Preparation method of high-yield-strength duplex stainless steel and duplex stainless steel

Abstract

The invention discloses a preparation method of high-yield-strength duplex stainless steel and the duplex stainless steel, belongs to the technical field of duplex stainless steel manufacturing through powder metallurgy, and solves the problems that when the duplex stainless steel is manufactured through an existing powder metallurgy hot isostatic pressing process, matching of a post-treatment process is unreasonable, harmful phases such as sigma phase are likely to be separated out from grain boundaries, and cost is low. Phase proportion imbalance and grain coarsening coexist, so that the plasticity and toughness of the duplex stainless steel are limited. The preparation method comprises the following steps: alloy smelting and powder preparation; packaging powder and packaging; carrying out furnace cooling after hot isostatic pressing densification; and carrying out high-temperature solid solution heat treatment to obtain the high-yield-strength duplex stainless steel. The high-yield-strength duplex stainless steel is prepared through the method. The high-yield-strength duplex stainless steel is excellent in performance.

Description

Technical Field

[0001] This invention relates to the fields of powder metallurgy and stainless steel material preparation technology, and particularly to a method for preparing high yield strength duplex stainless steel and the duplex stainless steel itself. Background Technology

[0002] Compared to conventional austenitic stainless steels (such as 316L), duplex stainless steels typically possess higher yield strength and superior pitting corrosion resistance. In applications requiring significantly higher strength and corrosion resistance than 316L, but without the need for expensive nickel-based alloys, duplex stainless steels offer the best overall cost-effectiveness. Some parts made from existing duplex stainless steels have complex shapes and can be manufactured using powder metallurgy hot isostatic pressing (PM-HIP), achieving near-net-shape / near-net-shape forming. This allows for the integrated creation of complex structures, significantly improving material utilization and drastically reducing machining and manufacturing cycles, thus making it a crucial direction for manufacturing complex components.

[0003] However, existing PM-HIP methods for preparing duplex stainless steel still face significant challenges: the coexistence of austenite and ferrite phases in duplex stainless steel, coupled with the complex phase transformations and element diffusion during high-temperature holding and cooling processes, particularly during the furnace cooling stage of HIP, easily leads to the precipitation of harmful intermetallic compounds such as σ phases at grain boundaries, resulting in decreased ductility and toughness. Eliminating these harmful phases often requires additional complex heat treatment processes, increasing costs and reducing economic viability. More critically, existing technologies often suffer from densification but struggle to further stabilize and improve yield strength: on the one hand, at lower solution temperatures, the re-dissolution of harmful phases is insufficient and phase proportion reconstruction is inadequate, making it difficult to increase yield strength; on the other hand, while higher solution temperatures facilitate the elimination of precipitated phases, they also result in grain coarsening and structural degradation, leading to unstable strength and ductility. Furthermore, factors such as phase imbalance caused by improper N content and oxygen control system (e.g., Al deoxidation) configuration, and increased sensitivity to inclusions / defects ultimately result in duplex stainless steel prepared by hot isostatic pressing having low yield strength, insufficient elongation, and poor plasticity, making it difficult to meet the comprehensive requirements of key components for high strength and stable reliability. Summary of the Invention

[0004] In view of the above, the present invention aims to provide a method for preparing high yield strength duplex stainless steel and duplex stainless steel, to solve one of the following technical problems: When manufacturing duplex stainless steel by existing powder metallurgy hot isostatic pressing (PM-HIP) process, the post-processing is not properly matched and the solid solution control is insufficient, which easily leads to the precipitation of harmful phases such as σ phase at grain boundaries, phase ratio imbalance and grain coarsening, thereby limiting the plasticity and toughness of duplex stainless steel.

[0005] The objective of this invention is mainly achieved through the following technical solutions: This invention provides a method for preparing high-yield-strength duplex stainless steel, comprising the following steps: Step 1: Alloy smelting and powder preparation; Step 2: Packaging and sealing the powder; Step 3: After hot isostatic pressing densification, the material is cooled in the furnace. Step 4: High-temperature solution heat treatment to obtain high-yield-strength duplex stainless steel.

[0006] Furthermore, in step 1, the particle size of the powder obtained from the powder preparation is controlled to be 53~180μm.

