Duplex stainless steel as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of iron-based alloys, and particularly relates to a duplex stainless steel material and a preparation method and application thereof. According to the method, the chemical component content of the duplex stainless steel is precisely regulated and controlled, the cold-rolled steel is rapidly placed in water after being subjected to heat treatment at the temperature of 1050-1140 DEG C, so that the surface temperature is reduced to 120 DEG C or below, and the steel can be effectively promoted to form a stable and uniform duplex structure in the process. Performance tests show that the percentage elongation after fracture of the steel at room temperature reaches 35.3-39.2%, excellent plasticity and toughness are shown, it is ensured that the material is not prone to brittle fracture when stressed, and the deformation requirement under the complex working condition can be better met; the hardness value is 28.3-29.1 HRC, which indicates that the material has higher strength, can bear higher load and pressure, and meets the use requirements in a high-pressure environment; and the corrosion rate is only 0.343-0.771 [mu] m / 48 h in a 65% + / -0.2% nitric acid environment, the extremely high corrosion resistance is highlighted, and long-term stable service can be achieved in a highly corrosive medium.

Description

Technical Field

[0001] This application belongs to the field of iron-based alloy technology, specifically relating to a duplex stainless steel material that combines austenitic and ferritic phases. Background Technology

[0002] To meet the ever-growing global demand for urea, urea production equipment is developing towards larger scale and higher temperature and pressure. Increasing the reaction temperature can significantly accelerate the reaction kinetics, shorten the time required to reach maximum yield, and increase unit capacity; while increasing system pressure helps shift the synthesis reaction equilibrium towards urea production, thereby improving the conversion rate of ammonia and carbon dioxide. This technological approach of improving efficiency and resource utilization by enhancing process conditions has become an important means for the industry to enhance its competitiveness.

[0003] However, this pursuit of high-efficiency production also brings significant engineering challenges. Increased reaction temperatures not only necessitate maintaining higher equilibrium pressures but also significantly accelerate the corrosion rate of equipment materials by high-temperature, high-concentration media, particularly the destructive effects of highly corrosive ammonium carbamate solutions under high temperature and pressure. Simultaneously, the continuous increase in operating pressure places extremely stringent demands on the material strength, fatigue resistance, and sealing reliability of critical pressure-bearing components such as reactors, high-pressure pipelines, and valves. Therefore, in advancing the scaling up and process intensification of urea plants, key technical challenges such as equipment corrosion protection and material selection must be addressed simultaneously to ensure the long-term, safe, and stable operation of the production system.

[0004] Patent application number 202380060752.4 discloses a duplex stainless steel that achieves excellent resistance to intergranular corrosion, especially when used as a welded joint, by optimizing the chemical composition (controlling C ≤ 0.030%, Cr 26.0~28.0%, Ni 6.0~10.0%, Mo 0.20~1.70%, W 2.00~3.00%, N 0.30~0.40% by mass percentage) and strictly controlling the microstructure (average ferrite thickness TF 2.50~4.50μm, sample standard deviation of ferrite thickness ΔTF ≤ 0.50μm, average austenite thickness TA 2.50~4.50μm). However, although existing duplex stainless steels have demonstrated superior strength and corrosion resistance compared to traditional austenitic steels in the urea industry, as production processes become increasingly complex and equipment operating environments become more extreme, there is still a need to further improve the mechanical strength and corrosion resistance of materials to meet the stringent requirements for long-term safe and stable operation under high temperature, high pressure, and highly corrosive media.

[0005] Application content

[0006] This application provides a duplex stainless steel material with high hardness, high elongation after fracture, and good corrosion resistance, which is achieved through the following technical solution:

[0007] A method for preparing duplex stainless steel includes the following steps: cold-rolled steel is heat-treated at 1050~1140℃, then placed in water to reduce the surface temperature of the steel to below 120℃, thereby obtaining duplex stainless steel; the raw materials for preparing the cold-rolled steel, calculated by mass percentage, include: 0.002%≤C≤0.025%, 0<Si≤1.0%, 0<Mn≤2.0%, 0<P≤0.02%, 0<S≤0.02%, 28.0%≤Cr≤31.0%, 6.5%≤Ni≤8.0%, 1.0%≤Mo≤3.0%, 0.30%≤N≤0.45%, 0<Cu≤1.0%, 0<W≤2.0%, 0<Al≤1.0%, 0.01%≤Ce≤0.10%, with the balance being Fe.

