Austenitic steel and preparation method therefor, and superconducting coil armor

Abstract

The present invention relates to an austenitic steel and a preparation method therefor, and a superconducting coil armor. The technical solution mainly used is: the austenitic steel comprises the following chemical components in percentages by mass: C: greater than 0 and less than or equal to 0.02 wt%, Mn: 3.5-8.0 wt%, Si: 0-1 wt%, Ni: 10-15 wt%, Cr: 19-25 wt%, Mo: 1-3 wt%, V: 0.1-0.3 wt%, Nb: 0.01-0.1 wt%, 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%, and the balance of Fe. The present invention is mainly used for providing an aging-resistant austenitic steel which still has a high strength and high toughness after a superconducting phase formation aging heat treatment so as to satisfy the strength and toughness requirements of a superconducting coil armor material.

Description

Austenitic steel and preparation method thereof, and superconducting coil armor

[0001] This disclosure claims priority to a Chinese patent application filed with the Patent Office of China on June 16, 2023, with application number 202310717046.4 and invention name “A kind of austenitic steel and its preparation method, and superconducting coil armor”, the entire contents of which are incorporated by reference into this disclosure. Technical Field

[0002] The present invention relates to the technical field of metal materials, in particular to austenitic steel and a preparation method thereof, and superconducting coil armor. Background Art

[0003] The China Fusion Engineering Test Reactor (CFETR) is a major scientific project independently designed and developed by China based on the construction experience of the International Fusion Experimental Reactor (ITER). Its purpose is to build a controllable "artificial sun", construct a commercial fusion demonstration reactor, and achieve continuous output of clean energy to completely solve the energy crisis.

[0004] As the main component of the fusion experimental reactor, the superconducting coil can provide a high magnetic field for the normal operation of the fusion reactor. However, the complex service environment poses a huge challenge to the superconducting coil armor material. In addition, during the manufacturing process of the superconducting coil, the superconducting wire must undergo aging heat treatment. The superconducting coil armor and superconducting wire as an integrated structure must also undergo superconducting phase aging heat treatment at 400-750°C. However, this heat treatment process will cause conventional austenitic steel to become sensitized, resulting in a significant reduction in toughness, plasticity and corrosion resistance. In addition, the yield strength of traditional low-temperature austenitic steels such as 316LN, JJ1, and JK2LB at 4.2K is less than 1300MPa.

[0005] Therefore, it is necessary to provide an aging-resistant structural steel that meets the strength and toughness requirements of superconducting coil armor materials and has the characteristics of excellent strength and toughness at ultra-low temperatures after aging heat treatment.

[0006] Summary of the Invention

[0007] In view of this, the present invention provides an austenitic steel and a preparation method thereof, and a superconducting coil armor. The main purpose is to provide an aging-resistant austenitic steel that still has high strength and high toughness after superconducting phase aging heat treatment, so as to meet the strength and toughness requirements of the superconducting coil armor material.

[0008] To achieve the above objectives, the present invention mainly provides the following technical solutions:

[0009] In one aspect, an embodiment of the present invention provides an austenitic steel, wherein the chemical composition of the austenitic steel, measured in percentage by mass, comprises:

[0010] C: greater than 0 and less than or equal to 0.02wt%;

[0011] Mn: 3.5-8.0 wt%;

[0012] Si: 0-1wt%;

[0013] Ni: 10-15wt%;

[0014] Cr: 19-25wt%;

[0015] Mo: 1-3 wt%;

[0016] V: 0.1-0.3 wt%;

[0017] Nb: 0.01-0.1wt%;

[0018] N: greater than 0.25wt% and less than 0.45wt%;

[0019] B: 10~65ppm;

[0020] Al: less than 0.03wt%;

[0021] Ti: less than 0.001wt%;

[0022] The balance is Fe.

[0023] Preferably, the internal adsorption capacity F(B) of the grain boundary of the austenitic steel for the B element is greater than 0.15, so as to increase the internal adsorption effect of the grain boundary on the B element, inhibit the precipitation of carbonitrides and intermetallic compounds, and thus obtain excellent low-temperature toughness; wherein, F(B) = [B] / (0.034[N]+0.23[C]); wherein [N], [C], and [B] are the mass percentages of N, C, and B, respectively.

[0024] Preferably, the mass percentage of C is less than or equal to 0.015 wt%.

[0025] Preferably, the mass percentage of Nb is 0.03-0.08 wt%.

