Heat-resistant steel for main steam pipeline of ultra-supercritical unit and preparation method of heat-resistant steel

Abstract

The invention discloses heat-resistant steel for a main steam pipeline of an ultra-supercritical unit and a preparation method of the heat-resistant steel. The heat-resistant steel comprises the following elements in percentage by mass: 0.08-0.12% of C; 0.35 to 0.55 percent of Si; mn: 0.55 to 0.65%; cr: 9.0 to 10.5%; 0.6 to 1.0 part of Mo; 2.0 to 3.5 parts of Co; 0.12 to 0.2 part of Zr; v: 0.22 to 0.35; 0.06 to 0.014% of Nb; 0.03 to 0.07% of N; b: 0.003 to 0.011% of the total weight; 0.6 to 1.0 part of Ni; 0.15 to 0.3 part of W; the mass ratio of Ce to La is (1: 1): (0.05-0.1); 0.01 to 0.05 percent of Hf, Ta and Sc; 0.01 to 0.03 percent of Ir, and the balance of Fe and impurities. Molybdenum, scandium and iridium are introduced, so that the high-temperature creep resistance, mechanical stability, strength and corrosion resistance of the heat-resistant steel are improved.

Description

Technical Field

[0001] The invention relates to the technical field of heat-resistant steel, and in particular to a heat-resistant steel for a main steam pipeline of an ultra-supercritical unit and a preparation method thereof. Background Art

[0002] In recent years, with the vigorous commissioning of ultra-supercritical power generating units with high power generation efficiency, my country's ultra-supercritical unit technology level, development speed, installed capacity and number of units have leapt to the forefront of the world. The heat-resistant components of the main steam boiler pipes of ultra-supercritical units are the bottleneck of manufacturing power plants.

[0003] The high-temperature steam pipelines of the 600℃ or 620℃ ultra-supercritical units currently in commercial operation or under construction often use the new martensitic heat-resistant steel P92 steel for the main steam pipelines and high-temperature reheat steam pipelines. When the temperature exceeds 630-650℃, the high-strength P92 steel commonly used in 600-620℃ ultra-supercritical units can no longer meet the thermal strength requirements of the 700℃ ultra-supercritical unit boilers, otherwise there will be problems of insufficient endurance strength and insufficient environmental corrosion resistance.

[0004] Therefore, in order to overcome the above problems, the present invention provides a heat-resistant steel for a main steam pipeline of an ultra-supercritical unit and a preparation method thereof. Summary of the invention

[0005] The purpose of the present invention is to provide a heat-resistant steel for the main steam pipe of an ultra-supercritical unit and a preparation method thereof in view of the defects of the prior art, so as to solve the problems raised by the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solution: A heat-resistant steel for the main steam pipeline of an ultra-supercritical unit, comprising the following elements in percentage by mass:

[0007] C: 0.08~0.12%; Si: 0.35~0.55%; Mn: 0.55~0.65%; Cr: 9.0~10.5%; Mo: 0.6~1.0%; Co: 2.0~3.5%; Zr: 0.12~0.2%; V: 0.22~0.35%; Nb: 0.06~0.014%; N: 0.03~0.07%; B: 0.003~0.011%; Ni: 0.6~1.0%; W: 0.15~0.3%; a mixture of Ce and La in a mass ratio of 1:1: 0.05~0.1%; Hf: 0.01~0.05%; Ta: 0.01~0.05%; Sc: 0.01~0.05%; Ir: 0.01~0.03%; the balance is Fe and impurities.

[0008] As a preferred technical solution of the present invention, Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, and S≤0.005wt.% in the impurities.

