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

Abstract

The invention relates to the technical field of extra-super duplex stainless steel, in particular to high-strength, high-toughness and corrosion-resistant extra-super duplex stainless steel and a preparation method thereof.The preparation method comprises the following steps that A, pure Ti powder and extra-super duplex stainless steel powder are evenly mixed and then dried, and composite powder is obtained; wherein the extra-super duplex stainless steel powder is prepared from the following components in percentage by mass: 0.01 to 0.02 percent of C, 0.3 to 0.4 percent of N, 26 to 28 percent of Cr, 6 to 7 percent of Ni, 4 to 5 percent of Mo, 0.2 to 0.4 percent of Si, 0.3 to 0.5 percent of Mn, 0.002 to 0.005 percent of S, 0.01 to 0.03 percent of O and the balance of Fe and impurities; b, in a nitrogen atmosphere, performing additive manufacturing on the composite powder by using a laser powder bed melting process to obtain a prefabricated part; and C, the prefabricated part is subjected to solution treatment, and the high-strength, high-toughness and corrosion-resistant super duplex stainless steel is obtained. According to the high-strength high-toughness corrosion-resistant super duplex stainless steel and the preparation method thereof, the obtained super duplex stainless steel has the characteristics of high strength, high toughness and corrosion resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of super duplex stainless steel, and particularly to a high-strength, high-toughness and corrosion-resistant super duplex stainless steel and a preparation method thereof. Background Art

[0002] At present, with the booming development of the industrial manufacturing and high-end equipment fields, the requirements for the performance of materials in various industries are becoming increasingly strict and diverse. Not only are materials required to have reliable practicality and durability, and be able to operate stably for a long time under complex working conditions, but also higher standards are put forward for key indicators such as strength, toughness and corrosion resistance in extreme environments. This change in market demand drives the stainless steel industry to continuously break through technical bottlenecks and accelerate product iteration and upgrading to meet the growing demand for high-performance materials.

[0003] As a representative of high-end materials in the stainless steel field, super duplex stainless steel has a unique duplex microstructure (i.e., ferrite and austenite) and high contents of chromium, molybdenum and nitrogen alloying elements, which make its performance significantly superior to that of duplex stainless steel and super duplex stainless steel, endowing it with excellent high strength, high toughness and corrosion resistance, and it is widely used in industries such as urea production, oil exploration and deep-sea exploration.

[0004] Existing super duplex stainless steel is mainly prepared by traditional casting-forging-hot rolling processes. However, due to its characteristics of high contents of chromium, molybdenum and nitrogen alloying elements, it faces many severe challenges in the manufacturing process. In the melting process, special pressurized equipment is required to complete it smoothly; in the forging and hot rolling stages, the control requirements for the temperature range are extremely strict. If the control is improper, the material is prone to cracking. In addition, the as-cast structure grains prepared by traditional processes are coarse, resulting in the mechanical properties of the material being difficult to reach the ideal state and unable to fully meet the use requirements of high strength and high toughness. The above limiting factors lead to the fact that the super duplex stainless steel prepared by the existing technology still has great potential for improvement in terms of strength, toughness and corrosion resistance.

[0005] In order to effectively overcome the defects existing in the existing technology, the existing technology has begun to try to use processes such as selective laser melting process or laser powder bed fusion process to prepare super duplex stainless steel by additive manufacturing with composite powder, that is, laying the composite powder, melting it layer by layer, and cooling and solidifying during the melting process, so that the material is stacked layer by layer to form a three-dimensional structure, thereby obtaining super duplex stainless steel. However, due to the differences in raw material selection and preparation processes, the super duplex stainless steel produced by the above methods still has deficiencies in terms of strength, toughness and corrosion resistance, and it is difficult to meet the use places with extremely high requirements for the above performances. Summary of the Invention

[0006] The object of the present invention is to provide a high-strength, high-toughness and corrosion-resistant super duplex stainless steel and a preparation method thereof. The obtained super duplex stainless steel has the characteristics of high strength, high toughness and corrosion resistance, so as to overcome the deficiencies in the prior art.

[0007] To achieve this purpose, the present invention adopts the following technical solutions:

[0008] A preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel, comprising the following steps:

[0009] A. After uniformly mixing pure Ti powder and super duplex stainless steel powder and drying, a composite powder is obtained; 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%, and the balance is Fe and impurities;

[0010] B. Under a nitrogen atmosphere, the composite powder is subjected to additive manufacturing by a laser powder bed fusion process to obtain a preform;

[0011] C. The preform is subjected to solution treatment to obtain a high-strength, high-toughness and corrosion-resistant super duplex stainless steel.

