Martensitic heat-resistant steel for use at 630°C or above and its manufacturing method

Abstract

The present invention belongs to the technical field of metallic materials, and discloses a martensitic heat-resistant steel for use at temperatures of 630° C. or higher and a manufacturing method thereof. The heat-resistant steel contains, by mass percentage, 0.01-0.14% C, 0.05-0.50% Si, 0.05-0.70% Mn, 8.5-13% Cr, 2.0-3.5% W, 0.1-0.7% Mo, 0.03-0.07% Nb, 0.1-0.3% V, 2.8-5% Co, 0.8-1.5% Cu, 0.1-0.5% Ni, 0.01-0.015% B, 0.01-0.08% N, 0.2-0.5% Ta, 0.1-0.5% Zr, and 0.01-0.3% Ce+Y, with the remaining components being Fe and unavoidable impurities. The heat-resistant steel of the present invention has good overall performance.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to a Chinese patent application bearing application number 2022107134571 and entitled "Martensitic heat-resistant steel for temperatures above 630°C and manufacturing method thereof," filed with the State Intellectual Property Office of the People's Republic of China on June 22, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present invention belongs to the technical field of metallic materials, and specifically relates to a martensitic heat-resistant steel for use at temperatures of 630°C or higher and a method for producing the same. [Background technology]

[0003] To achieve energy conservation and emission reduction goals, thermal power generation technology is currently evolving toward hyperparameterization, dual-stage reheating, and clean, high-efficiency technologies. Advanced ultra-supercritical coal-fired power generation technology is being fully utilized. For example, upgrading from subcritical to ultra-supercritical thermal power generation saves 68 grams of coal per kilowatt-hour of electricity, reducing coal consumption for power supply by 21%. This saves China 232 million tons of coal annually, equivalent to a reduction of CO2 emissions of 510 million tons per year. However, martensitic heat-resistant steel materials for large castings and forgings of hyperparameterized steam turbines remain a technological bottleneck limiting power plant parameter improvements. In particular, there are currently no mature rotor forgings for 630°C steam turbines, both domestically and internationally, and most rotors for 620°C steam turbines are imported.

[0004] Therefore, in order to break through key core technologies, domestic production of high-performance, highly reliable heat-resistant steel materials and products is urgently needed. As unit parameters improve, material performance requirements are also increasing, especially for strength, toughness, and durability under high stress and high temperature conditions. Currently, domestic and international research is primarily focused on adding elements such as W, Co, B, and N to further improve the high-temperature durability and oxidation resistance of heat-resistant steels. However, even with the addition of elements such as W, Co, B, Nb, and N, there are limitations to how much improvement can be made in the high-temperature durability and oxidation resistance of heat-resistant steels. Therefore, there is an urgent need to develop new strengthening methods to further improve the high-temperature properties and oxidation resistance of heat-resistant steels. Summary of the Invention [Problem to be solved by the invention]

[0005] Based on the above analysis, an object of the present invention is to provide a martensitic heat-resistant steel for use at temperatures above 630°C, which has good overall performance in terms of high-temperature strength, impact resistance, permanent creep strain resistance, oxidation resistance, etc., and is suitable for rotors of ultra-supercritical steam turbines with operating temperatures of 630°C or above, and a manufacturing method thereof.

[0006] In the present invention, numerous experimental studies were conducted to determine the material selection requirements for heat-resistant steels for steam turbine units in power plants under supercritical conditions. From the perspective of restricting the micromorphology of nitride precipitation phases in 10-12% Cr steels, a new martensitic heat-resistant steel was developed, in which tantalum is added to replace MX, resulting in a precipitation-strengthening phase of fine chromium-tantalum nitrides that are stable at high temperatures.

[0007] The object of the present invention is mainly achieved by the following technical solutions: The present invention provides a martensitic heat-resistant steel for use at temperatures of 630°C or higher, containing, by mass percentage, 0.01-0.14% C, 0.05-0.50% Si, 0.05-0.70% Mn, 8.5-13% Cr, 2.0-3.5% W, 0.1-0.7% Mo, 0.03-0.07% Nb, 0.1-0.3% V, 2.8-5% Co, 0.8-1.5% Cu, 0.1-0.5% Ni, 0.01-0.015% B, 0.01-0.08% N, 0.2-0.5% Ta, 0.1-0.5% Zr, and 0.01-0.3% Ce+Y, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, there is provided a martensitic heat-resistant steel for use at 630°C or higher, which contains, by mass percentage, 0.01 to 0.14% C, 0.05 to 0.50% Si, 0.05 to 0.70% Mn, 8.5 to 13% Cr, 2.0 to 3.5% W, 0.1 to 0.7% Mo, 0.03 to 0.07% Nb, 0.1 to 0.3% V, 2.8 to 5% Co, 0.8 to 1.2% Cu, 0.1 to 0.5% Ni, 0.01 to 0.015% B, 0.01 to 0.08% N, 0.28 to 0.45% Ta, 0.1 to 0.5% Zr, and 0.1 to 0.2% Ce+Y, with the remainder being Fe and unavoidable impurities.

