A method for preparing ultra-high strength stainless steel

Abstract

This invention relates to a method for preparing ultra-high strength stainless steel, belonging to the technical field of ultra-high strength stainless steel preparation, and aims to solve the problem of poor matching of strength, toughness, plasticity, and corrosion resistance in existing ultra-high strength stainless steels. The preparation method includes: raw material steel preparation; high-purity steel ingot preparation; homogenization treatment of the high-purity steel ingot; bar forging; preliminary heat treatment; performance heat treatment; the performance heat treatment includes solution treatment – ​​first cryogenic treatment – ​​first aging treatment – ​​second cryogenic treatment – ​​second aging treatment; wherein the holding temperature for the first aging treatment is 480℃~520℃; and the holding temperature for the second aging treatment is 490~520℃. The ultra-high strength stainless steel prepared by the method of this invention exhibits excellent strength, toughness, and corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength stainless steel preparation technology, and in particular to a method for preparing ultra-high strength stainless steel. Background Technology

[0002] With breakthroughs in metallurgical technology, microstructure design, and process innovation, ultra-high strength stainless steel has been continuously pursuing "ultimate strength" to meet the lightweight design requirements of key load-bearing components. Ultra-high strength stainless steel aligns with the design needs of miniaturization, lightweighting, and safe service life for aerospace structural components and has significant application and promotion value in fields such as deep-sea exploration and nuclear energy engineering. However, for ultra-high strength stainless steel, increased strength often leads to a decrease in plasticity and toughness; this "inverse relationship" has become a technical bottleneck limiting the widespread application of ultra-high strength stainless steel. Existing technologies, due to unsuitable processes, result in excessive amounts of other substances and inclusions in the preparation of ultra-high strength stainless steel, leading to poor matching of strength, toughness, plasticity, and corrosion resistance. Therefore, how to provide a method for preparing ultra-high strength stainless steel with excellent toughness has become an urgent problem to be solved. Summary of the Invention

[0003] In view of the above analysis, the present invention aims to provide a method for preparing ultra-high strength stainless steel to solve one of the following technical problems: the poor matching of strength, toughness, plasticity and corrosion resistance of existing ultra-high strength stainless steel.

[0004] This invention provides a method for preparing ultra-high strength stainless steel, comprising the following steps: Step 1: Raw material steel preparation; Step 2: High-purity steel ingot preparation; Step 3: High-purity steel ingot homogenization treatment; Step 4: Bar forging; Step 5: Preliminary heat treatment; Step 6: Performance heat treatment; Performance heat treatment includes solution treatment - first cryogenic treatment - first aging treatment - second cryogenic treatment - second aging treatment; wherein, the holding temperature for the first aging treatment is 480℃~520℃; the holding temperature for the second aging treatment is 490~520℃.

[0005] Furthermore, in step 6, the solution treatment includes the following steps: holding temperature of 1070℃~1090℃, holding time of 70~90min, and oil cooling to room temperature.

[0006] Furthermore, in step 6, the holding temperature for the first cryogenic treatment is -73 to -75°C, and the holding time is 8 to 9 hours.

[0007] Furthermore, in step 6, the holding temperature for the secondary cryogenic treatment is -73 to -75°C, and the holding time is 3 to 4 hours.

[0008] Furthermore, in step 4, the bar forging includes the following steps: S401, One-time forging: Heat the steel ingot after homogenization treatment in step 3 to 1150℃~1180℃, hold for 4~7 hours, then upset and draw into an octagonal cross-section forging; S402, Secondary Forging: The forgings in S401 are reheated in the furnace to 1120℃~1140℃ and held for 2~3 hours, then upset and drawn into an octagonal cross-section forging; S403, Three-stage forging: The forgings in S402 are reheated in the furnace to 1090℃~1110℃ and held for 1.5~3 hours, then upset and drawn into an octagonal cross-section forging; S404, Fourth-stage forging: The forgings in S403 are reheated in the furnace to 1050℃~1090℃ and held for 1.5~3 hours, then upset and drawn into square cross-section forgings; S405, Five-stage forging: The forgings in S404 are reheated in the furnace to 1050℃~1070℃ and held for 1.5~3 hours. After being drawn out, they are finished into round cross-section forgings.

[0009] Furthermore, in S401, the deformation during the upsetting process in a single forging is 22%~26%.

[0010] Furthermore, in S402, during the second forging process, the deformation during the upsetting process is 40%~48%.

[0011] Furthermore, in S403, during the three-stage forging process, the deformation during the upsetting process is 40%~48%.

[0012] Furthermore, in S405, the deformation amount of the five-stage forging is 72%~80%.

[0013] Furthermore, the composition of ultra-high strength stainless steel, by mass percentage, includes: C: 0.25%~0.30%, Mn: ≤0.01%, Cr: 9.00%~13.0%, Ni: 4.90%~5.50%, Mo: 2.00%~4.00%, V: 0.28%~0.32%, Al: 0.80~1.50%, Co: 14.5%~15.2%, W: 0.8%~1.0%, Ti: ≤0.015%, Nb: 0.01%~0.02%, Si: ≤0.05%, P: ≤0.01%, S: ≤0.001%, with the balance being Fe and other unavoidable impurities.

[0014] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: a) In the preparation method of the present invention, through the systematic coupling of raw material preparation, high-purity steel ingot smelting, homogenization treatment, multi-fire forging, pre-heat treatment and performance heat treatment, the composition fluctuation of ultra-high strength stainless steel bars is ≤0.5wt%, the grain size of the whole cross section is ASTM 7-8 grade, and excellent strength and toughness are guaranteed, while also ensuring excellent corrosion resistance.

[0015] (b) The method of this invention optimizes the selection of slag system and the addition of alloying materials to control the low gas content (H < 0.001%, O < 0.002%) of the initial steel ingot, shortening the subsequent vacuum induction melting time and avoiding Al contamination caused by Al2O3 crucible melting loss, thus laying the compositional foundation for the preparation of high-purity steel ingots. By employing segmented temperature control in vacuum induction melting and multi-parameter synergy in vacuum consumable remelting, precise control of easily segregating elements such as Cr-Ni-Mo is achieved, resulting in uniform steel ingot composition, a macroscopic segregation band width ≤ 50 μm, and a gas content [O] ≤ 15 ppm. Through staged temperature control and slow cooling, the diffusion distance of solute elements covers the dendrite spacing (50~200 μm), completely eliminating as-cast segregation, with compositional fluctuations ≤ 0.5 wt%, providing a uniform microstructure for subsequent forging.

[0016] c) The method of this invention employs a five-stage variable-temperature forging process. The first two stages involve high-temperature forging to break down the as-cast microstructure, while the latter three stages involve low-temperature forging with large deformation to refine the grains, ultimately resulting in a uniform and fine microstructure across the entire cross-section. Through a combination of normalizing and high-temperature tempering, forging stress is eliminated while suppressing grain coarsening, providing a defect-free pre-structure for performance heat treatment. Precise matching of solution treatment and aging processes achieves synergistic regulation of nanoscale precipitates (NiAl / M2C / Laves) and austenite, ultimately improving the material's strength-toughness ratio and enhancing corrosion resistance.

[0017] d) The ultra-high strength stainless steel of the present invention strictly controls the composition range of the core elements C, Cr, Ni, Mo, W, and Al, and precisely controls the synergistic quantitative relationship of different elements to ensure high strength of the material while avoiding excessive M. 23 The formation of C6 and M6C carbides maintains good corrosion resistance and toughness, ultimately resulting in ultra-high strength stainless steel with excellent strength-toughness matching.

[0018] e) The microstructure of the ultra-high strength stainless steel of the present invention is martensite + a small amount of austenite + uniformly dispersed nanoscale precipitates. The grains are fine and uniform, the volume fraction of austenite is 4.0%~6.5%, and the precipitates mainly include M2C phase, Laves phase, and NiAl phase. The above-mentioned microstructure ensures that the ultra-high strength stainless steel of the present application has good strength and toughness and good corrosion resistance.

