A preparation method of a cast 5A duplex stainless steel and its large castings

Abstract

The present invention belongs to the technical field of casting duplex stainless steel, and discloses a method for preparing a cast 5A duplex stainless steel and its large castings. The contents of Cr and Mo elements are controlled at the lower limit of the 5A steel standard, and at the same time, elements such as W, Nb, Ce, and La for refining the microstructure and microalloy strengthening are added. Through the coupling effect of these four elements, the temperature range for the precipitation of brittle phases (σ phase) can be avoided, and the precipitation of brittle phases in large castings related to pump bodies during slow cooling can be avoided, fundamentally solving the crack defects in the casting process of large castings. The finished product rate of the castings can be increased to more than 95%. At the same time, with the improvement of the technological yield rate and the implementation of cold box opening and shakeout, the casting production cost is greatly reduced, the safety risk in the production process is greatly reduced, and the labor intensity of workers is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal smelting, and particularly relates to a method for preparing a cast 5A duplex stainless steel and its large castings. Background Art

[0002] At present, fast reactors are the preferred reactor types for the fourth-generation advanced nuclear energy systems in the world. The advantage is that they can directly utilize the abandoned uranium isotopes. For example, the 600MW fast reactor circulating water pump system uses seawater as the heat transfer medium to cool the tertiary loop light water. These pump systems, including pump bodies, valves, impellers and other components, have the characteristics of large size (up to dozens of meters), complex shapes, and large differences in cross-sectional dimensions of parts. It is difficult to produce them by machining methods, and generally, casting methods are used to achieve integral forming. In addition, due to the strong corrosiveness of seawater, the pump system also faces problems such as fluid corrosion, erosion-corrosion and cavitation corrosion. Therefore, the materials of the pump body-related components are required to have high strength, high density, no casting defects, and strong chloride corrosion resistance and fluid erosion-corrosion resistance.

[0003] Super duplex stainless steel, as the third-generation duplex stainless steel, is characterized by ultra-low carbon, high chromium, high nickel, high molybdenum and high nitrogen. Its excellent mechanical properties and corrosion resistance make super duplex stainless steel an ideal choice for applications in harsh working conditions. Among them, ASTM A890 5A, as a representative of the third-generation duplex stainless steel, has a composition range of: C≤0.030%, Si≤1.0%, Mn≤1.5%, P≤0.040%, S≤0.040%, Cr: 24.0 - 26.0%, Ni: 6.0 - 8.0%, Mo: 4.0 - 5.0%, N: 0.10 - 0.30%, and has excellent comprehensive mechanical properties and cost performance.

[0004] However, with the increase in alloy content, the casting forming difficulty of 5A super duplex stainless steel has increased significantly. During the production process, brittle second phases represented by σ phase are likely to be generated in the 600 - 950°C stage, and the brittle decomposition of ferrite in the 400 - 500°C stage can easily lead to cracking of large castings. In addition, the casting structure is complex, there are many casting hot spots, the wall thickness varies greatly in local parts, and uneven wall thickness is likely to cause stress concentration, and casting defects such as shrinkage porosity and gas holes are likely to occur. The overall casting process is extremely difficult, and the scrap rate of large castings is as high as 70 - 90% or more.

[0005] The existing preparation process for casting of 5A duplex stainless steel is generally: smelting molten steel with standard composition through medium frequency induction furnace + AOD / VOD dual melting process, then adjusting the temperature of molten steel to 1550-1580℃ for pouring sand mold, waiting for the molten steel to solidify and the sand mold to cool to about 900-1000℃, hot opening and sand removal operation and cutting of the pouring system, and then quickly hoisting the casting for water cooling or cold water spraying to room temperature, and finally adjusting the duplex structure of the casting through 1120-1150℃ solid solution treatment and water cooling. However, hot opening and sand removal of large castings is very dangerous, with high work intensity and low production efficiency. Even so, the scrap rate of large castings is still as high as more than 50%.

