An austenitic steel and a method for producing the same, superconducting coil armor

Abstract

The application relates to an austenitic steel and a preparation method thereof and a superconducting coil armor, and mainly adopts the technical scheme that the chemical components of the austenitic steel include the following components in percentage by mass: C is greater than 0 and less than or equal to 0.02 wt%; Mn is 3.5-8.0 wt%; Si is 0-1 wt%; Ni is 10-15 wt%; Cr is 19-25 wt%; Mo is 1-3 wt%; V is 0.1 wt%-0.3 wt%; Nb is 0.01-0.1 wt%; N is greater than 0.25 wt% and less than 0.45 wt%; B is 10-65 ppm; Al is less than 0.03 wt%; Ti is less than 0.001 wt%; and the balance is Fe. The application mainly aims to provide an aging-resistant austenitic steel which 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.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal materials, and particularly relates to an austenitic steel, a preparation method thereof and a superconducting coil armor. BACKGROUND

[0002] The superconducting coil is a main component of a fusion experimental reactor, and can provide a high magnetic field for normal operation of the fusion reactor. However, a complex service environment brings great challenges to the superconducting coil armor material. In addition, in the manufacturing process of the superconducting coil, the superconducting wire material must be subjected to aging heat treatment, and the superconducting coil armor and the superconducting wire material must also be subjected to superconducting phase aging heat treatment at 400-750 DEG C. However, the heat treatment process will cause sensitization of the conventional austenitic steel, and the toughness, plasticity and corrosion resistance are significantly reduced. In addition, the yield strength of the traditional low-temperature austenitic steel such as 316LN, JJ1, JK2LB and the like at 4.2K is lower than 1300MPa.

[0003] Therefore, it is necessary to provide a time-aging-resistant structural steel which meets the strength and toughness requirements of the superconducting coil armor material and has excellent strength and toughness characteristics at ultra-low temperature after aging heat treatment. SUMMARY

[0004] Therefore, the present application provides an austenitic steel, a preparation method thereof and a superconducting coil armor, and mainly aims to provide a time-aging-resistant austenitic steel which 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.

[0005] To achieve the above-mentioned purpose, the present application mainly provides the following technical scheme:

[0006] In one aspect, the embodiment of the present application provides an austenitic steel, wherein the chemical composition of the austenitic steel includes, in terms of mass percentage:

[0007] C: greater than 0 and less than or equal to 0.02wt%;

[0008] Mn: 3.5-8.0wt%;

[0009] Si: 0-1wt%;

[0010] Ni: 10-15wt%;

[0011] Cr: 19-25wt%;

[0012] Mo: 1-3wt%;

[0013] V: 0.1-0.3wt%;

[0014] Nb: 0.01-0.1wt%;

[0015] N: more than 0.25wt%, less than 0.45wt%;

[0016] B: 10-65ppm;

[0017] Al: less than 0.03wt%;

[0018] Ti: less than 0.001wt%;

[0019] the balance being Fe.

[0020] Preferably, the internal adsorption capacity F(B) of the grain boundary of the austenitic steel to B element is greater than 0.15, so as to increase the internal adsorption of the grain boundary to B element, inhibit the precipitation of carbonitride and intermetallic compound, and thus obtain excellent low-temperature toughness; wherein, F(B) = [B] / (0.034[N]+0.23[C]); wherein, [N], [C], [B] are the mass percentage contents of N, C and B respectively.

[0021] Preferably, the mass percentage content of C is less than or equal to 0.015wt%.

[0022] Preferably, the mass percentage content of Nb is 0.03-0.08wt%.

[0023] Preferably, after the austenitic steel is subjected to aging heat treatment in the sensitization interval, no carbonitride and intermetallic compound is precipitated in the grain boundary; or after the austenitic steel is subjected to aging heat treatment in the sensitization interval, the equivalent particle diameter of the precipitates in the grain boundary is less than or equal to 50nm. It should be noted that the term "sensitization interval" mentioned in the present application refers to the heat treatment at 400-850℃ after solid solution treatment of the alloy material, and at this temperature, the austenitic stainless steel will cause intergranular chromium depletion due to the precipitation of chromium carbide, which increases the intergranular corrosion tendency of the material. This temperature range is the sensitization interval.