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

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

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

[0015] Further, the method of step B is:

[0016] In the forming chamber of a laser powder bed fusion equipment under a nitrogen atmosphere, the composite powder is evenly laid 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 fusion process, and the composite powder is cooled and solidified during the process of layer-by-layer melting to obtain a stainless-steel substrate with a preform attached thereto.

[0017] After the temperature of the forming chamber of the laser powder bed fusion equipment cools down to below 80 °C, the stainless-steel substrate with the preform attached thereto is taken out.

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

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

[0020] Further, in step B, before the composite powder is laid, it also includes preheating the stainless-steel substrate.

[0021] The method of the preheating treatment is: preheating the stainless-steel substrate at 150 - 180 °C for 5 - 10 min.

[0022] Further, in step C, the method of the solution treatment is: heating the electric furnace from room temperature to 1100 - 1200 °C, then placing the preform in the electric furnace, and after holding at 1100 - 1200 °C for 8 - 12 min, taking out the preform and immediately putting it into cold water for cooling.

[0023] A high-strength, high-toughness, corrosion-resistant super duplex stainless steel is prepared by using the above-mentioned preparation method of the high-strength, high-toughness, corrosion-resistant super duplex stainless steel. The yield strength of the high-strength, high-toughness, corrosion-resistant super duplex stainless steel is > 1100 MPa, the elongation at break is ≥ 45%, the product of strength and plasticity is 55 - 60 GPa·%, and the self-corrosion potential is -0.13 - -0.03 V SCE 。

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

[0025] 1. 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%, and the balance is Fe and impurities. Among them, Cr (26-28%) and Mo (4-5%) form a continuous passivation film (Cr2O3-MoO3 composite oxide film), and the PREN value (= %Cr + 3.3×%Mo + 16×%N) reaches 45-48, forming a high Cr-Mo corrosion-resistant matrix; N (0.3-0.4%) promotes the stability of austenite while enhancing the strength through interstitial solid solution strengthening; Ni (6-7%) is precisely regulated to obtain a balanced duplex structure (ferrite / austenite ≈ 1:1), which not only retains the high strength of ferrite but also exerts the high toughness of austenite; the ultra-low C (0.01-0.02%) design reduces the precipitation risk of grain boundary precipitates, ensuring the purity and corrosion resistance of the material. The above composition design of high Cr-Mo corrosion-resistant matrix + N interstitial strengthening + duplex stabilizing elements enables the composite powder added with super-duplex stainless steel powder to have the theoretical potential to achieve high strength, high toughness and good corrosion resistance after additive manufacturing.

[0026] 2. The excellent comprehensive properties of super-duplex stainless steel, such as strength, toughness and corrosion resistance, are all related to the austenite-ferrite two-phase balance. Therefore, in order to ensure the excellent comprehensive properties of the finally obtained super-duplex stainless steel, this technical solution performs a solution treatment on the preform to convert part of the ferrite into austenite to restore the austenite-ferrite two-phase balance (≈1:1) to ensure the performance of the product.

[0027] 3. To overcome the problem of strength decline introduced during the solution treatment process, pure Ti powder is introduced into the composite powder in this technical solution. During the additive manufacturing process, the pure Ti powder reacts with some nitrogen in the nitrogen atmosphere, in-situ precipitating TiN nanoparticles and the TiN nanoparticles are dispersedly distributed. The strengthening mechanism of TiN nanoparticles as a strengthening phase for super duplex stainless steel is mainly reflected in the following aspects: (1) The lattice misfit degree between TiN nanoparticles and austenite is relatively low, making them efficient heterogeneous nucleation cores during the solution treatment process, capable of effectively refining the austenite grain size, and compensating for the strength loss caused by heat treatment through grain boundary strengthening; (2) The uniformly distributed TiN nanoparticles can pin the grain boundaries and hinder the movement of dislocations, inhibiting grain growth and dislocation density decay, enabling the solution-treated duplex stainless steel to maintain high strength; (3) The refined austenite not only improves the uniformity of the austenite-ferrite two-phase distribution, enhances the dynamic recovery ability of dislocations during the deformation process, thereby improving toughness, but also the refined austenite is more uniform, improving the stability of austenite during the deformation process, which is also beneficial to improving toughness; (4) The refined austenite can block the corrosion channels, enhancing the pitting corrosion resistance, thereby improving the corrosion resistance. Description of the Drawings

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

[0029] Figure 2 Among them, (a) and (b) are both the TiN nanoparticle distribution diagrams in the super duplex stainless steel obtained in Example 1 of the present invention. Detailed Embodiments

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

[0031] A. After uniformly mixing pure Ti powder and super duplex stainless steel powder and drying, a composite powder is obtained; wherein, calculated by 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%, and the balance is Fe and impurities;

[0032] B. Under a nitrogen atmosphere, the composite powder is additively manufactured by a laser powder bed fusion process to obtain a preform;

[0033] C. The preform is solution-treated to obtain a high-strength, high-toughness and corrosion-resistant super duplex stainless steel.

