An austenitic heat-resistant steel and a method for manufacturing the same

Abstract

This invention discloses an austenitic heat-resistant steel and its preparation method, belonging to the field of metallic materials. The chemical composition of austenitic heat-resistant steel, by weight percentage, is as follows: Ni 22~26%, Cr 20~23%, Cu 2.0~5.0%, W 2.0~5.0%, Co 1.0~2.0%, Mn 0.2~0.5%, Nb 0.4~0.8%, Mo 0.4~0.6%, Si 0.2~0.5%, N 0.1~0.3%, C 0.05~0.1%, Al 0.005~0.01%, Ce 0.01~0.1%, with the balance being Fe and unavoidable impurities. The weight ratio of Ce to Nb is 1:(4~80). The average size of the austenite grains in the steel is ≤12 μm, the average size of the Nb-rich phase is 50~200 nm, and the number density of the Nb-rich phase is >100,000 grains / mm². 2 The preparation method includes: smelting, pre-alloyed wire feeding, two-stage hot rolling, ultra-rapid cooling, and multi-stage aging treatment. This invention aims to simultaneously solve the problems of coarse Nb-rich strengthening phases, increased Nb solid solution content in Nb-containing austenitic heat-resistant steel products, and uneven austenitic grain size in austenitic heat-resistant steel products.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, specifically relating to a method for achieving uniform grain size and refined Nb-rich precipitates in austenitic heat-resistant steel by precisely controlling the mixing ratio of Ce and Nb elements and the synergistic composite addition process. Background Technology

[0002] In recent years, my country's power industry has developed rapidly, but thermal power still dominates our energy structure. Currently, the country encourages clean energy and optimizes the development of coal-fired power, with the key being to improve the efficiency of coal-fired power generating units and reduce pollutant emissions. High-efficiency generating units place higher demands on the high-temperature performance of structural materials, leading to the widespread application of new austenitic heat-resistant stainless steel materials in key equipment for advanced coal-fired power generation.

[0003] High-Cr, high-Ni austenitic heat-resistant steels such as Fe-22Cr-25Ni type C-HRA-5 and Sanicro 25, Fe-20Cr-25Ni type NF709, and Fe-25Cr-22Ni type HR3C all incorporate Nb and other alloying elements to form Nb-rich strengthening phases such as CrNbN and NbX (X = C, N). Because Nb readily segregates and interacts strongly with Cr, N, and C to form Nb-rich phases like CrNbN and NbX, especially during solidification due to supercooling, these Nb-rich phases primarily aggregate between dendrites. They are large and continuously distributed, ranging in size from tens to hundreds of micrometers. These coarse Nb-rich phases cannot be eliminated through forging, hot working, or heat treatment, becoming the initiation sites for fatigue cracks and creep failure, negatively impacting high-temperature performance. This is a common problem in Nb- and N-containing austenitic heat-resistant steels. Regarding dispersion strengthening, studies have shown that its strengthening effect is closely related to the size and number of precipitated phase particles. The smaller the precipitated phase particles and the greater their number, the more obvious the strengthening effect.

[0004] Existing technologies have significant limitations in their solutions to the above problems: some solutions refine the microstructure by adding a single rare earth element, but do not consider the synergistic effect between rare earth and Nb elements, making it difficult to fundamentally suppress Nb segregation; some technologies improve the grain state by optimizing the heat treatment process, but have limited effectiveness in solving the problem of coarse Nb-rich phases; other studies have attempted to add multiple alloying elements to promote the formation of nano-reinforcing phases, but have not clarified the ratio of core elements, resulting in poor process stability and large fluctuations in performance.

[0005] For example, Chinese patent application No. 201710263001.9, published on June 27, 2017, discloses an austenitic heat-resistant steel and its preparation method. Based on HR3C austenitic heat-resistant steel, this method optimizes the alloy composition to achieve high strength, high toughness, and excellent microstructural stability. Its main strengthening phases are MX, Z(CrNbN) phase, and M... 23 C6 has a grain size of 50~80 μm, which has not yet reached the nanoscale.

[0006] Solid solubility refers to the maximum amount of solute in a solid solution, that is, the limiting solubility of a solute in a solvent. Solid solubility can be determined experimentally or calculated according to thermodynamic principles. Studying solid solubility has not only theoretical significance but also great practical significance because the magnitude of solid solubility and its change with temperature directly affect the properties and heat treatment behavior of alloys.

[0007] Existing technologies contain numerous methods for obtaining alloys with high solid solubility. For example, Chinese patent application No. 201710826388.4, published on February 23, 2018, discloses a method for preparing high-solid-solubility copper-tin-titanium alloys using electrical pulse assistance. This method involves repeatedly cold rolling and applying electrical pulses to a Cu-Sn-Ti alloy to obtain a high-solubility Cu-Sn-Ti alloy. Chinese patent application No. 201810527477.3, published on November 9, 2018, discloses a samarium-cobalt magnet and method for improving the solid solubility of Zr. This method involves adjusting the solution temperature to Tst = 1248-4... By applying WFe (%) ±3℃, the precipitation of Zr-rich second phase is suppressed, thereby increasing the solid solubility of Zr. Meanwhile, the high alloy content (mass percentage) of the aforementioned austenitic heat-resistant steel (≥50%) leads to abnormal grain growth during hot rolling, easily resulting in mixed crystal formation. However, neither of these methods can be applied to the preparation of austenitic stainless steel. Firstly, the multiple cold rolling and electro-pulse treatments result in a long process and high energy consumption, unsuitable for large-scale continuous production of stainless steel sheets. Secondly, controlling the solution treatment temperature within an extremely narrow range of ±3℃ poses a significant challenge to the uniformity and control precision of industrial heat treatment furnaces, leading to poor repeatability and stability.

[0008] Chinese patent application No. 202110844795.4, published on November 2, 2021, discloses a method for eliminating mixed grains in S31035 high-alloy austenitic heat-resistant steel. This method involves pinning grain boundaries by inducing the precipitation of the continuous grain boundary phase M23C6 during hot rolling, inhibiting recrystallized grain growth. Following hot rolling, a high-temperature solution treatment at 1200℃~1220℃ completely dissolves the M23C6 precipitates, resulting in complete recrystallization and ultimately eliminating mixed grains. However, this method does not actively control the Nb precipitate.