[0024] Preferably, the austenitic steel is subjected to superconducting phase aging heat treatment, which will precipitate dispersed compounds with a particle size of 10-200nm, so as to make the austenitic steel have excellent precipitation strengthening effect, thereby ensuring the low-temperature strength; wherein, the compounds include N, V and Nb three elements (the compounds mainly consist of N, V and Nb three elements, and additionally have Cr, Fe, Mo and other elements). It should be noted that the term "superconducting phase aging heat treatment" mentioned in the present application refers to: in order to make the Nb3Sn superconducting wire have superconducting characteristics, it needs to be subjected to heat treatment in the interval of 400-750℃, and the austenitic steel of the present application as the supporting and protective material of Nb3Sn must also passively accept the aging heat treatment process.

[0025] Preferably, during the superconducting phase aging heat treatment, 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, and inhibits the generation and expansion of intergranular cracks, so as to improve the low-temperature toughness and plasticity.

[0026] 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 >1300MPa, the tensile strength is >1700MPa, the elongation is >25%, the impact toughness is >100J, and the fracture toughness is >130MPa·m 1 / 2 .

[0027] In another aspect, the embodiments of the present application provide a preparation method of the austenitic steel described in any one of the above, which comprises the following steps:

[0028] 1) Preparing raw materials according to the chemical composition of the austenitic steel, performing a melting treatment, and obtaining an ingot after casting;

[0029] 2) Performing an electroslag remelting treatment on the ingot to obtain an electroslag ingot;

[0030] 3) Performing a forging treatment on the electroslag ingot to obtain a forged blank;

[0031] 4) Performing a solid solution treatment on the forged blank to obtain the austenitic steel.

[0032] Preferably, in the step 1), the N content in the ingot is greater than 0.25wt% and less than 0.45wt%; and / or the melting treatment is performed in a vacuum induction furnace; wherein the nitrogen atmosphere pressure value is 0.03-0.07MP during the melting process; and / or when the melting temperature is 1450-1520℃, nitrogen is continuously introduced to promote nitrogen alloying.

[0033] Preferably, in the step 3), the temperature of the forging treatment is 850-1200℃.

[0034] Preferably, the temperature of the solid solution treatment is 1000-1150℃; and preferably, the length of the solid solution treatment satisfies Hx(2-4min / mm), wherein H is the thickness of the ingot, in mm.

[0035] Preferably, in the step 4), the temperature of the solid solution treatment satisfies {1020+[Nb]x580+[V]x125}℃±(0-3)℃, wherein [Nb] and [V] are the mass percentage contents of Nb and V, respectively; and the length of the solid solution treatment satisfies Hx(2-4min / mm), wherein H is the thickness of the ingot, in mm.

[0036] Preferably, after the step 4), the method further comprises:

[0037] Step 5) subjecting the austenitic steel to superconducting phase aging treatment.

[0038] In still another aspect, the embodiments of the present application provide a superconducting coil armor, wherein the superconducting coil armor is made of the austenitic steel according to any one of the above.

[0039] Compared with the prior art, the austenitic steel and the preparation method thereof and the superconducting coil armor of the present application have at least the following beneficial effects:

