High aluminum austenitic alloy with excellent high temperature corrosion resistance and creep resistance

Abstract

According to the high-aluminum austenite alloy and the high-aluminum austenite centrifugal casting pipe provided by the invention, the high-aluminum austenite alloy and the high-aluminum austenite centrifugal casting pipe have excellent corrosion resistance and creep resistance at the temperature of 900 DEG C or above, and meanwhile, the high-aluminum austenite alloy and the high-aluminum austenite centrifugal casting pipe have required mechanical properties. The high-aluminum austenite alloy or the high-aluminum austenite centrifugal casting pipe comprises the following elements in percentage by weight: 0.3-0.7% of C; 0 to 0.5% of Mn; 0 to 0.5% of Si; 20 to 26% of Cr; 40 to 50% of Ni; 3.5 to 5% of Al; 0.01 to 0.3 percent of Ti; 0.01% to 0.3% of Zr; 0.1 to 1% of Nb; 0.01 to 2 percent of Ta; 0.01 to 1 percent of Mo; 0.01 to 1.9 percent of W; 0.001% to 0.04% of N; 0.03 to 0.3 percent of Re; and the balance of Fe and inevitable impurities. The invention also relates to a method for manufacturing the high-aluminum austenitic alloy and the high-aluminum austenitic centrifugally cast pipe according to the invention.

Description

[0001] This application is a divisional application of the patent application with the application date of January 8, 2021, application number 202110026450.8, and invention name “High aluminum austenitic alloy with excellent high temperature corrosion resistance and creep resistance”. Technical Field

[0002] The present invention relates to the field of austenitic alloys, in particular to a high-aluminum austenitic alloy with excellent high-temperature (≥900° C.) corrosion resistance and creep resistance. Background Art

[0003] Nickel-chromium austenitic heat-resistant alloys have long been widely used in the petrochemical industry. The equipment used in this industry (such as cracking tubes for steam cracking) must withstand combustion temperatures approaching 1100°C on the outside of the furnace tubes. Furthermore, the material must withstand carburization corrosion caused by hydrocarbon gases inside the furnace tubes, as well as high-temperature oxidation on the outer surface. Therefore, the material must exhibit excellent high-temperature and corrosion resistance under high-temperature environments, as well as high-temperature mechanical properties such as creep resistance and high-temperature ductility.

[0004] The two most commonly used nickel-chromium austenitic heat-resistant alloys are ZG45Ni35Cr25NbM and ZG50Ni45Cr35NbM (hereinafter referred to as 35 / 45 instead of ZG50Ni45Cr35NbM). The 35 / 45 alloy is used in higher temperatures and more corrosive environments. During operation, corrosive gases react with the alloy, causing high-temperature oxidation and corrosion. A thick metal oxide layer forms on the inner surface of the furnace tube, protecting the material from further oxidation and corrosion. The metal oxide layer formed in the 35 / 45 alloy is primarily a Cr2O3 + SiO2 composite oxide layer / film. This oxide layer is relatively stable below 1050°C and effectively prevents oxidation and carburization corrosion of the material. However, when the temperature is higher than 1050℃, the thermal stability of chromium oxide deteriorates. When the furnace tube is subjected to stress, the oxide layer is prone to cracks, which reduces its continuity and density, making it insufficient to continue to protect the material matrix, resulting in oxidation diffusion into the material and accelerated carburizing corrosion until the oxide layer and the matrix gradually crack and peel off.

[0005] Adding aluminum is one way to increase the resistance of 35 / 45 nickel-chromium austenitic alloy to oxidation and carburization. High aluminum contents form a thick, dense alumina layer on the alloy surface, which remains stable at temperatures above 1050°C in cracking furnaces. This gives the alloy excellent resistance to carburization and oxidation in high-temperature environments. However, increasing the aluminum content decreases the material's ductility. Consequently, heat-resistant alloys currently used in the petrochemical industry typically contain little or no aluminum.

[0006] The present invention proposes an austenitic alloy with a high aluminum content, ensuring high resistance to the environment (such as oxidation and carburizing corrosion) while ensuring mechanical properties that are at least as high as those of the alloys known to date. Summary of the Invention

[0007] The present invention aims to provide a high-aluminum austenitic alloy and a high-aluminum austenitic centrifugally cast pipe that exhibit excellent corrosion resistance and creep resistance at temperatures of 900°C and above, while also possessing desirable mechanical properties. The present invention also relates to methods for manufacturing the high-aluminum austenitic alloy and high-aluminum austenitic centrifugally cast pipe.

[0008] Specifically, the element composition of the high aluminum austenitic alloy or high aluminum austenitic centrifugal cast tube of the present invention is, in terms of weight percentage, C, 0.3-0.7%; Mn, 0-0.5%; Si, 0-0.5%; Cr, 20-26%; Ni, 40-50%; Al, 3.5-5%; Ti, 0.01-0.3%; Zr, 0.01-0.3%; Nb, 0.1-1%; Ta, 0.01-2%; Mo, 0.01-1%; W, 0.01-1.9%; N, 0.001-0.04%; Re, 0.03-0.3%; the balance is Fe and unavoidable impurities.

[0009] Preferably, the C content in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast tube of the present invention is 0.4-0.65%.

[0010] Preferably, the Mn content in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast pipe of the present invention is 0-0.4%.

[0011] Preferably, the Si content in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast tube of the present invention is 0-0.4%.

[0012] Preferably, the Ti content in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast tube of the present invention is 0.04-0.3%.

[0013] Preferably, the Ta content in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast tube of the present invention is 0.4-2%.