[0009] In summary, existing technologies cannot simultaneously address the issues of coarse Nb-rich strengthening phases, increased Nb solid solution content, and uneven austenitic grain size in Nb-rich, high-Cr, and high-Ni austenitic heat-resistant steel products. Therefore, an austenitic heat-resistant steel and its preparation method are proposed. Summary of the Invention

[0010] 1. The problem to be solved

[0011] One of the objectives of this invention is to provide an austenitic heat-resistant steel that simultaneously addresses the problems of coarse Nb-rich reinforcing phases, increased Nb solid solution content, and uneven austenitic grain size in Nb-containing austenitic heat-resistant steel products.

[0012] Another objective of this invention is to provide a method for preparing the aforementioned austenitic heat-resistant steel.

[0013] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: The first aspect of this invention provides an austenitic heat-resistant steel, the chemical composition of which, by weight percentage, is: Ni 22-26%, Cr 20-23%, Cu 2.0-5.0%, W 2.0-5.0%, Co 1.0-2.0%, Mn 0.2-0.5%, Nb 0.4-0.8%, Mo 0.4-0.6%, Si 0.2-0.5%, N 0.1-0.3%, C 0.05-0.1%, Al 0.005-0.01%, Ce 0.01-0.1%, with the balance being Fe and unavoidable impurities. The weight ratio of Ce to Nb is 1:(4-80). The steel has an average austenite grain size ≤12 μm, an average Nb-rich phase size of 50-200 nm, and an Nb-rich phase density >100,000 grains / mm². 2 .

[0014] Using the above technical solution, on the one hand, coarse grains are eliminated and homogenized (≤12 μm) by synergistic addition of Ce-Nb; on the other hand, the distribution, quantity, and size of the Nb-rich phase after aging are observed by transmission electron microscopy (TEM) to quantitatively reflect the change in Nb solid solubility. The average size of the Nb-rich phase in the austenitic heat-resistant steel of this invention is 50~200 nm, and the Nb-rich phase number density is >100,000 / mm. 2 This reflects the high solid solubility of Nb. Finally, comparing the room temperature and 700℃ high temperature tensile properties of austenitic heat-resistant steel before and after Ce-Nb synergistic addition, the yield strength, tensile strength and elongation after fracture of the austenitic heat-resistant steel of the present invention are significantly improved.

[0015] As one possible implementation, the number density of Nb-rich phases is 100,000 to 172,000 per mm. 2 .

[0016] As one possible implementation, the austenitic heat-resistant steel has a yield strength ≥475MPa, a tensile strength ≥790MPa, an elongation after fracture ≥45.4% at room temperature, and a tensile strength ≥600MPa at 700℃.

[0017] A second aspect of the present invention provides a method for preparing austenitic heat-resistant steel, comprising the steps of: S1. Smelting: The alloy containing Ni, Cr, Cu, W, Co, Mn, Mo, Si, N, C, Al, and Fe is smelted according to the above weight percentages to obtain molten steel; S2, Pre-alloyed wire feeding: When the molten steel after step S1 is smelted at 1550~1580℃, Ce-Nb pre-alloyed wire is fed into the depth of the molten steel through a wire feeding machine and evenly distributed to obtain austenitic stainless steel ingot. S3, Two-stage hot rolling: The austenitic stainless steel ingot obtained in step S2 is heated to 1200~1300℃ and held for 6~12h. During the holding process, Ce and Nb further synergistically interact, optimizing the distribution of precipitated phases. Then, two-stage hot rolling is performed to obtain hot-rolled steel. S4. Ultra-rapid cooling: The hot-rolled steel in step S3 is subjected to ultra-rapid cooling to suppress the secondary aggregation of Nb-rich phase. The cooling rate is ≥30℃ / s, and the steel is cooled to room temperature. S5. Multi-stage aging treatment: The hot-rolled steel that underwent ultra-rapid cooling in step S4 is subjected to solution treatment, followed by multi-stage aging treatment. The first stage of aging treatment is held at 700~800℃ for 1.5~3h to induce the initial precipitation of Nb. The second stage of aging treatment is held at 600~680℃ for 9~11h to promote the dispersion precipitation of nano-sized Nb-rich phase. Finally, it is air-cooled to room temperature to obtain austenitic heat-resistant steel. The synergistic effect of Ce and Nb in the staged aging treatment inhibits the coarsening of the Nb-rich phase and ensures that the size of the precipitated Nb-rich phase is stable at 50~200nm.

[0018] When the above technical solution is adopted, On the one hand, by precisely controlling the mixing ratio of Ce and Nb and the pre-alloyed wire feeding process, a "Ce-Nb synergistic mechanism" is constructed: The addition of rare earth element Ce can increase the nucleation temperature of the Nb-rich phase, allowing it to nucleate in the liquid phase region, significantly increasing the nucleation rate and promoting the dispersion and precipitation of the Nb-rich phase during solidification. Simultaneously, Ce and Nb form a thermodynamically stable composite system, inhibiting interdendritic segregation of Nb during solidification and suppressing its coarsening and aggregation. Ce-containing inclusions formed during solidification can act as heterogeneous nucleation sites for the Nb-rich phase, promoting its precipitation. This results in some Nb-rich phase adhering to the rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure (Nb-rich phase as the shell, Ce inclusion as the core). Simultaneously, the rare earth inclusions formed by adding an appropriate amount of Ce can serve as heterogeneous nucleation sites, promoting austenite nucleation, reducing system undercooling, limiting primary dendrites, promoting secondary dendrite growth, and refining the solidification structure. After the dendritic structure is refined, the proportion of grain boundaries increases significantly, and the distribution of Nb-rich precipitates between dendrites becomes more uniform and refined, breaking the continuous growth and aggregation of Nb-rich phases between dendrites. In the slab with Ce added but without Ce-Nb synergistic addition, the Nb-rich phase is mainly distributed in the core of the slab, exhibiting a coarse and continuous distribution; after adding an appropriate amount of Ce-Nb synergistically, the Nb-rich phase is more uniformly distributed in the Fe-22Cr-25Ni type heat-resistant steel slab, and its size decreases significantly. This improves the slab structure and compositional uniformity, thereby enhancing the slab quality.