[0040] The embodiments of the present application provide an austenitic steel, and the inventors of the present application have made a reasonable setting of the composition of the austenitic steel through sufficient research, and the composition of the austenitic steel specifically comprises, in terms of weight percentage, 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. Herein, (1) a small amount of Nb and V is added, that is, when the content of Nb is controlled to be 0.01-0.1wt% (preferably 0.03-0.08wt%) and the content of V is controlled to be 0.10-0.30wt%, the low-temperature toughness and plasticity can be obtained, and the precipitation strengthening effect can be maximized to obtain a better strength and toughness match. (2) Under the condition of containing a small amount of Nb and V, the austenitic steel of the present application further comprises B element, and the content of B element is strictly limited; so that the B element diffuses to the grain boundary under the action of the grain boundary energy in the aging process, occupies the grain boundary position, improves the grain boundary strength, and inhibits the generation and expansion of intergranular cracks, thereby improving the low-temperature toughness and plasticity. (3) Under the above condition, the present application further limits the matching relationship of N, C and B elements; mainly in the superconducting phase aging heat treatment process, N and C are also interstitial solid solution atoms, which will be adsorbed by the grain boundary at the same time to generate intergranular compounds such as Cr2N and C6, so there is a competitive relationship among the three elements at the grain boundary. The present application limits the relationship among the three elements of B, C and N to increase the internal adsorption of B element by the grain boundary and inhibit the precipitation of other carbonitrides, thereby obtaining excellent low-temperature toughness. 23 C6 and Cr2N, so there is a competitive relationship among the three elements at the grain boundary. The present application limits the relationship among the three elements of B, C and N to increase the internal adsorption of B element by the grain boundary and inhibit the precipitation of other carbonitrides, thereby obtaining excellent low-temperature toughness.

[0041] In summary, the present application has high strength and toughness by adding Nb, V and B simultaneously and controlling the adding amount of Nb, V, N, C and B, so that the steel has high strength and toughness after superconducting phase aging heat treatment, to meet the strength and toughness requirements of superconducting coil armor materials.

[0042] In another aspect, the present application provides a preparation method of the above-mentioned austenitic steel, which ensures that the composition meets the above requirements, and the temperature of the solid solution treatment is designed and selected, specifically, in the step 4), the temperature of the solid solution treatment meets {1020+[Nb]×580+[V]×125}℃±(0-3)℃, wherein [Nb] and [V] are the mass percentage of Nb and V respectively; by such setting, the precipitation strengthening effect of Nb and V elements can be more fully played, and the harmful MX phase of large size is further eliminated, to ensure low temperature strength and plasticity.

[0043] The above description is only a summary of the technical scheme of the present application, in order to more clearly understand the technical means of the present application, and the content of the specification can be implemented, the following preferred embodiments of the present application are described in detail with the help of the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 is the boride precipitation diagram in the austenitic steel;

[0045] Figure 2 is the BN impurity diagram in the austenitic steel;

[0046] Figure 3 is the scanning electron microstructure diagram of the austenitic steel prepared in example 1 after aging treatment;

[0047] Figure 4 is the scanning electron microstructure diagram of the austenitic steel prepared in comparative example 1 after aging treatment;

[0048] Figure 5 is the scanning electron microstructure diagram of the austenitic steel prepared in comparative example 2 after aging treatment;

[0049] Figure 6 is the scanning electron microstructure diagram of the austenitic steel prepared in example 2 after aging treatment;

[0050] Figure 7 is the scanning electron microstructure diagram of the austenitic steel prepared in comparative example 3 after aging treatment;

[0051] Figure 8 is the scanning electron microstructure diagram of the austenitic steel prepared in example 3 after aging treatment;

[0052] Figure 9A scanning electron micrograph of the austenitic steel prepared for Comparative Example 4 after aging treatment;

[0053] Figure 10 A scanning electron micrograph of the austenitic steel prepared for Comparative Example 5 after aging treatment. DETAILED DESCRIPTION

[0054] To further illustrate the technical means and effects taken by the present application to achieve the intended purposes, the specific embodiments, structures, features and effects according to the present application are described in detail below in conjunction with the accompanying drawings and preferred embodiments. In the following description, different "an embodiment" or "embodiments" do not necessarily refer to the same embodiment. In addition, the specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0055] In one aspect, the present embodiments provide an austenitic steel, the chemical composition of which includes, in terms of weight percentage:

[0056] C: greater than 0 and less than or equal to 0.02wt% (preferably less than 0.015wt%);

[0057] Mn: 3.5-8.0wt%;

[0058] Si: 0-1wt%;

[0059] Ni: 10-15wt%;

[0060] Cr: 19-25wt%;

[0061] Mo: 1-3wt%;

[0062] V: 0.1wt%-0.3wt%;

[0063] Nb: 0.01-0.1wt% (preferably 0.03-0.08wt%);

[0064] N: greater than 0.25wt% and less than 0.45wt%;

[0065] B: 10-65ppm;

[0066] Al: less than 0.03wt%;

[0067] Ti: less than 0.001wt%;

[0068] the balance being Fe.

