A bcc-dominant structure stainless steel based on non-equilibrium solidification and a method for manufacturing the same

Abstract

The application belongs to the technical field of stainless steel preparation, and particularly relates to a BCC dominant structure stainless steel based on non-equilibrium solidification and a preparation method thereof. The stainless steel comprises the following components in percentage by weight: Cr 16.0-17.5 wt.%, Ni 10.5-12.0 wt.%, Mo 3.5-8.0 wt.%, Mn≤1.0 wt.%, Si≤0.8 wt.%, C≤0.05 wt.%, and the balance of Fe and inevitable impurities. Through rapid melting and rapid solidification in the additive manufacturing process and the regulation of the BCC phase stable zone by Mo elements, the δ-Fe→γ-Fe diffusion type phase change is inhibited, the δ-Fe formed at high temperature is retained to room temperature, and thus the stainless steel organization dominated by the BCC structure δ-Fe phase is obtained. Compared with the existing 316L stainless steel, the material has higher strength and excellent corrosion resistance, and shows a wider passivation interval and obvious secondary passivation behavior, realizing the synergistic improvement of high strength and high corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel preparation technology, and particularly relates to a BCC-dominant structure stainless steel based on non-equilibrium solidification and its preparation method. Background Technology

[0002] Austenitic stainless steel (such as 316L) is widely used in marine engineering, chemical equipment, and medical devices due to its excellent corrosion resistance and good processing properties. Its matrix structure is mainly face-centered cubic (FCC) austenitic (γ-Fe) structure. Therefore, although it has good plasticity and toughness, its yield strength is generally low, making it difficult to meet the application requirements of high-load structural components.

[0003] To improve strength, ferritic stainless steel was developed. Traditional ferritic stainless steel typically employs a "reduced Ni, increased Cr" composition design approach, which involves reducing the content of the austenite-stabilizing element Ni while increasing the content of ferrite-stabilizing elements such as Cr, resulting in a body-centered cubic (BCC) ferrite microstructure at room temperature. However, Ni is a crucial element for improving the stability of the passivation film and resistance to pitting corrosion in stainless steel. Reducing the Ni content usually leads to a significant decrease in the material's resistance to chloride ion corrosion, making it difficult to meet the long-term service requirements of highly corrosive environments such as marine environments.

[0004] In recent years, additive manufacturing methods such as selective laser melting (SLM) have gained popularity due to their high cooling rates (up to 10⁻⁶ m / s). 6 (K / s) can form a non-equilibrium solidification structure significantly different from traditional metallurgical processes. Studies have shown that SLM-formed 316L stainless steel typically exhibits characteristics such as grain refinement, high dislocation density, and molten pool structure, which can improve material strength to some extent. However, since the traditional 316L composition system still belongs to a high-Ni stable austenitic system, its solidification and subsequent phase transformation processes tend to form and retain the FCC austenitic structure, which cannot fundamentally overcome the problem of low strength in austenitic stainless steel. Some studies have attempted to add ferrite-forming elements such as Mo to 316L to regulate the material structure and improve corrosion resistance; however, these efforts remain at the stage of FCC-dominated or FCC / BCC duplex structure, and have not yet achieved a stainless steel structure dominated by the BCC structure. In addition, when the Mo content is too high, it is easy to form Mo-rich brittle σ phase precipitation, leading to the formation of Cr / Mo depleted zones, which not only reduces the material's plasticity but also triggers micro-galvanic corrosion, thus weakening the material's corrosion resistance.

[0005] Therefore, how to achieve the formation of a BCC-dominant structure while maintaining the excellent corrosion resistance of 316L stainless steel, and simultaneously improve the mechanical properties and corrosion resistance of the material, has become a key technical problem that urgently needs to be solved in this field.

[0006] The information disclosed in this background section is only intended to enhance the understanding of the background technology of this application and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0007] To address the issues of low strength due to the predominantly FCC austenitic structure in existing 316L stainless steel, and the decline in corrosion resistance due to reduced Ni content in traditional ferritic stainless steel, this invention provides a BCC-dominant structure stainless steel based on non-equilibrium solidification and its preparation method. This method aims to achieve the formation of a BCC-dominant structure while maintaining the excellent corrosion resistance of 316L stainless steel, and simultaneously improve the mechanical properties and corrosion resistance of the material.

