Austenitic steel and its manufacturing method, armor for superconducting coils

Abstract

The present invention relates to austenitic steel, a method for producing the same, and armor for superconducting coils, and the main technical solutions used are as follows. By mass percentage, the chemical composition of the austenitic steel is: C: greater than 0.02 wt% or less, Mn: 3.5 to 8.0 wt%, Si: 0 to 1 wt%, Ni: 10 to 15 wt%, Cr: 19 to 25 wt%, Mo: 1 to 3 wt%, V: 0.1 wt% to 0.3 wt%, Nb: 0.01 to 0.1 wt%, N: greater than 0.25 wt% and less than 0.45 wt%, B: 10 to 65 ppm, Al: less than 0.03 wt%, Ti: less than 0.001 wt%, and the remainder: Fe. The present invention primarily provides age-resistant austenitic steel that maintains high strength and toughness even after undergoing superconducting phase formation aging heat treatment, and is used to meet the toughness requirements of armor materials for superconducting coils.

Description

Technical Field

[0001] This disclosure claims the priority of a Chinese patent application filed with the Chinese Patent Office on June 16, 2023, with an application number of 202310717046.4 and an invention title of "Austenitic Steel and Its Manufacturing Method, Armor for Superconducting Coils", and all of its contents are incorporated into this disclosure by reference.

[0002] The present invention relates to the technical field of metal materials, particularly to austenitic steel and its manufacturing method, and armor for superconducting coils.

Background Art

[0003] China's Compact High-Temperature Experimental Reactor (CFETR) is a major scientific project independently designed and developed by China after absorbing the construction experience of the International Thermonuclear Experimental Reactor (ITER). Its purpose is to build a controllable "artificial sun", construct a commercial fusion prototype reactor, achieve a sustainable supply of clean energy, and thoroughly solve the energy crisis.

[0004] As a main component of a fusion experimental reactor, a superconducting coil can provide a high magnetic field for the normal operation of the fusion reactor. However, the complex operating environment poses major challenges to the armor material for superconducting coils. In addition, during the manufacturing process of superconducting coils, the superconducting wire needs to undergo aging heat treatment, and the armor for superconducting coils and the superconducting wire also need to undergo superconducting phase formation aging heat treatment at 400 - 750 °C as an integral structure. However, this heat treatment process causes sensitization of ordinary austenitic steel, significantly reducing its toughness, plasticity, and corrosion resistance. Furthermore, the yield strength of conventional low-temperature austenitic steels, such as 316LN, JJ1, JK2LB, etc., at 4.2 K is all less than 1300 MPa.

[0005] Therefore, it is necessary to provide a time-resistant structural steel that meets the toughness requirements of the armor material for superconducting coils and has excellent toughness at extremely low temperatures even after undergoing aging heat treatment.

Summary of the Invention

[0006] In view of the above, the present invention provides austenitic steel, a method for manufacturing the same, and armor for superconducting coils. The main objective is to provide age-resistant austenitic steel that has high strength and high toughness even after undergoing superconducting phase formation aging heat treatment, thereby satisfying the toughness requirements for armor materials for superconducting coils. [Means for solving the problem]

[0007] To achieve the above objectives, the present invention primarily provides the following technical solutions.

[0008] In one embodiment, an embodiment of the present invention provides an austenitic steel, wherein the chemical composition of the austenitic steel is, by mass percentage, C:0 more than 0.02wt%, Mn: 3.5~8.0 wt%, Si: 0~1 wt%, Ni: 10-15 wt%, Cr: 19-25 wt%, Mo: 1-3 wt%, V: 0.1~0.3 wt%, Nb: 0.01~0.1wt%, N: greater than 0.25 wt% and less than 0.45 wt%, B: 10-65 ppm, Al: Less than 0.03 wt%, Ti: Less than 0.001 wt%, Remainder: Contains Fe.

[0009] Preferably, the intragrain adsorption capacity of the austenitic steel for element B is F(B) > 0.15, which enhances the intragrain adsorption effect for element B, suppresses the precipitation of carbonitrides and intermetallic compounds, and obtains excellent low-temperature toughness, where F(B) = [B] / (0.034[N] + 0.23[C]), where [N], [C], and [B] are the mass percentages of N, C, and B, respectively.

[0010] Preferably, the mass percentage of C is 0.015 wt% or less.

[0011] Preferably, the mass percentage of Nb is 0.03 to 0.08 wt%.

