High-nitrogen nickel-free austenitic stainless steel bracket and preparation method thereof

Abstract

The application provides a high-nitrogen nickel-free austenitic stainless steel bracket and a preparation method thereof, and belongs to the field of austenitic stainless steel. The method comprises the following steps: casting an initial molten steel into a consumable electrode, placing the consumable electrode in an electroslag remelting furnace, remelting and refining by using a mixed slag system of base slag and composite premelted nitride slag, and performing stepwise pressurization during the remelting process to obtain a steel ingot; homogenizing the steel ingot, and then performing multi-directional forging; performing high-temperature solid solution treatment on the forged blank, and then performing quenching; performing cold deformation processing on the plate, and then performing secondary solid solution treatment and quenching; machining the semi-finished product into the shape of a bracket, and then performing surface electrolytic polishing and passivation treatment to obtain the high-nitrogen nickel-free austenitic stainless steel bracket. Through the whole-process collaborative control from the material metallurgical source to the final forming, the harsh requirements of the high-nitrogen nickel-free austenitic stainless steel bracket on biocompatibility, mechanical properties and corrosion resistance as a long-term implanted instrument in the oral cavity are systematically met.

Description

Technical Field

[0001] This application relates to the field of austenitic stainless steel technology, and in particular to a high-nitrogen nickel-free austenitic stainless steel tray and its preparation method. Background Technology

[0002] Orthodontic brackets, as fixed medical devices placed in the oral cavity for extended periods, face extremely stringent requirements regarding material properties. First, they must possess excellent biocompatibility to avoid triggering allergic or toxic reactions, with the most critical aspect being the elimination of contact sensitization risks caused by nickel ion release. Second, they require sufficiently high mechanical properties, particularly yield strength, to withstand the continuous and complex forces applied by the orthodontic archwire without plastic deformation, ensuring treatment precision. Third, due to prolonged exposure to a salivary environment containing chloride ions, microorganisms, and fluctuating temperatures, the material must possess excellent corrosion resistance, especially resistance to pitting and crevice corrosion, to prevent excessive release of metal ions and material failure. Currently, widely used traditional austenitic stainless steel brackets (such as 304 and 316L) rely on nickel to stabilize the austenitic structure, posing a potential risk of nickel allergy. While some nickel-free alternatives, such as cobalt-chromium alloys or titanium alloys, offer better biocompatibility, they have limitations in terms of cost, processability, or specific strength. Developing a novel bracket material that combines nickel-free biocompatibility, high strength, and excellent corrosion resistance is an important direction in the field of orthodontic materials.

[0003] High-nitrogen nickel-free austenitic stainless steel is considered a highly promising solution, stabilizing austenite by replacing nickel with nitrogen and manganese, while nitrogen also significantly improves strength and corrosion resistance. However, its successful application in the fabrication of trays faces a series of technical challenges: First, how to achieve precise and stable addition of high nitrogen content (typically exceeding 0.4 wt%) during smelting and prevent its loss during subsequent processing; second, how to ensure a completely stable single austenitic structure without nickel, avoiding the precipitation of harmful phases, while refining the grains to optimize strength and toughness; third, how to achieve medical device-grade cleanliness and surface passivation through pure smelting and appropriate surface treatment, thereby fully meeting the requirements for biocompatibility, mechanical properties, and corrosion resistance. Existing conventional smelting and processing methods are insufficient to overcome these challenges simultaneously, necessitating a targeted, end-to-end collaboratively controlled fabrication method. Summary of the Invention

[0004] This application provides a high-nitrogen nickel-free austenitic stainless steel bracket and its preparation method to solve the following technical problem: how to meet the stringent requirements of high-nitrogen nickel-free austenitic stainless steel brackets as long-term intraoral implants for biocompatibility, mechanical properties and corrosion resistance.

[0005] In a first aspect, this application provides a method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray, the method comprising a first method or a second method; wherein The first method includes the following steps: S1, placing the raw material in a pressurized induction furnace, then filling it with a nitrogen-argon mixed gas, and melting it at a pressure of 0.4-0.6 MPa to obtain initial molten steel; S2, casting the initial molten steel into a consumable electrode, placing it in an electroslag remelting furnace, and remelting and refining it using a mixed slag system of base slag and composite pre-melted nitride slag, and performing stepped pressurization during the remelting process, and casting it to obtain a steel ingot with a set chemical composition; S3, homogenizing the steel ingot, and then performing multi-directional forging to obtain a forging billet; S4, subjecting the forging billet to high-temperature solution treatment. S5. The plate is subjected to cold deformation processing with a first preset deformation amount, followed by secondary solution treatment and quenching to obtain a semi-finished product; S6. The semi-finished product is machined into a groove shape, followed by surface electrolytic polishing and passivation treatment to obtain a high-nitrogen nickel-free austenitic stainless steel groove; wherein, by mass fraction, the composite pre-melted nitride slag is composed of the following chemical components: CrN: 50-65%, Ca3N2: 20-30%, Y2O3: 5-10%, CaF2: 5-10%; The second method includes the following steps: S1, melting the raw materials in a pressure induction furnace, filling with a nitrogen-argon mixed gas, and obtaining initial molten steel under a pressure of 0.4-0.6 MPa, then making the initial molten steel into stainless steel powder; S2, mixing the stainless steel powder with a binder at a mass ratio of 88:12-92:8, and obtaining injection molding feed after kneading and granulation; S3, injection molding the feed to obtain a green billet in the shape of a tray; S4, degreasing the green billet to remove the binder; S5, placing the degreased billet in a sintering furnace and sintering it under a nitrogen atmosphere at a sintering pressure of 0.5-1.0 MPa and a sintering temperature of 1200-1350℃ to obtain a high-nitrogen nickel-free austenitic stainless steel tray billet; S6, performing surface electrolytic polishing and passivation treatment on the tray billet to obtain a high-nitrogen nickel-free austenitic stainless steel tray.

[0006] Optionally, by mass fraction, the mixed slag system consists of 70-75% CaF2-Al2O3-CaO-based slag and 25-30% of the composite pre-melted nitride slag.

[0007] Optionally, the stepped pressurization is as follows: during the melting period of remelting, the pressure is controlled at 0.5 to 0.8 MPa; when the consumable electrode is melted to the remaining 1 / 2 to 2 / 3, the pressure is increased to 1.0 to 1.3 MPa within 5 to 15 minutes and maintained until the melting is completed.

[0008] Optionally, the set chemical composition, by mass fraction, is: C≤0.1%, Si≤1.0%, Mn: 10~14%, P≤0.03%, S≤0.03%, Cr: 16.0~18.0%, Mo: 3.0~4.0%, Ni≤0.10%, N: 0.4~0.9%, with the balance being Fe and unavoidable impurities.

[0009] Optionally, the specified chemical composition satisfies the following relationship: [Mn]≥10×[N]+6, and [Cr]+3.3×[Mo]≥28 In the formula, [Mn] is the value preceding the mass fraction % of Mn, [N] is the value preceding the mass fraction % of N, [Cr] is the value preceding the mass fraction % of Cr, and [Mo] is the value preceding the mass fraction % of Mo.

[0010] Optionally, the homogenization treatment is carried out at a temperature of 1150–1200°C for 2–4 hours. The total forging ratio of the multi-directional forging is ≥6, and the final forging temperature is ≥900℃.

[0011] Optionally, the high-temperature solution treatment temperature is 1100–1150°C, and the holding time is 1–2 hours; The quenching is water quenching.

[0012] Optionally, the first preset deformation amount is 30% to 50%; The secondary solution treatment is performed at a temperature of 1050–1100℃ for 1–2 hours.

[0013] Optionally, the passivation treatment is: immersing in a 20%–30% nitric acid solution for 10–30 minutes.

[0014] Secondly, this application provides a high-nitrogen nickel-free austenitic stainless steel support channel prepared by the method described in any one of the first aspects, wherein the matrix of the high-nitrogen nickel-free austenitic stainless steel support channel is a single austenitic structure with an average grain diameter ≤32μm and a non-metallic inclusion level ≤1.5.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing high-nitrogen nickel-free austenitic stainless steel brackets. Through the coordinated control of the entire process from the source of material metallurgy to the final forming, the method systematically meets the stringent requirements of high-nitrogen nickel-free austenitic stainless steel brackets as long-term intraoral implants for biocompatibility, mechanical properties and corrosion resistance.