[0069] Here, it needs to be explained that regarding the above component design:

[0070] (1) The present application sets the composition of the austenitic steel by sufficient research, and the composition is that the content of Nb is 0.01-0.1wt% (preferably 0.03-0.08wt%), and the content of V is 0.10-0.30wt%. It is found by research that when the content of Nb exceeds 0.1wt%, and the content of V exceeds 0.30wt%, Nb and V will combine with N element during the solidification of the ingot, and large-size primary MX phase is generated, which is a hard phase, and is not coordinated with the deformation of the matrix during the deformation process, thereby easily leading to the preferential generation of cracks at the interface of MX phase and matrix phase, and further easily leading to the initiation and expansion of the cracks, thereby reducing the low-temperature toughness and plasticity of the aging-resistant steel. However, during the superconducting phase aging heat treatment process 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 excellent precipitation strengthening effect, thereby ensuring the low-temperature strength. In summary, when the content of Nb is 0.01-0.1wt% (preferably 0.03-0.08wt%), and the content of V is 0.10-0.30wt%, the low-temperature toughness and plasticity can be obtained, and the precipitation strengthening effect can be maximized to obtain the optimal strength and toughness matching. It should be noted that Nb and V are both strong nitride-forming elements, and it is found by research that only if one of Nb and V is added, the precipitated phase during the aging process is less, and the precipitation strengthening effect is weak. In addition, if the content of Nb is controlled to be 0.03-0.08wt%, better strength and toughness matching can be obtained.

[0071] (2) Under the condition of containing trace Nb and V, the austenitic steel of the present application further adds B element in the chemical composition, and the content of B element is strictly limited.

[0072] During the superconducting phase aging heat treatment process at 400-750℃, the austenitic steel will have sensitization phenomenon, and a large amount of nitrides (mainly Cr2N) or carbides (M 23 C6) phase will occupy the grain boundary position, leading to the embrittlement of the grain boundary. The inventors of the present application find by research that B element will diffuse to the grain boundary under the action of the grain boundary energy during the aging process, and occupy the grain boundary position, thereby improving the strength of the grain boundary, inhibiting the generation and expansion of intergranular cracks, and improving the low-temperature toughness and plasticity. The present application limits the content of B element to be 10-65ppm. When the content of B is less than 10ppm, the B concentration in the matrix is low, and good toughening effect cannot be obtained, and when the content of B exceeds 65ppm, a large amount of borides (see Figure 1 ) and BN inclusions (see Figure 2 ) will be generated in the matrix, thereby significantly reducing the beneficial effect of B.

[0073] It should be noted that if Nb and V are not added, and only B is added, no aging precipitated phase is generated, and the strength is low.

[0074] (3) In the above conditions, the present application further limits the matching relationship of N, C and B. This is mainly because during the aging heat treatment in the superconducting phase, N and C are also interstitial solid solution atoms, which will be adsorbed by the grain boundary at the same time, and M 23 C6 and Cr2N are intergranular phases, so there is a competitive relationship between the three elements at the grain boundary. The present application limits the relationship between B, C and N to increase the internal adsorption of B elements by the grain boundary, and suppresses the precipitation of other carbonitrides, thereby obtaining excellent low-temperature toughness.

[0075] Specifically, the internal adsorption capacity F(B) of the grain boundary of the austenitic steel of the present application to B element is greater than 0.15, where F(B) = [B] / (0.034[N]+0.23[C]), [N], [C] and [B] are the mass percentage contents of N, C and B respectively. The present application limits the matching of N, C and B. Mainly because C and N elements as interstitial solid solution elements, compete with B element during heat treatment, quickly segregate to grain boundary to form large size carbonitride, reduce material toughness and plasticity, therefore reasonable control C, N, B, while ensuring the strength of the steel, use the grain boundary segregation advantage of B element, occupy the grain boundary position in advance, inhibit the nucleation of other carbonitrides at the grain boundary, and improve the plasticity and toughness of the austenitic steel of the present application.

