Austenitic steel with high stress relaxation resistance for lead-based reactor fasteners and method for its production

Abstract

The present application belongs to the field of austenitic stainless steel structural materials for high temperature, and particularly relates to a lead-based high stress relaxation resistant austenitic steel for fastener and a preparation method thereof. The chemical composition of the steel is as follows in terms of percentage by weight: C 0.007-0.03%; Si 3.0-4.0%; Mn ≤1.0%; S ≤0.005%; P ≤0.01%; Cr 15.0-17.0%; Ni 23.0-27.0%; Mo 0.5-2.0%; Ti 1.0-2.0%; Al 0.2-0.4%; O ≤0.003%; N ≤0.005%; and the balance being Fe. The Si element with strong oxidation resistance is added to the austenitic steel, and the preparation method comprises the following steps: batching, vacuum induction furnace smelting, vacuum consumable, low-temperature forging, solid solution treatment and aging treatment. The high stress relaxation resistant austenitic steel is successfully controlled, and the dual-scale nanoscale G phase (Ni 16 Ti6Si7) and Ni3(Al,Ti) phase synergistic strengthening is obtained through aging treatment, which improves the stress relaxation resistance of the austenitic steel. Meanwhile, the high Si ensures the lead-bismuth corrosion resistance of the steel, and solves the problem that the traditional austenitic steel for fastener cannot simultaneously satisfy the high stress relaxation resistance and the lead-bismuth corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature austenitic stainless steel structural materials, specifically relating to a high stress relaxation resistant austenitic steel for lead-based fasteners and its preparation method. Background Technology

[0002] Lead-cooled fast reactors (LTRs) exhibit enormous development potential due to their excellent neutron economy, sustainable development, high chemical stability, and safety. However, the internal operating environment of LTRs is extremely harsh, and fasteners are crucial structural components within the reactor, undertaking the vital tasks of connection and fixation. Only by simultaneously meeting requirements such as high-temperature resistance, stress relaxation resistance, and resistance to liquid lead-bismuth corrosion can the safe operation of the internal structural components be guaranteed.

[0003] Currently, while the austenitic stainless steels initially selected for fasteners both domestically and internationally possess certain resistance to lead-bismuth corrosion, their stress relaxation resistance is not ideal. Invention patent publications CN114657475A, CN114574765A, and CN114657465A all provide stress relaxation results for austenitic steels used in lead-based fasteners, but their stress relaxation resistance still fails to meet service requirements. Therefore, to ensure the reliability and durability of lead-based fasteners, it is necessary to develop an austenitic stainless steel for fasteners that combines higher stress relaxation resistance with resistance to liquid lead-bismuth corrosion through compositional design and microstructure control. This would overcome the technical barrier of insufficient stress relaxation resistance in existing structural materials for fasteners and provide a theoretical basis for future material design. Summary of the Invention

[0004] The purpose of this invention is to provide a high-stress-relaxation-resistant austenitic steel for lead-based fasteners and its preparation method, thereby obtaining austenitic stainless steel materials for fasteners that possess both excellent stress-relaxation resistance and resistance to liquid lead-bismuth corrosion.

[0005] The technical solution of this invention is:

[0006] A high-stress-relaxation-resistant austenitic steel for lead-based fasteners, the chemical composition of which is as follows by weight percentage:

[0007] C: 0.007–0.03%; Si: 3.0–4.0%; Mn ≤ 1.0%; S ≤ 0.005%; P ≤ 0.01%; Cr: 15.0–17.0%; Ni: 23.0–27.0%; Mo: 0.5–2.0%; Ti: 1.0–2.0%; Al: 0.2–0.4%; O ≤ 0.003%; N ≤ 0.005%; balance Fe.

[0008] The lead-based fasteners are made of austenitic steel with high resistance to stress relaxation, and the chromium equivalent is calculated according to formula (1):

[0009] Cr equivalent = 100 × (Cr + 1.5 × Mo + 2 × Si + 1.5 × Ti + 5.5 × Al) (1)

[0010] Nickel equivalent is calculated according to formula (2):

[0011] Ni equivalent = 100 × (Ni + 30 × C + 0.5 × Mn) (2)

[0012] The Cr equivalent and Ni equivalent satisfy the following conditions: Cr equivalent < 28; Ni equivalent > 23.

