A heat-resistant steel for high-temperature turbine rotor forgings and a method for producing the same

Abstract

The application provides a heat-resistant steel for high-temperature turbine rotor forgings, which comprises the following chemical elements in percentage by mass: C: 0.11-0.17%, Si: 0.02-0.095%, Mn: 0.22-0.5%, Cr: 10.6-11.8%, Co: 2.5-2.9%, Mo: 0.01-0.18%, W: 2.55-3.4%, V: 0.12-0.39%, Nb: 0.03-0.07%, Ni: 0.12-0.29%, Cu: 0.3-0.7%, B: 0.019-0.035%, N: 0.012-0.035%, Y: 0-0.5%, and the balance of Fe and inevitable impurities. The application also provides a preparation method of the heat-resistant steel and an application of the heat-resistant steel in turbine machinery. Compared with the prior art, the forged piece prepared from the material has excellent high-temperature creep strength, low-cycle fatigue performance and oxidation resistance, and can fully meet the use requirements of high-temperature turbine rotors with a working temperature of 650 DEG C or below. The forged piece preparation method is simple in operation and easy to implement production.

Description

Technical Field

[0001] This invention relates to the field of metal materials technology, and in particular to a heat-resistant steel for high-temperature turbine rotor forgings and its preparation method. Background Technology

[0002] A turbomachine is a machine that converts the energy in a fluid working medium into mechanical work. The working medium of a turbomachine can be a gas such as steam, fuel gas, or air, or a liquid such as water or oil. Generally, for turbomachines with a gaseous working medium, the higher the temperature and pressure of the working gas, the higher the energy conversion efficiency of the turbomachine. However, this also places higher demands on the performance of the turbomachine's components, especially those operating at high temperatures.

[0003] In particular, for thermal power generation, the steam turbine is the key component that converts the thermal energy of steam into mechanical work. Increasing steam temperature and pressure can improve power generation efficiency, reduce coal consumption, and lower CO2 and NO levels. X SO X The elimination of pollutants such as steam gases has significant economic and social benefits. Over the past few decades, steam parameters in steam turbines have been improved from subcritical and supercritical to supercritical parameters at levels of 620℃ and 630℃, and are currently being developed towards even higher temperature-rated turbine units. Increasing the steam temperature of steam turbines further deteriorates the working environment of high-temperature components, placing higher demands on the high-temperature strength and resistance to high-temperature oxidation of materials. Furthermore, with the emergence of concepts such as multi-energy complementarity and the gradual increase in new energy power supply, thermal power will become an important backup and peak-shaving power source. This will place greater demands on the operational flexibility of thermal power turbines. The fatigue performance of key turbine components, especially the low-cycle fatigue performance of rotor forgings, is a crucial factor affecting the flexibility and safety of the unit.

[0004] Forgings made from existing materials cannot simultaneously possess good high-temperature strength, low-cycle fatigue performance, and oxidation resistance, thus requiring further research. Summary of the Invention

[0005] To address the aforementioned deficiencies, the present invention aims to provide a heat-resistant steel for high-temperature turbine rotor forgings, which possesses excellent high-temperature strength, low-cycle fatigue performance, and oxidation resistance, and can meet the requirements for use of turbine machinery forgings with operating temperatures of 650℃ and below.

[0006] This invention provides a heat-resistant steel for high-temperature turbine rotor forgings, comprising the following chemical elements in the following mass percentages: C: 0.11-0.17%, Si: 0.02-0.095%, Mn: 0.22-0.5%, Cr: 10.6-11.8%, Co: 2.5-2.9%, Mo: 0.01-0.18%, W: 2.55-3.4%, V: 0.12-0.39%, Nb: 0.03-0.07%, Ni: 0.12-0.29%, Cu: 0.3-0.7%, B: 0.019-0.035%, N: 0.012-0.035%, Y: 0-0.5%, with the balance being Fe and unavoidable impurities.

[0007] Preferably, the heat-resistant steel comprises the following chemical elements in the following mass percentages: C: 0.12-0.15%, Si: 0.02-0.045%, Mn: 0.25-0.35%, Cr: 10.9-11.1%, Co: 2.65-2.85%, Mo: 0.01-0.09%, W: 3.05-3.35%, V: 0.31-0.37%, Nb: 0.04-0.06%, Ni: 0.12-0.24%, Cu: 0.45-0.65%, B: 0.019-0.027%, N: 0.0155-0.029%, Y: 0-0.5%, with the balance being Fe and unavoidable impurities.

[0008] Preferably, the mass percentage of Y is 0.26-0.34%.