[0019] Microstructural analysis of the austenitic stainless steel ingot after pre-alloying wire feeding in step S2 revealed that the equiaxed crystal zone of the billet expanded, the average size of columnar and equiaxed crystals decreased, the size of Nb-rich phase grains decreased and their distribution became more uniform, and the continuous distribution between dendrites changed to a discontinuous distribution, eliminating the phenomenon of enrichment and coarseness of Nb-rich phase in the billet; the Ce-containing inclusions formed during solidification can act as heterogeneous nucleation sites for Nb-rich phases, promoting the precipitation of Nb-rich phases, so that some Nb-rich phases adhere to rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure precipitation.

[0020] On the other hand, the adoption of the "two-stage hot rolling + ultra-rapid cooling + multi-stage aging treatment" process reduces the number of inclusions and eliminates the core-shell structure of Ce inclusions @ Nb-rich phases, encapsulating the Nb-rich phase in a dissolved matrix, thereby increasing the solid solution content of Nb in austenitic stainless steel. The Nb-rich phase and rare-earth Ce inclusions precipitate and distribute relatively uniformly and dispersedly in the billet. During the billet opening hot rolling process, the fine and uniformly distributed inclusions pin the grain boundaries, inhibiting grain recrystallization and abnormal growth, thus achieving austenitic grain homogenization. During the billet opening hot rolling process, larger Nb-rich phases fragment, further reducing their size and resulting in a more uniform distribution.

[0021] As one possible implementation, in step S1, a vacuum induction melting furnace is used for melting. After the raw materials are melted, the alloying elements are evenly dispersed by electromagnetic stirring for 10 to 15 minutes to ensure the uniformity of the matrix composition.

[0022] As one possible implementation, in step S2, the Ce-Nb pre-alloyed wire is a metallurgical material, referring to a composite wire made by pre-melting Ce and Nb into an alloy and processing it into wire, and then wrapping it with an iron strip. It belongs to the category of alloy cored wire, wherein the Fe content is 50~65 wt%, and the overall mass of the Ce-Nb pre-alloyed wire accounts for a very small proportion of the molten steel composition.

[0023] As one possible implementation, in step S2, the wire feeding speed of the wire feeder is controlled to be 2~4 m / min.

[0024] As one possible implementation, in step S2, while the Ce-Nb pre-alloyed wire is being fed into the depth of the molten steel, enhanced electromagnetic stirring and bottom-blowing argon gas are activated for 5 to 10 minutes to ensure that Ce and Nb elements are evenly distributed in the molten steel and form a synergistic system.

[0025] As one possible implementation, in step S3, the two-stage hot rolling includes: First stage rolling: large deformation rolling is carried out at temperatures above 1100℃, with a total deformation of ≥50%, and the initial coarse Nb-rich phase is broken up. The second stage of rolling: fine rolling is carried out at 900~950℃, with a total deformation of 30~50%, to refine the austenite grains.

[0026] Using the above technical solution, the microstructure of the austenitic stainless steel after the double-stage hot rolling and ultra-rapid cooling in step S4 is analyzed: On the one hand, the austenite grains are significantly refined, mainly due to the relatively uniform distribution of rare earth Ce inclusions, Nb-rich phases and large atomic radii of Ce in the billet, which effectively pin the grain boundaries, thereby hindering dynamic recrystallization and abnormal grain growth, resulting in refined austenite grains with uniform size distribution and eliminating mixed crystal phenomena.

[0027] On the other hand, compared with the as-cast microstructure, the larger Nb-rich phase particles are broken up after hot rolling, the average size is further reduced and the distribution is more uniform, resulting in finer and more dispersed Nb-rich phase precipitation. Compared with the Ce inclusions@Nb-rich phase core-shell structure in the as-cast microstructure, the inclusions after hot rolling are still Ce-containing inclusions, but the edges are no longer wrapped with Nb-rich phase. The Nb content is significantly reduced compared with the Ce inclusions@Nb-rich phase core-shell structure in the as-cast microstructure, and the Ce inclusions@Nb-rich phase core-shell structure disappears.

[0028] After hot rolling, the average austenite grain size of the Ce-Nb synergistic composite microalloyed billet is ≤12μm. This effect is attributed to the synergistic pinning effect of fine inclusions induced by Ce-Nb, Nb-rich precipitates, and solid-solution Ce atoms on grain boundaries, which hinders dynamic recrystallization and abnormal grain growth. At the same time, larger Nb-rich phase particles are broken during hot rolling, and the average size is further reduced to below 2μm, exhibiting a uniform and dispersed distribution. The core-shell structure of Ce inclusions@Nb-rich phases completely disappears, the inclusions are still Ce-containing, and there is no Nb-rich phase covering the edges, and the Nb element is fully dissolved back into the matrix.

[0029] As one possible implementation, in step S5, the solution treatment is: heating the ultra-rapidly cooled hot-rolled steel to 1150~1250℃ and holding it at that temperature for 1~2 hours.

[0030] 3. Beneficial effects Compared with the prior art, the beneficial effects of the present invention are as follows: (1) In the austenitic heat-resistant steel of the present invention, the Ce-Nb synergistic ratio is precisely controllable, forming a stable synergistic system, which significantly inhibits the segregation of Nb elements (the precipitation of Nb-rich phase during solidification). The size of the primary Nb-rich phase is reduced to below 5μm, and it is evenly distributed and discontinuous, eliminating the phenomenon of coarse aggregation of Nb-rich phase between dendrites. At the same time, the solidification structure is significantly refined, the proportion of equiaxed crystal zone in the billet is increased to more than 60%, and the average size of columnar crystals and equiaxed crystals is reduced by more than 30%, resulting in a significant improvement in the quality of the billet.