[0013] The lead-based fasteners are made of austenitic steel with high resistance to stress relaxation, and the microstructure of the steel is mono-austenitic.

[0014] The method for preparing austenitic steel with high resistance to stress relaxation for lead-based fasteners includes the following steps:

[0015] (1) Dual vacuum melting: Raw materials are proportioned according to the target chemical composition requirements, and the required steel ingots are obtained through a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting.

[0016] (2) Homogenization treatment: The smelted ingots are subjected to high-temperature homogenization treatment, and the ingots are heated to 1100-1150℃ and held for 8-48 hours.

[0017] (3) Hot forging: After the high temperature homogenization, the steel ingot is held at 1050-1100℃ for 2-8 hours and then forged. The initial forging temperature is 1020-1050℃ and the final forging temperature is 900-950℃. During the forging, the steel ingot is repeatedly forged with large reduction in the longitudinal-transverse-longitudinal directions. The number of repetitions is not less than 6. The deformation amount of a single forging is ≥10%, the total deformation amount is ≥80%, and the forging ratio is ≥6. After forging, the steel ingot is air-cooled to room temperature to obtain a forged bar.

[0018] (4) Heat treatment: The forged bar is subjected to solution treatment and aging treatment after forging.

[0019] The preferred method for preparing austenitic steel with high stress relaxation resistance for lead-based fasteners is as follows: single deformation amount is about 12%, total deformation amount is about 85%, and forging ratio is about 8%.

[0020] The method for preparing high stress relaxation resistant austenitic steel for lead-based fasteners, wherein the heat treatment process in step (4) is as follows:

[0021] (1) Solution treatment: keep at 1050-1180℃ for 1-4 hours, then water cool;

[0022] (2) The aging treatment is as follows: keep at 700-750℃ for 4-16 hours and then air cool; then keep at 550-650℃ for 16-48 hours and then air cool.

[0023] The method for preparing austenitic steel with high stress relaxation resistance for lead-based fasteners, wherein the steel has a residual stress of not less than 100 MPa after being held at 550°C and with an initial stress of 120 MPa for 400 hours.

[0024] The method for preparing austenitic steel with high resistance to stress relaxation for lead-based fasteners, wherein the steel, after being corroded in a liquid lead-bismuth eutectic alloy (45% Pb-Bi) at 550℃ for 3000 hours under saturated oxygen conditions, has an oxide film thickness of no more than 25 μm and exhibits excellent resistance to liquid lead-bismuth corrosion.

[0025] The method for preparing high-stress-relaxation-resistant austenitic steel for lead-based fasteners involves aging treatment to obtain a dual-scale nanoscale G phase (Ni). 16 Ti6Si7) and Ni3(Al,Ti) phases, dual-scale nanoscale G phase (Ni 16 The Ti6Si7 and Ni3(Al,Ti) phases are uniformly dispersed, with the G phase (Ni 16 The size of Ti6Si7 phase is 100–300 nm, and the size of Ni3(Al,Ti) phase is 10–50 nm.

[0026] The design concept of this invention has three aspects, as described below:

[0027] 1. This invention uses a combination of vacuum induction furnace melting and vacuum consumable metallurgy, and employs low-temperature forging to prepare austenitic stainless steel with high Si content, thus avoiding defects such as cracks during hot working.

[0028] 2. This invention utilizes a combination of solution treatment and aging to control the precipitation-strengthened phase, which exhibits excellent high-temperature stability, including the G phase (Ni). 16 The Ti6Si7) and Ni3(Al,Ti) phases significantly improve the strength of the austenitic matrix, thereby ensuring higher stress relaxation resistance of austenitic stainless steel.

[0029] 3. This invention further improves the resistance of austenitic stainless steel to liquid lead-bismuth corrosion by increasing the Si content (3.0-4.0 wt.%), thereby achieving excellent resistance to liquid lead-bismuth corrosion while maintaining higher stress relaxation resistance.

