High-strength high-toughness corrosion-resistant super duplex stainless steel and preparation method thereof

Abstract

The application relates to the technical field of extra super duplex stainless steel, in particular to high-strength high-toughness corrosion-resistant extra super duplex stainless steel and a preparation method thereof, which comprises the following steps: A. uniformly mixing pure Ti powder and extra super duplex stainless steel powder, drying to obtain composite powder; wherein, according to mass percentage, the extra super duplex stainless steel powder comprises the following components: C 0.01-0.02%, N 0.3-0.4%, Cr 26-28%, Ni 6-7%, Mo 4-5%, Si 0.2-0.4%, Mn 0.3-0.5%, S 0.002-0.005%, O 0.01-0.03%, and the balance of Fe and impurities; B. using a laser powder bed melting process to additively manufacture the composite powder under a nitrogen atmosphere to obtain a preform; and C. solid solution treating the preform to obtain high-strength high-toughness corrosion-resistant extra super duplex stainless steel. The high-strength high-toughness corrosion-resistant extra super duplex stainless steel and the preparation method thereof have the characteristics of high strength, high toughness and corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of super duplex stainless steel technology, and more particularly to a high-strength, high-toughness, corrosion-resistant super duplex stainless steel and its preparation method. Background Technology

[0002] With the booming development of industrial manufacturing and high-end equipment, various industries are placing increasingly stringent and diverse demands on material performance. Not only are materials required to possess reliable practicality and durability, and to operate stably for extended periods under complex conditions, but higher standards are also being set for key indicators such as strength, toughness, and corrosion resistance in extreme environments. This shift in market demand is driving the stainless steel industry to continuously break through technological bottlenecks and accelerate product iteration and upgrades to meet the ever-growing demand for high-performance materials.

[0003] Super duplex stainless steel, as a representative of high-end materials in the stainless steel field, has a unique duplex microstructure (i.e., ferrite and austenite) and high content of chromium, molybdenum and nitrogen alloying elements, which makes its performance significantly better than duplex stainless steel and super duplex stainless steel. It gives it excellent high strength, high toughness and corrosion resistance, and is widely used in industries such as urea production, oil extraction and deep-sea exploration.

[0004] Existing super duplex stainless steels are primarily manufactured using traditional casting-forging-hot rolling processes. However, due to their high chromium, molybdenum, and nitrogen alloying element content, their manufacturing process faces numerous severe challenges. The smelting stage requires specialized pressure equipment for successful completion; the forging and hot rolling stages demand extremely strict temperature control, as improper control can easily lead to cracking. Furthermore, the as-cast microstructure produced by traditional processes exhibits coarse grains, making it difficult to achieve ideal mechanical properties and fully meet the requirements for high strength and high toughness. These limitations mean that existing technologies for producing super duplex stainless steels still have significant potential for improvement in terms of strength, toughness, and corrosion resistance.

[0005] To effectively overcome the shortcomings of existing technologies, current techniques have begun to explore selective laser melting (SLM) or laser powder bed melting (LSB) processes to produce super duplex stainless steel through additive manufacturing of composite powders. This involves laying out composite powders, melting them layer by layer, and then cooling and solidifying them during the melting process, allowing the material to accumulate layer by layer to form a three-dimensional structure, thus obtaining super duplex stainless steel. However, due to differences in raw material selection and manufacturing processes, the super duplex stainless steel produced by these methods still has shortcomings in terms of strength, toughness, and corrosion resistance, making it difficult to meet the extremely high performance requirements of applications. Summary of the Invention

[0006] The purpose of this invention is to propose a high-strength, high-toughness, corrosion-resistant super duplex stainless steel and its preparation method. The obtained super duplex stainless steel has the characteristics of high strength, high toughness and corrosion resistance, so as to overcome the shortcomings of the prior art.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] A method for preparing a high-strength, high-toughness, corrosion-resistant super duplex stainless steel includes the following steps:

[0009] A. Pure Ti powder and super duplex stainless steel powder are mixed evenly and then dried to obtain a composite powder; wherein, calculated by mass percentage, the super duplex stainless steel powder comprises the following components: C 0.01-0.02%, N 0.3-0.4%, Cr 26-28%, Ni 6-7%, Mo 4-5%, Si 0.2-0.4%, Mn 0.3-0.5%, S 0.002-0.005%, O 0.01-0.03%, with the balance being Fe and impurities;

[0010] B. In a nitrogen atmosphere, composite powder is additively manufactured using laser powder bed melting process to obtain preforms;

[0011] C. Solution treatment is performed on the preforms to obtain high-strength, high-toughness, corrosion-resistant super duplex stainless steel.

[0012] Further, in step A, the mixing ratio of the pure Ti powder and the super duplex stainless steel powder is calculated to be (0.001~0.01):1 based on the mass ratio.

[0013] Further, in step A, the pure Ti powder, calculated by mass percentage, comprises 10-15% of a type I Ti powder with a particle size ≥10μm and <22μm, 35-45% of a type II Ti powder with a particle size ≥22μm and <35μm, 35-45% of a type III Ti powder with a particle size ≥35μm and <55μm, and 8-12% of a type IV Ti powder with a particle size ≥55μm and <65μm.

