A martensitic heat-resistant steel for 630°c or more and a method for manufacturing the same

Abstract

The application discloses a kind of martensitic heat-resistant steel for 630 ℃ above and preparation method thereof, belong to metal material technical field, for solving the poor stability of existing heat-resistant steel in long-time application, the problem of poor comprehensive performance.The above heat-resistant steel by mass percent, its composition includes:C:0.01%~0.06%, Si:0.05%~0.50%, Mn:0.05%~0.70%, Cr:11%~13%, W:2.0%~3.5%, Co:4%~5%, Cu:0.5%~1.5%, Ni:0.1%~0.5%, B:0.01%~0.015%, N:0.01%~0.08%, Ta:02%~0.5%, Zr:0.1%~0.5%, Ce+Y:0.01%~0.3%, the rest component is Fe and inevitable impurity.The heat-resistant steel of the application has good comprehensive performance such as high temperature strength, impact resistance, resistance to sustained creep and high temperature oxidation resistance, and is suitable for ultra-supercritical steam turbine rotor with working temperature of 630 ℃ and above.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials technology, specifically relating to a martensitic heat-resistant steel for temperatures above 630°C and its preparation method. Background Technology

[0002] To achieve energy conservation and emission reduction goals, thermal power generation technology is developing towards higher parameters, double reheat, and cleaner, more efficient methods, while fully utilizing advanced ultra-supercritical coal-fired power generation technology. For example, by upgrading thermal power generation from subcritical to ultra-supercritical conditions, 68g of coal can be saved per kilowatt-hour, reducing coal consumption for power generation by 21%. This could save my country 232 million tons of coal annually and reduce CO2 emissions by 510 million tons per year. However, the availability of martensitic heat-resistant steel materials for large castings and forgings in high-parameter steam turbines remains a technological bottleneck restricting the improvement of power plant parameters, especially for 630℃ steam turbine rotor forgings, for which there are currently no mature products available domestically or internationally.

[0003] Therefore, to overcome key core technologies, the localization of high-performance, high-reliability heat-resistant steel materials and products is urgently needed. The increasing parameters of power units also place higher demands on material performance, especially on the toughness and durability under high stress and high temperature conditions. Currently, domestic and international research mainly focuses on adding elements such as W, Co, B, and N to further improve the high-temperature durability and oxidation resistance of heat-resistant steel. However, adjusting elements such as W, Co, B, Nb, and N has limited effect on improving the high-temperature durability and oxidation resistance of heat-resistant steel. There is an urgent need to develop a new strengthening method to further optimize the high-temperature performance and oxidation resistance of heat-resistant steel. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a martensitic heat-resistant steel for temperatures above 630°C and its preparation method. The steel exhibits excellent comprehensive properties such as high-temperature strength, impact resistance, creep resistance, and oxidation resistance, and is suitable for ultra-supercritical steam turbine rotors with operating temperatures of 630°C and above.

[0005] To address the material selection requirements for heat-resistant steel used in power plant turbine units under ultra-supercritical conditions, this invention has conducted extensive experimental research. Starting from the perspective of controlling the microstructure of nitride precipitates in 10-12% Cr steel, a novel martensitic heat-resistant steel has been developed. By adding tantalum, high-temperature stable fine chromium-tantalum nitrides are used to replace MX as the precipitation strengthening phase.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] This invention provides a martensitic heat-resistant steel for temperatures above 630℃, comprising, by mass percentage: C: 0.01%–0.06%, Si: 0.05%–0.50%, Mn: 0.05%–0.70%, Cr: 11%–13%, W: 2.0%–3.5%, Co: 4%–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: 0.01%–0.3%, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, a martensitic heat-resistant steel for temperatures above 630°C comprises, by mass percentage: C: 0.01%–0.06%, Si: 0.05%–0.5%, Mn: 0.05%–0.70%, Cr: 11%–13%, W: 2.0%–3.5%, Co: 4%–5%, Cu: 0.8%–1.2%, Ni: 0.1%–0.5%, B: 0.006%–0.013%, N: 0.01%–0.08%, Ta: 0.28%–0.45%, Zr: 0.1%–0.5%, Ce+Y: 0.1%–0.2%, with the remainder being Fe and unavoidable impurities.