[0034] In order to overcome the deficiencies in strength, toughness, and corrosion resistance existing in the prior art, this technical solution proposes a preparation method for a high-strength, high-toughness, and corrosion-resistant super duplex stainless steel. By optimizing the formula and preparation method, in-situ precipitation of TiN nanoparticles is promoted during the preparation process. The characteristics of TiN nanoparticles as strengthening phases are fully utilized to optimize the performance of the super duplex stainless steel, so that the obtained super duplex stainless steel has high strength, high toughness, and corrosion resistance to meet the actual use requirements.

[0035] This technical solution is based on the composition of the super duplex stainless steel powder, laying a foundation for achieving high strength, high toughness, and corrosion resistance of the super duplex stainless steel. Specifically, calculated by 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%, and the balance is Fe and impurities. Among them, Cr (26 - 28%) and Mo (4 - 5%) form a continuous passivation film (Cr2O3 - MoO3 composite oxide film), and the PREN value (= %Cr + 3.3 × %Mo + 16 × %N) reaches 45 - 48, forming a high Cr - Mo corrosion-resistant matrix; N (0.3 - 0.4%) promotes austenite stability while enhancing strength through interstitial solid solution strengthening; Ni (6 - 7%) precisely regulates to obtain a balanced duplex structure (ferrite / austenite ≈ 1:1), retaining both the high strength of ferrite and the high toughness of austenite; the ultra-low C (0.01 - 0.02%) design reduces the precipitation risk of grain boundary precipitates, ensuring the purity and corrosion resistance of the material. The above composition design of high Cr - Mo corrosion-resistant matrix + N interstitial strengthening + duplex stabilizing elements enables the composite powder added with super duplex stainless steel powder to have the theoretical potential to achieve relatively high strength, relatively high toughness, and good corrosion resistance after additive manufacturing. It should be noted that the PREN value represents the pitting resistance equivalent value, and the larger the PREN value, the higher the corrosion resistance.

[0036] However, when using the laser powder bed fusion process for additive manufacturing, since the laser beam sweeps across the surface of the composite powder, only a very small amount of the composite powder is melted and then rapidly cooled and solidified, and the cooling and solidification rate reaches 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 failure of austenite to transform in time, making the preform only exhibit a single ferrite phase. As a result, the super-duplex stainless steel that should have presented an austenite-ferrite two-phase balance only shows a single ferrite phase. The single ferrite phase will have the following effects: (1) The TRIP effect of austenite (the TRIP effect refers to the phase transformation process of internal austenite to martensite in materials under external force. When the external stress reaches the critical value, austenite transforms into martensite through the shear mechanism. This phase transformation process absorbs energy and produces volume expansion, effectively delaying local stress concentration and significantly improving toughness) is missing, causing the toughness to drop sharply; (2) Since the solubility of N in ferrite is only 1 / 20 of that in austenite, the interstitial solid solution strengthening effect of N element will be lost; (3) It will lead to the aggravation of the segregation of Cr and Mo in the single phase, resulting in a cliff-like drop in the pitting corrosion resistance (the effective value of PREN drops from 45 to 28); (4) The continuous ferrite network becomes a rapid crack propagation channel, the stress corrosion sensitivity index increases, and the corrosion resistance decreases. That is, the excellent comprehensive properties such as strength, toughness, and corrosion resistance of the super-duplex stainless steel are all related to the austenite-ferrite two-phase balance. Therefore, in order to ensure the excellent comprehensive properties of the finally obtained super-duplex stainless steel, this technical solution performs a solution treatment on the preform to convert part of the ferrite into austenite to restore the austenite-ferrite two-phase balance (≈1:1) to ensure the performance of the product.

