Cobalt-free medium-entropy low-temperature steel, preparation method and application thereof

Abstract

The application provides a cobalt-free medium-entropy low-temperature steel and a preparation method and application thereof, relates to the technical field of stainless steel, and the cobalt-free medium-entropy low-temperature steel comprises a base alloy and a solid-solution N element; the base alloy is a FeMnCrNi medium-entropy alloy; in terms of mass percentage, the cobalt-free medium-entropy low-temperature steel comprises the following components in mass percentage: Mn: 14-20 wt%, Cr: 18-21 wt%, Ni: 15-20 wt%, N: 0.2-0.5 wt%, C: less than 0.01 wt%; and the rest is Fe element and inevitable impurities. The application can effectively improve the low-temperature yield strength of the material, and can also significantly improve the stability of austenite, so that the austenite does not occur deformation-induced martensite phase transition during deformation at low temperature, and the non-magnetic characteristic is maintained.

Description

Technical Field

[0001] This application relates to the field of stainless steel technology, and in particular to a cobalt-free medium-entropy low-temperature steel, its preparation method, and its application. Background Technology

[0002] Austenitic stainless steel, due to its excellent low-temperature yield strength, plasticity, non-magnetic properties, and weldability, and whose fracture toughness does not significantly decrease with decreasing temperature, can be used as a cryogenic support material for large high-field superconducting magnets in magnetic confinement fusion, accelerators, and MRI. Austenitic stainless steel has already been used in the coil box structure of magnetic confinement fusion superconducting high-field superconducting magnets. The core bottleneck currently facing cryogenic structural materials lies in the difficulty of synergistically matching high strength and high toughness under extremely low temperatures of 4.2K and even more stringent service conditions. While existing austenitic stainless steel can avoid the ductile-brittle transition, its strength level is already approaching its limit, making it difficult to meet the urgent requirements of next-generation compact fusion reactors and high-field superconducting magnets for lightweight and high-load-bearing structural materials.

[0003] Meanwhile, in extreme environments characterized by strong magnetic fields, high radiation, and complex thermo-mechanical multi-field coupling, materials must also possess non-magnetic properties, excellent fatigue resistance, fracture toughness, and structural stability. However, existing alloy systems often compromise on these aspects, exhibiting a technical paradox: "increased strength leads to decreased toughness, while low-temperature strengthening presents processing difficulties." The mechanical performance requirements for low-temperature materials are moving towards higher strength (yield strength exceeding 1200 MPa at 4.2 K) and superior low-temperature fracture toughness (K... IC ≥150MPa·m 1 / 2 The development of stainless steels, such as nitrogen-strengthened and low-carbon 316 (i.e., 316LN), used in the International Thermonuclear Experimental Reactor (ITER) can not meet this requirement.

[0004] In recent years, rapidly developing novel medium-high entropy alloys have emerged as potential high-performance low-temperature structural materials due to their excellent low-temperature toughness and yield strength ratio. Current research on medium-high entropy alloys based on FCC structures is primarily focused on Co-containing solid solution alloys. However, Co is expensive and undergoes significant nuclear transmutation under neutron irradiation, generating the highly radioactive nuclide cobalt-60. Therefore, Co is strictly limited in the structural materials of fusion devices.

[0005] Therefore, there is an urgent need to develop cobalt-free, high-performance, low-temperature structural materials. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this application is to provide a cobalt-free medium-entropy low-temperature steel, its preparation method, and its application. Without compromising the alloy's toughness and plasticity, and by combining the solid solution strengthening effect of nitrogen element, a high-strength and high-toughness medium-entropy low-temperature steel material and its preparation technology that can be used in the 4K temperature range are developed. The developed material properties are expected to meet the low-temperature mechanical performance requirements of coil box materials used in future nuclear fusion engineering reactors.

[0007] To achieve this objective, the following technical solution is adopted in this application: In a first aspect, this application provides a cobalt-free medium-entropy low-temperature steel, which comprises a matrix alloy and a solid-solution nitrogen element. The matrix alloy is a FeMnCrNi medium-entropy alloy.

[0008] By mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 14~20wt%, Cr: 18~21wt%, Ni: 15~20wt%, N: 0.2~0.5wt%, and C<0.01wt%. The remainder consists of Fe and unavoidable impurities.

[0009] In some technical solutions of this application, the average grain size of the cobalt-free medium-entropy low-temperature steel is 15~20μm.

[0010] In some technical solutions of this application, the cobalt-free medium-entropy low-temperature steel, by mass percentage, comprises Mn: 14~18wt%, Cr: 19~21wt%, Ni: 15~18wt%, N: 0.2~0.5wt%, C<0.009wt%; the remainder is Fe and unavoidable impurities. In some technical solutions of this application, the mass ratio of Mn to Ni in the cobalt-free medium-entropy low-temperature steel is 0.75. <Mn / Ni<1.3。

[0011] In some technical solutions of this application, the microstructure of the cobalt-free medium-entropy low-temperature steel includes an austenitic structure.

[0012] In some technical solutions of this application, the nitrogen element in the cobalt-free medium-entropy low-temperature steel is completely dissolved into the matrix alloy.

[0013] In some technical solutions of this application, the proportion of Cr2N precipitates in the cobalt-free medium-entropy low-temperature steel is ≤0.5%.