[0007] Furthermore, step 3 includes: placing the encapsulated casing in a hot isostatic pressing (HIP) apparatus, using high-purity argon as the pressure medium and protective atmosphere, heating and pressurizing to a temperature of 1100~1200℃ and a pressure of 100~160MPa, maintaining the temperature and pressure, and then cooling it with the furnace.

[0008] Furthermore, in step 3, the heating rate is 3~20℃ / min.

[0009] Furthermore, in step 3, the temperature and pressure are maintained for 1.5 to 4.5 hours.

[0010] Furthermore, in step 4, the solution heat treatment includes the following steps: heating the billet to 1125~1200℃ and holding it at that temperature, then water quenching it to obtain high yield strength duplex stainless steel.

[0011] Furthermore, in step 4, the heat preservation time t and the plate thickness d are controlled to conform to the following relationship: t = (2~3)d, where the unit of t is min and the unit of d is mm.

[0012] Furthermore, the composition of the high yield strength duplex stainless steel, by mass percentage, includes: C: ≤0.03%, Cr: 24.2%~25.5%, Mo: 3.5%~4.5%, Ni: 6.0%~8.0%, Mn: ≤1.0%, Si: 0.1%~0.5%, N: 0.08%~0.22%, W: 0.01%~0.32%, Cu: 0.05%~0.5%, Al: 0.005%~0.025%, P: ≤0.03%, S: ≤0.01%, O: ≤0.02%, with the balance being Fe and unavoidable impurities.

[0013] Furthermore, in the composition of high yield strength duplex stainless steel, K is controlled to be 0.32~0.39, where K = Ni eq / Cr eq Ni eq =Ni + 30(C + N) + 0.5Mn + 0.3Cu, Cr eq =Cr+Mo+1.5Si+0.5W, where Cr, Ni, Mo, Mn, Si, W, Cu, C, and N represent the mass percentage of the corresponding elements.

[0014] The present invention also provides a high yield strength duplex stainless steel, which is prepared by the above-described preparation method.

[0015] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: The preparation method of high yield strength duplex stainless steel of the present invention includes four stages: powder preparation, encapsulation, hot isostatic pressing densification, and high-temperature solution treatment. By precisely controlling the process parameters of each step, the present invention can achieve solid-state diffusion bonding and pore closure of powder particles under high temperature and high pressure below the melting point of the material, thereby obtaining a dense sintered body with uniform structure and fewer defects. Furthermore, the phase ratio and elemental distribution are effectively controlled by solution water quenching treatment at 1125℃~1200℃, which eliminates brittle precipitates such as σ phase while realizing a ferrite-dominated strengthening path, thereby obtaining a stable high-strength performance window with yield strength as the core indicator.

[0016] The hot isostatic pressing process for high yield strength duplex stainless steel of the present invention has a wide window, and the hot isostatic pressing temperature is applicable from 1125℃ to 1200℃, and the hot isostatic pressing pressure is applicable from 100MPa to 160MPa.

[0017] The high-yield-strength duplex stainless steel of this invention achieves excellent mechanical properties by precisely controlling the contents of Cr, Ni, Mo, N, W, Cu, and Al, and by precisely controlling the synergistic quantitative relationships of different elements. The low N content of this invention reduces the difficulty of powder preparation and improves the control of N, O, and inclusions during the powder preparation process. W, as a solid solution strengthening element, can exert a solid solution strengthening effect even when N content decreases, ensuring that the strength does not decrease. The addition of Al is used for deoxidation during the powder preparation process. Controlling the K value within an appropriate range ensures the formation of a high-ferrite content.

[0018] The high yield strength duplex stainless steel of the present invention has a ferrite + austenite duplex structure, wherein the austenite and ferrite are fine equiaxed grains uniformly dispersed, with continuous grain boundaries and a uniform structure; wherein the volume percentage of austenite is 4%~20%, and the volume percentage of ferrite is 80%~96%; the average grain size of both austenite and ferrite is maintained within 20μm. The above-mentioned fine grain + high ferrite content microstructure characteristics enable the material to achieve a synergistic improvement in ultra-high yield strength and plasticity without relying on large deformation cold working.