[0014] Preferably, the Mo content in the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast tube of the present invention is 0.2-1%.

[0015] Preferably, the W content in the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention is 0.4-1.9%.

[0016] Preferably, the nitrogen content in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast tube of the present invention is 0.006-0.035%.

[0017] Preferably, in the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast tube of the present invention, Re is Y, Hf and Ce, and the content of each is 0.01-0.1%.

[0018] Preferably, in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast tube of the present invention, the total content of Re is 0.08-0.3%.

[0019] Preferably or optionally, the high aluminum austenitic alloy or high aluminum austenitic centrifugally cast tube of the present invention further contains one or more of Cu, V, Co and B, wherein: Cu, ≤0.1%; V, ≤0.01%; Co, ≤0.03%; B, ≤0.1%.

[0020] The unavoidable impurities include one or more of S, P and O. Preferably, in the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugal casting tube of the present invention, S≤0.005%, P≤0.005%, and O≤0.005%.

[0021] Preferably, in the high aluminum austenitic alloy or high aluminum austenitic centrifugal casting tube of the present invention, the following elements are present: C, 0.4-0.65%; Mn, 0-0.4%; Si, 0-0.4%; Cr, 20-26%; Ni, 40-50%; Al, 3.5-5%; Ti, 0.04-0.3%; Zr, 0.01-0.3%; Nb, 0.1-1%; Ta, 0.4-2%; Mo, 0.2-1%; W, 0.4-1.9%; N, 0.006-0.035%; Re, 0.08-0.3%; Cu, ≤0.1%; V, ≤0.01%; Co, ≤0.03%; B, ≤0.1%; the balance is Fe and unavoidable impurities.

[0022] Preferably, the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention has a durability life measured under the test conditions of 1100° C. and 17 MPa of ≥100 hours, preferably ≥110 hours, and more preferably ≥115 hours.

[0023] Preferably, the average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention measured under the test conditions of 1050° C. and 15 MPa in the second stage of creep is ≤0.0005% / h, preferably ≤0.0003% / h.

[0024] Preferably, the average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention measured under the test conditions of 1050° C. and 20 MPa is ≤0.002% / h, preferably ≤0.0015% / h in the second stage of creep.

[0025] Preferably, the average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention measured under the test conditions of 1050° C. and 25 MPa is ≤0.01% / h, preferably ≤0.007% / h in the second stage of creep.

[0026] Preferably, the average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention measured under the test conditions of 1050° C. and 30 MPa is ≤0.05% / h, preferably ≤0.035% / h in the second stage of creep.

[0027] Preferably, the yield strength of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe measured under the test condition of 850°C is ≥120MPa, preferably ≥124MPa; the tensile strength is ≥185MPa, preferably ≥189MPa; and the elongation is ≥49%, preferably ≥50%.

[0028] Preferably, the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention has a yield strength measured under test conditions of 1050°C of ≥53MPa, preferably ≥55MPa; a tensile strength of ≥65MPa, preferably ≥67MPa; and an elongation of ≥59%, preferably ≥61%.

[0029] Preferably, the high aluminum austenitic alloy or high aluminum austenitic centrifugal cast pipe of the present invention has a carbon increase of less than 0.5% at a depth of 1 mm, preferably less than 0.45%, and a carbon increase of less than 0.05% at a depth of 2 mm, preferably less than 0.03%, under the test conditions of 1150°C / 7 days.

[0030] Preferably, the high-aluminum austenitic alloy centrifugally cast pipe of the present invention has an outer diameter of 60-250 mm and a wall thickness of 6-10 mm.

[0031] Preferably, the microstructure of the high aluminum austenitic alloy or high aluminum austenitic centrifugally cast tube of the present invention includes columnar crystals with a volume fraction of ≥80% and equiaxed crystals with a volume fraction of ≤20%, or consists of columnar crystals with a volume fraction of ≥80% and equiaxed crystals with a volume fraction of ≤20%.

[0032] Preferably, the high-aluminum austenite centrifugal casting tube of the present invention has columnar crystals near the outer wall in the wall thickness direction and uniform equiaxed crystals near the inner wall.

[0033] The present invention also provides a method for manufacturing the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugally cast pipe of the present invention, comprising:

[0034] 1) Melting: Melting the chemical components other than Al, Re, Ti, and Zr in the high-aluminum austenitic alloy or high-aluminum austenitic centrifugal cast tube in a medium frequency furnace according to the target chemical composition to obtain molten steel;

[0035] 2) Deoxidation and slag removal: deoxidation and slag removal are performed on the molten steel obtained in step 1);

[0036] 3) Adding Al: adding Al to the molten steel treated in step 2), and slagging after Al dissolves;

[0037] 4) Quenching and tempering: adding Re, Ti and Zr into the ladle, introducing the molten steel treated in step 3) into the ladle, dissolving Re, Ti and Zr and then slagging;

[0038] 5) Casting: Deslagging is performed before casting, and then the molten steel is poured into a metal mold. After cooling, a high-aluminum austenitic alloy or a high-aluminum austenitic centrifugal cast pipe is obtained.

[0039] Preferably, in step 1), raw materials are selected and prepared according to the target chemical composition, and the raw materials are smelted in the order of non-oxidizable to easily oxidizable.

[0040] Preferably, in step 1), Fe, Ni, C, Mn, Cr and Si are smelted in the order of Fe, Ni, C, Mn, FeCr and FeSi.

[0041] Preferably, in step 1), the contents of harmful elements such as Pb, Sn, Sb, Zn, As, and Bi in the molten steel are controlled to be lower than 50 ppm.