[0016] In ensuring biocompatibility, the core of this method lies in achieving complete nickel-free production. Through rigorous compositional design, the nickel content is controlled to an extremely low level, fundamentally eliminating the risk of oral mucosal allergies caused by nickel ion release. During the preparation process, a dual process of pressure induction melting and pressure electroslag remelting is carried out under a controlled atmosphere, effectively reducing the introduction of impurity elements. The final electrolytic polishing and passivation treatment not only removes the defect layer on the processed surface but also promotes the formation of a dense and stable oxide layer, greatly reducing the overall tendency for metal ion release in the complex oral cavity environment, thereby ensuring long-term biocompatibility.

[0017] In terms of achieving excellent mechanical properties, this method achieves a balance between high strength and good plasticity through the coordinated design of composition and process. The high nitrogen content in the material provides an essential solid solution strengthening effect. The key step S5, which involves cold working the sheet material with a certain amount of deformation followed by a secondary solution treatment, constitutes a classic deformation heat treatment process. Cold deformation introduces a high density of dislocations, significantly improving the material's yield and tensile strength, making it sufficient to withstand the continuous force applied by the orthodontic archwire without permanent deformation. The subsequent secondary solution treatment, through a recrystallization process, eliminates the internal stress and plasticity loss caused by work hardening, restoring the material's necessary ductility and toughness, ensuring the reliability of the bracket in clinical use.

[0018] To achieve superior corrosion resistance, this method establishes a multi-layered protection system from the bulk phase to the surface. In the bulk phase, precise composition control ensures sufficient chromium content to form a stable passivation film. Simultaneously, the high molybdenum content and high nitrogen content work synergistically to endow the material with extremely strong resistance to chloride-induced pitting and crevice corrosion in salivary environments. During the preparation process, the pressurized electroslag remelting step plays a decisive role. The composite functional slag system used deeply removes non-metallic inclusions such as oxides and sulfides from the molten steel; these inclusions are common starting points for corrosion initiation. For surface treatment, the final electrolytic polishing and passivation treatment makes the tray surface extremely smooth and forms a chromium-rich, uniform, and firmly adhered passivation film. This physicochemical barrier is the last and crucial line of defense for the tray to maintain its integrity and resist corrosion during long-term service.

[0019] In summary, this preparation method does not rely on a single step, but rather on a series of interconnected processes, including nickel-free component design, high-purity smelting, toughening deformation heat treatment, and final surface modification, to synergistically endow the material with the three-in-one comprehensive performance required for long-term oral implantation. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the first process of a method for preparing a high-nitrogen nickel-free austenitic stainless steel support groove provided in an embodiment of this application; Figure 2 This is a schematic diagram of the second process of a method for preparing a high-nitrogen nickel-free austenitic stainless steel support provided in an embodiment of this application. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0025] Figure 1 This is a schematic diagram of the first process of a method for preparing a high-nitrogen nickel-free austenitic stainless steel support groove provided in an embodiment of this application.

[0026] like Figure 1 As shown, this application provides a method for preparing a high-nitrogen nickel-free austenitic stainless steel support channel, which includes the following steps: S1. Place the raw material in a pressurized induction furnace, then fill it with a nitrogen-argon mixed gas, and melt it under a pressure of 0.4-0.6 MPa to obtain the initial molten steel; S2. The initial molten steel is cast into a consumable electrode and placed in an electroslag remelting furnace. A mixed slag system of base slag and composite pre-melted nitride slag is used for remelting and refining. During the remelting process, step-by-step pressurization is carried out, and the steel ingot with the set chemical composition is obtained by casting. S3. The steel ingot is homogenized and then multi-directional forged to obtain a forging billet; S4. The forging billet is subjected to high-temperature solution treatment and then quenched to obtain a plate with a single austenitic structure. S5. The sheet metal is subjected to cold deformation processing with a first preset deformation amount, followed by secondary solution treatment and quenching to obtain a semi-finished product. S6. The semi-finished product is machined into a tray shape, and then the surface is electrolytically polished and passivated to obtain a high-nitrogen nickel-free austenitic stainless steel tray.

[0027] Figure 2 This is a schematic diagram of the second process of a method for preparing a high-nitrogen nickel-free austenitic stainless steel support groove provided in an embodiment of this application; like Figure 2 As shown, this application provides a method for preparing a high-nitrogen nickel-free austenitic stainless steel support channel, which includes the following steps: S1. The raw materials are melted in a pressure induction furnace, and a nitrogen-argon mixed gas is introduced to obtain initial molten steel under a pressure of 0.4-0.6 MPa. The initial molten steel is then made into stainless steel powder. S2. The stainless steel powder and the binder are mixed at a mass ratio of 88:12 to 92:8, and then kneaded and granulated to obtain the injection molding feedstock. S3. The feed material is injection molded to obtain a green blank with a groove shape; S4. Degrease the green body to remove the binder; S5. Place the degreased billet in a sintering furnace and sinter it under a nitrogen atmosphere. The sintering pressure is 0.5-1.0 MPa and the sintering temperature is 1200-1350℃ to obtain a high-nitrogen nickel-free austenitic stainless steel support billet. S6. The tray blank is subjected to surface electrolytic polishing and passivation treatment to obtain a high-nitrogen nickel-free austenitic stainless steel tray.

[0028] The preparation method provided in this application is a complete technical solution for dental bracket applications, encompassing alloy melting, microstructure control, and final shaping. Each step is interconnected and based on well-defined metallurgical principles, collectively ensuring that the final product achieves the core characteristic of "high nitrogen and nickel-free" and meets the stringent requirements for biocompatibility, mechanical properties, and corrosion resistance as a long-term intraoral implant.

[0029] The core function of pressure induction melting in step S1 is to forcibly dissolve nitrogen in atomic form into liquid steel under pressure higher than atmospheric pressure, laying the foundation for high nitrogen content. According to Siefert's law, the solubility of nitrogen in liquid ferroalloys is proportional to the square root of its partial pressure. Melting in a nitrogen-argon mixed atmosphere of 0.4–0.6 MPa significantly increases the initial solubility of nitrogen in molten steel, creating the thermodynamic conditions for achieving the high nitrogen target of 0.4%–0.9%. Simultaneously, this closed pressurized environment effectively reduces the loss of volatile elements such as manganese, initially ensuring the accuracy of the alloy composition.

[0030] Step S2, pressurized electroslag remelting, is the most crucial metallurgical step in obtaining high-quality steel ingots, serving multiple purposes. First, it provides deep purification: as the molten metal droplets from the consumable electrode pass through the high-temperature liquid slag layer, they efficiently adsorb and remove non-metallic inclusions such as oxides and sulfides from the steel, significantly improving its purity. Second, it achieves precise and uniform nitrogen enrichment: using a specially formulated composite pre-melted nitride slag, nitrogen is transferred to the molten steel through a more stable slag-metal interface reaction. Combined with a "stepped pressurization" strategy—using 0.5-0.8 MPa during the melting period to stably establish the molten pool, and increasing to 1.0-1.3 MPa during the refining period—a higher nitrogen partial pressure is created during the refining stage, driving nitrogen to diffuse uniformly into the depths of the molten steel. This allows for precise and stable control of the nitrogen content within the target upper limit, ensuring a high degree of uniformity in the ingot composition. Furthermore, the process forms a top-down sequential solidification within the water-cooled crystallizer, which effectively reduces central porosity and segregation, resulting in a dense, uniform fine columnar crystal cast structure, providing excellent raw materials for subsequent hot working.

[0031] The homogenization treatment and multi-directional forging in step S3 aim to eliminate segregation, break up the as-cast structure, and densify the material. Homogenization treatment at 1150–1200℃ for 2–4 hours utilizes high temperature and long-term holding to promote atomic diffusion, thereby eliminating dendritic segregation generated during the solidification of high-alloy ingots and preventing subsequent precipitation of harmful phases or weakened properties due to localized compositional inhomogeneity. The subsequent large deformation (forging ratio ≥6) multi-directional forging thoroughly breaks up coarse as-cast grains and welds internal micropores. This is essentially a thermomechanical process that not only refines the grains but also provides a driving force for subsequent recrystallization. Controlling the final forging temperature to be no lower than 900℃ ensures that the material remains in the austenitic single-phase region throughout the entire processing, preventing processing difficulties or structural defects caused by entering the two-phase region at excessively low temperatures.