[0019] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0020] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0021] Figure 1 This is a physical image of the bar material according to Embodiment 1 of the present invention; Figure 2 This is a TEM image of the stainless steel in Example 1 of the present invention; Figure 3 This is a solid solution grain structure diagram of stainless steel according to Example 1 of the present invention; Figure 4 The image shows the morphology of M2C and Laves phases in the microstructure of Embodiment 1 of the present invention. Figure 5 This is a microstructure diagram of the NiAl phase in Example 1 of the present invention. Detailed Implementation

[0022] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0023] This invention provides a method for preparing ultra-high strength stainless steel, comprising the following steps: Step 1: Raw material steel preparation; Step 2: High-purity steel ingot preparation; Step 3: High-purity steel ingot homogenization treatment; Step 4: Bar forging; Step 5: Preliminary heat treatment; Step 6: Performance heat treatment; Performance heat treatment includes solution treatment - first cryogenic treatment - first aging treatment - second cryogenic treatment - second aging treatment; wherein, the holding temperature of the first aging treatment is 480℃~520℃, the holding time is 3~4h, and air cooling is performed; the holding temperature of the second aging treatment is 490~520℃, the holding time is 7~9h, and air cooling is performed.

[0024] Specifically, the composition of the aforementioned ultra-high strength stainless steel, by mass percentage, includes: C: 0.25%~0.30%, Mn: ≤0.01%, Cr: 9.00%~13.0%, Ni: 4.90%~5.50%, Mo: 2.00%~4.00%, V: 0.28%~0.32%, Al: 0.80~1.50%, Co: 14.5%~15.2%, W: 0.8%~1.0%, Ti: ≤0.015%, Nb: 0.01%~0.02%, Si: ≤0.05%, P: ≤0.01%, S: ≤0.001%, with the balance being Fe and other unavoidable impurities.

[0025] Specifically, in the composition of the aforementioned ultra-high strength stainless steel, the Cr / Ni ratio is 1.8 to 2.5, where Cr and Ni are the mass percentages of the corresponding elements.

[0026] Specifically, in the composition of the aforementioned ultra-high strength stainless steel, the C+Cr value is 9.6%~12.9%, where C and Cr are the mass percentages of the corresponding elements.

[0027] Specifically, in order to ensure that the above-mentioned ultra-high strength stainless steel has a good balance of strength and toughness, the value of Ni+Al is controlled at 5.9%~6.28%, where Ni and Al are the mass percentages of the corresponding elements.

[0028] The following details the function and dosage selection of the components contained in this invention: C: Carbon atoms, as important solid solution strengthening elements, significantly improve the strength and hardness of steel. During tempering, supersaturated carbon combines with strong carbide-forming elements (such as V, Mo, Nb, Ti, and Cr) to precipitate fine, dispersed alloy carbides. These nanoscale carbides possess extremely high hardness and thermal stability, strongly pinning dislocations and hindering their movement, thus producing a significant strength peak during tempering. However, excessive carbon content leads to a decrease in the weldability of steel. Furthermore, C has a strong affinity for Cr, forming a series of complex carbides, which in turn reduces the corrosion resistance of the steel. Simultaneously, the formed carbides reduce the toughness of the steel. Considering the above characteristics of carbon in steel and to obtain higher strength, the carbon content in the steel of this invention is controlled at 0.25%~0.30%.

[0029] Cr: Cr improves the hardenability and wear resistance of steel, and enhances its corrosion resistance and oxidation resistance. Cr has a strong affinity for C, promoting carbide formation and inhibiting grain growth under short-term heating. However, excessive carbide formation can disrupt the continuity of the matrix, thus affecting the toughness of the steel. Taking all factors into consideration, the chromium content in the steel of this invention is 9.00%~13.0%.

[0030] Ni: Ni is an important toughening element in stainless steel. Ni is an extremely strong austenite-forming element. At room temperature, it can transform the crystal structure of steel from ferrite (body-centered cubic structure) to austenite (face-centered cubic structure), lowering the ductile-brittle transition temperature and inhibiting crack initiation and propagation. Simultaneously, Ni enhances the passivation ability of stainless steel in reducing media (such as dilute sulfuric acid and organic acids), making the chromium oxide passivation film on the surface more stable and dense, and improving the stainless steel's resistance to cavitation and soil corrosion. In high-strength stainless steel applications, stress corrosion cracking is a common failure mode. Ni can significantly improve the steel's resistance to stress corrosion cracking in chloride environments and other media. However, if the amount of Ni is too high, the steel will lose its quenching ability, resulting in excessive retained austenite in the room temperature microstructure, reducing the steel's yield strength. Conversely, a lower Ni content is detrimental to maintaining toughness and ductility. Furthermore, Ni, as the main forming element of the NiAl phase in steel, can promote the formation of the NiAl phase. The NiAl phase has a low lattice mismatch with the martensitic matrix, providing a high precipitation strengthening increment. Based on the properties of nickel, and combined with the chromium and carbon content added to the steel, in order to ensure the acquisition of lath martensite and toughness, the nickel content in the steel of this invention is 4.90%~5.50%.

[0031] Mo: Mo is a crucial strengthening and corrosion-resistant element in high-strength stainless steel. In ultra-high-strength stainless steel, molybdenum can be expressed as molybdate (MoO4). 2- Molybdenum (Mo) is incorporated into the chromium oxide passivation film on the stainless steel surface in the form of molybdenum, making this protective film more stable, dense, and less susceptible to damage by corrosive media such as chloride ions. When the passivation film is locally damaged, Mo promotes the rapid formation of a new protective film in that area (i.e., "repassivation"), thereby preventing the further expansion of pitting corrosion. This is its core mechanism for resisting pitting corrosion. Mo is a strong carbide-forming element. During tempering or aging treatment, it precipitates in the form of fine, dispersed carbides (such as Mo2C). These precipitates effectively pin dislocations and hinder grain boundary slip, thereby producing a strong precipitation strengthening effect and significantly improving the steel's strength, hardness, and resistance to tempering softening. In addition, Mo can lower the Ms point of stainless steel, resulting in more austenite remaining in the steel after quenching, thus reducing the steel's strength. Therefore, based on the requirements for strength, toughness, and corrosion resistance, the Mo content in the steel of this invention is 2.00%~4.00%.

[0032] Co: In martensitic aging stainless steel, the core value of Co lies in its synergistic effect with Mo and Ti, promoting the precipitation of high-strength, high-toughness nanoscale intermetallic compounds and improving the steel's strength. Simultaneously, Co can inhibit and delay the recovery of martensitic dislocation substructures, maintaining a high dislocation density in martensitic laths, thus providing more nucleation sites for subsequent precipitate precipitation. Co is a strong austenite stabilizer (similar to Ni), significantly expanding the austenite phase region. Co promotes and optimizes the martensitic phase transformation, ensuring a complete and uniform austenitic structure before quenching (at solution treatment temperature). This is crucial for obtaining a fully martensitic matrix, avoiding the formation of harmful δ-ferrite, and thus improving the steel's strength and toughness. Considering both strength and toughness, the Co content in the steel of this invention is controlled at 14.5%~15.2%.

[0033] W: W is an extremely strong carbide-forming element, preferentially combining with C over Cr, mainly forming WC, W2C, and complex M6C (such as Fe3W3C, Fe4W2C) or M. 23 C6 (Cr rich in W) 23 C6-type carbides. These carbides (especially fine MC) strongly pin dislocations, producing a significant precipitation strengthening effect, which is one of the key mechanisms for achieving ultra-high strength. Tungsten carbides (such as W2C) generally have higher thermal stability than molybdenum carbides, maintaining their strengthening effect at higher temperatures and exhibiting stronger resistance to tempering softening. Furthermore, due to the larger size of W atoms, they strongly hinder dislocation slip and climb. In particular, W-containing ultra-high strength stainless steels typically exhibit better strength retention, creep resistance, and endurance strength at high temperatures (>600°C) than molybdenum-containing steels. This is crucial for applications such as aero-engine fasteners, turbine components, and high-temperature chemical equipment. However, adding W significantly increases the risk of forming harmful δ-ferrite. Based on the above characteristics and considering the strengthening effect of carbide formation, the tungsten content in the steel of this invention is controlled at 0.8%–1.0%.