[0006] The current mainstream solution focuses on the optimization design of sand casting and cooling systems, such as CN110242781A, which uses optimized sand cores and appropriate cold iron partitions to avoid macroscopic internal stress caused by huge differences in cooling rates in different parts of the parts. There are also considerations of adding a small amount of pure rare earth Ce (CN110242781A) or mixed rare earth elements La, Ce alloy (CN112410675A) to the basic alloy composition to try to obtain better mechanical properties, but all these improved casting processes still require hot unpacking operations, and do not fundamentally solve the problem of brittle cracking caused by the precipitation of a large amount of σ phase during the cooling process of the casting structure. Moreover, previous technologies did not consider the mutual coupling and proportional matching between the added alloy elements.

[0007] In summary, in order to better improve the safe production of large castings of 5A duplex stainless steel, reduce production costs, and increase the yield of large castings, it is urgently necessary to develop a 5A duplex stainless steel suitable for casting and a preparation method that does not require heat unpacking and prevents casting cracking. Summary of the invention

[0008] In order to solve the above technical problems, the object of the present invention is to provide a method for preparing cast 5A duplex stainless steel and large castings thereof, which can not only reduce the casting defects of 5A duplex stainless steel large castings and reduce the scrap rate, but also improve the output rate and finished product rate of large castings, improve production efficiency, and reduce production costs and safety risks.

[0009] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is as follows:

[0010] In the first aspect of the present invention, a cast 5A duplex stainless steel is proposed, comprising the following components: calculated by weight percentage, C ≤ 0.03%, Si ≤ 1%, Mn ≤ 1.5%, P ≤ 0.04%, S ≤ 0.02%, Cr 24.0 - 24.5%, Ni 6.0 - 7.0%, Mo 4.0 - 4.5%, N 0.20 - 0.30%, W 0.20 - 0.30%, Nb 0.20 - 0.30%, Re 0.02 - 0.035%, and the balance is Fe, wherein the PREN value: %Cr + 3.3%Mo + 16%N ≥ 40.

[0011] Preferably, the mass ratio of W to Nb is 1:1.

[0012] Preferably, Re is a La + Ce mixed rare earth, wherein the weight ratio of La to Ce is 1:(4 - 5).

[0013] In the second aspect of the present invention, a method for preparing a large casting of a cast 5A duplex stainless steel is proposed, comprising:

[0014] S1. Prepare raw materials according to the mass percentages of various elements in the cast 5A duplex stainless steel. Melting the remaining raw materials except the rare earth ferrosilicon alloy through an intermediate frequency induction circuit to obtain molten steel. Detect the alloy composition of the molten steel by a spectrometer and adjust according to the detection results until the alloy composition meets the designed composition requirements;

[0015] S2. Purify the molten steel through an AOD double refining process;

[0016] S3. Add rare earth elements to the molten steel before tapping the molten steel or in the ladle, and pour the molten steel into a sand mold;

[0017] S4. Air-cool the molten steel to room temperature, open the box to shake out the sand, cut off the gating system, and take out the casting;

[0018] S5. Transfer the casting to a heat treatment furnace for solution treatment;

[0019] S6. After the heat treatment is completed, cool the furnace to 1045 - 1080 °C and then water-cool the casting when it is taken out of the furnace.

[0020] Preferably, in step S1, the melting temperature is 1650 - 1680 °C.

[0021] Preferably, in step S1, the W and Nb elements are added in the form of a W-Nb master alloy.

[0022] Preferably, in step S3, it is pressed into the molten steel in the form of a rare earth ferrosilicon alloy, and the mass percentage of rare earth in the rare earth ferrosilicon alloy is 25 - 30%.

[0023] Preferably, in step S3, the pouring temperature is 1530-1580°C.

[0024] Preferably, in step S5, the solution treatment comprises: placing the casting into a heat treatment furnace at a furnace temperature of 1000°C, keeping the temperature for 30 minutes, heating the casting to 1130-1160°C at a rate of 80-150°C / h, and keeping the temperature for 3-3.5 hours.