[0019] The high yield strength duplex stainless steel of the present invention exhibits excellent properties, such as tensile strength R. m Approximately 800~940MPa, yield strength R p0.2 It has a strength of approximately 590~740MPa, an elongation at break (A) of approximately 18%~30%, and a reduction of area (Z) of approximately 50%~56%.

[0020] Other features and advantages of the invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description, claims, and drawings. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. Figure 1 Powder cross-section metallographic structure of Example 1; Figure 2 The metallographic structure of Example 2; Figure 3 The metallographic structure of Example 4; Figure 4 The metallographic structure of Example 5; Figure 5 This is a drawing of the hot isostatic pressing finished product; Figure 6 The microstructure is shown in Comparative Example 1. Detailed Implementation

[0022] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0023] This invention provides a method for preparing high-yield-strength duplex stainless steel, comprising the following steps: Step 1: Alloy smelting and powder preparation; Step 2: Packaging and sealing the powder; Step 3: Densification by hot isostatic pressing (PM-HIP) followed by furnace cooling; Step 4: High-temperature solution heat treatment to obtain high-yield-strength duplex stainless steel.

[0024] Specifically, in step 1 above, the alloy is melted into molten steel, then prepared into powder, and the target particle size range is obtained by sieving.

[0025] Specifically, in step 1 above, powder preparation can be carried out using VIGA (vacuum induction melting gas atomization), EIGA (electrode induction melting gas atomization), or PREP (plasma rotating electrode atomization) methods.

[0026] It should be noted that in step 1 above, considering that if the powder particle size is too small, the overall O content will be too high, and if it is too large, the packing density will be low, the porosity will be high and the grain size will be too large, resulting in reduced plasticity; therefore, the powder particle size obtained by powder preparation is controlled to be 53~180μm.

[0027] Specifically, in step 1 above, the sieved powder is preferably dried at low temperature (e.g., 80~150℃) under vacuum or inert atmosphere to reduce the risk of adsorbed water and surface oxidation, and is sealed and stored before being loaded into the powder to ensure that the oxygen content of the powder is stable and controlled.

[0028] It should be noted that in step 2 above, a sealed casing matching the shape of the target component is prepared (the casing is welded and has a vacuum / sealing structure). After the powder is loaded into the casing, vacuuming / degassing is performed and sealing is completed to ensure that the casing remains sealed during the subsequent hot isostatic pressing process, so that the powder can complete densification and diffusion metallurgical bonding under isostatic loading.

[0029] Specifically, in step 3 above, the encapsulated casing is placed in a hot isostatic pressing (HIP) apparatus, using high-purity argon as the pressure medium and protective atmosphere, and the temperature and pressure are increased to 1100~1200℃ and 100~160MPa, and the temperature and pressure are maintained for 1.5~4.5h, followed by cooling with the furnace.

[0030] Specifically, in step 3 above, an excessively rapid heating rate will lead to increased temperature difference and thermal stress, as well as uneven densification and metallurgical bonding; an excessively slow heating rate will prolong the residence time in the sensitive temperature zone, causing grain coarsening and precipitation of harmful phases, resulting in internal microcracks. Therefore, the heating rate should be controlled at 3~20℃ / min, for example, 3℃ / min, 10℃ / min, 15℃ / min, or 20℃ / min.

[0031] Specifically, in step 3 above, a simultaneous temperature and pressure loading method is preferred, that is, the temperature and pressure rise together and enter the heat preservation and pressure holding stage after reaching the target temperature and target pressure, so as to improve the uniformity of densification of various parts of the billet; after the heat preservation and pressure holding is completed, furnace cooling is adopted to make the billet obtain a near-net-shape state.