[0026] Preferably, after the austenitic steel undergoes aging heat treatment in the austenite sensitization range, no carbonitrides or intermetallic compounds precipitate at the grain boundaries; or after the austenitic steel undergoes aging heat treatment in the austenite sensitization range, the equivalent particle size of the grain boundary precipitates is ≤50nm. It should be noted that the term "sensitization range" mentioned in the present invention refers to the heat treatment of the alloy material at 400-850°C after solid solution treatment. At this temperature, austenitic stainless steel will cause intergranular chromium depletion due to the precipitation of chromium carbide, increasing the material's tendency to intergranular corrosion. This temperature range is the sensitization range.

[0027] Preferably, after the austenitic steel undergoes a superconducting phase-forming aging heat treatment, dispersed compounds with a particle size of 10-200 nm will precipitate, giving the austenitic steel an excellent precipitation strengthening effect, thereby ensuring low-temperature strength; wherein the compounds include three elements: N, V, and Nb (the compounds are mainly composed of N, V, and Nb, and also contain other elements such as Cr, Fe, and Mo). It should be noted that the term "superconducting phase-forming aging heat treatment" mentioned in the present invention refers to the fact that in order to make the Nb3Sn superconducting wire have superconducting properties, it must be placed in the range of 400-750°C for heat treatment, and the austenitic steel of the present invention, as a supporting and protective material for Nb3Sn, must also passively undergo an aging heat treatment process.

[0028] Preferably, during the superconducting phase aging heat treatment process of the austenitic steel, the B element is affected by the grain boundary energy and diffuses to the grain boundary, occupies the grain boundary position, improves the grain boundary strength, and inhibits the generation and expansion of intergranular cracks, so as to improve the low-temperature toughness and plasticity.

[0029] Preferably, after the superconducting phase aging heat treatment, the mechanical properties of the austenitic steel are as follows: at 4.2K, the yield strength of the austenitic steel is greater than 1300 MPa, the tensile strength is greater than 1700 MPa, the elongation is greater than 25%, the impact toughness is greater than 100 J, and the fracture toughness is greater than 130 MPa·m 1 / 2 .

[0030] On the other hand, an embodiment of the present invention provides a method for preparing the austenitic steel described in any one of the above items, comprising the following steps:

[0031] 1) preparing raw materials according to the chemical composition of the austenitic steel, performing smelting treatment, and casting to obtain an ingot;

[0032] 2) performing electroslag remelting on the ingot to obtain an electroslag ingot;

[0033] 3) forging the electroslag ingot to obtain a forging blank;

[0034] 4) performing a solution treatment on the forging blank to obtain the austenitic steel.

[0035] Preferably, in step 1), the nitrogen content in the ingot is ensured to be greater than 0.25wt% and less than 0.45wt%; and / or the smelting treatment is carried out in a vacuum induction furnace; wherein, the nitrogen atmosphere pressure is ensured to be 0.03-0.07MP during the smelting process; and / or when the smelting temperature is 1450-1520°C, nitrogen is continuously introduced to promote nitrogen alloying.

[0036] Preferably, in step 3), the temperature of the forging process is 850-1200°C.

[0037] Preferably, the temperature of the solution treatment is 1000-1150° C.; preferably, the duration of the solution treatment satisfies H×(2-4 min / mm), where H is the thickness of the steel ingot, in mm.

[0038] Preferably, in the step 4), the temperature of the solution treatment satisfies {1020+[Nb]×580+[V]×125}°C±(0-3)°C, where [Nb] and [V] are the mass percentages of Nb and V, respectively; and the duration of the solution treatment satisfies H×(2-4 min / mm), where H is the thickness of the steel ingot, in mm.

[0039] Preferably, after step 4), the method further comprises:

[0040] Step 5) performing superconducting phase aging treatment on the austenitic steel.

[0041] On the other hand, an embodiment of the present invention provides a superconducting coil armor, wherein the superconducting coil armor is made of any of the austenitic steels described above.