[0076] In addition, preferably, the present application further optimizes the solid solution heat treatment temperature. The inventors have found that during the solid solution heat treatment stage, (Nb, V)N and Cr(Nb, V)N will also precipitate, and although the composition is limited, the large size MX phase can be basically eliminated, but as the size of the ingot increases, segregation may cause micro-area composition fluctuation, and a small amount of MX harmful phase may still be generated, therefore the present application optimizes the solid solution heat treatment process, increases the precipitation of dispersed fine (Nb, V)N and Cr(Nb, V)N strengthening phases, and at the same time, makes the large size MX phase fully dissolve as much as possible in the solid solution stage, and reduces its damage to toughness and plasticity.

[0077] In summary, the present application provides an austenitic steel (austenitic steel with excellent super-low temperature strength and toughness, and resistance to aging) which has no carbonitride or intermetallic compound precipitated at the grain boundary, or the equivalent particle size of the grain boundary precipitates is less than or equal to 50 nm after aging heat treatment in the austenitic sensitive interval.

[0078] The application is described in detail as follows: the austenitic steel of the application is different from common austenitic steel in that the austenitic steel of the application still has good super-low-temperature strength and plasticity after aging treatment in the austenitic sensitization interval, that is, the austenitic steel of the application has good aging resistance. The application adjusts the component composition and ratio reasonably, so that no carbonitride and intermetallic compound is precipitated at the grain boundary or the equivalent particle diameter of the grain boundary precipitates is less than or equal to 50 nm after aging treatment in the austenitic sensitization interval, and therefore, the austenitic steel of the application has the aging resistance characteristic.

[0079] Preferably, for example, a specific process of the austenitic sensitization interval aging heat treatment or superconducting phase aging heat treatment is as follows:

[0080] 600-700℃×(30-200h) or

[0081] 150-220℃×30-60h+350-450℃×30-60h+600-750℃×40-60h.

[0082] Preferably, the mechanical properties of the austenitic steel of the application after the superconducting phase aging heat treatment are as follows: at 4.2K, the yield strength 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 .

[0083] On the other hand, the application also provides a preparation method of the austenitic steel, which comprises the following steps:

[0084] 1) preparing raw materials according to the chemical composition of the austenitic steel, performing melting treatment, and obtaining an ingot after casting; wherein, the N content in the ingot is greater than 0.25wt% and less than 0.45wt%.

[0085] The melting treatment is performed in a vacuum induction furnace, and the nitrogen atmosphere pressure value is ensured to be 0.03-0.07MPa during the melting process. Further, when the melting temperature is 1450-1520℃, nitrogen is continuously introduced to promote the alloying of nitrogen (to increase the solid solubility of nitrogen, so as to ensure that the nitrogen content is greater than 0.25%).

[0086] 2) performing electroslag remelting treatment on the ingot to obtain an electroslag ingot, which aims to reduce defects, purify the ingot, and greatly reduce the inclusion content in the steel;

[0087] 3) performing forging treatment on the electroslag ingot to obtain a forged blank, wherein the temperature of the forging treatment is 850-1200℃;

[0088] 4) performing solid solution treatment on the forged blank to obtain the austenitic steel.

[0089] The solid solution treatment process satisfies:

[0090] The temperature of the solid solution treatment is 1000-1150℃; or

[0091] The temperature of the solid solution treatment satisfies {1020+[Nb]×580+[V]×125}℃±(0-3)℃, wherein [Nb] and [V] are the mass percentage contents of Nb and V respectively; the time length of the solid solution treatment satisfies H×(2-4min / mm), wherein H is the thickness of the ingot, in mm.

[0092] Preferably, for example, the preparation method further comprises an aging heat treatment step; specifically, the austenitic steel after the cooling treatment is subjected to the aging heat treatment. Here, the aging heat treatment step is not necessary for preparing the austenitic steel of the present application. Here, the aging heat treatment step is an aging heat treatment in the austenite sensitization range, preferably a superconducting phase aging heat treatment, and more preferably, the aging heat treatment process is as follows:

[0093] 600-700℃×(30-200h) or

[0094] 150-220℃×30-60h+350-450℃×30-60h+600-750℃×40-60h;

[0095] That is, the aging heat treatment is at 640-700℃ for 30-200h, or the aging heat treatment is first at 150-220℃ for 30-60h, then heated to 350-450℃ for 30-60h, and then heated to 600-750℃ for 40-60h.