[0025] Preferably, in step S6, the water cooling method is circulating water cooling, wherein the circulating water temperature is ≤80°C.

[0026] Beneficial effects:

[0027] The present invention, through the composition design, can completely avoid the precipitation of intermetallic σ phase when the 5A duplex stainless steel is slowly cooled from the liquid phase to room temperature, and the casting structure is fine, which significantly reduces the structure and macro stress of large castings, improves the cracking tendency of large castings, and the casting yield rate can be increased to more than 95%;

[0028] Since the casting process of the present invention does not require hot box sand dropping and high temperature cutting and pouring system, the safety risk of the production process is greatly reduced and the labor intensity of workers is reduced. With the improvement of process yield and the implementation of hot charging furnace process, the casting production efficiency is greatly improved and the cost is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The metallographic structure of the casting after slow cooling in Example 1 of the present invention;

[0030] Figure 2 The scanning electron microstructure of the casting after slow cooling in Example 1 of the present invention;

[0031] Figure 3 The metallographic structure of the casting after solution treatment in Example 1 of the present invention;

[0032] Figure 4 The metallographic structure of the casting after slow cooling in Example 2 of the present invention;

[0033] Figure 5 The scanning electron microstructure of the casting after slow cooling in Example 2 of the present invention;

[0034] Figure 6 The metallographic structure of the casting after slow cooling in Comparative Example 1 of the present invention;

[0035] Figure 7 This is the metallographic structure of the casting after slow cooling in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will describe the specific embodiments of the present invention with reference to the accompanying drawings. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings, and other embodiments can also be obtained.

[0037] The present invention provides a cast 5A duplex stainless steel, comprising the following components: calculated by weight percentage, carbon (C) ≤ 0.03%, silicon (Si) ≤ 1%, manganese (Mn) ≤ 1.5%, phosphorus (P) ≤ 0.04%, sulfur (S) ≤ 0.02%, chromium (Cr) is 24.0 - 24.5%, nickel (Ni) is 6.0 - 7.0%, molybdenum (Mo) is 4.0 - 4.5%, nitrogen (N) is 0.20 - 0.30%, tungsten (W) is 0.20 - 0.30%, niobium (Nb) is 0.20 - 0.30%, rare earth element (Re) is 0.02 - 0.035%, and the balance is iron (Fe), wherein the PREN value: %Cr + 3.3%Mo + 16%N ≥ 40.

[0038] Preferably, the mass ratio of W to Nb is 1:1. More preferably, the elements W and Nb are added in the form of a W-Nb master alloy.

[0039] Preferably, Re is a mixed rare earth of La + Ce, wherein the weight ratio of La to Ce is 1:(4 - 5). More preferably, the rare earth element is added in the form of rare earth ferrosilicon alloy, wherein the mass percentage of rare earth in the rare earth ferrosilicon alloy is 25 - 30%.

[0040] Chromium is an important element in stainless steel, and the corrosion resistance of stainless steel increases with the increase of chromium content. However, chromium is also a ferrite-forming element. Adding chromium to steel can promote the formation of ferrite and has a tendency to increase the formation of intermetallic σ phase in stainless steel. Molybdenum can improve the pitting corrosion and crevice corrosion resistance of stainless steel, especially in a chloride ion environment, and its corrosion resistance is more prominent. Moreover, molybdenum is also a ferrite-forming element and can also increase the tendency to form intermetallic σ phase in stainless steel. In the present invention, the chromium element content is controlled at the lower limit of 24.0 - 24.5%, and the molybdenum element is controlled at the lower limit of 4.0 - 4.5%.

[0041] W and Nb are commonly used elements for grain refinement and significant microalloy strengthening, which can improve the toughness and cracking tendency of castings. However, W and Nb have high melting points and are prone to form carbides with carbon and become inclusions. Therefore, their contents cannot be too high. W and Nb can form an infinite solid solution, which helps to enter the ferrite. In the present invention, controlling the mass ratio of W to Nb to 1:1 can significantly inhibit the precipitation of intermetallic σ phase. The present invention designs the contents of W and Nb at a relatively low level of 0.20 - 0.30% and adds them in the form of a W-Nb master alloy.