[0042] Preferably, in step 1), samples are taken and sent to a laboratory for testing, and the chemical composition is adjusted according to the laboratory chemical analysis results.

[0043] Preferably, in step 2), the molten steel is heated to 1650±50° C., deoxidized with a deoxidizer, and then slag removed.

[0044] Preferably, in step 2), slag removal includes: covering the molten steel in the furnace with a slag-forming agent, commencing argon blowing from the furnace bottom, and then performing slag removal after the argon blowing. Preferably, argon blowing is continued for 3±1 minutes before performing slag removal. Blowing argon from the furnace bottom accelerates the rise of oxides, impurities, and gases in the molten steel, which are then bound by the slag-forming agent and removed simultaneously, thereby improving the purity of the molten steel.

[0045] Preferably, in step 3), the furnace mouth is covered with argon gas for protection to isolate the air from reacting with the molten steel surface.

[0046] Preferably, in step 3), argon is blown from the bottom of the furnace and covered with argon at the furnace mouth during the process of adding Al for dissolution. The purpose of the argon blowing from the bottom of the furnace and the argon covering at the furnace mouth is to ensure that the active elements added later are not burned or oxidized.

[0047] Preferably, in step 3), after Al is dissolved, the temperature of the molten steel is raised to 1680±50° C., and a slag forming agent is added to form slag and remove slag.

[0048] Preferably, in step 4), Re, Ti and Zr are added to the ladle, molten steel is introduced into the ladle, and the dissolution and homogenization process of Re, Ti and Zr is completed by pouring the molten steel; after the molten steel is poured, the surface of the molten steel in the ladle is covered with slag.

[0049] Preferably, in step 5), the molten steel in the ladle is quickly poured into a metal mold rotating at high speed on a centrifuge, and the centrifugal cast pipe is obtained after the molten steel is cooled. The casting time is as short as possible. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 : Average creep rate of the second stage of creep of the alloys of Examples 1, 3 and 4 of the present invention and Alloy 11 (35 / 45 alloy).

[0051] Figure 2 : Cyclic oxidation weight gain curves of the alloys of Examples 1, 3 and 4 of the present invention and Alloy 11 (34 / 45 alloy).

[0052] Figure 3 : High-temperature short-time tensile curves of the alloy of Example 1 of the present invention at 850, 950, 1050 and 1150°C respectively.

[0053] Figure 4 : High temperature short-time tensile curves of alloy No. 11 (34 / 45 alloy) at 850, 900, 1000 and 1050℃ respectively.

[0054] Figure 5 : The carbon addition percentages at different depths of the alloys of Examples 1-4 of the present invention and Alloy 11 (34 / 45 Alloy) under the test conditions of 1150°C / 7 days. DETAILED DESCRIPTION

[0055] To facilitate understanding of the features and effects of the present invention by those skilled in the art, the following provides a general description and definition of the terms and expressions used in the specification and claims. Unless otherwise indicated, all technical and scientific terms used herein have the ordinary meanings as understood by those skilled in the art regarding the present invention. In the event of conflict, the definitions in this specification shall prevail.

[0056] Herein, all features such as values, amounts, amounts, and concentrations defined in numerical ranges or percentage ranges are for brevity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to include and specifically disclose all possible subranges and individual values within the range (including integers and fractions).

[0057] When describing embodiments or examples herein, it should be understood that they are not intended to limit the present invention to these embodiments or examples. On the contrary, all alternatives, improvements, and equivalents of the methods and materials described herein are encompassed within the scope defined by the claims. It should be understood that within the scope of the present invention, the above-mentioned technical features of the present invention and the technical features specifically described below (such as in the examples) can be combined with each other to form a preferred technical solution.

[0058] In the present invention, the role of each element in the high aluminum austenitic alloy and the centrifugal cast pipe is as follows.

[0059] C: A carbide-forming element. C forms carbides such as M7C3, M23C6, and MC with medium-strong carbide-forming elements (Cr, Mo) or strong carbide-forming elements (Ti, V, Nb). During high-temperature aging, supersaturated dissolved carbon in the matrix precipitates as fine, dispersed secondary M23C6 particles, thereby improving the alloy's durability. However, excessive carbon content can reduce the alloy's toughness. Therefore, the C content must be appropriately selected to ensure both high-temperature durability and high-temperature plasticity. The C content in the alloy of the present invention is controlled within a range of 0.3-0.7%, preferably 0.4-0.65%.

[0060] Mn: Improves weldability and slows carbon diffusion. The Mn content in the alloy of the present invention is controlled to be below 0.5%. The Mn content is desirably kept as low as possible, preferably below 0.4%. In some embodiments, the Mn content is 0.01-0.4%.

[0061] Si: During the steelmaking process, Si acts as a strong deoxidizer to reduce the oxygen content in the molten steel, thereby improving the purity of the molten steel. During the high-temperature service of the material, an appropriate Si content can give the material good antioxidant and anti-carburization properties. Si has a stronger binding force with O than Cr, and like Cr, it can form a passivation film SiO2 in the alloy, which has higher antioxidant properties than Cr2O3. However, excessive Si addition will lead to deterioration of the mechanical properties of the alloy, affect welding performance, and reduce the durability. The Si content in the alloy of the present invention is controlled below 0.5%, preferably below 0.4%. In some embodiments, the Si content is 0.05-0.4%.