[0031] The resulting austenitic heat-resistant steel material exhibits comprehensive improvements in mechanical properties, with a room temperature yield strength ≥475MPa, tensile strength ≥790MPa, elongation after fracture ≥45.4%, and a high-temperature tensile strength ≥600MPa at 700℃, meeting the high-temperature service requirements of advanced coal-fired power generating units.

[0032] (2) In the preparation method of the austenitic heat-resistant steel of the present invention, on the one hand, the synergistic effect of double-stage hot rolling and ultra-fast cooling is adopted to make the austenitic grains after hot rolling highly homogeneous, with an average size ≤12μm and no mixed crystal phenomenon. The Nb-rich phase is further broken down to below 2μm and exhibits a diffuse distribution. The average size of the Nb-rich phase decreases, and the number density also decreases, which greatly increases the solid solubility of Nb element and increases the solid solubility by more than 2 times. On the other hand, the multi-stage aging process combined with the Ce-Nb synergistic inhibition effect is adopted to form a large number of 50~200nm nanoscale (aging precipitation of secondary Nb-rich phase) secondary Nb-rich precipitates after aging. Their average size decreases significantly and the number density increases significantly, indicating that the solid solubility of Nb element is improved, which significantly improves the strengthening effect of the material and avoids the problem of coarsening of precipitates in traditional aging. Attached Figure Description

[0033] Figure 1 This is a SEM image of the austenitic heat-resistant steel in Comparative Example 1 of this invention, showing a coarse and continuous Nb-rich phase distribution between dendrites.

[0034] Figure 2 This is an EDS point scan analysis image of the coarse Nb-rich interdendritic phase in the austenitic heat-resistant steel of Comparative Example 1 of this invention.

[0035] Figure 3 This is a SEM image showing the discontinuous distribution of Nb-rich phase in the austenitic heat-resistant steel billet in Example 1 of the present invention.

[0036] Figure 4 This is an EDS point scan analysis diagram of the discontinuously distributed Nb-rich phase in the austenitic heat-resistant steel billet in Example 1 of the present invention.

[0037] Figure 5 This is a SEM image of the Ce inclusions @ Nb-rich phase core-shell structure in the austenitic heat-resistant steel of Embodiment 1 of the present invention.

[0038] Figure 6 This is an EDS analysis diagram of the Ce inclusions @ Nb-rich phase core-shell structure in the austenitic heat-resistant steel of Example 1 of the present invention.

[0039] Figure 7 This is a SEM image of the mixed crystals that appeared in Comparative Example 1 of the present invention after conventional hot rolling.

[0040] Figure 8 This is a SEM image of the coarse Nb-rich phase that appeared in Comparative Example 1 of the present invention after conventional hot rolling.

[0041] Figure 9 This is a SEM image showing the refined and uniformly distributed austenitic grains of the austenitic heat-resistant steel after two-stage hot rolling and ultra-rapid cooling in Example 1 of the present invention.

[0042] Figure 10This is a SEM image of the refined distribution of Nb-rich phase after two-stage hot rolling and ultra-rapid cooling of austenitic heat-resistant steel in Example 1 of the present invention.

[0043] Figure 11 This is a SEM image of the Ce inclusions@Nb-rich phase core-shell structure in the austenitic heat-resistant steel of Embodiment 1 of the present invention after double-stage hot rolling and ultra-rapid cooling, showing the re-dissolution of the Nb-rich phase.

[0044] Figure 12 This is an energy-dissipating surface scan (EDS) image of the Nb-rich phase re-dissolution of Ce inclusions@Nb-rich phase core-shell structure in austenitic heat-resistant steel in Example 1 of the present invention after double-stage hot rolling and ultra-rapid cooling.

[0045] Figure 13 This is a TEM image of the initial coarse Nb-rich phase particles in the austenitic heat-resistant steel sample after conventional aging in Comparative Example 1 of this invention.

[0046] Figure 14 This is a TEM image of a large number of nanoscale Nb-rich phases precipitated in the multi-stage aging sample of austenitic heat-resistant steel in Example 1 of the present invention. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0048] Table 1 shows the chemical composition of Examples 1, 2, and 3 of the present invention. In Table 1, Comparative Example 1 is the composition of austenitic heat-resistant steel without the addition of Ce, and the compositions of Examples 1, 2, and 3 are based on the composition of Comparative Example 1 with the addition of different proportions of Ce-Nb.

[0049] Table 1. Chemical composition (wt%, balance Fe) of various embodiments and comparative examples of the present invention.

[0050] Example 1 This embodiment describes a method for preparing austenitic heat-resistant steel, comprising the following steps: S1, Smelting: The alloy of Ni, Cr, Cu, W, Co, Mn, Mo, Si, N, C, Al, and Fe was prepared according to the mass percentage of alloying elements in Table 1. It was then smelted in a vacuum induction melting furnace. Note that after the raw materials were melted, the alloying elements were evenly dispersed by electromagnetic stirring for 15 minutes to ensure the uniformity of the matrix composition and obtain molten steel.

[0051] S2, Pre-alloyed wire feeding: When the molten steel after step S1 is controlled at 1550℃, Ce-Nb pre-alloyed wire is fed into the depth of the molten steel through a wire feeder. At the same time, the enhanced electromagnetic stirring and bottom blowing argon are turned on for 5 minutes to ensure that Ce and Nb elements are evenly distributed in the molten steel and form a synergistic system to obtain austenitic stainless steel ingot. The wire feeding speed of the wire feeder is controlled at 3 m / min, and the Fe content in the Ce-Nb pre-alloyed wire is 50~65wt%.

[0052] S3. Two-stage hot rolling: The austenitic stainless steel ingot obtained in step S2 is heated to 1250℃ and held for 6~12h. During the holding process, Ce and Nb further synergistically interact, optimizing the distribution of precipitated phases. Subsequently, two-stage hot rolling is performed to obtain hot-rolled steel. First stage rolling: large deformation rolling at 1200℃, with a total deformation of 70%, breaking the initial coarse Nb-rich phase; The second stage of rolling: fine rolling is carried out at 950℃ with a total deformation of 40% to refine the austenite grains.