[0030] The content of the main elements in this invention is described as follows:

[0031] C: 0.007~0.03%

[0032] In this invention, to allow more Ti to participate in the formation of the G phase (Ni) in the steel 16 For the Ti6Si7 and Ni3(Al,Ti) phases, the C content needs to be strictly controlled. C has a strong affinity for Ti, easily forming TiC, while excessive C content will react with Cr in the steel to form M.23 C6 affects the formation of the Cr2O3 passivation film and can actually worsen the steel's resistance to liquid lead-bismuth corrosion. Therefore, the C content in this invention is 0.007–0.03%.

[0033] Ti: 1.0~2.0%; Al: 0.2~0.4%;

[0034] Ti and Al are constituent elements of the Ni3(Al,Ti) phase. The content of Ti and Al plays a crucial role in the steel of this invention. The content of these two elements must be maintained in a certain ratio and within a certain range; otherwise, defects may occur or the alloy properties may be reduced. Therefore, in this invention, the Ti content is controlled at 1.0–2.0%, and the Al content is controlled at 0.2–0.4%.

[0035] Si: 3.0–4.0 wt%

[0036] The higher the Si content, the better the resistance to liquid lead-bismuth corrosion. Si readily combines with O to form a stable SiO2 oxide film. Therefore, adding Si to steel will preferentially form a continuous and dense Si-containing oxide barrier under high-temperature and oxygen-containing environments, which can hinder further corrosion by liquid lead-bismuth and ensure the steel's excellent resistance to liquid lead-bismuth corrosion. However, excessive Si content will form silicides, deteriorating the mechanical and corrosion resistance properties of the steel. Therefore, considering all factors, the Si content in this invention is 3.0–4.0 wt%, preferably 3.5–4.0 wt%.

[0037] Cr: 15.0–17.0 wt%

[0038] Cr is one of the fundamental elements in austenitic stainless steel. The stainless and corrosion-resistant properties of austenitic stainless steel are mainly due to Cr promoting passivation and maintaining the steel in a stable passive state. Similarly, this effect of Cr results in a continuous and dense Cr₂O₃ passivation film on the steel surface, which hinders ion migration and the dissolution of elements into liquid lead and bismuth, thereby improving the steel's resistance to liquid lead and bismuth corrosion. This effect of Cr is mutually reinforcing with that of Si, further enhancing the resistance to liquid lead and bismuth corrosion. However, Cr is a ferrite-forming element and readily forms molybdenum (M) with C. 23 C6. Therefore, the Cr content is controlled between 15.0 and 17.0 wt%.

[0039] Ni: 23.0–27.0 wt%

[0040] Ni is another fundamental element in austenitic stainless steel. Its main function is to form and stabilize austenite, giving the steel a fully austenitic microstructure and improving the thermodynamic stability of austenitic stainless steel. Ni also forms the G phase (Ni... 16The Ni content in the steel of this invention is controlled at 23.0–27.0 wt%, primarily forming the Ti6Si7 and Ni3(Al,Ti) phases, and plays a role in precipitation hardening. Therefore, taking all factors into consideration, the Ni content in the steel of this invention is controlled at 23.0–27.0 wt%.

[0041] Mo: 0.5–2.0 wt%

[0042] Mo is an element that forms, stabilizes, and expands the ferrite phase region. To maintain a single austenitic structure, the addition of Mo to the steel of this invention is accompanied by an increase in the Ni content. The main role of Mo in the steel of this invention is to improve the high-temperature strength of the steel. As the Mo content in the steel increases, the high-temperature creep resistance of the steel improves, but Mo promotes the precipitation of intermetallic phases in austenitic stainless steel, such as the σ phase and the Laves phase, reducing the stability of the microstructure. Therefore, considering all factors, the Mo content in the steel of this invention is 0.5–1.0 wt%.