[0032] Specifically, in step 3 above, powder metallurgy hot isostatic pressing (PM-HIP) essentially applies isotropic high pressure and high temperature below the material's melting point, causing powder particles to achieve densification and metallurgical bonding through sintering and diffusion mechanisms, resulting in a high-density near-net-shape billet. Considering that too low a hot isostatic pressing temperature leads to insufficient powder plasticity, resulting in poor density and deteriorated plasticity, and insufficient diffusion driving force and incomplete pore closure between particles, unbonded interfaces and micropore defects are easily left behind, thus reducing the plasticity and reduction of area of ​​the finished product; while too high a temperature is beneficial for rapid densification, it significantly exacerbates two-phase grain coarsening, causing a decrease in yield strength and even performance degradation. Furthermore, too low a pressure makes it difficult to effectively promote pore collapse and interface contact expansion, resulting in insufficient density and increased defect sensitivity; too high a pressure increases cladding / billet deformation and excessive residual stress, and may induce localized inhomogeneity. At the same time, excessively high temperature and high pressure processes have poor applicability, high equipment requirements, and cannot be widely promoted. Therefore, the temperature is controlled at 1100~1200℃, for example 1100℃, 1110℃, 1120℃, 1130℃, 1140℃, 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃; and the pressure is controlled at 100~160MPa, for example 100MPa, 110MPa, 120MPa, 130MPa, 140MPa, 150MPa, 160MPa.

[0033] Specifically, in step 3 above, if the hot isostatic pressing time is too long, the grain size will become significantly coarsened; if the time is too short, sintering will be insufficient, reducing the elongation. Therefore, the holding time should be controlled to be 1.5~4.5h, for example, 1.5h, 2h, 3h, 4h, 4.5h.

[0034] It should be noted that after hot isostatic pressing and furnace cooling, the material undergoes an austenitization-precipitation process and tends to produce brittle precipitates at the grain boundaries. In particular, Cr-rich σ phases may precipitate and form a three-phase structure of "ferrite + austenite + σ phase", which has an adverse effect on ductility and fracture toughness. Therefore, solution water quenching is required to effectively dissolve the precipitates and finally obtain a uniform ferrite-austenite dual-phase structure.

[0035] Specifically, in step 4 above, the surface cladding material of the billet is removed, followed by solution heat treatment to eliminate the brittle grain boundary phase that may precipitate during the furnace cooling stage and obtain a stable two-phase matrix structure, laying the microstructure foundation for ensuring excellent performance.

[0036] Specifically, in step 4 above, the solution heat treatment includes the following steps: heating the billet to 1125~1200℃ and holding it at that temperature, and then water quenching it to obtain high yield strength duplex stainless steel.

[0037] It should be noted that in step 4 above, excessively high solution holding temperatures will promote grain growth, causing the ferrite grain size to continue to increase, weakening the fine-grain strengthening effect, and making it difficult to guarantee the elongation after fracture due to the decrease in austenite volume fraction. If the solution holding temperature is too low, the brittle precipitates such as the σ phase will not be able to fully dissolve, and the redistribution of elements and phase ratio reconstruction between the two phases will be insufficient. The increase in ferrite content will be limited, or the phase ratio will not enter the design range where ferrite is dominant, making it difficult to form a ferrite-dominated yield strength gain path, thus limiting the increase in yield strength. Therefore, the solution holding temperature should be controlled at 1125~1200℃, for example, 1125℃, 1150℃, 1175℃, 1200℃.

[0038] It should be noted that in step 4 above, if the holding time for solid solution is too short, the harmful phase cannot be fully dissolved and the phase ratio cannot be fully balanced; if the holding time for solid solution is extended, it will not cause abnormal growth of grain size, but it will waste energy. Therefore, the holding time t and the plate thickness d should be controlled to conform to the following relationship: t = (2~3)d, where the unit of t is min and the unit of d is mm.

[0039] Furthermore, in step 4 above, the water quenching preferably uses a cooling medium with sufficient flow rate and temperature difference control to ensure the cooling rate, thereby promoting the full re-dissolution of the σ phase and inhibiting the re-precipitation of harmful phases.

[0040] Specifically, the composition of the above-mentioned high yield strength duplex stainless steel, by mass percentage, includes: C: ≤0.03%, Cr: 24.2%~25.5%, Mo: 3.5%~4.5%, Ni: 6.0%~8.0%, Mn: ≤1.0%, Si: 0.1%~0.5%, N: 0.08%~0.22%, W: 0.01%~0.32%, Cu: 0.05%~0.5%, Al: 0.005%~0.025%, P: ≤0.03%, S: ≤0.01%, O: ≤0.02%, with the balance being Fe and unavoidable impurities.