[0009] A method for preparing heat-resistant steel for a main steam pipeline of an ultra-supercritical unit, the specific steps are as follows:

[0010] S1: The alloy raw materials are melted by high vacuum arc melting method, and electromagnetic stirring is performed during the melting process to ensure uniform composition. After melting, the cast alloy is obtained by suction casting system;

[0011] S2: The cast alloy is subjected to two normalizing treatments:

[0012] S21: The first normalizing treatment is heating to 1050-1150℃, keeping the temperature for 2.5-3.0h and then air cooling to refine the grains and eliminate internal stress;

[0013] S22: Second normalizing treatment, heating to 900-1000℃, keeping warm for 1.5-2.5h and then air cooling, used to improve the toughness and creep resistance of the material;

[0014] S3: Quenching the alloy after normalizing treatment, specifically:

[0015] S31: heating the alloy to 980-1020°C, keeping the temperature for 1.5-2.5 hours, and then rapidly cooling to room temperature to form a martensitic structure, which is used to improve the hardness and strength of the material;

[0016] S4: Tempering the quenched alloy, specifically:

[0017] S41: The alloy is heated to 600-700°C, kept warm and then air-cooled to precipitate strengthening phases to improve the high-temperature strength and stability of the material;

[0018] S5: The prepared heat-resistant steel is shot peened to introduce compressive stress on the surface of the material by spraying small particles at high speed, thereby forming a dense FeCr2O4 protective layer to improve the material's antioxidant properties.

[0019] As a preferred technical solution of the present invention, step S1 comprises the following steps of preparing the cast alloy by high vacuum arc melting:

[0020] S11: The interior of the melting furnace is made to reach a high vacuum state by evacuating the vacuum. After the interior of the melting furnace reaches a high vacuum state, an appropriate amount of argon gas is introduced as a protective gas to make the gas pressure in the furnace reach -0.5Mpa;

[0021] S12: Deoxidation of pure titanium by melting with an arc gun;

[0022] S13: Move the arc gun to the melting crucible position, increase the arc gun current to 230A to start melting the alloy, and continue electromagnetic stirring for 30s after the alloy raw materials are melted, maintain a constant current for 30s, and then reduce the arc gun current to 0A, and the magnetic stirring current is 2A;

[0023] S14: Repeat the step S13 of melting the alloy 4 times to make the alloy composition uniform. After the melting is completed, an initial cast alloy is obtained;

[0024] S15: placing the initially cast alloy cooled in the melting crucible into the suction casting crucible through a long-handled ladle, repeating the alloy melting operation, and repeating the melting to obtain molten metal liquid, at which time the arc gun current is increased to 270-280A;

[0025] S16: After the initial cast alloy in the suction casting crucible is completely melted, the molten alloy is sucked into a copper mold through a suction casting system to obtain a cast alloy steel ingot.

[0026] As a preferred technical solution of the present invention, the electromagnetic stirring current in step S13 is 2A.

[0027] As a preferred technical solution of the present invention, the vacuum in step S11 is 10 -5 Pa.

[0028] As a preferred technical solution of the present invention, the alloy after normalizing treatment uses quenching oil with a quenching medium of 60° C. during quenching, and a structure mainly composed of martensite is formed after quenching.

[0029] As a preferred technical solution of the present invention, the tempering treatment in step S41 adopts a temperature of 600-700°C, a heating rate of 80°C / h, a holding time of 1.5-4h, and then air-cooling to room temperature.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The heat-resistant steel for the main steam pipe of an ultra-supercritical unit provided by the present invention can significantly improve the high-temperature endurance strength of the alloy by adding nitrogen, vanadium and niobium; adding boron and cobalt and controlling the content of molybdenum can improve the creep resistance of the alloy; adding chromium can enhance the high-temperature endurance strength and creep resistance of the alloy; adding silicon can improve the oxidation resistance.

[0032] 2. The present invention provides a heat-resistant steel for the main steam pipe of an ultra-supercritical unit, which suppresses the segregation of impurities on grain boundaries under high temperature and corrosive conditions by controlling aluminum, titanium, phosphorus and sulfur at lower levels.