[0008] The following details the function and dosage selection of the components in the duplex stainless steel material of this application:

[0009] C: The addition of carbon can significantly improve the strength of steel. If the carbon content is too low, the solid solution strengthening effect is not obvious; however, if the content is too high, carbides are easily precipitated during the manufacturing process, resulting in local chromium depletion at grain boundaries and deteriorating corrosion resistance. Therefore, this application takes into account both the strength and corrosion resistance effects and controls its content to be between 0.002% and 0.025%.

[0010] Si: Silicon is added as a deoxidizer during steel manufacturing. Theoretically, a high content is beneficial for deoxidation, but excessive content will increase the tendency for intermetallic phase precipitation and reduce nitrogen solubility. Therefore, its content is controlled to be ≤1.0% in this application.

[0011] Mn: Manganese is a good deoxidizer, which can increase the solid solubility of nitrogen and can also combine with sulfur to play a role in sulfur fixation. However, excessive content may have a negative impact on the stability of the structure. Therefore, its content is controlled to be ≤2.0% in this application.

[0012] P: Phosphorus is a harmful impurity element that reduces both the plasticity and corrosion resistance of materials, so the lower the phosphorus content, the better. Therefore, in this application, its content is controlled to be ≤0.02%.

[0013] S: Sulfur is a harmful impurity element that significantly reduces the hot workability of materials, easily forms non-metallic inclusions, and affects corrosion resistance. Therefore, its content is controlled to be ≤0.02% in this application.

[0014] Cr: Chromium readily forms a protective passivation film. In ammonia synthesis environments, chromium content is crucial for the corrosion resistance of materials. It is also a ferrite-forming element, requiring a balance between hot workability and corrosion resistance. Therefore, in this application, its content is controlled at 28.0% to 31.0%.

[0015] Ni: Nickel is an austenitizing and stabilizing element that can effectively improve the hot workability of materials and enhance their strength through solid solution strengthening. When coexisting with chromium, it can significantly enhance the corrosion resistance of stainless steel. Therefore, its content is controlled at 6.5% to 8.0% in this application.

[0016] Mo: Molybdenum is a ferrite-forming element that can significantly improve the hardness, strength, and corrosion resistance of stainless steel. However, excessive amounts can lead to the risk of intermetallic phase precipitation. Therefore, its content is controlled within the range of 1.0% to 3.0% in this application.

[0017] Nitrogen (N) is a strong austenite-forming element that influences the distribution of chromium, molybdenum, and nickel in both austenitic and ferrite phases. Higher nitrogen content results in a greater relative proportion of chromium and molybdenum in the austenitic phase, making the austenite more corrosion-resistant while maintaining structural stability. However, nitrogen has limited solubility, and excessively high nitrogen content increases the risk of chromium nitride formation, thus affecting corrosion resistance. Therefore, in this application, its content is controlled at 0.30–0.45%.

[0018] Cu: Copper can improve resistance to general corrosion in acidic environments, but high copper content will reduce resistance to pitting and crevice corrosion. Therefore, its content is controlled to be ≤1.0% in this application.

[0019] W: Tungsten increases resistance to pitting and crevice corrosion, but excessive amounts increase the risk of intermetallic phase precipitation, especially when combined with high Cr and Mo contents. Therefore, its content is controlled to ≤2.0% in this application.

[0020] Al: Aluminum is added as a good deoxidizer during the steel manufacturing process, but excessive aluminum will reduce the purity and toughness of the steel. Therefore, its content is controlled to be ≤1.0% in this application.

[0021] Ce: Cerium, as a rare earth element, can effectively deoxidize, but excessive content significantly affects the weldability of materials. Therefore, its content is controlled at 0.01~0.10% in this application.