[0014] In some technical solutions of this application, the cobalt-free medium-entropy low-temperature steel has a yield strength greater than 1.0 GPa and an elongation greater than 30% at 77K.

[0015] In some technical solutions of this application, the tensile strength of the cobalt-free medium-entropy low-temperature steel at 77K is greater than 1.5GPa.

[0016] In some technical solutions of this application, the cobalt-free medium-entropy low-temperature steel has a yield strength greater than 1.3 GPa at 4.2 K.

[0017] In some technical solutions of this application, the tensile strength of the cobalt-free medium-entropy low-temperature steel at 4.2K is greater than 1.8GPa.

[0018] Secondly, this application provides a method for preparing the cobalt-free medium-entropy low-temperature steel described in the first aspect, the method comprising the following steps: (1) The raw materials of the base alloy are smelted and forged to obtain medium-entropy alloy forging materials. Nitrogen element is introduced during the smelting process.

[0019] (2) The medium-entropy alloy forging material described in step (1) is subjected to heat treatment and hot rolling in sequence to obtain medium-entropy alloy plate.

[0020] (3) The medium-entropy alloy plate described in step (2) is annealed to obtain the medium-entropy low-temperature steel.

[0021] In some technical solutions of this application, the method of introducing nitrogen element in step (1) includes: adding it by means of chromium nitride and / or manganese nitride alloys, and / or, introducing nitrogen gas during the smelting process.

[0022] In some technical solutions of this application, the pressure inside the furnace during the smelting process in step (1) is 100~105 kPa.

[0023] In some technical solutions of this application, the melting is vacuum arc melting.

[0024] In some of the technical solutions of this application, the melting is carried out in an argon atmosphere.

[0025] In some technical solutions of this application, the initial forging temperature is 900~1100℃.

[0026] In some technical solutions of this application, the final forging temperature is 850~950℃.

[0027] In some technical solutions of this application, the temperature of the heat treatment in step (2) is 1000~1200℃.

[0028] In some technical solutions of this application, the heat treatment time is 0.1~1h.

[0029] In some technical solutions of this application, the initial rolling temperature of the hot rolling is 900~1100℃.

[0030] In some technical solutions of this application, the final rolling temperature of the hot rolling is 850~950℃.

[0031] In some technical solutions of this application, the rolling reduction of the hot rolling is 50-70%.

[0032] In some technical solutions of this application, the annealing temperature is 1000~1200℃.

[0033] In some technical solutions of this application, the annealing process takes 0.1 to 1.0 hours.

[0034] As a technical solution of this application, the preparation method includes the following steps: (1) The raw material of the base alloy is vacuum arc melted in an argon atmosphere. The pressure inside the furnace during the vacuum arc melting process is 100~105kPa. Nitrogen element is introduced during the melting process. The nitrogen element is introduced by means of adding chromium nitride and / or manganese nitride alloys; and / or, nitrogen gas is introduced during the melting process; and then forging is performed. The initial forging temperature is 900~1100℃ and the final forging temperature is 850~950℃ to obtain medium entropy alloy forging material.

[0035] (2) The medium entropy alloy forging material described in step (1) is heat-treated at 1000~1200℃ for 0.1~1h, and then hot-rolled. The initial rolling temperature is 900~1100℃, the final rolling temperature is 850~950℃, and the rolling reduction is 50~70%, to obtain medium entropy alloy plate. (3) The medium-entropy alloy plate in step (2) is annealed at 1000~1200℃ for 0.1~1.0h to obtain the medium-entropy low-temperature steel.

[0036] Thirdly, this application provides an application of the cobalt-free, medium-entropy, low-temperature steel described in the first aspect in nuclear fusion, accelerators, or hydrogen energy.

[0037] In some of the technical solutions of this application, the cobalt-free medium-entropy low-temperature steel is used in a temperature range of 4.2K to 77K.

[0038] Compared with the prior art, this application has at least the following beneficial effects: The cobalt-free medium-entropy low-temperature steel provided in this application adopts an N-strengthened FeMnCrNi-based medium-entropy alloy. By eliminating the expensive Co element, it not only achieves the core advantage of low cost, but also exhibits excellent strength-plasticity-toughness synergistic matching ability in extremely low temperature environments. It has become an ideal candidate material to replace traditional 316LN stainless steel and Co-containing high-entropy alloys, and has broad prospects in applications such as nuclear fusion, accelerators, and hydrogen energy. Attached Figure Description

[0039] Figure 1 This is a comparison diagram of the electron backscattering diffraction bands of the cobalt-free, medium-entropy, low-temperature steel prepared in Example 1 of this application.

[0040] Figure 2 These are the stress-strain curves at room temperature of the cobalt-free medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application.

[0041] Figure 3 This is a stress-strain curve at 77K for the cobalt-free medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application.

[0042] Figure 4 This is a stress-strain curve at 4.2K for the cobalt-free medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application.

[0043] Figure 5 These are XRD patterns of cobalt-free medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application. Detailed Implementation

[0044] To facilitate understanding of this application, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of this application.

[0045] The purpose of this application is to provide a cobalt-free medium-entropy low-temperature steel, its preparation method, and its application. By combining reasonable element control to increase the solid solubility of nitrogen with heat treatment processes, a medium-entropy alloy with excellent mechanical properties at low temperatures can be prepared, solving the technical bottleneck problem that the yield strength of existing low-temperature structural materials is too low to meet the material requirements of future fusion devices.