[0033] 3. The present invention forms a stable oxide film and carbide phase by introducing rare earth elements (a mixture of Ce and La), tantalum, hafnium, scandium and iridium, which greatly enhances the high-temperature oxidation resistance and creep resistance of heat-resistant steel.

[0034] 4. The present invention provides a heat-resistant steel for the main steam pipe of an ultra-supercritical unit, which is finely controlled by normalizing and tempering, so that the alloy elements are completely dissolved in austenite and finely dispersed M23C6 carbides are precipitated. The introduction of tantalum and hafnium further enhances the stability of the carbides, and the introduction of iridium and scandium improves the high-temperature stability and creep resistance of the material, thereby obtaining a strong and tough martensitic heat-resistant steel.

[0035] In summary, the present invention provides a method for preparing heat-resistant steel for the main steam pipe of an ultra-supercritical unit. The method can prepare heat-resistant steel for the main steam pipe of an ultra-supercritical unit having excellent high-temperature mechanical properties and high-temperature creep properties and meeting the use requirements at a steam temperature of 650°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The present invention is a flow chart of a method for preparing heat-resistant steel for a main steam pipeline of an ultra-supercritical unit.

[0037] Figure 2 is a metallographic diagram of the martensitic heat-resistant steel prepared in Example 1 of the present invention; DETAILED DESCRIPTION

[0038] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0039] Embodiment 1: According to the attached Figure 1-2 As shown, the present embodiment provides a method for preparing a heat-resistant steel for the main steam pipeline of an ultra-supercritical unit, wherein the mass ratio of each component in the alloy is: C: 0.10%; Si: 0.4%; Mn: 0.5%; Cr: 9.50%; Mo: 0.8%; Co: 2.50%; Zr: 0.1%; V: 0.20%; Nb: 0.050%; N: 0.020%; B: 0.0020%; Ni: 0.6%; W: 0.15%; rare earth elements (a mixture of Ce and La, a mass ratio of 1:1): 0.07%; Hf: 0.03%; Ta: 0.04%; Sc: 0.03%; Ir: 0.02%; the remainder is industrial pure iron and impurities, wherein the impurities are controlled as follows: Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, S≤0.005wt.%.

[0040] The preparation method comprises the following steps:

[0041] Step 1: The melting crucible heating method adopts high vacuum arc melting technology, adds alloy raw materials into the melting crucible, and puts pure titanium into the impurity removal crucible;

[0042] Step 2: Use the molecular pump vacuum system to make the inside of the melting furnace reach a high vacuum (10 -5 Pa) state, after the inside of the melting furnace reaches a high vacuum state, introduce an appropriate amount of argon as a protective gas to make the pressure in the furnace reach -0.5MPa;

[0043] Step 3: Use an inverter arc welding machine as a power source to melt pure titanium. Increase the arc gun current to 340A. When the pure titanium melts and the surface of the pure titanium liquid metal becomes red and translucent, continue heating for 20 seconds, then reduce the arc gun current to 0A.

[0044] Step 4: After the pure titanium metal liquid is cooled, repeat the pure titanium smelting step in step 3 for 3 times to fully remove the oxygen in the smelting furnace;

[0045] Step 5: Use the inverter arc welding machine as the power source to melt the alloy, move the arc gun to the melting crucible position, increase the arc gun current to 230A to start melting the alloy, and continue electromagnetic stirring for 30s after the alloy raw materials are melted, maintain a constant current for 30s, then reduce the arc gun current to 0A, and the magnetic stirring current is 2A;

[0046] Step 6: Repeat the alloy smelting step in step 5 for 3 times to make the initial cast alloy composition uniform. Place the cooled initial cast alloy in the smelting crucible into the suction casting crucible through a long-handled ladle, and repeat the alloy smelting operation to obtain molten metal liquid. At this time, the arc gun current is increased to 270-280A.