[0009] Furthermore, the microstructure of the heat-resistant steel is a fully tempered martensite structure and a precipitated phase, and the precipitated phase is a dispersedly distributed M 23 It contains C6-type carbides, CrTaN phase, Laves phase and nano-dispersion strengthened Cu phase.

[0010] Compared with the prior art, the martensitic heat-resistant steel of the present invention for temperatures above 630°C has the following advantages: On the other hand, in addition to the addition of small amounts of V and Nb, 0.2 to 0.5% of Ta is also added. The appropriate amount of Ta forms the CrTaN phase, which is resistant to coarsening. This prevents the formation of the Cr(V,Nb)N phase, which is very susceptible to coarsening over the long term. The coarsening of this phase reduces durability, and also prevents the transformation of MX into Cr(Nb,V)N, which coarsens over the long term and causes a sudden drop in performance. On the other hand, in the present invention, the carbon content is limited to a relatively low level, and finely dispersed M with a relatively low C content is obtained. 23 The formation of C6 plays a role in dispersion strengthening, and adding a certain amount of Cu element suppresses the formation of high-temperature ferrite. In addition, the precipitation of nano-copper-rich phase compensates for the lack of V and Nb, plays a role in precipitation strengthening, and can improve the durability limit of heat-resistant steel. Adding an appropriate amount of B element contributes to the formation of M. 23 Substitution at the C element position of C6 gives M 23 (C,B)6 is formed, which results in M ​​near the prior austenite grain boundary. 23 The coarsening rate of C6 is reduced, significantly improving creep strength, an appropriate amount of W element is used to improve endurance limit, a small amount of Ni element is used, and a relatively high Co content improves matrix toughness. The rational element composition largely prevents the formation of high-temperature ferrite, which provides a wider temperature range for forging and heat treatment processes in actual production.

[0011] The present invention also provides a method for producing a martensitic heat-resistant steel for use at 630°C or higher, comprising the following steps: In step S1, raw materials are weighed according to the content of each component in the composition, and are smelted in a vacuum induction furnace and poured into an ingot. The content of impurity elements is strictly limited, and the composition is, in mass percentage, C 0.01-0.14%, Si 0.05-0.50%, Mn 0.05-0.70%, Cr 8.5-13%, W 2.0-3.5%, Mo 0.1-0.7%, Nb 0.03-0.07%, V 0.1-0.3%, Co 2.8-5%, Cu 0.8-1.5%, Ni 0.1-0.5%, B 0.01-0.015%, N 0.01-0.08%, Ta 0.2-0.5%, Zr 0.1-0.5%, Ce+Y The content is 0.01 to 0.3%, and the remaining components are Fe and unavoidable impurities. In step S2, the ingot obtained in step S1 is subjected to high-temperature homogenization treatment, kept at a constant temperature, and then cooled to room temperature in each furnace to obtain a blank. In step S3, the blank obtained in step S2 is forged by the rolling method, the upsetting method, or the stretching method, and when the temperature during the forging process is lower than the finish forging temperature, it is returned to the furnace to be heated and then forged again, and after forging, it is furnace cooled to room temperature. In step S4, the forged bar obtained in step S3 is subjected to normalizing and two tempering heat treatments to obtain a chromium-tantalum nitride strengthened martensitic heat-resistant steel, wherein the first tempering temperature is lower than the second tempering temperature.

[0012] Furthermore, in step S2, the temperature of the high-temperature homogenization treatment is 1160 to 1200° C., and the temperature retention time is 4 to 8 hours.

[0013] Furthermore, in step S3, the rough forging temperature is 1160 to 1200°C and the finish forging temperature is 850 to 950°C in the forging process.

[0014] Furthermore, step S4 includes the following steps. Step S401 is normalizing, and the normalizing step involves heating the forged bar to 1050 to 1150°C, keeping the temperature, and then air-cooling it to room temperature. Step S402 is the first tempering, and the first tempering step involves heating the forged bar to 600 to 700°C, keeping the temperature, and then air-cooling it to room temperature. Step S403 is the second tempering, and the second tempering step involves heating the forged bar to 680 to 780°C, keeping the temperature, and then air-cooling it to room temperature.