[0012] Preferably, the austenitic steel, after undergoing aging heat treatment in the austenite sensitization region, is free from the deposition of carbonitrides and intermetallic compounds at the grain boundaries, or, after undergoing aging heat treatment in the austenite sensitization region, has an equivalent grain size of grain boundary precipitates of ≤50 nm. The term "sensitization region" as used in this invention refers to a heat treatment performed at 400-850°C after solution treatment of the alloy material. At this temperature, austenitic stainless steel experiences chromium deficiency at the grain boundaries due to the deposition of chromium carbide, increasing the material's tendency for intergranular corrosion, and this temperature range constitutes the sensitization region.

[0013] Preferably, the austenitic steel undergoes a superconducting phase formation aging heat treatment, which precipitates dispersed compounds with a particle size of 10 to 200 nm, thereby imparting an excellent precipitation strengthening effect to the austenitic steel and guaranteeing low-temperature strength. Here, the compound contains three elements: N, V, and Nb (the compound is mainly composed of three elements: N, V, and Nb, and also contains other elements such as Cr, Fe, and Mo). The term "superconducting phase formation aging heat treatment" as used in this invention refers to the aging heat treatment process that the austenitic steel of this invention passively undergoes as a support and protective material for the Nb3Sn, which is necessary to heat-treat the Nb3Sn superconducting wire at a temperature of 400 to 750°C in order to impart superconducting properties to it.

[0014] Preferably, in the austenitic steel, during the superconducting phase formation aging heat treatment process, element B diffuses into the grain boundaries due to the action of grain boundary energy, occupies the grain boundary positions, increases grain boundary strength, suppresses the generation and expansion of grain boundary cracks, and improves low-temperature toughness and plasticity.

[0015] Preferably, after the austenitic steel undergoes the aging heat treatment for superconducting phase generation, at 4.2 K, the yield strength of the austenitic steel > 1300 MPa, the tensile strength > 1700 MPa, the elongation rate > 25%, the impact toughness > 100 J, and the fracture toughness > 130 MPa·m 1 / 2 It has the mechanical properties as described above.

[0016] In another aspect, the embodiments of the present invention provide a method for manufacturing the austenitic steel according to any one of the above items. Step 1) of preparing raw materials based on the chemical composition of the austenitic steel, performing smelting treatment, and obtaining an ingot after casting; Step 2) of performing electro-slag remelting treatment on the ingot to obtain an electro-slag ingot; Step 3) of performing forging treatment on the electro-slag ingot to obtain a forging material; It includes Step 4) of performing a solution treatment on the forging material to obtain the austenitic steel.

[0017] Preferably, in Step 1), it is necessary to ensure that the content of N in the ingot is more than 0.25 wt% and less than 0.45 wt%, and / or the smelting treatment is carried out in a vacuum induction furnace. Here, it is guaranteed that the pressure value of the nitrogen atmosphere in the smelting process is 0.03 - 0.07 MP, and / or when the smelting temperature is 1450 - 1520 °C, nitrogen gas is continuously introduced to promote the alloying of nitrogen.

[0018] Preferably, in Step 3), the temperature of the forging treatment is 850 - 1200 °C.

[0019] Preferably, the temperature of the solution treatment is 1000 - 1150 °C. Preferably, the time of the solution treatment satisfies H×(2 - 4 min / mm), where H is the thickness of the steel block in mm.

[0020] Preferably, in step 4), the temperature of the solution treatment satisfies {1020 + [Nb]×580 + [V]×125}°C ± (0 to 3)°C, where [Nb] and [V] are the mass percentages of Nb and V respectively, and the time of the solution treatment satisfies H×(2 to 4 min / mm), where H is the thickness of the steel ingot in mm.

[0021] Preferably, after step 4), The method further includes step 5) of performing a superconducting phase formation aging treatment on the austenitic steel.

[0022] In a further aspect, an embodiment of the present invention provides an armor for a superconducting coil, and the armor for the superconducting coil is made of the austenitic steel described in any one of the above items.

Advantages of the Invention

[0023] Compared with the prior art, the austenitic steel, its manufacturing method, and the armor for a superconducting coil of the present invention have at least the following beneficial effects.