[0009] Furthermore, the microstructure of the heat-resistant steel is a fully tempered martensite structure with precipitates, the precipitates including finely dispersed M... 23 C6 type carbides, CrTaN phase, a small amount of fine Laves phase and nanoscale dispersion-strengthened Cu phase.

[0010] The present invention also provides a method for preparing martensitic heat-resistant steel for temperatures above 630°C, comprising the following steps:

[0011] Step S1: Determine the proportion of raw materials according to the content of each component in the above composition ratio, melt and cast them into ingots in a vacuum induction furnace, and strictly control the content of impurity elements;

[0012] Step S2: The ingot obtained in step S1 is subjected to high-temperature homogenization treatment, kept at the temperature, and then cooled to room temperature in the furnace to obtain the billet;

[0013] Step S3: The billet obtained in step S2 is forged by rounding, upsetting and drawing. If the temperature is lower than the final forging temperature during the forging process, it needs to be reheated in the furnace before forging. After forging, the furnace is cooled to room temperature.

[0014] Step S4: The forged bar obtained in step S3 is subjected to normalizing and two tempering heat treatments to obtain chromium-tantalum nitride reinforced martensitic heat-resistant steel; wherein the first tempering temperature is lower than the second tempering temperature.

[0015] Furthermore, in step S2, the high-temperature homogenization temperature is 1160–1200℃, and the holding time is 4–8 hours.

[0016] Furthermore, in step S3, the initial forging temperature is 1160–1200℃, and the final forging temperature is 850–950℃.

[0017] Further, step S4 includes the following steps:

[0018] S401: Normalizing. The normalizing process involves heating the forged bar to 1050-1150℃ and holding it at that temperature, then air cooling it to room temperature.

[0019] S402: First tempering. The first tempering process involves heating the forged bar to 600-700℃, holding it at that temperature, and then air cooling it to room temperature.

[0020] S403: Second tempering. The second tempering process involves heating the forged bar to 680-780℃, holding it at that temperature, and then air cooling it to room temperature.

[0021] Furthermore, in step S401, the heating rate is ≤100℃ / h, and the holding time is 1~10h.

[0022] Furthermore, in step S402, the heat preservation time is 5 to 10 hours.

[0023] Furthermore, in step S403, the heat preservation time is 5 to 10 hours.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] 1. In the preparation process of the martensitic heat-resistant steel above 630℃ provided by this invention, the microstructure obtained is a complete tempered martensite structure plus precipitates by precisely controlling the process parameters such as the time and temperature of high-temperature homogenization treatment, forging temperature, normalizing temperature and time, and secondary tempering temperature and time. The precipitates include finely dispersed M... 23 C6 type carbides, CrTaN phase, a small amount of fine Laves phase, and nanoscale dispersed reinforced Cu phase. This ensures the excellent room temperature strength, high temperature strength, and resistance to creep and oxidation of the heat-resistant steel, making it suitable for ultra-supercritical steam turbine rotors above 630℃.

[0026] 2. The martensitic heat-resistant steel with a temperature above 630℃ provided by this invention does not contain V or Nb elements, thereby avoiding the formation of the Cr(V,Nb)N phase, which is very easy to form and coarsen during the creep quenching process. The coarsening of this phase will deteriorate the creep quenching performance. Instead, this invention adds 0.2% to 0.5% Ta element. An appropriate amount of Ta element will form a CrTaN phase that is not easily coarsented, thereby avoiding the sharp drop in performance caused by the coarsening of MX to Cr(Nb,V)N during long-term service.

[0027] 3. This invention controls the carbon content to a low level, thereby forming finely dispersed M particles through a lower C content. 23 C6 plays a role in dispersion strengthening; by adding a certain amount of Cu, the formation of high-temperature ferrite is inhibited, while the precipitation of nanoscale copper-rich phases can compensate for the lack of V and Nb, thus playing a role in precipitation strengthening and improving the creep strength of heat-resistant steel; adding an appropriate amount of B replaces M. 23 The position of the C element in C6 forms M. 23 (C,B)6, thereby reducing M near the original austenite grain boundaries 23 The C6 coarsening rate significantly improves creep strength; the use of an appropriate amount of W element improves creep strength; the use of a small amount of Ni element and a higher Co content improves the toughness of the matrix; through reasonable element ratio, the formation of high-temperature ferrite can be greatly avoided, providing a larger temperature window for forging and heat treatment processes in actual production.