[0015] Furthermore, in step S401, the temperature rise rate is 100° C. / hour or less, and the temperature retention time is 1 to 10 hours.

[0016] Furthermore, in step S402, the warming time is 5 to 10 hours.

[0017] Furthermore, in step S403, the warming time is 5 to 10 hours. [Effects of the Invention]

[0018] Compared with the prior art, the manufacturing process of martensitic heat-resistant steel for temperatures above 630°C provided by the present invention precisely limits the process parameters, such as the time and temperature of high-temperature homogenization, the forging temperature, the temperature and time of normalizing, and the temperature and time of two temperings, to ensure that the microstructure obtained is a complete tempered martensite structure and precipitated phases, and the precipitated phases are finely dispersed and distributed M. 23 Contains C6-type carbides, CrTaN phase, a small amount of fine Laves phase, and nano-dispersion strengthened Cu phase. This heat-resistant steel guarantees excellent room temperature strength, high temperature strength, permanent creep resistance, and oxidation resistance, making it suitable for use in rotors of ultra-supercritical steam turbines operating at temperatures above 630°C.

[0019] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention may be realized and obtained by means of the particular points pointed out in the description and drawings. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 shows the as-cast structure of the heat-resistant steel according to Example 1 of the present invention. [Figure 2] FIG. 2 shows the homogenized structure of the heat-resistant steel according to Example 1 of the present invention. [Figure 3] FIG. 3 shows the structure of the heat-resistant steel according to Example 1 of the present invention after quenching and tempering. DETAILED DESCRIPTION OF THE INVENTION

[0021] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which are part of the present invention and serve to explain the principles of the present invention together with an outline of the invention.

[0022] The present invention provides a martensitic heat-resistant steel for use at temperatures of 630°C or higher, containing, by mass percentage, 0.01-0.14% C, 0.05-0.50% Si, 0.05-0.70% Mn, 8.5-13% Cr, 2.0-3.5% W, 0.1-0.7% Mo, 0.03-0.07% Nb, 0.1-0.3% V, 2.8-5% Co, 0.8-1.5% Cu, 0.1-0.5% Ni, 0.01-0.015% B, 0.01-0.08% N, 0.2-0.5% Ta, 0.1-0.5% Zr, and 0.01-0.3% Ce+Y, with the remainder being Fe and unavoidable impurities.

[0023] Generally, after long-term use, 9-12% Cr steels form coarse chromium vanadium or chromium niobium nitride phases, i.e., CrVN and CrNbN. Compared with conventional steels, the heat-resistant steels provided by the present invention have a relatively low C content, a relatively high Co content, and contain strengthening elements such as Ta and Cu, as well as rare earth elements Ce and Y. The heat-resistant steels provided by the present invention do not contain V or Nb, thereby avoiding the formation of the coarse Cr(V,Nb)N phase, which is very likely to form over a long period of time and reduces durability.

[0024] In the present invention, 0.2 to 0.5% of Ta element is added, and an appropriate amount of Ta element forms a CrTaN phase that is resistant to coarsening. During long-term use, it is possible to avoid a sudden decrease in performance due to coarsening caused by MX transforming into Cr(Nb,V)N.

[0025] The heat-resistant steel provided by the present invention has a relatively low C content and finely dispersed M 23 The formation of C6 plays a role in dispersion strengthening. 0.8 to 1.5% of Cu element is added. On the one hand, Cu element acts as an austenite forming element to suppress the formation of high temperature ferrite. On the other hand, the precipitation of nano-copper rich phase compensates for the lack of V and Nb, plays a role in precipitation strengthening, and improves the endurance limit of heat-resistant steel. An appropriate amount of B element is added to M 23 Substitution at the C element position of C6 gives M 23 (C,B)6 can be formed, which allows M near the prior austenite grain boundary to be formed. 23 The coarsening rate of C6 is reduced, significantly improving creep strength, an appropriate amount of W element is used to improve endurance limit, a small amount of Ni element is used, and a relatively high Co content improves matrix toughness. The rational element composition also largely prevents the formation of high-temperature ferrite, which provides a wider temperature range for forging and heat treatment processes in actual production.