[0014] Further, in step A, the super duplex stainless steel powder, calculated by mass percentage, comprises 10-15% of a type I stainless steel powder with a particle size ≥10μm and <22μm, 35-45% of a type II stainless steel powder with a particle size ≥22μm and <35μm, 35-45% of a type III stainless steel powder with a particle size ≥35μm and <55μm, and 8-12% of a type IV stainless steel powder with a particle size ≥55μm and <65μm.

[0015] Furthermore, the method for step B is as follows:

[0016] In the forming chamber of a laser powder bed melting equipment under a nitrogen atmosphere, composite powder is evenly spread on the surface of a stainless steel substrate. The composite powder is melted layer by layer according to the parameters of the laser powder bed melting process. During the layer-by-layer melting process, the composite powder is cooled and solidified to obtain a stainless steel substrate with preforms attached.

[0017] Once the temperature of the forming chamber of the laser powder bed melting equipment has cooled to below 80°C, the stainless steel substrate with the preform attached is removed.

[0018] The preform attached to the surface of the stainless steel substrate is peeled off to obtain the preform.

[0019] Furthermore, in step B, the laser power of the laser powder bed melting process is 230-270W, the scanning speed is 1000-1200mm / s, the powder thickness is 0.02-0.04mm, and the scanning interval is 0.08-0.10mm.

[0020] Furthermore, in step B, the stainless steel substrate is preheated before the composite powder is laid.

[0021] The preheating treatment method is as follows: preheat the stainless steel substrate at 150-180°C for 5-10 minutes.

[0022] Further, in step C, the solution treatment method is as follows: the electric furnace is heated from room temperature to 1100-1200°C, the preform is then placed in the electric furnace, and after being kept at 1100-1200°C for 8-12 minutes, the preform is taken out and immediately placed in cold water for cooling.

[0023] A high-strength, high-toughness, corrosion-resistant, and super duplex stainless steel is prepared using the aforementioned method. The high-strength, high-toughness, corrosion-resistant, and super duplex stainless steel exhibits a yield strength > 1100 MPa, an elongation at break ≥ 45%, a strength-ductility product of 55–60 GPa·%, and a self-corrosion potential of -0.13 to -0.03 V. SCE .

[0024] The technical solution provided by this invention may include the following beneficial effects:

[0025] 1. According to the mass percentage, the super duplex stainless steel powder includes the following components: C 0.01~0.02%, N 0.3~0.4%, Cr 26~28%, Ni 6~7%, Mo 4~5%, Si 0.2~0.4%, Mn 0.3~0.5%, S 0.002~0.005%, O 0.01~0.03%, with the balance being Fe and impurities. In this composition, Cr (26-28%) and Mo (4-5%) form a continuous passivation film (Cr2O3-MoO3 composite oxide film), with a PREN value (=%Cr+3.3×%Mo+16×%N) reaching 45-48, forming a high Cr-Mo corrosion-resistant matrix. N (0.3-0.4%) enhances strength through interstitial solid solution strengthening while promoting austenite stability. Ni (6-7%) is precisely controlled to achieve a balanced duplex microstructure (ferrite / austenite ≈ 1:1), retaining the high strength of ferrite while leveraging the high toughness of austenite. The ultra-low C (0.01-0.02%) design reduces the risk of grain boundary precipitates, ensuring the purity and corrosion resistance of the material. This compositional design—high Cr-Mo corrosion-resistant matrix + N interstitial strengthening + duplex stabilizing elements—gives the composite powder containing super duplex stainless steel powder the theoretical potential to achieve high strength, high toughness, and good corrosion resistance after additive manufacturing.

[0026] 2. The superior comprehensive properties of super duplex stainless steel, such as strength, toughness, and corrosion resistance, are all related to the austenite-ferrite two-phase equilibrium. Therefore, to ensure the excellent comprehensive properties of the final super duplex stainless steel, this technical solution involves solution treatment of the preform to partially transform the ferrite into austenite, thereby restoring the austenite-ferrite two-phase equilibrium (≈1:1) and ensuring the product's performance.

[0027] 3. To overcome the strength reduction problem introduced during solution treatment, this technical solution introduces pure Ti powder into the composite powder. During additive manufacturing, the pure Ti powder reacts with some of the nitrogen in the nitrogen atmosphere, precipitating TiN nanoparticles in situ, and the TiN nanoparticles are dispersedly distributed. The strengthening mechanism of TiN nanoparticles as a strengthening agent for super duplex stainless steel is mainly reflected in the following aspects: (1) TiN nanoparticles have a low degree of lattice mismatch with austenite, making them efficient heterogeneous nucleation cores during solid solution treatment, which can effectively refine the austenite grain size and compensate for the strength loss caused by heat treatment through grain boundary strengthening; (2) Uniformly distributed TiN nanoparticles can pin grain boundaries and hinder dislocation movement, inhibit grain growth and dislocation density decay, so that solid solution duplex stainless steel can maintain high strength; (3) Refined austenite not only improves the uniformity of the austenite-ferrite two-phase distribution and enhances the dynamic recovery ability of dislocations during deformation, thereby improving toughness, but also the refined austenite is more uniform, which improves the stability of austenite during deformation and is also conducive to improving toughness; (4) Refined austenite can block corrosion channels and improve pitting corrosion resistance, thereby improving corrosion resistance. Attached Figure Description

[0028] Figure 1 (a) and (b) in the figure are EBSD phase ratio diagrams of the super duplex stainless steel obtained in Example 1 of the present invention on the forming surface and in the forming direction, respectively.