[0047] After the initial cast alloy in the suction casting crucible melts into liquid, the liquid alloy is sucked into the copper mold through the suction casting system to obtain a cast alloy with a size of Φ6mm×6cm;

[0048] Step 7: First normalizing treatment: heat the cast alloy to 1100°C at a rate of 400°C / h, keep it at that temperature for 3.0 hours, and then air cool it; Second normalizing treatment: heat the normalized alloy to 950°C, keep it at that temperature for 2.0 hours, and then air cool it again;

[0049] Step 8: After the alloy has been normalized twice, it is heated to 1000°C, kept at this temperature for 2.0 hours, and then rapidly cooled to room temperature using quenching oil at 60°C as the quenching medium to form a martensitic structure;

[0050] Step 9: The quenched alloy was heated to 650°C at a rate of 80°C / h, kept at this temperature for 4.0 hours, and then air-cooled to room temperature;

[0051] Step 10: Shot peening is performed on the heat-resistant steel after tempering. By spraying small particles at high speed, compressive stress is introduced into the surface of the material to form a dense FeCr2O4 protective layer to improve the material's oxidation resistance.

[0052] Embodiment 2:

[0053] The present embodiment provides a method for preparing heat-resistant steel for the main steam pipeline of an ultra-supercritical unit, wherein the mass ratios of the components in the alloy are: C: 0.12%; Si: 0.50%; Mn: 0.60%; Cr: 10.0%; Mo: 0.9%; Co: 3.0%; Zr: 0.15%; V: 0.30%; Nb: 0.07%; N: 0.05%; B: 0.004%; Ni: 0.7%; W: 0.2%; rare earth elements (a mixture of Ce and La, a mass ratio of 1:1): 0.08%; Hf: 0.04%; Ta: 0.05%; Sc: 0.04%; Ir: 0.03%; the remainder is industrial pure iron and impurities, wherein the impurities are controlled as follows: Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, and S≤0.005wt.%.

[0054] The preparation method comprises the following steps:

[0055] Step 1: The melting crucible heating method adopts high vacuum arc melting technology, adds alloy raw materials into the melting crucible, and puts pure titanium into the impurity removal crucible;

[0056] Step 2: Use the molecular pump vacuum system to make the inside of the melting furnace reach a high vacuum (10 -5 Pa) state, after the inside of the melting furnace reaches a high vacuum state, introduce an appropriate amount of argon as a protective gas to make the pressure in the furnace reach -0.5MPa;

[0057] Step 3: Use an inverter arc welding machine as a power source to melt pure titanium. Increase the arc gun current to 340A. When the pure titanium melts and the surface of the pure titanium liquid metal becomes red and translucent, continue heating for 20 seconds, then reduce the arc gun current to 0A.

[0058] Step 4: After the pure titanium metal liquid is cooled, repeat the pure titanium smelting step in step 3 for 3 times to fully remove the oxygen in the smelting furnace;

[0059] Step 5: Use the inverter arc welding machine as the power source to melt the alloy, move the arc gun to the melting crucible position, increase the arc gun current to 230A to start melting the alloy, and continue electromagnetic stirring for 30s after the alloy raw materials are melted, maintain a constant current for 30s, then reduce the arc gun current to 0A, and the magnetic stirring current is 2A;

[0060] Step 6: Repeat the alloy smelting step in step 5 for 3 times to make the initial cast alloy composition uniform, place the cooled initial cast alloy in the smelting crucible into the suction casting crucible through a long-handled ladle, repeat the alloy smelting operation to obtain molten metal liquid, and at this time, the arc gun current is increased to 270-280A;

[0061] After the initial cast alloy in the suction casting crucible melts into liquid, the liquid alloy is sucked into the copper mold through the suction casting system to obtain a cast alloy with a size of Φ6mm×6cm;

[0062] Step 7: First normalizing treatment: heat the cast alloy to 1150°C at a rate of 400°C / h, keep it at that temperature for 3.0 hours, and then air cool it. Second normalizing treatment: heat the normalized alloy to 900°C, keep it at that temperature for 2.0 hours, and then air cool it again;