[0022] Furthermore, it should be noted that the comprehensive performance of the steel in this application is achieved through heat treatment, and the heat treatment temperature must be strictly controlled. If the temperature is too high, the two-phase ratio will be imbalanced, the interlayer spacing and grain size will be coarse, and the performance will be deteriorated; if the temperature is too low, the residual stress during the cold working process will not be completely eliminated, and the microstructure will be unstable.

[0023] Preferably, the method for preparing the duplex stainless steel material includes the following steps: forging a steel ingot containing the raw materials to obtain a steel billet, wherein the forging ratio is 3.0 to 5.0.

[0024] Preferably, the method for preparing the duplex stainless steel includes the following steps: hot extruding a steel billet to obtain a pre-deformed steel; wherein the temperature of the steel billet during hot extrusion is higher than the recrystallization temperature of the steel billet.

[0025] Preferably, a preheating stage is provided before hot extrusion, wherein the temperature of the billet during the preheating stage is 1020~1100℃, and the temperature of the billet during hot extrusion is 1130~1230℃.

[0026] Preferably, the method for preparing the duplex stainless steel includes the following steps: cold rolling the pre-deformed steel to obtain steel of the target size, wherein the deformation during the cold rolling process is 30-70%.

[0027] Preferably, the process includes the following steps: after heat treatment, the cold-rolled steel is cooled to below 120°C at a cooling rate of more than 30°C / s.

[0028] A duplex stainless steel material is prepared by any of the preparation methods described above.

[0029] Preferably, the duplex stainless steel has a room temperature elongation after fracture of 35.3~39.2% and a hardness of 28.3~29.1 HRC.

[0030] Preferably, the corrosion rate of the duplex stainless steel in a 65% ± 0.2% nitric acid environment is 0.343~0.771 μm / 48h. The test method is based on Method C of ASTM A262, "Standard Method for Determining the Intergranular Corrosion Susceptibility of Austenitic Stainless Steel".

[0031] The duplex stainless steel described in any of the preceding claims is used as a carrier in reactions that produce corrosive products, corrosive byproducts, or corrosive intermediates. Preferably, the duplex stainless steel is used in urea production.

[0032] Compared with the prior art, this application has the following beneficial effects:

[0033] This application achieves this by precisely controlling the chemical composition of duplex stainless steel and rapidly immersing cold-rolled steel in water after heat treatment at 1050~1140℃ to lower the surface temperature to below 120℃. This process effectively promotes the formation of a stable and uniform duplex microstructure. Performance tests show that the elongation at room temperature reaches 35.3~39.2%, exhibiting excellent plasticity and toughness, ensuring that the material is not prone to brittle fracture under stress and can better adapt to deformation requirements under complex working conditions. The hardness value is 28.3~29.1HRC, indicating that the material has high strength and can withstand large loads and pressures, meeting the requirements for use in high-pressure environments. In a 65%±0.2% nitric acid environment, the corrosion rate is only 0.343~0.771μm / 48h, highlighting extremely strong corrosion resistance and enabling long-term stable service in highly corrosive media. This duplex stainless steel, with its stable structure, high toughness, and excellent corrosion resistance, perfectly meets the stringent requirements of high-temperature, high-pressure, and highly corrosive environments such as urea production. It not only ensures the long-term safe and stable operation of equipment but also improves the service life and operating efficiency of the equipment.

[0034] This application, by limiting the forging ratio to a range of not less than 3.0 during the forging process, forces the coarse casting structure inside the steel ingot to break down and reconstruct through sufficient plastic deformation, resulting in a more uniform element distribution and avoiding performance fluctuations caused by component segregation. Simultaneously, it promotes a more reasonable ratio between the two phases and a more regular interlayer spacing. This homogenized internal structure not only reduces stress concentration within the material but also ensures consistency of mechanical properties (such as strength and toughness) across different parts. Furthermore, it enhances the uniformity of corrosion-resistant media adhesion to the material surface, reducing localized corrosion and thus further improving the material's mechanical and corrosion resistance properties.