[0008] This invention proposes a BCC-dominant stainless steel based on non-equilibrium solidification, wherein the stainless steel is prepared by additive manufacturing and comprises the following components by weight percentage: Cr: 16.0~17.5 wt.%, Ni: 10.5~12.0 wt.%, Mo: 3.5~8.0 wt.%, Mn≤1.0 wt.%, Si≤0.8 wt.%, C≤0.05 wt.%, balance being Fe and unavoidable impurities; The stainless steel is characterized by a body-centered cubic (BCC) structure with δ-Fe phase as the dominant structure, and the proportion of δ-Fe phase is not less than 90%.

[0009] Preferably, the Mo content is 4 wt.%, and the Mo exists in the matrix in a solid solution state.

[0010] Furthermore, the stainless steel has a yield strength of not less than 877 MPa and a tensile strength of not less than 893 MPa.

[0011] Furthermore, the pitting potential of the stainless steel is higher than that of conventional rolled stainless steel, and the stainless steel exhibits a pitting potential of 0.5~1.0 V in an artificial seawater environment. SCE The high-potential region exhibits secondary passivation behavior, which corresponds to MoO4²⁻. - Participated in the in-situ restoration process.

[0012] This invention also proposes a method for preparing BCC-dominant stainless steel based on non-equilibrium solidification, comprising the following steps: S1. Provide stainless steel powder, said stainless steel powder comprising, by weight percentage, the following components: Cr: 16.0~17.5 wt.%, Ni: 10.5~12.0 wt.%, Mo: 3.5~8.0 wt.%, Mn≤1.0 wt.%, Si≤0.8 wt.%, C≤0.05 wt.%, balance being Fe and unavoidable impurities; S2. The stainless steel powder is melted and shaped layer by layer using an additive manufacturing method, so that the molten pool undergoes a rapid melting and non-equilibrium rapid solidification process, suppressing the δ→γ diffusion phase transformation, thereby obtaining stainless steel with δ-Fe phase as the dominant structure.

[0013] Preferably, the additive manufacturing method is selective laser melting technology.

[0014] Preferably, the process parameters of the selective laser melting technology include: laser power 230~260 W, scanning speed 550~900 mm / s, scanning spacing 0.07~0.10 mm, powder thickness 0.02~0.04 mm, and protective atmosphere is nitrogen or argon.

[0015] Preferably, the cooling rate during the rapid solidification process is 10. 5 ~10 6 K / s.

[0016] Preferably, the stainless steel powder has a particle size of 15~45 μm.

[0017] Preferably, the sphericity of the stainless steel powder is ≥ 95%.

[0018] During the equilibrium solidification of ordinary Fe-Cr-Ni stainless steel, the molten alloy typically first forms a high-temperature body-centered cubic (BCC) structure of δ-Fe. Subsequently, as the temperature decreases, δ-Fe further transforms into a face-centered cubic (FCC) structure of γ-Fe austenite. When further cooled to lower temperatures, in low-Ni or ordinary carbon steel systems, γ-Fe may undergo a solid-state phase transformation to form a room-temperature stable BCC structure of α-Fe. However, for 316L stainless steel with a high Ni content, since Ni is a strong austenite stabilizing element, γ-Fe can still be stably retained at room temperature. Therefore, traditional 316L stainless steel usually ultimately has an FCC austenite structure.