[0063] Step 8: After the alloy has been normalized twice, it is heated to 1020°C, kept at this temperature for 2.5 hours, and then rapidly cooled to room temperature using quenching oil at 60°C as the quenching medium to form a martensitic structure;

[0064] Step 9: The quenched alloy was heated to 650°C at a rate of 80°C / h, kept at this temperature for 4.0 hours, and then air-cooled to room temperature;

[0065] Step 10: Shot peening is performed on the heat-resistant steel after tempering. By spraying small particles at high speed, compressive stress is introduced into the surface of the material to form a dense FeCr2O4 protective layer to improve the material's oxidation resistance.

[0066] Embodiment 3:

[0067] The present embodiment provides a method for preparing heat-resistant steel for the main steam pipeline of an ultra-supercritical unit, wherein the mass ratio of each component in the alloy is: C: 0.09%; Si: 0.45%; Mn: 0.55%; Cr: 9.20%; Mo: 0.7%; Co: 2.8%; Zr: 0.12%; V: 0.25%; Nb: 0.06%; N: 0.04%; B: 0.003%; Ni: 0.9%; W: 0.25%; rare earth elements (a mixture of Ce and La, a mass ratio of 1:1): 0.06%; Hf: 0.02%; Ta: 0.03%; Sc: 0.02%; Ir: 0.01%, and the balance is industrial pure iron and impurities, wherein the impurities are controlled as follows: Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, and S≤0.005wt.%.

[0068] The preparation method comprises the following steps:

[0069] Step 1: The melting crucible heating method adopts high vacuum arc melting technology, adds alloy raw materials into the melting crucible, and puts pure titanium into the impurity removal crucible;

[0070] Step 2: Use the molecular pump vacuum system to make the inside of the melting furnace reach a high vacuum (10 -5 Pa) state, after the inside of the melting furnace reaches a high vacuum state, introduce an appropriate amount of argon as a protective gas to make the pressure in the furnace reach -0.5MPa;

[0071] Step 3: Use an inverter arc welding machine as a power source to melt pure titanium. Increase the arc gun current to 340A. When the pure titanium melts and the surface of the pure titanium liquid metal becomes red and translucent, continue heating for 20 seconds, then reduce the arc gun current to 0A.

[0072] Step 4: After the pure titanium metal liquid is cooled, repeat the pure titanium smelting step in step 3 for 3 times to fully remove the oxygen in the smelting furnace;

[0073] Step 5: Use the inverter arc welding machine as the power source to melt the alloy, move the arc gun to the melting crucible position, increase the arc gun current to 230A to start melting the alloy, and continue electromagnetic stirring for 30s after the alloy raw materials are melted, maintain a constant current for 30s, then reduce the arc gun current to 0A, and the magnetic stirring current is 2A;

[0074] Step 6: Repeat the alloy smelting step in step 5 for 3 times to make the initial cast alloy composition uniform. Place the cooled initial cast alloy in the smelting crucible into the suction casting crucible through a long-handled ladle, and repeat the alloy smelting operation to obtain molten metal liquid. At this time, the arc gun current is increased to 270-280A.

[0075] After the initial cast alloy in the suction casting crucible melts into liquid, the liquid alloy is sucked into the copper mold through the suction casting system to obtain a cast alloy with a size of Φ6mm×6cm;

[0076] Step 7: First normalizing treatment: heat the cast alloy to 1050°C at a rate of 400°C / h, keep it at that temperature for 2.5 hours, and then air cool it. Second normalizing treatment: heat the normalized alloy to 930°C, keep it at that temperature for 1.5 hours, and then air cool it again;

[0077] Step 8: After the alloy has been normalized twice, it is heated to 1000°C, kept at this temperature for 2.0 hours, and then rapidly cooled to room temperature using quenching oil at 60°C as the quenching medium to form a martensitic structure;

[0078] Step 9: The quenched alloy was heated to 620°C at a rate of 80°C / h, kept at this temperature for 3.5 hours, and then air-cooled to room temperature;

[0079] Step 10: Shot peening is performed on the heat-resistant steel after tempering. By spraying small particles at high speed, compressive stress is introduced into the surface of the material to form a dense FeCr2O4 protective layer to improve the material's oxidation resistance.