[0035] This application demonstrates that preheating before hot extrusion effectively softens the steel billet, reduces its deformation resistance, and improves its plasticity, making the billet easier to shape during subsequent hot extrusion and reducing defects such as cracks caused by excessive deformation resistance. Simultaneously, preheating also reduces the power and strength requirements of the extrusion equipment, extends its service life, lowers production energy consumption and equipment maintenance costs, and improves production stability and efficiency.

[0036] This application uses a hot extrusion temperature higher than the recrystallization temperature. Within this temperature range, the grains inside the material undergo dynamic recrystallization, significantly reducing deformation resistance and making it easier to form according to the mold shape, thus reducing energy consumption during the forming process. After hot extrusion, the grains in the material structure are severely broken and sufficiently refined, resulting in a rough tube with uniform properties and structure, laying a good foundation for subsequent processing. Furthermore, controlling the hot extrusion temperature between 1130 and 1230℃ precisely avoids the drawbacks of excessively low or high temperatures: when the temperature is below 1130℃, the material's resistance to hot deformation is high, easily leading to stalling and affecting production continuity; when the temperature is above 1230℃, although the material's resistance to hot deformation decreases, its plasticity is greatly reduced, easily causing the tube to crack. This temperature range can reduce deformation resistance while ensuring the material's plasticity, ensuring stable quality of the rough tube.

[0037] This application controls the deformation during cold rolling to 30-70%, which can not only enable the material to obtain higher strength through work hardening, but also promote a finer and more uniform microstructure. It avoids problems such as insufficient microstructure and insufficient performance improvement caused by too small deformation, as well as material embrittlement and cracking caused by too large deformation. Thus, while ensuring the dimensional accuracy of the material, its mechanical properties are further optimized.

[0038] This application describes a rapid cooling method that reduces the temperature of cold-rolled steel to below 120°C after heat treatment at a rate greater than 30°C / s. This rapid cooling effectively suppresses the precipitation of harmful phases such as the σ phase and Cr2N. The precipitation of the σ phase leads to a sharp decrease in the material's toughness, while the precipitation of Cr2N causes chromium depletion near the grain boundaries, reducing the material's corrosion resistance. Rapid cooling stabilizes the material's microstructure in an ideal two-phase structure, thus avoiding the decline in mechanical properties and corrosion resistance caused by the precipitation of harmful phases, ensuring excellent and stable performance of the final product. Attached Figure Description

[0039] The attached diagram will be briefly described below:

[0040] Figure 1 This is a microstructure diagram of the finished tube used for ferrite content testing in Example 1 of this application.

[0041] Figure 2 This is a microstructure diagram of the finished tube used for ferrite content testing in Example 2 of this application. Detailed Implementation

[0042] The present application will now be further described by way of specific embodiments. Those skilled in the art will be able to implement the present application based on these descriptions. Furthermore, the embodiments of the present application described below are generally only a part of the embodiments of the present application, and not all of the embodiments. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present application without inventive effort should fall within the scope of protection of the present application.

[0043] Example 1

[0044] A method for preparing a seamless duplex stainless steel pipe with a specification of Φ31×2.5mm for synthetic ammonia includes the following steps:

[0045] S1. Smelting:

[0046] Using molten iron as the main raw material, molten steel is smelted to meet the composition requirements of the super duplex stainless steel seamless pipe for synthetic ammonia environment. That is, the composition of the molten steel reaches the following proportions, by weight percentage: C: 0.011%, Si: 0.38%, Mn: 1.00%, P: 0.013%, S: 0.004%, Cr: 28.54%, Ni: 6.69%, Mo: 2.21%, N: 0.34%, Cu: 0.32%, W: 0.80%, Al: 0.83%, Ce: 0.013%, with the balance being Fe and other unavoidable impurities.

[0047] S2. Making tube blanks:

[0048] Molten steel is smelted into steel ingots using the die casting method. The steel ingots are then forged. The forged round tube blanks are straightened, peeled, subjected to non-destructive testing, physical and chemical testing, and dimensional testing to obtain round tube blanks (diameter 204mm) with qualified physical and chemical properties. The forging ratio of the entire process is 3.5.