[0042] Most importantly, rare earths Ce and La are added in the present invention. Ce and La have the functions of surface activity and grain refinement elements, and the function of Ce is stronger than that of La. The present invention uses a relatively low-cost rare earth ferrosilicon alloy. Ce+La enriched at the grain boundaries can refine the grains, change the grain boundary state, improve the plasticity and toughness of the steel, and the cracking tendency of the casting. At the same time, Ce+La also has the functions of purification and microalloying. It can not only desulfurize and degas, but also change the morphology, size and distribution of oxides and sulfides in the steel by forming sulfur oxides, which has a very favorable impact on the properties of the steel. However, pure rare earth alloys are prone to oxidation, not easily dissolved into the matrix, and will form large inclusions. Therefore, too low rare earth content has no effect, and too high content is prone to generate more slag. The present invention designs the La+Ce content to be 0.020-0.035%, the ratio of La:Ce is 1:(4-5), and the rare earth ferrosilicon alloy is pressed into the molten steel or the ladle, so that the rare earth is more easily dissolved into the ferrite, thereby significantly inhibiting the precipitation of the σ phase.

[0043] Through the design of each important element in the present invention, when the molten steel is slowly cooled from the liquid phase to room temperature, the precipitation of the intermetallic σ phase can be completely avoided, and the casting structure is fine, significantly reducing the structure and macroscopic stress of the large casting, and improving the cracking tendency of the large casting.

[0044] Based on the above-mentioned cast 5A duplex stainless steel, the present invention also proposes a preparation method for large castings, including steps such as preliminary melting in an intermediate frequency induction furnace, AOD double refining, adding rare earth ferrosilicon alloy, pouring, air cooling, solution treatment and water cooling. The specific steps are as follows:

[0045] S1. Prepare raw materials according to the mass percentages of each element in the cast 5A duplex stainless steel. The remaining raw materials except the rare earth ferrosilicon alloy are sampled and melted through an intermediate frequency induction circuit to obtain molten steel. The alloy composition of the molten steel is detected by a spectrometer and adjusted according to the detection results until the alloy composition meets the designed composition requirements;

[0046] S2. Purify the molten steel through the AOD double refining process;

[0047] S3. Before tapping the molten steel or in the ladle, press the rare earth ferrosilicon alloy into the molten steel and pour the molten steel into a sand mold;

[0048] S4. Air cool the molten steel to room temperature, open the box and shake out the sand, cut off the gating system, and take out the casting;

[0049] S5. Transfer the casting to a heat treatment furnace for solution treatment;

[0050] S6. After the heat treatment is completed, cool the furnace to 1045-1080°C and water cool the casting when it is taken out of the furnace.

[0051] Preferably, in step S1, the melting temperature is 1650 - 1680 °C.

[0052] Preferably, in step S3, the pouring temperature is 1530 - 1580 °C.

[0053] Preferably, in step S5, the solution treatment includes: loading the casting into a heat treatment furnace with a furnace temperature of 1000 °C, holding for 30 min, heating up to 1130 - 1160 °C at a rate of 80 - 150 °C / h, and holding for 3 - 3.5 h.

[0054] Preferably, in step S6, the water cooling method is circulating water cooling, where the circulating water temperature ≤ 80 °C.

[0055] The technical solution of the present invention will be introduced in detail below with specific embodiments.

[0056] Example 1

[0057] In this embodiment, by weight percentage, its designed chemical composition is: C ≤ 0.03%, Si ≤ 1%, Mn ≤ 1.5%, P ≤ 0.04%, S ≤ 0.02%, Cr 24.0 - 24.5%, Ni 6.0 - 7.0%, Mo 4.0 - 4.5%, N 0.20 - 0.30%, W 0.27%, Nb 0.27%, Re 0.03%, and the rest is Fe, PREN value: %Cr + 3.3%Mo + 16%N is 42.1.