[0080] Comparative Example 1: The process steps of Comparative Example 1 are the same as those of Example 1, and the chemical composition thereof does not include manganese, cobalt, molybdenum, tantalum, hafnium, scandium, and iridium elements. The mass ratios of the components in the alloy are: C: 0.10%; Si: 0.4%; Mn: 0.5%; Cr: 9.50%; Mo: 0.8%; Co: 2.50%; Zr: 0.1%; V: 0.20%; Nb: 0.050%; N: 0.020%; B: 0.0020%; Ni: 0.6%; W: 0.15%, and the remainder is industrial pure iron and impurities, wherein the impurities are controlled as follows: Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, and S≤0.005wt.%.

[0081] Comparative Example 2: The process steps of Comparative Example 2 are the same as those of Example 1, and the chemical composition thereof does not include manganese, cobalt, molybdenum, tantalum, hafnium, scandium, iridium and rare earth elements. The mass ratios of the components in the alloy are: C: 0.12%; Si: 0.50%; Mn: 0.60%; Cr: 10.0%; Mo: 0.9%; Co: 3.0%; Zr: 0.15%; V: 0.30%; Nb: 0.07%; N: 0.05%; B: 0.004%; Ni: 0.7%; W: 0.2%, and the remainder is industrial pure iron and impurities, wherein the impurities are controlled as follows: Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, and S≤0.005wt.%.

[0082] The samples prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to mechanical property tests, and the specific performance indicators are shown in Tables 1 and 2. Table 1 shows the performance indicators of the examples and comparative examples at room temperature, and Table 2 shows the performance indicators of the examples and comparative examples at 630°C. Among them, the room temperature tensile strength test standard refers to GB / T228.1-2010; the room temperature yield strength test standard refers to GB / T 7314-2017: the impact energy detection test standard refers to GB / T 229-2007. The high temperature tensile strength refers to GB / T228.2-2015; the high temperature yield strength method refers to ASTM E21; the 180MPa creep rupture test method refers to ASTM E139-200.

[0083] Table 1: Room temperature performance of examples and comparative examples

[0084] serial number Tensile strength / MPa Yield strength / MPa Impact energy / J(normal temperature) Example 1 632 528 121 Example 2 650 557 132 Example 3 645 547 125 Comparative Example 1 557 462 103 Comparative Example 2 525 432 97

[0085] Table 2: High temperature performance of examples and comparative examples

[0086]

[0087] As can be seen from Table 1 and Table 2, compared with the comparative example, the comprehensive performance of the embodiment at room temperature and high temperature shows significant advantages. This advantage is attributed to the synergistic effect between the introduced elements, such as tantalum, hafnium, scandium, iridium, and a mixture of rare earth elements cerium and lanthanum. The introduction of each element not only improves its individual performance, but more importantly, through the interaction between each other, a significant improvement in the comprehensive performance of the material is achieved. The addition of these elements plays a key role in improving the performance of heat-resistant steel. Specifically, the addition of tantalum and hafnium helps to form stable carbides and oxides at high temperatures, and the dispersed distribution of these phases significantly enhances the high-temperature strength and creep resistance of the alloy; the introduction of scandium improves the grain refinement effect, thereby enhancing the mechanical properties and toughness of the material, especially at high temperatures; iridium further improves the durability of the material in harsh environments due to its excellent oxidation resistance under extreme high temperature conditions; the mixture of rare earth elements cerium and lanthanum can form a dense oxide protective layer, effectively reducing the peeling and growth of the oxide layer, and significantly improving the material's oxidation resistance and long-term service performance.