[0042] Compared with the prior art, the austenitic steel and its preparation method, and the superconducting coil armor of the present invention have at least the following beneficial effects:

[0043] An embodiment of the present invention provides an austenitic steel. The inventors of the present invention have rationally set the composition of the austenitic steel through sufficient research, specifically as follows: in terms of weight percentage, the chemical composition of the austenitic steel includes C: greater than 0 and less than or equal to 0.02wt% (preferably less than or equal to 0.15wt%); Mn: 3.5-8.0wt%; Si: 0-1wt%; Ni: 10-15wt%; Cr: 19-25wt%; Mo: 1-3wt%; V: 0.1wt%-0.3wt%; Nb: 0.01-0.1wt% (preferably 0.03-0.08wt%); N: greater than 0.25 and less than 0.45wt%; B: 10-65ppm; Al: less than 0.03wt%; Ti: less than 0.001wt%; and the balance is Fe. Here, (1) a trace amount of Nb and V is added, that is, when the Nb content is controlled at 0.01-0.1wt% (preferably 0.03-0.08wt%) and the V content is controlled at 0.10-0.30wt%, it is possible to obtain better low-temperature toughness and plasticity while maximizing its precipitation strengthening effect, thereby obtaining better strength-toughness matching. (2) Under the condition of containing trace amounts of Nb and V, the chemical composition of the austenitic steel of the present invention is further added with B element, and the content of B element is strictly limited; so that during the aging process, the B element is affected by the grain boundary energy and will diffuse to the grain boundary, occupy the grain boundary position, improve the grain boundary strength, and inhibit the generation and expansion of intergranular cracks, thereby improving the low-temperature toughness and plasticity. (3) Under the above conditions, the present invention further limits the matching relationship of the three elements N, C, and B; mainly during the superconducting phase aging heat treatment process, since N and C are also interstitial solid solution atoms, they will be simultaneously adsorbed by the grain boundary to generate M 23 C6 and Cr2N are intergranular compounds, so these three elements compete with each other at the grain boundaries. The present invention limits the relationship between the three elements B, C, and N to increase the internal adsorption effect of the grain boundary on the B element and inhibit the precipitation of other carbonitrides, thereby obtaining excellent low-temperature toughness.

[0044] In summary, the present invention adds Nb, V, and B simultaneously and controls the addition amounts of Nb, V, N, C, and B to achieve a synergistic effect, so that the steel of the present invention still has high strength and high toughness after superconducting phase aging heat treatment, thereby meeting the strength and toughness requirements of superconducting coil armor materials.

[0045] On the other hand, an embodiment of the present invention proposes a method for preparing the above-mentioned austenitic steel. While ensuring that the composition meets the above-mentioned requirements, the temperature of the solid solution treatment is designed and selected. Specifically, in the step 4), the temperature of the solid 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 Nb and V elements can be more fully exerted, and the large-sized MX harmful phase can be further eliminated to ensure low-temperature strong plasticity.

[0046] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a diagram of boride precipitation in austenitic steel;

[0048] Figure 2 is a diagram of BN impurities in austenitic steel;

[0049] FIG3 is a scanning electron micrograph of the austenitic steel prepared in Example 1 after aging treatment;

[0050] FIG4 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 1 after aging treatment;

[0051] FIG5 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 2 after aging treatment;

[0052] FIG6 is a scanning electron micrograph of the austenitic steel prepared in Example 2 after aging treatment;

[0053] FIG7 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 3 after aging treatment;

[0054] FIG8 is a scanning electron micrograph of the austenitic steel prepared in Example 3 after aging treatment;

[0055] FIG9 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 4 after aging treatment;

[0056] FIG10 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 5 after aging treatment. DETAILED DESCRIPTION

[0057] To further illustrate the technical means and effects employed by the present invention to achieve its intended objectives, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention is provided in conjunction with the accompanying drawings and preferred embodiments. In the following description, different references to "one embodiment" or "embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics of one or more embodiments may be combined in any suitable manner.

[0058] In one aspect, an embodiment of the present invention provides an austenitic steel, wherein the chemical composition of the austenitic steel comprises, in terms of weight percentage:

[0059] C: greater than 0 and less than or equal to 0.02 wt% (preferably less than 0.015 wt%);

[0060] Mn: 3.5-8.0 wt%;

[0061] Si: 0-1wt%;

[0062] Ni: 10-15wt%;

[0063] Cr: 19-25wt%;

[0064] Mo: 1-3 wt%;

[0065] V: 0.1wt%~0.3wt%;

[0066] Nb: 0.01-0.1 wt% (preferably 0.03-0.08 wt%);

[0067] N: greater than 0.25wt% and less than 0.45wt%;

[0068] B: 10-65ppm;

[0069] Al: less than 0.03wt%;

[0070] Ti: less than 0.001wt%;

[0071] The balance is Fe.