[0058] The basic raw materials are allocated according to the chemical composition. Among them, W and Nb are added in the form of master alloy (W:Nb weight ratio is 1:1), melted at 1680 °C using an intermediate frequency induction furnace, and the preliminary alloy composition is adjusted by a spectrometer until the designed composition requirements are met.

[0059] The molten steel is purified by the AOD double refining process.

[0060] Before tapping the molten steel or in the ladle, rare earth ferrosilicon alloy (where Ce:La is 4.5:1, addition amount is 6.4 Kg / t of molten steel) is pressed into the molten steel, poured at a temperature of 1550 °C, and the molten steel is poured into a sand mold.

[0061] Air-cooled to room temperature, the mold is opened to shake out the sand, the gating system is cut off, the casting is taken out, and it is observed that the casting has no macroscopic large cracks and fine cracks, and the surface is dense. Small pieces of samples are cut from the casting, the alloy composition is analyzed by a spectrometer, and rare earth La and Ce in the alloy are analyzed by inductively coupled plasma emission spectrometry. The results are shown in Table 1, the composition meets the standard requirements, and the PREN value is 42.1. Its metallographic structure is as Figure 1As shown, it can be seen from the figure that its as-cast structure is fine, with only austenite and ferrite structures, and the precipitation of σ-phase is hardly observable. Only through magnified scanning electron microscopy (such as Figure 2 ) can a very small amount of σ-phase be observed. Analyzing its σ-phase content is only about 0.5%, indicating that this 5A steel component can completely inhibit the precipitation of σ-phase during the slow cooling process, thus avoiding the cracking of the casting.

[0062] Transfer the casting to a heat treatment furnace. The casting is loaded into a heat treatment furnace with a furnace temperature of 1000 °C. After holding for 30 minutes, it is heated to 1150 °C at a rate of 150 °C / h and held for 3.5 h.

[0063] After the heat treatment is completed, the furnace is cooled to 1080 °C, and then taken out of the furnace and water-cooled. Water cooling is carried out with circulating water, and the water temperature ≤ 80 °C; the produced impeller casting has a dense structure and meets the requirements of ASMEⅧ, DIV.1, APP.7 after penetrant inspection; small pieces of samples are cut out from the large casting for metallographic structure observation, as Figure 3 shown. Its structure is composed of two phases, austenite and ferrite, and the ferrite content is 54.4%. The mechanical properties of the samples are tested according to the ASTM A370 method. Its tensile strength is 767 MPa, the elongation rate is 31%, its low-temperature (-46 °C) impact resistance is 107 AKV / J, and the average hardness is 247 HB. The mechanical properties are excellent. According to ASTM G48-3 method A, in a 6% FeCl 3 solution at 50 °C for 24 h crevice corrosion test, the weight loss is 0.0016 g, there is no pitting corrosion, and the corrosion rate is 0.471 g / m 2 , and the corrosion resistance is excellent.

[0064] Example 2

[0065] In this example, in terms of weight percentage, its designed chemical composition is: C ≤ 0.03%, Si ≤ 1%, Mn ≤ 1.5%, P ≤ 0.04%, S ≤ 0.02%, Cr 24.0 - 24.5%, Ni 6.0 - 7.0%, Mo 4.0 - 4.5%, N 0.20 - 0.30%, W 0.22%, Nb 0.22%, Re 0.02%, and the rest is Fe.

[0066] Allocate the basic raw materials according to the chemical composition. Among them, W and Nb are added in the form of master alloys (W:Nb weight ratio is 1:1). Melting is carried out in an intermediate frequency induction furnace at 1650 °C, and a spectrometer is used to adjust the preliminary alloy composition until the designed composition requirements are met.

[0067] Purify the molten steel through the AOD double refining process.

[0068] Before tapping the molten steel or in the ladle, press the rare earth ferrosilicon alloy (where Ce:La is 5:1 and the addition amount is 4.2 Kg / t of molten steel) into the molten steel, pour at a temperature of 1560 °C, and pour the molten steel into a sand mold.