[0088] The present invention introduces scandium, tantalum, hafnium, iridium and rare earth elements (a mixture of Ce and La) so that the elements can produce a synergistic effect and jointly improve the comprehensive performance of the heat-resistant steel.

[0089] In the present invention, tantalum and hafnium can react with carbon to form stable carbides, including TaC and HfC. These carbides have excellent thermal stability at high temperatures, can pin grain boundaries and dislocations, prevent grain growth, and thus significantly improve the high-temperature creep resistance of the material. This synergistic effect makes the introduction of tantalum and hafnium the key to maintaining structural stability of the material at extreme temperatures. Iridium reacts with oxygen at high temperatures to form iridium oxide (IrO2), which can form a dense protective layer on the surface of the material to prevent further penetration of oxygen and delay oxidation reactions. The composite oxide layer formed by iridium, tantalum and hafnium under high temperature conditions, including Ir-Ta-O and Ir-Hf-O, can further enhance the oxidation resistance of the material. Scandium can effectively refine the grains and improve the strength and toughness of the material. Scandium can also interact with alloying elements such as chromium, molybdenum, tantalum, and hafnium to form a composite phase that can reduce grain boundary segregation at high temperatures and enhance the comprehensive mechanical properties of the material.

[0090] The present invention uses a mixture of Ce and La with a rare earth element mass ratio of 1:1 in order to form a stable rare earth oxide film, including La2O3 and CeO2, in a high temperature environment. These oxide films form a dense protective layer on the surface of the material, significantly improving the material's antioxidant ability, and can also reduce the segregation of grain boundary impurities, further improving the material's high temperature stability and long-term service performance.

[0091] In addition, in the heat-resistant steel of the present invention, conventional alloying elements Cr, Ni, Mn, Si, and Mo react with elements such as carbon and nitrogen at high temperatures to generate fine and uniformly distributed metal carbonitrides or intermetallic compounds, which exist as strengthening phases on the martensite matrix and are promoted to disperse and precipitate through the tempering process. During the tempering process, these strengthening phases can effectively hinder dislocation movement and grain growth, thereby improving the comprehensive mechanical properties of the material. The Cr2O3 oxide film generated by chromium and the composite protective layers such as IrO2 and CeO2 formed by iridium and rare earth elements synergize to further enhance the antioxidant capacity, while elements such as nickel, scandium, and tantalum enhance the high-temperature creep resistance of the material by pinning grain boundaries and refining grains. The combination of manganese and rare earth elements helps to inhibit impurity segregation at grain boundaries and ensure the stability of grain boundaries. Silicon further enhances the antioxidant properties of the material by generating a composite oxide film together with chromium, while molybdenum significantly enhances the high-temperature strength and durability by combining with tantalum and hafnium to form a composite carbide. These conventional alloying elements work together with newly added scandium, iridium, tantalum, hafnium and rare earth elements to form a strong and durable structure in high temperature environments, significantly improving the high temperature and long-lasting performance of heat-resistant steel.

[0092] The above embodiments only express the implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. A heat-resistant steel for main steam pipeline of ultra-supercritical unit, characterized by: The mass percentage of each element contained is: C: 0.08~0.12%; Si: 0.35~0.55%; Mn: 0.55~0.65%; Cr: 9.0~10.5%; Mo: 0.6~1.0%; Co: 2.0~3.5%; Zr：0.12～0.2％； V: 0.22~0.35%; Nb: 0.06~0.014%; N: 0.03~0.07%; B: 0.003~0.011%; Ni: 0.6-1.0%; W: 0.15-0.3%; a mixture of Ce and La in a mass ratio of 1:1: 0.05-0.1%; Hf: 0.01-0.05%; Ta: 0.01-0.05%; Sc: 0.01-0.05%; Ir: 0.01-0.03%; the remainder is Fe and impurities.