[0043] The advantages and beneficial effects of this invention are:

[0044] 1. The steel of this invention achieves a dual-scale nanoscale G phase (Ni) by adding Si, Ti, and Al elements and combining them with an aging heat treatment process. 16 The uniform dispersion of Ti6Si7) and Ni3(Al,Ti) phases, with the G phase (Ni 16 The Ti6Si7 phase has a size of 100–300 nm, while the Ni3(Al,Ti) phase has a size of 10–50 nm. The synergistic strengthening of the two phases improves the stress relaxation resistance of the steel.

[0045] 2. By increasing the Si content, the steel of this invention forms a continuous and dense SiO2 oxide film under saturated oxygen conditions of liquid lead-bismuth at 550℃. Together with the Cr2O3 oxide film, it ensures the steel's oxidation resistance and resistance to liquid lead-bismuth corrosion. This successfully produces an austenitic stainless steel that combines high stress relaxation resistance and resistance to liquid lead-bismuth corrosion, breaking through the technical barrier that the structural materials initially selected for fasteners could not simultaneously achieve higher stress relaxation resistance and resistance to liquid lead-bismuth corrosion. Attached Figure Description

[0046] Figure 1 This is a microstructure diagram of the G phase after solution treatment and aging treatment in Example 3.

[0047] Figure 2 This is a microstructure diagram of the Ni3(Al,Ti) phase after solution treatment and aging treatment in Example 3.

[0048] Figure 3 The graph shows the relationship between residual stress and time for steels of Example 3, Comparative Example 1, and Comparative Example 3 at 550°C and with an initial stress of 120 MPa.

[0049] Figure 4The oxide film morphology of Example 3 after corrosion in a liquid lead-bismuth alloy (45% Pb-Bi) at 550°C and saturated oxygen concentration for 3000 hours.

[0050] Figure 5 The oxide film morphology of Comparative Example 1 after corrosion in liquid lead-bismuth alloy (45% Pb-Bi) at 550℃ with saturated oxygen concentration for 3000 hours. Detailed Implementation

[0051] The following embodiments will further supplement the description of the present invention. The steel in the embodiments and the steel in the comparative examples were all melted in a vacuum induction furnace and smelted in a vacuum arc remelting process, forged and subsequently heat-treated, and then processed into stress relaxation and corrosion samples, and finally subjected to performance testing.

[0052] In specific implementation, the preparation method of the high stress relaxation resistant austenitic steel for lead-based fasteners of the present invention (example) is as follows:

[0053] (1) Steel ingots are obtained by mixing raw materials according to the chemical composition described in this invention and then performing vacuum induction smelting and casting.

[0054] (2) Remove the surface oxide scale from the steel ingot obtained by vacuum induction smelting and cut both ends flat to make a consumable electrode rod;

[0055] (3) The consumable electrode rod is further purified and smelted in a vacuum consumable melting furnace to obtain a high-purity consumable ingot.

[0056] (4) The ingot is homogenized at 1150℃ and held for 8 hours. Then it is held at 1050℃ for 8 hours before forging. The initial forging temperature is 1030℃. The initial forging is carried out by repeated large-reduction forging in three directions: longitudinal, transverse and longitudinal. The cycle is repeated 6 times. The deformation amount of each forging is about 12%, the total deformation amount is about 85%, and the total forging ratio is about 8. Then it is forged into a Φ25mm round bar. The final forging temperature is 920℃. After forging, it is air-cooled to room temperature.

[0057] (5) The forged round bars were cut and subjected to solution aging treatment. First, they were held at 1150℃ for 1 hour and then water-cooled to room temperature. Then, they were held at 700℃ for 16 hours and air-cooled to room temperature. Finally, they were held at 600℃ for 48 hours and air-cooled to room temperature to obtain a uniformly dispersed precipitation-hardened phase G (Ni). 16 Ti6Si7) and Ni3(Al,Ti) phases;

[0058] (6) Cut relevant performance samples from the heat-treated bar stock and conduct mechanical property tests.

[0059] The present invention will now be described through different embodiments and comparative examples. These embodiments are for illustrative purposes only, and the present invention is not limited to these embodiments.