[0072] Here, it is necessary to explain the above-mentioned component design:

[0073] (1) The present invention has rationally set the composition of austenitic steel through thorough research, wherein the Nb content is 0.01-0.1wt% (preferably 0.03-0.08wt%) and the V content is 0.10-0.30wt%. The research found that when the Nb content exceeds 0.1wt% and the V content exceeds 0.30wt%, during the solidification process of the steel ingot, Nb and V will combine with the N element to form a large-sized primary MX phase. This phase is a hard phase and is not coordinated with the matrix deformation during deformation, which easily leads to cracks preferentially generated at the interface between the MX phase and the matrix, and further easily leads to crack initiation and expansion, thereby reducing the low-temperature toughness and ductility of the aging-resistant steel. However, during the superconducting phase aging heat treatment at 400-750℃, the addition of trace Nb and V elements will lead to the precipitation of fine dispersed (Nb, V)N and Cr(Nb, V)N, thereby having an excellent precipitation strengthening effect, thereby ensuring its low-temperature strength. In summary, when the Nb content is 0.01-0.1wt% (preferably 0.03-0.08wt%) and the V content is 0.10-0.30wt%, it is possible to achieve better low-temperature toughness and ductility while maximizing its precipitation strengthening effect, thereby achieving a better match between strength and toughness. It should be noted that both Nb and V are strong nitride-forming elements. Studies have found that if only one of the Nb and V elements is added, fewer precipitates will form during the aging process, and the precipitation strengthening effect will be weaker. In addition, if the Nb content is controlled at 0.03-0.08wt%, a better match between strength and toughness can be achieved.

[0074] (2) Under the condition of containing trace amounts of Nb and V, the chemical composition of the austenitic steel of the present invention further adds B element, and the content of B element is strictly limited.

[0075] During the superconducting phase aging heat treatment process at 400-750℃, austenitic steel will be sensitized, and a large amount of nitrides (mainly Cr2N) or carbides (M 23 C6) phase will occupy the grain boundary position, causing the grain boundary to become brittle. The inventors of the present invention have found through research that: during the aging process, the B element will be affected by the grain boundary energy and will diffuse to the grain boundary position, occupy the grain boundary position, increase the grain boundary strength, inhibit the generation and expansion of intergranular cracks, and thus improve the low-temperature toughness and plasticity. The present invention limits the content of the B element to 10-65ppm. When the B content is lower than 10ppm, the B concentration in the matrix is ​​low and a good toughening effect cannot be obtained. When the B content exceeds 65ppm, a large amount of boride (see Figure 1) and BN inclusions (see Figure 2) will be generated in the matrix, significantly reducing the beneficial effects of B.

[0076] It should be noted that if only B is added without adding Nb and V, no aging precipitation phase is generated and the strength is low.

[0077] (3) Under the above conditions, the present invention further limits the matching relationship of the three elements N, C, and B. This is mainly because during the superconducting phase aging heat treatment process, since N and C are also interstitial solid solution atoms, they will be adsorbed by the grain boundary at the same time, generating M 23 C6 and Cr2N are along the crystal phase, so these three elements compete with each other at the grain boundaries. The present invention limits the relationship between the three elements B, C, and N to increase the internal adsorption effect of the grain boundary on the B element and inhibit the precipitation of other carbonitrides, thereby obtaining excellent low-temperature toughness.

[0078] Specifically, the austenitic steel of the present invention has an internal adsorption capacity F(B) for the B element at the grain boundaries greater than 0.15, 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. The present invention limits the matching of the N, C, and B elements primarily because, as interstitial solid solution elements, C and N compete with B during heat treatment, rapidly segregating toward the grain boundaries to form large-sized carbonitrides, which reduces the material's toughness and ductility. Therefore, by rationally controlling the C, N, and B values, while ensuring the steel's strength, the advantage of B's ​​grain boundary segregation is utilized to preemptively occupy the grain boundaries, inhibiting the nucleation of other carbonitrides at the grain boundaries and thereby improving the ductility and toughness of the austenitic steel of the present invention.

[0079] In addition, the present invention preferably further optimizes the solution heat treatment temperature. The inventors' research has found that during the solution heat treatment stage, dispersed and fine (Nb, V)N and Cr(Nb, V)N will also precipitate. While large-scale primary MX phases can be essentially eliminated through compositional limitations, as the ingot size increases, segregation can lead to micro-region composition fluctuations, and small amounts of harmful MX phases may still form. Therefore, the present invention optimizes the solution heat treatment process, increasing the precipitation of dispersed and fine (Nb, V)N and Cr(Nb, V)N strengthening phases while minimizing the potential for full dissolution of large-scale MX phases during the solution stage, minimizing their impact on toughness and ductility.