[0024] The embodiments of the present invention provide austenitic steel, and the inventors of the present invention have rationally set the composition of the austenitic steel through thorough research, specifically as follows: In weight percentage, the chemical composition of the austenitic steel is C: greater than 0.02 wt% or less (preferably 0.15 wt% or less), Mn: 3.5 to 8.0 wt%, Si: 0 to 1 wt%, Ni: 10 to 15 wt%, Cr: 19 to 25 wt%, Mo: 1 to 3 wt%, V: 0.1 wt% to 0.3 wt%, Nb: 0.01 to 0.1 wt% (preferably 0.03 to 0.08 wt%), N: greater than 0.25 and less than 0.45 wt%, B: 10 to 65 ppm, Al: less than 0.03 wt%, Ti: less than 0.001 wt%, and the remainder: Fe. Here, (1) when small amounts of Nb and V are added, that is, when the Nb content is controlled to 0.01~0.1 wt% (preferably 0.03~0.08 wt%) and the V content is controlled to 0.10~0.30 wt%, good low-temperature toughness and plasticity can be obtained, and the precipitation strengthening effect can be maximized, achieving an excellent balance of toughness. (2) Under conditions containing small amounts of Nb and V, element B is further added to the chemical composition of the austenitic steel of the present invention, and the content of element B is strictly limited. As a result, element B diffuses into the grain boundaries due to the action of grain boundary energy during the aging process, occupies the grain boundary position, increases grain boundary strength, suppresses the generation and expansion of grain boundary cracks, and improves low-temperature toughness and plasticity. (3) Under the above conditions, the present invention further restricts the composition relationship of the three elements N, C, and B. Mainly, in the process of superconducting phase formation aging heat treatment, both N and C are interstitial solid solution atoms, and at the same time they are subjected to the adsorption effect of grain boundaries, M 23 Since C6 and Cr2N are involved in the formation of grain boundary compounds, these three elements are in a competitive relationship at the grain boundaries. The present invention aims to enhance the internal adsorption effect of element B at the grain boundaries by restricting the relationship between the three elements B, C, and N, thereby suppressing the precipitation of other carbonitrides and obtaining excellent low-temperature toughness.

[0025] As described above, the present invention involves simultaneously adding Nb, V, and B, and controlling the amounts of Nb, V, N, C, and B added to exert a synergistic effect. As a result, the steel of the present invention retains high strength and toughness even after undergoing superconducting phase formation aging heat treatment, meeting the toughness requirements for armor materials for superconducting coils.

[0026] On the other hand, the embodiments of the present invention propose a method for producing the above-mentioned austenitic steel, ensuring that the components satisfy the above requirements, and designing and selecting the temperature for the solution treatment. Specifically, in step 4), the temperature for the solution treatment satisfies {1020 + [Nb] × 580 + [V] × 125}℃ ± (0~3)℃, where [Nb] and [V] are the mass percentages of Nb and V, respectively. By setting it in this way, the precipitation strengthening effect of elements Nb and V can be more fully exerted, and larger MX harmful phases can be removed, ensuring low-temperature strength and plasticity.

[0027] The above description is merely an outline of the technical solutions of the present invention. To provide a clearer understanding of the technical means of the present invention and to enable implementation according to the specifications, preferred embodiments of the present invention will be described in detail below, in combination with the drawings. [Brief explanation of the drawing]

[0028] [Figure 1] Figure 1 shows the precipitation of boride in austenitic steel. [Figure 2] Figure 2 shows a diagram of BN inclusions in austenitic steel. [Figure 3] Figure 3 is a scanning electron microscope diagram of the austenitic steel produced in Example 1 after aging treatment. [Figure 4] Figure 4 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 1 after aging treatment. [Figure 5] Figure 5 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 2 after aging treatment. [Figure 6] Figure 6 is a scanning electron microscope diagram of the austenitic steel produced in Example 2 after aging treatment. [Figure 7] Figure 7 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 3 after aging treatment. [Figure 8]Figure 8 is a scanning electron microscope diagram of the austenitic steel produced in Example 3 after aging treatment. [Figure 9] Figure 9 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 4 after aging treatment. [Figure 10] Figure 10 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 5 after aging treatment. [Modes for carrying out the invention]

[0029] To further describe the technical means and effects used by the present invention to achieve a predetermined objective, specific embodiments, structures, features, and effects based on the present invention will be described in detail below, in combination with the drawings and preferred embodiments. In the following description, different “Embodiments” or “Embodiments” do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0030] In one embodiment, an embodiment of the present invention provides an austenitic steel, wherein the chemical composition of the austenitic steel is, by weight percentage, C: greater than 0.02 wt% or less (preferably less than 0.015 wt%) Mn: 3.5~8.0 wt%, Si: 0~1 wt%, Ni: 10-15 wt%, Cr: 19-25 wt%, Mo: 1-3 wt%, V: 0.1 wt% ~ 0.3 wt%, Nb: 0.01~0.1wt% (preferably 0.03~0.08wt%) N: greater than 0.25 wt% and less than 0.45 wt%, B: 10-65 ppm, Al: Less than 0.03 wt%, Ti: Less than 0.001 wt%, Remainder: Contains Fe.

[0031] The following points need to be explained regarding the design of the above components.