[0009] The present invention also provides a method for preparing the aforementioned heat-resistant steel, comprising the following steps: weighing and mixing raw materials according to element ratio, then casting them into electrode rods after smelting, ladle refining and vacuum degassing, the electrode rods being electroslag remelted and cooled and solidified to form steel ingots, the steel ingots being forged into turbine rotor blanks and subjected to heat treatment.

[0010] Preferably, the heat treatment includes one quenching process and two tempering processes.

[0011] Preferably, the quenching temperature is 1100℃-1150℃, the first tempering temperature is 580℃-660℃, and the second tempering temperature is 670℃-730℃.

[0012] The present invention also provides the use of the aforementioned heat-resistant steel in turbine machinery.

[0013] Preferably, the turbine is a steam turbine.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] Carbon (C) ensures the hardenability of steel, promotes martensitic transformation, and is an important element for carbide formation, thus improving the strength of steel. However, excessive C reduces the toughness of the material and is detrimental to low-cycle fatigue performance. Therefore, the C content is 0.11-0.17%, and more preferably 0.12-0.15%.

[0016] Si and Mn are deoxidizers and desulfurizers in molten steel. Si can improve the oxidation resistance of materials, while Mn can improve the hardenability and strength of steel. However, Si and Mn will reduce the plasticity and toughness of materials. Therefore, the Si content is 0.02-0.095% and the Mn content is 0.22-0.50%, and more preferably, the Si content is 0.02-0.045% and the Mn content is 0.25-0.35%.

[0017] Cr is an important element providing steel with oxidation resistance and is also a significant carbide-forming element; however, excessive Cr content can easily lead to the formation of δ-ferrite and, after long-term service, the formation of Z-phase, thus reducing the material's long-term high-temperature strength. Therefore, the Cr content is 10.6-11.8%, and more preferably 10.9-11.1%.

[0018] Co is an important solid solution strengthening element and can also inhibit the formation of δ-ferrite; however, excessive Co content will promote carbide coarsening and reduce the low-cycle fatigue performance of the material. Therefore, the Co content is 2.5-2.9%, and more preferably 2.65-2.85%.

[0019] Mo and W are two important solid solution strengthening elements that play a crucial role in the high-temperature strength of materials. However, excessive Mo content can lead to the formation of large primary carbides, reducing the low-cycle fatigue performance of the material. Conversely, excessive W content can reduce the material's processability and increase the likelihood of segregation during manufacturing. Therefore, the optimal Mo content is 0.01-0.18% and the optimal W content is 2.55-3.4%, with a more preferred Mo content of 0.01-0.09% and a preferred W content of 3.05-3.35%.

[0020] V and Nb are important forming elements of fine dispersed carbonitrides in steel, increasing the high-temperature strength of the material. However, excessive V content will reduce the toughness of the material, and excessive Nb content will easily form coarse primary NbC phase, reducing the low-cycle fatigue performance of the material. Therefore, the V content is 0.12-0.39%, and the Nb content is 0.03-0.07%, with a more preferred V content of 0.31-0.37% and an Nb content of 0.04-0.06%.

[0021] Ni can increase the hardenability of materials and has a significant impact on their toughness; however, excessive Ni content will reduce the long-term high-temperature strength of the material. Therefore, the Ni content is 0.12-0.29%, and more preferably 0.12-0.24%.

[0022] Cu can form a dispersed copper-rich phase, improving the high-temperature strength and low-cycle fatigue performance of the material; however, excessive Cu content can lead to coarsening of the copper-rich phase, reducing the strengthening effect and decreasing the material's plasticity. Therefore, the Cu content is typically 0.3-0.7%, with a more preferred Cu content of 0.45-0.65%.

[0023] Boron (B) can inhibit the growth of grain boundary carbides and improve the high-temperature long-term strength of materials. Nitrogen (N) is an important forming element of fine dispersed carbonitrides in steel, which can improve the strength and low-cycle fatigue performance of materials. However, excessive B and N content will form coarse BN phases, reducing low-cycle fatigue performance. Therefore, the optimal B content is 0.019-0.035% and the optimal N content is 0.012-0.035%, with a more preferred B content of 0.019-0.027% and an optimal N content of 0.0155-0.029%.

[0024] Y is a rare earth element. Adding trace amounts of Y can further improve the oxidation resistance, strength, and low-cycle fatigue performance of materials; however, excessive Y content weakens the strengthening effect. Therefore, the Y content is 0-0.5%, and more preferably 0.26-0.34%.