[0028] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0029] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0030] Figure 1 Microstructure of the heat-resistant steel in Example 1 (as-cast state);

[0031] Figure 2 The homogenized microstructure of the heat-resistant steel in Example 1;

[0032] Figure 3 The quenched and tempered microstructure of the heat-resistant steel in Example 1. Detailed Implementation

[0033] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and are used together with the description of the invention to explain the principles of the invention.

[0034] This invention provides a martensitic heat-resistant steel for temperatures above 630℃, comprising, by mass percentage: C: 0.01%–0.06%, Si: 0.05%–0.50%, Mn: 0.05%–0.70%, Cr: 11%–13%, W: 2.0%–3.5%, Co: 4%–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: 0.01%–0.3%, with the remainder being Fe and unavoidable impurities.

[0035] Under normal circumstances, after prolonged service, 9-12% Cr steel will form coarse chromium-vanadium or chromium-niobium nitride phases, namely CrVN and CrNbN. Compared with the prior art, the heat-resistant steel provided by this invention has a lower C content, a higher Co content, and adds strengthening elements such as Ta and Cu, as well as Ce+Y rare earth elements. The heat-resistant steel provided by this invention does not contain V or Nb elements, which can avoid the formation of the Cr(V,Nb)N phase, which is very easy to form and coarsen during the creep erosion process. The coarsening of this phase will deteriorate the creep erosion performance.

[0036] The present invention adds 0.2% to 0.5% of Ta element. An appropriate amount of Ta element will form a CrTaN phase that is not easily coarsened, which can avoid the sudden drop in performance caused by the coarsening of MX to Cr(Nb,V)N during long-term service.

[0037] The heat-resistant steel provided by this invention forms finely dispersed M through a low C content. 23 C6 plays a role in dispersion strengthening; the addition of 0.8% to 1.5% Cu element, on the one hand, acts as an austenite forming element to inhibit the formation of high-temperature ferrite, and on the other hand, the precipitation of nanoscale copper-rich phase can compensate for the lack of V and Nb, playing a precipitation strengthening role and improving the creep strength of heat-resistant steel; an appropriate amount of B element can replace M 23 The position of the C element in C6 forms M. 23 (C,B)6, thereby reducing M near the original austenite grain boundaries 23 The C6 coarsening rate significantly improves creep strength; the use of an appropriate amount of W element improves endurance strength; and the use of a small amount of Ni element and a higher Co content improves matrix toughness. Through a reasonable element ratio, the formation of high-temperature ferrite can also be greatly avoided, providing a larger temperature window for forging and heat treatment processes in actual production.

[0038] The heat-resistant steel provided by this invention possesses high strength, resistance to creep, and oxidation resistance. The martensitic heat-resistant steel prepared by this invention exhibits a room temperature yield strength greater than 660 MPa, a tensile strength greater than 850 MPa, an elongation greater than 16%, a reduction of area greater than 50%, and an impact energy greater than 20 J. At 630℃, its yield strength is greater than 280 MPa, its tensile strength greater than 380 MPa, its elongation greater than 22%, its reduction of area greater than 62%, its creep rupture time at 650℃ and 180 MPa is greater than 3500 h, and its oxidation weight gain (400 h) at 650℃ is 0.3 mg / m³. 2 The following features excellent overall performance.

[0039] Specifically, the functions of each component in the above-mentioned martensitic heat-resistant steel used at temperatures above 630℃ are as follows:

[0040] C: An important precipitation-enhancing element, M 23 C6 and MX dispersion strengthening improve creep resistance; C is a strong austenite stabilizing element, which can reduce the formation of δ-ferrite; improve hardenability and precipitation strengthening; excessive carbon content can lead to excessive consumption of solid solution elements (such as Cr, W), reduce grain boundary corrosion resistance, deteriorate weldability, and have a negative impact on creep resistance; too low carbon content will result in insufficient strengthening and reduce strength and hardness. Therefore, the mass percentage of C in this invention is controlled at 0.01% to 0.06%.

[0041] Si (Si) is beneficial for improving the strength and steam corrosion resistance of the material matrix; increased Si content leads to a sharp increase in oxidation resistance; however, increased Si content promotes the formation of high-temperature ferrite, lowering its formation temperature, which has an adverse effect on the forging temperature range. Furthermore, excessively high Si content is detrimental to the impact toughness of the material, and the creep strength decreases with increasing Si content. Therefore, the mass percentage of Si in this invention is controlled between 0.05% and 0.50%.