[0034] Al: Al can form extremely fine, dispersed intermetallic compounds Ni3Al, NiAl, and Ni3(Al,Ti) with Ni. The Ni3Al and NiAl precipitates have an ordered superlattice structure, are coherent with the matrix, and can very effectively hinder dislocation movement, resulting in a significant increase in strength. Al is the main forming element of Al2O3, a type D inclusion in ultra-high strength stainless steel. At the same time, Al is a ferrite stabilizing element. The presence of Al2O3 and the formation of ferrite will seriously impair the toughness of the steel. Therefore, considering all factors, the Al content in the steel of this invention is controlled at 0.80~1.50%.

[0035] Si: Si promotes the coarsening of inclusions (such as SiO2) in ultra-high strength stainless steel, leading to a decrease in toughness; Si is a strong ferrite stabilizing element, and the precipitation of ferrite disrupts the continuity of martensite, resulting in a decrease in strength. Therefore, the Si content in the steel of this invention is controlled to be ≤0.05%.

[0036] Mn: Mn is the main forming element of MnS, a type A inclusion in ultra-high strength stainless steel. On the one hand, MnS inclusions induce stress concentration, reducing the fatigue performance of the steel; on the other hand, MnS inclusions in Cl... - Dissolution in the environment forms corrosion pits, reducing the steel's corrosion and stress corrosion resistance. Therefore, the Mn content in the steel of this invention is controlled to be ≤0.01%.

[0037] P: P tends to accumulate at grain boundaries in steel, leading to grain boundary embrittlement, reducing grain boundary bonding energy, increasing temper brittleness, and reducing toughness. Therefore, the P content in the steel of this invention is controlled to be ≤0.01%.

[0038] S: S is the main forming element of MnS in Class A inclusions of ultra-high strength stainless steel. MnS inclusions themselves or at their interface with the base metal preferentially dissolve in corrosive media (especially those containing chloride ions), becoming nucleation sites for pitting corrosion and reducing the steel's corrosion and stress corrosion resistance. Therefore, the S content in the steel of this invention is controlled to be ≤0.001%.

[0039] Ti: Ti has a strong affinity for N and C. Excessive titanium will form coarse primary TiN or Ti(C,N) inclusions (usually formed during the solidification of molten steel) and large secondary TiC precipitates, which seriously impair toughness, plasticity, and fatigue properties. Therefore, the Ti content in the steel of this invention is controlled to be ≤0.015%.

[0040] Specifically, in order to further improve the overall performance of the above-mentioned ultra-high strength stainless steel, the composition of the above-mentioned ultra-high strength stainless steel, by mass percentage, includes: C: 0.25%~0.29%, Mn: ≤0.008%, Cr: 9.45%~12.65%, Ni: 4.90%~5.3%, Mo: 2.5%~3.9%, V: 0.28%~0.32%, Al: 0.80~1.3%, Co: 14.6%~15.2%, W: 0.9%~1.0%, Ti: ≤0.015%, Nb: 0.01%~0.015%, Si: ≤0.05%, P: ≤0.01%, S: ≤0.001%, with the balance being Fe and other unavoidable impurities.

[0041] Preferably, in the composition of the above-mentioned ultra-high strength stainless steel, the C+Cr value is 9.7%~12.9% and the Ni+Al value is 5.9%~6.25%.

[0042] Specifically, in step 1 above, the preparation of raw material steel includes the following steps: S101. Prepare metal raw materials according to the target composition of high-purity steel ingots, including scrap steel, pig iron, Mo-Fe alloy, and Cr-Fe alloy; S102, Charging and Melting: The metal raw materials in S101 are charged into the electric arc furnace in sequence, the electrodes are lowered and electricity is applied to melt them; S103. Slag preparation: The slag composition by mass percentage is: CaF2 and Si-Ca alloy are added to the surface of molten steel in a mass ratio of 7:3 as refining slag. S104. Refining: including oxidative refining and reduction refining. The reduction refining process is judged by the steel slag turning grayish-white. S105. Casting: Molten steel is cast into primary steel ingots using a continuous casting machine.

[0043] Preferably, in step 1, the addition of Mo-Fe and Cr-Fe can reduce the use of molybdenum strips and high-purity metallic chromium in the subsequent process, thereby reducing material costs.

[0044] Specifically, the order of loading metal raw materials is as follows: scrap steel and pig iron are added during the melting period, and ferromolybdenum and ferrochrome are added during the refining period.

[0045] Specifically, the slag composition in S103, by mass percentage, is as follows: CaF2 and Si-Ca alloy are added to the surface of the molten steel in a ratio of 7:3 as refining slag. Through diffusion deoxidation and precipitation deoxidation, oxygen elements are adsorbed into the slag material. In addition, through the argon blowing process at the bottom of the furnace, nitrogen elements in the steel float to the surface along with oxygen elements. Subsequently, vacuum degassing further reduces the content of H, O, and N.

[0046] Specifically, S105 is cast using ingot casting at a rate of 4-5 min / ton. The cooling rate of the initial steel ingot is controlled to be ≤50℃ / h. Too fast a cooling rate leads to the enrichment of elements such as C, P, and S between dendrites, forming banded segregation (which is difficult to eliminate in subsequent heat treatment); too slow a cooling rate leads to severe macroscopic segregation, such as the C and S concentration in the core of the steel ingot reaching twice the average value.

[0047] Specifically, Al is the main forming element of the NiAl / Ni3Al phase, the main precipitated strengthening phase in ultra-high strength stainless steel. In addition, Al is the main forming element of Al2O3, a type D inclusion in ultra-high strength stainless steel. The presence of Al2O3 will seriously damage the toughness of the steel. Therefore, the O content should be strictly controlled during the smelting process to reduce the formation of Al2O3 inclusions. The formation of Al2O3 inclusions seriously damages the toughness and fatigue performance of the finished bar stock.

[0048] To ensure low Al2O3 inclusion content in the high-purity steel ingots produced in step 3, on the one hand, by selecting a reasonable slag system, the low gas content of the initial steel ingots is controlled. For example, in the initial steel ingots, H < 0.001%, N < 0.008%, and O < 0.002%. Controlling the initial steel ingots to have low hydrogen, low nitrogen, and low oxygen content can shorten the vacuum induction smelting time in the subsequent high-purity steel ingot preparation process, preventing the Al2O3 crucible from melting due to excessive smelting time, thus allowing Al to enter the molten steel. On the other hand, the addition of Mo-Fe and Cr-Fe alloy materials can shorten the vacuum induction smelting time in the subsequent high-purity steel ingot preparation process, preventing the Al2O3 crucible from melting due to excessive smelting time, thus allowing Al to enter the molten steel.

[0049] Specifically, step 2, the preparation of high-purity steel ingots, includes vacuum induction melting and vacuum arc remelting. Vacuum induction melting includes the following steps: S201, Raw material preparation: including raw steel prepared by S105, low S, low P, low Si, low Mn, low Al metallic chromium, nickel plates, molybdenum bars, cobalt plates, and high-purity carbon raisers. S202, Furnace loading and material processing: Place the raw materials prepared in S201 into the crucible in sequence, and melt them under vacuum, controlling the vacuum degree to be less than 5Pa. S203 Refining: After the raw materials in S202 are fully melted, adjust the melting temperature to 1510~1580℃ and start refining. Perform electromagnetic stirring for 10 minutes every 20 minutes. Take samples of the composition and gas in front of the furnace. The refining ends after the gas content meets the requirements. S204. Based on the furnace sample taken after the refining period, add materials and stir to obtain a uniform melt; S205, Casting: The melt obtained in S204 is cast into a consumable electrode under vacuum; S206. Cooling and Annealing: After casting, the consumable electrode is cooled under vacuum for 55-65 minutes, then the vacuum is broken and the mold is cooled for 3-5 hours. After demolding, the consumable electrode is sent for hot annealing or slow-cooled for 48 hours before annealing.