[0026] The heat-resistant steel provided by the present invention has properties such as high strength, permanent creep strain resistance, and oxidation resistance. The martensitic heat-resistant steel produced by the present invention has a yield strength of more than 660 MPa, a tensile strength of more than 850 MPa, an elongation of 16% or more, a cross-sectional reduction of 50% or more, and an impact energy of 20 J or more at room temperature. At 630°C, the yield strength is more than 280 MPa, the tensile strength is more than 380 MPa, the elongation is 22% or more, and the cross-sectional reduction is 62% or more. The creep rupture time at 650°C and 180 MPa is more than 3500 hours, and the weight gain due to oxidation resistance at 650°C (400 hours) is 0.3 mg / m². 2 The overall performance is excellent.

[0027] Specifically, in the martensitic heat-resistant steel for temperatures above 630°C, the role of each component is as follows: C is an important precipitate strengthening element, and M 23 C6 and MX strengthen the dispersion and improve permanent creep strain resistance. C is a strong austenite stabilizing element that can reduce the formation of δ-ferrite, improving hardenability and precipitation strengthening. If the carbon content is too high, it will lead to excessive consumption of solid solution elements (e.g., Cr, W), reducing intergranular corrosion resistance, degrading weldability, and negatively affecting permanent creep strain resistance. If the carbon content is too low, insufficient strengthening will occur, resulting in reduced strength and hardness. Therefore, in the present invention, the mass percentage of C is limited to 0.01 to 0.14%.

[0028] Silicon contributes to improving the strength and steam corrosion resistance of the material matrix. As the Si content increases, oxidation resistance improves dramatically. Increasing the Si content promotes ferrite formation at high temperatures and lowers the high-temperature ferrite formation temperature, which negatively affects the forging temperature range. Furthermore, too high a Si content is detrimental to the impact strength of the material, and the endurance limit of the material decreases with increasing Si content. Therefore, the present invention limits the Si mass percentage to 0.05 to 0.50%.

[0029] Mn improves strength, improves hot workability, and stabilizes P, S, etc. If the content is less than 0.2%, Mn cannot play a significant role. If the content is more than 1%, a second phase may appear in the structure, which is detrimental to the impact strength of the material. Therefore, in the present invention, the mass percentage of Mn is limited to 0.05 to 0.70%.

[0030] Cr is the most important corrosion-resistant and oxidation-resistant element. Cr itself has excellent creep strain resistance properties, and is the main element that improves the steam oxidation and corrosion resistance of heat-resistant steels, and can also improve the high-temperature strength of steels. When present in sufficient amounts, Cr reacts with O to form a Cr2O3 protective film on the surface of the alloy matrix, preventing the diffusion of O atoms and metal ions, thereby slowing the progression of oxidation. Cr is also an important precipitate-strengthening element, and acts together with C and M. 23 It can strengthen the steel by forming C6 precipitates. If the Cr content is too high, δ ferrite is formed, which reduces the high-temperature strength. Therefore, the Cr content range is set to 9-13%.

[0031] W is a typical solid solution strengthening element, and its solid solution strengthening effect is more obvious than that of Mo. 23 It can stabilize the fine distribution of C6 and promote its precipitation strengthening. Increasing the W element can significantly improve the high-temperature strength and creep properties of heat-resistant steel. If the W element content is less than 2.0%, the long-term creep requirements of heat-resistant steel for temperatures above 630°C cannot be met. If the W content exceeds 3.5%, ferrite formation occurs at high temperatures. In addition, increasing the W content gradually reduces the weldability of the heat-resistant steel. Therefore, in this invention, the W mass percentage is limited to 2.0 to 3.5%.

[0032] Regarding Mo, the combined addition of W and Mo can improve the toughness and plasticity of the material, the hot workability is good, and the impact strength is improved.

[0033] Co is an austenite stabilizing element, suppresses the formation of δ ferrite, improves the high temperature strength of the material, and 23 Coarsening of C6 is suppressed. In the present invention, the mass percentage of Co is limited to 2.8 to 5%.

[0034] Regarding Cu, Cu element can suppress the formation of δ-ferrite. Furthermore, adding Cu helps enhance the solid solution strengthening role of W, thereby improving the high-temperature creep strength of W-containing martensitic heat-resistant steel. Cu itself can also play a role in precipitation strengthening by existing as nano-Cu-rich particles. When the Cu content is relatively low, Cu exists mainly in a solid solution form, and the strengthening effect is relatively weak. However, when the Cu content is relatively high, it has a significant impact on thermoplasticity at high temperatures. Therefore, in the present invention, the mass percentage of Cu is limited to 0.5 to 1.5%.

[0035] Ni is a typical austenite-forming element, which can improve toughness and balance with the Cr equivalent of the material. In the present invention, the mass percentage of Ni is limited to 0.1 to 0.5%.