[0096] The present application is further illustrated by the following specific examples:

[0097] Example 1

[0098] In this example, an austenitic steel is prepared, wherein the component ratio of the austenitic steel of this example is shown in Table 1. Specifically, the preparation method comprises the following steps:

[0099] 1) Prepare raw materials according to the chemical composition of the austenitic steel, perform melting treatment, and after casting, obtain an ingot; wherein, and ensure that the N content of the ingot is greater than 0.25wt% and less than 0.45wt%.

[0100] 2) Perform electroslag remelting treatment on the ingot to obtain an electroslag ingot, the purpose of which is to reduce defects, purify the ingot, and significantly reduce the inclusion content in the steel.

[0101] 3) Perform forging treatment on the electroslag ingot, with an initial forging temperature of 1200℃ and a final forging temperature of 900℃, to obtain a forged blank;

[0102] 4) solid-solution treating the forged blank to obtain the austenitic steel, wherein the solid-solution treating temperature is 1050°C. The duration of the solid-solution treatment satisfies H x (4 min / mm), wherein H is the thickness of the ingot, in mm.

[0103] Example 2

[0104] In this example, an austenitic steel is prepared, wherein the component proportions of the austenitic steel of this example are shown in Table 1.

[0105] The preparation steps of the austenitic steel of this example are the same as those of Example 1.

[0106] Example 3

[0107] In this example, an austenitic steel is prepared, wherein the component proportions of the austenitic steel of this example are shown in Table 1.

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

[0109] Comparative Example 1

[0110] In this example, an austenitic steel is prepared, wherein the component proportions of the austenitic steel of this example are shown in Table 1.

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

[0112] Comparative Example 2

[0113] In this example, an austenitic steel is prepared, wherein the component proportions of the austenitic steel of this example are shown in Table 1.

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

[0115] Comparative Example 3

[0116] In this example, an austenitic steel is prepared, wherein the component proportions of the austenitic steel of this example are shown in Table 1.

[0117] The preparation steps of the austenitic steel of this example are the same as those of Example 1.

[0118] Comparative Example 4

[0119] In this example, an austenitic steel is prepared, wherein the component proportions of the austenitic steel of this example are shown in Table 1.

[0120] The preparation steps of the austenitic steel of this example are the same as those of Example 1.

[0121] Comparative Example 5

[0122] Austenitic steel of Comparative Example 5 was prepared, wherein the component ratio of the austenitic steel of Comparative Example 5 is shown in Table 1.

[0123] The preparation steps of the austenitic steel of Comparative Example 5 are the same as those of Example 1.

[0124] The chemical components of the austenitic steel prepared in Examples 1-3 and Comparative Examples 1-5 are shown in Table 1.

[0125] Table 1

[0126]

[0127] The properties of the austenitic steel 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℃×48h+400℃×48h+665℃×50h.

[0128] Table 2

[0129]

[0130] Wherein, Figure 3 is a scanning electron micrograph of the austenitic steel prepared in Example 1 after aging treatment (the arrow in points to the grain boundary); Figure 3 Figure 4 is a scanning electron micrograph of the austenitic steel prepared in Comparative Example 1 after aging treatment (the arrow in points to the grain boundary); Figure 4 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.

[0131] Here, according to Table 1, Table 2, Figures 3-9 It can be seen that:

[0132] (1) It can be clearly seen from the comparison of Table 1 and Table 2 that the comprehensive performance of the austenitic steel prepared in Examples 1-3 of the present application after aging heat treatment is obviously better than that of Comparative Examples 1-5, which contains trace amounts of Nb and V, and the content of B element is limited to 10-65ppm.​​

[0133] (2) By comparing the microstructure of the austenitic steel prepared by Example 1 Figure 3 ) and Comparative Example 1 Figure 4 ), Comparative Example 2 Figure 5 ), it can be seen that:

[0134] When containing an appropriate amount of B element, the austenitic steel after aging heat treatment, the grain boundary is clean, and no obvious precipitate phase is precipitated (see Example 1 and Figure 3 ).