[0042] Mn: Improves strength and hot working performance, and can also stabilize P, S, etc. When the content is below 0.2%, Mn has no significant effect; when the content is above 1%, a second phase may appear in the microstructure, which is detrimental to the impact toughness of the material. Therefore, the mass percentage of Mn in this invention is controlled between 0.05% and 0.70%.

[0043] Cr: The most critical element for corrosion and oxidation resistance. Cr itself possesses excellent resistance to creep deformation and is a key element in heat-resistant steels to improve resistance to steam oxidation and corrosion. It also enhances the high-temperature strength of steel. In sufficient quantities, Cr can react with O to form a Cr₂O₃ protective film on the alloy matrix surface, preventing the diffusion of O atoms and metal ions, thus slowing down the oxidation process. Furthermore, Cr is an important precipitation strengthening element, capable of reacting with C to form M₂O₃. 23C6 precipitation strengthening; compared to 9% or lower Cr content, 12% Cr steel exhibits significantly better oxidation resistance in heat-resistant steel than 9% Cr steel. When the Cr content is too high, δ-ferrite will form, reducing high-temperature thermal strength; therefore, the Cr content range is set at 11-13%.

[0044] W: A typical solid solution strengthening element, its solid solution strengthening effect is more obvious than that of Mo, and it can stabilize M. 23 The fine distribution of C6 promotes precipitation strengthening; the increase of W significantly improves the high-temperature strength and creep properties of heat-resistant steel. Below 2.0% W content, the long-term creep requirements for heat-resistant steel at temperatures of 630℃ and above cannot be met, while W exceeding 3.5% leads to the formation of high-temperature ferrite. Furthermore, the weldability of heat-resistant steel gradually deteriorates with increasing W content. Therefore, the mass percentage of W in this invention is controlled between 2.0% and 3.5%.

[0045] Co is an austenite stabilizing element that inhibits the formation of δ-ferrite, improves the high-temperature strength of materials, and inhibits M. 23 C6 coarsening; in this invention, the mass percentage of Co is controlled at 4% to 5%.

[0046] Cu: Cu can inhibit the formation of δ-ferrite. Simultaneously, the addition of Cu enhances the solid solution strengthening effect of W, improving the high-temperature creep strength of W-containing martensitic heat-resistant steel. Cu itself, existing as nano-Cu-rich particles, also plays a precipitation strengthening role. When the Cu content is low, it mainly exists in solid solution, resulting in a relatively weak strengthening effect; when the Cu content is high, it severely reduces high-temperature plasticity. Therefore, in this invention, the mass percentage of Cu is controlled between 0.5% and 1.5%.

[0047] Ni: A typical austenite-forming element that can improve toughness and balance the Cr equivalent of the material; in this invention, the mass percentage of Ni is controlled at 0.1% to 0.5%.

[0048] B: For high-Cr martensitic heat-resistant steel, element B can replace M. 23 The position of the C element in C6 forms M. 23 (C,B)6,M 23 (C,B)6 has a slow ripening rate and good creep properties, thereby reducing the amount of molten metal near the original austenite grain boundaries. 23 C6 coarsening rate, inhibiting M 23 C6 coarsening improves the creep strength of steel; B can purify grain boundaries and form M. 23 (C0.85B0.15)6 carborides; however, excessively high boron content reduces high-temperature plasticity and increases the risk of forging cracks. Therefore, the mass percentage of boron in this invention is controlled between 0.01% and 0.015%.

[0049] Nitrogen (N) can form fine, dispersed second-phase particles with V, Nb, and Ta, significantly improving the high-temperature creep strength of the material. However, when the N content is too high, it combines with boron (B) to form coarse BN particles, severely weakening the strength and toughness of the steel. It also consumes boron used for grain boundary strengthening, seriously impairing the high-temperature creep strength of the steel. Therefore, in this invention, the mass percentage of N is controlled between 0.01% and 0.08%.