[0036] Regarding B, in the case of high Cr martensitic heat-resistant steel, the B element is M 23 Substitution at the C element position of C6 gives M 23 (C,B)6 can be formed, and M 23 (C,B)6 has a slow aging rate and good durability, which contributes to the formation of M near the prior austenite grain boundaries. 23 By reducing the coarsening rate of C6, M 23 It suppresses the coarsening of C6 and improves the creep strength of steel. B purifies the grain boundary and forms carbon boride M 23 (C 0.85 B 0.15 )6 can be formed. If the B content is too high, the thermoplasticity at high temperatures decreases and the risk of cracking during forging increases. Therefore, in the present invention, the mass percentage of B is limited to 0.01 to 0.015%.

[0037] N forms finely dispersed second-phase particles with V, Nb, and Ta, significantly improving the high-temperature endurance limit of the material. However, if the N content is too high, it combines with B to form coarse BN particles, significantly weakening the strength and toughness of the steel. Furthermore, it consumes B, which strengthens the grain boundaries, significantly reducing the high-temperature endurance limit of the steel. Therefore, in the present invention, the mass percentage of N is limited to 0.01 to 0.08%.

[0038] Ta increases the Cr content in this steel to approximately 12%. When aged and used for long periods at 600-650°C, Cr in the matrix aggregates into MX, further forming Cr(Nb,V)N. This consumes Cr from the matrix, leading to coarsening of the precipitates and further promoting the formation of Cr(Nb,V)N. 12Cr steel is more prone to forming coarse chromium vanadium or chromium niobium nitride phases than 9Cr steel, which is unfavorable for dislocation pinning and lath, thereby affecting the endurance limit of the steel. The addition of an appropriate amount of Ta suppresses the transformation of MX to Cr(Nb,V)N, resulting in the formation of a phase primarily composed of CrTaN. This CrTaN phase is less likely to coarsen than the Cr(Nb,V)N phase and exists as a finely dispersed precipitate, contributing to precipitation strengthening and thereby resolving the problems associated with increased Cr content. Too high a Ta content makes it difficult for MX to completely dissolve in the matrix, resulting in the formation of Ta-rich particles, which increases the difficulty of smelting. Therefore, in the present invention, the mass percentage of Ta is limited to 0.2 to 0.5%.

[0039] Zr tends to reduce the growth of austenite grains as the Zr content increases and also reduces the size of inclusions, so in the present invention the Zr mass percentage is limited to 0.1 to 0.5%.

[0040] Regarding rare earth elements, Ce+Y are rare earth elements, and adding a small amount can improve the high-temperature mechanical properties and corrosion resistance of heat-resistant steel. Adding a combination of rare earth elements exerts a synergistic effect, purifying and strengthening grain boundaries, and limiting the number and shape of inclusions, thereby improving high-temperature strength and oxidation resistance. The total amount of rare earth added in the present invention is 0.01-0.3%.

[0041] Preferably, the martensitic heat-resistant steel for use at 630°C or higher contains, by mass percentage, 0.01 to 0.14% C, 0.05 to 0.50% Si, 0.05 to 0.70% Mn, 8.5 to 13% Cr, 2.0 to 3.5% W, 0.1 to 0.7%, Mo, 0.03 to 0.07%, Nb, 0.1 to 0.3%, Co, 2.8 to 5%, Cu, 0.8 to 1.2%, Ni, 0.1 to 0.5%, B, 0.01 to 0.015%, N, 0.01 to 0.08%, Ta, 0.28 to 0.45%, Zr, and Ce+Y, with the remainder being Fe and unavoidable impurities.

[0042] The present invention also provides a method for producing a martensitic heat-resistant steel for use at 630°C or higher, comprising the following steps: In step S1, raw materials are weighed according to the content of each component in the composition, and are smelted in a vacuum induction furnace and poured into an ingot. The content of impurity elements is strictly limited, and the composition is, in mass percentage, C 0.01-0.14%, Si 0.05-0.50%, Mn 0.05-0.70%, Cr 8.5-13%, W 2.0-3.5%, Mo 0.1-0.7%, Nb 0.03-0.07%, V 0.1-0.3%, Co 2.8-5%, Cu 0.8-1.5%, Ni 0.1-0.5%, B 0.01-0.015%, N 0.01-0.08%, Ta 0.2-0.5%, Zr 0.1-0.5%, Ce+Y The content is 0.01 to 0.3%, and the remaining components are Fe and unavoidable impurities. In step S2, the ingot obtained in step S1 is subjected to high-temperature homogenization treatment, kept at a constant temperature, and then cooled to room temperature in each furnace to obtain a blank. In step S3, the blank obtained in step S2 is forged by the rolling method, the upsetting method, or the stretching method, and when the temperature during the forging process is lower than the finish forging temperature, it is returned to the furnace to be heated and then forged again, and after forging, it is furnace cooled to room temperature. In step S4, the forged bar obtained in step S3 is subjected to normalizing and two tempering heat treatments to obtain a chromium-tantalum nitride strengthened martensitic heat-resistant steel, wherein the first tempering temperature is lower than the second tempering temperature.