[0062] Cr: It is the main element for high-temperature oxidation resistance and high-temperature corrosion resistance, and can improve the thermal strength of the alloy. When the Cr content is sufficient, an oxide film will be formed on the surface of the alloy, which will inhibit the formation of coke deposition and increase the carburizing resistance of the alloy. The Cr content in the alloy of the present invention is controlled to be 20-26%. If the Cr content is too high, the material will be prone to rapid or gradual precipitation of the ferrite phase, the microstructural stability of the material will be reduced under high-temperature working conditions, and the high-temperature mechanical properties of the material, especially the endurance performance, will decrease; at the same time, it will promote the formation of the ferrite phase, and will also lead to a decrease in the welding performance of the material, resulting in the inability to replace spare parts by welding in the later stage.

[0063] Nickel is one of the most important alloying elements in heat-resistant alloys. Its primary function is to stabilize the γ region, allowing the alloy to achieve a fully austenitic structure. This in turn imparts high strength, ductility, and toughness, while also ensuring good high-temperature strength and creep resistance. The high price of nickel directly determines the final price of the product. Considering both cost and performance, the nickel content in the alloy of the present invention is controlled to 40-50%.

[0064] Al: An essential element for forming an aluminum oxide layer in the alloy of the present invention at high temperatures. The alloy of the present invention has a relatively high Al content, exceeding 3.5%, which ensures the formation of a continuous and dense aluminum oxide layer on the alloy surface. Furthermore, considering that excessive Al content reduces the alloy's toughness at room temperature, making machining difficult and increasing machining costs, the Al content in the alloy of the present invention is controlled within a range of 3.5-5%.

[0065] Ti: During high-temperature aging, secondary carbides gradually form. The addition of Ti improves the thermodynamic stability of the secondary precipitates, M23C6, maintaining a uniform dispersion over time and thereby enhancing the alloy's high-temperature creep strength. Furthermore, Ti inhibits the transformation of the primary precipitates, MC, into G phase, indirectly improving their stability and, consequently, the alloy's high-temperature creep strength. The Ti content in the alloy of this invention is controlled to be 0.01-0.3%, preferably 0.04-0.3%.

[0066] Zr: As a strong oxidant, the addition of Zr can reduce the oxygen content in the molten steel during the smelting process, thereby ensuring the absorption of other core elements. The Zr content in the alloy of the present invention is controlled to be 0.01-0.3%.

[0067] Nb: A precipitation-strengthening element, it reduces creep rate and improves creep resistance. Nb is also a primary forming element of the carbides M7C3, M23C6, and MC, making them very stable at high temperatures. Nb also forms carbonitrides, which alter the carbide morphology and refine M23C6, resulting in a uniform dispersion and thus improving the alloy's high-temperature creep strength. Given its high cost, the Nb content in the alloy of this invention is controlled below 1%, preferably between 0.1% and 1%.

[0068] Ta: It contributes to both solid solution strengthening and precipitation strengthening. Ta has a strong affinity for interstitial atoms like carbon, and the resulting precipitates are very stable at high temperatures. Ta also helps improve the alloy's high-temperature transient strength and creep properties. In the present invention, the Ta content in the alloy is controlled to 0.01-2%, preferably 0.4-2%.

[0069] Mo: Mo atoms are mostly dissolved in the γ matrix. Mo atoms are larger than nickel and iron atoms, which also improves yield strength. Furthermore, the addition of molybdenum forms M6C carbides, which are fine and dispersed and also contribute to strengthening. Mo also refines austenite grains, and fine grains contribute to improved alloy plasticity. In the present invention, the Mo content in the alloy is controlled at 0.01-1%, preferably 0.2-1%.

[0070] W: It provides solid solution strengthening. W dissolves in the gamma matrix. Tungsten, with its relatively large atomic radius, causes significant lattice expansion within the matrix, inhibiting dislocation motion and increasing yield strength. Tungsten also reduces the stacking fault energy of the gamma matrix, which effectively improves the creep properties of the high-temperature alloy. In the present invention, the W content in the alloy is controlled to 0.01-1.9%, preferably 0.4-1.9%.

[0071] N: N can form carbonitrides with Nb and C, changing the carbide morphology, refining M23C6 and making it uniformly dispersed, thereby improving the high-temperature creep strength of the alloy. The N content in the alloy of the present invention is controlled at 0.001-0.04%, preferably 0.006-0.035%.

[0072] Re (rare earth element): The rare earth elements in the heat-resistant alloy of the present invention include at least one of Ce, Y, and Hf. Rare earth elements help refine and stabilize secondary precipitates, thereby improving the material's high-temperature mechanical properties. Furthermore, rare earth elements help densify the oxide layer, primarily composed of chromium oxide and silicon oxide, thereby enhancing the product's high-temperature oxidation resistance. The total Re content in the alloy of the present invention can be within the range of 0.03-0.3%, preferably 0.08-0.3%. Ce, Y, and Hf can each be added in amounts of 0.01-0.1%.

[0073] The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe of the present invention can be manufactured by a method comprising the following steps:

[0074] 1) Melting: Melting the chemical components other than Al, Re, Ti, and Zr in the high-aluminum austenitic alloy or high-aluminum austenitic centrifugal cast tube in a medium frequency furnace according to the target chemical composition to obtain molten steel;

[0075] 2) Deoxidation and slag removal: deoxidation and slag removal are performed on the molten steel obtained in step 1);

[0076] 3) Adding Al: adding Al to the molten steel treated in step 2), and slagging after Al dissolves;

[0077] 4) Quenching and tempering: adding Re, Ti and Zr into the ladle, introducing the molten steel treated in step 3) into the ladle, dissolving Re, Ti and Zr and then slagging;

[0078] 5) Casting: Deslagging is performed before casting, and then the molten steel is poured into a metal mold. After cooling, a high-aluminum austenitic alloy or a high-aluminum austenitic centrifugal cast pipe is obtained.