[0080] In summary, the austenitic steel provided by the present invention (austenitic steel that is resistant to aging and has excellent ultra-low temperature strength and toughness) has no carbonitrides or intermetallic compounds precipitated at the grain boundaries after aging heat treatment in the austenite sensitization range, or the equivalent particle size of the grain boundary precipitates is ≤50nm.

[0081] The details are as follows: The austenitic steel of the present invention differs from conventional austenitic steel in that it maintains excellent ultra-low temperature strength and ductility after aging in the austenite sensitization range. In other words, the austenitic steel of the present invention exhibits excellent aging resistance. By rationally adjusting the composition and ratio of the components, the present invention ensures that after aging in the austenite sensitization range, no carbonitrides or intermetallic compounds precipitate at grain boundaries, or that the equivalent particle size of the precipitates at grain boundaries is ≤50 nm. Consequently, the austenitic steel of the present invention exhibits aging resistance.

[0082] Preferably, for example, a specific process of the austenite sensitization range aging heat treatment or superconducting phase aging heat treatment is:

[0083] 600~700℃×(30~200h) or

[0084] 150~220℃×30~60h+350~450℃×30~60h+600~750℃×40~60h.

[0085] Preferably, the mechanical properties of the austenitic steel of the present invention after superconducting phase aging heat treatment are as follows: yield strength>1300MPa, tensile strength>1700MPa, elongation>25%, impact toughness>100J, fracture toughness>130MPa·m at 4.2K 1 / 2 .

[0086] On the other hand, an embodiment of the present invention further provides a method for preparing the above-mentioned austenitic steel, comprising the following steps:

[0087] 1) preparing raw materials according to the chemical composition of the austenitic steel, performing smelting treatment, and casting to obtain an ingot; wherein, the nitrogen content in the ingot is ensured to be greater than 0.25wt% and less than 0.45wt%.

[0088] The smelting process is carried out in a vacuum induction furnace, and the nitrogen atmosphere pressure is maintained at 0.03-0.07 MPa during the smelting process. Furthermore, when the smelting temperature is 1450-1520°C, nitrogen is continuously introduced to promote nitrogen alloying (increasing the solid solubility of nitrogen to ensure that the nitrogen content is greater than 0.25%).

[0089] 2) The ingot is subjected to electroslag remelting treatment to obtain electroslag ingots, the purpose of which is to reduce defects, purify the steel ingots, and significantly reduce the inclusion content in the steel;

[0090] 3) forging the electroslag ingot to obtain a forging blank, wherein the forging temperature is 850-1200° C.;

[0091] 4) performing a solution treatment on the forging blank to obtain the austenitic steel.

[0092] Wherein, the solution treatment process satisfies:

[0093] The temperature of the solution treatment is 1000-1150°C; or

[0094] The temperature of the solution treatment satisfies {1020+[Nb]×580+[V]×125}°C±(0-3)°C, where [Nb] and [V] are the mass percentages of Nb and V, respectively; the duration of the solution treatment satisfies H×(2-4 min / mm), where H is the thickness of the steel ingot, in mm.

[0095] Preferably, for example, the preparation method further includes an aging heat treatment step; specifically, aging the cooled austenitic steel. The aging heat treatment step is not essential for preparing the austenitic steel of the present invention. The aging heat treatment step is an austenite sensitization range aging heat treatment, preferably a superconducting phase aging heat treatment. Preferably, the specific aging heat treatment process is:

[0096] 600~700℃×(30~200h) or

[0097] 150~220℃×30~60h+350~450℃×30~60h+600~750℃×40~60h;

[0098] That is to say, the aging heat treatment is to keep the temperature at 640-700℃ for 30-200h, or the aging heat treatment is to keep the temperature at 150-220℃ for 30-60h, then heat it to 350-450℃ for 30-60h, and then continue to heat it to 600-750℃ for 40-60h.

[0099] The present invention is further described below by means of specific examples:

[0100] Example 1

[0101] This embodiment prepares an austenitic steel, wherein the composition ratio of the austenitic steel of this embodiment is shown in Table 1. Specifically, the preparation method includes the following steps:

[0102] 1) preparing raw materials according to the chemical composition of the austenitic steel, performing smelting treatment, and casting to obtain an ingot; wherein, the nitrogen content of the ingot is ensured to be greater than 0.25wt% and less than 0.45wt%.