[0041] The following details the function and dosage selection of the components contained in this invention: C: Although the addition of carbon can significantly improve the hardness and strength of steel, due to the high diffusion capacity of carbon, it easily combines with Cr to form M. 23 C6 type carbides mainly precipitate rapidly within a temperature range of 700~900℃ (≤0.5 hours), or precipitate after prolonged holding at 550~700℃. The precipitation of carbides leads to localized Cr depletion, increasing the material's susceptibility to intergranular corrosion and worsening its corrosion resistance. Therefore, this invention comprehensively considers the combined effects of strength and corrosion resistance, controlling the C content to ≤0.03%.

[0042] Cr: The main element that ensures the corrosion resistance of stainless steel, and also a ferrite-forming element. Improper control of Cr content will lead to an imbalance in the phase ratio and reduce strength. Considering all factors, its content should be controlled at 24.2%~25.5%.

[0043] Ni is an element that strongly forms and stabilizes austenite and expands the austenite phase region. Considering that this invention aims to reduce the difficulty of preparing metal powder and achieve the goal of easy powder manufacturing, the Ni content is controlled at 6.0%~8.0%.

[0044] Mn can increase the solubility of N in steel and inhibit the precipitation of harmful phase chromium nitride, but excessive Mn content will reduce the toughness and corrosion resistance of stainless steel. Therefore, this invention controls the Mn content to ≤1.0%.

[0045] Mo: In addition to improving the corrosion resistance of stainless steel in oxidizing media, it also has a good effect on improving the corrosion resistance of stainless steel in reducing media. At the same time, in order to ensure the resistance to pitting corrosion, alloying elements such as Cr and Mo should not be too low. Taking all factors into consideration, the Mo content should be controlled at 3.5%~4.5%.

[0046] Si is added as a deoxidizer during the smelting process, which can achieve a deoxidation effect. However, excessive addition will deteriorate the material's resistance to intergranular corrosion. Therefore, this invention controls the Si content to be between 0.1% and 0.5%.

[0047] N: Like Ni, N is an element that strongly forms and stabilizes austenite and expands the austenite phase region. N can also significantly improve the pitting corrosion resistance of the austenite phase in stainless steel, thereby improving the overall pitting corrosion resistance of stainless steel. Excessive N content will increase the difficulty of manufacturing metal powder. Therefore, this invention controls the N content to be between 0.08% and 0.22% by taking into account the characteristics of subsequent hot isostatic pressing process and the matching of strength and toughness.

[0048] W, as a solid solution strengthening element, can exert a solid solution strengthening effect when nitrogen content is reduced, ensuring that strength is not reduced while increasing corrosion resistance. Therefore, this invention controls the W content to be between 0.01% and 0.32%, taking into account the characteristics of subsequent hot isostatic pressing processes and the matching of strength and toughness.

[0049] Cu: can improve the corrosion resistance of products, reduce oxygen content during metal powder preparation, and improve the quality of metal powder. In this invention, the Cu content is controlled at 0.05%~0.5%.

[0050] Al is a good deoxidizing element in the preparation of metal powders. However, it is important to consider that it can form AlN in the nitrogen-containing environment of the components in this invention, which would lead to a decrease in the ductility and toughness of the final hot isostatic pressed product. Therefore, in order to achieve a good deoxidation effect and avoid harmful effects, the Al content in this invention is controlled at 0.005%~0.025%. S significantly reduces the hot working properties of steel and readily reacts with Mn to form MnS inclusions, reducing the material's resistance to pitting corrosion and crevice corrosion. Therefore, this invention controls its content to ≤0.01%.

[0051] P: It is a harmful impurity element that not only deteriorates the plasticity of the material, but also reduces its corrosion resistance. Therefore, the present invention controls its content to be ≤0.03%.

[0052] Specifically, in order to further optimize the performance of the above-mentioned high yield strength duplex stainless steel, the composition of the above-mentioned high yield strength duplex stainless steel, by mass percentage, includes: C: 0.003%~0.029%, Cr: 24.3%~25.2%, Mo: 3.5%~4.3%, Ni: 6.1%~7.1%, Mn: 0.2%~0.6%, Si: 0.2%~0.45%, N: 0.08%~0.19%, W: 0.05%~0.25%, Cu: 0.1%~0.4%, Al: 0.01%~0.025%, P: ≤0.01%, S: ≤0.01%, O: ≤0.02%, with the balance being Fe and unavoidable impurities.