[0050] Ta: In this steel, as the Cr content increases to around 12%, during long-term aging at 600-650℃ and use, Cr in the matrix will aggregate towards MX and further form Cr(Nb,V)N, thus consuming the Cr in the matrix and causing coarsening of the precipitates. This further promotes 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 detrimental to pinning dislocations and laths, thus affecting the steel's creep strength. Adding an appropriate amount of Ta will inhibit the transformation of MX to Cr(Nb,V)N, instead forming a phase dominated by CrTaN. This CrTaN phase is less prone to coarsening than the Cr(Nb,V)N phase and exists as a fine, dispersed precipitate, playing a precipitation strengthening role, thus solving the problems caused by the increased Cr content. If the Ta content is too high, it is difficult to completely dissolve in the matrix and exist as Ta-rich particles, increasing the difficulty of smelting. Therefore, in this invention, the mass percentage of Ta is controlled between 0.2% and 0.5%.

[0051] Zr: The growth tendency of austenite grains decreases with increasing Zr content, and the size of inclusions is reduced. Therefore, the mass percentage of Zr in this invention is controlled at 0.1% to 0.5%.

[0052] Rare earth elements: Ce+Y are rare earth elements. Adding a small amount can improve the high-temperature mechanical properties and corrosion resistance of heat-resistant steel. The addition of mixed rare earth elements can exert a synergistic effect, purifying and strengthening grain boundaries, controlling the number and morphology of inclusions, thereby improving high-temperature strength and oxidation resistance. The overall rare earth addition amount in this invention is 0.01%-0.3%.

[0053] Preferably, a martensitic heat-resistant steel for temperatures above 630°C comprises, by mass percentage: C: 0.01%–0.06%, Si: 0.05%–0.5%, Mn: 0.05%–0.70%, Cr: 11%–13%, W: 2.0%–3.5%, Co: 4%–5%, Cu: 0.8%–1.2%, Ni: 0.1%–0.5%, B: 0.006%–0.013%, N: 0.01%–0.08%, Ta: 0.28%–0.45%, Zr: 0.1%–0.5%, Ce+Y: 0.1%–0.2%, with the remainder being Fe and unavoidable impurities.

[0054] The present invention also provides a method for preparing martensitic heat-resistant steel for temperatures above 630°C, comprising the following steps:

[0055] Step S1: Determine the proportion of raw materials according to the content of each component in the above composition ratio, melt and cast them into ingots in a vacuum induction furnace, and strictly control the content of impurity elements;

[0056] Step S2: The ingot obtained in step S1 is subjected to high-temperature homogenization treatment, kept at the temperature, and then cooled to room temperature in the furnace to obtain the billet;

[0057] Step S3: The billet obtained in step S2 is forged by rounding, upsetting and drawing. If the temperature is lower than the final forging temperature during the forging process, it needs to be reheated in the furnace before forging. After forging, the furnace is cooled to room temperature.

[0058] Step S4: The forged bar obtained in step S3 is subjected to normalizing and two tempering heat treatments to obtain chromium-tantalum nitride reinforced martensitic heat-resistant steel; wherein the first tempering temperature is lower than the second tempering temperature.

[0059] Specifically, in step S2 above, the purpose of the high-temperature homogenization treatment is to eliminate the segregation of high-temperature ferrite, precipitates, and alloying elements in the ingot. Excessively high homogenization temperatures can lead to severe grain coarsening and the formation of high-temperature ferrite, while excessively low temperatures will fail to effectively eliminate precipitates such as δ-ferrite and M3B2, as well as elemental segregation, in the casting structure. Through extensive experimental research, the high-temperature homogenization temperature was controlled at 1160–1200℃. The holding time was controlled at 4–8 hours. A holding time that is too short is insufficient to eliminate the high-temperature ferrite, precipitates, and elemental segregation in the ingot, while a holding time that is too long may cause overheating and burning, resulting in coarse grains.

[0060] Specifically, the forging process in step S3 above can achieve the goals of compacting defects, refining grains, and homogenizing the microstructure (as shown in Example 1, the quenched and tempered microstructure of the heat-resistant steel is as follows). Figure 3 As shown in the figure, this lays a good foundation for improving the comprehensive performance of heat-resistant steel. When the initial forging temperature is higher than 1200℃, high-temperature ferrite will form, increasing the risk of forging cracks. Moreover, excessively high initial forging temperatures will cause severe grain coarsening, making it more difficult to control the grain size in subsequent forging processes. If there are excessively large grains and mixed grains after forging, it will affect the final mechanical properties and also affect flaw detection. Therefore, this invention controls the initial forging temperature to be 1160~1200℃ and the final forging temperature to be 850~950℃. Within the forging temperature range given in this invention, the steel has good plasticity. Compared with the prior art, the initial forging temperature of this invention is about 100℃ higher than that of similar steels, greatly expanding the forging temperature range.