[0032] The high-temperature solution treatment in step S4 is to obtain a homogeneous and uniform austenitic structure. Holding the forged billet at 1100–1150°C followed by water quenching serves to completely dissolve any carbides, nitrides, or intermediate phases that may have precipitated during hot working into the austenitic matrix. The subsequent rapid water quenching aims to "freeze" this high-temperature supersaturated single-phase solid solution state to room temperature. This is a crucial step in ensuring the material achieves optimal corrosion resistance and excellent ductility and toughness, as it avoids the "chromium depletion" phenomenon caused by carbonitride precipitation at grain boundaries.

[0033] The core objective of step S5, cold deformation and secondary solution treatment, is to achieve an optimized balance between material strength and plasticity. First, the sheet material undergoes 30%–50% cold deformation. This introduces numerous crystal defects through dislocation multiplication and entanglement, resulting in a significant work hardening effect. This substantially increases yield strength and tensile strength to meet the mechanical requirements of the bracket's resistance to orthogonal forces. Subsequently, a secondary solution treatment at 1050–1100℃ for 1–2 hours, followed by water quenching, is performed. This is a recrystallization annealing process: severely distorted old grains are replaced by new, strain-free, fine equiaxed grains. This eliminates work hardening and internal stress, restoring the material's good plasticity and formability, while maintaining a high strength level due to grain refinement. This "deformation + recrystallization" process is key to precisely controlling the balance between material strength and toughness.

[0034] Step S6, precision machining and surface treatment, ultimately achieves the product form and optimizes its surface condition to improve service performance. First, precision machining transforms the semi-finished product into a bracket shape with complex grooves, wing plates, and other structures. Then, electropolishing is performed, using electrochemical dissolution to uniformly remove microscopic defects, burrs, and deformed layers generated during machining, resulting in a bright and smooth surface, thereby significantly reducing the preferential sites for corrosion initiation. Finally, passivation treatment is performed, using oxidizing acids such as nitric acid to transform the chromium-rich layer on the bracket surface into an extremely thin, dense, and chemically stable chromium oxide passivation film. This film serves as a physical barrier against chloride ions, acids, and enzymes in the oral cavity environment, and is a crucial final guarantee for achieving long-term biocompatibility.

[0035] In summary, this method systematically solves the key technical challenges in the preparation of high-nitrogen nickel-free stainless steel, such as precise control of nitrogen content, structural stability, strengthening and toughening, and biocompatibility, through the coordinated control of the entire process: "pressure melting to control nitrogen and maintain composition → electroslag remelting to purify and homogenize → thermomechanical processing to densify → solution treatment to obtain single-phase structure → deformation heat treatment to adjust strength and toughness → surface treatment to improve corrosion resistance". Ultimately, it produces a high-performance special tray.

[0036] In some embodiments, the mixed slag system consists of 70-75% CaF2-Al2O3-CaO-based slag and 25-30% composite pre-melted nitride slag by mass fraction.

[0037] Slag composition (70-75% base slag, 25-30% composite pre-melted nitride slag): This composition ensures a balance between the metallurgical effect and nitrogen enrichment efficiency in the electroslag remelting process. Sufficient base slag (70-75%) guarantees good slag fluidity, a stable arc, and adequate deep purification capabilities, efficiently removing inclusions. Simultaneously, the 25-30% composite pre-melted nitride slag provides a sufficient and controllable nitrogen source, achieving stable and uniform nitrogen enrichment through interfacial reaction with the molten steel, avoiding process instability due to an excessively high nitrogen source ratio or insufficient nitrogen enrichment due to an excessively low ratio.

[0038] In some embodiments, the composite premelted nitride slag, by mass fraction, is composed of the following chemical components: CrN: 50-65%, Ca3N2: 20-30%, Y2O3: 5-10%, CaF2: 5-10%.

[0039] Composite pre-melted nitride slag composition: This design constitutes a multifunctional synergistic system. CrN, as the primary nitrogen supplier compatible with the molten steel composition, ensures basic nitrogen supply strength and stability through its high proportion (50-65%). Ca3N2 (20-30%) acts as both a flux and a nitrogen carrier, lowering the slag melting point. Its decomposition product, calcium (Ca), also provides deep deoxidation and desulfurization, creating a low-oxygen potential environment to significantly improve nitrogen recovery. Y2O3 (5-10%) acts as a stabilizer, reducing slag erosion of the furnace lining and refining the solidification structure of the steel. CaF2 (5-10%) is mainly used to further improve slag fluidity and promote mass transfer processes in the refining reaction.

[0040] In some embodiments, the stepwise pressurization is as follows: the pressure is controlled at 0.5 to 0.8 MPa during the melting period of remelting; when the consumable electrode is melted to the remaining 1 / 2 to 2 / 3, the pressure is increased to 1.0 to 1.3 MPa within 5 to 15 minutes and maintained until the melting is completed.

[0041] A stepped pressurization regime (0.5–0.8 MPa during melting, increasing to 1.0–1.3 MPa during refining): This pressure regime is designed to match the remelting process. A moderate pressure (0.5–0.8 MPa) during melting is beneficial for the stable establishment of the molten pool, avoiding violent boiling and splashing. After the electrode has melted to 1 / 2–2 / 3 and entered the refining phase, the pressure is increased to 1.0–1.3 MPa. At this point, the liquid metal pool is stable, and the increased nitrogen partial pressure most effectively drives nitrogen atoms to diffuse and dissolve deep into the molten metal, achieving efficient and uniform "high-pressure nitriding" and ensuring that the final ingot nitrogen content remains stable within the target upper limit range (e.g., close to 0.9%).

[0042] In some embodiments, the chemical composition is set as follows by mass fraction: C≤0.1%, Si≤1.0%, Mn:10~14%, P≤0.03%, S≤0.03%, Cr:16.0~18.0%, Mo:3.0~4.0%, Ni≤0.10%, N:0.4~0.9%, with the balance being Fe and unavoidable impurities.

[0043] In the composition design of this high-nitrogen nickel-free austenitic stainless steel, the elements work synergistically to give the material the key properties suitable for orthodontic brackets.

[0044] Carbon (C≤0.1%) is strictly limited to low carbon levels, with the primary aim of minimizing the formation of chromium carbides (such as Cr) with chromium (Cr). 23 This mitigates the risk of C6, thereby avoiding intergranular corrosion susceptibility caused by chromium depletion at grain boundaries, which is crucial for ensuring the corrosion resistance of long-term implants.

[0045] Silicon (Si≤1.0%) is mainly used as a deoxidizer in the smelting process. Its content is controlled at a low level to ensure that the deoxidation task is completed without significantly impairing the plasticity, toughness and cold workability of the material. The latter is especially important for grooves that are precision machined into complex shapes.

[0046] Manganese (Mn: 10–14%) plays a central role in this design. In a high-nitrogen and nickel-free system, the primary function of high manganese content is to expand and stabilize the austenite phase region, completely replacing the austenitizing effect of nickel in traditional stainless steel. Secondly, it can significantly increase the solubility of nitrogen (N) in steel, providing the necessary conditions for introducing high nitrogen content. In addition, manganese also contributes to a certain solid solution strengthening effect.

[0047] Nitrogen (N: 0.4–0.9%) is the characteristic element and key strengthening phase in this design. It works synergistically with manganese to stabilize the austenitic microstructure, achieving complete nickel-free composition. The solid solution of nitrogen atoms in the austenitic matrix produces a strong solid solution strengthening effect, significantly improving the material's strength and hardness to meet the mechanical requirements of the bracket against deformation. Simultaneously, nitrogen significantly improves the material's resistance to pitting and crevice corrosion, with effects superior to traditional elements like chromium and molybdenum.

[0048] Chromium (Cr: 16.0–18.0%) is fundamental to corrosion resistance. It promotes the formation of an extremely thin, dense, and strongly adhering chromium oxide (Cr2O3) passivation film on the material surface. This passivation film is a physicochemical barrier that protects the material from electrochemical corrosion by various media, including saliva.

[0049] Molybdenum (Mo: 3.0–4.0%) is a key element for enhancing corrosion resistance, particularly against pitting and crevice corrosion. It strengthens the stability of the passivation film, especially in environments containing chloride ions (such as saliva), effectively inhibiting the initiation and spread of localized corrosion. The high molybdenum content (3–4%) provides additional corrosion resistance to this material in harsh oral environments.