[0050] Preferably, the vacuum induction melting in step 2 is carried out in a 6-ton or 12-ton vacuum induction furnace.

[0051] Specifically, the raw material preparation in S201 also includes cleaning the metal materials, alloy materials, and raw steel used. After cleaning, the surface of the raw materials is free of oil, rust, dust, and other dirt. The raw material preparation also includes crushing the primary steel ingots obtained from electric furnace smelting in S105.

[0052] Specifically, the charging sequence in S202 is as follows: 70% of the raw steel is added to the bottom of the crucible first to quickly form a molten pool in the early stages of melting; the second batch consists of high-melting-point and non-oxidizing furnace materials, such as nickel plates, molybdenum bars, cobalt plates, and high-purity carbon raisers, placed in the lower high-temperature zone of the crucible; the remaining raw steel is added to the top layer to ensure that the metal in the middle is fully melted. The charging of raw materials must follow the principle of tighter packing at the top and looser packing at the bottom to avoid bridging. Easily oxidized furnace materials, such as metallic chromium, should be added during the refining stage under conditions of good deoxidation of the molten metal.

[0053] It should be noted that, in order to accurately control the final composition, only a portion of the raw materials are added when S202 is loaded into the furnace, and some elements such as chromium, nickel, molybdenum, cobalt and carbon, which are easily burned or require precise control, are reserved to be added during the refining period.

[0054] Considering that excessive power during the melting period would result in rapid melting, hindering gas removal from the raw materials, while insufficient power would prevent complete melting, the power during the melting period in the S202 furnace is controlled at 500~1000KW when using a 6-ton vacuum induction furnace for melting, for example, 500KW, 600KW, 700KW, 800KW, 900KW, or 1000KW. When using a 12-ton vacuum induction furnace for melting, the power during the melting period in the S202 furnace is controlled at 800~1200KW, for example, 800KW, 900KW, 1000KW, 1100KW, or 1200KW.

[0055] Specifically, in S203, the melting temperatures are 1510℃, 1530℃, 1540℃, 1560℃, 1570℃, and 1580℃.

[0056] It should be noted that the power during the refining period is controlled at 300~500KW, such as 300KW, 400KW, and 5000KW. During the refining period, the composition is finely adjusted according to the real-time detection of chemical composition to achieve the target value.

[0057] Specifically, in S203, the required gas content is: [O]≤0.0015%, [N]≤0.0015%.

[0058] In the vacuum induction melting process, the key operations for controlling the gas content include: (1) rationally selecting the melting power, adjusting the power according to different furnace types (e.g., 500~1000kW for a 6-ton furnace and 800~11200kW for a 12-ton furnace), to avoid excessive power causing the material to melt too quickly and affecting gas removal, or insufficient power causing the raw material to melt incompletely; (2) optimizing the alloy material addition method, reserving easily oxidized metal chromium for addition after good deoxidation during the refining period, and adding high-melting-point and non-oxidized nickel plates, molybdenum bars, and cobalt plates during furnace loading. The material is placed in the high-temperature zone at the bottom of the crucible and a batch feeding strategy is adopted (70% of the raw steel is added at the bottom to form a molten pool, alloy material is added in the middle, and the remaining 30% of the raw steel is covered at the end) to reduce oxidation and gas residue. At the same time, during the refining period, high temperature (1510~1580℃), high vacuum and electromagnetic stirring are used to promote carbon-oxygen reaction, further reducing the gas content ([O]≤0.0015%, [N]≤0.0015%). The refining temperature and chemical composition are adjusted in real time according to the actual composition and gas content in front of the furnace to ensure that the final composition is accurate and controllable.

[0059] Specifically, S205 is cast using the top-pouring method, with a pouring temperature of the steel's melting point +60~70℃, for example, a pouring temperature of 1540~1560℃, such as 1540℃, 1550℃, or 1560℃. The vacuum degree during pouring is less than 5Pa, and the pouring speed is 1~2 tons / minute, such as 1 ton / minute, 1.5 tons / minute, or 2 tons / minute.

[0060] Specifically, the S205 casting process is a crucial step in obtaining consumable electrodes with low segregation, high purity, and high uniformity. When using a 12t vacuum induction furnace for melting, the melt obtained from S204 needs to be transferred to a tundish preheated to above 1000℃ and poured into two molds in two stages to prepare two consumable electrodes. When using a 6t furnace, the melt is directly poured into the mold to prepare one consumable electrode. During the casting process, the nozzle size of the consumable electrode must be strictly controlled to φ40mm. If it is too small, it will cause the molten steel to block and prevent casting; if it is too large, it will cause the molten steel to flow too fast and cause uneven solidification. The tundish must be preheated to above 1000℃; otherwise, the high-temperature molten steel will adhere to the ladle surface and cannot flow smoothly into the mold.

[0061] After casting, the consumable electrode is cooled under vacuum for 55-65 minutes (e.g., 55, 60, 65 minutes) and then the vacuum is broken. The mold is then cooled for 3-5 hours (e.g., 3 hours, 4 hours, 5 hours). After demolding, the consumable electrode is either heated to anneal or slowly cooled for 48 hours before annealing.

[0062] After being cooled under vacuum for 55-65 minutes, the vacuum is broken to allow the molten steel to solidify stably under vacuum conditions, ensuring the quality of the electrode. Slow cooling for 48 hours ensures the ingot uniformly transforms into a martensitic structure, followed by annealing. Annealing eliminates the electrode's microstructure and thermal stress, ensuring a smooth subsequent consumable remelting process.

[0063] Specifically, if the annealing holding temperature in S206 is too high, the electrode will transform from martensite to austenite, and then back to martensite upon cooling. Due to the different crystal structures of martensite and austenite, the phase transformation process will cause the electrode volume to expand and contract, ultimately leading to electrode cracking. If the annealing holding temperature is too low, the stress in the electrode cannot be removed. During the electrode's self-consumption process, the stress release will cause instability in the self-consumption curve, ultimately affecting the metallurgical quality of the self-consumption ingot. Therefore, the annealing holding temperature in S206 should be controlled at 650~680℃, for example, 650℃, 660℃, 670℃, 680℃; and the holding time should be >20h, for example, 22~30h, for example, 22h, 24h, 26h, 28h, 30h.

[0064] Specifically, in step 2, vacuum arc remelting includes the following steps: S207. Electrode preparation: The surface of the consumable electrode after S206 annealing is machined to remove defects. Fe-based dummy electrodes are used as conductive media. The consumable electrode is installed with the filling end facing down and vacuum treatment is performed. S208, Arc Initiation Stage: Confirm that the vacuum level is ≤0.5Pa, the electrodes are aligned, and the water and helium cooling pipes are unobstructed before initiating the arc; S209, Normal smelting stage; S210, Feeding stage: Feeding begins when the electrode weighs 340~350kg. After feeding is completed, a consumable steel ingot is formed. S211, annealing of consumable steel ingots.

[0065] Specifically, the S207 vacuum control limit vacuum degree is ≤0.5Pa, and the leakage rate is ≤2Pa / min.

[0066] Preferably, the S208 arc ignition stage employs current and voltage control. The current setting during the arc ignition stage is 6000~12000A, the voltage setting is 21~24.5V, and the arc ignition time is ≥60min, ensuring the activity of the molten pool and the uniformity of the melting rate. Steel contains elements such as Cr, Ni, Co, and Mo, which are easily segregated and difficult to dissolve, thus requiring high current and high voltage for arc ignition and melting. For example, the current setting during the arc ignition stage is 6000A, 7000A, 8000A, 9000A, 10000A, 11000A, or 12000A, and the voltage setting is 21V, 22V, 23V, 24V, or 24.5V.