[0025] Compared to existing technologies, the forgings prepared from the materials of this invention exhibit superior high-temperature creep strength, low-cycle fatigue performance, and oxidation resistance, fully meeting the requirements for high-temperature turbine rotors operating at temperatures of 650℃ and below. The forging preparation method of this invention is simple to operate and easy to implement in production. Attached Figure Description

[0026] Figure 1 This is a comparison chart showing the weight gain of oxidation at 650°C for the four embodiments of the present invention and the weight gain of oxidation at 620°C for 13Cr9Mo2Co1NiVNbNB. Detailed Implementation

[0027] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0028] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified and changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0029] The present invention provides a heat-resistant steel for high-temperature turbine rotor forgings, comprising the following chemical elements in the following mass percentages: C: 0.11-0.17%, Si: 0.02-0.095%, Mn: 0.22-0.5%, Cr: 10.6-11.8%, Co: 2.5-2.9%, Mo: 0.01-0.18%, W: 2.55-3.4%, V: 0.12-0.39%, Nb: 0.03-0.07%, Ni: 0.12-0.29%, Cu: 0.3-0.7%, B: 0.019-0.035%, N: 0.012-0.035%, Y: 0-0.5%, with the balance being Fe and unavoidable impurities.

[0030] Preferably, the heat-resistant steel of the present invention comprises the following chemical elements in the following mass percentages: C: 0.12-0.15%, Si: 0.02-0.045%, Mn: 0.25-0.35%, Cr: 10.9-11.1%, Co: 2.65-2.85%, Mo: 0.01-0.09%, W: 3.05-3.35%, V: 0.31-0.37%, Nb: 0.04-0.06%, Ni: 0.12-0.24%, Cu: 0.45-0.65%, B: 0.019-0.027%, N: 0.0155-0.029%, Y: 0-0.5%, with the balance being Fe and unavoidable impurities.

[0031] Furthermore, the preferred mass percentage of Y is 0.26-0.34%.

[0032] Based on the total mass of the heat-resistant steel, the main components and corresponding mass percentages of the aforementioned impurities are P≤0.015% and S≤0.01%.

[0033] This invention also provides a method for preparing turbine rotor forgings, comprising the following steps: weighing and mixing raw materials according to element ratios, then smelting in an electric arc furnace, refining in a ladle, and degassing under vacuum, followed by casting into electrode rods; the electrode rods undergo electroslag remelting and cooling solidification to form steel ingots; the steel ingots are forged into turbine rotor blanks and subjected to heat treatment. The heat treatment includes one quenching and two tempering processes. The quenching temperature is 1100℃-1150℃, the first tempering temperature is 580℃-660℃, and the second tempering temperature is 670℃-730℃.

[0034] This invention provides the chemical compositions of four examples of forgings and a comparative example of 13Cr9Mo2Co1NiVNbNB (disclosed by Chinese Invention Patent CN103074550B), the chemical compositions being expressed as mass percentages with the balance being Fe. See Table 1.

[0035] Table 1. Chemical composition analysis results of forgings (wt.%)

[0036]

[0037]

[0038] Example 1

[0039] By mass percentage, 0.115% C, 0.086% Si, 0.24% Mn, 11.43% Cr, 2.76% Co, 0.025% Mo, 3.29% W, 0.22% V, 0.06% Nb, 0.27% Ni, 0.63% Cu, 0.021% B, 0.0319% N, 0.45% Y, and the balance Fe are mixed in a specific ratio. After being smelted in an electric arc furnace, refined in a ladle, and degassed under vacuum, the mixture is cast into electrode rods. The electrode rods are then subjected to electroslag remelting and cooling solidification to form steel ingots. The steel ingots are then subjected to a series of processes including forging, heat treatment, and machining to produce 25-ton thermal power turbine rotor forgings with a maximum diameter of 1.1 meters.

[0040] The heat treatment includes one quenching and two tempering processes. The quenching temperature is 1110℃, the first tempering temperature is 600℃, and the second tempering temperature is 690℃.

[0041] Example 2

[0042] By mass percentage, 0.132% C, 0.044% Si, 0.32% Mn, 11.05% Cr, 2.84% Co, 0.078% Mo, 3.11% W, 0.33% V, 0.04% Nb, 0.14% Ni, 0.52% Cu, 0.024% B, 0.0161% N, 0.27% Y, and the balance Fe are mixed in a specific ratio. After being smelted in an electric arc furnace, refined in a ladle, and degassed under vacuum, the mixture is cast into electrode rods. The electrode rods are then subjected to electroslag remelting and cooling solidification to form steel ingots. The steel ingots are then subjected to a series of processes including forging, heat treatment, and machining to produce a 1-ton supercritical carbon dioxide rotor forging with an outer diameter of 0.4 meters.

[0043] The heat treatment includes one quenching and two tempering processes. The quenching temperature is 1140℃, the first tempering temperature is 620℃, and the second tempering temperature is 710℃.