[0049] S3. Hot extrusion blanking:

[0050] S31. The round tube blank is cut, drilled, chamfered, and cleaned.

[0051] S32. The cleaned round tube blank is preheated in an annular furnace at a temperature of 1060℃ for 3.5 hours.

[0052] S33. The round tube billet exiting the ring furnace is subjected to induction heating once at a temperature of 1150℃ for 7 minutes.

[0053] S34. Expand the hole of the round tube blank after one induction heating, and then use glass powder to lubricate the inner surface of the round tube blank after the hole expansion.

[0054] S35. The expanded round tube billet is sent into a secondary furnace for induction heating at a temperature of 1161℃ for 7 minutes.

[0055] S36. After the secondary induction heating, the round tube blank is lubricated on the outer surface with glass powder, and then transferred to the extruder for hot extrusion. The hot-extruded tube blank is quickly put into water.

[0056] S37. The extruded tube blank is straightened, cut, and inspected to obtain a hot-extruded tube with a specification of Φ82×8.2mm.

[0057] S4, Cold-rolled:

[0058] The extruded rough tube is cold rolled and degreased to obtain an intermediate tube of Φ31×2.5mm. The cold deformation process route is Φ80×8.2mm→Φ45×4.5mm→Φ31×2.5mm.

[0059] S5, Heat Treatment

[0060] S51. Heat treatment is performed on the intermediate pipe Φ31×2.5mm at a temperature of 1100℃ and a holding time of 5min. After completion, the pipe is quickly immersed in water to cool it to below 120℃.

[0061] S52. After heat treatment, the steel pipe is straightened, pickled, cut, and inspected in sequence to obtain ammonia duplex stainless steel seamless pipe.

[0062] Example 2

[0063] It is basically the same as Example 1, except that:

[0064] S1. Smelting:

[0065] Using molten iron as the main raw material, molten steel is smelted to meet the composition requirements of the super duplex stainless steel seamless pipe for synthetic ammonia environment. That is, the composition of the molten steel reaches the following proportions, by weight percentage: C: 0.018%, Si: 0.52%, Mn: 0.88%, P: 0.011%, S: 0.008%, Cr: 29.27%, Ni: 6.84%, Mo: 2.24%, N: 0.38%, Cu: 0.40%, W: 0.75%, Al: 0.64%, Ce: 0.023%, with the balance being Fe and other unavoidable impurities.

[0066] Example 3

[0067] It is basically the same as Example 1, except that:

[0068] S1. Smelting:

[0069] Using molten iron as the main raw material, molten steel is smelted to meet the composition requirements of the super duplex stainless steel seamless pipe for synthetic ammonia environment. Specifically, the composition of the molten steel reaches the following proportions by weight percentage: C: 0.015%, Si: 0.53%, Mn: 0.96%, P: 0.009%, S: 0.006%, Cr: 28.85%, Ni: 7.02%, Mo: 1.93%, N: 0.42%, Cu: 0.38%, W: 0.96%, Al: 0.67%, Ce: 0.017%, with the balance being Fe and other unavoidable impurities.

[0070] Example 4

[0071] It is basically the same as Example 1, except that:

[0072] S2. Making tube blanks:

[0073] Molten steel is smelted into steel ingots using the die casting method, and then the steel ingots are forged. The forged round tube blanks are straightened, peeled, subjected to non-destructive testing, physical and chemical testing, and dimensional testing to obtain round tube blanks (diameter 204mm) with qualified physical and chemical properties. The forging ratio of the whole process is 4.3.

[0074] Example 5

[0075] It is basically the same as Example 1, except that:

[0076] S3. Hot extrusion blanking:

[0077] S31. The round tube blank is cut, drilled, chamfered, and cleaned.

[0078] S32. The cleaned round tube blank is preheated in an annular furnace at a temperature of 1070℃ for 3 hours.

[0079] S33. The round tube billet exiting the ring furnace is subjected to induction heating once at a temperature of 1150℃ for 7 minutes.

