. The total vacuum time is 40-60 minutes, and the holding time for the ultimate vacuum of 0.1 Torr or below is 25-45 minutes. Soft blowing time is 20-30 minutes.

[0043] The casting process uses AG-5 protective slag and argon gas protection during casting, controlling low superheat, with a casting temperature of 1520℃-1540℃. Argon gas is filled into the ingot mold 30 minutes before casting to reduce the oxygen content inside the mold to ≤50ppm. An argon gas hood protects the casting process, effectively solving problems such as secondary oxidation of the molten steel during casting.

[0044] 3. Vacuum consumable remelting

[0045] The ratio of the electrode rod diameter d to the crystallizer inner diameter D is d / D = 0.7-0.85. The stable remelting stage requires the molten pool to be active to the edge. (1) Before arc ignition of self-consumable remelting, confirm the vacuum level: ≤0.3pa, leakage rate: ≤0.4Pa / min, electrode alignment, and unobstructed water cooling pipes.

[0046] (2) Arc ignition stage: The arc ignition stage is controlled by current and voltage. In order to ensure the activity of the molten pool and the uniformity of the melting rate, the current setting value of the arc ignition stage is 7000-8000A, the voltage setting value is 21-24V, and the arc ignition time is 60-90min.

[0047] (3) Melting stage: In order to ensure the stability of the molten pool, the melting drop and melting rate are controlled in the melting stage. The melting rate is set at 4.0-6.0 kg / min and the melting drop time is set at 0.05-0.15s / drop.

[0048] (4) During the self-consumption process, the furnace body and the crystallizer are kept cooled by circulating water, and the crystallizer and the self-consumption ingot are cooled by He gas.

[0049] Note: A lower vacuum degree of ≤0.3Pa is required for self-consumption, which can further remove gases during the remelting process. By selecting appropriate crucible ratio, melting rate control and cooling process requirements, the size of the molten pool during the remelting process can be controlled. The size and activity state of the molten pool determine issues such as inclusion improvement, solidification segregation degree and composition uniformity during the remelting process.

[0050] 4. Forging Production

[0051] The steel ingots smelted above are subjected to high-temperature diffusion at 1180~1210℃ for 25h-40h and then free forging (forging ratio>10) to form bars (equivalent diameter range D≤500mm). The forged bars are then air-cooled and tested.

[0052] against For sizes ranging from 90-600mm, production requires 6t and 12t steel ingots. The process involves multiple upsetting and drawing, successive cooling, and low-pressure forging at low rates (70-120mm / s) and low pressure (20-50MN) to ensure consistent material quality across the entire cross-section. The ingot holding time is 3-5 hours + 0.5 hours. (50mm), that is, based on the forging and holding time of 3-5h, the forging time is increased by 0.5h for every 50mm of diameter, the single-fire forging ratio is ≥4, and the total forging ratio is ≥10.

[0053] In some preferred embodiments, the reheating temperature for each forging pass is 1030-1130°C, the temperature drop between adjacent passes is 10-50°C, and the reheating time for each pass is 2-3 hours; for For specifications of 90-280mm, the forging holding time is 3-6 hours. Specifications: 280-600mm, forging and holding time: 6-10h.

[0054] In some preferred embodiments, 90- 280mm bar stock using 6t ( The production of 660mm steel ingots involves a forging and holding time of 3-6 hours; the specific forging process is shown in Table 1.

[0055] Table 1:

[0056] > 280~ 600mm bar stock uses 12t ( The forging process involves producing 920mm steel ingots with a holding time of 6-10 hours. Specific forging procedures are shown in Table 2.

[0057] Table 2:

[0058] In this invention, the high purity of the bar stock is ensured by employing an LF+VOD+LF+VD refining process. Specifically, on the one hand, a specific ratio of special lime, fluorite powder, and ferrosilicon is used as the reducing slag system. On the other hand, a special vacuum degassing process is designed to ensure extremely low oxygen and sulfur content (O≤15ppm, S≤6ppm) in the base material. Furthermore, the forging stage employs multiple upsetting and drawing, sequential cooling, low-pressure forging rate, and low-pressure forging processes to ensure consistent quality and uniform microstructure across the entire cross-section of the material. The content of A, B, C, and D type inclusions (coarse and fine series) is ≤1.0 grade. This solves the problems of ultra-pure smelting, high alloy segregation, and low-cost industrial production of this new material, and produces large-size bars with high purity and low segregation.