[0019] The BCC-dominant structure in this invention is not obtained through traditional heat treatment, but rather relies on a non-equilibrium microstructure formed by instantaneous melting and ultrafast solidification during additive manufacturing. The BCC phase in this invention primarily originates from δ-Fe retained during the non-equilibrium rapid solidification process, rather than α-Fe formed by traditional solid-state phase transformations. Traditional α-Fe typically undergoes a solid-state phase transformation path of "liquid phase → δ-Fe → γ-Fe → α-Fe," requiring a long diffusion process and easily leading to grain coarsening and elemental segregation. In contrast, the δ-Fe in this application originates from a direct solidification process of "liquid phase → δ-Fe," and the δ-Fe → γ-Fe transformation is suppressed under ultrafast cooling conditions, thereby achieving room-temperature retention of the high-temperature BCC phase. Because δ-Fe forms during the rapid solidification stage, its grains are finer and more uniform, and the diffusion time of elements such as Mo is shortened, which is more conducive to the uniform solidification of Mo and suppresses the precipitation of Mo-rich σ-phase and the formation of Cr / Mo depleted regions. Therefore, compared with the traditional α-Fe structure, the δ-Fe dominant structure in this application not only has higher strength and structural stability, but is also more conducive to maintaining excellent corrosion resistance, reflecting the unique phase transformation path regulation mechanism under the non-equilibrium solidification conditions of additive manufacturing.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention utilizes the ultra-fast cooling effect under non-equilibrium solidification conditions in additive manufacturing to achieve a large amount of retention of the high-temperature BCC phase δ-Fe at room temperature, thus obtaining a BCC-dominant structure. Specifically, when the Mo content is 4 wt.%, the BCC phase accounts for 92.2%; when the Mo content is 8 wt.%, the BCC phase accounts for 99%, breaking through the traditional microstructure characteristics of 316L stainless steel, which is dominated by FCC austenite. This fundamental breakthrough successfully utilizes the rapid cooling nature of 3D printing to retain the thermodynamically high-temperature stable BCC structure at room temperature, opening up a new path for obtaining high-performance ferritic stainless steel; (2) Based on maintaining a high-Ni stable austenitic system (Ni≈10~12 wt.%), this invention achieves the formation of BCC-dominant structure through the synergistic effect of Mo composition regulation and non-equilibrium rapid solidification, without the need for the traditional "reducing Ni and increasing Cr" ferritic design, thus achieving a synergistic improvement in mechanical properties and corrosion resistance. (3) This invention achieves effective solid solution of Mo in the matrix by controlling the Mo content. Under SLM ultrafast cooling conditions, the effective solid solution limit of Mo in 316L stainless steel is about 4 wt.%. Within this window, Mo atoms can be completely dissolved in the matrix, avoiding the risk of precipitation of brittle σ phase and formation of Cr / Mo depletion zone due to excessive Mo content as traditionally understood, ensuring the high uniformity of the chemical composition of the matrix, and eliminating the hidden danger of micro-galvanic corrosion from the source; (4) The fine-grained BCC-dominated structure formed by this invention significantly improves the mechanical properties of the material. When the Mo content is 4 wt.%, the yield strength reaches 877 MPa and the tensile strength reaches 893 MPa; when the Mo content is 8 wt.%, the yield strength reaches 1005 MPa and the tensile strength reaches 1023 MPa. Compared with Comparative Example 1, the yield strength is increased by up to about 610 MPa and the tensile strength is increased by up to about 505 MPa. (5) The Mo-rich passivation film formed by this invention exhibits excellent stability. Polarization curve results show that when the Mo content is 4 wt.%, it has the most positive self-corrosion potential, the lowest corrosion current density, and the highest pitting potential; simultaneously, it exhibits stability within the range of 0.5~1.0 V. SCE The presence of a distinct secondary passivation plateau under high potential conditions indicates that MoO4² - It enables in-situ dynamic repair of the passivation film. Unlike traditional 316L stainless steel, which mainly relies on static passivation film protection, the Mo-rich passivation film in this invention can undergo a "dissolution-migration-redeposition" process under high potential conditions, giving the damaged passivation film a certain dynamic repair capability, thereby improving the material's resistance to pitting corrosion failure. Attached Figure Description

[0021] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the specific embodiments will be briefly introduced below. Obviously, the following description is only a part of the embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 Scanning electron microscope images of Comparative Example 1(a), Comparative Example 2(b), Example 1(c), and Example 2(d).

[0023] Figure 2 The XRD patterns are for the examples and comparative examples.

[0024] Figure 3 The images are EBSD images of comparative examples and embodiments, where a~d are the inverse pole figures (IPF) of comparative example 1, comparative example 2, embodiment 1 and embodiment 2, respectively; a1~d1 are the phase distribution diagrams of the corresponding samples.

[0025] Figure 4 The stress-strain curves are shown for the examples and comparative examples.

[0026] Figure 5 The polarization curves are for the examples and comparative examples.