[0135] And Comparative Example 1, because the B content is less than 0.0010%, the internal adsorption effect of the grain boundary on the B element is weak, therefore, there are a large number of intergranular precipitates at the grain boundary, see Figure 4 .

[0136] And when the B content exceeds the limited standard of the present application, such as Comparative Example 2 (see Figure 5 ), a large amount of BN inclusions are formed, thereby damaging the low temperature toughness and plasticity.

[0137] (3) By comparing Example 2 and Comparative Example 3, it can be seen that:

[0138] When containing trace amounts of Nb and V, that is, 0.03-0.08wt% Nb, 0.1-0.3wt% V (Example 2), the microstructure of the austenitic steel is as shown in Figure 6 , and after aging heat treatment, fine and dispersed Nb and V-containing precipitates can be formed in the grains.

[0139] And when the Nb content exceeds 0.08wt% (especially more than 0.1wt%), and the V content exceeds 0.3wt% (Comparative Example 3), the microstructure of the austenitic steel is as shown in Figure 7 , large size MX phase is precipitated, thereby damaging the toughness.

[0140] (4) By comparing Example 3 and Comparative Example 4, it can be seen that:

[0141] When containing trace amounts of Nb and V, and limiting the B element content to 10-65ppm, F(B) meets the limited requirements of the present application, the microstructure of the austenitic steel of Example 3 is as shown in Figure 8 , after aging treatment, a large number of fine and dispersed precipitates can be seen, and the grain boundary is relatively clean without precipitates.

[0142] And when the F(B) of the B element does not meet the requirements, such as Comparative Example 4, see Figure 9 , a large amount of large size carbonitride in the austenitic steel will also precipitate at the grain boundary after aging treatment, thereby significantly reducing the toughness and plasticity at 4.2K.

[0143] (5) By comparing Example 1 with Comparative Example 5, it can be seen that:

[0144] When the content of B element is limited to 10-65 ppm, but Nb and V are not added, as in Comparative Example 5, see Figure 10 It is shown that, although the austenitic steel with better toughness and plasticity is obtained, the strength of the austenitic steel is low due to the absence of precipitated phase.

[0145] The above comparison of Examples 1-3 and Comparative Examples 1-5 mainly proves that, in the formula design of the austenitic steel of the present application, by simultaneously adding Nb, V and B and controlling the addition amounts of Nb, V, N, C and B, the steel of the present application has a synergistic effect and 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.

[0146] Further, the present application proves by the following Example 4 that, on the basis of the formula design of the austenitic steel of the present application, the design of the solid solution treatment temperature can further improve the performance of the austenitic steel, as follows:

[0147] Example 4:

[0148] In this example, the austenitic steel is prepared, wherein the composition of the austenitic steel of this example is the same as that of Example 2. The difference between the preparation method of the austenitic steel of this example and that of Example 2 is that:

[0149] In step 4), the forged blank is subjected to solid solution treatment to obtain the austenitic steel, wherein the solid solution treatment temperature is 1098℃ (satisfying the formula {1020+[Nb]×580+[V]×125}℃±(0-3)℃).

[0150] The other steps are exactly the same as those of Example 2.

[0151] Here, the performance of the austenitic steel prepared in Example 2 and Example 4 after superconducting phase aging heat treatment is shown in Table 3. The specific process of the superconducting phase aging heat treatment is 210℃×48h+400℃×48h+665℃×50h.

[0152] Table 3

[0153]

[0154] As can be seen from Table 3, by designing the solid solution treatment temperature, the performance (strength and toughness) of the austenitic steel prepared in Example 4 is further improved compared with Example 2. The main reason is that the inventors of the present application have found that the large-size primary MX phase that damages the toughness and plasticity is mainly the compound of Nb and V elements. Although the large-size primary MX phase can be basically eliminated by the composition limitation of the present application, with the increase of the size of the steel ingot, the segregation may cause the micro-area composition fluctuation, and a small amount of harmful MX phase may still be generated. Therefore, the inventors further optimize the solid solution heat treatment process to make the large-size MX phase fully dissolve in the solid solution stage, reduce its damage to the toughness and plasticity, and further improve the performance of the austenitic steel.