[0029] Figure 2 (a) and (b) in the figure are both distribution diagrams of TiN nanoparticles in the super duplex stainless steel obtained in Example 1 of the present invention. Detailed Implementation

[0030] This technical solution provides a method for preparing high-strength, high-toughness, corrosion-resistant super duplex stainless steel, including the following steps:

[0031] A. Pure Ti powder and super duplex stainless steel powder are mixed evenly and then dried to obtain a composite powder; wherein, calculated by mass percentage, the super duplex stainless steel powder comprises the following components: C 0.01-0.02%, N 0.3-0.4%, Cr 26-28%, Ni 6-7%, Mo 4-5%, Si 0.2-0.4%, Mn 0.3-0.5%, S 0.002-0.005%, O 0.01-0.03%, with the balance being Fe and impurities;

[0032] B. In a nitrogen atmosphere, composite powder is additively manufactured using laser powder bed melting process to obtain preforms;

[0033] C. Solution treatment is performed on the preforms to obtain high-strength, high-toughness, corrosion-resistant super duplex stainless steel.

[0034] To overcome the shortcomings of existing technologies, such as insufficient strength, toughness, and corrosion resistance, this technical solution proposes a method for preparing high-strength, high-toughness, and corrosion-resistant super duplex stainless steel. By optimizing the formulation and preparation method, TiN nanoparticles are precipitated in situ during the preparation process. The characteristics of TiN nanoparticles as a reinforcing phase are fully utilized to optimize the performance of the super duplex stainless steel, resulting in a super duplex stainless steel with high strength, high toughness, and corrosion resistance to meet practical application requirements.

[0035] This technical solution is based on the composition of super duplex stainless steel powder, laying the foundation for achieving the high strength, high toughness, and corrosion resistance of super duplex stainless steel. Specifically, calculated by mass percentage, the super duplex stainless steel powder comprises the following components: C 0.01–0.02%, N 0.3–0.4%, Cr 26–28%, Ni 6–7%, Mo 4–5%, Si 0.2–0.4%, Mn 0.3–0.5%, S 0.002–0.005%, O 0.01–0.03%, with the balance being Fe and impurities. In this composition, Cr (26-28%) and Mo (4-5%) form a continuous passivation film (Cr2O3-MoO3 composite oxide film), with a PREN value (=%Cr+3.3×%Mo+16×%N) reaching 45-48, forming a high Cr-Mo corrosion-resistant matrix. N (0.3-0.4%) enhances strength through interstitial solid solution strengthening while promoting austenite stability. Ni (6-7%) is precisely controlled to achieve a balanced duplex microstructure (ferrite / austenite ≈ 1:1), retaining the high strength of ferrite while leveraging the high toughness of austenite. The ultra-low C (0.01-0.02%) design reduces the risk of grain boundary precipitates, ensuring the purity and corrosion resistance of the material. This compositional design—high Cr-Mo corrosion-resistant matrix + N interstitial strengthening + duplex stabilizing elements—gives the composite powder containing super duplex stainless steel powder the theoretical potential to achieve high strength, high toughness, and good corrosion resistance after additive manufacturing. It should be noted that the PREN value represents the pitting resistance equivalent value; the higher the PREN value, the higher the corrosion resistance.

[0036] However, in additive manufacturing using laser powder bed melting, only a small amount of composite powder is melted as the laser beam sweeps across the surface, and then it rapidly cools and solidifies, with a cooling and solidification rate reaching 10. 3 ~10 8K / s, the cooling and solidification rate is extremely fast, which inhibits the diffusion-type phase transformation of austenite, resulting in the austenite not being able to transform in time, so that the preform only presents a single ferrite phase, thus the super duplex stainless steel that should present an austenite-ferrite two-phase balance only presents a single ferrite phase. The single ferrite phase will have the following effects: (1) TRIP effect of austenite (TRIP effect refers to the phase transformation process of internal austenite to martensite when the material is subjected to external force. When the external stress reaches the critical value, austenite transforms into martensite through the shear mechanism. (1) The phase transformation process absorbs energy and generates volume expansion, effectively delaying local stress concentration and significantly improving toughness. (2) The solubility of N in ferrite is only 1 / 20 of that in austenite, which will cause the loss of the interstitial solid solution strengthening effect of N element. (3) It will lead to the aggravation of the segregation of Cr and Mo in the single phase, resulting in a cliff drop in pitting corrosion resistance (PREN effective value drops from 45 to 28). (4) The continuous ferrite network becomes a channel for rapid crack propagation, the stress corrosion sensitivity index increases, and the corrosion resistance decreases. That is, the excellent comprehensive properties of super duplex stainless steel, such as strength, toughness and corrosion resistance, are all related to the balance of austenite-ferrite two phases. Therefore, in order to ensure the excellent comprehensive properties of the final super duplex stainless steel, this technical solution performs solid solution treatment on the preform to convert part of the ferrite into austenite in order to restore the balance of austenite-ferrite two phases (≈1:1) to ensure the performance of the product.