[0027] Figure 6XPS spectra of Mo 3d (5 / 2, 3 / 2) for the secondary passivation films of Example 1 (a) and Example 2 (b). Detailed Implementation

[0028] This invention proposes a BCC-dominant stainless steel based on non-equilibrium solidification and its preparation method. To facilitate understanding of this invention by those skilled in the art, the specific embodiments of this invention are described below with reference to the accompanying drawings.

[0029] In this invention, unless otherwise specified, the equipment and raw materials used are commercially available or commonly used in the art. The methods in the following embodiments, unless otherwise specified, are conventional methods in the art. Unless otherwise indicated, all parts and percentages are by weight. Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0030] Example 1 The additive manufacturing process for BCC structural stainless steel is as follows: S1. Prepare stainless steel powder for later use: The powder composition is Cr: 17.0 wt.%, Ni: 12.0 wt.%, Mo: 4 wt.%, Mn: 1.0 wt.%, Si: 0.5 wt.%, C: 0.018 wt.%, with the balance being Fe and unavoidable impurities.

[0031] The powder was pre-alloyed 316L stainless steel powder using gas atomization. The powder quality parameters are as follows: the powder particle size is 15~45 μm, the average diameter is 21.81 μm, there is no hollow powder, the sphericity is above 95%, no inclusions were detected, and the oxygen content is 186 ppm. S2. The powder described in step S1 is shaped using selective laser melting (SLM) additive manufacturing method to obtain a printed product.

[0032] The SLM process parameters are as follows: spot diameter is 100 μm, laser power is 230 W, scanning spacing is 0.10 mm, scanning speed is 886 mm / s, powder thickness is 0.02 mm, protective atmosphere is nitrogen, and the density of the printed product is 98.5%.

[0033] Example 2 The additive manufacturing process for BCC structural stainless steel is as follows: S1. Prepare stainless steel powder for later use: The powder composition is Cr: 16.6 wt.%, Ni: 11.76 wt.%, Mo: 8 wt.%, Mn: 1.0 wt.%, Si: 0.5 wt.%, C: 0.018 wt.%, with the balance being Fe and unavoidable impurities.

[0034] Pre-alloyed 316L stainless steel powder was used, and the powder quality parameters were the same as in Example 1.

[0035] S2. The powder described in step S1 is shaped using the SLM additive manufacturing method to obtain a printed product.

[0036] The SLM process parameters are the same as in Example 1.

[0037] Comparative Example 1 The difference from Example 1 is that the SLM additive manufacturing method in Example 1 is replaced by a rolling process, and the Mo content is adjusted to 2 wt.%, that is, Comparative Example 1 uses an alloy material with the same chemical composition as Comparative Example 2.

[0038] Comparative Example 2 The additive manufacturing process for BCC structural stainless steel is as follows: S1. Prepare stainless steel powder for later use: The powder composition is Cr: 16.32 wt.%, Ni: 11.52 wt.%, Mo: 2 wt.%, Mn: 1.0 wt.%, Si: 0.5 wt.%, C: 0.018 wt.%, with the balance being Fe and unavoidable impurities.

[0039] Pre-alloyed 316L stainless steel powder was used, and the powder quality parameters were the same as in Example 1.

[0040] S2. The powder described in step S1 is shaped using the SLM additive manufacturing method to obtain a printed product.

[0041] The SLM process parameters are the same as in Example 1.

[0042] Structural comparison (1) Microstructure like Figure 1 As shown in (a), Comparative Example 1 has regular polygonal austenite grains, such as Figure 1 As shown in (b-d), Comparative Example 2 and Examples 1-2 prepared by SLM all exhibit the irregular grain morphology and clear melt pool boundaries characteristic of rapid solidification. The addition of Mo introduced key microstructural changes. In the sample of Example 1, the microstructure was relatively uniform. However, in the sample of Example 2, a large number of diffusely distributed "grayish-white" precipitates were observed, indicating that the solid solution limit was exceeded. When the Mo content was controlled at 4 wt.% or less, it was within the non-equilibrium solid solution limit of SLM, the matrix chemical composition was uniform, there were no Mo-rich σ phase precipitates and no Cr / Mo depleted regions in the microstructure, thereby reducing the tendency for local micro-galvanic corrosion.