[0046] As a specific embodiment of this application, a cobalt-free medium-entropy low-temperature steel is provided, which comprises a matrix alloy and a solid-solution nitrogen element. The matrix alloy is a FeMnCrNi medium-entropy alloy.

[0047] By mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 14~20wt%, Cr: 18~21wt%, Ni: 15~20wt%, N: 0.2~0.5wt%, and C<0.01wt%. The remainder consists of Fe and unavoidable impurities.

[0048] The cobalt-free medium-entropy low-temperature steel, by mass percentage, comprises Mn: 14~20wt%, for example, 14wt%, 14.7wt%, 15.4wt%, 16wt%, 16.7wt%, 17.4wt%, 18wt%, 18.7wt%, 19.4wt%, or 20wt%, etc., but is not limited to the listed values; other unlisted values ​​within this range also apply. Cr: 18~21wt%, for example, 18wt%, 18.4wt%, 18.7wt%, 19wt%, 19.4wt%, 19.7wt%, 20wt%, 20.4wt%, 20.7wt%, or 21wt%, etc., but is not limited to the listed values; other unlisted values ​​within this range also apply. Ni: 15~20wt%, for example, it can be 15wt%, 15.6wt%, 16.2wt%, 16.7wt%, 17.3wt%, 17.8wt%, 18.4wt%, 18.9wt%, 19.5wt%, or 20wt%, etc., but is not limited to the listed values. Other unlisted values ​​within this range also apply. N: 0.2~0.5wt%, for example, it can be 0.2wt%, 0.24wt%, 0.27wt%, 0.3wt%, 0.34wt%, 0.37wt%, 0.4wt%, 0.44wt%, 0.47wt%, or 0.5wt%, etc., but is not limited to the listed values. Other unlisted values ​​within this range also apply. C < 0.01wt%, for example, it can be 0.001wt%, 0.002wt%, 0.003wt%, 0.004wt%, 0.005wt%, 0.006wt%, 0.007wt%, 0.008wt%, 0.009wt%, or 0.0099wt%, etc., but is not limited to the listed values. Other unlisted values ​​within this range also apply.

[0049] In this application, the nitrogen content is increased. The addition of nitrogen not only provides a strong solid solution strengthening effect and effectively improves the low-temperature yield strength of the material, but also significantly improves the stability of austenite, so that deformation at low temperatures will not induce martensitic phase transformation and maintain non-magnetic properties.

[0050] Co is strictly restricted in the structural materials of fusion devices. However, current high-performance low-temperature structural materials are generally based on high-entropy alloys in FCC structures, and the research on high-entropy alloys in FCC structures is based on solid solution alloys containing Co.

[0051] Therefore, there is a need to develop cobalt-free high-performance low-temperature materials. This application improves the low-temperature mechanical properties of stainless steel by strictly controlling the content of Mn, Cr, Ni, N and C elements in cobalt-free medium-entropy low-temperature steel, which combines the solid solution strengthening effect of N element without compromising the toughness and plasticity of the alloy.

[0052] In some specific embodiments, the average grain size of the cobalt-free medium-entropy cryogenic steel is 15~20 μm.

[0053] The average grain size of the cobalt-free medium-entropy cryogenic steel is 15~20 μm, for example, it can be 15 μm, 15.6 μm, 16.2 μm, 16.7 μm, 17.3 μm, 17.8 μm, 18.4 μm, 18.9 μm, 19.5 μm or 20 μm, but it is not limited to the listed values, and other unlisted values within this range are also applicable.

[0054] In the present application, by strictly controlling the contents of Mn, Cr, Ni, N and C elements in the cobalt-free medium-entropy cryogenic steel and controlling the average grain size within the range of 15~20 μm, the solid solution strengthening effect of N element can be combined without impairing the toughness and plasticity of the alloy, so as to better improve the low-temperature mechanical properties of stainless steel.

[0055] In some specific embodiments, based on mass percentage, the cobalt-free medium-entropy cryogenic steel comprises Mn: 14~18 wt%, Cr: 19~21 wt%, Ni: 15~18 wt%, N: 0.2~0.5 wt%, C < 0.009 wt%; the rest is Fe element and unavoidable impurities.

[0056] In the present application, the content of Cr is controlled within the range of 19~21 wt%. When the Cr content is too high, it will lead to the precipitation of BCC phase, thereby reducing the strength; when the Cr content is too low, it will lead to the formation of ferrite phase in the structure, thereby reducing the plasticity and toughness.

[0057] In some specific embodiments, the mass ratio of Mn to Ni in the cobalt-free medium-entropy cryogenic steel satisfies 0.75 < Mn / Ni < 1.3, for example, it can be 0.75, 0.82, 0.88, 0.94, 1, 1.06, 1.12, 1.18, 1.24 or 1.3, but it is not limited to the listed values, and other unlisted values within this range are also applicable.

[0058] In the present application, the mass ratio of Mn to Ni is controlled within the above range to ensure that N has an appropriate solid solubility in the matrix. When the Mn content is too high, it will lead to an increase in the solid solubility of nitrogen, resulting in decreased properties and a significant reduction in toughness; when the Mn content is too low, it will reduce the solid solubility of nitrogen, leading to excessive precipitation of nitride phases and reduced strength properties.

[0059] In some specific embodiments, the structure of the cobalt-free medium-entropy cryogenic steel includes an austenite structure.