[0103] 2) The ingot is subjected to electroslag remelting treatment to obtain electroslag ingots, the purpose of which is to reduce defects, purify the steel ingots, and significantly reduce the inclusion content in the steel.

[0104] 3) forging the electroslag ingot at an initial forging temperature of 1200° C. and a final forging temperature of 900° C. to obtain a forging blank;

[0105] 4) performing a solution treatment on the forging blank to obtain the austenitic steel, wherein the solution treatment temperature is 1050° C. and the duration of the solution treatment satisfies H×(4 min / mm), where H is the thickness of the steel ingot in mm.

[0106] Example 2

[0107] In this embodiment, an austenitic steel is prepared. The composition ratio of the austenitic steel in this embodiment is shown in Table 1.

[0108] The preparation steps of the austenitic steel in this embodiment are the same as those in Example 1.

[0109] Example 3

[0110] In this embodiment, an austenitic steel is prepared. The composition ratio of the austenitic steel in this embodiment is shown in Table 1.

[0111] The preparation steps of the austenitic steel in this embodiment are the same as those in Example 1.

[0112] Comparative Example 1

[0113] Comparative Example 1: An austenitic steel was prepared. The composition ratio of the austenitic steel in Comparative Example 1 is shown in Table 1.

[0114] The preparation steps of the austenitic steel in Comparative Example 1 are the same as those in Example 1.

[0115] Comparative Example 2

[0116] Comparative Example 2: An austenitic steel was prepared. The composition ratio of the austenitic steel in Comparative Example 2 is shown in Table 1.

[0117] The preparation steps of the austenitic steel in Comparative Example 2 are the same as those in Example 1.

[0118] Comparative Example 3

[0119] Comparative Example 3: An austenitic steel was prepared. The composition ratio of the austenitic steel in Comparative Example 3 is shown in Table 1.

[0120] The preparation steps of the austenitic steel in Comparative Example 3 are the same as those in Example 1.

[0121] Comparative Example 4

[0122] Comparative Example 4: An austenitic steel is prepared. The composition ratio of the austenitic steel in Comparative Example 4 is shown in Table 1.

[0123] The preparation steps of the austenitic steel in Comparative Example 4 are the same as those in Example 1.

[0124] Comparative Example 5

[0125] Comparative Example 5: An austenitic steel was prepared. The composition ratio of the austenitic steel in Comparative Example 5 is shown in Table 1.

[0126] The preparation steps of the austenitic steel in Comparative Example 5 are the same as those in Example 1.

[0127] The chemical compositions of the austenitic steels prepared in Examples 1-3 and Comparative Examples 1-5 are shown in Table 1.

[0128] Table 1

[0129] The properties of the austenitic steels prepared in Examples 1-3 and Comparative Examples 1-5 after superconducting phase aging heat treatment are shown in Table 2. The specific process of the superconducting phase aging heat treatment is 210°C×48h+400°C×48h+665°C×50h.

[0130] Table 2

[0131] Among them, Figure 3 is a scanning electron micrograph of the austenitic steel prepared in Example 1 after aging treatment (the arrows in Figure 3 indicate grain boundaries); Figure 4 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 1 after aging treatment (the arrows in Figure 4 indicate grain boundaries); Figure 5 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 2 after aging treatment; Figure 6 is a scanning electron micrograph of the austenitic steel prepared in Example 2 after aging treatment; Figure 7 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 3 after aging treatment; Figure 8 is a scanning electron micrograph of the austenitic steel prepared in Example 3 after aging treatment; Figure 9 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 4 after aging treatment. Figure 10 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 5 after aging treatment.

[0132] Here, according to Table 1, Table 2, and Figures 3-9, we can see that:

[0133] (1) It can be clearly seen from the comparison of Table 1 and Table 2 that the austenitic steels prepared in Examples 1-3 of the present invention, which contain trace amounts of Nb and V and the content of the B element is limited to 10-65 ppm, have significantly better comprehensive properties after aging heat treatment than Comparative Examples 1-5.

[0134] (2) By comparing the microstructures of the austenitic steels prepared in Example 1 ( FIG. 3 ), Comparative Example 1 ( FIG. 4 ), and Comparative Example 2 ( FIG. 5 ), it can be seen that:

[0135] When an appropriate amount of element B is contained, the grain boundaries of the austenitic steel after aging heat treatment are clean and no obvious precipitation phase is precipitated (see Example 1 and FIG3 ).