[0080] S34. Expand the hole of the round tube blank after one induction heating, and then use glass powder to lubricate the inner surface of the round tube blank after the hole expansion.

[0081] S35. The expanded round tube billet is sent into a secondary furnace for induction heating at a temperature of 1183℃ for 6 minutes.

[0082] S36. After the secondary induction heating, the round tube blank is lubricated on the outer surface with glass powder, and then transferred to the extruder for hot extrusion. The hot-extruded tube blank is quickly put into water.

[0083] S37. The extruded tube blank is straightened, cut, and inspected to obtain a hot-extruded tube with a specification of Φ82×8.2mm.

[0084] Example 6

[0085] It is basically the same as Example 1, except that:

[0086] S3. Hot extrusion blanking:

[0087] S31. The round tube blank is cut, drilled, chamfered, and cleaned.

[0088] S32. The cleaned round tube blank is preheated in an annular furnace at a temperature of 1050℃ for 3 hours.

[0089] S33. The round tube billet exiting the ring furnace is subjected to induction heating once at a temperature of 1140℃ for 7 minutes.

[0090] S34. Expand the hole of the round tube blank after one induction heating, and then use glass powder to lubricate the inner surface of the round tube blank after the hole expansion.

[0091] S35. The expanded round tube blank is sent into a secondary furnace for induction heating at a temperature of 1200℃ for 6 minutes.

[0092] S36. After the secondary induction heating, the round tube blank is lubricated on the outer surface with glass powder, and then transferred to the extruder for hot extrusion. The hot-extruded tube blank is quickly put into water.

[0093] S37. The extruded tube blank is straightened, cut, and inspected to obtain a hot-extruded tube with a specification of Φ82×8.2mm.

[0094] Example 7

[0095] It is basically the same as Example 1, except that:

[0096] S4, Cold-rolled:

[0097] The extruded rough tube is cold rolled and degreased to obtain an intermediate tube with a diameter of Φ31×2.5mm. The cold deformation process route is Φ80×7mm→Φ43×4.1mm→Φ31×2.5mm.

[0098] Example 8

[0099] It is basically the same as Example 1, except that:

[0100] S5, Heat Treatment

[0101] S51. Heat treatment is performed on the intermediate pipe Φ31×2.5mm at a temperature of 1060℃ and a holding time of 10min. After completion, the pipe is quickly immersed in water to cool it to below 120℃.

[0102] S52. After heat treatment, the steel pipe is straightened, pickled, cut, and inspected in sequence to obtain ammonia duplex stainless steel seamless pipe.

[0103] Example 9

[0104] It is basically the same as Example 1, except that:

[0105] S5, Heat Treatment

[0106] S51. Heat treatment is performed on the intermediate pipe Φ31×2.5mm at a temperature of 1130℃ and a holding time of 5min. After completion, the pipe is quickly immersed in water to cool it to below 120℃.

[0107] S52. After heat treatment, the steel pipe is straightened, pickled, cut, and inspected in sequence to obtain ammonia duplex stainless steel seamless pipe.

[0108] Comparative Example 1

[0109] It is basically the same as Example 1, except that:

[0110] S1. Smelting:

[0111] Using molten iron as the main raw material, molten steel is smelted to meet the composition of the synthetic ammonia duplex stainless steel seamless pipe, that is, the composition of the molten steel reaches the following proportions, by weight percentage: C: 0.011%, Si: 0.43%, Mn: 1.02%, P: 0.009%, S: 0.006%, Cr: 27.36%, Ni: 7.11%, Mo: 2.14%, N: 0.34%, Cu: 0.36%, W: 0.76%, Al: 0.81%, Ce: 0.012%, with the balance being Fe and other unavoidable impurities.

[0112] Comparative Example 2

[0113] It is basically the same as Example 1, except that:

[0114] S1. Smelting:

[0115] Using molten iron as the main raw material, molten steel is smelted to meet the composition of the synthetic ammonia duplex stainless steel seamless pipe, that is, the composition of the molten steel reaches the following proportions, by weight percentage: C: 0.009%, Si: 0.36%, Mn: 1.04%, P: 0.014%, S: 0.005%, Cr: 28.63%, Ni: 6.82%, Mo: 2.10%, N: 0.28%, Cu: 0.38%, W: 0.93%, Al: 0.78%, Ce: 0.015%, with the balance being Fe and other unavoidable impurities.