[0059] Example 1

[0060] Steel ingots are obtained by smelting in a 50t refining furnace and vacuum self-consumption furnace in a 6t self-consumption furnace. After forging in a 45MN furnace, 6t (Φ660mm) steel ingots are subjected to high-temperature diffusion at 1180℃ for 25h to produce Φ300mm bars.

[0061] 1. Raw Material Batching: A reasonable material structure is employed, using low-phosphorus, low-sulfur scrap steel (P≤0.025%, S≤0.005%), nickel plates (purity ≥99.95%), cobalt plates (purity ≥99.9%), low-phosphorus ferrochrome (P≤0.025%), ferromolybdenum, and cobalt plates, strictly prohibiting the mixing of low-quality scrap. Surface moisture and oil contamination are strictly controlled to minimize the introduction of H and O gases during smelting. The furnace charge ratio is strictly designed based on the target composition (C: 0.015%, Mn: 0.50%, Si: 0.25%, Cr: 11.00%, Ni: 8%, Co: 5%, Mo: 2.0%, V: 0.15%) to ensure subsequent composition adjustment margins and smelting process stability.

[0062] 2. Electric arc furnace (electric furnace + alloy melting furnace + LF + VOD + LF + VD) smelting

[0063] (1) Melting stage: A 50-ton closed electric arc furnace and a 20-ton alloy melting furnace are used to melt the material. Argon gas is introduced into the electric arc furnace to form a slight positive pressure to suppress hydrogen and nitrogen absorption. After melting and clearing, oxidation, decarburization and dephosphorization operations are carried out. The ladle is hoisted to the LF furnace for pre-adjustment.

[0064] (2) Pre-adjustment of LF furnace

[0065] The LF refining furnace adds 1.0% lime to create a high-alkalinity, strongly reducing slag system for deoxidation, desulfurization, and alloying.

[0066] Desulfurization to S: 0.0015%, heating to 1630℃, changing the pit, removing all desulfurization slag, and transferring to a VOD furnace. (3) VOD Refining ① Close the lid and evacuate the vacuum. After the vacuum reaches 12 kPa, slowly blow oxygen through the oxygen gun. After 5 minutes, start the main oxygen blowing.

[0067] ② After stopping oxygen blowing, immediately switch to vacuum carbon deoxidation, evacuate to the limit: ≤0.3 Torr, and hold for 20 minutes.

[0068] ③ Break the vacuum, add pre-reduced slag CaO:CaF2:Al2O3:SiO2:FeSi=16:3:2:2:3, close the lid and evacuate the vacuum for slag removal for 20 minutes, break the vacuum, take a sample, lift the ladle out and remove all the slag, and transfer the ladle to the LF furnace.

[0069] (4) LF furnace refining

[0070] ① After entering the pit, feed Al line (without considering burn-off, target 0.090%), use 250kg of special lime and 500kg of refining slag to re-form slag, adjust to white slag, and maintain white slag during the refining process.

[0071] ② Use Al particles and ferrosilicon powder for slag surface deoxidation.

[0072] ③ Adjust the steel composition according to the target composition of the process. Ensure that elements such as C, Cr, Ni, Mo, and Co are within the design range. Perform vacuum degassing to ensure extremely low hydrogen, oxygen, and nitrogen content.

[0073] ④ At 1625℃, switch to VD.

[0074] (5) VD furnace

[0075] Argon gas was used for stirring during the vacuum process. The total vacuum time was 50 minutes, with a minimum ultimate vacuum of 0.1 Torr maintained for 30 minutes. Soft blowing time was 25 minutes.

[0076] (6) AG-5 protective slag is used during the casting process, and argon gas is used for casting to control the low superheat. The casting temperature is 1530℃. Argon gas is filled into the ingot mold 30 minutes before casting to reduce the oxygen content in the mold to 45ppm. Argon gas cover is used to protect the casting process, thereby effectively solving the problem of secondary oxidation of molten steel during the casting process.