[0069] Air-cool to room temperature, remove the mold from the sand after opening the box, cut off the gating system, take out the casting, and observe that there are no macroscopic large cracks and fine cracks on the casting, and the surface is dense. Cut out small pieces of samples from the casting, analyze the alloy composition using a spectrometer, and analyze the rare earth La and Ce in the alloy using inductively coupled plasma optical emission spectrometry. The results are shown in Table 1. The composition meets the standard requirements, and the PREN value is 41.7. Its metallographic structure is as Figure 4 shown. It can be seen from the figure that its as-cast structure is relatively fine, basically only austenite and ferrite structures, and only a small amount of σ-phase precipitates. Only through magnified scanning electron microscopy (such as Figure 5 ), can a very small amount of σ-phase be observed. Analyzing its σ-phase content is only about 1%, indicating that this 5A steel with this composition can completely inhibit the precipitation of σ-phase during the slow cooling process, thus avoiding the cracking of the casting.

[0070] Transfer the casting to a heat treatment furnace. Load the casting into a heat treatment furnace with a furnace temperature of 1000 °C. After holding for 30 minutes, heat up to 1130 °C at a rate of 80 °C / h and hold for 3 h.

[0071] After the heat treatment, cool the furnace to 1080 °C, take out the furnace and cool it with water. Use water cooling as circulating water cooling, and the water temperature ≤ 80 °C; The produced casting has a dense structure, excellent mechanical properties and corrosion resistance. Cut out small pieces of samples from the large casting. Its structure is composed of two phases, austenite and ferrite, and the ferrite content is 55.1%.

[0072] Comparative Example 1

[0073] In this comparative example, by weight percentage, its designed chemical composition is: C ≤ 0.03%, Si ≤ 1%, Mn ≤ 1.5%, P ≤ 0.04%, S ≤ 0.02%, Cr 24.0 - 24.5%, Ni 6.0 - 7.0%, Mo 4.0 - 4.5%, N 0.20 - 0.30%, and the rest is Fe.

[0074] Mix the basic raw materials according to the chemical composition, melt them in an intermediate frequency induction furnace at 1680 °C, and adjust the preliminary alloy composition using a spectrometer until the designed composition requirements are met.

[0075] Purify the molten steel through the AOD double refining process.

[0076] Pour at a temperature of 1580 °C, and pour the molten steel into a sand mold.

[0077] Air-cooled to room temperature, the mold was opened and shakeout was carried out, the gating system was cut off, the casting was taken out, and it was observed that macroscopic open cracks occurred in the casting. The casting was unqualified and no subsequent heat treatment process was carried out. Small pieces of samples were cut from the crack-free part of the casting, the alloy composition was analyzed by a spectrometer, and rare earth La and Ce in the alloy were analyzed by inductively coupled plasma emission spectrometry. The results are shown in Table 1. The composition meets the standard requirements and the PREN value is 41.1. Its metallographic structure is as Figure 6 shown. It can be seen from the figure that its as-cast structure is coarse, there are many σ precipitates at the grain boundaries and form a network, and the content of σ precipitates reaches as high as 28.5%. This is similar to the metallographic structure of general 5A duplex stainless steel castings.

[0078] Comparative Example 2

[0079] In this comparative example, in terms of weight percentage, its designed chemical composition is: C≤0.03%, Si≤1%, Mn≤1.5%, P≤0.04%, S≤0.02%, Cr 24.0 - 24.5%, Ni 6.0 - 7.0%, Mo 4.0 - 4.5%, N 0.20 - 0.30%, W 0.5%, Nb 0.20%, Re 0.012%, and the rest is Fe.

[0080] The basic raw materials were allocated according to the chemical composition. Among them, W-Nb was added in the form of master alloy (weight ratio of W:Nb is 2.5:1). Melting was carried out in an intermediate frequency induction furnace at 1650°C, and the preliminary alloy composition was adjusted by a spectrometer until the designed composition requirements were met.