[0061] Specifically, step S4 above includes the following steps:

[0062] S401: Normalizing. The normalizing process involves heating the forged bar to 1050-1150℃ and holding it at that temperature, then air cooling it to room temperature.

[0063] S402: First tempering. The first tempering process involves heating the forged bar to 600-700℃, holding it at that temperature, and then air cooling it to room temperature.

[0064] S403: Second tempering. The second tempering process involves heating the forged bar to 680-780℃, holding it at that temperature, and then air cooling it to room temperature.

[0065] Specifically, in S401 above, considering that excessively rapid heating rate during heat treatment of large parts can lead to huge internal and external temperature differences, and excessively rapid heating rate may cause hot cracking, therefore, according to the actual heat treatment rules of large parts, the forging bar is placed in the heating furnace at room temperature with a heating rate ≤100℃ / h.

[0066] Specifically, in S401 above, excessively long holding time will cause severe grain coarsening, while insufficient holding time will prevent the forging from being fully heated. Therefore, the holding time should be controlled to be 1–10 hours.

[0067] Specifically, in the S402 process described above, the first tempering promotes the precipitation of CrTaN, fixing the nitrogen element into the CrTaN phase. This is because around 650℃ is the temperature range in which CrTaN most readily precipitates in 12% Cr steel. Tempering at this temperature allows the formation of a very fine and dispersed CrTaN phase. This CrTaN phase, compared to other MX phases, exhibits higher Cr content. 23 C6 and Laves precipitates are less likely to grow; excessively high tempering temperatures will result in the precipitation of large amounts of carbides, causing widening of the martensite laths, reducing dislocation density, and leading to a severe decrease in strength. Tempering temperatures that are too low are insufficient to promote the precipitation of CrTaN. Excessive holding time softens the properties, while insufficient holding time hinders heat penetration and element diffusion. Therefore, the tempering temperature should be controlled at 600–700℃, and the holding time at 5–10 hours.

[0068] Specifically, in the S403 process described above, the second tempering serves to form a fully tempered martensite structure, resulting in excellent 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 too low, and if the holding time is too long, the precipitates will grow larger, failing to meet the performance requirements for strength and impact value; if the holding time is too short, the forging will not be sufficiently heat-through to form tempered martensite. Therefore, the tempering temperature should be controlled at 680–780℃, and the holding time at 5–10 hours.

[0069] It should be noted that the above-mentioned normalizing temperature can be used to determine M. 23The complete elimination of precipitates such as C6, M3B2, and Laves ensures that alloying elements are fully dissolved in the matrix, and the grain size is controlled to within grade 2, while preventing the formation of δ-ferrite. The first low-temperature tempering promotes the precipitation of CrTaN, fixing nitrogen elements into the CrTaN phase. The second high-temperature tempering tempers the newly transformed martensite, thus ensuring that the final microstructure consists entirely of uniform tempered martensite and precipitates, with tempered martensite and precipitates accounting for 98% and 2%, respectively. The precipitates are finely and dispersedly distributed at lath boundaries and grain boundaries, playing a significant strengthening role.

[0070] The microstructure of the heat-resistant steel after the above heat treatment is a fully tempered martensite structure with precipitates, the precipitates being mainly fine and dispersed M-phase. 23 C6, CrTaN, a small amount of fine Laves phase and nanoscale dispersion-strengthened Cu phase.

[0071] It should be noted that the heat-resistant steel treated as described above has a room temperature yield strength greater than 660 MPa, a tensile strength greater than 850 MPa, an elongation greater than 16%, a reduction of area greater than 50%, and an impact energy greater than 20 J. At 630℃, its yield strength is greater than 280 MPa, its tensile strength greater than 380 MPa, its elongation greater than 22%, its reduction of area greater than 62%, its creep rupture time at 650℃ and 180 MPa is greater than 3500 h, and its oxidation weight gain (400 h) at 650℃ is 0.3 mg / m³. 2 The following is a summary of the product's specifications. It boasts excellent performance.

[0072] The advantages of precise control over the composition and process parameters of the steel of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0073] Example 1

[0074] This embodiment provides a martensitic heat-resistant steel for temperatures above 630℃ and its preparation method.