[0077] 3. Vacuum consumable remelting: The ratio of the electrode rod diameter d to the crystallizer inner diameter D is d / D=0.8. The stable remelting stage requires the molten pool to be active to the edge. (1) Before arc ignition by self-consumable remelting, confirm that the vacuum degree is 0.3pa, the leakage rate is 0.4Pa / min, the electrode is aligned, and the water cooling pipe is unobstructed.

[0078] (2) Arc ignition stage: The arc ignition stage is controlled by current and voltage. In order to ensure the activity of the molten pool and the uniformity of the melting rate, the current setting value of the arc ignition stage is 7500A, the voltage setting value is 22V, and the arc ignition time is 60min.

[0079] (3) Melting stage: In order to ensure the stability of the molten pool, the melting droplet and melting rate are controlled in the melting stage. The melting rate is set at 50 kg / min and the melting droplet time is set at 0.10s.

[0080] (4) During the self-consumption process, the furnace body and the crystallizer are kept cooled by circulating water, and the crystallizer and the self-consumption ingot are cooled by He gas.

[0081] 4. Forging Production

[0082] The steel ingots smelted above were subjected to high-temperature diffusion at 1200℃ for 25 hours followed by 45MN rapid forging to prepare bars. 300mm forged bar stock was inspected after air cooling.

[0083] The forging process is carried out as follows:

[0084] Example 2

[0085] Using the manufacturing method provided by this invention, smelting was carried out in different 50-ton refining furnaces, Consac's 12-ton consumable furnace, and 60MN forging, with a capacity of 6 tons. A 660mm steel ingot was prepared by holding it at 1210℃ for 35 hours, with the remaining operations being the same as in Example 1. 350mm bar stock.

[0086] Example 3

[0087] Using the manufacturing method provided by this invention, smelting is carried out in a 50-ton refining furnace, INTECO's 12-ton consumable furnace, and 45MN forging, resulting in 12t ( A 920mm steel ingot was heated at 1210℃ for 40 hours, and the remaining operations were the same as in Example 1. 400mm bar stock.

[0088] Comparative Example 1

[0089] The slag system used for VOD refining is: special lime: fluorite powder: Al particles: ferrosilicon = 12:3:2:3. The remaining operations are the same as in Example 1. The inclusions in the prepared Φ400mm bars did not meet the requirements.

[0090] Comparative Example 2

[0091] VD furnace: Argon gas was used for stirring during the vacuum process; total vacuum time: 25 min; holding time below ultimate vacuum of 1.0 Torr: 10 min. Soft blowing time: 10 minutes. The pouring temperature was 1538℃; all other operations were the same as in Example 1. 400mm bar stock. Testing revealed that the carbon content exceeded the standard, the H+O+N content did not meet the requirement of ≤80ppm, and the inclusion density (D) did not meet the standard requirements.

[0092] Comparative Example 3

[0093] The vacuum self-consumption melting rate is unstable, with large fluctuations, ranging from 5.0 to 7.5 kg / min. There was a power outage during the melting process, and low-magnification detection revealed a ring-shaped pattern of grade B, and the inclusions did not meet the requirements for flaw detection.

[0094] Comparative Example 4

[0095] The difference from Example 1 is that the forging upsetting process was controlled with a reduction rate of 120 mm / s, a pressure of 60 MN, and a reduction of 150 mm. Testing revealed that the room temperature impact and low temperature impact results did not meet the requirements, and the grain size was uneven, ranging from 3 to 5 grades.

[0096] Multiple batches of bars were prepared according to the above process requirements. The chemical composition (Table 3), inclusions (Table 4), grain size (Table 5), and mechanical properties (Table 6) are tested as follows: Table 3: Chemical Composition

[0097] Table 4: Results of Inclusion Detection

[0098] Table 5: Grain size results

[0099] The results of microscopic grain size observation in Examples 1-3 are as follows: Figure 1 , Figure 2 and Figure 3 As shown.