[0053] Specifically, to ensure the feasibility of powder preparation and to guarantee the balance between corrosion resistance and strength, the composition of the aforementioned high-yield-strength duplex stainless steel should have a value of 100Cr + 330×(Mo + 0.5W) + 1600N of 38 or higher. Here, Cr, Mo, W, and N refer to the mass percentage of the corresponding elements; for example, 24.7% is represented as 0.247. Preferably, the value of 100Cr + 330×(Mo + 0.5W) + 1600N is between 38 and 43.

[0054] Specifically, in this invention, W provides the main solid solution strengthening compensation, while Cu takes into account both powder quality and auxiliary strengthening. To avoid insufficient yield due to low N without compensation, or manufacturing risks introduced by over-alloying, the value of 100N + 25W + 10Cu is controlled to be 0.13~0.23, where W, N, and Cu refer to the mass percentage of the corresponding elements. Preferably, the value of 100N + 25W + 10Cu is 0.13~0.21.

[0055] Specifically, in this invention, in order to achieve a balance between deoxygenation and avoiding the risk of AlN, 100Al > 35O + 0.04 × (100N - 0.09) + 0.005 - Cu is controlled.

[0056] Specifically, in order to characterize the phase ratio stability, an austenite stability index K is introduced in this invention to constrain the relative strength of the "austenite formation tendency / ferrite formation tendency", K=Ni eq / Creq , where Ni eq =Ni + 30(C + N) + 0.5Mn + 0.3Cu, Cr eq =Cr+Mo+1.5Si+0.5W, where the symbols for each element (Cr, Ni, Mo, Mn, Si, W, Cu, C, N) represent the mass percentage of the corresponding element.

[0057] Specifically, Cr eq Ni reflects the overall contribution of ferrite-forming elements. eq Reflecting the comprehensive contribution of austenite-forming elements, this invention controls the composition range and combines it with the solution heat treatment window to keep K within a controllable range that can form a certain amount of austenite without causing excessive austenite content. K is controlled to be 0.32~0.39, thereby obtaining 4%~20% austenite volume fraction in the solution state while maintaining a high ferrite content (see Table 5). This allows the material to obtain an excellent combination of strength and plasticity without relying on large deformation cold working.

[0058] Specifically, the microstructure of the high-yield-strength duplex stainless steel after step 4 is a ferrite + austenite duplex structure, in which austenite and ferrite are uniformly dispersed as fine equiaxed grains, with a generally uniform grain morphology and no obvious preferred orientation; the volume percentage of austenite is 4%~20% (e.g., 4%~12%), and the volume percentage of ferrite is 80%~96% (e.g., 88%~96%); the average grain size of both austenite and ferrite is maintained within 20 μm, for example, the average grain size of austenite is 5.5~8.5 μm, and the average grain size of ferrite is 10~19 μm. This microstructure characteristic of fine grains and high ferrite content allows the material to achieve an excellent combination of strength and ductility without relying on large deformation cold working.

[0059] Specifically, the high yield strength duplex stainless steel treated in step 4 above exhibits excellent properties, such as tensile strength R... m Approximately 800~940MPa, yield strength R p0.2 It has a strength of approximately 590~740MPa, an elongation at break (A) of approximately 18%~30%, and a reduction of area (Z) of approximately 50%~56%.

[0060] The preparation method of high yield strength duplex stainless steel of the present invention includes four stages: powder preparation, encapsulation, hot isostatic pressing densification, and solution heat treatment. By precisely controlling the process parameters of each step, the present invention can achieve solid-state diffusion bonding and pore closure of powder particles under high temperature and high pressure below the melting point of the material, thereby obtaining a dense sintered body with uniform structure and fewer defects. Furthermore, the phase ratio and elemental distribution are effectively controlled by solution water quenching treatment at 1125℃~1200℃, which eliminates brittle precipitates such as σ phase while realizing a ferrite-dominated strengthening path, thereby obtaining a stable high-strength performance window with yield strength as the core indicator.