[0043] Specifically, the purpose of the high-temperature homogenization treatment in step S2 above is to eliminate the segregation of high-temperature ferrite, precipitates, and alloy elements in the ingot. If the high-temperature homogenization temperature is too high, the grains will become significantly coarser and high-temperature ferrite will form. If the temperature is too low, the precipitates, such as delta ferrite and M3B2, and element segregation in the cast structure will not be effectively eliminated. Based on extensive experimental research, the high-temperature homogenization temperature is limited to 1160-1200°C. The holding time is limited to 4-8 hours. A shorter holding time is insufficient to eliminate the high-temperature ferrite, precipitates, and element segregation in the ingot. A longer holding time may result in overheating or overburning, resulting in grain coarsening.

[0044] Specifically, the forging in step S3 achieves the goals of crushing defects, refining grains, and homogenizing the structure (the structure of the heat-resistant steel according to Example 1 after quenching and tempering is shown in Figure 3), thereby laying a good foundation for improving the overall performance of the heat-resistant steel. If the roughing temperature is higher than 1200°C, high-temperature ferrite will form, increasing the risk of forging cracks. Furthermore, if the roughing temperature is too high, the grains will significantly coarsen, making it more difficult to control the grain size in the subsequent forging process. If the grains are too large or mixed crystals are present after forging, this will affect both the final mechanical properties and flaw detection. Therefore, in this invention, the roughing temperature is limited to 1160-1200°C and the finish forging temperature is limited to 850-950°C. Within the forging temperature range specified in this invention, the steel has good plasticity. Compared to the prior art, the roughing temperature in this invention is approximately 100°C higher than that of the same steel, significantly expanding the forging temperature range.

[0045] Specifically, the above step S4 includes the following steps. Step S401 is normalizing, and the normalizing process involves heating the forged bar to 1050 to 1150°C, keeping the temperature, and then air-cooling it to room temperature. Step S402 is the first tempering, and the first tempering step involves heating the forged bar to 600 to 700°C, keeping the temperature, and then air-cooling it to room temperature. Step S403 is the second tempering, and the second tempering process involves heating the forged bar to 680 to 780°C, keeping the temperature, and then air-cooling it to room temperature.

[0046] Specifically, in the above S401, when large forgings are heat treated, if the heating rate is too fast, the temperature difference between the inside and outside will be very large, and if the heating rate is too fast, thermal cracking may occur. Therefore, based on the actual rules for heat treatment of large forgings, the forged bar is placed in a heating furnace at room temperature, and the heating rate is not more than 100°C / hour.

[0047] Specifically, in the above S401, if the temperature-holding time is too long, the grains will become significantly coarser, and if it is too short, the heat treatment of the forged product will be insufficient. Therefore, the temperature-holding time is limited to 1 to 10 hours.

[0048] Specifically, in the above S402, the role of the first tempering is to promote the precipitation of CrTaN and fix the N element in the CrTaN phase. This is because around 650°C is the temperature range in which CrTaN in 12% Cr steel is most likely to precipitate. Tempering at this temperature allows the formation of an extremely finely dispersed CrTaN phase, which then dissolves in the other MX and Cr 23 It is more difficult to grow than precipitates such as C6 and Laves. If the tempering temperature is too high, many carbides will precipitate, widening the lath width of the martensite structure and reducing the dislocation density, resulting in a significant decrease in strength. If the tempering temperature is too low, it will be insufficient to promote the precipitation of CrTaN. If the heat-holding time is too long, performance will decrease, and if it is too short, it will be insufficient for heat treatment and element diffusion. Therefore, the tempering temperature should be limited to 600-700°C and the heat-holding time to 5-10 hours.

[0049] Specifically, in the S403 steel, the second tempering step is intended to fully form tempered martensite and achieve good overall performance. If the tempering temperature is too high, the strength will be insufficient; if the tempering temperature is too low, the impact strength will be significantly reduced; if the soaking time is too long, the precipitates will become too large, and neither the strength nor the impact strength will meet the performance requirements. If the soaking time is too short, the forging will not be sufficiently heat-treated and the martensite will not be formed. Therefore, the tempering temperature should be limited to 680-780°C, and the soaking time to 5-10 hours.