[0100] Table 6: Results of Mechanical Property Tests

[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 manufacturing large-size Cr-Ni-Co-Mo stainless steel, characterized in that the steps include... include: S1. According to the target composition, the raw materials are prepared, and after melting and cleaning, they are oxidized, dephosphorized, and decarburized until C≤0.10% and P≤0.006%, and then transferred to the LF furnace; S2. Add 0.5-1.0% lime to the LF furnace to form slag, desulfurize to ≤0.002%, then raise the temperature to 1620-1650℃, remove all desulfurization slag, and transfer to the VOD furnace; S3. VOD furnace refining includes: S31. After evacuating to 10-15 kPa, oxygen blowing is used for decarburization, with main oxygen blowing for 10-15 minutes; S32. After stopping oxygen blowing, evacuate to ≤0.3 Torr and maintain for 15-20 minutes; S33. After adding pre-reducing slag into the furnace, vacuum slag for 15-20 minutes, remove the slag, and then transfer it to the LF furnace; The chemical composition of the reducing slag by mass ratio is CaO:CaF2:Al2O3:SiO2:FeSi = (14-18):(2:4):(1-3):(1-3):(2:4); S4.LF furnace refining: S41. Feed Al line, then use special lime and refining slag to re-form slag, adjust to slag white and maintain it; S42. Deoxidize the slag surface to ≤20ppm; S43. Adjust the target composition of the molten steel to the design range, and adjust the temperature to 1615℃-1635℃ to switch to a VD furnace; Argon gas is used to lift and stir in the S5.VD furnace. The total time under vacuum is 40-60 min, the vacuum degree is maintained below 0.1 Torr for 25-45 min, and the soft blowing time is 20-30 min. S6. Cast into a consumable electrode rod; S7. Vacuum arc remelting yields steel ingots; S8 steel ingots are subjected to high-temperature diffusion and forging to obtain the target large-size Cr-Ni-Co-Mo stainless steel.

2. The preparation method according to claim 1, characterized in that, The Cr-Ni-Co-Mo stainless steel, by mass percentage, has the following composition: C≤0.03%, Si≤0.50%, Mn≤0.70%, S≤0.0012%, P≤0.008%, Cr: 10.50-12.00%, Ni: 7.50-9.00%, Mo: 1.80-2.20%, Co: 4.0-5.5%, V: ≤0.30%.

3. The preparation method according to claim 1, characterized in that, In step S41, during the Al wire feeding process, the Al wire quantity is calculated based on a target of 0.08-0.10%, ignoring burn loss. And / or, the mass ratio of the special lime to the refining slag is 1:2, and the refining slag is 55% Al2O3 and 45% SiO2.

4. The preparation method according to claim 1, characterized in that, In step S42, Al particles and ferrosilicon powder are added to the slag surface deoxidation process in a mass ratio of 5:

6.

5. The preparation method according to claim 1, characterized in that, In step S6, protective slag is added during the casting process, and the casting temperature is 1520-1540℃; And / or, fill with argon gas 30 minutes before pouring to reduce the oxygen content in the mold to ≤50ppm.

6. The preparation method according to claim 1, characterized in that, In step S7, the vacuum self-consumption control vacuum degree is ≤0.3Pa, and the leakage rate is ≤0.4Pa / min; And / or, the current value of the arc ignition stage of the vacuum self-consuming process is 7000-8000A, the voltage value is 21-24V, and the arc ignition time is 60-90min; And / or, the melting rate of the vacuum self-consuming process is 4.0-6.0 kg / min, and the droplet melting time is 0.05-0.15 s per drop.

7. The preparation method according to claim 1, characterized in that, In step S8, the high-temperature diffusion conditions are 1180-1210℃, and the temperature is maintained for 25-40 hours.

8. The preparation method according to claim 1, characterized in that, The large-size Cr-Ni-Co-Mo stainless steel is 90-600mm, the steel ingot mentioned in step S7 is 6-12t.

9. The preparation method according to claim 8, characterized in that, In step S8, the forging process adopts multiple upsetting and drawing, sequential cooling, low-pressure forging rate and low-pressure forging, single-fire forging ratio ≥4, total forging ratio ≥10, and forging holding time of 3-5h, with an increase of 0.5h for every 50mm of diameter.

10. The preparation method according to claim 8, characterized in that, The reheating temperature for each forging process is 1030-1130℃, the temperature drop between adjacent forging processes is 10-50℃, and the reheating time for each forging process is 2-3 hours. And / or, for For specifications of 90-280mm, the forging holding time is 3-6 hours. Specifications: 280-600mm, forging and holding time: 6-10h.

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