[0067] Specifically, in S209, droplet and droplet speed control is used during the normal smelting stage. The smelting speed is set at 3.7~4.7 kg / min, and the droplet time is set at 0.1~0.4 s to ensure the stability of the molten pool. Since the steel contains elements such as Cr, Ni, Co, and Mo, which are easily segregated and difficult to dissolve, a relatively low smelting speed and short droplet time are used during the normal smelting stage to allow the normal electrodes to melt slowly.

[0068] Preferably, in S210, feeding begins when the electrode has 340-350 kg remaining. During the feeding stage, the melting rate and current are reduced: melting rate: 4.2-3.7-3.2-2.7-2.2 kg / min; current: 7000-5000-3000-2000-500A, ensuring the remaining weight is not less than 60 kg. A 660 mm steel ingot should have a 100 mm filling length, and the remaining weight should be at least 20 mm thick. Therefore, feeding begins when the electrode has 340-350 kg remaining, ensuring the remaining weight is not less than 60 kg.

[0069] Specifically, in S211, the annealing of consumable steel ingots includes the following steps: the consumable steel ingot is molded for ≥60 min and then demolded. After demolding, it is slowly cooled in a barrel for ≥36 h (or heated) and then annealed. The holding temperature for annealing is AC1 point -10~50℃, and the holding time is greater than 20 h, for example, 24~30 h.

[0070] Specifically, in S211, the annealing holding temperature is 650~680℃, for example, 650℃, 670℃, 680℃.

[0071] It should be noted that during the S208~S210 smelting process, helium cooling is started when the ingot weight is ≥300kg. This is because excessive helium pressure could break through the solidified steel ingot, causing molten pool leakage and affecting the metallurgical quality of the consumable ingot. Therefore, the helium pressure is set to 300~500Pa, for example, 300Pa, 400Pa, or 500Pa.

[0072] It is worth noting that the vacuum arc remelting process requires monitoring to ensure that the process curve is normal and free from fluctuations.

[0073] Specifically, the diameter of the consumable steel ingot in S211 above is 660mm.

[0074] Specifically, S211 also includes checking whether the surface of the annealed consumable steel ingot is in good condition, free from defects such as cracks and folds, and whether the chemical composition of the ingot meets the standard requirements.

[0075] Specifically, the homogenization treatment of high-purity steel ingots in step 3 includes the following process: preheating below 600℃, heating to 800℃~1000℃ and holding, raising to 1180℃~1250℃ and holding, and slowly cooling to room temperature. To promote the homogenization of the alloy composition of the consumable steel ingot, the temperature is raised to 800℃~1000℃ (e.g., 800℃, 850℃, 900℃, 950℃, 1000℃) at a rate controlled below 80℃ / h (e.g., 50~80℃ / h), and held for at least 2 hours (e.g., 2~5 hours); subsequently, the temperature is raised to 1150℃~1250℃ (e.g., 1150℃, 1180℃, 1200℃, 1220℃, 1250℃) at a rate below 100℃ / h (e.g., 80~100℃ / h), held for at least 30 hours (e.g., 40~60 hours), and then slowly cooled to room temperature.

[0076] Specifically, in step 3, considering that the higher the preheating temperature, the more intense the reaction between the steel ingot surface and the oxidizing atmosphere (air, water vapor, CO2, etc.) in the furnace, the more likely a thick and dense oxide scale will be formed. This not only wastes metal materials but also increases the difficulty and allowance of subsequent forging or machining. Furthermore, excessively high preheating temperatures can cause the temperature of the steel ingot, either locally or overall, to exceed the material's phase transformation temperature (Ac3) or grain coarsening temperature, leading to rapid grain growth and severely reducing the material's plasticity and toughness. During the heating process, the surface of the steel ingot heats up quickly and expands significantly, while the core heats up slowly and expands less. If the preheating temperature is too low, the temperature difference between the surface and the core will be very large, generating enormous thermal stress. When the thermal stress exceeds the strength limit of the steel ingot at that temperature, cracks will occur. In addition, excessively low temperatures can prevent the effective elimination of dendrite segregation and carbide segregation in the core of the steel ingot, making it extremely easy for cracks or performance defects to occur in the core during subsequent forging or rolling. Therefore, the preheating temperature should be controlled at 500~600℃, for example, 500℃, 550℃, 600℃.

[0077] Specifically, the bar forging in step 4 includes the following steps: S401, One-time forging: Heat the steel ingot after homogenization treatment in step 3 to 1150℃~1180℃, hold for 4~7 hours, then upset and draw it into an octagonal cross-section forging with a diameter of φ680mm. S402, Secondary Forging: The forgings in S401 are reheated in the furnace to 1120℃~1140℃ and held for 2~3 hours, then upset and drawn into φ680mm octagonal cross-section forgings; S403, Three-stage forging: The forgings in S402 are reheated in the furnace to 1090℃~1110℃ and held for 1.5~3 hours, then upset and drawn into an octagonal cross-section forging with a diameter of φ680mm; S404, Four-stage forging: The forgings in S403 are reheated in the furnace to 1050℃~1090℃ and held for 1.5~3 hours, then upset and drawn to a square cross-section forging with a side length of 650mm~700mm; S405, Five-stage forging: The forgings in S404 are reheated in the furnace to 1050℃~1070℃ and held for 1.5~3 hours. After being drawn out, they are finished into forgings with a φ300mm round cross section.

[0078] Preferably, during the S401 single-fire forging, the upsetting process reduces the consumable steel ingot from an initial height of 1700mm~1800mm by 600mm~700mm, and after pressing the clamp at the filling end, it is drawn into an octagonal cross-section forging with a diameter of φ680mm. The deformation during the upsetting process in the single-fire forging is controlled to be 22%~26%, for example, 22%, 24%, or 26%. Preferably, the deformation during the upsetting process in the single-fire forging is 22%~24%.

[0079] Specifically, during the S402 two-stage forging, the upsetting process presses the forging down by 600mm~800mm and elongates it to the original size of φ680mm octagonal cross-section forging; the deformation during the upsetting process in the two-stage forging is controlled to be 40%~48%, for example 40%, 42%, 44%, 46%, 48%.

[0080] Specifically, during the S403 three-stage forging process, the upsetting process presses the forging down by 600mm~800mm and elongates it to the original size of φ680mm octagonal cross-section forging. The deformation during the upsetting process in the three-stage forging process is controlled to be 40%~48%, for example, 40%, 42%, 44%, 46%, 48%.

[0081] Specifically, during the four-stage forging of S404, the upsetting process presses the forging down by 600mm~800mm and elongates it into a square cross-section forging with a side length of 650mm~700mm. The deformation during the upsetting process in the four-stage forging is controlled to be 45%~50%, for example, 45%, 47%, 48%, 49%, and 50%.

[0082] It should be noted that when performing the S405 five-stage forging, the material is directly drawn to φ320mm and then rounded, and finished to a φ300mm round cross-section forging. The deformation of the five-stage forging is controlled to be 72%~80%, for example, 72%, 74%, 76%, 78%, 80%.

[0083] In step 4 above, by reducing the forging temperature of each forging pass and increasing the deformation amount of the last forging pass during the bar forging process, the grain structure is refined, and the grain grade from the core to the edge of the forging is ≥6, preferably 7~8.

[0084] In the five-stage forging process of the present invention, the first and second stages mainly involve breaking the as-cast structure, so the forging temperature is relatively high. By reducing the heating temperature of the third, fourth, and fifth stages and increasing the deformation amount of the last stage, a uniform and fine grain structure can be obtained.