[0044] Example 3

[0045] By mass percentage, 0.145% C, 0.025% Si, 0.43% Mn, 10.73% Cr, 2.57% Co, 0.153% Mo, 2.81% W, 0.29% V, 0.05% Nb, 0.21% Ni, 0.39% Cu, 0.033% B, 0.0239% N, 0.33% Y, and the balance Fe are mixed in a specific ratio. After being smelted in an electric arc furnace, refined in a ladle, and degassed under vacuum, the mixture is cast into electrode rods. The electrode rods are then subjected to electroslag remelting and cooling solidification to form steel ingots. The steel ingots undergo a series of processes including forging, heat treatment, and machining to produce 2-ton industrial turbine rotor forgings with an outer diameter of 0.6 meters.

[0046] The heat treatment includes one quenching and two tempering processes. The quenching temperature is 1120℃, the first tempering temperature is 610℃, and the second tempering temperature is 700℃.

[0047] Example 4

[0048] By mass percentage, 0.16% C, 0.035% Si, 0.28% Mn, 10.92% Cr, 2.88% Co, 0.045% Mo, 3.18% W, 0.37% V, 0.053% Nb, 0.18% Ni, 0.46% Cu, 0.027% B, 0.0138% N, and the balance Fe are mixed in a specific ratio, smelted in an electric arc furnace, refined in a ladle, and degassed under vacuum before being cast into electrode rods. The electrode rods are then subjected to electroslag remelting and cooling solidification to form steel ingots. After a series of processes including forging, heat treatment, and machining, the steel ingots are used to produce 2.4-ton air turbine rotor forgings with an outer diameter of 0.8 meters.

[0049] The heat treatment includes one quenching and two tempering processes. The quenching temperature is 1130℃, the first tempering temperature is 590℃, and the second tempering temperature is 720℃.

[0050] Table 2 shows the room temperature mechanical properties (including yield strength R) of the forgings in Examples 1-4 of this invention. p0.2 Tensile strength R m Elongation after fracture (A) and reduction of area (Z). According to Table 2 and CN103074550B, the forgings provided in Examples 1-4 of this invention have room temperature strength and plasticity comparable to 13Cr9Mo2Co1NiVNbNB.

[0051] Table 2 Mechanical properties of forgings at room temperature

[0052]

[0053] Table 3 shows the high-temperature creep strength of the forgings in Examples 1-4 of this invention, obtained by extrapolation from creep test data, after working at 650°C for 100,000 hours. As can be seen from Table 3, the high-temperature creep strength of Examples 1-4 of this invention after working at 650°C for 100,000 hours all reached above 115 MPa, demonstrating excellent high-temperature creep strength.

[0054] Table 3 Durability of Embodiments of the Invention

[0055]

[0056] Table 4 shows the low-cycle fatigue test results of the forgings of Examples 1-4 of the present invention at room temperature and 650°C. According to Table 4, Examples 1-4 of the present invention have excellent low-cycle fatigue performance at both room temperature and 650°C.

[0057] Table 4 Low-cycle fatigue performance (cycles) of embodiments of the present invention

[0058]

[0059] like Figure 1 As shown, steam oxidation tests were conducted on the forgings of Examples 1-4 of the present invention at 650°C for different oxidation times (100h, 500h, 1000h, and 2000h). Under the same oxidation time conditions, the oxidation weight gain of the forgings of Examples 1-4 of the present invention was lower than that of 13Cr9Mo2Co1NiVNbNB under a steam environment at 620°C. This demonstrates that the forgings of Examples 1-4 of the present invention exhibit excellent resistance to steam oxidation at 650°C.

[0060] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing heat-resistant steel for high-temperature turbine rotor forgings, characterized in that, The heat-resistant steel comprises the following chemical elements by mass percentage: C: 0.12-0.15%, Si: 0.02-0.045%, Mn: 0.25-0.35%, Cr: 10.9-11.1%, Co: 2.65-2.85%, Mo: 0.01-0.09%, W: 3.05-3.35%, V: 0.31-0.37%, Nb: 0.04-0.06%, Ni: 0.12-0.24%, Cu: 0.45-0.65%, B: 0.019-0.027%, N: 0.0155-0.029%, Y: 0.26-0.34%, with the balance being Fe and unavoidable impurities. Based on the total mass of the heat-resistant steel, the main components of the impurities and their corresponding mass percentages are P≤0.015% and S≤0.01%. The preparation method includes the following steps: weighing raw materials according to element ratio, mixing them, smelting them in an electric arc furnace, refining them in a steel ladle, and degassing them in a vacuum before casting them into electrode rods; the electrode rods are then subjected to electroslag remelting and cooling solidification to form steel ingots; the steel ingots are forged into turbine rotor blanks and subjected to heat treatment. The heat treatment includes one quenching process and two tempering processes; The quenching temperature is 1100℃-1150℃, the first tempering temperature is 580℃-660℃, and the second tempering temperature is 670℃-730℃.

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