[0037] At the same time, in the laser powder bed fusion process, the non-equilibrium structure formed by rapid cooling and solidification causes the metal atoms in the molten pool to have no time to arrange orderly, resulting in a large number of non-equilibrium defects such as dislocations, vacancies, and sub-grain boundaries, forming a non-equilibrium structure with a high dislocation density. However, during the solution treatment process, this non-equilibrium structure will undergo significant evolution: under the action of high-temperature thermal activation, the high-density dislocations are recombined through mechanisms such as slip, climb, and annihilation, resulting in a significant reduction in the dislocation density and weakening the dislocation strengthening effect; at the same time, the increase in the grain boundary mobility promotes the merger and growth of fine grains, weakening the fine grain strengthening (Hall-Petch effect). The above two factors together lead to a decrease in strength. In addition, the restoration of the austenite ratio increases the effective slip system, the reduction of the residual dislocation hindrance effect (caused by the significant reduction in the dislocation density) improves the dislocation movement ability, and the reduction of the grain boundary number (caused by the merger and growth of fine grains) relieves the stress concentration. Coupled with the release of the residual stress introduced by rapid cooling and solidification in the laser powder bed fusion process, the above changes synergistically promote the uniform movement of dislocations during deformation, thus significantly improving the toughness of the material.

[0038] To overcome the problem of strength decline introduced during the solution treatment process, pure Ti powder is introduced into the composite powder in this technical solution. During the additive manufacturing process, the pure Ti powder reacts with some nitrogen in the nitrogen atmosphere to in-situ precipitate TiN nanoparticles, and the TiN nanoparticles are dispersedly distributed. The strengthening mechanism of TiN nanoparticles as a strengthening phase for super duplex stainless steel is mainly reflected in the following aspects:

[0039] (1) The lattice misfit degree between TiN nanoparticles and austenite is relatively low, making them efficient heterogeneous nucleation cores during the solution treatment process, capable of effectively refining the austenite grain size, and compensating for the strength loss caused by heat treatment through grain boundary strengthening;

[0040] (2) The uniformly distributed TiN nanoparticles can pin the grain boundaries and hinder the movement of dislocations, inhibiting grain growth and dislocation density decay, enabling the solution-treated duplex stainless steel to maintain high strength;

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

[0042] (4) The refined austenite can block the corrosion channels, enhancing the pitting corrosion resistance, thereby improving the corrosion resistance.

[0043] In summary, this technical solution lays a foundation for the finally obtained super duplex stainless steel to have high strength, high toughness and corrosion resistance by introducing super duplex stainless steel powder into the composite powder. Combining with the introduction of pure Ti powder into the composite powder, it reacts with nitrogen during the additive manufacturing process to in-situ precipitate TiN nanoparticles. The above TiN nanoparticles can overcome the problem of strength decline introduced during the solution treatment process, and at the same time strengthen the properties of super duplex stainless steel powder after additive manufacturing and solution treatment, realizing the synergistic improvement of "high strength-high toughness-corrosion resistance", making the finally obtained super duplex stainless steel have high strength, high toughness and corrosion resistance.

[0044] It should be noted that since nitrogen can improve the strength and corrosion resistance of super duplex stainless steel. When a large amount of nitrogen in the composite powder reacts with pure Ti powder to form TiN nanoparticles, it will lead to a decline in the strength and corrosion resistance of super duplex stainless steel. Therefore, this technical solution selects nitrogen as the forming atmosphere, which can reduce the loss of nitrogen in the composite powder, thus also being beneficial to ensuring the performance of the product.

[0045] Meanwhile, in this technical solution, pure Nb powder cannot be used to replace pure Ti powder, and the reasons are as follows: (1) The chemical activity of Nb is much lower than that of Ti. Under process conditions such as additive manufacturing, the tendency of pure Nb powder to react with nitrogen to form nitride particles is low, and it is difficult to effectively precipitate nitride particles in-situ to strengthen the material. (2) Oxygen elements will inevitably be introduced during the preparation and processing of super duplex stainless steel powder, which affects the quality of the formed parts. The ability of Nb element to combine with oxygen is weak, and it cannot form oxides to consume oxygen elements; while Ti element is extremely easy to react with oxygen to form TiO particles, and TiN is easily precipitated on the surface of TiO particles, which can not only harmlessize oxygen elements, but also serve as the nucleation core of nitrides, improving the nucleation rate and quantity of nitrides; (3) The price of pure Nb powder is higher than that of pure Ti powder. Using pure Nb powder to replace pure Ti powder will lead to a substantial increase in cost. It should be noted that although a small amount of TiN@TiO composite particles (with titanium nitride as the shell and titanium oxide as the core) may be formed in the pure Ti powder in this technical solution, it mainly forms TiN nanoparticles, and the formation of a small amount of composite particles does not affect the product performance.

[0046] Preferably, calculated by mass percentage, the super duplex stainless steel powder 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%, and the balance is Fe and impurities.

[0047] This technical solution further optimizes the ratio of the super duplex stainless steel powder, thereby being more conducive to ensuring the performance of the super duplex stainless steel.