[0060] In some specific embodiments, all N elements in the cobalt-free medium-entropy cryogenic steel are solid-dissolved into the matrix alloy.

[0061] In some specific embodiments, the cobalt-free medium-entropy low-temperature steel includes a precipitate phase with a proportion of ≤0.5%, the precipitate phase including Cr2N, for example, it can be 0.5%, 0.48%, 0.45%, 0.43%, 0.4%, 0.38%, 0.37%, 0.35%, 0.3%, 0.28%, 0.25%, 0.2%, 0.18%, 0.15%, 0.1%, 0.08%, 0.05%, 0.01%, or 0, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. The cobalt-free medium-entropy low-temperature steel does not contain Cr2N precipitate phase.

[0062] The microstructure of the cobalt-free medium-entropy low-temperature steel provided in this application, through the adjustment of elements and processes, can achieve a microstructure with an extremely low proportion of precipitates, ≤0.1%, or even no Cr2N precipitates. This can better improve the low-temperature yield strength and low-temperature stability of the cobalt-free medium-entropy low-temperature steel.

[0063] The testing methods for the proportion of precipitated phase in this application include image analysis of electron backscatter diffraction band patterns and analysis of the area proportion of precipitated phase through multi-region statistical testing of TEM.

[0064] In some specific embodiments, the cobalt-free medium-entropy low-temperature steel has a yield strength greater than 1.0 GPa at 77 K, for example, it can be 1.01 GPa, 1.3 GPa, 1.5 GPa, 1.7 GPa, 1.9 GPa, 2.2 GPa, 2.4 GPa, 2.6 GPa, 2.8 GPa or 3.0 GPa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable. The elongation is greater than 30%, for example, it can be 30.1%, 32.4%, 34.6%, 36.8%, 39%, 41.2%, 43.4%, 45.6%, 47.8% or 50%, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0065] In some specific embodiments, the tensile strength of the cobalt-free medium-entropy low-temperature steel at 77K is greater than 1.5GPa, for example, it can be 1.51GPa, 1.57GPa, 1.62GPa, 1.68GPa, 1.73GPa, 1.79GPa, 1.84GPa, 1.9GPa, 1.95GPa or 2.0GPa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0066] In some specific embodiments, the cobalt-free medium-entropy low-temperature steel has a yield strength greater than 1.3 GPa at 4.2 K, for example, it can be 1.31 GPa, 1.39 GPa, 1.47 GPa, 1.54 GPa, 1.62 GPa, 1.7 GPa, 1.77 GPa, 1.85 GPa, 1.93 GPa or 2.0 GPa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0067] In some specific embodiments, the tensile strength of the cobalt-free medium-entropy low-temperature steel at 4.2K is greater than 1.8GPa, for example, it can be 1.81GPa, 1.89GPa, 1.97GPa, 2.04GPa, 2.12GPa, 2.2GPa, 2.27GPa, 2.35GPa, 2.43GPa or 2.5GPa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0068] In some specific embodiments, the elongation at break of the cobalt-free medium-entropy low-temperature steel is above 30%, for example, it can be 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 42%, or 45%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0069] As another specific embodiment of this application, a method for preparing cobalt-free medium-entropy low-temperature steel as described in any of the above specific embodiments is provided, the preparation method comprising the following steps: (1) The raw materials of the base alloy are smelted and forged to obtain medium-entropy alloy forging materials. Nitrogen element is introduced during the smelting process.

[0070] (2) The medium-entropy alloy forging material described in step (1) is subjected to heat treatment and hot rolling in sequence to obtain medium-entropy alloy plate.

[0071] (3) The medium-entropy alloy plate described in step (2) is annealed to obtain the medium-entropy low-temperature steel.

[0072] This application employs hot rolling to allow direct rolling at high temperatures, eliminating the need for intermediate annealing and other processes. This results in a shorter production process, lower energy consumption, and is particularly suitable for large-scale production. Furthermore, the slow cooling after high-temperature rolling leads to a uniform microstructure and low internal stress, effectively preventing subsequent processing deformation caused by residual stress in cold-rolled materials. Hot rolling also allows for the welding of internal microcracks, porosity, and other defects.

[0073] In some specific embodiments, the introduction of nitrogen in step (1) includes: adding it by means of chromium nitride and / or manganese nitride alloys, and / or introducing nitrogen gas during the smelting process.

[0074] In some specific embodiments, the pressure inside the furnace during the smelting process in step (1) is 100~105 kPa, for example, it can be 100 kPa, 100.6 kPa, 101.2 kPa, 101.7 kPa, 102.3 kPa, 102.8 kPa, 103.4 kPa, 103.9 kPa, 104.5 kPa or 105 kPa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0075] In some specific embodiments, the melting is vacuum arc melting.

[0076] In some specific embodiments, the melting is carried out in an argon atmosphere.