[0079] In step 1), raw materials can be selected and prepared according to the target chemical composition. The raw materials are preferably smelted in the order of least oxidizable to most oxidizable, for example, Fe, Ni, C, Mn, Cr, and Si are smelted in the order of Fe, Ni, C, Mn, FeCr, and FeSi. In step 1), the content of harmful elements such as Pb, Sn, Sb, Zn, As, and Bi in the molten steel can be controlled to be less than 50 ppm by optimizing the raw materials. In step 1), samples can be taken for laboratory testing, and the chemical composition can be adjusted based on the laboratory chemical analysis results.

[0080] In step 2), the molten steel can be heated, and then deoxidized with a deoxidizer before slagging. Preferably, the molten steel is heated to 1650±50°C, and then deoxidized and slag-beaten. In step 2), slagging preferably includes: covering the molten steel in the furnace with a slagging agent, starting argon blowing from the furnace bottom; and slagging after argon blowing. Preferably, argon blowing is performed for 3±1 minutes before slagging. During slagging, oxides, impurities and gases in the molten steel are removed by adding a slagging agent and blowing argon from the furnace bottom, thereby improving the purity of the molten steel. Before deoxidation, the temperature of the molten steel in the furnace is controlled by controlling the power of the medium frequency furnace.

[0081] In step 3), it is preferred to cover the furnace mouth with argon to isolate the air from reacting with the surface of the molten steel. In step 3), it is preferred to keep the furnace bottom blowing argon and the furnace mouth covering protection with argon during the process of adding Al blocks for dissolution. Bottom blowing argon is to introduce argon gas into the bottom of the furnace for bubbling, so that the slag in the molten steel adheres, which helps to remove the slag. Furnace mouth argon covering protection is to replace the air at the furnace mouth with argon to prevent the added Al from being oxidized by oxygen in the air. One of the characteristics of the alloy of the present invention is that it contains Al. The present invention adopts furnace bottom blowing argon and furnace mouth covering protection with argon when adding Al for dissolution, which ensures that the added Al is not burned and oxidized. During the Al dissolution process, the temperature of the molten steel in the furnace can be controlled by controlling the power of the medium frequency furnace to avoid accidents caused by excessive temperature. In step 3), after Al is dissolved, the molten steel can be heated, and then a slag-making agent can be added to slag and slag. Preferably, slag is removed after the molten steel is heated to 1680±50℃.

[0082] In step 4), Re, Ti, and Zr can be added to a ladle, and molten steel can be introduced into the ladle. The pouring of the molten steel completes the dissolution and homogenization of the Re and other raw materials. After the molten steel is poured, the surface of the molten steel in the ladle is slag-coated. One of the characteristics of the alloy of the present invention is that it contains Re. By adding Re to the molten steel, the present invention improves the castability of the molten steel and enhances the performance of the alloy.

[0083] In step 5), slag removal can be performed when the molten steel reaches the pouring temperature. Those skilled in the art can determine the pouring temperature based on the amount of molten steel, mold size, and other factors. After slag removal, the molten steel in the ladle can be poured into a high-speed rotating metal mold in a centrifuge. After the molten steel cools, a centrifugal cast pipe is obtained. The casting time should be as short as possible.

[0084] In some embodiments, the high-aluminum austenitic centrifugally cast tube of the present invention is manufactured using a method comprising the following steps:

[0085] Step 1: Select and prepare raw materials according to the target chemical composition. The raw materials are smelted in the order of non-oxidizable to easily oxidizable (for example, Fe, Ni, C, Mn, Cr, Si are smelted in the order of Fe, Ni, C, Mn, FeCr, FeSi), and the chemical components other than Al, Re, Ti, and Zr are melted to obtain molten steel; the raw materials are preferably selected to control the content of harmful elements such as Pb, Sn, Sb, Zn, As, and Bi in the molten steel to be less than 50 ppm respectively; chemical composition samples are taken and sent to the laboratory for testing, and the chemical composition is adjusted according to the laboratory chemical analysis results;

[0086] Step 2: After the chemical composition is verified, the molten steel is heated to 1650±50℃, deoxidized with a deoxidizer and then slag removed. A slag forming agent is used to cover the molten steel in the furnace, and argon blowing is started at the bottom of the furnace. After blowing argon for 3±1 minutes, slag removal is performed.

[0087] Step 3: Cover the furnace mouth with argon to prevent air from reacting with the molten steel surface; add Al blocks for dissolution, and keep the furnace bottom blown with argon and the furnace mouth covered with argon for protection during this process; after Al is dissolved, heat and stir, and after the molten steel is heated to 1680±50℃, add slag-making agent to make slag and prepare for unloading;

[0088] Step 4: Add rare earth, Ti, and Zr into the ladle; introduce molten steel into the ladle, and complete the dissolution and homogenization process of rare earth and other raw materials through the pouring process of molten steel; after the molten steel is poured, the surface of the molten steel in the ladle is covered with slag;

[0089] Step 5: Before transferring the ladle to the centrifuge, after the temperature of the molten steel reaches the pouring temperature, the slag is removed from the ladle for the last time, and then the molten steel in the ladle is quickly poured into the high-speed rotating metal mold on the centrifuge. After the molten steel cools, a centrifugal casting tube is obtained.