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

[0049] By limiting the mixing ratio of the pure Ti powder and the super duplex stainless steel powder in this technical solution, it is beneficial to increase the proportion of TiN nanoparticles in the super duplex stainless steel while saving costs, so as to make full use of TiN nanoparticles to optimize the performance of the super duplex stainless steel and improve the performance of the product.

[0050] Further explanation, in step A, calculated by mass percentage, the pure Ti powder comprises 10 - 15% of a first type of Ti powder with a particle size of ≥10μm and <22μm, 35 - 45% of a second type of Ti powder with a particle size of ≥22μm and <35μm, 35 - 45% of a third type of Ti powder with a particle size of ≥35μm and <55μm, and 8 - 12% of a fourth type of Ti powder with a particle size of ≥55μm and <65μm.

[0051] In this technical solution, by optimizing the mesh number and its proportion of various Ti powder materials in pure Ti powder, not only does it have a large specific surface area, ensuring sufficient contact with nitrogen gas, improving the conversion rate of pure Ti powder into TiN nanoparticles, but also it avoids the agglomeration phenomenon caused by overly fine pure Ti powder, ensuring the uniformity of in-situ synthesis of TiN nanoparticles. Additionally, the pure Ti powder with the above particle size gradation has good fluidity, enabling it to flow and spread smoothly, reducing the generation of defects such as uneven powder accumulation and pores, thereby ensuring the performance of the product.

[0052] Further explanation, in step A, calculated by mass percentage, the super duplex stainless steel powder includes 10 - 15% of a first type of stainless steel powder material with a particle size of ≥10μm and <22μm, 35 - 45% of a second type of stainless steel powder material with a particle size of ≥22μm and <35μm, 35 - 45% of a third type of stainless steel powder material with a particle size of ≥35μm and <55μm, and 8 - 12% of a fourth type of stainless steel powder material with a particle size of ≥55μm and <65μm.

[0053] By optimizing the mesh number and its proportion of various stainless steel powder materials in the super duplex stainless steel powder, not only does the composite powder added with the super duplex stainless steel powder have good fluidity, enabling it to flow and spread smoothly, reducing the problems of local powder accumulation or sparseness caused by uneven powder spreading during the additive manufacturing process, but also it can make the composite powder form an appropriate packing density, enabling it to melt to form a stable molten pool, and the obtained microstructure after cooling and solidification is denser, reducing defects such as pores and cracks, thereby improving the performance of the finally obtained super duplex stainless steel.

[0054] Further explanation, the method of step B is as follows:

[0055] In the forming chamber of a laser powder bed fusion device under a nitrogen atmosphere, the composite powder is evenly laid on the surface of a stainless steel substrate, and the composite powder is melted layer by layer according to the parameters of the laser powder bed fusion process, and the composite powder is cooled and solidified during the process of layer-by-layer melting to obtain a stainless steel substrate with a preform attached;

[0056] After the temperature of the forming chamber of the laser powder bed fusion device cools below 80°C, the stainless steel substrate with the preform attached is taken out;

[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 below 80 °C before taking out the stainless steel substrate with the preform attached, it is avoided to directly take out the stainless steel substrate with the preform attached at high temperature, and due to the large temperature gradient between the preform and the external environment, sharp thermal stress is generated, resulting in defects such as deformation and cracking of the preform, which is conducive to ensuring the performance of the product. It should be noted that the peeling method can be wire cutting, and the specific method is not limited here.

[0059] Further explanation, in step B, the laser power of the laser powder bed melting process is 230 - 270 w, the scanning speed is 1000 - 1200 mm / s, the powder laying thickness is 0.02 - 0.04 mm, and the scanning spacing is 0.08 - 0.10 mm.

[0060] By optimizing the parameters of the laser powder bed melting process, it is beneficial to improve the density of the preform, thereby improving its performance.

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

[0062] Further explanation, in step B, before laying the composite powder, it also includes preheating the stainless steel substrate;

[0063] The method of the preheating treatment is: preheating the stainless steel substrate at 150 - 180 °C for 5 - 10 min.

[0064] During the laser powder bed melting (LPBF) process, the rapid heating and melting of the laser beam on the composite powder will cause a significant temperature difference between the stainless steel substrate and the melted composite powder. The above temperature difference is likely to cause a large temperature difference between the preform obtained after the composite powder melts and cools and solidifies and the stainless steel substrate, thereby generating large thermal stress and causing the stainless steel substrate or the preform to be easily deformed. Therefore, in this technical solution, by preheating the stainless steel substrate at 150 - 180 °C for 5 - 10 min, the stainless steel substrate can have a certain initial temperature before the melting of the composite powder starts, reducing the temperature difference between the stainless steel substrate and the melted composite powder, thereby reducing the risk of deformation of the stainless steel substrate or the preform caused by thermal stress.