[0037] Meanwhile, in laser powder bed melting, the non-equilibrium structure formed by rapid cooling and solidification prevents metal atoms in the molten pool from arranging themselves in an orderly manner, resulting in a large number of non-equilibrium defects, such as dislocations, vacancies, and subgrain boundaries, forming a non-equilibrium structure with high dislocation density. However, during solution treatment, this non-equilibrium structure undergoes significant evolution: under high-temperature thermal activation, high-density dislocations reorganize through mechanisms such as slip, climb, and annihilation, leading to a significant reduction in dislocation density and weakening the dislocation strengthening effect; simultaneously, increased grain boundary mobility promotes the merging and growth of fine grains, weakening the fine-grain strengthening (Hall-Petch effect). Both of these factors contribute to a decrease in strength. In addition, the restoration of the austenite ratio increases the effective slip system, the reduction of the residual dislocation hindering effect (caused by the significant reduction in dislocation density) improves dislocation mobility, and the reduction in the number of grain boundaries (caused by the merging and growth of fine grains) alleviates stress concentration. Furthermore, the residual stress introduced by rapid cooling and solidification in laser powder bed melting is released. These changes synergistically promote the uniform movement of dislocations during deformation, thereby significantly improving the toughness of the material.

[0038] To overcome the strength reduction problem introduced during solution treatment, this technical solution introduces pure Ti powder into the composite powder. During additive manufacturing, the pure Ti powder reacts with some of the nitrogen in a nitrogen atmosphere, resulting in the in-situ precipitation of TiN nanoparticles, which are dispersed in a dispersed manner. The strengthening mechanism of TiN nanoparticles as a strengthening agent for special super duplex stainless steel is mainly reflected in the following aspects:

[0039] (1) The low lattice mismatch between TiN nanoparticles and austenite makes them efficient heterogeneous nucleation cores during solid solution treatment, which can effectively refine the austenite grain size and compensate for the strength loss caused by heat treatment through grain boundary strengthening.

[0040] (2) Uniformly distributed TiN nanoparticles can pin grain boundaries and hinder dislocation movement, suppress grain growth and dislocation density decay, so that solid solution duplex stainless steel can maintain high strength.

[0041] (3) Refined austenite not only improves the uniformity of the austenite-ferrite two-phase distribution and enhances the dynamic recovery ability of dislocations during deformation, thereby improving toughness, but also the refined austenite is more uniform, which improves the stability of austenite during deformation and is also conducive to improving toughness.

[0042] (4) Refined austenite can block corrosion channels and improve pitting resistance, thereby improving corrosion resistance.

[0043] In summary, this technical solution lays the foundation for the high strength, high toughness, and corrosion resistance of the final super duplex stainless steel by introducing ultra-super duplex stainless steel powder into the composite powder. Furthermore, by introducing pure Ti powder into the composite powder, TiN nanoparticles are precipitated in situ during the additive manufacturing process through reaction with nitrogen. These TiN nanoparticles overcome the strength reduction problem introduced during solution treatment and simultaneously enhance the performance of the super duplex stainless steel powder after additive manufacturing and solution treatment, achieving a synergistic improvement in "high strength, high toughness, and corrosion resistance," resulting in a final super duplex stainless steel with high strength, high toughness, and corrosion resistance.

[0044] It should be noted that nitrogen can enhance the strength and corrosion resistance of super duplex stainless steel. However, when a large amount of nitrogen in the composite powder reacts with pure Ti powder to form TiN nanoparticles, the strength and corrosion resistance of the super duplex stainless steel will decrease. Therefore, this technical solution uses nitrogen as the forming atmosphere, which can reduce the loss of nitrogen in the composite powder, thereby helping to ensure the performance of the product.

[0045] Meanwhile, pure Nb powder cannot be used to replace pure Ti powder in this technical solution for the following reasons: (1) The chemical reactivity of Nb is much lower than that of Ti. Under additive manufacturing and other process conditions, pure Nb powder has a low tendency to react with nitrogen to generate nitride particles, making it difficult to effectively precipitate nitride particles in situ to strengthen the material. (2) Oxygen elements are inevitably introduced during the preparation and processing of super duplex stainless steel powder, which affects the quality of the formed parts. Nb has a weak ability to combine with oxygen and cannot form oxides to consume oxygen. Ti, on the other hand, reacts readily with oxygen to generate TiO particles, and TiN is easily precipitated on the surface of TiO particles. This can both neutralize oxygen and serve as a nucleation core for nitrides, increasing the nucleation rate and quantity of nitrides. (3) Pure Nb powder is more expensive than pure Ti powder. Using pure Nb powder to replace pure Ti powder will lead to a significant increase in cost. It should be noted that although pure Ti powder in this technical solution may form a small amount of TiN@TiO composite particles (with titanium nitride as the shell and titanium oxide as the core), it mainly forms TiN nanoparticles. The formation of a small amount of composite particles does not affect the product performance.

[0046] Preferably, the super duplex stainless steel powder comprises the following components by mass percentage: C 0.017%, N 0.33%, Cr 27.23%, Ni 6.75%, Mo 4.67%, Si 0.31%, Mn 0.41%, S 0.0035%, O 0.019%, with the balance being Fe and impurities.

[0047] This technical solution further optimizes the formulation of super duplex stainless steel powder, which is more conducive to ensuring the performance of super duplex stainless steel.

[0048] To further explain, in step A, the mixing ratio of the pure Ti powder and the super duplex stainless steel powder, calculated by mass ratio, is (0.001~0.01):1.

[0049] This technical solution limits the mixing ratio of pure Ti powder and super duplex stainless steel powder, which helps to increase the proportion of TiN nanoparticles in super duplex stainless steel while saving costs. This allows for the full utilization of TiN nanoparticles to optimize the performance of super duplex stainless steel and improve product performance.