[0090] The microstructure of the high-aluminum austenitic alloy and high-aluminum austenitic centrifugally cast pipe of the present invention comprises, or is composed of, columnar crystals having a volume fraction of 80% or greater and equiaxed crystals having a volume fraction of 20% or less. In a preferred embodiment, the high-aluminum austenitic centrifugally cast pipe of the present invention has columnar crystals near the outer wall along the wall thickness direction and uniform equiaxed crystals near the inner wall.

[0091] The high-aluminum austenitic alloy centrifugally cast pipe of the present invention may have an outer diameter of 60-250 mm, such as 60-70 mm, and a wall thickness of 6-10 mm, such as 7-8 mm.

[0092] The high-aluminum austenitic alloy and the high-aluminum austenitic centrifugally cast pipe of the present invention have excellent corrosion resistance and creep resistance at temperatures of 900° C. and above, and also have required mechanical properties.

[0093] Compared with 35 / 45 alloy, the high aluminum austenitic alloy and high aluminum austenitic centrifugal casting pipe of the present invention have:

[0094] (1) Longer durability: The durability measured under the test conditions of 1100°C and 17 MPa is ≥100 hours, preferably ≥110 hours, and more preferably ≥115 hours;

[0095] (2) Smaller creep rate: the average creep rate of the second stage of creep measured under the test conditions of 1050°C and 15 MPa is ≤0.0005% / h, preferably ≤0.0003% / h; the average creep rate of the second stage of creep measured under the test conditions of 1050°C and 20 MPa is ≤0.002% / h, preferably ≤0.0015% / h; the average creep rate of the second stage of creep measured under the test conditions of 1050°C and 25 MPa is ≤0.01% / h, preferably ≤0.007% / h; the average creep rate of the second stage of creep measured under the test conditions of 1050°C and 30 MPa is ≤0.05% / h, preferably ≤0.035% / h;

[0096] (3) Better oxidation resistance: The air temperature is raised to 950℃ at a rate of 600℃ / h, maintained for 4 hours, and then cooled to room temperature to measure its weight gain. After this process is repeated 19 times, the weight gain of the alloy is ≤0.3g / m 2 , preferably ≤0.15g / m 2 ;

[0097] (4) Better anti-carburization performance: Under the test conditions of 1150℃ / 7 days, the carbon increase at a depth of 1mm is less than 0.5%, preferably less than 0.45%, and the carbon increase at a depth of 2mm is less than 0.05%, preferably less than 0.03%.

[0098] At the same time, the high-aluminum austenitic alloy and high-aluminum austenitic centrifugally cast pipe of the present invention have good strength and elongation at high temperatures: the yield strength measured under the test conditions of 850°C is ≥120MPa, for example ≥124MPa, the tensile strength is ≥185MPa, for example ≥189MPa; the elongation is ≥49%, for example ≥50%; the yield strength measured under the test conditions of 1050°C is ≥53MPa, for example ≥55MPa; the tensile strength is ≥65MPa, for example ≥67MPa; the elongation is ≥59%, for example ≥61%.

[0099] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0100] The high-aluminum austenite centrifugal cast pipes of Examples 1-7 and Comparative Examples 8-10 were manufactured by the following method:

[0101] Step 1: Select and prepare raw materials according to the target chemical composition. The raw materials are smelted in the order of non-oxidizable to easily oxidizable. The chemical components other than Al, Re, Ti, and Zr are melted to obtain molten steel. Among them, Fe, Ni, C, Mn, Cr, and Si are smelted in the order of Fe, Ni, C, Mn, FeCr, and FeSi. The contents of harmful elements such as Pb, Sn, Sb, Zn, As, and Bi in the molten steel are controlled to be less than 50 ppm respectively. Chemical composition samples are taken and sent to the laboratory for testing. According to the laboratory chemical analysis results, the chemical composition in the furnace is adjusted.

[0102] Step 2: After the chemical composition is verified, the molten steel is heated to 1650℃, deoxidized with a deoxidizer and then slag removed. A slag forming agent is used to cover the molten steel in the furnace, and argon blowing is started at the bottom of the furnace. After blowing argon for 3 minutes, slag removal is performed.

[0103] Step 3: Cover the furnace mouth with argon to prevent air from reacting with the molten steel surface; add Al blocks for dissolution, and keep the furnace bottom blown with argon and the furnace mouth covered with argon for protection during this process; after Al is dissolved, heat and stir, and after the molten steel is heated to 1680℃, add slag-making agent to make slag and remove slag, and prepare to be discharged from the furnace;

[0104] Step 4: Add rare earth, Ti, and Zr into the ladle; introduce molten steel into the ladle, and complete the dissolution and homogenization process of rare earth and other raw materials through the pouring process of molten steel; after the molten steel is poured, the surface of the molten steel in the ladle is covered with slag;

[0105] Step 5: Transfer the ladle to the centrifuge. After the temperature reaches the pouring temperature, carry out the last slag removal in the ladle. Then the molten steel in the ladle is quickly poured into the high-speed rotating metal mold on the centrifuge. After the molten steel cools down, a centrifugal casting tube is obtained.

[0106] The outer diameter of the centrifugal casting pipe in the embodiment of the present invention is 66 mm, the wall thickness is 7 mm, and the microstructure is composed of columnar crystals with a volume fraction of ≥80% and equiaxed crystals with a volume fraction of ≤20%. The columnar crystals are close to the outer wall in the wall thickness direction, and the uniform equiaxed crystals are close to the inner wall.