[0075] The chemical composition of this embodiment, by weight percentage, includes: C: 0.05%, Si: 0.30%, Mn: 0.50%, Cr: 12.0%, W: 3.0%, Co: 4.5%, Ni: 0.2%, Cu: 1.0%, B: 0.006%, N: 0.045%, Ta: 0.32%, Zr: 0.3%, Ce+Y: 0.15%, with the remainder being Fe and unavoidable impurities.

[0076] Methods for preparing heat-resistant steel include:

[0077] Step S1: According to the alloy composition ratio, melt and cast the alloy into ingots in a vacuum induction furnace, strictly controlling the content of impurity elements;

[0078] Step S2: The ingot obtained in step S1 is subjected to high-temperature homogenization treatment, and then cooled to room temperature in the furnace to obtain billet; wherein, the high-temperature homogenization temperature is 1180℃ and the holding time is 5h.

[0079] Step S3: The billet obtained in step S2 is forged by rounding, upsetting, and drawing. The initial forging temperature is 1180℃ and the final forging temperature is 950℃. If the temperature is lower than the final forging temperature during the forging process, it needs to be reheated in the furnace before forging.

[0080] Step S4: The forged bar obtained in step S3 is subjected to normalizing and two tempering heat treatments to obtain chromium-tantalum nitride reinforced martensitic heat-resistant steel; wherein, the normalizing temperature is 1100℃ and the holding time is 5h; the first tempering temperature is 650℃ and the first tempering holding time is 6h; the second tempering temperature is 740℃ and the second tempering holding time is 6h.

[0081] The chemical composition of the steels in Examples 1-4 and Comparative Examples 1-2 is shown in Table 1. The process steps of Examples 2-4 are the same as those in Example 1. The specific process parameters are shown in Table 2. The properties of Examples 1-4 and Comparative Examples 1-2 are shown in Tables 3 and 4. The metallographic structure of Examples 1-4 and Comparative Examples 1-2 is shown in Table 5.

[0082] Comparative Example 1 did not contain Cu, Ta, Zr, or rare earth elements in its chemical composition; Comparative Example 2 did not contain Ta in its chemical composition. The specific chemical compositions are shown in Table 1. The process steps for Comparative Examples 1-2 were the same as those for Example 1, and the specific process parameters are shown in Table 2. The performance indicators of Comparative Examples 1-4 are compared with those of Examples 1-4 in Table 3. As can be seen from Table 3, the performance of the examples is superior to that of the control examples.

[0083] Table 1 Chemical composition (wt%) of the examples and comparative examples

[0084] element Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 C 0.05 0.04 0.05 0.03 0.15 0.05 Mn 0.5 0.5 0.1 0.3 0.73 0.5 Si 0.3 0.4 0.2 0.1 0.35 0.3 Cr 12 11.5 12.5 11.8 10.40 11 Mo - - - - 0.45 - W 3.0 2.5 2.8 3.3 2.21 3.0 Cu 1.0 0.8 1.2 1.1 - 1.0 Co 4.5 4.1 4.2 4.5 3.07 4.5 Nb - - - - 0.05 - Ni 0.2 0.3 0.4 0.3 0.32 0.2 V - - - - 0.21 - B 0.006 0.008 0.010 0.013 0.01 0.006 N 0.045 0.03 0.06 0.04 0.014 0.045 Ta 0.32 0.39 0.45 0.28 - - Zr 0.3 0.1 0.5 0.4 - 0.3 Ce+Y 0.15 0.1 0.2 0.18 - 0.15

[0085] Table 2. Specific process parameters for the examples and comparative examples.

[0086]

[0087] Table 3. Room temperature performance of the examples and comparative examples

[0088]

[0089] Table 4 Performance at 630°C for Examples and Comparative Examples

[0090]

[0091] Table 5. Metallographic structures of the examples and comparative examples.