[0050] To further explain, in step A, the pure Ti powder, calculated by mass percentage, comprises 10-15% of a type I Ti powder with a particle size ≥10μm and <22μm, 35-45% of a type II Ti powder with a particle size ≥22μm and <35μm, 35-45% of a type III Ti powder with a particle size ≥35μm and <55μm, and 8-12% of a type IV Ti powder with a particle size ≥55μm and <65μm.

[0051] This technical solution optimizes the mesh size and proportion of various Ti powders in pure Ti powder, ensuring a larger specific surface area for sufficient contact with nitrogen and improving the conversion rate of pure Ti powder to TiN nanoparticles. It also avoids agglomeration caused by excessively fine pure Ti powder, guaranteeing the uniformity of in-situ synthesis of TiN nanoparticles. Furthermore, the pure Ti powder with the aforementioned particle size distribution exhibits good flowability, allowing for smooth flow and spreading, reducing uneven powder accumulation and porosity, thereby ensuring product performance.

[0052] To further explain, in step A, the super duplex stainless steel powder, calculated by mass percentage, comprises 10-15% of a type I stainless steel powder with a particle size ≥10μm and <22μm, 35-45% of a type II stainless steel powder with a particle size ≥22μm and <35μm, 35-45% of a type III stainless steel powder with a particle size ≥35μm and <55μm, and 8-12% of a type IV stainless steel powder with a particle size ≥55μm and <65μm.

[0053] By optimizing the mesh size and proportion of various stainless steel powders in the super duplex stainless steel powder, the composite powder with added super duplex stainless steel powder not only has better fluidity, enabling it to flow and spread smoothly, reducing the problem of local powder accumulation or sparseness caused by uneven powder spreading during additive manufacturing, but also allows the composite powder to form a suitable packing density, enabling it to melt and form a stable molten pool. After cooling and solidification, the resulting microstructure is more compact, reducing defects such as pores and cracks, thereby improving the performance of the final super duplex stainless steel.

[0054] To further explain, the method in step B is as follows:

[0055] In the forming chamber of a laser powder bed melting equipment under a nitrogen atmosphere, composite powder is evenly spread on the surface of a stainless steel substrate. The composite powder is melted layer by layer according to the parameters of the laser powder bed melting process. During the layer-by-layer melting process, the composite powder is cooled and solidified to obtain a stainless steel substrate with preforms attached.

[0056] Once the temperature of the forming chamber of the laser powder bed melting equipment has cooled to below 80°C, the stainless steel substrate with the preform attached is removed.

[0057] The preform attached to the surface of the stainless steel substrate is peeled off to obtain the preform.

[0058] By optimizing the specific method of step B, and waiting for the temperature of the forming chamber of the laser powder bed melting equipment to cool down to below 80°C before removing the stainless steel substrate with the attached preform, the direct removal of the stainless steel substrate with the attached preform at high temperatures is avoided. This prevents the preform from being directly removed due to the large temperature gradient between the preform and the external environment, which could generate rapid thermal stress and lead to defects such as deformation and cracking. This helps ensure product performance. It should be noted that the peeling method can be wire cutting; the specific method is not limited here.

[0059] To further explain, in step B, the laser power of the laser powder bed melting process is 230-270W, the scanning speed is 1000-1200mm / s, the powder thickness is 0.02-0.04mm, and the scanning interval is 0.08-0.10mm.

[0060] Optimizing the parameters of the laser powder bed melting process can improve the density of the preforms, thereby enhancing their performance.

[0061] Preferably, in step B, the laser power of the laser powder bed melting process is 250W, the scanning speed is 1100mm / s, the powder thickness is 0.03mm, and the scanning interval is 0.09mm.

[0062] To further explain, step B includes preheating the stainless steel substrate before laying the composite powder.

[0063] The preheating treatment method is as follows: preheat the stainless steel substrate at 150-180°C for 5-10 minutes.

[0064] During laser powder bed melting (LPBF), the rapid heating and melting of the composite powder by the laser beam creates a significant temperature difference between the stainless steel substrate and the molten composite powder. This temperature difference can lead to a large temperature difference between the preform obtained after the composite powder melts and solidifies and the stainless steel substrate, resulting in significant thermal stress and making the stainless steel substrate or preform prone to deformation. Therefore, this technical solution preheats the stainless steel substrate at 150–180°C for 5–10 minutes, ensuring that the stainless steel substrate has a certain initial temperature before the composite powder melting begins. This reduces the temperature difference between the stainless steel substrate and the molten composite powder, thereby lowering the risk of deformation of the stainless steel substrate or preform due to thermal stress.

[0065] To further explain, in step C, the solution treatment method is as follows: the electric furnace is heated from room temperature to 1100-1200°C, the preform is then placed in the electric furnace, and after being kept at 1100-1200°C for 8-12 minutes, the preform is removed and immediately placed in cold water for cooling.

[0066] By optimizing the specific methods of solution treatment, it is beneficial to partially transform ferrite into austenite, thereby restoring the austenite-ferrite two-phase equilibrium and ensuring product performance.

[0067] A high-strength, high-toughness, corrosion-resistant, and super duplex stainless steel is prepared using the aforementioned method. The high-strength, high-toughness, corrosion-resistant, and super duplex stainless steel exhibits a yield strength > 1100 MPa, an elongation at break ≥ 45%, a strength-ductility product of 55–60 GPa·%, and a self-corrosion potential of -0.13 to -0.03 V. SCE .