[0043] (2) Chemical structure X-ray diffraction (XRD) analysis, such as Figure 2 As shown, the main diffraction peaks of Comparative Example 1 and Comparative Example 2 (Mo: 2 wt.%) both correspond to the austenite (γ-Fe, FCC) phase. With the increase of Mo addition, the intensity of the austenite diffraction peaks in Example 1 (Mo: 4 wt.%) and Example 2 (Mo: 8 wt.%) significantly decreased, and ferrite (δ-Fe, BCC) diffraction peaks appeared.

[0044] Under the ultrafast cooling conditions of SLM, the material solidification process is in a significantly non-equilibrium state. With increasing Mo content, Mo, as a strong ferrite-forming element, can expand the BCC phase stability region and increase the tendency for δ-Fe formation. When the Mo content reaches approximately 4 wt.%, the molten pool solidification path preferentially enters the BCC phase region; subsequently, due to the extremely high cooling rate of SLM (10⁻⁶ wt.%), the solidification occurs further. 5 ~10 6 The K / s ratio significantly inhibited elemental diffusion and the δ→γ diffusion-type phase transformation, allowing the δ-Fe formed at high temperature to be retained at room temperature, thus forming a BCC-dominant structure. In Comparative Example 2, due to the low Mo content (2 wt.%), although it also underwent the SLM rapid solidification process, the driving force for BCC phase stability was insufficient, and the final microstructure was still dominated by FCC austenite.

[0045] (3) Phase distribution Electron backscatter diffraction (EBSD) analysis, such as Figure 3 As shown, Comparative Example 1 exhibits a typical large-size equiaxed crystal structure with uniform grain size. Figure 3 a). In contrast, Comparative Example 2 ( Figure 3 b) Example 1 ( Figure 3 c) and Example 2 ( Figure 3 d) All exhibit the non-equilibrium microstructure characteristics unique to rapid solidification: irregular grain morphology and clear melt pool boundaries, which are closely related to the heterogeneous nucleation and growth caused by the extremely high cooling rate and directional heat flow of the SLM process.

[0046] With increasing Mo content, the average grain size of the SLM-formed samples showed a refining trend. This refining effect is mainly attributed to two mechanisms: First, the solute dragging effect. As a large atomic radius element, Mo, after dissolving in the austenitic matrix, produces significant lattice distortion, which strongly drags grain boundary migration at the solidification front and during subsequent solid-state phase transformations, inhibiting grain growth. Second, the nucleation-promoting effect. The addition of Mo may reduce the solid / liquid interface energy or, by altering local undercooling, provide more opportunities for heterogeneous nucleation during solidification, thereby refining the grains. Grain refinement generally helps improve the mechanical properties of materials and provides favorable matrix conditions for the formation of a more uniform passivation film.

[0047] Figure 3 Phase distribution diagram in Figure 3 (a1, b1, c1, d1) provide direct evidence of phase composition, with phase proportions obtained statistically from the EBSD phase distribution map. Comparative Examples 1 and 2, with the addition of 2 wt.% Mo, are dominated by face-centered cubic (FCC) austenite (γ) phase (marked in green in the figure). As the Mo content increases to 4% and 8%, the content of body-centered cubic (BCC) phase (marked in red in the figure) in the samples increases significantly, with the BCC phase accounting for 92.2% in Example 1 and 99% in Example 2. Mo is a strong ferrite-forming element, which inhibits the complete transformation of austenite during the rapid solidification of SLM and promotes the retention or formation of ferrite (δ-Fe).

[0048] It should be noted that existing EBSD and XRD results can only characterize the presence of the BCC structural phase in the material, but cannot directly distinguish whether it belongs to δ-Fe or α-Fe. This invention, combining the characteristics of SLM non-equilibrium solidification process and the phase transformation law of Fe-Cr-Ni system, further analyzes the origin of this BCC phase.

[0049] During the solidification of Fe-based stainless steel, δ-Fe is a high-temperature stable BCC phase, typically forming at temperatures above 1394℃, and is a typical primary solidification phase. α-Fe, on the other hand, usually forms at lower temperatures (below 912℃) and is a product of subsequent solid-state phase transformation. The SLM process has an ultra-high cooling rate of 105~106 K / s and an extremely short solidification time, which significantly suppresses the δ→γ diffusion-type phase transformation, allowing the high-temperature-formed δ-Fe to be directly retained to room temperature. In contrast, the formation of α-Fe usually requires a γ→α solid-state diffusion transformation, which is not the normal solidification path of the SLM process and is therefore extremely difficult to achieve in SLM. Therefore, the dominant BCC phase in this invention primarily originates from δ-Fe retained to room temperature, rather than α-Fe formed by traditional solid-state phase transformation.