[0065] Further explanation, in step C, the method of the solution treatment is: heating the electric furnace from room temperature to 1100 - 1200 °C, then placing the preform in the electric furnace, and after holding at 1100 - 1200 °C for 8 - 12 min, taking out the preform and immediately putting it into cold water for cooling.

[0066] By optimizing the specific method of solution treatment, it is beneficial to partially transform ferrite into austenite to restore the austenite-ferrite two-phase balance and ensure the performance of the product.

[0067] A high-strength, high-toughness, corrosion-resistant special super duplex stainless steel is prepared by using the preparation method of the above-mentioned high-strength, high-toughness, corrosion-resistant special super duplex stainless steel. The yield strength of the high-strength, high-toughness, corrosion-resistant special super duplex stainless steel is > 1100 MPa, the elongation at break is ≥ 45%, the product of strength and plasticity is 55 - 60 GPa·%, and the self-corrosion potential is -0.13 - -0.03 V. SCE 。

[0068] This solution also proposes a high-strength, high-toughness, corrosion-resistant special super duplex stainless steel, whose yield strength is > 1100 MPa, the elongation at break is ≥ 45%, the product of strength and plasticity is 55 - 60 GPa·%, and the self-corrosion potential is -0.13 - -0.03 V. SCE 。And for the yield strength, the higher the strength; the higher the elongation at break and the product of strength and plasticity, the higher the toughness; the higher the self-corrosion potential, the higher the corrosion resistance. Therefore, the special 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 described below through specific embodiments.

[0070] Performance test method:

[0071] (1) The yield strength, elongation at break and product of strength and plasticity of the special super duplex stainless steel were measured by using a universal tensile testing machine;

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

[0073] Example 1

[0074] A. After uniformly mixing pure Ti powder and super-duplex stainless steel powder and drying, a composite powder is obtained. Among them, calculated by mass percentage, the super-duplex stainless steel powder includes 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%, and the balance is Fe and impurities. Among them, calculated by mass ratio, the mixing ratio of pure Ti powder and the super-duplex stainless steel powder is 0.001:1. Calculated by mass percentage, the pure Ti powder includes 10% of a first type of Ti powder with a particle size of ≥10 μm and <22 μm, 40% of a second type of Ti powder with a particle size of ≥22 μm and <35 μm, 40% of a third type of Ti powder with a particle size of ≥35 μm and <55 μm, and 10% of a fourth type of Ti powder with a particle size of ≥55 μm and <65 μm. Calculated by mass percentage, the super-duplex stainless steel powder includes 10% of a first type of stainless steel powder with a particle size of ≥10 μm and <22 μm, 40% of a second type of stainless steel powder with a particle size of ≥22 μm and <35 μm, 40% of a third type of stainless steel powder with a particle size of ≥35 μm and <55 μm, and 10% of a fourth type of stainless steel powder with a particle size of ≥55 μm and <65 μm.

[0075] B. The stainless steel substrate is preheated at 160 °C for 10 min to obtain a preheated stainless steel substrate. In the forming chamber of a laser powder bed fusion equipment under a nitrogen atmosphere, the composite powder is uniformly laid on the surface of the preheated stainless steel substrate, and the composite powder is melted layer by layer according to the parameters of the laser powder bed fusion process. During the process of layer-by-layer melting, the composite powder cools and solidifies to obtain a stainless steel substrate with a preform attached. When the temperature of the forming chamber of the laser powder bed fusion equipment cools to 75 °C, the stainless steel substrate with the preform attached is taken out. The preform attached to the surface of the stainless steel substrate is wire-cut to obtain the preform. Among them, the parameters of the laser powder bed fusion process are: laser power is 250 w, scanning speed is 1100 mm / s, powder laying thickness is 0.03 mm, and scanning spacing is 0.09 mm.

[0076] C. The electric furnace is heated from room temperature to 1180 °C, and then the preform is placed in the furnace. After holding at 1180 °C for 10 min, the preform is taken out and immediately put into cold water for cooling to obtain a high-strength, high-toughness and corrosion-resistant super-duplex stainless steel.

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

[0078] The distribution map of TiN nanoparticles in the high-strength, high-toughness, corrosion-resistant super duplex stainless steel obtained in Example 1 is as shown in Figure 2 (a) and (b) in it. It can be seen from the figure that the TiN nanoparticles are dispersedly distributed.