[0107] The chemical compositions and contents of the centrifugally cast pipes of the present invention's embodiments and comparative examples are shown in Table 1. Herein, alloys 1-7 correspond to embodiments 1-7, respectively; alloys 8-10 correspond to comparative examples 8-10, respectively; alloy 11 is a conventional alloy material ZG50Ni45Cr35NbM (35 / 45 alloy) with a carbon content of 0.44%; and alloy 12 is a conventional alloy material ZG50Ni45Cr35NbM (35 / 45 alloy) with a carbon content of 0.45%.

[0108] Table 1: Composition of Example and Comparative Example Alloys (wt%) Balance Fe

[0109] alloy C Mn Si Cr Ni Al Ti Zr Nb Ta Mo W N Re 1 0.4 0.33 0.07 24 48 4.5 0.2 0.24 0.5 0.4 0.5 0.8 0.034 0.08 2 0.32 0.25 0.26 25 40 4 0.27 0.25 0.9 0.9 0.6 0.4 0.006 0.18 3 0.63 0.28 0.4 24 50 3.5 0.08 0.1 1 1.2 1 1.5 0.024 0.22 4 0.52 0.04 0.34 26 47 4.3 0.2 0.23 0.7 0.6 0.2 1.9 0.009 0.08 5 0.47 0.21 0.1 20 44 3.8 0.1 0.09 0.1 2 0.8 1.1 0.012 0.29 6 0.7 0.11 0.4 22 44 5 0.3 0.28 0.3 1.7 0.6 0.9 0.034 0.15 7 0.45 0.15 0.26 23 42 4.9 0.04 0.02 1 0.8 0.3 0.6 0.017 0.24 8 0.4 0.43 0.05 29 48 4.6 0.2 0.23 0.5 0.4 0.51 0.8 0.033 0.05 9 0.52 0.39 0.34 26 47 6 0.2 0.24 0.7 0.6 0.2 1.9 0.008 0.08 10 0.47 0.21 0.1 20 44 7.5 0.1 0.9 0.1 2 0.8 1.1 0.012 0.29 11 0.44 1.2 1.4 35 45 - 0.1 - 0.7 - - 0.8 0.003 - 12 0.45 1.1 1.3 35 45 - 0.08 - 0.6 - - 0.7 0.003 -

[0110] Durability: The durability of the alloy was measured under the test conditions of 1100°C / 17 MPa according to ASTM E139-11. The results are shown in Table 2.

[0111] As shown in Table 2, the alloys of the embodiments of the present invention have better endurance at 1100°C / 17 MPa than the alloys of the comparative example and alloys No. 11 and No. 12 of the prior art.

[0112] Table 2: Endurance of each alloy at 1100℃ / 17MPa

[0113] alloy Durability (h) 1 127 2 131 3 138 4 118 5 154 6 123 7 126 8 86 9 104 10 97 11 108 12 100

[0114] Creep rate: At 1050℃, different stresses were applied to the alloy, and the lengths at different times were measured using an extensometer. The deformation was derived from the time derivative to obtain the deformation rate. The average results of the deformation rate in the second stage of creep are shown in Table 3. For ease of comparison, the average creep rate of the second stage of creep was obtained by taking the logarithm of the pressure and creep average rate. Figure 1 Table 3 and Figure 1 The 35 / 45 alloy is alloy No. 11.

[0115] From Table 3 and Figure 1 It can be seen that at the same pressure and temperature, the average creep rate of the second stage of creep of the alloy of the present invention is significantly lower than that of the comparative alloy. Therefore, the creep resistance of the alloy of the present invention is significantly better than that of the comparative alloy 35 / 45.

[0116] Table 3: Average creep rate of the second stage of creep of alloys at 1050℃ and different pressures

[0117]

[0118] Cyclic oxidation: To simulate the actual conditions of the alloy during use, a cyclic oxidation test was conducted on the alloy. The air temperature was raised to 950℃ at a rate of 600℃ / h, maintained for 4 hours, and then cooled to room temperature to measure its weight gain. This process was repeated. The test results are shown in Table 4 and Figure 2 As shown in Table 4 and Figure 2 The 35 / 45 alloy is alloy No. 11.

[0119] From Table 4 and Figure 2 It can be seen that the oxidation resistance of the alloy of the present invention is significantly better than that of the 35 / 45 alloy.

[0120] Table 4: Weight gain of alloy after cyclic oxidation

[0121]

[0122] High temperature short time tensile test: refer to ASTM E21-05 to measure the yield, tensile and elongation test of the alloy at 850, 950, 1050 and 1150℃. The results are shown in Table 5. Figure 3 and Figure 4 shown. Figure 3 The alloy in is alloy No. 1. Figure 4 The alloy in is alloy No. 11.

[0123] By comparing the alloy of Example 1 with the 35 / 45 alloy, it can be seen that the alloy of Example 1 has good strength and elongation at high temperatures even though it contains a higher content of aluminum.

[0124] Table 5-1: Results of high temperature short time tensile tests of example alloys and comparative alloys at different temperatures

[0125]

[0126] Table 5-2: Results of high temperature short time tensile tests of example alloys and comparative alloys at different temperatures

[0127]

[0128] Carburizing test: solid carburizing agent was placed in the test pipe section, dried, welded and sealed, placed in 1150℃ environment, kept warm for 7 days, and the increase in carbon content per millimeter from the inner surface to the outer surface of the alloy was measured. The results are shown in Table 6 and Figure 5 shown. Figure 5 The 35 / 45 alloy is alloy No. 11.

[0129] From Table 6 and Figure 5 It can be seen that the carburization amount of the alloy of the present invention is significantly lower than that of the comparative alloy, indicating that the alloy of the present invention has good anti-carburization performance.