[0116] Comparative Example 3

[0117] It is basically the same as Example 1, except that:

[0118] S1. Smelting:

[0119] Using molten iron as the main raw material, molten steel is smelted to meet the composition of the synthetic ammonia duplex stainless steel seamless pipe, that is, the composition of the molten steel reaches the following proportions, by weight percentage: C: 0.012%, Si: 0.55%, Mn: 1.14%, P: 0.012%, S: 0.007%, Cr: 28.58%, Ni: 6.84%, Mo: 1.94%, N: 0.33%, Cu: 0.41%, W: 1.01%, Al: 0.13%, Ce: 0.014%, with the balance being Fe and other unavoidable impurities.

[0120] Comparative Example 4

[0121] It is basically the same as Example 1, except that:

[0122] S1. Smelting:

[0123] Using molten iron as the main raw material, molten steel is smelted to meet the composition of the synthetic ammonia duplex stainless steel seamless pipe, that is, the composition of the molten steel reaches the following proportions, by weight percentage: C: 0.011%, Si: 0.36%, Mn: 0.93%, P: 0.015%, S: 0.005%, Cr: 28.54%, Ni: 6.92%, Mo: 2.06%, N: 0.33%, Cu: 0.42%, W: 0.87%, Al: 1.08%, Ce: 0.011%, with the balance being Fe and other unavoidable impurities.

[0124] Comparative Example 5

[0125] It is basically the same as Example 1, except that:

[0126] S5, Heat Treatment

[0127] S51. Heat treatment is performed on the intermediate pipe Φ31×2.5mm at a temperature of 1030℃ and a holding time of 5min. After completion, the pipe is quickly immersed in water to cool it to below 120℃.

[0128] S52. After heat treatment, the steel pipe is straightened, pickled, cut, and inspected in sequence to obtain ammonia duplex stainless steel seamless pipe.

[0129] Comparative Example 6

[0130] It is basically the same as Example 1, except that:

[0131] S5, Heat Treatment

[0132] S51. Heat treatment is performed on the intermediate pipe Φ31×2.5mm at a temperature of 1160℃ and a holding time of 5min. After completion, the pipe is quickly immersed in water to cool it to below 120℃.

[0133] S52. After heat treatment, the steel pipe is straightened, pickled, cut, and inspected in sequence to obtain ammonia duplex stainless steel seamless pipe.

[0134] Comparative Example 7

[0135] It is basically the same as Example 1, except that:

[0136] S5, Heat Treatment

[0137] S51. Heat-treat the intermediate tube Φ31×2.5mm at a temperature of 1100℃ for 5 minutes, and then air-cool it to room temperature.

[0138] S52. After heat treatment, the steel pipe is straightened, pickled, cut, and inspected in sequence to obtain ammonia duplex stainless steel seamless pipe.

[0139] Performance testing

[0140] The super duplex stainless steel seamless pipes for synthetic ammonia environment prepared in Examples 1-9 and Comparative Examples 1-7 were sampled and their mechanical properties were tested. The test results are shown in Tables 1-3.

[0141] Samples of the super duplex stainless steel seamless pipes for synthetic ammonia environment prepared in Examples 1-9 and Comparative Examples 1-7 were taken and subjected to metallographic analysis (including ferrite content, harmful phases and non-metallic inclusions). The test results are shown in Table 4.

[0142] Samples of the super duplex stainless steel seamless pipes for synthetic ammonia environment prepared in Examples 1-9 and Comparative Examples 1-7 were taken and tested for resistance to nitric acid corrosion. Detailed requirements are shown in Table 5, and test results are shown in Table 6.