[0050] The normalizing temperature mentioned above is M 23 The alloying elements are completely dissolved in the matrix by completely eliminating precipitates such as C6, M3B2, and Laves, while the grain size is limited to Grade 2 or less and no delta ferrite is formed. The first low-temperature tempering promotes the precipitation of CrTaN and fixes the N element in the CrTaN phase. The second high-temperature tempering tempers the newly transformed martensite, ensuring that the final structure is a uniform mixture of tempered martensite and precipitates. The proportions of tempered martensite and precipitates are 98% and 2%, respectively. The precipitates are finely dispersed at lath and grain boundaries, providing excellent strengthening properties.

[0051] The structure of the heat-resistant steel that has undergone the above heat treatment is a fully tempered martensite structure plus a precipitated phase, which is mainly composed of finely dispersed M 23 C6, CrTaN, a small amount of fine Laves phase and nano-dispersion strengthening Cu phase.

[0052] The heat-resistant steel that has undergone the above heat treatment has a yield strength of more than 660 MPa, a tensile strength of more than 850 MPa, an elongation of 16% or more, a cross-sectional shrinkage of 50% or more, and an impact energy of 20 J or more at room temperature. At 630°C, the yield strength is more than 280 MPa, the tensile strength is more than 380 MPa, the elongation is 22% or more, and the cross-sectional shrinkage is 62% or more. The creep rupture time at 650°C and 180 MPa is more than 3500 hours, and the weight gain due to oxidation resistance at 650°C (400 hours) is 0.3 mg / m 2 Below is the result: Excellent performance.

[0053] The following specific examples and comparative examples demonstrate the advantages of the precise limitations of the steel composition and process parameters according to the present invention.

[0054] Example 1 This embodiment provides a martensitic heat-resistant steel for use at temperatures of 630°C or higher and a method for manufacturing the same.

[0055] The chemical composition of this example is, by weight percentage, C 0.05%, Si 0.30%, Mn 0.50%, Cr 12.0%, W 3.0%, Mo 0.2%, Nb 0.05%, V 0.2%, Co 4.5%, Ni 0.2%, Cu 1.0%, B 0.01%, N 0.045%, Ta 0.32%, Zr 0.3%, Ce+Y 0.15%, and the remaining components are Fe and unavoidable impurities.

[0056] The manufacturing method of heat-resistant steel includes the following: In step S1, the alloy is smelted in a vacuum induction furnace according to the alloy composition ratio, and poured into an ingot, and the content of impurity elements is strictly limited. In step S2, the ingot obtained in step S1 is subjected to high-temperature homogenization treatment, and then cooled to room temperature in each furnace to obtain a blank, where the high-temperature homogenization temperature is 1180°C and the heat retention time is 5 hours. In step S3, the blank obtained in step S2 is forged using the rolling method, the upsetting method, or the stretching method. The rough forging temperature is 1180°C, and the finish forging temperature is 950°C. If the temperature in the forging process is lower than the finish forging temperature, it needs to be returned to the furnace and heated before being forged again. In step S4, the forged bar obtained in step S3 is subjected to normalizing and two tempering heat treatments to obtain a chromium-tantalum nitride strengthened martensitic heat-resistant steel, in which the normalizing temperature is 1100°C, the heat-holding time is 5 hours, the first tempering temperature is 650°C, the heat-holding time for the first tempering is 6 hours, and the second tempering temperature is 740°C, the heat-holding time for the second tempering is 6 hours.

[0057] The chemical compositions of the steels according to Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 1. The process steps of Examples 2 to 4 are the same as those of Example 1, and the specific process parameters are shown in Table 2. The performance of Examples 1 to 4 and Comparative Examples 1 and 2 is shown in Tables 3 and 4, and the metal structures of Examples 1 to 4 and Comparative Examples 1 and 2 are shown in Table 5.

[0058] Comparative Example 1 does not contain Cu, Ta, Zr or rare earth elements in its chemical composition, and Comparative Example 2 contains Ta in its chemical composition, with specific chemical compositions as shown in Table 1. The process steps of Comparative Examples 1 and 2 are the same as those of Example 1, with specific process parameters shown in Table 2, and a comparison of each performance index with Examples 1 to 4 is shown in Table 3. As can be seen from Table 3, the Examples are superior in each performance compared to the Comparative Examples. JPEG0007770561000001.jpg37170JPEG0007770561000002.jpg252170JPEG0007770561000003.jpg123170

[0059] The above-mentioned are only specific preferred embodiments of the present invention, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that are easily thought up by a person skilled in the art within the technical scope disclosed in the present invention are intended to be included in the protection scope of the present invention.