[0077] In some specific embodiments, the initial forging temperature is 900~1100℃, for example, it can be 900℃, 920℃, 945℃, 960℃, 980℃, 1010℃, 1030℃, 1050℃, 1070℃ or 1100℃, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0078] In some specific embodiments, the final forging temperature is 850~950℃, for example, it can be 850℃, 860℃, 870℃, 880℃, 895℃, 900℃, 910℃, 920℃, 930℃ or 950℃, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0079] In some specific embodiments, the heat treatment temperature in step (2) is 1000~1200℃, for example, it can be 1000℃, 1020℃, 1045℃, 1060℃, 1080℃, 1110℃, 1130℃, 1150℃, 1170℃ or 1200℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0080] In some specific embodiments, the heat treatment time is 0.1 to 1 hour, for example, it can be 0.1 hour, 0.2 hour, 0.3 hour, 0.4 hour, 0.5 hour, 0.6 hour, 0.7 hour, 0.8 hour, 0.9 hour or 1 hour, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0081] In some specific embodiments, the initial rolling temperature of the hot rolling is 900~1100℃, for example, it can be 900℃, 920℃, 945℃, 960℃, 980℃, 1010℃, 1030℃, 1050℃, 1070℃ or 1100℃, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0082] In some specific embodiments, the final rolling temperature of the hot rolling is 850~950℃, for example, it can be 850℃, 860℃, 870℃, 880℃, 895℃, 900℃, 910℃, 920℃, 930℃ or 950℃, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0083] In some specific embodiments, the hot rolling reduction is 50-70%, for example, it can be 50%, 53%, 55%, 57%, 59%, 62%, 64%, 66%, 68% or 70%, etc., but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0084] In some specific embodiments, the annealing temperature is 1000~1200℃, for example, it can be 1000℃, 1020℃, 1045℃, 1060℃, 1080℃, 1110℃, 1130℃, 1150℃, 1170℃ or 1200℃, but is not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0085] This application controls the annealing temperature within the above-mentioned range, which on the one hand can effectively control the grain size; on the other hand, it can effectively avoid the formation of Cr2N, so that N atoms are dissolved in the matrix, thereby improving the low-temperature strength, while avoiding the adverse effects of Cr2N precipitation on plasticity and toughness.

[0086] In some specific embodiments, the annealing time is 0.1 to 1.0 h, for example, it can be 0.1 h, 0.2 h, 0.3 h, 0.4 h, 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h or 1.0 h, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0087] As a technical solution of this application, the preparation method includes the following steps: (1) The raw material of the base alloy is vacuum arc melted in an argon atmosphere. The pressure inside the furnace during the vacuum arc melting process is 100~105kPa. Nitrogen element is introduced during the melting process. The nitrogen element is introduced by means of adding chromium nitride and / or manganese nitride alloys; and / or, nitrogen gas is introduced during the melting process; and then forging is performed. The initial forging temperature is 900~1100℃ and the final forging temperature is 850~950℃ to obtain medium entropy alloy forging material.

[0088] (2) The medium entropy alloy forging material described in step (1) is heat-treated at 1000~1200℃ for 0.1~1h, and then hot-rolled. The initial rolling temperature is 900~1100℃, the final rolling temperature is 850~950℃, and the reduction is 50~70%, to obtain medium entropy alloy plate. (3) The medium-entropy alloy plate in step (2) is annealed at 1000~1200℃ for 0.1~1.0h to obtain the medium-entropy low-temperature steel.

[0089] As another specific embodiment of this application, an application of cobalt-free medium-entropy low-temperature steel in nuclear fusion, accelerators or hydrogen energy is provided in the above specific embodiment.

[0090] In some specific embodiments, the cobalt-free medium-entropy low-temperature steel is used in environments ranging from 4.2K to 77K.

[0091] The following detailed description uses specific examples.

[0092] Example 1 This embodiment provides a cobalt-free medium-entropy low-temperature steel, which comprises a matrix alloy and dissolved nitrogen (N) element. The matrix alloy is a FeMnCrNi medium-entropy alloy. By mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 15.88 wt%, Cr: 20.02 wt%, Ni: 16.08 wt%, N: 0.27 wt%, C: 0.0064 wt%, with the remainder being Fe element and unavoidable impurities. The average grain size of the cobalt-free medium-entropy low-temperature steel is 16.75 μm. The mass ratio of Mn to Ni in the cobalt-free medium-entropy low-temperature steel is 0.9876.

[0093] The microstructure of the cobalt-free medium-entropy low-temperature steel is basically austenitic.

[0094] This embodiment also provides a method for preparing the above-mentioned cobalt-free medium-entropy low-temperature steel, the method comprising the following steps: (1) The raw material of the base alloy is vacuum arc melted in an argon atmosphere. The pressure inside the furnace during the vacuum arc melting process is 101 kPa. Nitrogen element is introduced during the melting process. The nitrogen element is introduced by adding it through chromium nitride and / or manganese nitride alloy according to the nitrogen content. Then, it is forged. The initial forging temperature of the forging is 1052℃ and the final forging temperature is 910℃ to obtain medium entropy alloy forging material.

[0095] (2) The medium entropy alloy forging material described in step (1) is heat-treated at 1150℃ for 0.3h and then hot-rolled. The initial rolling temperature is 1000℃, the final rolling temperature is 900℃, and the rolling reduction is 60%, to obtain medium entropy alloy plate. (3) The medium-entropy alloy plate described in step (2) is annealed at 1100℃ for 0.5h to obtain the medium-entropy low-temperature steel.

[0096] Example 2 This embodiment provides a cobalt-free medium-entropy low-temperature steel, which comprises a matrix alloy and dissolved nitrogen (N) element. The matrix alloy is a FeMnCrNi medium-entropy alloy. By mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 16.39 wt%, Cr: 20.02 wt%, Ni: 16.12 wt%, N: 0.37 wt%, C: 0.0076 wt%, with the remainder being Fe element and unavoidable impurities. The average grain size of the cobalt-free medium-entropy low-temperature steel is 15.89 μm. The mass ratio of Mn to Ni in the cobalt-free medium-entropy low-temperature steel is 1.0167.