[0061] The hot isostatic pressing process for high yield strength duplex stainless steel of the present invention has a wide window, and the hot isostatic pressing temperature is applicable from 1125℃ to 1200℃, and the hot isostatic pressing pressure is applicable from 100MPa to 160MPa.

[0062] The high-yield-strength duplex stainless steel of this invention achieves excellent mechanical properties and corrosion resistance by precisely controlling the contents of Cr, Ni, Mo, N, W, Cu, and Al, and by precisely controlling the synergistic quantitative relationships of different elements. The low N content of this invention reduces the difficulty of powder preparation and improves the control of N, O, and inclusions during the powder preparation process. W, as a solid solution strengthening element, can exert a solid solution strengthening effect even when N content decreases, ensuring that the strength does not decrease. The addition of Al is used for deoxidation during the powder preparation process. Controlling the K value within an appropriate range ensures the formation of a high-ferrite content.

[0063] The following specific embodiments and comparative examples demonstrate the advantages of precise control over the composition and process parameters of the high yield strength duplex stainless steel of the present invention.

[0064] Examples 1-6: The embodiments of the present invention provide a high yield strength duplex stainless steel and its preparation method. The chemical composition of the high yield strength duplex stainless steel in Examples 1-6 is shown in Table 1.

[0065] The preparation methods of Examples 1-6 include: Step 1: Alloy smelting and gas atomization powder production; Step 2: Packaging and sealing the powder; Step 3: After hot isostatic pressing densification, the material is cooled in the furnace. Step 4: Solution heat treatment to obtain duplex stainless steel.

[0066] In step 1, after the alloy is melted into molten steel, pre-alloyed powder is prepared by atomization with high-purity argon gas, and powder with a size of 53~180μm is obtained by sieving.

[0067] In step 2, stainless steel is selected as the cladding material to prepare a sealed cladding that matches the shape of the target component (the cladding is welded and has a vacuum / sealing structure). After the atomized powder is loaded into the cladding, vacuuming / degassing is performed and sealing is completed to ensure that the cladding remains sealed during the subsequent hot isostatic pressing process, so that the powder can be densified and diffusion metallurgically bonded under isostatic load.

[0068] In step 3, the encapsulated casing is placed in a hot isostatic pressing (HIP) apparatus, using high-purity argon as the pressure medium and protective atmosphere. The temperature and pressure are increased to 1100~1200℃ and 100~160MPa, and held at these temperatures and pressures for 2.5~3.5 hours, followed by furnace cooling. The heating rate is 7~8℃ / min.

[0069] In step 4, the solution heat treatment includes the following steps: heating the billet to 1125~1200℃ and holding it for 25~35 minutes, and then water quenching to obtain high yield strength duplex stainless steel.

[0070] Table 2 shows the specific process parameters for Examples 1-6, Table 3 shows the mechanical property data, Table 4 shows the grain size information, and Table 5 shows the phase ratio information. Figure 1 Powder cross-section metallographic structure of Example 1; Figure 2 The metallographic structure of Example 2; Figure 3 The metallographic structure of Example 4; Figure 4 The metallographic structure of Example 5; Figure 5 This is a drawing of the hot isostatic pressing finished product.

[0071] The inventors conducted extensive research during the research process, and some suboptimal solutions are presented here as comparative examples.

[0072] Comparative Example 1 This comparative example provides a duplex stainless steel and its preparation method. The composition is shown in Table 1 below. The preparation method is generally consistent with the example, but the solution treatment is adjusted to water quenching at 1000 ℃ for 30 min. In this comparative example, due to the low solution temperature, the σ phase is not completely eliminated, resulting in a low elongation. Figure 6 The microstructure is shown in Comparative Example 1.

[0073] Comparative Example 2 This comparative example provides a duplex stainless steel and its preparation method. The composition is shown in Table 1 below. The preparation method is generally consistent with the example, but water quenching at 1220 ℃ for 30 min is used.

[0074] The nitrogen content in this comparative example is relatively high. High nitrogen significantly stabilizes austenite, and the solid solution phase ratio approaches 1:1, resulting in a yield strength R. p0.2 The pressure dropped to 540 MPa, indicating that the high solution temperature could not offset the phase imbalance caused by the high nitrogen content.