[0130] Table 6: Carburizing test results of example alloys and comparative alloys at 1150°C for 7 days (%)

[0131]

Claims

1. A high-aluminum austenitic alloy or a high-aluminum austenitic centrifugally cast pipe, characterized in that: The element composition of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugal cast pipe is as follows: C, 0.4-0.65%; Mn, 0-0.4%; Si, 0-0.4%; Cr, 20-26%; Ni, 40-50%; Al, 3.5-5%; Ti, 0.04-0.3%; Zr, 0.01-0.3%; Nb, 0.1-1%; Ta, 0.4-2%; Mo, 0.2-1%; W, 0.4-1.9%; N, 0.006-0.035%; Re, 0.08-0.3%; the balance is Fe and unavoidable impurities.

2. The high-aluminum austenite alloy or high-aluminum austenite centrifugally cast pipe according to claim 1, characterized in that: Re is Y, Hf and Ce, and the content of each of Y, Hf and Ce is 0.01-0.1%.

3. The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe according to claim 1, characterized in that: The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe further contains one or more of Cu, V, Co and B.

4. The high-aluminum austenite alloy or high-aluminum austenite centrifugally cast pipe according to claim 3, characterized in that: The elemental composition of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe has one or more of the following characteristics: Cu content ≤ 0.1%; V content ≤ 0.01%; Co content ≤ 0.03%; and B content ≤0.1%.

5. The high-aluminum austenite alloy or high-aluminum austenite centrifugally cast pipe according to claim 1, characterized in that: The unavoidable impurities include one or more of S, P and O. Preferably, the S content of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugal cast tube is ≤0.005%, the P content is ≤0.005%, and the O content is ≤0.005%.

6. The high-aluminum austenite alloy or high-aluminum austenite centrifugally cast pipe according to claim 1, characterized in that: The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe has one or more of the following properties: The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe has a durability of ≥100 hours, preferably ≥110 hours, and more preferably ≥115 hours, measured under test conditions of 1100°C and 17 MPa; The average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe in the second stage of creep measured under the test conditions of 1050° C. and 15 MPa is ≤0.0005% / h, preferably ≤0.0003% / h; The average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe in the second stage of creep measured under the test conditions of 1050° C. and 20 MPa is ≤0.002% / h, preferably ≤0.0015% / h; The average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe in the second stage of creep measured under the test conditions of 1050° C. and 25 MPa is ≤0.01% / h, preferably ≤0.007% / h; The average creep rate of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe in the second stage of creep measured under the test conditions of 1050° C. and 30 MPa is ≤0.05% / h, preferably ≤0.035% / h; The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe has a yield strength of ≥120 MPa, a tensile strength of ≥185 MPa, and an elongation of ≥49% measured under the test condition of 850°C; The high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe has a yield strength of ≥53 MPa, a tensile strength of ≥65 MPa, and an elongation of ≥59% measured under the test condition of 1050° C.; and Under the test conditions of 1150°C / 7 days, the carbon increase of the high-aluminum austenitic alloy or the high-aluminum austenitic centrifugal cast pipe at a depth of 1 mm is less than 0.5%, and the carbon increase at a depth of 2 mm is less than 0.05%.

7. The high-aluminum austenite centrifugal cast pipe according to claim 1, characterized in that: The high-aluminum austenitic alloy centrifugally cast pipe has an outer diameter of 60-250 mm and a wall thickness of 6-10 mm.

8. The high-aluminum austenite centrifugal cast pipe according to claim 1, characterized in that: The microstructure of the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast tube includes columnar crystals with a volume fraction ≥80% and equiaxed crystals with a volume fraction ≤20%; preferably, the high-aluminum austenitic centrifugally cast tube has columnar crystals near the outer wall in the wall thickness direction and uniform equiaxed crystals near the inner wall.

9. A method for manufacturing the high-aluminum austenitic alloy or high-aluminum austenitic centrifugally cast pipe according to claims 1 to 8, comprising the following steps: 1) Melting: Melting the chemical components other than Al, Re, Ti, and Zr in the high-aluminum austenitic alloy or high-aluminum austenitic centrifugal cast tube in a medium frequency furnace according to the target chemical composition to obtain molten steel; 2) Deoxidation and slag removal: deoxidation and slag removal are performed on the molten steel obtained in step 1); 3) Adding Al: adding Al to the molten steel treated in step 2), and slagging after Al dissolves; 4) Quenching and tempering: adding Re, Ti and Zr into the ladle, introducing the molten steel treated in step 3) into the ladle, dissolving Re, Ti and Zr and then slagging; 5) Casting: Deslagging is performed before casting, and then the molten steel is poured into a metal mold. After cooling, a high-aluminum austenitic alloy or a high-aluminum austenitic centrifugal cast pipe is obtained.

10. The method according to claim 9, wherein The method has one or more of the following characteristics: In step 1), the contents of Pb, Sn, Sb, Zn, As, and Bi in the molten steel are controlled to be less than 50 ppm respectively; In step 2), the molten steel is heated to 1650±50° C., deoxidized with a deoxidizer, and then slag removed; In step 2), slag removal includes: using a slag-forming agent to cover the molten steel in the furnace, starting argon blowing at the bottom of the furnace, and then performing slag removal after argon blowing, wherein the argon blowing time is preferably 3±1 min; In step 3), the furnace mouth is covered with argon gas for protection to isolate the air from reacting with the molten steel surface; In step 3), during the process of adding Al and dissolving Al, argon is blown at the bottom of the furnace and argon is covered at the furnace mouth for protection; and In step 3), after Al is dissolved, the temperature is raised to 1680±50° C., and a slag forming agent is added to form slag and remove slag.

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