[0053] S4. Ultra-rapid cooling: The hot-rolled steel in step S3 is subjected to ultra-rapid cooling to suppress the secondary aggregation of Nb-rich phase. The cooling rate is 50℃ / s, and the steel is cooled to room temperature.

[0054] S5, Multi-level Time-sensitivity Processing: The hot-rolled steel that has undergone ultra-rapid cooling in step S4 is subjected to solution treatment, which involves heating the ultra-rapidly cooled hot-rolled steel to 1200°C and holding it at that temperature for 1 hour.

[0055] Then, a tiered timeliness process is implemented: The first stage of aging treatment involves holding at 750℃ for 2 hours to induce the initial precipitation of Nb. The second-stage aging treatment involves holding at 650℃ for 10 hours to promote the dispersion and precipitation of nano-sized Nb-rich phases; finally, the steel is air-cooled to room temperature to obtain austenitic heat-resistant steel.

[0056] The synergistic effect of Ce and Nb in the graded aging treatment suppresses the coarsening of the Nb-rich phase and ensures that the size of the precipitated phase is stable in the range of 50 to 200 nm.

[0057] Comparative Example 1 The preparation method of the austenitic heat-resistant steel in Comparative Example 1 is basically the same as that in Example 1, except that Ce was not added to the molten steel.

[0058] (1) In Example 1 and Comparative Example 1, the microstructure of the smelted austenitic heat-resistant steel billets was analyzed respectively: The solidification structure of the austenitic heat-resistant steel in Comparative Example 1 has coarse dendrites, coarse intergranular distribution, and continuous Nb-rich phase, as shown in its SEM image. Figure 1 As shown, the EDS point scan analysis of the coarse Nb-rich phase is as follows: Figure 2 As shown.

[0059] In Example 1, the equiaxed grain region of the austenitic heat-resistant steel billet expanded, the average size of columnar and equiaxed grains decreased, the size of the Nb-rich phase decreased and its distribution became more uniform, changing from a continuous distribution among dendrites to a discontinuous distribution. This eliminated the enrichment and coarseness of the Nb-rich phase in the billet. SEM images are shown below. Figure 3 As shown, the discontinuous distribution of EDS point scan analysis is as follows: Figure 4 As shown; It can be seen that the Ce-containing inclusions formed during the solidification of the Ce-Nb synergistic composite austenitic heat-resistant steel in Example 1 can act as heterogeneous nucleation sites for the Nb-rich phase, promoting the precipitation of the Nb-rich phase. This results in some of the Nb-rich phase adhering to the rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure. EDS surface scanning analysis of the Ce inclusion@Nb-rich phase core-shell structure is shown below. Figure 5 and Figure 6 As shown.

[0060] (2) In Example 1 and Comparative Example 1, the microstructure of the hot-rolled steel was analyzed as follows: In Comparative Example 1, mixed grains appeared after hot rolling, with a few grains reaching the millimeter scale. The average size of the austenite grains was 18.5 μm, as shown in the SEM image. Figure 7 As shown, the Nb-rich phases are large and clustered together, with an average size of 2.8 μm and a number density of 10158 phases / mm. 2 Its distribution and size in the matrix are as follows Figure 8 As shown.

[0061] In Example 1, after the austenitic heat-resistant steel billet was hot-rolled, the austenitic grains were significantly refined, with an average grain size of 11.5 μm, as shown in the SEM image of the microstructure. Figure 9 As shown, this is mainly attributed to the relatively uniform distribution of rare earth Ce inclusions, Nb-rich phases, and the large atomic radius of the solid-solution Ce in the cast billet. These elements pin the grain boundaries and subgrain boundaries, thus hindering dynamic recrystallization and resulting in refined austenite grains with a uniform size distribution. On the other hand, compared to the as-cast microstructure, the size of the Nb-rich phase is further reduced and its distribution is more uniform after hot rolling, leading to finer and more dispersed Nb-rich phase precipitation, as shown in the SEM images. Figure 10 As shown, the average size of the Nb-rich phase is 1.85 μm, and the number density is 8962 phases / mm². 2 Compared with the sample of Comparative Example 1, the average size decreased by 35.7% and the number density decreased by 11.8%, indicating that the solid solubility of Nb in Comparative Example 1 was increased by 2.1 times.

[0062] Although the Ce inclusions@Nb-rich phase core-shell structure precipitated in the as-cast microstructure of Example 1, accounting for 35% of the total Ce inclusions, and the inclusions remained Ce-containing after hot rolling, the edges were no longer encapsulated by the Nb-rich phase. The Nb content was significantly reduced compared to the as-cast Ce inclusions@Nb-rich phase core-shell structure, accounting for only 2% of the total Ce inclusions. The encapsulated Nb-rich phase dissolved back into the matrix, greatly improving the solid solubility of Nb. EDS surface scanning analysis of the Ce inclusions showed... Figure 11 and Figure 12 As shown.

[0063] (3) The microstructure of the stainless steel after multi-stage aging treatment in Example 1 and Comparative Example 1 is as follows: Figure 13 The image shows a TEM image of the initial coarse Nb-rich phase particles in the sample of Comparative Example 1. The nanoscale precipitates are scarce, with an average Nb-rich phase size of 449 nm and a number density of 52,742 particles / mm². 2 .

[0064] Figure 14 The image shows a TEM image of the austenitic heat-resistant steel sample after two-stage hot rolling and ultra-rapid cooling in Example 1. A large number of nano-sized Nb-rich phases can be observed in the image, which further confirms that the solid solubility of Nb in Ce-Nb synergistic composite austenitic heat-resistant steel is significantly increased after two-stage hot rolling and ultra-rapid cooling + solution treatment.

[0065] Performance Evaluation: Comparing the distribution and quantity of Nb-rich phase precipitation in the austenitic heat-resistant steel of Example 1 and Comparative Example 1, it was found that the sample of Example 1 contained more nanoscale secondary Nb-rich phase precipitation, with a size of 112 nm and a number density of 121,259 phases / mm. 2 Compared with the sample of Comparative Example 1, the average size decreased by 75.1% and the number density increased by 56.5%, indicating that the solid solubility of Nb in austenitic heat-resistant steel was significantly improved before and after the addition of Ce-Nb synergistic composite.