[0050] Nickel (Ni≤0.10%) is strictly limited to extremely low levels (“nickel-free”) with the fundamental aim of completely eliminating contact allergic reactions to the skin and mucous membranes that may be caused by nickel ions, and meeting biocompatibility requirements, which is a mandatory requirement for medical devices that are placed in the oral cavity for a long time.

[0051] Phosphorus (P≤0.03%) and sulfur (S≤0.03%) are impurity elements that cannot be completely avoided, and their contents are strictly controlled. Low phosphorus helps prevent grain boundary embrittlement, while low sulfur reduces the formation of harmful inclusions such as manganese sulfide. Together, they ensure the high purity of the material, which is beneficial to plasticity, toughness, fatigue performance, and corrosion resistance.

[0052] Iron (Fe) serves as the matrix element, forming the macroscopic framework of the material.

[0053] In some implementations, the chemical components are set to satisfy the following relationship: [Mn]≥10×[N]+6, and [Cr]+3.3×[Mo]≥28 In the formula, [Mn] is the value preceding the mass fraction % of Mn, [N] is the value preceding the mass fraction % of N, [Cr] is the value preceding the mass fraction % of Cr, and [Mo] is the value preceding the mass fraction % of Mo.

[0054] The compositional relationships for steel ([Mn]≥10×[N]+6 and [Cr]+3.3×[Mo]≥28): These two relationships are the core criteria for material design. The manganese-nitrogen relationship ensures the thermodynamic stability of the austenitic structure: This relationship ensures that within a set nitrogen content range, there is sufficient manganese to maintain the supersaturated solid solution of nitrogen in austenite, preventing the precipitation of brittle chromium nitride (Cr2N) during processing or cooling. High manganese (10–14%) can expand the austenitic region and increase nitrogen solubility. This inequality ensures that under any compositional combination, there is sufficient manganese to stabilize nitrogen in solution, preventing the solubility limit from being exceeded due to excessively high nitrogen content (e.g., reaching 0.9%), thereby avoiding the precipitation of brittle chromium nitride (e.g., Cr2N) during solidification or cooling. The chromium-molybdenum relationship is a quantitative guarantee of corrosion resistance: Chromium is a passivation film forming element, and molybdenum is an effective element against pitting corrosion. This inequality requires that the base part of the pitting equivalent (PREN) ([Cr] + 3.3 × [Mo]) be no less than 28. Combined with the contribution of high nitrogen (N) (the coefficient of nitrogen in the PREN formula is usually 16 to 30), it can ensure that the material has extremely strong overall resistance to pitting corrosion and meet the requirements for long-term use in the humid environment of the oral cavity containing chloride ions.

[0055] In some embodiments, the homogenization treatment is carried out at a temperature of 1150–1200°C for 2–4 hours. The total forging ratio of multi-directional forging is ≥6, and the final forging temperature is ≥900℃.

[0056] Homogenization treatment (1150–1200℃, 2–4h): This high-temperature, long-duration treatment aims to eliminate dendritic segregation that occurs during ingot solidification. At 1150–1200℃, the diffusion capacity of alloying elements (such as Mo and Cr) is significantly enhanced. The 2–4h holding time provides sufficient time for atomic diffusion, thereby achieving a high degree of compositional homogeneity in the cross-section and longitudinal section of the ingot, laying the microstructural foundation for obtaining products with uniform properties.

[0057] Forging parameters (total forging ratio ≥6, final forging temperature ≥900℃): A large forging ratio (≥6) ensures that the coarse cast grain structure is thoroughly broken down through intense multi-directional plastic deformation, welding together the porosity and voids inside the ingot, making the material denser and significantly refining the grains. Controlling the final forging temperature to be no less than 900℃ aims to ensure that the entire hot deformation process is completed within the single austenitic phase region of the material, avoiding the induction of uneven deformation or precipitation of harmful phases in the two-phase region at lower temperatures, thereby obtaining a uniform and fine processed structure.

[0058] In some embodiments, the high-temperature solution treatment temperature is 1100–1150°C, and the holding time is 1–2 hours; Quenching is water quenching.

[0059] High-temperature solution treatment (1100–1150℃, 1–2 h, water quenching): This step is crucial for obtaining the optimal initial state. Holding at an austenitizing temperature of 1100–1150℃ for 1–2 h is sufficient to completely dissolve any carbides, nitrides, or intermediate phases that may precipitate after hot working into the austenitic matrix. The subsequent rapid water quenching preserves this supersaturated single-phase solid solution state to room temperature, resulting in a homogeneous and pure single austenitic microstructure. This provides the material with optimal initial corrosion resistance (no risk of grain boundary chromium depletion) and good plasticity reserves.

[0060] In some embodiments, the first preset deformation amount is 30% to 50%; The temperature for the secondary solution treatment is 1050–1100℃, and the time is 1–2 hours.

[0061] Deformation heat treatment parameters (cold deformation 30%–50%, secondary solution treatment 1050–1100℃ / 1–2h): This process is the core of controlling the final mechanical properties. The 30%–50% cold deformation introduces high-density dislocations, resulting in strong work hardening and significantly improving the material strength (especially yield strength), meeting the requirements for deformation resistance in the slot. The subsequent secondary solution treatment at 1050–1100℃ for 1–2h is essentially a precise recrystallization annealing process. It eliminates the internal stress from cold deformation and recrystallizes the distorted grains into fine new equiaxed grains. This restores the material's good plasticity and toughness while maintaining a high strength level through grain refinement, ultimately achieving an optimized balance between strength and toughness.

[0062] In some embodiments, the passivation treatment is to soak in a 20%–30% nitric acid solution for 10–30 minutes.

[0063] Passivation treatment (20%–30% HNO3, immersion for 10–30 min): This is the final crucial step in imparting excellent corrosion resistance and biocompatibility to the product. Nitric acid at a concentration of 20%–30% is a strong oxidizing medium. Within this concentration and 10–30 min time range, it effectively and uniformly dissolves trace amounts of active substances such as iron remaining on the surface after machining and electropolishing of the mounting bracket, while simultaneously promoting the selective oxidation of chromium, forming an extremely thin (usually nanoscale), dense, and chemically extremely stable chromium-rich oxide (mainly Cr2O3) passivation film on the alloy surface. This film serves as a physical and chemical barrier against pitting and crevice corrosion in the complex environment of the oral cavity over the long term.

[0064] This application systematically solves the three core challenges faced by applying high-nitrogen nickel-free stainless steel to dental brackets through a set of interconnected, end-to-end manufacturing technologies.

[0065] First, addressing the challenge of precisely adding and maintaining stable high nitrogen content, this application employs a staged pressure metallurgical strategy. In the pressure induction melting stage, nitrogen partial pressure is utilized to increase the initial solubility of nitrogen in the molten steel. More crucially, the subsequent pressure electroslag remelting process innovatively uses a specially formulated composite pre-melted nitride slag as a nitrogen carrier. Stable and uniform nitrogen transfer is achieved through slag-metal interface reactions, combined with a stepped pressure process, applying higher pressure during metal refining and solidification to strongly suppress nitrogen escape. Subsequent hot working and heat treatment involve strict temperature and time control, and final quenching dissolves supersaturated nitrogen atoms into the matrix, thus achieving precise control of nitrogen content throughout the entire process from melting to finished product.

[0066] Secondly, to achieve microstructural stability and toughness under nickel-free conditions, this application employs a synergistic approach involving both compositional design and deformation heat treatment. The material utilizes a high-manganese and high-nitrogen composition, which synergistically replaces the austenite stabilizing effect of nickel, thermodynamically ensuring the stability of the austenite phase. In terms of processing, the original grains are broken down through multi-directional forging with large deformation, followed by high-temperature solution treatment to dissolve potentially precipitated harmful phases and obtain a single austenite. Subsequently, cold working with a predetermined deformation amount, combined with a secondary solution treatment, utilizes deformation energy storage to drive recrystallization, thereby obtaining a fine-grained, uniform austenite microstructure. This improves strength while maintaining good plasticity, optimizing the strength-toughness balance.

[0067] Finally, to achieve medical device-grade cleanliness and surface condition, this application constructs a complete system from bulk purification to surface modification. Bulk purity primarily relies on the deep refining capability of the pressurized electroslag remelting process. The composite slag system used efficiently adsorbs and removes non-metallic inclusions from the molten steel, and further purifies the steel using the deoxidizing and desulfurizing effects of calcium, fundamentally improving the metallurgical quality of the material. In the surface treatment stage, the formed trays are electrolytically polished to remove surface defect layers and embedded impurities, obtaining a smooth and clean surface. Finally, a dense and stable chromium-rich oxide film is formed on the surface through chemical passivation. This passivation film acts as a physicochemical barrier against corrosion in the oral cavity environment, and together with the high-purity matrix, it ensures the material's excellent corrosion resistance and long-term biocompatibility.