[0075] Comparative Example 3 This comparative example provides a duplex stainless steel and its preparation method. The composition is shown in Table 1 below, and the preparation method is consistent with the example.

[0076] In this comparative example, the Al content was reduced to 0.002%, resulting in an increase in O content to 0.035%. The low Al content in this comparative example led to insufficient oxygen control, which increased the sensitivity to inclusions / defects and reduced the yield strength. This demonstrates that the control of O content in this invention is necessary and effective.

[0077] From the mechanical performance results (R in the table) p0.2 As the core indicator), Examples 1–6 show a clear and adjustable strength window after “PM-HIP densification + solution water quenching”: its solution-treated R p0.2 The strength is 590~740 MPa, while the tensile strength Rm is 800~940 MPa. The strength is improved while maintaining a high level of plasticity, with A=18%~30% and Z=50%~56%.

[0078] The comparative results demonstrate that the components and process parameters of this invention need to be precisely controlled; improper control of components and process parameters will result in performance not meeting requirements.

[0079] Table 1. Main chemical components (%)

[0080] Table 2 Process Parameters

[0081] Table 3 Mechanical Properties

[0082] Table 4 Grain Size

[0083] Table 5 Phase Proportions

[0084] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing high yield strength duplex stainless steel, characterized in that, Includes the following steps: Step 1: Alloy smelting and powder preparation; Step 2: Packaging and sealing the powder; Step 3: After hot isostatic pressing densification, the material is cooled in the furnace. Step 4: High-temperature solution heat treatment to obtain high-yield-strength duplex stainless steel.

2. The method for preparing high yield strength duplex stainless steel according to claim 1, characterized in that, In step 1, the particle size of the powder obtained by powder preparation is controlled to be 53~180μm.

3. The method for preparing high yield strength duplex stainless steel according to claim 1, characterized in that, Step 3 includes: placing the encapsulated casing in a hot isostatic pressing (HIP) apparatus, using high-purity argon as the pressure medium and protective atmosphere, heating and pressurizing to a temperature of 1100~1200℃ and a pressure of 100~160MPa, maintaining the temperature and pressure, and then cooling it with the furnace.

4. The method for preparing high yield strength duplex stainless steel according to claim 3, characterized in that, In step 3, the heating rate is 3~20℃ / min.

5. The method for preparing high yield strength duplex stainless steel according to claim 3, characterized in that, In step 3, the temperature and pressure are maintained for 1.5 to 4.5 hours.

6. The method for preparing high yield strength duplex stainless steel according to claim 1, characterized in that, In step 4, the solution heat treatment includes the following steps: heating the billet to 1125~1200℃ and holding it at that temperature, then water quenching it to obtain high yield strength duplex stainless steel.

7. The method for preparing high yield strength duplex stainless steel according to claim 6, characterized in that, In step 4, the heat preservation time t and the plate thickness d are controlled to conform to the following relationship: t = (2~3)d, where the unit of t is min and the unit of d is mm.

8. The method for preparing high yield strength duplex stainless steel according to claim 1, characterized in that, The high yield strength duplex stainless steel comprises, by mass percentage: C: ≤0.03%, Cr: 24.2%~25.5%, Mo: 3.5%~4.5%, Ni: 6.0%~8.0%, Mn: ≤1.0%, Si: 0.1%~0.5%, N: 0.08%~0.22%, W: 0.01%~0.32%, Cu: 0.05%~0.5%, Al: 0.005%~0.025%, P: ≤0.03%, S: ≤0.01%, O: ≤0.02%, with the balance being Fe and unavoidable impurities.

9. The method for preparing high yield strength duplex stainless steel according to any one of claims 1 to 8, characterized in that, In the composition of the high yield strength duplex stainless steel, K is controlled to be 0.32~0.39, where K = Ni eq / Cr eq Ni eq =Ni + 30(C + N) + 0.5Mn + 0.3Cu, Cr eq =Cr+Mo+1.5Si+0.5W, where Cr, Ni, Mo, Mn, Si, W, Cu, C, and N represent the mass percentage of the corresponding elements.

10. A high yield strength duplex stainless steel, characterized in that, The high yield strength duplex stainless steel is prepared by the preparation method described in any one of claims 1 to 9.

Images