[0085] It should be noted that if the forging heating temperature in the first firing stage is too high, oxidation or even localized melting may occur at the grain boundaries, resulting in an "overheating" defect. Even if the overheating temperature is not reached, excessively high temperatures will cause the austenite grains to grow rapidly, leading to a significant decrease in the material's impact toughness and plasticity, increasing the risk of forging cracks. In addition, excessively high temperatures will produce too much δ-ferrite, a brittle phase that will disrupt the continuity of the matrix, causing uneven deformation and significantly increasing the risk of forging cracks. If the forging heating temperature in the first firing stage is too low, the deformation resistance will increase sharply, placing higher demands on the equipment, and may also cause the ingot to be "unbreakable," preventing effective deformation in the first firing stage and resulting in insufficient forging penetration. In the first forging process, excessive deformation during upsetting causes violent metal flow, generating tensile stress that can lead to surface or internal cracking. Simultaneously, excessive deformation causes a rapid temperature rise in the billet, exceeding the allowable final forging temperature and resulting in overheating or burning. Alternatively, it can lead to the formation of quenched martensite during subsequent cooling, increasing the tendency to crack. Excessive deformation can also excessively elongate grains and inclusions, forming a strong fibrous structure, which can cause severe anisotropy in subsequent forming processes. Insufficient deformation, particularly in the upsetting process, prevents effective pressure transfer to the ingot core. This results in the retention of coarse dendrites, network carbides, and other cast structures, causing microstructural inheritance and poor uniformity in subsequent forging processes, ultimately leading to substandard mechanical properties. The triaxial compressive stress generated during upsetting is crucial for preventing internal defects such as porosity, shrinkage cavities, and gas pores in the welded ingot center. Insufficient deformation prevents sufficient plastic flow and atomic diffusion around these defects, allowing them to remain as crack initiators that may propagate during subsequent forging or heat treatment, resulting in failed flaw detection. Therefore, it is necessary to precisely control the process parameters of the above-mentioned single-fire forging.

[0086] It should be noted that the excessively high heating temperatures during the second to fourth forging processes cause the recrystallized grains to grow rapidly, forming coarse austenite grains. This coarse-grained structure is directly inherited after the final heat treatment, leading to a significant decrease in the material's impact toughness and fatigue performance. Furthermore, excessively high local temperatures or uneven deformation can cause abnormal grain growth in some areas, while other areas remain fine-grained, resulting in a mixed grain structure of coarse and fine grains. Conversely, excessively low temperatures significantly increase material strength, making forging difficult and placing heavy loads on equipment. At low temperatures, plasticity decreases, making it easier for transverse or longitudinal cracks to form on the surface during deformation. Additionally, if the temperature is too low, complete dynamic or static recrystallization cannot occur after deformation, leaving elongated, unrefined deformed grains in the microstructure. This leads to severe anisotropy, resulting in coarse or uneven grains even after subsequent heat treatment. The work hardening caused by low-temperature deformation cannot be eliminated through recrystallization, leading to significant residual stress inside the billet, which may trigger delayed cracking during cooling or subsequent heating. Excessive deformation during the upsetting process results in excessively high stored distortion energy, which may trigger secondary recrystallization during subsequent heating. This can lead to a few grains abnormally engulfing surrounding fine grains, forming localized giant grains. Severe deformation can significantly increase the core temperature of the billet (by 50-100°C). If the core temperature exceeds the allowable upper limit of the initial forging temperature, it can cause core overheating or even burning, resulting in irreparable damage. Insufficient deformation, on the other hand, fails to bring the entire cross-section above the critical deformation level. The core may not recrystallize, retaining coarse grains; partial recrystallization may occur on the surface, forming a fine-grained layer. Small deformation results in fewer dynamically recrystallized grains, and insufficient driving force for subsequent static recrystallization. The accumulation of multiple small deformations may, in turn, lead to unstable grain growth due to "strain accumulation." Therefore, precise control of the forging process parameters for the second to fourth forging stages is necessary.

[0087] Excessive heating temperature in the fifth forging pass can cause the already refined grains to grow again, destroying the refining results of the previous passes and leading to coarse grains and decreased toughness and fatigue performance after final heat treatment. High-strength stainless steel exhibits a sharp decrease in plasticity below the final forging temperature, making it highly susceptible to surface transverse cracks or corner cracking during deformation. Furthermore, low-temperature deformation can cause pre-existing micro-defects to expand into central cracks. At excessively low temperatures, recrystallization does not occur after deformation, retaining fibrous, elongated grains. This "cold-hardened" structure may not be completely eliminated during subsequent solution treatment, resulting in severe anisotropy and transverse properties (impact, elongation) far lower than longitudinal properties. Moreover, low-temperature forging generates significant work-hardening residual stress, which may trigger delayed cracking during cooling or subsequent heat treatment, or cause stress corrosion cracking in the parts during use. Excessive deformation during the upsetting process can lead to abnormally large individual grains, forming "coarse grain rings" or isolated coarse grains that cannot be eliminated in subsequent heat treatments and must be scrapped. It can also produce a strong deformation texture (preferred grain orientation), causing extreme anisotropy; for example, high longitudinal impact toughness but very low transverse toughness. Insufficient deformation directly results in coarse grains, or may induce localized coarse grains due to uneven local deformation. Insufficient deformation prevents the deformation from penetrating the entire cross-section, resulting in a layered structure with large surface deformation and small core deformation, leading to extremely uneven properties. Therefore, precise control of the aforementioned forging process parameters is necessary.

[0088] Specifically, the preparatory heat treatment in step 5 includes normalizing and high-temperature tempering.

[0089] Specifically, the normalizing process in step 5 includes the following steps: heating the forged bar to 1000~1020℃, holding the bar at that temperature for no less than 3 hours (e.g., 3~4 hours), and then air cooling to room temperature. Normalizing can promote the homogenization of the grain structure as much as possible while ensuring that the grain structure does not coarsen significantly.

[0090] Specifically, the high-temperature tempering process in step 5 includes the following steps: heating to 660~680℃ and holding for no less than 20 hours (e.g., 22~24 hours), furnace cooling to below 300℃, and air cooling to room temperature.

[0091] Preferably, the high-temperature tempering cooling process in step 5 is furnace cooling to room temperature. The main purpose of the high-temperature tempering process is to eliminate the internal stress caused by the forging process and prevent the bar from cracking.

[0092] It should be noted that the performance heat treatment in step 6 includes solution treatment, primary aging, and secondary aging, specifically including the following steps: S601, Solution treatment: heat preservation temperature 1070℃~1090℃, heat preservation time 70~90min, oil cooling to room temperature, first deep cryogenic treatment temperature -73~-75℃, heat preservation time 8~9h; S602, First aging treatment: Insulation temperature 480℃~520℃, insulation time 3~4h, air cooling; S603, Secondary cryogenic treatment: Insulation temperature -73~-75℃, insulation time 3~4h; S604, Secondary aging treatment: Insulation temperature 490~520℃, insulation time 7~9h, air cooling.

[0093] It should be noted that in S601, excessively high solution treatment temperatures or excessively long holding times can lead to abnormal grain growth, forming coarse austenite grains. This coarse-grained structure will be inherited after aging, resulting in a significant decrease in the material's impact toughness. Excessively high solution temperatures will form a large amount of high-temperature δ-ferrite. This brittle phase remains in the matrix after cooling, disrupting matrix continuity, reducing plasticity and toughness, and potentially inducing microcracks during subsequent aging. Conversely, excessively low temperatures or insufficient time will prevent the complete dissolution of carbides, δ-ferrite, Laves phases, etc., remaining in the forged state. These undissolved phases become crack initiators in the matrix, reducing toughness. Insufficient solution treatment will prevent elements such as Cr, Mo, Ti, and Al from uniformly diffusing into the matrix, resulting in residual microsegregation. This leads to uneven precipitation of strengthening phases during subsequent aging (local depletion, local enrichment), ultimately causing large fluctuations in the material's strength and hardness, and a decrease in corrosion resistance. Therefore, the solution treatment temperature is controlled at 1070℃~1090℃, for example, 1070℃, 1080℃, 1090℃; the holding time is 70~90min, for example, 70min, 80min, 90min.