[0068] In summary, this application does not rely on a single technical point, but rather forms a complete solution through multi-level technological innovation and integration of component design, pressure melting, tissue regulation and surface engineering, thereby simultaneously meeting the comprehensive and stringent requirements of long-term oral implantable devices for biosafety, mechanical properties and corrosion resistance.

[0069] Based on a general inventive concept, this application provides a high-nitrogen nickel-free austenitic stainless steel support channel prepared by any of the above methods. The matrix of the high-nitrogen nickel-free austenitic stainless steel support channel is a single austenitic structure with an average grain diameter ≤32μm and a non-metallic inclusion level ≤1.5.

[0070] The high-nitrogen nickel-free austenitic stainless steel support channel prepared in this application achieves an excellent microstructure with a single austenitic structure, an average grain diameter not exceeding 32 micrometers, and non-metallic inclusions controlled to within level 1.5. This is achieved through a systematic control technology covering the entire process from alloy design and metallurgical preparation to subsequent processing. These three core indicators correspond to the phase structure stability, microstructure fineness, and metallurgical purity of the material, respectively, and their specific implementation path is based on well-defined materials science principles.

[0071] Achieving the goal of a single austenitic microstructure is rooted in precise alloy composition design and key heat treatment processes. In terms of composition, a synergistic strategy of high manganese and high nitrogen was employed. Manganese, as a strong austenite-forming element, effectively expands the austenite phase region, while nitrogen is an extremely efficient austenite stabilizer and strengthening element. This combination thermodynamically completely replaces the role of nickel in traditional austenitic stainless steel, ensuring that austenite is the only stable phase structure across a wide range from room temperature to processing temperature, laying the compositional foundation for obtaining a single-phase microstructure. In terms of processing, by subjecting the hot-forged material to high-temperature solution treatment at 1100 to 1150 degrees Celsius followed by water quenching, trace amounts of second phase that may precipitate during hot working are completely dissolved back into the matrix. This uniformly composed, supersaturated single-phase austenite state is then rapidly fixed to room temperature, resulting in a pure and uniform single austenitic microstructure.

[0072] Achieving a fine-grained microstructure is primarily accomplished through a combination of thermomechanical processing and subsequent recrystallization heat treatment. First, the steel ingot undergoes multi-directional hot forging with a forging ratio of at least 6. This intense plastic deformation process thoroughly breaks down the coarse, original cast grains, transforming them into a fine, deformed microstructure filled with high-density dislocations. Simultaneously, it welds internal defects, providing a nucleation basis and energy driving force for grain refinement. Subsequently, after subjecting the material to 30% to 50% cold deformation, a secondary solution treatment is performed at 1050 to 1100 degrees Celsius. This treatment is essentially a controlled recrystallization annealing process. The distortion energy stored from cold deformation drives the formation of new, strain-free, fine equiaxed austenite grains. Precise control of process parameters suppresses excessive grain growth, ultimately resulting in a uniform, fine-grained microstructure with an average diameter of no more than 32 micrometers. This simultaneously enhances the material's strength and toughness.

[0073] Achieving extremely high metallurgical purity, i.e., extremely low levels of non-metallic inclusions, primarily relies on advanced smelting and refining technologies. The entire purification process begins with pressure induction melting, a step conducted under inert gas protection, which reduces oxidation loss and impurity introduction at the source. The decisive purification step lies in the subsequent pressure electroslag remelting process. In this process, a mixed slag system composed of a base slag and a specific functional composite pre-melted nitride slag is employed. When the consumable electrode melts and molten metal droplets pass through the high-temperature slag layer, the high-basicity slag system efficiently removes suspended oxides and sulfides and other non-metallic inclusions from the molten steel through powerful adsorption and chemical reactions. In particular, the components in the functional slag also play a role in deep deoxidation and desulfurization, further reducing the source of inclusion formation. At the same time, the unique top-down sequential solidification method of electroslag remelting facilitates the flotation and separation of residual inclusions, ultimately obtaining high-purity steel ingots with inclusion levels no higher than 1.5.

[0074] In summary, this application establishes thermodynamic stability through a high-manganese, high-nitrogen alloy design, refines the microstructure through a combination of large-deformation forging and cold-deformation-induced recrystallization, and achieves deep purification through a dual-process of pressurized induction melting combined with a functionalized slag system and pressurized electroslag remelting. These three technological pillars are interconnected and work synergistically to ensure that the final tray product possesses excellent and reliable microstructure and performance characteristics.

[0075] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0076] Example 1 This embodiment provides a method for preparing a high-nitrogen nickel-free austenitic stainless steel support channel, including the following steps: S1. Raw Material Preparation and Smelting: C: 0.08%, Si: 0.5%, Mn: 13.9%, P: <0.02%, S: <0.02%, Cr: 17.2%, Mo: 3.5%, Ni: <0.05%, N: ~0.75%, Fe: balance (for batching). The calculated masses of electrolytic iron, metallic chromium, metallic manganese, ferromolybdenum, ferrosilicon, and other raw materials are placed in a pressure induction melting furnace. After evacuation, a mixture of high-purity nitrogen and argon (nitrogen partial pressure 60%) is introduced, and the total pressure inside the furnace is maintained at 0.5 MPa. Smelting is carried out under this atmosphere to fully melt and homogenize the alloying elements, obtaining molten steel with an initial nitrogen content of approximately 0.5%. Subsequently, this molten steel is cast into consumable electrodes suitable for subsequent electroslag remelting.

[0077] S2. Pressurized Electroslag Remelting and Refining: First, prepare a composite pre-melted nitride slag: Weigh out powders of CrN: 60%, Ca3N2: 25%, Y2O3: 8%, and CaF2: 7% by mass fraction, mix them evenly, and pre-melt them at 1650℃ under argon protection. After casting into ingots, crush them into 10-15mm particles. Subsequently, a mixed slag system is pre-added to the bottom of the water-cooled crystallizer of the electroslag remelting furnace. This mixed slag system consists of 75% traditional CaF2-Al2O3-CaO-based slag (CaF2: 50%, Al2O3: 25%, CaO: 25%) and 25% of the above-mentioned composite pre-melted nitride slag. The consumable electrode is then loaded into the furnace, and remelting begins.

[0078] The remelting process employs a stepped pressure control: Melting period: Control the furnace pressure to 0.6MPa, and after arc ignition, melt the consumable electrode smoothly to establish a molten metal pool.

[0079] Refining period: When the consumable electrode is melted down to about 60%, the furnace pressure is gradually increased to 1.3 MPa over a period of about 10 minutes and maintained until the melting is completed.

[0080] The high-pressure environment at this stage significantly improves the solubility of nitrogen in the molten steel and achieves precise and uniform nitrogen enrichment through the slag-metal reaction (mainly through the decomposition of CrN and Ca3N2). After smelting, the steel is cast under pressure protection to obtain a steel ingot with uniform chemical composition, high purity, and a stable nitrogen content of 0.78%. Testing shows that its composition satisfies the relationship defined in the claims: [Mn] = 13.9 ≥ 10 × [N] (0.78) + 6 = 13.8, and [Cr] + 3.3 × [Mo] = 17.2 + 11.55 = 28.75 ≥ 28.

[0081] S3 and S4, hot working and initial solution treatment The obtained steel ingots were homogenized by annealing at 1180℃ for 3 hours to eliminate dendritic segregation. Subsequently, the ingots were forged at 1150℃ and subjected to multi-directional forging with a total forging ratio of 8. The final forging temperature was controlled above 920℃, resulting in a slab with a thickness of 30mm. The forged slabs were then subjected to high-temperature solution treatment: held at 1120℃ for 1.5 hours to ensure complete dissolution of all alloying elements, followed immediately by water quenching. Metallographic examination confirmed that the microstructure after treatment was a complete monolithic austenite with an average grain size of approximately 50μm (ASTM grade 6).