[0066] Then, room temperature tensile tests were conducted on the austenitic heat-resistant steels in Example 1 and Comparative Example 1. The room temperature yield strength and tensile strength of the austenitic heat-resistant steel sample in Example 1 were 475 MPa and 790 MPa, respectively, with an elongation after fracture of 45.4%. In contrast, the room temperature yield strength and tensile strength of the sample in Comparative Example 1 were 407 MPa and 722 MPa, respectively, with an elongation after fracture of 40.7%. The strength and plasticity of the austenitic heat-resistant steel were improved after the addition of Ce-Nb synergistic composite.

[0067] Example 2 The preparation method of austenitic heat-resistant steel in Example 2 is basically the same as that in Example 1. The difference is that the composition of the molten steel is different, as shown in Table 1. The weight ratio of Ce to Nb is controlled at 1:10.4.

[0068] Microstructural analysis of the pre-alloyed wire feed billet in Example 2 showed that, compared to Comparative Example 1, the equiaxed grain region of the austenitic heat-resistant steel billet in Example 2 was expanded, the average size of columnar and equiaxed grains decreased, and the size of the Nb-rich phase decreased and its distribution became more uniform, changing from a continuous distribution among dendrites to a discontinuous distribution, thus eliminating the phenomenon of enriched and coarse Nb-rich phase in the billet. Ce-containing inclusions formed during solidification could act as heterogeneous nucleation sites for the Nb-rich phase, promoting its precipitation. This resulted in some Nb-rich phase adhering to rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure, accounting for 43% of the total Ce-containing inclusions.

[0069] In Example 2, the hot-rolled microstructure analysis showed that after hot rolling of the austenitic heat-resistant steel billet, the austenite grains were significantly refined, with an average grain size of 8.8 μm. Furthermore, compared to the as-cast microstructure, the size of the Nb-rich phase was further reduced and its distribution became more uniform after hot rolling, with an average Nb-rich phase size of 1.66 μm and a number density of 7761 phases / mm². 2 Compared with the comparative sample of Comparative Example 1, the average austenite grain size and the average size of the Nb-rich phase both decreased significantly. Compared to the Ce inclusions@Nb-rich phase core-shell structure in the as-cast microstructure, the hot-rolled inclusions were still Ce-containing inclusions, but the edges were no longer wrapped with Nb-rich phase. The Nb content was significantly reduced compared to the as-cast Ce inclusions@Nb-rich phase core-shell structure, accounting for only 1% of the Ce inclusions. The wrapped Nb-rich phase dissolved back into the matrix, greatly increasing the Nb solubility, indicating that the Nb solubility was 2.8 times higher than in Comparative Example 1.

[0070] Performance evaluation of austenitic heat-resistant steel after multi-stage aging treatment in Example 2: Compared with the distribution and quantity of Nb-rich phase precipitation in the austenitic heat-resistant steel of Comparative Example 1, it was found that the sample of Example 2 had more nanoscale secondary Nb-rich phase precipitation, with a size of 95 nm and a number density of 152,173 / mm. 2 Compared with the sample of Comparative Example 1, the average size decreased by 78.9% and the number density increased by 63.3%, indicating that the solid solubility of Nb in the austenitic heat-resistant steel of Example 2 was significantly improved.

[0071] Tensile tests were conducted at room temperature and 700℃. The austenitic heat-resistant steel in Example 2 had a room temperature yield strength of 482 MPa, a tensile strength of 798 MPa, and an elongation after fracture of 46.1%; at 700℃, the tensile strength was 625 MPa and the elongation was 39.0%. The combined effect of the synergistic effect and the strengthening effect of the composite process resulted in the best overall performance.

[0072] Example 3 The preparation method of austenitic heat-resistant steel in Example 3 is basically the same as that in Example 1. The difference is that the composition of the molten steel is different, as shown in Table 1. The weight ratio of Ce to Nb is controlled at 1:6.1.

[0073] Microstructural analysis of the pre-alloyed wire-feeded billet in Example 3 showed that, compared to Comparative Example 1, the equiaxed grain region of the austenitic heat-resistant steel billet in Example 3 was expanded, the average size of columnar and equiaxed grains decreased, and the size of the Nb-rich phase decreased and its distribution became more uniform, changing from a continuous distribution among dendrites to a discontinuous distribution, thus eliminating the phenomenon of enriched and coarse Nb-rich phase in the billet. Ce-containing inclusions formed during solidification could act as heterogeneous nucleation sites for the Nb-rich phase, promoting its precipitation. This resulted in some Nb-rich phase adhering to rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure, accounting for 44% of the total Ce-containing inclusions.

[0074] In Example 3, the hot-rolled microstructure analysis showed that after hot rolling of the austenitic heat-resistant steel billet, the austenite grains were significantly refined, with an average grain size of 9.1 μm. Furthermore, compared to the as-cast microstructure, the size of the Nb-rich phase was further reduced and its distribution became more uniform after hot rolling, with an average Nb-rich phase size of 1.70 μm and a number density of 8062 phases / mm². 2 Compared with the control sample without Ce-Nb synergistic addition, the average austenite grain size and the average size of the Nb-rich phase both decreased significantly. Compared to the Ce inclusions@Nb-rich phase core-shell structure in the as-cast microstructure, the hot-rolled inclusions, while still Ce-containing, no longer had Nb-rich phase encapsulation at the edges. The Nb content was significantly reduced compared to the as-cast Ce inclusions@Nb-rich phase core-shell structure, accounting for only 2.5% of the Ce inclusions. The encapsulated Nb-rich phase dissolved back into the matrix, greatly increasing the Nb solid solubility, indicating that the Nb solid solubility was increased by 3.1 times compared to Control Example 1.