[0094] It should be noted that in S602, excessively high holding temperatures or excessively long holding times during a single aging treatment can lead to the aggregation and growth of strengthening phase particles, resulting in the loss of coherent or semi-coherent relationships. The dislocation cutting mechanism will switch to a bypass mechanism, significantly reducing the strengthening effect (hardness and strength decrease). After the strengthening phase coarsens, alloying elements in the matrix will further precipitate, weakening the matrix solid solution strengthening effect and causing overall softening. If the aging temperature is too low or the holding time is too short, the amount of strengthening phase precipitated will be insufficient and the particles will be small, failing to achieve the peak strengthening effect, and the strength and hardness of the material will be lower than expected. Low aging temperatures cannot effectively eliminate the thermal and structural stresses generated during solid solution treatment, making parts prone to delayed cracking or stress corrosion cracking during use. Insufficient aging and uneven distribution of the strengthening phase (e.g., more grain boundary precipitation and less intragranular precipitation) will result in large dispersion of material properties and poor stability. Therefore, the temperature for a single aging treatment is controlled at 480℃~520℃, for example, 480℃, 490℃, 500℃, 510℃, 520℃; the holding time is 3~5h, for example, 3h, 3.5h, 4h, 4.5h, 5h.

[0095] It should be noted that in S604, excessively high holding temperatures or excessively long holding times during the secondary aging treatment can lead to the aggregation and growth of strengthening phase particles, resulting in the loss of coherent or semi-coherent relationships. The dislocation cutting mechanism shifts to a bypass mechanism, significantly reducing the strengthening effect (lower hardness and strength). After the strengthening phase coarsens, alloying elements in the matrix further precipitate, weakening the matrix solid solution strengthening effect and causing overall softening. Conversely, excessively low aging temperatures or short holding times result in insufficient strengthening phase precipitation and small particles, failing to achieve the peak strengthening effect, leading to lower-than-expected strength and hardness. Low aging temperatures cannot effectively eliminate the thermal and structural stresses generated during solution treatment, making parts prone to delayed cracking or stress corrosion cracking during use. Insufficient aging leads to uneven distribution of the strengthening phase (e.g., more precipitation at grain boundaries and less within grains), resulting in high material property dispersion and poor stability. Therefore, the temperature for the secondary aging treatment is controlled at 490℃~520℃, for example, 490℃, 500℃, 510℃, 520℃; the holding time is 7~9h, for example, 7h, 8h, 9h.

[0096] Preferably, in S604, the temperature of the secondary aging treatment is 510℃.

[0097] Specifically, the microstructure of the aforementioned ultra-high strength stainless steel consists of martensite + a small amount of austenite + uniformly dispersed nanoscale precipitates, with fine and uniform grains and a grain size of 7 to 8.

[0098] Specifically, in the microstructure of the aforementioned ultra-high strength stainless steel, the volume fraction of austenite is 4.0% to 6.5%, for example, 4.5% to 6.5%.

[0099] Specifically, in the microstructure of the aforementioned ultra-high strength stainless steel, the precipitated phases mainly include M2C phase, Laves phase, and NiAl phase; among which, the mass fraction of M2C phase is 0.9~1.5%, the mass fraction of Laves phase is 5.9~6.8%, and the mass fraction of NiAl phase is 2.0~3.0%.

[0100] Specifically, in the microstructure of the aforementioned ultra-high strength stainless steel, the M2C phase is needle-shaped, rich in C, Cr, and Mo, and its size is mainly distributed in the range of 10~30 nm; the laves phase is spherical, rich in Mo and Fe, and its size is mainly distributed in the range of 10~20 nm; and the NiAl phase is mainly distributed in the range of 5~15 nm.

[0101] Preferably, in the microstructure of the above-mentioned ultra-high strength stainless steel, the mass fraction of the M2C phase is 0.9~1.4%, the mass fraction of the laves phase is 5.9~6.7%, and the mass fraction of the NiAl phase is 2.1~3.0%.

[0102] Specifically, the ultra-high strength stainless steel of this invention exhibits good strength and toughness, and excellent corrosion resistance. For example, its tensile strength is above 2400 MPa, such as 2420~2450 MPa; its yield strength is above 1980 MPa, such as 1982~2100 MPa; its elongation after fracture is above 9%, such as 9%~10.5%; its reduction of area is above 40%, such as 42%~45%; and its fracture toughness is 42 MPa·m. 1 / 2 The above, for example, 42.9~45.6 MPa·m 1 / 2 The pitting potential is 0.132~0.148 V. SEC .

[0103] The preparation method of the ultra-high strength stainless steel of the present invention will be described below with reference to specific embodiments.

[0104] Examples 1-4 of the present invention provide a method for preparing ultra-high strength stainless steel. The chemical composition of the stainless steel in Examples 1-4 is shown in Table 1. The raw material steel preparation process parameters and the element control of the primary steel ingot are shown in Table 2. The vacuum induction melting process parameters are shown in Table 3. The vacuum self-consumable remelting process parameters are shown in Table 4. The forging process parameters are shown in Table 5. The heat treatment process parameters are shown in Table 6. The properties of the stainless steel bar are shown in Table 7. The microstructure is shown in Table 8.

[0105] The preparation methods of the ultra-high strength stainless steel in Examples 1-4 include the following steps: Step 1, Preparation of raw material steel: S101. Prepare metal raw materials according to the target composition of high-purity steel ingots, including scrap steel, pig iron, Mo-Fe alloy, and Cr-Fe alloy; S102, Charging and Melting: The metal raw materials in S102 are charged into the electric arc furnace in sequence, the electrodes are lowered and electricity is applied to melt them; S103. Slag preparation: The slag composition by mass percentage is: CaF2 and Si-Ca alloy are added to the surface of molten steel in a mass ratio of 7:3 as refining slag. S104. Refining: including oxidative refining and reduction refining. The reduction refining process is judged by the steel slag turning grayish-white. S105, Casting: Molten steel is cast into primary steel ingots using a continuous casting machine; Step 2, Preparation of high-purity steel ingots: S201, Raw material preparation: including raw steel prepared by S105, low S, P, Si, Mn, Al metallic chromium, nickel plates, molybdenum bars, cobalt plates, and high-purity carbon raisers; S202, Furnace loading and material processing: Place the raw materials prepared in S201 into the crucible in sequence, and melt them under vacuum, controlling the vacuum degree to be less than 5Pa. S203 Refining: After the raw materials are fully melted, adjust the melting temperature to 1510-1580℃ and start refining. Stir electromagnetically for 10 minutes every 20 minutes. Take samples of the composition and gas in front of the furnace. Refining ends after the gas content meets the requirements. S204. Composition Adjustment: Based on the furnace sample taken after the refining period, add materials and stir to obtain a uniform melt; S205, Casting: The melt obtained in S204 is cast into a φ580mm consumable electrode under vacuum; the vacuum degree is less than 5Pa, and the casting speed is 1~2 tons / minute; S206. Cooling and Annealing: After casting, the consumable electrode is cooled under vacuum for 55-65 minutes, then the vacuum is broken, the mold is cooled for 3-5 hours, and after demolding, the consumable electrode is sent for hot annealing or slow-cooled for 48 hours before annealing. S207. Electrode preparation: The surface of the consumable electrode after S206 annealing is machined to remove defects. Fe-based dummy electrodes are used as conductive media. The consumable electrode is installed with the filling end facing down and vacuum treatment is performed. S208, Arc Initiation Stage: Confirm vacuum level ≤ 0.5 Pa, electrode alignment, and unobstructed water and helium cooling pipes before initiating the arc; S209, Normal smelting stage; S210, Feeding stage: Feeding begins when the electrode has 340kg remaining. After feeding is completed, a consumable steel ingot is formed. S211, Consumable steel ingot annealing; Step 3: Homogenization treatment of high-purity steel ingots; Step 4, bar forging: S401, One-time forging: Heat the consumable steel ingot obtained in step 2 to 1150℃~1180℃, hold for 4~7 hours, then upset and draw it into an octagonal cross-section forging with a diameter of φ680mm. S402, Secondary Forging: The forgings in S401 are reheated in the furnace to 1120℃~1140℃ and held for 2~3 hours, then upset and drawn into φ680mm octagonal cross-section forgings; S403, Three-stage forging: The forgings in S402 are reheated in the furnace to 1090℃~1110℃ and held for 1.5~3 hours, then upset and drawn into an octagonal cross-section forging with a diameter of φ680mm; S404, Four-stage forging: The forgings in S403 are reheated in the furnace to 1050℃~1090℃ and held for 1.5~3 hours, then upset and drawn to a square cross-section forging with a side length of 650mm~700mm; S405, Five-stage forging: The forgings in S404 are reheated in the furnace to 1050℃~1070℃ and held for 1.5~3 hours. After being drawn out, they are finished into forgings with a φ300mm round cross section. Step 5: Preparatory heat treatment; Step 6, Performance Heat Treatment: S601, Solution treatment: Temperature 1070℃~1090℃, holding time 70~90min, oil cooling to room temperature, first cryogenic treatment temperature -73~-75℃, holding time 8~9h; S602, Single aging treatment: Temperature 480℃~520℃, holding time 3~5h, air cooling; S603, Secondary cryogenic treatment: Temperature -73~-75℃, holding time is 3~4h; S604, Secondary aging treatment: Temperature 490~520℃, holding time 7~9h, air cooling.