[0032] (1) Through thorough research, the present invention rationally sets the composition of the austenitic steel, wherein the Nb content in the composition is 0.01 to 0.1 wt% (preferably 0.03 to 0.08 wt%) and the V content is 0.10 to 0.30 wt%. Research has shown that when the Nb content exceeds 0.1 wt% and the V content exceeds 0.30 wt%, during the solidification process of the steel ingot, Nb and V combine with the N element to form a large-sized primary MX phase. This phase is a hard phase and does not conform to the deformation of the substrate during the deformation process, which makes it easier for cracks to preferentially occur at the phase boundary between the MX phase and the substrate, leading to the initiation and expansion of cracks, thereby reducing the low-temperature toughness and plasticity of the aging steel. However, during the superconducting phase formation aging heat treatment process at 400-750°C, the addition of trace amounts of Nb and V elements causes finely dispersed (Nb,V)N and Cr(Nb,V)N to precipitate, resulting in an excellent precipitation strengthening effect and guaranteeing low-temperature strength. Therefore, when the Nb content is 0.01-0.1 wt% (preferably 0.03-0.08 wt%) and the V content is 0.10-0.30 wt%, excellent low-temperature toughness and plasticity can be obtained, and the precipitation strengthening effect can be maximized, achieving an excellent balance of toughness. It should be noted that both Nb and V are elements that form strong nitrides, and research has shown that when only one of the Nb or V elements is added, the precipitated phase is small during the aging process, and the precipitation strengthening effect is weak. Furthermore, when the Nb content is controlled to 0.03-0.08 wt%, a better balance of toughness can be obtained.

[0033] (2) Under conditions containing trace amounts of Nb and V, element B is further added to the chemical composition of the austenitic steel of the present invention, and the content of element B is strictly limited.

[0034] During the superconducting phase formation aging heat treatment process at 400-750°C, sensitization occurs in austenitic steel, resulting in the formation of large amounts of nitrides (mainly Cr2N) or carbides (M 23The C6 phase occupies the grain boundary location and embrittles the grain boundary. Through research, the inventors of this invention discovered that during the aging process, element B diffuses into the grain boundary due to the action of grain boundary energy, occupies the grain boundary location, increases grain boundary strength, suppresses the generation and expansion of grain boundary cracks, and improves low-temperature toughness and plasticity. This invention limits the content of element B to 10 to 65 ppm. If the B content is less than 10 ppm, the concentration of B in the substrate is low and a good toughness improvement effect cannot be obtained, and if the B content exceeds 65 ppm, a large amount of boride (see Figure 1) and BN inclusions (see Figure 2) are generated in the substrate, and the beneficial effects of B are significantly reduced.

[0035] Furthermore, if only B is added and Nb and V are not added, no aging precipitate phase is formed, resulting in low strength.

[0036] (3) In addition to satisfying the above conditions, the present invention further restricts the composition relationships of the three elements N, C, and B. Primarily, in the process of superconducting phase formation aging heat treatment, both N and C are interstitial solid solution atoms and are simultaneously subjected to grain boundary adsorption, M 23 In order to form the C6 and Cr2N grain boundary phases, these three elements are in a competitive relationship at the grain boundaries. By restricting the relationship between the three elements B, C, and N, this invention enhances the internal adsorption effect of element B at the grain boundaries, suppresses the precipitation of other carbonitrides, and obtains excellent low-temperature toughness.

[0037] Specifically, the intrinsic adsorption capacity of the austenitic steel of the present invention for element B at grain boundaries is F(B)>0.15, where F(B)=[B] / (0.034[N]+0.23[C]), and [N], [C], and [B] are the mass percentages of N, C, and B, respectively. The main reason for limiting the composition of elements N, C, and B in this invention is that elements C and N, as interstitial solid solution elements, compete with element B during the heat treatment process, rapidly segregating at grain boundaries and generating large-sized carbonitrides, thereby reducing the toughness and plasticity of the material. Therefore, by rationally controlling C, N, and B, the strength of the steel is guaranteed, and the advantage of element B's grain boundary segregation is utilized to occupy the grain boundary position early, suppressing nucleation of other carbonitrides at grain boundaries, and improving the plasticity and toughness of the austenitic steel of the present invention.

[0038] Preferably, the present invention further optimizes the solution heat treatment temperature. According to the inventors' research, during the solution heat treatment stage, dispersed fine (Nb,V)N and Cr(Nb,V)N also precipitate, and by limiting the components, large-sized primary MX phases can be basically removed. However, as the size of the steel ingot increases, segregation causes fluctuations in the micro-region components, and small amounts of harmful MX phases may still be generated. Therefore, the present invention optimizes the solution heat treatment process to increase the precipitation of dispersed fine (Nb,V)N and Cr(Nb,V)N strengthening phases, and to sufficiently redissolve the largest possible MX phases during the solution treatment stage, thereby reducing damage to toughness and plasticity.