[0097] The microstructure of the cobalt-free medium-entropy low-temperature steel is basically austenitic.

[0098] This embodiment also provides a method for preparing the above-mentioned cobalt-free medium-entropy low-temperature steel, the method comprising the following steps: (1) The raw material of the matrix alloy is vacuum arc melted in an argon atmosphere. The pressure inside the furnace during the vacuum arc melting process is 100 kPa. Nitrogen element is introduced during the melting process. The method of introducing nitrogen element includes: passing nitrogen gas during the melting process. Then forging is carried out. The initial forging temperature of the forging is 900℃ and the final forging temperature is 850℃ to obtain medium entropy alloy forging material.

[0099] (2) The medium entropy alloy forging material described in step (1) is heat-treated at 1200℃ for 0.1h and then hot-rolled. The initial rolling temperature is 900℃, the final rolling temperature is 850℃, and the rolling reduction is 70%, to obtain medium entropy alloy plate. (3) The medium-entropy alloy plate in step (2) is annealed at 1200℃ for 0.1h to obtain the medium-entropy low-temperature steel.

[0100] Example 3 This embodiment provides a cobalt-free medium-entropy low-temperature steel, which comprises a matrix alloy and dissolved nitrogen (N) element. The matrix alloy is a FeMnCrNi medium-entropy alloy. By mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 17.01 wt%, Cr: 20.25 wt%, Ni: 17.02 wt%, N: 0.47 wt%, C: 0.0085 wt%, with the remainder being Fe element and unavoidable impurities. The average grain size of the cobalt-free medium-entropy low-temperature steel is 16.24 μm. The mass ratio of Mn to Ni in the cobalt-free medium-entropy low-temperature steel is 0.9994.

[0101] The microstructure of the cobalt-free medium-entropy low-temperature steel is basically austenitic.

[0102] This embodiment also provides a method for preparing the above-mentioned cobalt-free medium-entropy low-temperature steel, the method comprising the following steps: (1) The raw material of the matrix alloy is vacuum arc melted in an argon atmosphere. The pressure inside the furnace during the vacuum arc melting process is 102 kPa. Nitrogen element is introduced during the melting process. The method of introducing nitrogen element includes: passing nitrogen gas during the melting process. Then forging is carried out. The initial forging temperature of the forging is 1100℃ and the final forging temperature is 950℃ to obtain medium entropy alloy forging material.

[0103] (2) The medium entropy alloy forging material described in step (1) is heat-treated at 1000℃ for 0.1h and then hot-rolled. The initial rolling temperature is 1100℃, the final rolling temperature is 950℃, and the rolling reduction is 55%, to obtain medium entropy alloy plate. (3) The medium-entropy alloy plate in step (2) is annealed at 1000℃ for 1.0h to obtain the medium-entropy low-temperature steel.

[0104] Example 4 This embodiment provides a cobalt-free medium-entropy low-temperature steel. Except that the cobalt-free medium-entropy low-temperature steel has a Mn content of 20wt% and a Ni content of 15.0wt%, that is, a mass ratio of Mn to Ni of 1.3333, the rest is the same as in Embodiment 1, and will not be repeated here.

[0105] Example 5 This embodiment provides a cobalt-free medium-entropy low-temperature steel. Except that the cobalt-free medium-entropy low-temperature steel has a Mn content of 14wt% and a Ni content of 20.0wt%, that is, a Mn to Ni mass ratio of 0.7, the rest is the same as in Embodiment 1, and will not be repeated here.

[0106] Example 6 This embodiment provides a cobalt-free medium-entropy low-temperature steel. Except for the Cr content of 18wt% in the cobalt-free medium-entropy low-temperature steel, the other contents are the same as those in Example 1, and will not be repeated here.

[0107] Example 7 This embodiment provides a cobalt-free medium-entropy low-temperature steel. Except for the annealing temperature, which is higher than 1300℃, the cobalt-free medium-entropy low-temperature steel is the same as that in Embodiment 1, and will not be described again here.

[0108] Example 8 This embodiment provides a cobalt-free medium-entropy low-temperature steel. Except for the annealing temperature, which is relatively low at only 900°C, the cobalt-free medium-entropy low-temperature steel is the same as that in Embodiment 1, and will not be described again here.

[0109] Example 9 This embodiment provides a cobalt-free medium-entropy low-temperature steel. Except for the hot rolling in step (2) being replaced by cold rolling, and the cold rolling temperature being room temperature of 30°C, the cobalt-free medium-entropy low-temperature steel is the same as that in embodiment 1, and will not be repeated here.

[0110] This embodiment will cause the steel to crack, making it difficult to obtain the target product.

[0111] Comparative Example 1 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the N content of 0.01 wt%, the cobalt-free medium-entropy low-temperature steel is the same as that in Example 1, and will not be described again here.

[0112] Comparative Example 2 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the N content of 0.52 wt%, the cobalt-free medium-entropy low-temperature steel is the same as that in Example 1, and will not be described again here.

[0113] Comparative Example 3 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the Cr content of 22wt% in the cobalt-free medium-entropy low-temperature steel, the other contents are the same as those in Example 1, and will not be repeated here.