[0106] like Figure 4 The image shown is a microstructure of the M2C and Laves phases in Example 1. Figure 5 The image shows the NiAl phase morphology in the microstructure of Example 1. It can be seen that the stainless steel of this invention contains three nanoscale precipitates: needle-like precipitates and two spherical precipitates of different sizes. The spherical precipitates within the red box are the Laves phase, and the needle-like precipitates within the yellow box are the M2C carbide precipitates. The corresponding EDS surface scan results show that the M2C phase is rich in C, Cr, and Mo, with a size mainly distributed in the range of 10–30 nm; the Laves phase is rich in Mo and Fe, with a size mainly distributed in the range of 10–20 nm; and the NiAl phase is mainly distributed in the range of 5–15 nm.

[0107] The inventors conducted extensive experimental research during the research process, and some solutions with suboptimal results are now presented as comparative examples.

[0108] The difference between Comparative Examples 1-4 and Example 1 lies in the alloy design composition.

[0109] In Comparative Example 1, the Cr / Ni ratio does not meet the requirement of 1.8~2.5; in Comparative Example 2, the V content does not meet the requirements of this invention; in Comparative Example 3, the Mo content does not meet the requirements of this invention; and in Comparative Example 4, the Al content does not meet the requirements of this invention.

[0110] The difference between Comparative Examples 5-12 and Example 1 lies in the preparation parameters.

[0111] In Comparative Example 5, the H, N, and O content in the initial steel ingot was too high, leading to an increase in inclusion content, disrupting the continuity of the matrix, and consequently resulting in lower toughness. In Comparative Example 6, the solution treatment temperature was too low; in Comparative Example 7, the solution treatment holding time was too short; in Comparative Example 8, the primary aging temperature was too low; in Comparative Example 9, the primary aging holding time was too short; in Comparative Example 10, the secondary aging holding time was too short; in Comparative Example 11, the forging temperatures for the third, fourth, and fifth heat treatments were too high; in Comparative Example 12, the deformation amount in the fifth heat treatment was low. The chemical composition of the comparative example is shown in Table 1, the raw material steel preparation process parameters and the element control of the primary steel ingot are shown in Table 2, the vacuum induction melting process parameters are shown in Table 3, the vacuum arc remelting process parameters are shown in Table 4, the forging process parameters are shown in Table 5, the heat treatment process parameters are shown in Table 6, and the properties of stainless steel bars are shown in Table 7.

[0112] Table 1 Chemical composition, wt%

[0113] Table 2. Element control status of primary steel ingots, wt%

[0114] Table 3 Vacuum Induction Melting Process Parameters

[0115] Table 4 Vacuum self-consuming remelting process parameters

[0116] Table 5 Specific process parameters for forging

[0117] Table 6 Heat treatment process parameters

[0118] Table 7 Performance parameters of stainless steel bars

[0119] Table 8 Microstructure results of stainless steel

[0120] In the preparation method of this invention, through the systematic coupling of raw material preparation, high-purity steel ingot smelting, homogenization treatment, multi-fire forging, pre-heat treatment, and performance heat treatment, ultra-high strength stainless steel bars are achieved with a compositional fluctuation of ≤0.5wt%, a full-section grain size of ASTM 7-8, and excellent strength and toughness, while also ensuring excellent corrosion resistance. For example, the tensile strength is above 2400MPa, such as 2420~2450MPa; the yield strength is above 1980MPa, such as 1982~2100MPa; the elongation after fracture is above 9%, such as 9%~10.5%; the reduction of area is above 40%, such as 42%~45%; and the fracture toughness is 42MPa·m. 1 / 2 The above, for example, 42.9~45.6 MPa·m 1 / 2 The pitting potential is 0.132~0.148 V. SEC It solved the problem of strength and toughness imbalance and segregation control in large-diameter bars (Φ≥300mm).

[0121] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing ultra-high strength stainless steel, characterized in that, The preparation method includes the following steps: Step 1: Preparation of raw material steel; Step 2: Preparation of high-purity steel ingots; Step 3: Homogenization treatment of high-purity steel ingots; Step 4: Bar forging; Step 5: Preparatory heat treatment; Step 6, Performance heat treatment; Performance heat treatment includes solution treatment - first cryogenic treatment - first aging treatment - second cryogenic treatment - second aging treatment; wherein, the holding temperature for the first aging treatment is 480℃~520℃; the holding temperature for the second aging treatment is 490~520℃.

2. The preparation method according to claim 1, characterized in that, In step 6, the solution treatment includes the following steps: the holding temperature is 1070℃~1090℃, the holding time is 70~90min, and the solution is cooled to room temperature.

3. The preparation method according to claim 1, characterized in that, In step 6, the temperature for the first cryogenic treatment is -73 to -75°C, and the holding time is 8 to 9 hours.

4. The preparation method according to claim 1, characterized in that, In step 6, the holding temperature for the secondary cryogenic treatment is -73 to -75°C, and the holding time is 3 to 4 hours.

5. The preparation method according to claim 1, characterized in that, In step 4, the bar forging includes the following steps: S401, One-time forging: Heat the steel ingot after homogenization treatment in step 3 to 1150℃~1180℃, hold for 4~7 hours, then upset and draw into an octagonal cross-section forging; S402, Secondary Forging: The forgings in S401 are reheated in the furnace to 1120℃~1140℃ and held for 2~3 hours, then upset and drawn into an octagonal cross-section forging; S403, Three-stage forging: The forgings in S402 are reheated in the furnace to 1090℃~1110℃ and held for 1.5~3 hours, then upset and drawn into an octagonal cross-section forging; S404, Fourth-stage forging: The forgings in S403 are reheated in the furnace to 1050℃~1090℃ and held for 1.5~3 hours, then upset and drawn into square cross-section forgings; S405, Five-stage forging: The forgings in S404 are reheated in the furnace to 1050℃~1070℃ and held for 1.5~3 hours. After being drawn out, they are finished into round cross-section forgings.

6. The preparation method according to claim 5, characterized in that, In S401, during the single forging process, the deformation amount during the upsetting process is 22%~26%.

7. The preparation method according to claim 5, characterized in that, In S402, during the second forging process, the deformation amount during the upsetting process is 40%~48%.

8. The preparation method according to claim 5, characterized in that, In S403, during the three-stage forging process, the deformation during the upsetting process is 40%~48%.

9. The preparation method according to claim 5, characterized in that, In S405, the deformation amount of the five-stage forging is 72%~80%.

10. The preparation method according to any one of claims 1 to 9, characterized in that, The composition of the ultra-high strength stainless steel, by mass percentage, includes: C: 0.25%~0.30%, Mn: ≤0.01%, Cr: 9.00%~13.0%, Ni: 4.90%~5.50%, Mo: 2.00%~4.00%, V: 0.28%~0.32%, Al: 0.80~1.50%, Co: 14.5%~15.2%, W: 0.8%~1.0%, Ti: ≤0.015%, Nb: 0.01%~0.02%, Si: ≤0.05%, P: ≤0.01%, S: ≤0.001%, with the balance being Fe and other unavoidable impurities.

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