[0060] Example 1

[0061] In this example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.008%; Si: 3.77%; Mn: 0.56%; S: 0.0017%; P: 0.009%; Cr: 16.46%; Ni: 24.4%; Mo: 1.19%; Ti: 1.68%; Al: 0.28%; O: 0.002%; N: 0.005%; balance Fe. The Cr equivalent is 29.85 ≥ 28, and the Ni equivalent is 24.92 > 23.

[0062] Example 2

[0063] In this example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.015%; Si: 3.01%; Mn: 0.54%; S: 0.001%; ​​P: 0.007%; Cr: 15.17%; Ni: 25.3%; Mo: 1.17%; Ti: 1.58%; Al: 0.33%; O: 0.002%; N: 0.003%; balance Fe. The Cr equivalent is 27.13 < 28, and the Ni equivalent is 26.02 > 23.

[0064] Example 3

[0065] In this example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.012%; Si: 3.49%; Mn: 0.44%; S: 0.002%; P: 0.006%; Cr: 15.46%; Ni: 24.4%; Mo: 0.97%; Ti: 1.55%; Al: 0.32%; O: 0.0029%; N: 0.0045%; balance Fe. The Cr equivalent is 27.98 < 28, and the Ni equivalent is 24.98 > 23.

[0066] Example 4

[0067] In this example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.0074%; Si: 3.52%; Mn: 0.56%; S: 0.001%; ​​P: 0.006%; Cr: 14.96%; Ni: 24.8%; Mo: 1.01%; Ti: 1.03%; Al: 0.24%; O: 0.001%; ​​N: 0.0038%; the balance is Fe. The Cr equivalent is 26.38 < 28, and the Ni equivalent is 25.30 > 23.

[0068] Example 5

[0069] In this example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.008%; Si: 3.66%; Mn: 0.60%; S: 0.0015%; P: 0.006%; Cr: 15.02%; Ni: 24%; Mo: 1.16%; Ti: 1.22%; Al: 0.28%; O: 0.0018%; N: 0.0035%; balance Fe. The Cr equivalent is 27.45 < 28, and the Ni equivalent is 24.54 > 23.

[0070] Comparative Example 1

[0071] In this comparative example, the steel's chemical composition did not include a certain amount of Si. The chemical composition of the steel, by weight percentage, was: C: 0.0078%; Si: 0.35%; Mn: 0.56%; S: 0.0018%; P: 0.008%; Cr: 15.19%; Ni: 25.12%; Mo: 1.05%; Ti: 1.65%; Al: 0.26%; O: 0.002%; N: 0.005%; with the balance being Fe. The Cr equivalent was 21.37 < 28, and the Ni equivalent was 25.63 > 23. Other hot working methods and heat treatment processes were the same as in Examples 1-5.

[0072] As shown in Table 2, compared with Example 3, the resistance to liquid lead-bismuth corrosion is reduced because a certain amount of Si element is not added and a SiO2 oxide film is not formed.

[0073] Comparative Example 2

[0074] In this comparative example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.0078%; Si: 3.55%; Mn: 0.56%; S: 0.0018%; P: 0.008%; Cr: 15.23%; Ni: 25.02%; Mo: 1.05%; Ti: 1.62%; Al: 0.29%; O: 0.002%; N: 0.005%; with the balance being Fe. The Cr equivalent is 27.93 < 28, and the Ni equivalent is 25.53 > 23. Unlike Examples 1-5, Comparative Example 2 did not undergo aging heat treatment. As shown in Table 2, its resistance to liquid lead-bismuth corrosion is comparable to Examples 3-5, but its stress relaxation resistance at 550°C, as shown in Table 1, is lower than that of Examples 3-5.