[0079] Example 2

[0080] A. After uniformly mixing pure Ti powder and super duplex stainless steel powder and drying, a composite powder is obtained. Among them, calculated by mass percentage, the super duplex stainless steel powder includes 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%, and the balance is Fe and impurities. Among them, calculated by mass ratio, the mixing ratio of pure Ti powder and the super duplex stainless steel powder is 0.005:1. Calculated by mass percentage, the pure Ti powder includes 8% of the first type of Ti powder with a particle size of ≥10 μm and <22 μm, 35% of the second type of Ti powder with a particle size of ≥22 μm and <35 μm, 45% of the third type of Ti powder with a particle size of ≥35 μm and <55 μm, and 12% of the fourth type of Ti powder with a particle size of ≥55 μm and <65 μm. Calculated by mass percentage, the super duplex stainless steel powder includes 8% of the first type of stainless steel powder with a particle size of ≥10 μm and <22 μm, 35% of the second type of stainless steel powder with a particle size of ≥22 μm and <35 μm, 45% of the third type of stainless steel powder with a particle size of ≥35 μm and <55 μm, and 12% of the fourth type of stainless steel powder with a particle size of ≥55 μm and <65 μm.

[0081] B. The 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, the composite powder is uniformly laid on the surface of the preheated stainless steel substrate, and 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 cools and solidifies to obtain a stainless steel substrate with a preform attached. When the temperature of the forming chamber of the laser powder bed melting equipment cools to 70°C, the stainless steel substrate with the preform attached is taken out. The preform attached to the surface of the stainless steel substrate is wire-cut to obtain the preform. Among them, the parameters of the laser powder bed melting process are: laser power is 250w, scanning speed is 1000mm / s, powder laying thickness is 0.03mm, and scanning spacing is 0.08mm.

[0082] C. The electric furnace is heated from room temperature to 1100°C and maintained at 1100°C. Subsequently, the preform is placed in the electric furnace. After holding for 12 minutes, the preform is taken out and immediately placed in cold water for cooling to obtain a high-strength, high-toughness, corrosion-resistant super duplex stainless steel.

[0083] Example 3

[0084] A. After uniformly mixing pure Ti powder and super-duplex stainless steel powder and drying, a composite powder is obtained. Among them, calculated by mass percentage, the super-duplex stainless steel powder includes 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%, and the balance is Fe and impurities. Among them, calculated by mass ratio, the mixing ratio of pure Ti powder and the super-duplex stainless steel powder is 0.005:1. Calculated by mass percentage, the pure Ti powder includes 15% of a first type of Ti powder with a particle size of ≥10 μm and <22 μm, 40% of a second type of Ti powder with a particle size of ≥22 μm and <35 μm, 35% of a third type of Ti powder with a particle size of ≥35 μm and <55 μm, and 10% of a fourth type of Ti powder with a particle size of ≥55 μm and <65 μm. Calculated by mass percentage, the super-duplex stainless steel powder includes 15% of a first type of stainless steel powder with a particle size of ≥10 μm and <22 μm, 40% of a second type of stainless steel powder with a particle size of ≥22 μm and <35 μm, 35% of a third type of stainless steel powder with a particle size of ≥35 μm and <55 μm, and 10% of a fourth type of stainless steel powder with a particle size of ≥55 μm and <65 μm.

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

[0086] C. The electric furnace is heated from room temperature to 1100°C, and the preform is placed in the electric furnace at 1100°C. After holding for 12 min, the preform is taken out and put into cold water for cooling to obtain a high-strength, high-toughness and 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, except that pure Ti powder is not added in Comparative Example 1, and in step B, only the super-duplex stainless steel powder is additively manufactured by the laser powder bed fusion 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 solid 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 (%) Product of strength and 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] From the performance test results in Table 1, it can be seen that the super duplex stainless steel obtained by the preparation method of the present technical solution has a yield strength of >1100MPa, an elongation at break of ≥45%, a strength-ductility product of 55-60GPa·%, and a self-corrosion potential of -0.13--0.03V. SCE The yield strength is higher; the elongation at break and the strength-ductility product are higher, and the toughness is also higher; the higher the self-corrosion potential is, the higher the corrosion resistance is. Therefore, the super duplex stainless steel obtained by this technical solution has high strength, high toughness and corrosion resistance.

[0095] Since pure Ti powder is not added in Comparative Example 1, TiN nanoparticles cannot be generated, and thus TiN nanoparticles cannot be used as a reinforcement phase to improve the properties of strength, toughness and corrosion resistance, thereby reducing the strength, toughness and corrosion resistance of the obtained super duplex stainless steel.