[0075] Performance evaluation of austenitic heat-resistant steel after multi-stage aging treatment in Example 3: Compared with the distribution and quantity of Nb-rich phase precipitation in the austenitic heat-resistant steel of Comparative Example 1, it was found that the sample of Example 3 had more nanoscale secondary Nb-rich phase precipitation, with a size of 84 nm and a number density of 162,459 / mm². 2 Compared with the sample of Comparative Example 1, the average size decreased by 81.3% and the number density increased by 67.5%, indicating that the solid solubility of Nb in the austenitic heat-resistant steel of Example 3 was significantly improved. Tensile tests were conducted at room temperature and 700℃. The austenitic heat-resistant steel in Example 3 had a room temperature yield strength of 488 MPa, a tensile strength of 805 MPa, and an elongation after fracture of 45.8%; at 700℃, the tensile strength was 632 MPa and the elongation was 38.8%, showing a more significant improvement in strength.

[0076] Example 4 The preparation method of austenitic heat-resistant steel in Example 4 is basically the same as that in Example 1. The difference is that the composition of the molten steel is different, as shown in Table 1. The weight ratio of Ce to Nb is controlled at 1:4.

[0077] Microstructure analysis of the pre-alloyed wire feed billet in Example 4 showed that, compared to Comparative Example 1, the equiaxed grain region of the austenitic heat-resistant steel billet in Example 4 was expanded, the average size of columnar and equiaxed grains decreased, and the size of the Nb-rich phase decreased and its distribution became more uniform, changing from a continuous distribution among dendrites to a discontinuous distribution, thus eliminating the phenomenon of enriched and coarse Nb-rich phase in the billet. Ce-containing inclusions formed during solidification could act as heterogeneous nucleation sites for the Nb-rich phase, promoting its precipitation. This resulted in some Nb-rich phase adhering to rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure, accounting for 35% of the total Ce-containing inclusions.

[0078] In Example 4, the hot-rolled microstructure analysis showed that after hot rolling of the austenitic heat-resistant steel billet, the austenite grains were significantly refined, with an average grain size of 11.8 μm. Furthermore, compared to the as-cast microstructure, the size of the Nb-rich phase was further reduced and its distribution became more uniform after hot rolling, with an average Nb-rich phase size of 2.44 μm and a number density of 10079 phases / mm². 2 Compared with the comparative sample of Comparative Example 1, the average austenite grain size and the average size of the Nb-rich phase both decreased significantly. Compared to the Ce inclusions@Nb-rich phase core-shell structure in the as-cast microstructure, the hot-rolled inclusions were still Ce-containing inclusions, but the edges were no longer wrapped with Nb-rich phase. The Nb content was significantly reduced compared to the as-cast Ce inclusions@Nb-rich phase core-shell structure, accounting for only 1.7% of the Ce inclusions. The wrapped Nb-rich phase dissolved back into the matrix, greatly increasing the Nb solid solubility, indicating that the Nb solid solubility was 2.4 times higher than in Comparative Example 1.

[0079] Performance evaluation of austenitic heat-resistant steel after multi-stage aging treatment in Example 4: Compared with the distribution and quantity of Nb-rich phase precipitation in the austenitic heat-resistant steel of Comparative Example 1, it was found that the sample of Example 4 had more nanoscale secondary Nb-rich phase precipitation, with a size of 144 nm and a number density of 101058 phases / mm. 2 Compared with the sample of Comparative Example 1, the average size decreased by 67.9% and the number density increased by 47.8%, indicating that the solid solubility of Nb in the austenitic heat-resistant steel of Example 4 was significantly improved.

[0080] Tensile tests were conducted at room temperature and 700℃. The austenitic heat-resistant steel in Example 4 had a room temperature yield strength of 471 MPa, a tensile strength of 764 MPa, and an elongation after fracture of 45.1%; at 700℃, the tensile strength was 621 MPa and the elongation was 38.2%. The combined effect of the synergistic effect and the strengthening effect of the composite process resulted in the best overall performance.

[0081] Example 5 The preparation method of austenitic heat-resistant steel in Example 5 is basically the same as that in Example 1, except that the composition of the molten steel is different, as shown in Table 1, wherein the weight ratio of Ce to Nb is controlled at 1:80.

[0082] Microstructural analysis of the pre-alloyed wire feed billet in Example 5 showed that, compared to Comparative Example 1, the equiaxed grain region of the austenitic heat-resistant steel billet in Example 5 was expanded, the average size of columnar and equiaxed grains decreased, and the size of the Nb-rich phase decreased and its distribution became more uniform, changing from a continuous distribution among dendrites to a discontinuous distribution, thus eliminating the phenomenon of enriched and coarse Nb-rich phase in the billet. The Ce-containing inclusions formed during solidification could act as heterogeneous nucleation sites for the Nb-rich phase, promoting its precipitation. This resulted in some Nb-rich phase adhering to rare earth inclusions, forming a Ce inclusion@Nb-rich phase core-shell structure, accounting for 59% of the total Ce-containing inclusions.

[0083] In Example 5, the hot-rolled microstructure analysis showed that after hot rolling of the austenitic heat-resistant steel billet, the austenite grains were significantly refined, with an average grain size of 10.9 μm. Furthermore, compared to the as-cast microstructure, the size of the Nb-rich phase was further reduced and its distribution became more uniform after hot rolling, with an average Nb-rich phase size of 1.96 μm and a number density of 9189 phases / mm². 2 Compared with the comparative sample of Comparative Example 1, the average austenite grain size and the average size of the Nb-rich phase both decreased significantly. Compared to the Ce inclusions@Nb-rich phase core-shell structure in the as-cast microstructure, the hot-rolled inclusions were still Ce-containing inclusions, but the edges were no longer wrapped with Nb-rich phase. The Nb content was significantly reduced compared to the as-cast Ce inclusions@Nb-rich phase core-shell structure, accounting for only 3.8% of the Ce inclusions. The wrapped Nb-rich phase dissolved back into the matrix, greatly increasing the Nb solid solubility, indicating that the Nb solid solubility was 2.2 times higher than in Comparative Example 1.