[0075] Comparative Example 3

[0076] In this comparative example, the chemical composition of the steel, by weight percentage, is as follows: C: 0.007%; Si: 3.51%; Mn: 0.62%; S: 0.0017%; P: 0.006%; Cr: 14.95%; Ni: 25.31%; Mo: 1.01%; Ti: 1.66%; Al: 0.33%; O: 0.002%; N: 0.005%; with the balance being Fe. The Cr equivalent is 27.79 < 28, and the Ni equivalent is 25.83 > 23. Unlike Examples 1-5, Comparative Example 3 underwent multi-stage aging treatment: after holding at 700℃ for 16 h and at 600℃ for 48 h, it was held at 550℃ for 48 h, and finally air-cooled to room temperature. As shown in Table 2, the resistance to liquid lead-bismuth corrosion is comparable to that of Examples 3-5, but the stress relaxation resistance at 550℃ is slightly higher than that of Examples 3-5, as shown in Table 1.

[0077] The residual stress values ​​of the above embodiments and comparative examples after being maintained at 550°C and with an initial stress of 120 MPa for 400 hours are shown in Table 1.

[0078] Table 1

[0079]

[0080] Table 1 shows that by controlling the Cr and Ni equivalents to obtain a single austenitic structure, the high hot strength of the steel can be significantly improved. In Example 1, due to the insufficient Cr equivalent, a two-phase structure appeared, resulting in reduced resistance to stress relaxation. The results of Examples 3, 4, and 5 indicate that increasing the Ti / Al ratio and (Ti+Al) content in the austenitic steel of this invention can improve the steel's resistance to stress relaxation. Furthermore, compared to Example 3, the residual stress in Comparative Example 3 is slightly higher than that in Example 3 after multi-stage aging treatment. However, Comparative Example 3 has the disadvantages of a long preparation cycle and high cost.

[0081] like Figure 1 , Figure 2 As shown, the transmission microstructure of the steel in Example 3 is a single austenitic structure with dual-scale nanoscale G phase (Ni). 16 The Ti6Si7 and Ni3(Al,Ti) phases are uniformly dispersed, with the G phase (Ni 16 The size of Ti6Si7 phase is 100–300 nm, and the size of Ni3(Al,Ti) phase is 10–50 nm.

[0082] like Figure 3As shown, at 550°C and with an initial stress of 120 MPa, the residual stress versus time graphs of Example 3 and Comparative Example 3 showed that the stress of Example 3 and Comparative Example 3 decreased by a much smaller amount over time than that of Comparative Example 2. That is, Example 3 and Comparative Example 3 had higher residual stress than Comparative Example 2, while Comparative Example 3 showed better stress relaxation resistance due to multi-stage aging treatment.

[0083] The oxide film thickness values ​​of Examples 1-5 and the comparative examples after corrosion in saturated oxygen concentration and 550°C liquid lead-bismuth alloy (45% Pb-Bi) for 3000 hours are shown in Table 2.

[0084] Table 2

[0085]

[0086] Table 2 shows that the addition of Si significantly improves the resistance to lead-bismuth corrosion in austenitic steel (Comparative Example 1 did not add an appropriate amount of Si as required), exhibiting excellent resistance to lead-bismuth corrosion. In Example 2, due to the Si content being less than 3.5%, a dense and continuous SiO2 oxide film could not be formed, resulting in an oxide film thickness exceeding 25 μm.

[0087] like Figure 4 , Figure 5 As shown, the oxide film morphology of the steel in Example 3 and Comparative Example 1 after corrosion in saturated oxygen concentration and 550°C liquid lead-bismuth alloy (45% Pb-Bi) for 3000 hours shows that the oxide film thickness of Comparative Example 1 is relatively thick, up to 36.7 μm, and its resistance to liquid lead-bismuth alloy corrosion is poor. The oxide film thickness of the steel in Example 3 is less than 25 μm, showing excellent resistance to liquid lead-bismuth corrosion.

[0088] The results show that this invention, by adding Si, an element with strong oxidation resistance, to austenitic steel, and using the following preparation method: batching → vacuum induction furnace melting → vacuum arc remelting → low-temperature forging → solution treatment + aging treatment, successfully produced austenitic steel with high stress relaxation resistance. The aging treatment yielded a dual-scale nanoscale G phase (Ni). 16 The synergistic strengthening of Ti6Si7 and Ni3(Al,Ti) improves the stress relaxation resistance of austenitic steel, while the high Si content ensures the steel's resistance to lead-bismuth corrosion. This solves the problem that traditional austenitic steels for fasteners cannot simultaneously meet the requirements of high stress relaxation resistance and resistance to liquid lead-bismuth corrosion, providing a new technical route for lead-based reactor fastener structural materials used in the nuclear energy field.