[0114] Comparative Example 4 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the Mn content of 22wt% in the cobalt-free medium-entropy low-temperature steel, the other contents are the same as those in Example 1, and will not be repeated here.

[0115] Comparative Example 5 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the Mn content of 12wt% in the cobalt-free medium-entropy low-temperature steel, the other contents are the same as those in Example 1, and will not be repeated here.

[0116] Comparative Example 6 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the Ni content of 22wt% in the cobalt-free medium-entropy low-temperature steel, the other contents are the same as those in Example 1, and will not be repeated here.

[0117] Comparative Example 7 This comparative example provides a cobalt-free medium-entropy low-temperature steel. Except for the Ni content of 14wt%, the cobalt-free medium-entropy low-temperature steel is the same as that in Example 1, and will not be repeated here.

[0118] The electron backscattering diffraction bands of the cobalt-free, medium-entropy, low-temperature steel prepared in Example 1 are shown in the figure below. Figure 1 As shown, from Figure 1It can be seen that the grain size distribution of the cobalt-free medium-entropy low-temperature steel prepared in this embodiment is in the range of 15~20μm, and the grain size distribution is relatively uniform.

[0119] Figure 2 The stress-strain curves at room temperature of the cobalt-free, medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application are shown. Figure 2 It can be seen that Embodiments 1 and 2 of this application have significantly better room temperature mechanical properties compared to Comparative Example 1.

[0120] Figure 3 The stress-strain curves of the cobalt-free medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application are shown at 77K. Figure 4 The stress-strain curves of the cobalt-free, medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application are shown at 4.2K. Figure 3 and Figure 4 It can be seen that the cobalt-free medium-entropy low-temperature steels obtained in Examples 1-2 of this application have significantly better mechanical properties at temperatures of 77K and 4.2K.

[0121] Figure 5 The XRD patterns of cobalt-free, medium-entropy low-temperature steels prepared in Examples 1 (0.27N), 2 (0.37N), and Comparative Example 1 (0.01N) of this application are shown. Figure 5 It can be seen that in Comparative Example 1, only 0.01 wt% N was added, which formed HCP martensite and BCC martensite, while the cobalt-free medium-entropy low-temperature steels obtained in Examples 1-2 of this application do not contain martensite structure.

[0122] Test methods: The grain size of austenitic stainless steel was determined by scanning electron microscopy with electron backscatter diffraction. The tensile strength, yield strength and elongation after fracture of austenitic stainless steel at 4.2K, 77K and room temperature were determined by GBT 228.4-2019 Metallic Materials - Tensile Testing - Part 4: Liquid Helium Test Method.

[0123] The test results of the above embodiments and comparative examples are shown in Table 1.

[0124] Table 1 In Table 1, YS represents the yield strength, which refers to the yield strength when the plastic elongation is specified as 0.2%, denoted as Rp. 0.2The unit is MPa (megapascal). UTS represents tensile strength, and its unit is MPa (megapascal). EL represents elongation after fracture, and its unit is % (percentage). " / " in the table indicates that there is no relevant data.

[0125] The following points can be observed from Table 1: (1) As can be seen from the comprehensive examples 1 to 3, the average grain size of the cobalt-free medium-entropy low-temperature steel provided in this application is <20μm, and the microstructure is FCC phase, in which the precipitated phase Cr2N is basically undetectable. It has excellent mechanical properties. Specifically, the yield strength at room temperature is above 400MPa, the tensile strength is above 700MPa, the yield strength at 77K is above 1000MPa, the tensile strength is above 1500MPa, the yield strength at 4.2K is above 1300MPa, the tensile strength is above 1830MPa, and the elongation at break is above 30%.

[0126] (2) Combining Examples 1 and Examples 4-5, it can be seen that in Example 4, the mass ratio of Mn to Ni is 1.3333, and the relatively increased Mn content leads to an increase in nitrogen solid solubility, resulting in an increase in the final grain size, while the yield strength and elongation after fracture both decrease. In Example 5, the mass ratio of Mn to Ni is 0.7, which is relatively low, leading to a relatively increased Ni content and a decrease in N solid solubility, resulting in the precipitation of the Cr2N phase, and a significant decrease in mechanical properties compared to Example 1. This shows that by setting the Mn to Ni ratio within a reasonable range, this application can significantly improve the strength and toughness of cobalt-free medium-entropy low-temperature steel.

[0127] (3) Combining Examples 1 and 6 with Comparative Example 3, it can be seen that the Cr content in the cobalt-free medium-entropy low-temperature steel in Example 6 is 18wt%, which is relatively low, leading to the precipitation of the Cr2N phase, resulting in a decrease in both strength and toughness compared to Example 1. In Comparative Example 3, the Cr content is relatively high, leading to the precipitation of the Cr2N phase, resulting in a significant decrease in both strength and toughness compared to Example 1. In summary, it can be seen that by controlling the Cr content within a reasonable range, this application can improve the mechanical properties of low-temperature steel.

[0128] (4) As can be seen from Examples 1 and 7-8, the annealing temperature in Example 7 was too high, resulting in a significant increase in grain size and ultimately a significant decrease in yield strength at low temperature compared to Example 1; the annealing temperature in Example 8 was too low, causing the precipitation of Cr2N phase in the austenitic microstructure, and both its yield strength and tensile strength decreased compared to Example 1. This indicates that by controlling the annealing temperature within a reasonable range, this application can improve the mechanical properties of low-temperature steel.