[0039] Based on the above, the austenitic steel provided by the present invention (austenitic steel possessing aging resistance and excellent cryogenic toughness) is characterized by the absence of carbonitride and intermetallic compound precipitation at grain boundaries after aging heat treatment in the austenite sensitization region, or by an equivalent grain size of grain boundary precipitates of ≤50 nm.

[0040] More specifically, the difference between the austenitic steel of the present invention and ordinary austenitic steel is that the austenitic steel of the present invention retains good cryogenic strength and plasticity even after undergoing aging treatment in the austenite sensitization region; in other words, the austenitic steel of the present invention has good aging resistance. By rationally adjusting the component composition and mixing ratio, the present invention prevents the deposition of carbonitrides and intermetallic compounds at grain boundaries or the equivalent grain size of grain boundary precipitates is ≤50 nm after aging treatment in the austenite sensitization region, thus the austenitic steel of the present invention has aging resistance.

[0041] Preferably, for example, the specific process of the aging heat treatment of the austenite sensitization region or the superconducting phase formation aging heat treatment is: 600-700°C × (30-200 hours) or The time intervals are 150-220°C for 30-60 hours + 350-450°C for 30-60 hours + 600-750°C for 40-60 hours.

[0042] Preferably, the austenitic steel of the present invention, after undergoing superconducting phase formation aging heat treatment, exhibits a yield strength >1300 MPa, tensile strength >1700 MPa, elongation >25%, impact toughness >100 J, and fracture toughness >130 MPa·m at 4.2 K. 1 / 2 It possesses the mechanical property of being such.

[0043] In another embodiment, the embodiment of the present invention further provides a method for producing the above-mentioned austenitic steel, which includes the following steps.

[0044] 1) Prepare raw materials based on the chemical composition of the austenitic steel, perform smelting, and obtain an ingot after casting. Ensure that the nitrogen content in the ingot is greater than 0.25 wt% and less than 0.45 wt%.

[0045] The smelting process is carried out in a vacuum induction furnace, ensuring that the nitrogen atmosphere pressure is 0.03-0.07 MPa during the smelting process. Furthermore, when the smelting temperature is 1450-1520°C, nitrogen gas is continuously introduced to promote nitrogen alloying (ensureing a nitrogen content greater than 0.25% by improving the solid solubility of the nitrogen element).

[0046] 2) The ingot is subjected to electroslag remelting treatment to obtain an electroslag ingot. The purpose of this is to reduce defects, refine the steel ingot, and significantly reduce the inclusion content in the steel.

[0047] 3) The electroslag ingot is subjected to a forging process to obtain a forged material. The temperature of the forging process is 850 to 1200°C.

[0048] 4) The forged material is subjected to solution treatment to obtain the austenitic steel.

[0049] The solution treatment process described above is The solution treatment temperature must be 1000-1150°C, or The solution treatment temperature satisfies {1020 + [Nb] × 580 + [V] × 125}℃ ± (0~3)℃, where [Nb] and [V] are the mass percentages of Nb and V, respectively, and the solution treatment time satisfies H × (2~4 min / mm), where H is the thickness of the steel ingot and the unit is mm.

[0050] Preferably, for example, the manufacturing method further includes an aging heat treatment step, specifically, an aging heat treatment on the cooled austenitic steel. In this case, the aging heat treatment step is not an essential step for manufacturing the austenitic steel of the present invention. Furthermore, the aging heat treatment step is an aging heat treatment in the austenite sensitization region, and preferably, a superconducting phase formation aging heat treatment. Preferably, the specific aging heat treatment process is: 600~700℃ × (30~200h) or, The time intervals are 150-220°C for 30-60 hours + 350-450°C for 30-60 hours + 600-750°C for 40-60 hours.

[0051] In other words, aging heat treatment is performed by maintaining a temperature of 640-700°C for 30-200 hours, or by first maintaining a temperature of 150-220°C for 30-60 hours, then raising the temperature to 350-450°C and maintaining it for 30-60 hours, and then continuing to raise the temperature to 600-750°C and maintaining it for 40-60 hours.

[0052] The present invention will be further described below with reference to specific examples. [Examples]

[0053] This embodiment produces austenitic steel, and the component ratios of the austenitic steel in this embodiment are shown in Table 1. Specifically, the manufacturing method includes the following steps.

[0054] 1) Prepare raw materials based on the chemical composition of the austenitic steel, perform smelting, and obtain an ingot after casting. Ensure that the N content of the ingot is greater than 0.25 wt% and less than 0.45 wt%.

[0055] 2) The ingot is subjected to electroslag remelting treatment to obtain an electroslag ingot. The purpose of this is to reduce defects, refine the steel ingot, and significantly reduce the inclusion content in the steel.