[0155] In summary, the present application provides an austenitic steel and a preparation method thereof, and a superconducting coil armor. The austenitic steel of the present application has no nitride precipitates at the grain boundary or the equivalent particle diameter of the grain boundary precipitates is less than or equal to 50 nm after the aging heat treatment in the austenite sensitive interval. After the superconducting phase aging heat treatment, the yield strength σ s of the austenitic steel is greater than 1300 MPa, the tensile strength σ b is greater than 1700 MPa, the elongation A is greater than 30%, and the impact toughness AKV2 is greater than 150 J at 4.2 K. Therefore, the austenitic steel provided by the present application still has the high-strength and high-toughness performance of the aging-resistant austenitic steel after the superconducting phase aging heat treatment, and can meet the strength and toughness requirements of the superconducting coil armor material.

[0156] The above description is only a preferred embodiment of the present application, and does not limit the present application in any form. Any simple modification, equivalent change and modification of the above embodiment according to the technical essence of the present application still belongs to the scope of the technical solution of the present application.

Claims

1. An austenitic steel, characterized in that, The austenitic steel has the following chemical composition in 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: 1-3wt%; 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%; O: less than 30ppm; the balance being Fe; wherein the internal adsorption capacity F(B) of the grain boundary of the austenitic steel to B element is greater than 0.15, so as to increase the internal adsorption of the grain boundary to B element, inhibit the precipitation of carbonitride and intermetallic compound, 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.

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

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

4. The austenitic steel according to claim 1, characterized in that, After the austenitic steel is subjected to aging heat treatment in the austenite sensitization interval, no carbonitride and intermetallic compound is precipitated at the grain boundary; or After the austenitic steel is subjected to aging heat treatment in the austenite sensitization interval, the equivalent particle diameter of the precipitates at the grain boundary is less than or equal to 50nm.

5. The austenitic steel according to claim 1, characterized in that, After the austenitic steel is subjected to superconducting phase aging heat treatment, a dispersed compound with a particle size of 10-200nm is precipitated, so as to enable the austenitic steel to have excellent precipitation strengthening effect, and thus ensure low-temperature strength; wherein the compound comprises N, V and Nb.

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

7. A method of producing an austenitic steel according to any one of claims 1 to 6, characterised in that, It comprises the following steps: 1) preparing raw materials according to the chemical composition of the austenitic steel, performing melting treatment, and obtaining a cast ingot after casting; 2) performing electroslag remelting treatment on the cast ingot to obtain an electroslag ingot; 3) performing forging treatment on the electroslag ingot to obtain a forged blank; 4) performing solid solution treatment on the forged blank to obtain the austenitic steel.

8. The method of manufacturing an austenitic steel according to claim 7, characterized in that, In the step 1): It is necessary to ensure that the N content in the cast ingot is greater than 0.25wt% and less than 0.45wt%.

9. The method of manufacturing an austenitic steel according to claim 7, characterized in that, In the step 1): The melting treatment is performed in a vacuum induction furnace; wherein the nitrogen atmosphere pressure value is ensured to be 0.03-0.07MP during the melting process.

10. The method of manufacturing an austenitic steel according to claim 7, characterized in that, In the step 4), the temperature of the solid solution treatment is 1000-1150℃.

11. The method of manufacturing an austenitic steel according to claim 7, characterized in that, In the step 4), the temperature of the solid solution treatment satisfies {1020+[Nb]×580+[V]×125}℃, wherein [Nb] and [V] are the mass percentages of Nb and V respectively; the time length of the solid solution treatment satisfies H×(2-4min / mm), wherein H is the thickness of the ingot, in mm.

12. The method of manufacturing an austenitic steel according to claim 7, characterized in that, After the step 4), it further comprises: Step 5) performing superconducting phase aging heat treatment on the austenitic steel.

13. A superconducting coil armour characterised in that, The superconducting coil armor is made of the austenitic steel according to any one of claims 1-6.

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