[0096] In Comparative Example 2, no solid solution treatment was performed, resulting in the super duplex stainless steel retaining only a single ferrite phase, which 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 the N element will be lost; (3) The segregation of Cr and Mo in the single phase will be aggravated, resulting in a cliff-like drop in pitting corrosion resistance (PREN effective value drops from 45 to 28); (4) The continuous ferrite network becomes a rapid crack propagation channel, the stress corrosion sensitivity index increases, and the corrosion resistance decreases. It should be noted that although the single ferrite phase will cause the interstitial solid solution strengthening effect of the N element to be lost, the yield strength test result is higher based on the high strength of ferrite.

[0097] The technical principles of the present invention have been described above in connection with specific embodiments. These descriptions are only for explaining the principles of the present invention and cannot be construed in any way as limiting the scope of protection of the present invention. Based on the explanations herein, those skilled in the art can readily conceive of other specific embodiments of the present invention without creative efforts, and these embodiments will all fall within the scope of protection of the present invention.

Claims

1. A preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel, characterized in that, It includes the following steps: A. After uniformly mixing pure Ti powder and super-duplex stainless steel powder and drying, a composite powder is obtained; wherein, calculated by 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%, and the balance is Fe and impurities; B. Under a nitrogen atmosphere, the composite powder is additively manufactured by a laser powder bed fusion process to obtain a preform; C. The preform is solution-treated to obtain a high-strength, high-toughness and corrosion-resistant super-duplex stainless steel.

2. The preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel according to claim 1, characterized in that, In step A, calculated by mass ratio, the mixing ratio of the pure Ti powder and the super-duplex stainless steel powder is (0.001-0.01):

1.

3. The preparation method of a high-strength, high-toughness, corrosion-resistant super duplex stainless steel according to claim 1, characterized in that In step A, calculated by mass percentage, the pure Ti powder includes 10-15% of a first type of Ti powder with a particle size of ≥10μm and <22μm, 35-45% of a second type of Ti powder with a particle size of ≥22μm and <35μm, 35-45% of a third type of Ti powder with a particle size of ≥35μm and <55μm, and 8-12% of a fourth type of Ti powder with a particle size of ≥55μm and <65μm.

4. The preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel according to claim 1, characterized in that, In step A, calculated by mass percentage, the super-duplex stainless steel powder includes 8-12% of a first type of stainless steel powder with a particle size of ≥10μm and <22μm, 35-45% of a second type of stainless steel powder with a particle size of ≥22μm and <35μm, 35-45% of a third type of stainless steel powder with a particle size of ≥35μm and <55μm, and 8-12% of a fourth type of stainless steel powder with a particle size of ≥55μm and <65μm.

5. The preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel according to claim 1, characterized in that, The method of step B is: In the forming chamber of a laser powder bed fusion device under a nitrogen atmosphere, the composite powder is uniformly laid on the surface of a stainless steel substrate, and the composite powder is melted layer by layer according to the parameters of the laser powder bed fusion process, and the composite powder is cooled and solidified 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 fusion device cools to below 80°C, the stainless steel substrate with the preform attached is taken out; The preform attached to the surface of the stainless steel substrate is peeled off to obtain a preform.

6. The preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel according to claim 5, characterized in that, In step B, the laser power of the laser powder bed fusion process is 230-270w, the scanning speed is 1000-1200mm / s, the powder laying thickness is 0.02-0.04mm, and the scanning spacing is 0.08-0.10mm.

7. The preparation method of a high-strength, high-toughness and corrosion-resistant super duplex stainless steel according to claim 5, characterized in that, In step B, before laying the composite powder, it also includes preheating the stainless steel substrate; The method of the preheating treatment is: preheating the stainless steel substrate at 150-180°C for 5-10min.

8. A 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: heat the electric furnace from room temperature to 1100 - 1200 °C, then place the preform in the furnace, keep it at 1100 - 1200 °C for 8 - 12 minutes, take out the preform and immediately put it into cold water for cooling.

9. A high-strength, high-toughness, corrosion-resistant super duplex stainless steel, characterized in that: Prepared by using the preparation method of the high-strength, high-toughness and corrosion-resistant super duplex stainless steel as described in claims 1 to 8, wherein the yield strength of the high-strength, high-toughness and corrosion-resistant super duplex stainless steel is > 1100 MPa, the elongation at break is ≥ 45%, the product of strength and plasticity is 55 - 60 GPa·%, and the self-corrosion potential is -0.13 - -0.03 V SCE .

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