[0056] 3) The electroslag ingot is subjected to a forging process, with an initial forging temperature of 1200°C and a finish forging temperature of 900°C to obtain a forged material.

[0057] 4) The forged material is subjected to solution treatment to obtain the austenitic steel. The temperature of the solution treatment is 1050°C. The duration of the solution treatment satisfies the formula H × (4 min / mm), where H is the thickness of the steel ingot and the unit is mm. [Examples]

[0058] This embodiment produces austenitic steel, and the component ratios of the austenitic steel in this embodiment are shown in Table 1.

[0059] The manufacturing steps for the austenitic steel in this embodiment are the same as in Example 1. [Examples]

[0060] This embodiment produces austenitic steel, and the component ratios of the austenitic steel in this embodiment are shown in Table 1.

[0061] The manufacturing steps for the austenitic steel in this embodiment are the same as in Example 1.

[0062] Comparative Example 1 Comparative Example 1 produced austenitic steel, and the component ratios of the austenitic steel of Comparative Example 1 are shown in Table 1.

[0063] The manufacturing steps for the austenitic steel in Comparative Example 1 are the same as those in Example 1.

[0064] Comparative Example 2 Comparative Example 2 produced austenitic steel, and the component ratios of the austenitic steel of Comparative Example 2 are shown in Table 1.

[0065] The manufacturing steps for the austenitic steel in Comparative Example 2 are the same as those in Example 1.

[0066] Comparative Example 3 Comparative Example 3 produced austenitic steel, and the component ratios of the austenitic steel of Comparative Example 3 are shown in Table 1.

[0067] The manufacturing steps for the austenitic steel in Comparative Example 3 are the same as those in Example 1.

[0068] Comparative Example 4 Comparative Example 4 produced austenitic steel, and the component ratios of the austenitic steel of Comparative Example 4 are shown in Table 1.

[0069] The manufacturing steps for the austenitic steel in Comparative Example 4 are the same as those in Example 1.

[0070] Comparative Example 5 Comparative Example 5 produced austenitic steel, and the component ratios of the austenitic steel of Comparative Example 5 are shown in Table 1.

[0071] The manufacturing steps for the austenitic steel in Comparative Example 5 are the same as those in Example 1.

[0072] The chemical composition of the austenitic steel produced in Examples 1-3 and Comparative Examples 1-5 is shown in Table 1.

[0073] [Table 1]

[0074] The performance of the austenitic steels produced in Examples 1-3 and Comparative Examples 1-5 after superconducting phase formation aging heat treatment is shown in Table 2. The specific process for superconducting phase formation aging heat treatment is 210°C × 48h + 400°C × 48h + 665°C × 50h.

[0075] [Table 2]

[0076] Figure 3 is a scanning electron microscope diagram of the austenitic steel produced in Example 1 after aging treatment (arrows in Figure 3 indicate grain boundaries), Figure 4 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 1 after aging treatment (arrows in Figure 4 indicate grain boundaries), Figure 5 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 2 after aging treatment, Figure 6 is a scanning electron microscope diagram of the austenitic steel produced in Example 2 after aging treatment, Figure 7 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 3 after aging treatment, Figure 8 is a scanning electron microscope diagram of the austenitic steel produced in Example 3 after aging treatment, Figure 9 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 4 after aging treatment, and Figure 10 is a scanning electron microscope diagram of the austenitic steel produced in Comparative Example 5 after aging treatment.

[0077] From Tables 1 and 2, and Figures 3-9, the following can be seen.

[0078] (1) As is clear from the comparison of Table 1 and Table 2, the austenitic steel produced in Examples 1 to 3 of the present invention, which contain trace amounts of Nb and V and limit the content of element B to 10 to 65 ppm, has clearly superior overall performance after aging heat treatment compared to Comparative Examples 1 to 5.

[0079] (2) The following has been found from a comparison of the microstructures of the austenitic steel produced in Example 1 (Figure 3), Comparative Example 1 (Figure 4), and Comparative Example 2 (Figure 5).

[0080] When containing an appropriate amount of element B, austenitic steel exhibits clean grain boundaries and no obvious precipitation of precipitated phases after aging heat treatment (see Example 1 and Figure 3).

[0081] On the other hand, Comparative Example 1 has a B content of less than 0.0010%, and the intragrain adsorption effect of the grain boundaries on element B is weak, resulting in a large amount of grain boundary precipitate phase at the grain boundaries, as shown in Figure 4.

[0082] Furthermore, when the B content exceeded the limiting criteria of the present invention, a large amount of BN inclusions were formed, as shown in Comparative Example 2 (see Figure 5), and its low-temperature toughness and plasticity were impaired.

[0083] (3) The following has been found from a comparison between Example 2 and Comparative Example 3.