[0129] (5) In Comparative Example 1, the N content was too low, resulting in the inability to form an austenitic structure. Consequently, the yield strength and tensile strength of the resulting steel were significantly lower than those in Example 1. In Comparative Example 2, the N content was too high, making it prone to cracking and difficult to obtain the target product. In Comparative Example 4, the Mn content was too high, leading to the precipitation of the Cr2N phase and a significant decrease in mechanical properties. In Comparative Example 5, the Mn content was too low, resulting in a decrease in the solid solubility of N and the precipitation of the Cr2N phase, which also significantly reduced mechanical properties. In Comparative Example 6, the Ni content was too high, making it difficult to dissolve such a high content of N, resulting in a significant decrease in strength. In Comparative Example 7, the Ni content was too low, which also led to the precipitation of the Cr2N phase and a significant decrease in mechanical properties. In summary, it can be seen that this application can improve the mechanical properties of low-temperature steel by optimizing the elemental composition and controlling the grain size and microstructure.

[0130] This application illustrates its detailed features through the above embodiments, but it is not limited to these detailed features, meaning that this application does not necessarily rely on them for implementation. Those skilled in the art should understand that any improvements to this application, equivalent substitutions for selected technical features, additions of auxiliary technical features, and selection of specific methods all fall within the protection and disclosure scope of this application.

Claims

1. A cobalt-free, medium-entropy, low-temperature steel, characterized in that, The cobalt-free medium-entropy low-temperature steel comprises a matrix alloy and a solid-solution nitrogen element; the matrix alloy is a FeMnCrNi medium-entropy alloy. By mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 14~20wt%, Cr: 18~21wt%, Ni: 15~20wt%, N: 0.2~0.5wt%, C<0.01wt%; the remainder is Fe and unavoidable impurities.

2. The cobalt-free, medium-entropy, low-temperature steel according to claim 1, characterized in that, The average grain size of the cobalt-free medium-entropy low-temperature steel is 15~20μm; And / or, by mass percentage, the cobalt-free medium-entropy low-temperature steel comprises Mn: 14~18wt%, Cr: 19~21wt%, Ni: 15~18wt%, N: 0.2~0.5wt%, C<0.009wt%; the remainder being Fe and unavoidable impurities; And / or, the mass ratio of Mn to Ni in the cobalt-free medium-entropy low-temperature steel is 0.

75. <Mn / Ni<1.3。 3. The cobalt-free, medium-entropy, low-temperature steel according to claim 1 or 2, characterized in that, The microstructure of the cobalt-free medium-entropy low-temperature steel includes an austenitic structure; In the cobalt-free medium-entropy low-temperature steel, all nitrogen elements are dissolved into the matrix alloy. The proportion of Cr2N precipitates in the cobalt-free medium-entropy low-temperature steel is ≤0.5%.

4. The cobalt-free, medium-entropy, low-temperature steel according to claim 1 or 2, characterized in that, The cobalt-free medium-entropy low-temperature steel satisfies at least one of the following conditions: A. The cobalt-free medium-entropy low-temperature steel has a yield strength greater than 1.0 GPa and an elongation greater than 30% at 77K. B. The tensile strength of the cobalt-free medium-entropy low-temperature steel at 77K is greater than 1.5 GPa; C. The cobalt-free medium-entropy low-temperature steel has a yield strength greater than 1.3 GPa at 4.2 K; D. The tensile strength of the cobalt-free medium-entropy low-temperature steel at 4.2K is greater than 1.8GPa.

5. A method for preparing cobalt-free, medium-entropy, low-temperature steel according to any one of claims 1 to 4, characterized in that, The preparation method includes the following steps: (1) The raw materials of the base alloy are smelted and forged to obtain medium-entropy alloy forging materials; nitrogen element is introduced during the smelting process; (2) The medium-entropy alloy forging material described in step (1) is subjected to heat treatment and hot rolling in sequence to obtain medium-entropy alloy plate; (3) The medium-entropy alloy plate described in step (2) is annealed to obtain the medium-entropy low-temperature steel.

6. The preparation method according to claim 5, characterized in that, The methods of introducing nitrogen in step (1) include: adding it by means of chromium nitride and / or manganese nitride alloys; and / or, introducing nitrogen gas during the smelting process.

7. The preparation method according to claim 5 or 6, characterized in that, The pressure inside the furnace during the smelting process described in step (1) is 100~105 kPa; The melting is a vacuum arc melting process; The melting was carried out in an argon atmosphere; The initial forging temperature is 900~1100℃, and the final forging temperature is 850~950℃.

8. The preparation method according to claim 5 or 6, characterized in that, The heat treatment temperature in step (2) is 1000~1200℃; The heat treatment time is 0.1~1h; The initial rolling temperature of the hot rolling is 900~1100℃, and the final rolling temperature is 850~950℃; The rolling reduction of the hot rolling process is 50-70%.

9. The preparation method according to claim 5 or 6, characterized in that, The annealing temperature is 1000~1200℃; And / or, the annealing process takes 0.1 to 1.0 hours.

10. The application of the cobalt-free, medium-entropy, low-temperature steel according to any one of claims 1 to 3 in nuclear fusion, accelerators, or hydrogen energy.

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