Claims

1. A martensitic heat-resistant steel for temperatures of 630°C or higher, The composition, expressed by mass percentage, includes C 0.01 to 0.14%, Si 0.05 to 0.50%, Mn 0.05 to 0.70%, Cr 8.5 to 13%, W 2.0 to 3.5%, Mo 0.1 to 0.7%, Nb 0.03 to 0.07%, V 0.1 to 0.3%, Co 2.8 to 5%, Cu 0.8 to 1.5%, Ni 0.1 to 0.5%, B 0.01 to 0.015%, N 0.01 to 0.08%, Ta 0.2 to 0.5%, Zr 0.1 to 0.5%, Ce+Y 0.01 to 0.3%, and the remaining components are Fe and inevitable impurities. Martensitic heat-resistant steel for temperatures above 630°C.

2. The composition, expressed by mass percentage, includes C 0.01 to 0.14%, Si 0.05 to 0.50%, Mn 0.05 to 0.70%, Cr 8.5 to 13%, W 2.0 to 3.5%, Mo 0.1 to 0.7%, Nb 0.03 to 0.07%, V 0.1 to 0.3%, Co 2.8 to 5%, Cu 0.8 to 1.2%, Ni 0.1 to 0.5%, B 0.01 to 0.015%, N 0.01 to 0.08%, Ta 0.28 to 0.45%, Zr 0.1 to 0.5%, Ce+Y 0.1 to 0.2%, and the remaining components are Fe and inevitable impurities. The martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 1.

3. The microstructure of the heat-resistant steel is a fully tempered martensite structure and a precipitated phase, and the precipitated phase is a dispersedly distributed M ２３ C ６ carbides, CrTaN phase, Laves phase and nano-dispersion strengthened Cu phase, The martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 1 or 2.

4. The raw materials are weighed according to the content of each component in the component composition, and are smelted in a vacuum induction furnace and poured into an ingot. The content of impurity elements is strictly limited, and the component composition is, in mass percentage, C 0.01-0.14%, Si 0.05-0.50%, Mn 0.05-0.70%, Cr 8.5-13%, W 2.0-3.5%, Mo 0.1-0.7%, Nb 0.03-0.07%, V 0.1-0.3%, Co 2.8-5%, Cu 0.8-1.5%, Ni 0.1-0.5%, B 0.01-0.015%, N 0.01-0.08%, Ta 0.2-0.5%, Zr 0.1-0.5%, Ce+Y Step S1, comprising 0.01 to 0.3% of iron and unavoidable impurities; Step S2: The ingot obtained in step S1 is subjected to high-temperature homogenization treatment, kept at a constant temperature, and then cooled to room temperature in each furnace to obtain a blank. Step S3: Forging the blank obtained in step S2 by a rolling method, an upsetting method, or a stretching method. When the temperature in the forging process is lower than the finishing temperature, the blank is returned to the furnace and heated, and then forged again. After forging, the furnace is cooled to room temperature. Step S4: performing normalizing and two tempering heat treatments on the forged bar obtained in step S3 to obtain a chromium-tantalum nitride strengthened martensitic heat-resistant steel, wherein the first tempering temperature is lower than the second tempering temperature. The method for producing the martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 1.

5. In step S2, the temperature of the high-temperature homogenization treatment is 1160 to 1200°C, and the heat-retention time is 4 to 8 hours. The method for producing a martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 4.

6. In step S3, the rough forging temperature is 1160 to 1200°C and the finish forging temperature is 850 to 950°C. The method for producing a martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 4.

7. Step S4 Step S401: Normalizing, in which the normalizing process is to heat the forged bar to 1050 to 1150°C, keep the temperature, and then air-cool it to room temperature; Step S402: First tempering, in which the forged bar is heated to 600 to 700 ° C., kept at that temperature, and then air-cooled to room temperature; A second tempering step includes step S403 of heating the forged bar to 680 to 780 ° C, maintaining the temperature, and then air-cooling the bar to room temperature. The method for producing a martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 4.

8. In step S401, the temperature rising rate is 100°C / hour or less, and the temperature keeping time is 1 to 10 hours. The method for producing a martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 7.

9. In step S402, the warming time is 5 to 10 hours. The method for producing a martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 7.

10. In step S403, the warming time is 5 to 10 hours. The method for producing a martensitic heat-resistant steel for use at temperatures of 630°C or higher according to claim 7.

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