[0050] Performance Comparison (1) Mechanical properties Mechanical tests were conducted on the stainless steels prepared in each embodiment and comparative example. The performance test results are shown in Table 1. Compared with Comparative Example 1, the hardness, yield strength, and tensile strength of Comparative Example 2 and Examples 1-2, prepared using the SLM process, were significantly improved. Among them, Example 2 had the highest strength level, with a hardness of 362.70 HV, a yield strength of 1005 MPa, and a tensile strength of 1023 MPa. The strength improvement is attributed to the ultrafine grain structure brought about by the rapid solidification of SLM and the solid solution strengthening effect of Mo. Figure 4It can be seen that although Example 2 has the highest strength, its fracture strain is significantly reduced, indicating that excessive Mo leads to a decrease in the plasticity of the material. In contrast, Example 1 maintains a high yield strength and tensile strength while still having a large plastic deformation range and better fracture strain, demonstrating the best strength-plasticity matching performance. Comparative Example 1 has the largest fracture strain but the lowest strength, exhibiting an overall low strength and high plasticity characteristic.

[0051] Although Example 2 has higher strength, the precipitation of Mo-rich σ phase reduces the uniformity of its microstructure and may lead to local Cr / Mo depletion, thereby reducing long-term corrosion resistance and ductility-toughness matching performance. Therefore, 4 wt.% Mo is the optimal window to balance BCC-dominant structure, corrosion resistance, microstructure stability and comprehensive mechanical properties.

[0052] Table 1. Test results of the mechanical properties of stainless steel in the embodiments and comparative examples of the present invention.

[0053] (2) Corrosion resistance Artificial seawater was used as the corrosive medium. At 20°C, a three-electrode system was used to perform potentiodynamic polarization tests on the samples of each embodiment and comparative example. The saturated calomel electrode (SCE) was used as the reference electrode, the platinum sheet as the auxiliary electrode, and the sample as the working electrode. The polarization curves obtained are shown below. Figure 5 As shown. Plotting the logarithm of corrosion current density on the x-axis and the applied potential on the y-axis, compared to Comparative Example 1, the pitting potentials of Comparative Example 2 and Examples 1-2 obtained using the SLM process all shifted positively, indicating a significant improvement in corrosion resistance. Example 1 showed the best performance, with the most positive self-corrosion potential (-0.27 V). SCE It also has the widest passivation range (0.41~1.20 V). SCE ) and the highest pitting potential (1.20 V) SCE This indicates that its passivation film has the strongest stability and resistance to pitting corrosion; Example 2 is second best, and Comparative Example 2 is also better than Comparative Example 1. At the same time, Examples 1 and 2 show obvious secondary passivation plateaus, which means that the MoO4² produced by corrosion is relatively stable. - It can migrate to the active site for redeposition, achieving in-situ repair of the damaged passivation film. This is attributed to the optimized microstructure and Mo-rich passivation film formed by the combined action of Mo composition regulation and SLM rapid solidification, which effectively inhibits the occurrence and development of corrosion. Combined with mechanical property data, it is confirmed that this scheme achieves simultaneous improvement of the mechanical strength and corrosion resistance of 316L stainless steel.

[0054] X-ray photoelectron spectroscopy (XPS) analysis was performed on the secondary passivation films of Examples 1 and 2, such as... Figure 6 As shown, Mo in the secondary passivation film 6+ (MoO4)2- The significant increase in the proportion of solid-solid Mo proves that solid-solid Mo dissolves into migratory Mo at high potentials. 6+ Mo 6+ Soluble in solution and adsorbed / deposited in the form of molybdate on active sites of the film (such as pits about to break), effectively blocking Cl. - Offensive action, achieving in-situ dynamic repair through "dissolution-migration-redeposition," is the core chemical mechanism for secondary passivation. Example 1 with 4% Mo added ( Figure 6 a) Compared to Example 2 with 8% Mo added ( Figure 6 b) Has higher Mo 6+ The ratio demonstrates its stronger dynamic repair capability of the passivation film.