2. The heat-resistant steel for main steam pipeline of ultra-supercritical unit according to claim 1, characterized in that: Among the impurities, Al≤0.01wt.%, Ti≤0.01wt.%, P≤0.008wt.%, and S≤0.005wt.%.

3. A method for preparing heat-resistant steel for main steam pipeline of ultra-supercritical unit according to any one of claims 1 to 2, characterized in that: The specific steps are as follows: S1: The alloy raw materials are melted by high vacuum arc melting method, and electromagnetic stirring is performed during the melting process to ensure uniform composition. After melting, the cast alloy is obtained by suction casting system; S2: The as-cast alloy is subjected to two fire treatments: S21: The first normalizing treatment is heating to 1050-1150℃, keeping the temperature for 2.5-3.0h and then air cooling to refine the grains and eliminate internal stress; S22: Second normalizing treatment, heating to 900-1000℃, keeping warm for 1.5-2.5h and then air cooling, used to improve the toughness and creep resistance of the material; S3: Quenching the alloy after normalizing treatment, specifically: S31: heating the alloy to 980-1020°C, keeping the temperature for 1.5-2.5 hours, and then rapidly cooling to room temperature to form a martensitic structure, which is used to improve the hardness and strength of the material; S4: Tempering the quenched alloy, specifically: S41: The alloy is heated to 600-700°C, kept warm and then air-cooled to precipitate strengthening phases to improve the high-temperature strength and stability of the material; S5: The prepared heat-resistant steel is shot peened to introduce compressive stress on the surface of the material by spraying small particles at high speed, thereby forming a dense FeCr2O4 protective layer to improve the material's antioxidant properties.

4. The method for preparing heat-resistant steel for main steam pipeline of ultra-supercritical unit according to claim 3, characterized in that: Step S1: The steps of preparing the cast alloy by high vacuum arc melting method include the following: S11: The interior of the melting furnace is made to reach a high vacuum state by evacuating the vacuum. After the interior of the melting furnace reaches a high vacuum state, an appropriate amount of argon gas is introduced as a protective gas to make the gas pressure in the furnace reach -0.5Mpa; S12: Deoxidation of pure titanium by melting with an arc gun; S13: Move the arc gun to the melting crucible position, increase the arc gun current to 230A to start melting the alloy, and continue electromagnetic stirring for 30s after the alloy raw materials are melted, maintain a constant current for 30s, and then reduce the arc gun current to 0A, and the magnetic stirring current is 2A; S14: Repeat the step S13 of melting the alloy 4 times to make the alloy composition uniform. After the melting is completed, an initial cast alloy is obtained; S15: placing the initially cast alloy cooled in the melting crucible into the suction casting crucible through a long-handled ladle, repeating the alloy melting operation, and repeating the melting to obtain molten metal liquid, at which time the arc gun current is increased to 270-280A; S16: After the initial cast alloy in the suction casting crucible is completely melted, the molten alloy is sucked into a copper mold through a suction casting system to obtain a cast alloy steel ingot.

5. The method for preparing heat-resistant steel for main steam pipeline of ultra-supercritical unit according to claim 4, characterized in that: The electromagnetic stirring current in step S13 is 2A.

6. The method for preparing heat-resistant steel for main steam pipeline of ultra-supercritical unit according to claim 4, characterized in that: The vacuum in step S11 is 10 -5 Pa.

7. The method for preparing heat-resistant steel for main steam pipeline of ultra-supercritical unit according to claim 3, characterized in that: The alloy after normalizing treatment uses quenching oil at 60°C as the quenching medium during the quenching process, and a martensite-based structure is formed after quenching.

8. The method for preparing heat-resistant steel for main steam pipeline of ultra-supercritical unit according to claim 3, characterized in that: The tempering treatment in step S41 adopts a temperature of 600-700° C., a heating rate of 80° C. / h, a holding time of 1.5-4 h, and then air-cooling to room temperature.

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