[0081] The molten steel was purified by the AOD double refining process.

[0082] Pouring was carried out at a temperature of 1550°C, and the molten steel was poured into a sand mold.

[0083] Air-cooled to room temperature, the mold was opened and shakeout was carried out, the gating system was cut off, the casting was taken out, and it was observed that macroscopic open cracks occurred in the casting. The casting was unqualified and no subsequent heat treatment process was carried out. Small pieces of samples were cut from the crack-free part of the casting, the alloy composition was analyzed by a spectrometer, and rare earth La and Ce in the alloy were analyzed by inductively coupled plasma emission spectrometry. The results are shown in Table 1. The composition meets the standard requirements and the PREN value is 42.3. Its metallographic structure is as Figure 7 shown. It can be seen from the figure that its as-cast structure is relatively fine, but there are many massive σ precipitates, and the content of σ precipitates reaches as high as 24.7%. This is the main reason for the macroscopic cracking of the casting.

[0084] Table 1 Casting compositions of examples and comparative examples (mass percentage%)

[0085]

[0086] The above has elaborated in detail on the embodiments provided by the present invention. Specific examples have been used herein to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the core idea of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principles of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A cast 5A duplex stainless steel, characterized in that: The invention comprises the following components: calculated by weight percentage, C≤0.03%, Si≤1%, Mn≤1.5%, P≤0.04%, S≤0.02%, Cr 24.0-24.5%, Ni 6.0-7.0%, Mo4.0-4.5%, N 0.20-0.30%, W 0.20-0.30%, Nb 0.20-0.30%, Re 0.02-0.035%, and the balance is Fe, wherein the PREN value is: %Cr+3.3%Mo+16%N≥40; The mass ratio of W and Nb is 1:1, and Re is a mixed rare earth of La+Ce, wherein the weight ratio of La and Ce is 1:(4-5).

2. A method for preparing a large casting of 5A duplex stainless steel, characterized in that: The method of preparing a large casting using the cast 5A duplex stainless steel as claimed in claim 1 comprises: S1. Prepare raw materials according to the mass percentage of each element in the cast 5A duplex stainless steel, sample the remaining raw materials except the rare earth elements and melt them in a medium frequency induction furnace to obtain molten steel, detect the alloy composition of the molten steel by a spectrometer, and adjust according to the test results until the alloy composition meets the designed composition requirements; S2, purifying the molten steel through AOD double refining process; S3, before the molten steel is tapped or in the ladle, rare earth elements are added to the molten steel, and the molten steel is poured into the sand mold; S4, air-cooling the molten steel to room temperature, unpacking and removing the sand, cutting off the pouring system, and taking out the casting; S5, transferring the casting to a heat treatment furnace for solution treatment; S6. After the heat treatment is released, the furnace is cooled to 1045-1080°C and then taken out of the furnace and cooled in water.

3. The preparation method according to claim 2, characterized in that: In step S1, the melting temperature is 1650-1680°C.

4. The preparation method according to claim 2, characterized in that: In step S1, W and Nb elements are added to form a W—Nb master alloy.

5. The preparation method according to claim 2, characterized in that: In step S3, rare earth elements are pressed into the molten steel in the form of rare earth ferrosilicon alloy, wherein the mass percentage of rare earth in the rare earth ferrosilicon alloy is 25-30%.

6. The preparation method according to claim 2, characterized in that: In step S3, the pouring temperature is 1530-1580°C.

7. The preparation method according to claim 2, characterized in that: In step S5, the solution treatment includes: placing the casting into a heat treatment furnace at a furnace temperature of 1000°C, keeping the temperature for 30 minutes, heating the temperature to 1130-1160°C at a rate of 80-150°C / h, and keeping the temperature for 3-3.5 hours.

8. The preparation method according to claim 2, characterized in that: In step S6, the water cooling method is circulating water cooling, wherein the circulating water temperature is ≤80°C.

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