[0084] Performance evaluation of austenitic heat-resistant steel after multi-stage aging treatment in Example 5: Compared with the distribution and quantity of Nb-rich phase precipitation in the austenitic heat-resistant steel of Comparative Example 1, it was found that the sample of Example 5 had more nanoscale secondary Nb-rich phase precipitation, with a size of 165 nm and a number density of 171,431 phases / mm². 2Compared with the sample of Comparative Example 1, the average size decreased by 63.3% and the number density increased by 69.2%, indicating that the solid solubility of Nb in the austenitic heat-resistant steel of Example 2 was significantly improved.

[0085] Tensile tests were conducted at room temperature and 700℃. The austenitic heat-resistant steel in Example 5 had a room temperature yield strength of 482 MPa, a tensile strength of 768 MPa, and an elongation after fracture of 44.9%; at 700℃, the tensile strength was 603 MPa and the elongation was 37.6%. The combined effect of the synergistic effect and the strengthening effect of the composite process resulted in the best overall performance.

[0086] A comparison of Examples 1, 2, and 3 reveals that when the weight ratio of Ce to Nb is controlled within the range of 1:(4~80), the two can form a highly efficient synergistic effect. Combined with the composite process of "two-stage hot rolling + ultra-fast cooling + multi-stage aging", the entire process of "refining the as-cast structure - controlling the grain size of hot rolling - improving the solid solubility of Nb - strengthening through aging precipitation" can be optimized, while simultaneously solving the three core problems of coarse Nb-rich phase, insufficient Nb solid solubility, and uneven grain size.

[0087] In Example 2, when Ce / Nb = approximately 1:10, the synergistic effect and the strengthening effect of the composite process are optimally combined, taking into account the comprehensive needs of microstructure refinement, precipitate control, and performance improvement. The Ce-Nb synergistic composite microalloying method and composite process system of this invention provide a novel technical path for solving the core technical challenges of high-Cr, high-Ni austenitic heat-resistant steel, and has significant theoretical and engineering application value.

[0088] The above detailed embodiments provide a specific description of the analytical methods involved in this invention. It should be noted that the above description is only intended to help those skilled in the art better understand the methods and ideas of this invention, and is not intended to limit the scope of the invention. Without departing from the principles of this invention, those skilled in the art can make appropriate adjustments or modifications to this invention, and such adjustments and modifications should also fall within the protection scope of this invention.

Claims

1. An austenitic heat-resistant steel, characterized in that: Its chemical composition by weight percentage is as follows: Ni 22~26%, Cr 20~23%, Cu 2.0~5.0%, W 2.0~5.0%, Co 1.0~2.0%, Mn 0.2~0.5%, Nb 0.4~0.8%, Mo 0.4~0.6%, Si 0.2~0.5%, N 0.1~0.3%, C 0.05~0.1%, Al 0.005~0.01%, Ce 0.01~0.1%, with the balance being Fe and unavoidable impurities. The weight ratio of Ce to Nb is 1:(4~80). The average size of the austenite grains in the steel is ≤12μm, the average size of the Nb-rich phase is 50~200 nm, and the number density of the Nb-rich phase is >100,000 / mm². 2 .

2. The austenitic heat-resistant steel according to claim 1, characterized in that: The number density of the Nb-rich phase is 100,000 to 172,000 / mm². 2 .

3. An austenitic heat-resistant steel according to claim 1 or 2, characterized in that: Austenitic heat-resistant steel has a yield strength ≥475MPa, tensile strength ≥790MPa, elongation after fracture ≥45.4% at room temperature, and tensile strength ≥600MPa at 700℃.

4. A method for preparing the austenitic heat-resistant steel according to any one of claims 1 to 3, characterized in that: step include: S1. Smelting: Smelting an alloy containing Ni, Cr, Cu, W, Co, Mn, Mo, Si, N, C, Al, and Fe to obtain molten steel; S2, Pre-alloyed wire feeding: When the molten steel after step S1 is smelted at 1550~1580℃, Ce-Nb pre-alloyed wire is fed into the depth of the molten steel through a wire feeding machine and evenly distributed to obtain austenitic stainless steel ingot. S3, Two-stage hot rolling: The austenitic stainless steel ingot obtained in step S2 is heated to 1200~1300℃ and held for 6~12h; then two-stage hot rolling is performed to obtain hot-rolled steel; S4. Ultra-rapid cooling: The hot-rolled steel in step S3 is subjected to ultra-rapid cooling at a rate of ≥30℃ / s until it reaches room temperature. S5. Multi-stage aging treatment: The hot-rolled steel that has undergone ultra-rapid cooling in step S4 is subjected to solution treatment, followed by multi-stage aging treatment to obtain austenitic heat-resistant steel.

5. The method for preparing an austenitic heat-resistant steel according to claim 4, characterized in that: In step S3, the two-stage hot rolling includes: First stage rolling: large deformation rolling is carried out at temperatures above 1100℃, with a total deformation of ≥50%; Second stage rolling: finish rolling at 900~950℃, with a total deformation of 30~50%.

6. The method for preparing an austenitic heat-resistant steel according to claim 4, characterized in that: In step S5, the solution treatment is to heat the ultra-rapidly cooled hot-rolled steel to 1150~1250℃ and hold it for 1~2 hours.

7. The method for preparing an austenitic heat-resistant steel according to claim 4, characterized in that: In step S5, the multi-stage aging treatment includes: the first stage of aging treatment is to keep warm at 700~800℃ for 1.5~3h; the second stage of aging treatment is to keep warm at 600~680℃ for 9~11h; and finally, air cooling to room temperature.

8. A method for preparing an austenitic heat-resistant steel according to any one of claims 4 to 7, characterized in that: In step S1, a vacuum induction melting furnace is used for melting. After the raw materials are melted, the alloying elements are evenly dispersed by electromagnetic stirring for 10 to 15 minutes.

9. A method for preparing an austenitic heat-resistant steel according to claim 8, characterized in that: In step S2, the feeding speed of the wire feeder is controlled to be 2~4 m / min.

10. The method for preparing an austenitic heat-resistant steel according to claim 8, characterized in that: In step S2, while the Ce-Nb pre-alloyed wire is being fed into the depth of the molten steel, enhanced electromagnetic stirring and bottom-blowing argon gas are activated, and the stirring time is 5-10 minutes.

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