[0084] When trace amounts of Nb and V are present, i.e., when 0.03 to 0.08 wt% of Nb and 0.1 to 0.3 wt% of V are present (Example 2), the microstructure of the austenitic steel is shown in Figure 6, and after aging heat treatment, a finely dispersed precipitate phase containing Nb and V can be formed within the crystal.

[0085] On the other hand, when the Nb content exceeded 0.08 wt% (especially exceeding 0.1 wt%) and the V content exceeded 0.3 wt% (Comparative Example 3), the microstructure of the austenitic steel was as shown in Figure 7, with large-sized MX phases precipitating and toughness being impaired.

[0086] (4) The following has been found from a comparison between Example 3 and Comparative Example 4.

[0087] When containing trace amounts of Nb and V, and limiting the content of element B to 10-65 ppm, and F(B) satisfies the limiting requirements of the present invention, the microstructure characteristics of the austenitic steel of Example 3 are shown in Figure 8. After aging treatment, a large amount of finely dispersed precipitate phase precipitates, the grain boundaries are clean, and there are no precipitates.

[0088] On the other hand, when element B F(B) does not meet the requirements, as shown in Comparative Example 4 and Figure 9, a large amount of large carbonitrides precipitate at the grain boundaries in the austenitic steel after aging treatment, and the toughness and plasticity at 4.2K are significantly reduced.

[0089] (5) The following has been found from a comparison of Example 1 and Comparative Example 5.

[0090] While limiting the content of element B to 10-65 ppm, if Nb and V are not added, an austenitic steel with good toughness and plasticity is obtained, as shown in Comparative Example 5 and Figure 10. However, since no precipitate phase is formed, the strength of the austenitic steel is low.

[0091] By comparing Examples 1-3 and Comparative Examples 1-5, the following was primarily demonstrated: In the compounding design of the austenitic steel of the present invention, by simultaneously adding Nb, V, and B, and controlling the amounts of Nb, V, N, C, and B added to exert their synergistic effect, the steel of the present invention retains high strength and toughness even after undergoing superconducting phase formation aging heat treatment, thus meeting the toughness requirements for armor materials for superconducting coils.

[0092] Furthermore, through the following Example 4, the present invention has demonstrated that the performance of austenitic steel can be further improved by designing the solution treatment temperature based on the austenitic steel compound design of the present invention. Specifically, this is as follows. [Examples]

[0093] This embodiment produces austenitic steel, and the composition of the austenitic steel in this embodiment is the same as in Example 2. The differences between the method of producing the austenitic steel in this embodiment and that of Example 2 are as follows.

[0094] In step 4), the forged material is subjected to solution treatment to obtain the austenitic steel. The temperature of the solution treatment is 1098°C (satisfying the formula {1020 + [Nb] × 580 + [V] × 125}°C ± (0~3)°C).

[0095] The other steps are exactly the same as in Example 2.

[0096] Here, the performance of the austenitic steel produced in Example 2 and Example 4 after undergoing superconducting phase formation aging heat treatment is shown in Table 3. The specific process for superconducting phase formation aging heat treatment is 210°C × 48h + 400°C × 48h + 665°C × 50h.

[0097] [Table 3]

[0098] As can be seen from Table 3, by designing the solution treatment temperature, the performance (toughness) of the austenitic steel produced in Example 4 was further improved compared to Example 2. The main reason for this is that, according to the inventor's research, the large-sized primary MX phase that impairs toughness and plasticity is mainly composed of Nb and V elements, and although the component limitation of the present invention can basically remove the large-sized primary MX phase, as the size of the steel ingot increases, the micro-region components fluctuate due to segregation, and a small amount of harmful MX phase may still be generated. Therefore, the inventor further optimized the solution heat treatment process to sufficiently remelt the large-sized MX phase in the solid solution stage, reducing damage to toughness and plasticity, and thereby further improving the performance of the austenitic steel.

[0099] Based on the above, embodiments of the present invention provide austenitic steel, a method for manufacturing the same, and armor for superconducting coils. The austenitic steel of these embodiments undergoes aging heat treatment in the austenite sensitization region, after which there is no nitride precipitation at the grain boundaries, or the equivalent grain size of grain boundary precipitates is ≤50 nm. After the austenitic steel undergoes aging heat treatment to form a superconducting phase, the yield strength σ at 4.2 K is s >1300MPa, tensile strength σ b The austenitic steel has a MPa pressure of >1700 MPa, elongation A > 30%, and impact toughness AKV2 > 150 J. Therefore, the austenitic steel provided in the embodiment of the present invention is an age-resistant austenitic steel that retains high strength and toughness even after undergoing superconducting phase formation aging heat treatment, and can satisfy the toughness requirements for armor materials for superconducting coils.