[0068] This scheme also proposes a high-strength, high-toughness, corrosion-resistant super duplex stainless steel with a yield strength >1100MPa, elongation at break ≥45%, strength-ductility product of 55-60GPa·%, and self-corrosion potential of -0.13 to -0.03V. SCE Furthermore, the higher the yield strength, the higher the elongation at break and the strength-ductility product, the higher the toughness; the higher the self-corrosion potential, the higher the corrosion resistance. Therefore, the super duplex stainless steel obtained by this technical solution has high strength, high toughness and corrosion resistance.

[0069] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0070] Performance testing methods:

[0071] (1) The yield strength, elongation at break and strength-ductility product of super duplex stainless steel were measured using a universal tensile testing machine.

[0072] (2) The self-corrosion potential of super duplex stainless steel was measured using an electrochemical workstation. The corrosion solution was a NaCl solution with a NaCl content of 3.5% by mass percentage.

[0073] Example 1

[0074] A. Pure Ti powder and super duplex stainless steel powder are mixed evenly and then dried to obtain a composite powder. The super duplex stainless steel powder, by mass percentage, comprises the following components: C 0.017%, N 0.33%, Cr 27.23%, Ni 6.75%, Mo 4.67%, Si 0.31%, Mn 0.41%, S 0.0035%, O 0.019%, with the balance being Fe and impurities. The mixing ratio of pure Ti powder to the super duplex stainless steel powder is 0.001:1. The pure Ti powder, by mass percentage, comprises 10% Ti powder with a particle size ≥10μm and <22μm, 40% Ti powder with a particle size ≥22μm and <35μm, and 40% Ti powder with a particle size ≥35μm and <55μm. The powder comprises 10% Ti powder of four types with a particle size ≥ 55 μm and < 65 μm; and, by mass percentage, the super duplex stainless steel powder comprises 10% Ti powder of type I stainless steel with a particle size ≥ 10 μm and < 22 μm, 40% Ti powder of type II stainless steel with a particle size ≥ 22 μm and < 35 μm, 40% Ti powder of type III stainless steel with a particle size ≥ 35 μm and < 55 μm, and 10% Ti powder of type IV stainless steel with a particle size ≥ 55 μm and < 65 μm.

[0075] B. A stainless steel substrate is preheated at 160°C for 10 minutes to obtain a preheated stainless steel substrate. In the forming chamber of a laser powder bed melting equipment under a nitrogen atmosphere, composite powder is evenly spread on the surface of the preheated stainless steel substrate. The composite powder is melted layer by layer according to the parameters of the laser powder bed melting process, and the composite powder cools and solidifies during the layer-by-layer melting process to obtain a stainless steel substrate with a preform attached. After the temperature of the forming chamber of the laser powder bed melting equipment cools down to 75°C, the stainless steel substrate with the preform attached is removed. The preform attached to the surface of the stainless steel substrate is wire-cut to obtain the preform. The parameters of the laser powder bed melting process are: laser power of 250W, scanning speed of 1100mm / s, powder thickness of 0.03mm, and scanning spacing of 0.09mm.

[0076] C. Heat the electric furnace from room temperature to 1180℃, then place the preform inside the furnace and hold it at 1180℃ for 10 minutes. Remove the preform and immediately place it in cold water to cool, thus obtaining a high-strength, high-toughness, corrosion-resistant super duplex stainless steel.

[0077] The EBSD phase ratio diagrams of the high-strength, high-toughness, corrosion-resistant super duplex stainless steel obtained in Example 1 on the forming surface and in the forming direction are shown below. Figure 1 As shown in (a) and (b) in the figure, the ratio of ferrite to austenite is close to 1:1.

[0078] The TiN nanoparticle distribution diagram in the high-strength, high-toughness, corrosion-resistant super duplex stainless steel obtained in Example 1 is shown below. Figure 2 As shown in (a) and (b) in the figure, it can be seen that the TiN nanoparticles are diffusely distributed.

[0079] Example 2

[0080] A. Pure Ti powder and super duplex stainless steel powder are mixed evenly and then dried to obtain a composite powder. The super duplex stainless steel powder, by mass percentage, comprises the following components: C 0.01%, N 0.34%, Cr 26.5%, Ni 6.65%, Mo 4.35%, Si 0.28%, Mn 0.37%, S 0.003%, O 0.018%, with the balance being Fe and impurities. The mixing ratio of pure Ti powder to the super duplex stainless steel powder is 0.005:1 by mass. The pure Ti powder comprises, by mass percentage, 8% of a type I Ti powder with a particle size ≥10μm and <22μm, 35% of a type II Ti powder with a particle size ≥22μm and <35μm, and 45% of a type III Ti powder with a particle size ≥35μm and <55μm. The powder comprises 12% Ti powder of four types with a particle size ≥55μm and <65μm; and, by mass percentage, the super duplex stainless steel powder comprises 8% Ti powder of type I stainless steel with a particle size ≥10μm and <22μm, 35% Ti powder of type II stainless steel with a particle size ≥22μm and <35μm, 45% Ti powder of type III stainless steel with a particle size ≥35μm and <55μm, and 12% Ti powder of type IV stainless steel with a particle size ≥55μm and <65μm.