[0136] In Comparative Example 1, since the B content is lower than 0.0010%, the internal adsorption effect of the B element on the grain boundary is weak. Therefore, a large number of intergranular precipitation phases exist at the grain boundary, as shown in FIG4 .

[0137] When the B content exceeds the limit of the present invention, as in Comparative Example 2 (see FIG5 ), a large amount of BN inclusions are formed, thereby damaging the low-temperature toughness and plasticity.

[0138] (3) By comparing Example 2 and Comparative Example 3, it can be seen that:

[0139] When containing trace amounts of Nb and V, i.e., 0.03-0.08 wt% Nb and 0.1-0.3 wt% V (Example 2), the microstructure of the austenitic steel is shown in FIG6 . After aging heat treatment, fine dispersed precipitates containing Nb and V can be formed in the grains.

[0140] When the Nb content exceeds 0.08 wt% (especially exceeding 0.1 wt%) and the V content exceeds 0.3 wt% (Comparative Example 3), the microstructure of the austenitic steel is shown in FIG7 , and large-sized MX phase precipitates, thereby damaging the toughness.

[0141] (4) By comparing Example 3 and Comparative Example 4, it can be seen that:

[0142] When containing trace amounts of Nb and V, and the B element content is limited to 10-65 ppm, and F(B) meets the 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 number of fine dispersed precipitates can be seen, and the grain boundaries are relatively clean and free of precipitates.

[0143] However, when the B element limits F(B) and does not meet the requirements, as shown in Comparative Example 4, a large number of large-sized carbonitrides in the austenitic steel will also precipitate at the grain boundaries after aging treatment, resulting in a significant decrease in 4.2K toughness and ductility.

[0144] (5) By comparing Example 1 and Comparative Example 5, it can be seen that:

[0145] When the B content is limited to 10-65 ppm without adding Nb and V, as shown in Comparative Example 5 and FIG10 , although austenitic steel with excellent toughness and ductility is obtained, the strength of the austenitic steel is relatively low because no precipitate phase is generated.

[0146] The above comparison of Examples 1-3 and Comparative Examples 1-5 mainly proves that in the austenitic steel formula design of the present invention, by simultaneously adding Nb, V, and B and controlling the addition amounts of Nb, V, N, C, and B, a synergistic effect is achieved, so that the steel of the present invention still has high strength and high toughness after superconducting phase aging heat treatment, thereby meeting the strength and toughness requirements of superconducting coil armor materials.

[0147] Furthermore, the present invention demonstrates through the following Example 4 that, based on the design of the austenitic steel formulation of the present invention, the design of the solution treatment temperature can further improve the performance of the austenitic steel, as follows:

[0148] Example 4:

[0149] This embodiment prepares an austenitic steel, wherein the composition of the austenitic steel of this embodiment is the same as that of Example 2. The difference between the preparation method of the austenitic steel of this embodiment and that of Example 2 is:

[0150] In step 4): the forging blank is subjected to a solution treatment to obtain the austenitic steel, wherein the solution treatment temperature is 1098°C (satisfying the formula {1020+[Nb]×580+[V]×125}°C±(0~3)°C).

[0151] The other steps are exactly the same as those in Example 2.

[0152] Here, the properties of the austenitic steels prepared in Example 2 and Example 4 after the superconducting phase aging heat treatment are shown in Table 3. The specific process of the superconducting phase aging heat treatment is 210°C×48h+400°C×48h+665°C×50h.

[0153] Table 3

[0154] As can be seen from Table 3, by designing the solution treatment temperature, the performance (strength and toughness) of the austenitic steel prepared in Example 4 is further improved compared to that in Example 2. The main reason is that the inventors of the present invention have found that the large-sized primary MX phase that impairs toughness and ductility is mainly a compound of Nb and V. Although the composition limitation of the present invention can basically eliminate large-sized primary MX phases, as the ingot size increases, segregation may cause micro-region composition fluctuations, and a small amount of harmful MX phases may still be generated. Therefore, the inventors further optimized the solution heat treatment process to fully dissolve the large-sized MX phase in the solution stage, reducing its damage to toughness and ductility, thereby further improving the performance of the austenitic steel.