[0143] Table 1: Room temperature tensile properties of Examples 1-9 and Comparative Examples 1-7

[0144]

[0145] Table 2: High-temperature tensile properties of Examples 1-9 and Comparative Examples 1-7

[0146]

[0147] Table 3: Hardness test results (HRC) of Examples 1-9 and Comparative Examples 1-7

[0148]

[0149] Table 4: Metallographic structure test results of Examples 1-9 and Comparative Examples 1-7

[0150]

[0151] Table 5: Test conditions for the resistance to nitric acid corrosion in Examples 1-9 and Comparative Examples 1-7

[0152]

[0153] Table 6: Test results of nitric acid corrosion resistance of Examples 1-9 and Comparative Examples 1-7

[0154]

[0155] As can be seen from Tables 1-4 and 6, the super duplex stainless steel seamless pipe for synthetic ammonia environments prepared by this application through controlling the content of chemical components and optimizing the preparation process exhibits good strength and toughness, stable performance and microstructure, and excellent corrosion resistance. Comparing the nitric acid corrosion resistance test results of Examples 1-9 and Comparative Example 1, it is evident that the Cr element content has a significant effect on improving the corrosion resistance of the material. Comparing the room temperature tensile results of Examples 1-9 and Comparative Example 2, it is evident that the N content has a significant impact on strengthening the mechanical properties of the material. Comparing the performance test results of Examples 1-9 and Comparative Examples 3 and 4, it is evident that the Al content has a significant impact on the microstructure stability of the material. Low Al content results in excessively high oxygen content and high content of oxide inclusions (Type D and Type DS). Excessively high Al content leads to a large amount of Al2O3 inclusions (Type B), resulting in poor corrosion resistance.

Claims

1. A method for preparing duplex stainless steel, characterized in that, Includes the following steps: After heat treatment at 1050~1140℃, cold-rolled steel is placed in water to reduce the surface temperature of the steel to below 120℃, thus obtaining duplex stainless steel. The raw materials for preparing the cold-rolled steel, calculated by mass percentage, include: 0.002%≤C≤0.025%, 0<Si≤1.0%, 0<Mn≤2.0%, 0<P≤0.02%, 0<S≤0.02%, 28.0%≤Cr≤31.0%, 6.5%≤Ni≤8.0%, 1.0%≤Mo≤3.0%, 0.30%≤N≤0.45%, 0<Cu≤1.0%, 0<W≤2.0%, 0<Al≤1.0%, 0.01%≤Ce≤0.10%, with the balance being Fe.

2. The method for preparing duplex stainless steel according to claim 1, characterized in that, The process includes the following steps: forging a steel ingot containing the raw materials to obtain a steel billet, wherein the forging ratio is 3.0 to 5.

0.

3. The method for preparing duplex stainless steel according to claim 2, characterized in that, Includes the following steps: A steel billet is hot-extruded to obtain a pre-deformed steel product; the temperature of the steel billet during hot extrusion is higher than the recrystallization temperature of the steel billet.

4. The method for preparing duplex stainless steel according to claim 3, characterized in that, A preheating stage is also provided before hot extrusion, wherein the temperature of the billet in the preheating stage is 1020~1100℃, and the temperature of the billet during hot extrusion is 1130~1230℃.

5. The method for preparing duplex stainless steel according to claim 3, characterized in that, Includes the following steps: The pre-deformed steel is cold-rolled to obtain steel of the target size, wherein the deformation during the cold rolling process is 30-70%.

6. The method for preparing duplex stainless steel according to claim 1, characterized in that, Includes the following steps: After heat treatment, the cold-rolled steel is cooled to below 120°C at a cooling rate of more than 30°C / s.

7. A duplex stainless steel material, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 6.

8. A duplex stainless steel material according to claim 7, characterized in that, The duplex stainless steel has a room temperature elongation after fracture of 35.3~39.2% and a hardness of 28.3~29.1 HRC.

9. A duplex stainless steel material according to claim 8, characterized in that, The corrosion rate of the duplex stainless steel in a 65%±0.2% nitric acid environment is 0.343~0.771μm / 48h.

10. The duplex stainless steel material according to any one of claims 7 to 9 is used as a carrier in reactions that produce corrosive products or corrosive byproducts or corrosive intermediates.

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