[0100] The foregoing are merely preferred embodiments of the present invention and do not impose any formal limitations on the present invention. Any simple modifications, equivalent changes, and modifications made to the above embodiments in accordance with the technical spirit of the present invention are all included within the scope of the technical solutions of the present invention.

Claims

1. Austenitic steel, wherein the chemical composition of the austenitic steel is, by mass percentage, C: more than 0 and less than 0.02wt%, Mn: 3.5 to 8.0 wt%, Si: 0-1wt%, Ni: 10-15wt%, Cr: 19-25wt%, Mo: 1 to 3 wt%, V: 0.1wt% to 0.3wt%, Nb: 0.01 to 0.1 wt%, N: greater than 0.25 wt% and less than 0.45 wt%, B: 10 to 65 ppm, Al: Less than 0.03 wt%, Ti: Less than 0.001 wt%, Remainder: Austenitic steel characterized by containing Fe.

2. The austenitic steel according to claim 1, characterized in that the intragrain adsorption capacity of the austenitic steel for element B is F(B) > 0.15, thereby enhancing the intragrain adsorption effect of element B, suppressing the precipitation of carbonitrides and intermetallic compounds, and obtaining excellent low-temperature toughness, where F(B) = [B] / (0.034 [N] + 0.23 [C]), where [N], [C], and [B] are the mass percentages of N, C, and B, respectively.

3. The austenitic steel according to claim 1, characterized in that the mass percentage of C is 0.015 wt% or less.

4. The austenitic steel according to claim 1, characterized in that the mass percentage of Nb is 0.03 to 0.08 wt%.

5. The austenitic steel, after undergoing age heat treatment in the austenite sensitization region, is free from the deposition of carbonitrides and intermetallic compounds at the grain boundaries, or The austenitic steel according to claim 1, characterized in that, after undergoing age heat treatment in the austenite sensitization region, the equivalent grain size of the grain boundary precipitates is ≤ 50 nm.

6. The austenitic steel according to claim 1, characterized in that, after undergoing superconducting phase formation aging heat treatment, dispersed compounds with a particle size of 10 to 200 nm precipitate, thereby imparting an excellent precipitation strengthening effect to the austenitic steel and guaranteeing low-temperature strength, and the compound contains three elements: N, V, and Nb.

7. The austenitic steel according to claim 1, characterized in that, during the superconducting phase formation aging heat treatment process, element B diffuses into the grain boundaries due to the action of grain boundary energy, occupies the grain boundary positions, increases grain boundary strength, suppresses the generation and expansion of grain boundary cracks, and improves low-temperature toughness and plasticity.

8. The austenitic steel undergoes superconducting phase formation aging heat treatment, At 4.2K, the austenitic steel exhibits a yield strength of >1300 MPa, tensile strength of >1700 MPa, elongation of >25%, impact toughness of >100 J, and fracture toughness of >130 MPa·m. １／２ The austenitic steel according to claim 1, characterized in that it has the mechanical property of being

9. A method for producing austenitic steel according to any one of claims 1 to 8, Step 1) involves preparing raw materials based on the chemical composition of the austenitic steel, performing a smelting process, and obtaining an ingot after casting. Step 2) involves performing an electroslag remelting treatment on the aforementioned ingot to obtain an electroslag ingot, Step 3) is to perform a forging process on the electroslag ingot to obtain a forging material, A method for producing austenitic steel, characterized by including step 4) a solution treatment of a forged material to obtain the austenitic steel.

10. In step 1) above, A method for producing austenitic steel according to claim 9, characterized in that it is necessary to ensure that the N content in the ingot is greater than 0.25% and less than 0.45 wt%.

11. In step 1) above, A method for producing austenitic steel according to claim 9, characterized in that the smelting process is carried out in a vacuum induction furnace, and the pressure value of the nitrogen atmosphere is guaranteed to be 0.03 to 0.07 MPa during the smelting process.

12. The method for producing austenitic steel according to claim 9, characterized in that, in step 4), the temperature of the solution treatment is 1000 to 1150°C.

13. The method for producing austenitic steel according to claim 9, characterized in that, in step 4), the temperature of the solution treatment satisfies {1020 + [Nb] × 580 + [V] × 125} °C ± (0 to 3) °C, where [Nb] and [V] are the mass percentages of Nb and V, respectively, and the time of the solution treatment satisfies H × (2 to 4 min / mm), where H is the thickness of the steel ingot and the unit is mm.

14. After step 4) above, The method for producing austenitic steel according to claim 9, further comprising step 5) a superconducting phase formation aging treatment of the austenitic steel.

15. Armor for a superconducting coil, characterized in that the armor for the superconducting coil is manufactured from austenitic steel as described in any one of claims 1 to 8.

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