[0081] B. A stainless steel substrate is preheated at 180°C for 5 minutes to obtain a preheated stainless steel substrate. In the forming chamber of a laser powder bed melting equipment under a nitrogen atmosphere, composite powder is evenly spread on the surface of the preheated stainless steel substrate. The composite powder is melted layer by layer according to the parameters of the laser powder bed melting process, and the composite powder cools and solidifies during the layer-by-layer melting process to obtain a stainless steel substrate with a preform attached. After the temperature of the forming chamber of the laser powder bed melting equipment cools down to 70°C, the stainless steel substrate with the preform attached is removed. The preform attached to the surface of the stainless steel substrate is wire-cut to obtain the preform. The parameters of the laser powder bed melting process are: laser power of 250W, scanning speed of 1000mm / s, powder thickness of 0.03mm, and scanning spacing of 0.08mm.

[0082] C. Heat the electric furnace from room temperature to 1100℃ and hold at 1100℃. Then place the preform in the electric furnace, hold for 12 minutes, remove the preform and immediately put it into cold water to cool, thus obtaining high-strength, high-toughness, corrosion-resistant super duplex stainless steel.

[0083] Example 3

[0084] A. Pure Ti powder and super duplex stainless steel powder are mixed evenly and then dried to obtain a composite powder. The super duplex stainless steel powder, by mass percentage, comprises the following components: C 0.02%, N 0.35%, Cr 27.3%, Ni 6.3%, Mo 4.5%, Si 0.3%, Mn 0.3-0.5%, S 0.002%, O 0.015%, with the balance being Fe and impurities. The mixing ratio of pure Ti powder to the super duplex stainless steel powder is 0.005:1 by mass. The pure Ti powder comprises, by mass percentage, 15% of a type I Ti powder with a particle size ≥10μm and <22μm, 40% of a type II Ti powder with a particle size ≥22μm and <35μm, and 35% of a type III Ti powder with a particle size ≥35μm and <55μm. The powder comprises 10% Ti powder of four types with a particle size ≥ 55 μm and < 65 μm; and, by mass percentage, the super duplex stainless steel powder comprises 15% Ti powder of type I stainless steel with a particle size ≥ 10 μm and < 22 μm, 40% Ti powder of type II stainless steel with a particle size ≥ 22 μm and < 35 μm, 35% Ti powder of type III stainless steel with a particle size ≥ 35 μm and < 55 μm, and 10% Ti powder of type IV stainless steel with a particle size ≥ 55 μm and < 65 μm.

[0085] B. A stainless steel substrate is preheated at 150°C for 10 minutes to obtain a preheated stainless steel substrate. In the forming chamber of a laser powder bed melting equipment under a nitrogen atmosphere, composite powder is evenly spread on the surface of the preheated stainless steel substrate. The composite powder is melted layer by layer according to the parameters of the laser powder bed melting process, and the composite powder cools and solidifies during the layer-by-layer melting process to obtain a stainless steel substrate with a preform attached. After the temperature of the forming chamber of the laser powder bed melting equipment cools down to below 80°C, the stainless steel substrate with the preform attached is removed. The preform attached to the surface of the stainless steel substrate is wire-cut to obtain the preform. The parameters of the laser powder bed melting process are: laser power of 270W, scanning speed of 1200mm / s, powder thickness of 0.03mm, and scanning spacing of 0.08mm.

[0086] C. Heat the electric furnace from room temperature to 1100℃, place the preform in the electric furnace at 1100℃, hold for 12 minutes, remove the preform and cool it in cold water to obtain high-strength, high-toughness, corrosion-resistant super duplex stainless steel.

[0087] Comparative Example 1

[0088] The preparation method of Comparative Example 1 is the same as that of Example 1. The difference is that pure Ti powder was not added in Comparative Example 1, and in step B, only the super duplex stainless steel powder was additively manufactured using laser powder bed melting process to obtain the preform.

[0089] Comparative Example 2

[0090] The preparation method of Comparative Example 2 is the same as that of Example 1, except that step C is missing in Comparative Example 2. That is, no solution treatment is performed in Comparative Example 2.

[0091] The yield strength, elongation at break, strength-ductility product, and self-corrosion potential of the super duplex stainless steels prepared in the examples and comparative examples were measured, and the results are shown in Table 1 below.

[0092] Table 1. Test results of relevant properties of super duplex stainless steel

[0093] Experiment number Yield strength (MPa) Elongation at break (%) High plasticity (GPa·%) <![CDATA[Self - corrosion potential (V SCE )]]> Example 1 1130 48.9 59.8 -0.03 Example 2 1108 47.8 57.4 -0.08 Example 3 1113 46.2 55.6 -0.13 Comparative Example 1 892 36.5 32.7 -0.31 Comparative Example 2 1339 18.2 25.9 -0.58

[0094] As shown in Table 1, the super duplex stainless steel prepared using the method described in this paper has a yield strength > 1100 MPa, an elongation at break ≥ 45%, a strength-ductility product of 55–60 GPa·%, and a self-corrosion potential of -0.13 to -0.03 V. SCE Furthermore, higher yield strength, higher elongation at break, and higher strength-ductility product all contribute to higher toughness; higher self-corrosion potential also results in higher corrosion resistance. Therefore, the super duplex stainless steel obtained by this technical solution possesses high strength, high toughness, and corrosion resistance.