[0082] S5, Deformation Heat Treatment The solution-treated sheet was cold-rolled with a first pre-set deformation of 40% (i.e., rolling from 30 mm to 18 mm). This cold deformation process significantly improved the strength and hardness of the material. Subsequently, the cold-rolled sheet underwent a second solution treatment: holding at 1070℃ for 1 hour, followed by water quenching. This treatment induced complete recrystallization, eliminated work hardening, and yielded fine and uniform austenitic grains. The average grain size of the treated sheet was refined to 25 μm (ASTM grade 8), while maintaining both high strength and good plasticity.

[0083] S6. Precision machining and surface treatment The treated sheet metal was then machined into standard-sized square wire arch brackets using a five-axis precision CNC machine. The machined brackets were first electropolished, removing approximately 20 μm of the surface layer under specific electrolyte and process parameters to obtain a mirror-smooth, stress-free surface. Subsequently, a passivation treatment was performed: the brackets were completely immersed in a 25% nitric acid solution at room temperature for 20 minutes. After removal, they were rinsed thoroughly with deionized water and dried. This process formed a dense, chemically stable chromium-rich oxide film on the bracket surface.

[0084] Example 2 This embodiment provides a method for preparing a high-nitrogen nickel-free austenitic stainless steel support channel, the steps of which are as follows: S1. Raw Material Preparation and Smelting: The raw materials are batched according to the following mass fractions (C: 0.07%, Si: 0.6%, Mn: 12.1%, P: <0.02%, S: <0.02%, Cr: 16.8%, Mo: 3.4%, Ni: <0.05%, N: ~0.60%, Fe balance). The raw materials are placed in a pressure induction furnace, evacuated, and then filled with a nitrogen-argon mixture (N2 partial pressure 55%). The furnace pressure is maintained at 0.45 MPa for smelting to obtain molten steel with an initial nitrogen content of 0.42%, which is then cast into consumable electrodes.

[0085] S2. Pressurized electroslag remelting and refining: A composite pre-melted nitride slag (CrN: 62%, Ca3N2: 23%, Y2O3: 7%, CaF2: 8%) was prepared, pre-melted under argon protection at 1620℃, and then crushed into 8-12mm particles. The mixed slag system consisted of 72% CaF2-Al2O3-CaO-based slag (CaF2: 48%, Al2O3: 26%, CaO: 26%) and 28% composite slag, which was laid into the crystallizer. Stepped pressurization: The pressure was 0.7MPa during the melting period. When the electrode was 55% full, the pressure was increased to 1.1MPa within 8 minutes and maintained until the end, resulting in a steel ingot with a nitrogen content of 0.61%.

[0086] S3. Homogenization treatment and multi-directional forging: The steel ingot is homogenized and annealed at 1160℃ for 4 hours, then forged at 1140℃ for multi-directional forging with a total forging ratio of 7 and a final forging temperature of 930℃ or higher, resulting in a 28mm slab.

[0087] S4. High-temperature solution treatment and quenching: The slab is held at 1100℃ for 2 hours for high-temperature solution treatment, followed by water quenching to obtain a single austenitic structure with a grain size of 55μm (ASTM 5.5 grade).

[0088] S5. Cold deformation and secondary solution treatment: The sheet is cold rolled to deform by 45% (28mm→15.4mm), held at 1050℃ for 1.5h for secondary solution treatment, and then water quenched to refine the grain to 28μm (ASTM 7.5 grade).

[0089] S6. Machining and Surface Treatment: The product is machined into a tray shape using a five-axis machine tool, electropolished to remove the 18μm surface layer, and then passedivated by soaking in a 22% nitric acid solution at room temperature for 15 minutes. Finally, it is rinsed with deionized water and dried to obtain the finished product.

[0090] Example 3 This embodiment provides a method for preparing a high-nitrogen nickel-free austenitic stainless steel support channel, the steps of which are as follows: S1. Raw Material Preparation and Smelting: Materials are batched according to mass fraction (C: 0.09%, Si: 0.4%, Mn: 14.5%, P: <0.02%, S: <0.02%, Cr: 17.8%, Mo: 3.8%, Ni: <0.05%, N: ~0.85%, Fe balance). The raw materials are placed in a pressure induction furnace, evacuated, and then filled with a nitrogen-argon mixture (N2 partial pressure 65%). The furnace pressure is maintained at 0.55 MPa for smelting to obtain molten steel with an initial nitrogen content of 0.65%, which is then cast into consumable electrodes.

[0091] S2. Pressurized electroslag remelting and refining: A composite pre-melted nitride slag (CrN: 55%, Ca3N2: 28%, Y2O3: 9%, CaF2: 8%) was prepared, pre-melted under argon protection at 1680℃, and then broken into 12-16mm particles. The mixed slag system consisted of 73% CaF2-Al2O3-CaO-based slag (CaF2: 46%, Al2O3: 27%, CaO: 27%) and 27% composite slag, which was laid into the crystallizer. Stepped pressurization: The pressure was 0.8MPa during the melting period. When 65% of the electrode remained, the pressure was increased to 1.2MPa within 12 minutes and maintained until the end, resulting in a steel ingot with a nitrogen content of 0.84%.

[0092] S3. Homogenization treatment and multi-directional forging: The steel ingot is homogenized and annealed at 1190℃ for 2.5h, then forged at 1160℃ for multi-directional forging, with a total forging ratio of 9 and a final forging temperature of 940℃ or higher, to form a 32mm slab.

[0093] S4. High-temperature solution treatment and quenching: The slab is held at 1140℃ for 1.2h for high-temperature solution treatment, followed by water quenching to obtain a single austenitic structure with a grain size of 48μm (ASTM 6.2 grade).

[0094] S5. Cold deformation and secondary solution treatment: The sheet is cold rolled to deform by 35% (32mm→20.8mm), held at 1080℃ for 1.2h for secondary solution treatment, and then water quenched to refine the grains to 22μm (ASTM 8.5 grade).

[0095] S6. Machining and Surface Treatment: The product is machined into a tray shape using a five-axis machine tool, electropolished to remove the 22μm surface layer, and then passedivated by soaking in a 28% nitric acid solution at room temperature for 25 minutes. Finally, it is rinsed with deionized water and dried to obtain the finished product.

[0096] Example 4 Raw material preparation: Industrial pure iron, metallic chromium, metallic manganese, metallic molybdenum, chromium nitride, and high-purity nitrogen are used as raw materials, and the ingredients are prepared according to the target composition (mass fraction): C: 0.08%, Si: 0.5%, Mn: 13.9%, P: <0.02%, S: <0.02%, Cr: 17.2%, Mo: 3.5%, Ni: <0.05%, N: ~0.75%, Fe: balance.

[0097] S1. Melting and Powdering: The prepared raw materials are placed in a pressure induction furnace, evacuated to 5 Pa, and then filled with a mixture of high-purity nitrogen and argon (nitrogen partial pressure 0.25 MPa, total pressure 0.5 MPa). Melting is carried out at 1580℃. After the alloying elements are fully melted and homogenized, the furnace is held at this temperature for 15 minutes to obtain the initial molten steel. Subsequently, the molten steel is powdered through a tightly coupled gas atomizer under nitrogen protection at 0.6 MPa. The atomizing gas is high-purity nitrogen, and the superheat is controlled at 150℃. High-nitrogen nickel-free austenitic stainless steel spherical powder with a particle size distribution D50 of 18 μm is collected.

[0098] S2. Feed Preparation: Weigh the above-mentioned stainless steel powder and multi-component binder (composed of 65% paraffin, 30% polypropylene, and 5% stearic acid) at a mass ratio of 90:10. Mix in an internal mixer at 150°C for 120 minutes to ensure uniform mixing of the powder and binder. Then cool, crush, and granulate the mixture to obtain the feed for injection molding.

[0099] S3. Injection Molding: The feed material is placed in a screw-type injection molding machine. Under the process parameters of injection temperature 160℃, injection pressure 80MPa, and mold temperature 40℃, a green blank with a standard square wire arch support shape is injection molded.

[0100] S4. Degreasing Treatment: The green body is placed in a vacuum degreasing furnace and degreased using a two-step method. First, under a nitrogen atmosphere, the temperature is increased to 400°C at a rate of 2°C / min and held for 60 minutes to remove most of the paraffin and stearic acid. Then, the temperature is switched to a vacuum state (10 Pa) and increased to 600°C at a rate of 5°C / min, held for 90 minutes to completely remove the remaining binder.