[0092]

[0093] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A martensitic heat-resistant steel for use at 630°C or more, characterized in that, The components include, by mass percentage: C: 0.01%~0.05%, Si: 0.2%~0.50%, Mn: 0.05%~0.70%, Cr: 11%~13%, W: 2.5%~3.0%, Co: 4.1%~5%, Cu: 0.8%~1.5%, Ni: 0.1%~0.5%, B: 0.01%~0.015%, N: 0.03%~0.08%, Ta: 0.2%~0.5%, Zr: 0.1%~0.5%, Ce+Y: 0.01%-0.3%, and the rest components are Fe and inevitable impurities; The microstructure of the heat-resistant steel is a complete tempered martensite structure + precipitates, the precipitates including fine and dispersed M 23 C6-type carbides, CrTaN phase, a small amount of fine Laves phase and nanoscale dispersed strengthening Cu phase; The heat-resistant steel is prepared by the following steps: Step S1: the raw material is proportioned according to the component content in the above component proportioning, is smelted in a vacuum induction furnace, and is poured into an ingot, and the content of impurity elements is strictly controlled; Step S2: the ingot obtained in step S1 is subjected to high-temperature homogenization treatment, is kept warm, and then is cooled to room temperature with the furnace to obtain a blank; Step S3: the blank obtained in step S2 is forged by means of rounding, upsetting and drawing, and is reheated and forged again after being reheated in a furnace below the final forging temperature, and is cooled to room temperature after forging; Step S4: the forged rod obtained in step S3 is subjected to normalizing and twice tempering heat treatment to obtain the martensitic heat-resistant steel; wherein the first tempering temperature is lower than the second tempering temperature; In step S3, the initial forging temperature of the forging is 1160-1200℃; In step S4, the first tempering temperature is 600-700℃, and the holding time is 5-10h; the second tempering temperature is 680-780℃, and the holding time is 5-10h; The heat-resistant steel is suitable for a supercritical steam turbine rotor forging with a working temperature of 630℃ and above; The yield strength of the heat-resistant steel at 630 DEG C is greater than 280 Mpa, the tensile strength is greater than 380 Mpa, the elongation is greater than 22%, the reduction of area is greater than 62%, the creep rupture time at 650 DEG C and 180 MPa is greater than 3500 h, and the weight gain after oxidation at 650 DEG C for 400 h is 0.3 mg / m 2 The following.

2. The heat-resistant steel according to claim 1, characterized in that, The components include, by mass percentage: C: 0.01%~0.05%, Si: 0.2%~0.50%, Mn: 0.05%~0.70%, Cr: 11%~13%, W: 2.5%~3.0%, Co: 4.1%~5%, Cu: 0.8%~1.2%, Ni: 0.1%~0.5%, B: 0.01%~0.013%, N: 0.03%~0.08%, Ta: 0.28%~0.45%, Zr: 0.1%~0.5%, Ce+Y: 0.1%-0.2%, and the rest components are Fe and inevitable impurities.

3. A preparation method of a martensitic heat-resistant steel for 630℃ and above, for preparing the heat-resistant steel of any one of claims 1-2, comprising the following steps: Step S1: the raw material is proportioned according to the component content in the above component proportioning, is smelted in a vacuum induction furnace, and is poured into an ingot, and the content of impurity elements is strictly controlled; Step S2: the ingot obtained in step S1 is subjected to high-temperature homogenization treatment, is kept warm, and then is cooled to room temperature with the furnace to obtain a blank; Step S3: the blank obtained in step S2 is forged by means of rounding, upsetting and drawing, and is reheated and forged again after being reheated in a furnace below the final forging temperature, and is cooled to room temperature after forging; Step S4: subjecting the forged bar obtained in step S3 to a normalizing and twice tempering heat treatment to obtain the martensitic heat-resistant steel; wherein, The first tempering temperature is lower than the second tempering temperature; In step S3, the initial forging temperature of the forging is 1160-1200℃; In step S4, the first tempering temperature is 600-700℃, and the holding time is 5-10h; the second tempering temperature is 680-780℃, and the holding time is 5-10h.

4. The production method according to claim 3, characterized by, In step S2, the high-temperature homogenization temperature is 1160-1200℃, and the holding time is 4-8h.

5. The production method according to claim 3, wherein In step S3, the initial forging temperature is 1180-1200℃, and the final forging temperature is 850-950℃.

6. The preparation method according to claim 3, characterized in that, The step S4 comprises the following steps: S401: normalizing, the normalizing process is to heat the forging bar to 1050-1150℃ and hold, and then air cool to room temperature; S402: first tempering, the first tempering process is to heat the forging bar to 600-650℃, and hold for 5-10h, and then air cool to room temperature; S403: second tempering, the second tempering process is to heat the forging bar to 680-740℃, and hold for 5-10h, and then air cool to room temperature.

7. The production method according to claim 6, wherein In step S401, the heating rate is ≤100℃ / h, and the holding time is 1-10h.

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