[0095] Since pure Ti powder was not added in Comparative Example 1, TiN nanoparticles could not be generated. Therefore, the properties of TiN nanoparticles as a reinforcing phase to improve strength, toughness and corrosion resistance could not be utilized, resulting in a decrease in the strength, toughness and corrosion resistance of the obtained super duplex stainless steel.

[0096] In Comparative Example 2, no solution treatment was performed, resulting in the super duplex stainless steel retaining only a single ferrite phase. This will have the following effects: (1) The TRIP effect of austenite is lost, causing a sharp drop in toughness; (2) Since the solubility of N in ferrite is only 1 / 20 of that in austenite, the interstitial solid solution strengthening effect of N will be lost; (3) The segregation of Cr and Mo in the single phase will be aggravated, leading to a precipitous drop in pitting corrosion resistance (PREN effective value drops from 45 to 28); (4) The continuous ferrite network becomes a channel for rapid crack propagation, increasing the stress corrosion susceptibility index and decreasing corrosion resistance. It should be noted that although the single ferrite phase will cause the loss of the interstitial solid solution strengthening effect of N, the high strength of ferrite results in a higher yield strength.

[0097] The technical principles of the present invention have been described above with reference to specific embodiments. These descriptions are merely for explaining the principles of the invention and should not be construed as limiting the scope of protection of the invention in any way. Based on this explanation, those skilled in the art can readily conceive of other specific embodiments of the invention without inventive effort, and these embodiments will all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a high-strength, high-toughness, corrosion-resistant, super duplex stainless steel, characterized in that, Includes the following steps: A. Pure Ti powder and super duplex stainless steel powder are mixed evenly and then dried to obtain a composite powder; wherein, calculated by mass percentage, the super duplex stainless steel powder comprises the following components: C 0.01~0.02%, N 0.3~0.4%, Cr 26~28%, Ni 6~7%, Mo 4~5%, Si 0.2~0.4%, Mn 0.3~0.5%, S 0.002~0.005%, O 0.01~0.03%, with the balance being Fe and impurities; B. In a nitrogen atmosphere, composite powder is additively manufactured using laser powder bed melting process to obtain preforms; C. Solution treatment of the preforms yields high-strength, high-toughness, corrosion-resistant super duplex stainless steel; In step A, the mixing ratio of the pure Ti powder and the super duplex stainless steel powder is calculated to be (0.001~0.01):1 based on the mass ratio. In step A, the pure Ti powder, calculated by mass percentage, comprises 10-15% of a type I Ti powder with a particle size ≥10μm and <22μm, 35-45% of a type II Ti powder with a particle size ≥22μm and <35μm, 35-45% of a type III Ti powder with a particle size ≥35μm and <55μm, and 8-12% of a type IV Ti powder with a particle size ≥55μm and <65μm.

2. The method for preparing a high-strength, high-toughness, corrosion-resistant, super duplex stainless steel according to claim 1, characterized in that, In step A, the super duplex stainless steel powder, calculated by mass percentage, comprises 8-12% of a type I stainless steel powder with a particle size ≥10μm and <22μm, 35-45% of a type II stainless steel powder with a particle size ≥22μm and <35μm, 35-45% of a type III stainless steel powder with a particle size ≥35μm and <55μm, and 8-12% of a type IV stainless steel powder with a particle size ≥55μm and <65μm.

3. The method for preparing a high-strength, high-toughness, corrosion-resistant, super duplex stainless steel according to claim 1, characterized in that, The method for step B is as follows: In the forming chamber of a laser powder bed melting equipment under a nitrogen atmosphere, composite powder is evenly spread on the surface of a stainless steel substrate. The composite powder is melted layer by layer according to the parameters of the laser powder bed melting process. During the layer-by-layer melting process, the composite powder is cooled and solidified to obtain a stainless steel substrate with preforms attached. Once the temperature of the forming chamber of the laser powder bed melting equipment has cooled to below 80°C, the stainless steel substrate with the preform attached is removed. The preform attached to the surface of the stainless steel substrate is peeled off to obtain the preform.

4. The method for preparing a high-strength, high-toughness, corrosion-resistant, super duplex stainless steel according to claim 3, characterized in that, In step B, the laser power of the laser powder bed melting process is 230-270W, the scanning speed is 1000-1200mm / s, the powder thickness is 0.02-0.04mm, and the scanning interval is 0.08-0.10mm.

5. The method for preparing a high-strength, high-toughness, corrosion-resistant, super duplex stainless steel according to claim 3, characterized in that, In step B, the stainless steel substrate is preheated before the composite powder is laid. The preheating treatment method is as follows: preheat the stainless steel substrate at 150-180°C for 5-10 minutes.

6. The method for preparing a high-strength, high-toughness, corrosion-resistant, super duplex stainless steel according to claim 1, characterized in that, In step C, the solution treatment method is as follows: the electric furnace is heated from room temperature to 1100-1200°C, the preform is then placed in the furnace, and after being kept at 1100-1200°C for 8-12 minutes, the preform is taken out and immediately placed in cold water for cooling.

7. A high-strength, high-toughness, corrosion-resistant super duplex stainless steel, characterized in that: The high-strength, high-toughness, corrosion-resistant, super duplex stainless steel described in claims 1-6 is prepared using the same method. The high-strength, high-toughness, corrosion-resistant, super duplex stainless steel has a yield strength > 1100 MPa, an elongation at break ≥ 45%, a strength-ductility product of 55-60 GPa·%, and a self-corrosion potential of -0.13 to -0.03V. SCE .

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