[0155] In summary, the embodiments of the present invention provide an austenitic steel and a preparation method thereof, and a superconducting coil armor; wherein, after the austenitic steel of this embodiment undergoes an aging heat treatment in the austenite sensitization range, no nitrides are precipitated at the grain boundaries, or the equivalent particle size of the precipitates at the grain boundaries is ≤50nm. After the austenitic steel undergoes a superconducting phase aging heat treatment, the yield strength σ s >1300MPa, tensile strength σ b>1700MPa, elongation A>30%, impact toughness AKV2>150J. Therefore, the austenitic steel provided by the embodiment of the present invention still has high strength and high toughness after the superconducting phase aging heat treatment, which can meet the strength and toughness requirements of superconducting coil armor materials.

[0156] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiment based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. An austenitic steel, characterized in that: The chemical composition of the austenitic steel includes, by mass percentage: C: greater than 0 and less than or equal to 0.02wt%; Mn: 3.5-8.0wt%; Si: 0-1wt%; Ni: 10-15wt%; Cr: 19-25wt%; Mo: 1-3 wt%; V: 0.1wt%~0.3wt%; Nb: 0.01-0.1wt%; N: greater than 0.25wt% and less than 0.45wt%; B: 10-65ppm; Al: less than 0.03wt%; Ti: less than 0.001wt%; The balance is Fe.

2. The austenitic steel according to claim 1, characterized in that: The internal adsorption capacity F(B) of the grain boundary of the austenitic steel for the B element is greater than 0.15, so as to increase the internal adsorption effect of the grain boundary on the B element, inhibit the precipitation of carbonitrides and intermetallic compounds, and thus obtain excellent low-temperature toughness; wherein, F(B)=[B] / (0.034[N]+0.23[C]); wherein [N], [C], and [B] are the mass percentages of N, C, and B, respectively.

3. The austenitic steel according to claim 1 or 2, characterized in that: The mass percentage of C is less than or equal to 0.015wt%.

4. The austenitic steel according to any one of claims 1 to 3, characterized in that: The mass percentage of Nb is 0.03-0.08wt%.

5. The austenitic steel according to any one of claims 1 to 4, characterized in that: After the austenitic steel is subjected to aging heat treatment in the austenite sensitization range, no carbonitride or intermetallic compound is precipitated at the grain boundary; or After the austenitic steel is subjected to aging heat treatment in the austenite sensitization range, the equivalent particle size of the grain boundary precipitates is ≤50nm.

6. The austenitic steel according to any one of claims 1 to 5, characterized in that: After the superconducting phase aging heat treatment, the austenitic steel will precipitate dispersed compounds with a particle size of 10 to 200 nm, so that the austenitic steel has an excellent precipitation strengthening effect, thereby ensuring low-temperature strength; wherein the compounds include three elements: N, V, and Nb.

7. The austenitic steel according to any one of claims 1 to 6, characterized in that: During the superconducting phase aging heat treatment process of the austenitic steel, the B element is affected by the grain boundary energy, diffuses to the grain boundary, occupies the grain boundary position, improves the grain boundary strength, inhibits the generation and expansion of intergranular cracks, and thus improves the low-temperature toughness and plasticity.

8. The austenitic steel according to any one of claims 1 to 7, characterized in that: After the austenitic steel is subjected to superconducting phase aging heat treatment, the mechanical properties are as follows: At 4.2K, the yield strength of the austenitic steel is greater than 1300MPa, the tensile strength is greater than 1700MPa, the elongation is greater than 25%, the impact toughness is greater than 100J, and the fracture toughness is greater than 130MPa·m 1 / 2 .

9. The method for preparing austenitic steel according to any one of claims 1 to 8, characterized in that: It includes the following steps: 1) preparing raw materials according to the chemical composition of the austenitic steel, performing smelting treatment, and obtaining an ingot after casting; 2) performing electroslag remelting on the ingot to obtain an electroslag ingot; 3) forging the electroslag ingot to obtain a forging blank; 4) performing a solid solution treatment on the forging blank to obtain the austenitic steel.

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

11. The method for preparing austenitic steel according to claim 9, characterized in that: In the step 1): The smelting process is carried out in a vacuum induction furnace; wherein, during the smelting process, the nitrogen atmosphere pressure is ensured to be 0.03-0.07MP.

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

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

14. The method for preparing austenitic steel according to claim 9, characterized in that: After step 4), the method further comprises: Step 5) performing superconducting phase aging treatment on the austenitic steel.

15. A superconducting coil armor, characterized in that: The superconducting coil armor is made of the austenitic steel described in any one of claims 1-8.

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