[0101] S5. Sintering: The degreased green body is transferred to a pressure sintering furnace. Under the protection of flowing high-purity nitrogen (purity ≥99.999%), the temperature is increased to 1280℃ at a rate of 10℃ / min and held at this temperature for 120 minutes. The nitrogen pressure inside the furnace is maintained at 0.8MPa throughout the sintering process. After sintering, the green body is cooled to below 200℃ in the furnace and removed from the furnace to obtain a high-density grooved green body. Testing shows that the relative density of the green body is ≥98%.

[0102] S6. Surface Treatment: The sintered tray blank is machined to remove residual traces such as injection gates. Then, it undergoes electropolishing with a mixed solution of phosphoric acid and sulfuric acid at 70℃, 12V, and 3 minutes. After polishing, it is passivated by immersing in a 25% nitric acid solution for 20 minutes, ultimately obtaining a bright, corrosion-resistant high-nitrogen nickel-free austenitic stainless steel tray.

[0103] Comparative Example 1 This comparative example is modified from the one disclosed in Example 1 as follows: In step S2, instead of using composite pre-melted nitride slag, only traditional CaF2-Al2O3-CaO-based slag is used for electroslag remelting. The remelting process pressure is kept constant at 0.8 MPa, without step-by-step pressurization. The remaining steps and parameters are exactly the same as in Example 1.

[0104] Results: The nitrogen content of the obtained steel ingot was only 0.35%, far below the minimum design target of 0.4%. Due to the insufficient nitrogen content, a small amount of δ-ferrite was found in the microstructure of the material after subsequent solution treatment, instead of pure austenite. The final yield strength of the groove was 651.3 MPa, with a significant decrease in corrosion resistance and a pitting potential Eb of +432.6 mV (vs. SCE). This indicates that high and stable nitrogen alloying cannot be achieved solely through vapor-phase nitriding; the lack of precise nitrogen supply from the composite slag system and the synergistic effect of stepped pressurization were the key reasons for the failure.

[0105] Comparative Example 2 This comparative example is based on Example 1, with the following modifications: The manganese content of the ingredients in step S1 was adjusted to 10.0%, while the nitrogen content target remained at ~0.75%, and other components remained unchanged. This resulted in [Mn](10.0) < 10 × [N](7.5) + 6, which does not satisfy the compositional relationship defined in the claims. All parameters of the preparation process remained consistent with those in Example 1.

[0106] Results: Due to insufficient manganese content to stabilize the high-nitrogen austenite, a large number of Cr2N nitrides precipitated in a network distribution in the microstructure after the high-temperature solution treatment in step S4. These brittle phases led to a sharp deterioration in the material's plasticity, resulting in edge cracking during cold rolling and preventing the achievement of the preset 40% deformation. Ultimately, the elongation of the material was less than 10%, and the area around the nitrides became a corrosion initiation point, severely impairing corrosion resistance. This demonstrates that the specified manganese-nitrogen relationship is a necessary condition for ensuring microstructure uniformity and processing performance.

[0107] Comparative Example 3 This comparative example is based on Example 1, with the following modifications: Step S5 is omitted, i.e., the cold deformation and secondary solution treatment are cancelled. After obtaining the solution-treated sheet in step S4, the machining and surface treatment in step S6 are performed directly.

[0108] Results: Due to the lack of a crucial deformation heat treatment strengthening step, the material relied solely on solid solution strengthening, resulting in low strength. The final yield strength of the bracket was 552.7 MPa, which fails to meet the mechanical requirements for bracket resistance to archwire deformation forces in orthodontic treatment. Furthermore, the grains were not refined, with the average size remaining at approximately 50 μm (ASTM grade 6). This indicates that a cold deformation combined with secondary solid solution treatment is indispensable for achieving the high strength and fine-grained microstructure required for brackets.

[0109] Comparative Example 4 This comparative example is based on Example 1, with the following modifications: The electropolishing and passivation treatment in step S6 is omitted. After machining to obtain the tray shape, only routine ultrasonic cleaning is performed, followed by drying to obtain the final product.

[0110] Results: Tool marks, microcracks, and embedded impurities left by machining on the tray surface were not removed, resulting in a high surface roughness (Ra). A complete and dense passivation film could not be formed on the unpassivated surface. In the artificial saliva corrosion test, the pitting potential Eb of this comparative tray was significantly reduced to below +300mV (vs. SCE), and the release of metal ions (such as Cr and Mn) in the static immersion test was far greater than that of the sample in Example 1. This highlights the decisive role of electropolishing and passivation in obtaining an extremely corrosion-resistant surface and ensuring long-term biocompatibility.

[0111] Comparative Example 5 This comparative example is based on Example 1, with the following modifications: The stepped pressurization in step S2 is changed to constant high pressure, that is, the pressure is maintained at 1.3 MPa from the beginning to the end of remelting. The remaining steps and parameters are exactly the same as in Example 1.

[0112] Results: Applying excessively high pressure in the early stages of melting led to arc instability, violent churning of the molten pool, and difficulty in controlling the refining process. The resulting steel ingot exhibited significant discontinuities and compositional fluctuations in its macrostructure, with extremely uneven nitrogen content distribution (0.90% at the head, 0.65% at the tail). This inhomogeneity resulted in large localized differences in microstructure properties after subsequent heat treatment, affecting product consistency. This indicates that the relatively low pressure in the early stages of the "stepped pressurization" process is crucial for ensuring a stable melting process and is a prerequisite for achieving efficient and uniform nitrogen addition in the later stages.

[0113] To evaluate the performance of the high-nitrogen nickel-free austenitic stainless steel support trays prepared in this application, systematic tests were conducted on the final products of Examples 1-4 and Comparative Examples 1-5. All tests were performed according to national standards or internationally accepted methods, and the specific measurement methods and measured data are as follows.

[0114] (1) Mechanical property determination The test method was based on GB / T228.1-2021 Metallic materials, tensile testing—Part 1: Tests at room temperature. Standard tensile specimens were wire-cut from the finished bracket substrate and tested on a universal testing machine. The test results are shown in Table 1.

[0115] Table 1 Mechanical properties of high-nitrogen nickel-free austenitic stainless steel support channels Note: Comparative Example 2 was made of a brittle material that cracked during the processing of the standard tensile specimen, making it impossible to complete an effective test.

[0116] As shown in Table 1, the mechanical properties of the products prepared by the method of this application in Examples 1 to 3 exhibit excellent balances of high strength and good plasticity, with yield strength ranging from 818.5 to 876.4 MPa, tensile strength from 1001.3 to 1098.6 MPa, and elongation after fracture from 32.5% to 38.2%. In contrast, the properties of the comparative examples deteriorated significantly: Comparative Example 1, due to the lack of a composite slag system and stepped pressurization, had insufficient nitrogen content, resulting in a substantial decrease in both strength (yield strength 651.3 MPa) and plasticity (elongation after fracture 25.1%). Comparative Example 2, due to insufficient manganese content, could not stabilize the high-nitrogen austenite, resulting in extremely brittle material that cracked during sample processing. Comparative Example 3, due to the omission of crucial cold deformation and secondary solution treatment, relied solely on solution strengthening, resulting in a severely insufficient yield strength (552.7 MPa). Although the matrix mechanical properties (yield strength 841.5 MPa) of Comparative Example 4 were comparable to those of the Example, Comparative Example 5, due to the use of unreasonable constant high-pressure melting, resulted in uneven microstructure and composition, and significant fluctuations in mechanical property data.

[0117] (2) Corrosion resistance test The test method was based on ASTM G61-86 (2018), "Standard Test Method for Cyclic Potential Polarization Measurements to Evaluate Pitting Potential of Stainless Steel and Related Alloys". An electrochemical workstation was used, with a modified Fusayama artificial saliva (pH=6.8) at 37±1℃ as the electrolytic cell and a saturated calomel electrode (SCE) as the reference electrode. The scan rate was 0.5 mV / s, and the pitting potential (Eb) was defined as the potential at which the anodic current density reached 100 μA / cm². The results are shown in Table 2.