[0089] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing austenitic steel with high resistance to stress relaxation for lead-based fasteners, characterized in that, The chemical composition of this steel, by weight percentage, is as follows: C: 0.007~0.03%; Si: 3.0~4.0%; Mn≤1.0%; S≤0.005%; P≤0.01%; Cr: 15.0~17.0%; Ni: 23.0~27.0%; Mo: 0.5~2.0%; Ti: 1.0~2.0%; Al: 0.2~0.4%; O≤0.003%; N≤0.005%; balance Fe; The method for preparing austenitic steel with high resistance to stress relaxation for lead-based fasteners includes the following steps: (1) Dual vacuum melting: The raw materials are proportioned according to the target chemical composition requirements, and the required ingots are obtained through a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting. (2) Homogenization treatment: The smelted ingots are subjected to high-temperature homogenization treatment, and the ingots are heated to 1100~1150 ℃ and held for 8~48 hours. (3) Hot forging: After the high temperature homogenization of the ingot is held at 1050~1100℃ for 2~8 hours, it is forged. The initial forging temperature is 1020~1050℃ and the final forging temperature is 900~950℃. During the forging, the longitudinal-transverse-longitudinal three-way repeated large reduction forging is carried out. The number of repetitions is not less than 6, the single deformation amount is ≥10%, the total deformation amount is ≥80%, the forging ratio is ≥6, and the forging bar is obtained by air cooling to room temperature after forging. (4) Heat treatment: The forged bar is subjected to solution treatment and aging treatment after forging; The heat treatment process in step (4) is as follows: (1) Solution treatment: keep at 1050~1180℃ for 1~4 hours, then water cool; (2) The aging treatment is as follows: keep at 700~750℃ for 4~16 hours and air cool; then keep at 550~650℃ for 16~48 hours and air cool. Aging treatment yielded dual-scale nanoscale G-phase Ni 16 Ti6Si7 and Ni3(Al,Ti) phases, dual-scale nanoscale G phase Ni 16 Ti6Si7 and Ni3(Al,Ti) phases are uniformly dispersed, with G phase Ni 16 The Ti6Si7 phase has a size of 100~300nm, while the Ni3(Al,Ti) phase has a size of 10~50nm.

2. The method for preparing high-stress-relaxation-resistant austenitic steel for lead-based fasteners according to claim 1, characterized in that, Chromium equivalent is calculated according to formula (1): Cr equivalent = 100 × (Cr + 1.5 × Mo + 2 × Si + 1.5 × Ti + 5.5 × Al) (1) Nickel equivalent is calculated according to formula (2): Ni equivalent = 100 × (Ni + 30 × C + 0.5 × Mn) (2) The Cr equivalent and Ni equivalent satisfy the following conditions: Cr equivalent < 28; Ni equivalent > 23.

3. The method for preparing high-stress-relaxation-resistant austenitic steel for lead-based fasteners according to claim 1, characterized in that, The microstructure of this steel is a single austenite.

4. The method for preparing high-stress-relaxation-resistant austenitic steel for lead-based fasteners according to claim 1, characterized in that, The single deformation amount is 12%, the total deformation amount is 85%, and the forging ratio is 8%.

5. The method for preparing high-stress-relaxation-resistant austenitic steel for lead-based fasteners according to claim 1, characterized in that, The steel has a residual stress of not less than 100 MPa after being held at 550℃ with an initial stress of 120 MPa for 400 hours.

6. The method for preparing high-stress-relaxation-resistant austenitic steel for lead-based fasteners according to claim 1, characterized in that, Under saturated oxygen conditions, after 3000 hours of corrosion in a 45% Pb-Bi liquid lead-bismuth eutectic alloy at 550℃, the oxide film thickness of this steel does not exceed 25μm, demonstrating excellent resistance to liquid lead-bismuth corrosion.

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