[0055] In summary, through the synergistic effect of Mo composition regulation and non-equilibrium rapid solidification process, a BCC microstructure dominated by δ-Fe was achieved while maintaining the high-Ni corrosion-resistant system of 316L stainless steel. Specifically, Mo expanded the stable region of the BCC phase, while the ultra-fast cooling during additive manufacturing significantly suppressed the δ→γ diffusion-type phase transformation, allowing the δ-Fe formed at high temperatures to be retained at room temperature. The results of the examples show that when the Mo content is 2 wt.%, the material is still dominated by the FCC austenitic phase; when the Mo content is increased to 4 wt.%, a BCC-dominated structure is obtained without the observation of Mo-rich σ phase precipitation, exhibiting excellent strength-ductility matching and corrosion resistance; when the Mo content is further increased to 8 wt.%, although the material strength is further improved, precipitates appear in the microstructure, indicating that the non-equilibrium solid solution limit has been exceeded. Therefore, approximately 4 wt.% Mo is the preferred composition window for the system of this invention, which can achieve a BCC-dominant structure, complete solid solution without precipitation, and synergistic improvement in mechanical properties and corrosion resistance, while also endowing the material with excellent pitting corrosion resistance, stable passivation behavior, and secondary passivation self-healing ability.

[0056] The embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A stainless steel with a BCC-dominant structure based on non-equilibrium solidification, characterized in that, The stainless steel is prepared by additive manufacturing and comprises the following components by weight percentage: Cr: 16.0~17.5 wt.%, Ni: 10.5~12.0 wt.%, Mo: 3.5~8.0 wt.%, Mn≤1.0 wt.%, Si≤0.8 wt.%, C≤0.05 wt.%, balance being Fe and unavoidable impurities; The stainless steel is characterized by a body-centered cubic (BCC) structure with δ-Fe phase as the dominant structure, and the proportion of δ-Fe phase is not less than 90%.

2. The BCC-dominant structure stainless steel based on non-equilibrium solidification according to claim 1, characterized in that: The Mo content is 4 wt.%, and the Mo exists in the matrix in a solid solution state.

3. The BCC-dominant structure stainless steel based on non-equilibrium solidification according to claim 1, characterized in that: The yield strength of the stainless steel is not less than 877 MPa, and the tensile strength is not less than 893 MPa.

4. The BCC-dominant structure stainless steel based on non-equilibrium solidification according to claim 1, characterized in that: The stainless steel is subjected to a temperature of 0.5~1.0 V in an artificial seawater environment. SCE The high-potential region exhibits secondary passivation behavior.

5. A method for preparing BCC-dominant stainless steel based on non-equilibrium solidification, characterized in that, Includes the following steps: S1. Provide stainless steel powder, wherein the stainless steel powder comprises the following components by weight percentage: Cr: 16.0~17.5 wt.%, Ni: 10.5~12.0 wt.%, Mo: 3.5~8.0 wt.%, Mn≤1.0 wt.%, Si≤0.8 wt.%, C≤0.05 wt.%, balance being Fe and unavoidable impurities; S2. The stainless steel powder is melted and shaped layer by layer using an additive manufacturing method, so that the molten pool undergoes a rapid melting and non-equilibrium rapid solidification process, suppressing the δ→γ diffusion phase transformation, thereby obtaining stainless steel with δ-Fe phase as the dominant structure.

6. The preparation method according to claim 5, characterized in that: The additive manufacturing method is selective laser melting technology.

7. The preparation method according to claim 6, characterized in that, The process parameters of the selective laser melting technology include: laser power 230~260 W, scanning speed 550~900 mm / s, scanning spacing 0.07~0.10 mm, powder thickness 0.02~0.04 mm, and protective atmosphere of nitrogen or argon.

8. The preparation method according to claim 5, characterized in that: The cooling rate during the rapid solidification process is 10. 5 ~10 6 K / s.

9. The preparation method according to claim 5, characterized in that: The stainless steel powder has a particle size of 15~45 μm.

10. The preparation method according to claim 5, characterized in that: The sphericity of the stainless steel powder is ≥ 95%.

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