[0118] Table 2 Corrosion Resistance of High-Nitrogen Nickel-Free Austenitic Stainless Steel Supports As shown in Table 2, the pitting potentials of Examples 1 to 3 in artificial saliva ranged from 895.3 to 977.8 mV, exhibiting extremely excellent pitting corrosion resistance. The nitrogen content (~0.75%) of the material in Example 4 reached the level of high-nitrogen stainless steel, and its composition was uniform, which is the basis for its high corrosion resistance. The final electropolishing and passivation treatment removed surface defects, forming a complete and dense passivation film. Therefore, its pitting potential in artificial saliva was comparable to that of the optimal casting process sample (such as Example 3), demonstrating excellent pitting corrosion resistance. The corrosion resistance of the comparative examples deteriorated to varying degrees: Comparative Example 1 also experienced a sharp decrease in pitting potential (432.6 mV) due to severely insufficient nitrogen content. Comparative Example 2, due to the presence of a network of brittle chromium nitride precipitates in its microstructure, had these phases surrounding sensitive points where corrosion occurred rapidly, exhibiting porous corrosion and an extremely low pitting potential (+155.2 mV). Comparative Example 3 showed acceptable substrate corrosion resistance (935.1 mV), but no further surface optimization was performed. Comparative Example 4, lacking electropolishing and passivation, suffered from machined surface damage and a fragmented passivation film, resulting in the worst corrosion resistance (285.4 mV) among all samples. Comparative Example 5, due to its uneven composition and microstructure, exhibited a pitting potential (+902.1 mV) that, while numerically acceptable, fluctuated significantly, leading to poor reliability.

[0119] (3) Microstructure and purity determination The test methods were based on GB / T13298-2015 "Methods for Examination of Microstructure of Metals". Metallographic samples were prepared, and the microstructure was observed using an optical microscope and a scanning electron microscope (SEM). The grain size was evaluated according to GB / T6394-2017 "Methods for Determination of Average Grain Size of Metals". The inclusion level was evaluated according to GB / T10561-2005 "Standard Rating Chart Microscopic Examination Method for Determination of Non-metallic Inclusion Content in Steel". The test results are shown in Table 3.

[0120] Table 3. Microstructure and purity of high-nitrogen nickel-free austenitic stainless steel support channels Table 3 shows the microstructure and purity data. Examples 1 to 3 all yielded a single austenitic microstructure with an average grain size of 21.6 to 28.5 micrometers, and the non-metallic inclusion level was well controlled (all ≤1.2 grade). However, Comparative Example 1, due to improper composition design, exhibited harmful δ-ferrite phases and coarse grains (52.3 micrometers). Comparative Example 2 showed a large amount of continuous network chromium nitride precipitation, severely disrupting the microstructure continuity. Comparative Example 3, lacking a deformation heat treatment step to refine the grains, had the largest grain size (51.7 micrometers). Comparative Example 4 had a microstructure similar to the examples. Comparative Example 5, due to improper melting process, resulted in drastic fluctuations in grain size within the range of 26 to 58 micrometers, leading to an uneven microstructure.

[0121] (4) Biocompatibility determination Test method: According to the biological evaluation standard for medical devices YY / T0127.2-2023 / ISO7405:2018 Dentistry – Biological evaluation of dental medical devices – Part 2: In vitro cytotoxicity tests of dental materials, the MTT assay was used to test the relative proliferation rate (RGR) of the material extract on L929 mouse fibroblasts. The results are shown in Table 4.

[0122] Table 4. Biocompatibility of High-Nitrogen Nickel-Free Austenitic Stainless Steel Trays As shown in Table 4, the relative cell proliferation rates of Examples 1 to 3 were all above 97.8%, with a toxicity rating of 0, indicating complete compliance. However, Comparative Example 4, lacking surface passivation treatment, had a poor surface condition, leading to easier release of metal ions and a decreased relative cell proliferation rate to 85.2%, resulting in a toxicity rating of 1. This directly demonstrates the indispensable role of electropolishing and passivation in ensuring the biocompatibility of the final product.

[0123] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0124] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0125] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a high-nitrogen nickel-free austenitic stainless steel support channel, characterized in that, The method is either the first method or the second method; wherein The first method includes the following steps: S1, placing the raw material in a pressurized induction furnace, then filling it with a nitrogen-argon mixed gas, and melting it under a pressure of 0.4-0.6 MPa to obtain initial molten steel; S2, casting the initial molten steel into a consumable electrode, placing it in an electroslag remelting furnace, and remelting and refining it using a mixed slag system of base slag and composite pre-melted nitride slag, and performing stepped pressurization during the remelting process, and casting it to obtain a steel ingot with a set chemical composition; S3, homogenizing the steel ingot, and then performing multi-directional forging to obtain a forging billet; S4. The forging billet is subjected to high-temperature solution treatment and then quenched to obtain a plate with a single austenitic structure; S5. The plate is subjected to cold deformation processing with a first preset deformation amount, followed by secondary solution treatment and quenching to obtain a semi-finished product; S6. The semi-finished product is machined into a groove shape, followed by surface electrolytic polishing and passivation treatment to obtain a high-nitrogen nickel-free austenitic stainless steel groove; wherein, by mass fraction, the composite pre-melted nitride slag is composed of the following chemical components: CrN: 50-65%, Ca3N2: 20-30%, Y2O3: 5-10%, CaF2: 5-10%; The second method includes the following steps: S1, melting the raw materials in a pressure induction furnace, filling with a nitrogen-argon mixed gas, and obtaining initial molten steel under a pressure of 0.4-0.6 MPa, then making the initial molten steel into stainless steel powder; S2, mixing the stainless steel powder with a binder at a mass ratio of 88:12-92:8, and obtaining injection molding feed after kneading and granulation; S3, injection molding the feed to obtain a green billet in the shape of a tray; S4, degreasing the green billet to remove the binder; S5, placing the degreased billet in a sintering furnace and sintering it under a nitrogen atmosphere at a sintering pressure of 0.5-1.0 MPa and a sintering temperature of 1200-1350℃ to obtain a high-nitrogen nickel-free austenitic stainless steel tray billet; S6, performing surface electrolytic polishing and passivation treatment on the tray billet to obtain a high-nitrogen nickel-free austenitic stainless steel tray.

2. The method for preparing the high-nitrogen nickel-free austenitic stainless steel support channel according to claim 1, characterized in that, By mass fraction, the mixed slag system consists of 70-75% CaF2-Al2O3-CaO-based slag and 25-30% of the composite pre-melted nitride slag.

3. The method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray according to claim 1, characterized in that, The stepped pressurization is as follows: during the melting period of remelting, the pressure is controlled at 0.5 to 0.8 MPa; when the consumable electrode is melted to the remaining 1 / 2 to 2 / 3, the pressure is increased to 1.0 to 1.3 MPa within 5 to 15 minutes and maintained until the melting is completed.

4. The method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray according to claim 1, characterized in that, The specified chemical composition, by mass fraction, is as follows: C≤0.1%, Si≤1.0%, Mn: 10~14%, P≤0.03%, S≤0.03%, Cr: 16.0~18.0%, Mo: 3.0~4.0%, Ni≤0.10%, N: 0.4~0.9%, with the balance being Fe and unavoidable impurities.

5. The method for preparing the high-nitrogen nickel-free austenitic stainless steel support tray according to claim 4, characterized in that, The specified chemical composition satisfies the following relationship: [Mn]≥10×[N]+6, and [Cr]+3.3×[Mo]≥28 In the formula, [Mn] is the value preceding the mass fraction % of Mn, [N] is the value preceding the mass fraction % of N, [Cr] is the value preceding the mass fraction % of Cr, and [Mo] is the value preceding the mass fraction % of Mo.

6. The method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray according to claim 1, characterized in that, The homogenization process is carried out at a temperature of 1150–1200°C for 2–4 hours. The total forging ratio of the multi-directional forging is ≥6, and the final forging temperature is ≥900℃.

7. The method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray according to claim 1, characterized in that, The high-temperature solution treatment is performed at a temperature of 1100–1150°C for 1–2 hours. The quenching is water quenching.

8. The method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray according to claim 1, characterized in that, The first preset deformation amount is 30% to 50%; The secondary solution treatment is performed at a temperature of 1050–1100℃ for 1–2 hours.

9. The method for preparing a high-nitrogen nickel-free austenitic stainless steel support tray according to claim 1, characterized in that, The passivation treatment is as follows: soaking in a 20% to 30% nitric acid solution for 10 to 30 minutes.

10. A high-nitrogen nickel-free austenitic stainless steel support tray prepared by the method according to any one of claims 1 to 9, characterized in that, The matrix of the high-nitrogen nickel-free austenitic stainless steel support is a single austenitic structure with an average grain diameter ≤32μm and a non-metallic inclusion level ≤1.5.

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