Casting powder for austenitic stainless steel

Abstract

The invention relates to the technical field of casting powder, in particular to austenitic stainless steel casting powder which comprises the following raw materials: calcium oxide, aluminum oxide, magnesium oxide, silicon dioxide, calcium fluoride, modified cerium-lanthanum composite oxide, modified nano silicon carbide and a boric oxide-phosphorus pentoxide compound. According to the preparation method, diethylenetriamine pentaacetic acid is used as a structure-directing agent, so that atomic-scale uniform mixing of cerium ions and lanthanum ions is realized; carrying out medium-temperature heat treatment so as to carry out controllable pyrolysis to obtain a precursor with high reaction activity; under a continuous casting high-temperature environment, cerium and lanthanum oxides can be quickly and uniformly melted into the slag and are preferentially combined with active oxygen in the slag to generate a compact and stable composite phase barrier layer such as lanthanum cerate, so that high-content active alloy elements such as Cr and Ni in the 904 steel can be effectively prevented from diffusing to the slag, an interface oxidation reaction is inhibited, the permeation rate of a corrosive medium on an interface is reduced, and the corrosion resistance of the 904 steel is improved. Therefore, the defects of over-thick oxide skin on the surface of the casting blank, scabbing and the like are reduced.

Description

Technical Field

[0001] This invention relates to the field of protective slag technology, and more specifically, to protective slag for austenitic stainless steel. Background Technology

[0002] 904 super austenitic stainless steel, as a high-alloy corrosion-resistant material, has a chromium (Cr) content of 20%-26% and a nickel (Ni) content of 24%-26% in its chemical composition, supplemented by alloying elements such as molybdenum (Mo) and copper (Cu). It possesses excellent resistance to pitting corrosion, crevice corrosion, and strong oxidizing media corrosion, and is widely used in extreme working conditions such as petrochemicals, seawater desalination, nuclear power equipment, and chemical reactors in strong acid environments. Continuous casting is the core link in the large-scale production of 904 super austenitic stainless steel, and the protective slag, as a key auxiliary material in the continuous casting process, directly determines the surface quality and internal structure stability of the billet. Its core functions include isolating air to prevent oxidation of the billet surface, lubricating the inner wall of the crystallizer to reduce the resistance of billet pulling, absorbing inclusions in the molten steel, and regulating the surface temperature field of the billet to inhibit crack formation.

[0003] However, due to the extremely high content of active alloying elements such as Cr and Ni in 904 steel, these elements are prone to undergo interfacial oxidation reactions with oxygen in the protective slag under the high temperature (above 1400℃) environment of continuous casting, generating highly stable oxides. These oxides will continue to accumulate at the protective slag-cast billet interface, which not only changes the initial chemical composition of the protective slag, but also causes irreversible fluctuations in its key process properties such as melting rate, viscosity, and fluidity (i.e., property instability), thereby destroying the lubrication and isolation effect of the protective slag. In view of this, we propose a protective slag for austenitic stainless steel. Summary of the Invention

[0004] The purpose of this invention is to provide a protective slag for austenitic stainless steel, in order to solve the problem mentioned in the background art that active alloying elements easily react with oxygen in the protective slag to form highly stable oxides. These oxides will continue to accumulate at the protective slag-cast billet interface, which not only changes the initial chemical composition of the protective slag, but also causes irreversible fluctuations (i.e., property instability) in its key process properties such as melting rate, viscosity, and fluidity, thereby destroying the lubrication and isolation effect of the protective slag.

[0005] To achieve the above objectives, the present invention provides a protective slag for austenitic stainless steel, comprising the following raw materials: calcium oxide, aluminum oxide, magnesium oxide, silicon dioxide, calcium fluoride, modified cerium-lanthanum composite oxide, modified nano-silicon carbide, and boron oxide-phosphorus pentoxide composite. The modified cerium-lanthanum composite oxide was prepared by complexation co-precipitation with diethylenetriaminepentaacetic acid followed by heat treatment in a medium-temperature inert atmosphere. Modified nano-silicon carbide is prepared by grafting silicon carbide with octaaminopropyl cage-like polysilsesquioxane.

[0006] Preferably, the composition comprises 20-28 parts by weight of calcium oxide, 4-8 parts by weight of aluminum oxide, 5-10 parts by weight of magnesium oxide, 30-38 parts by weight of silicon dioxide, 6-10 parts by weight of calcium fluoride, 3-5 parts by weight of modified cerium-lanthanum composite oxide, 2-4 parts by weight of modified nano silicon carbide, and 8-12 parts by weight of boron oxide-phosphorus pentoxide composite.

[0007] Preferably, the modified cerium-lanthanum composite oxide is prepared by the following method: Cerium nitrate and lanthanum oxide are mixed at a mass ratio of 1:0.5 and dissolved in nitric acid solution to form a cerium nitrate-lanthanum nitrate mixed solution; ammonia water with pH 9-10 is added dropwise to the mixed solution, and the mixture is stirred at 300-400 rpm for 2-3 hours to generate cerium-lanthanum composite oxide. Cerium-lanthanum composite oxide was mixed with diethylenetriaminepentaacetic acid solution at a mass ratio of 1:8-10; the mixture was refluxed at 80-100℃ and stirred at 150-200 rpm for 6-8 hours; after the reaction was completed, the reaction solution was cooled to room temperature and the solid product was separated by filtration; the solid product was washed 3-4 times with deionized water to remove unreacted diethylenetriaminepentaacetic acid and byproducts. The washed solid product was dried at 110-120℃ for 10-12h to obtain a precursor of diethylenetriaminepentaacetic acid-cerium lanthanum composite oxide; subsequently, it was heat-treated at 250-280℃ under nitrogen protection for 2-4h to obtain a modified cerium-lanthanum composite oxide.

[0008] Preferably, the concentration of the nitric acid solution is 0.05-0.1 mol / L.

[0009] Preferably, the diethylenetriaminepentaacetic acid solution is prepared by dissolving diethylenetriaminepentaacetic acid in deionized water to obtain a solution with a mass concentration of 5-10%, and adjusting the pH to 6-7 with sodium hydroxide at a concentration of 0.5-1 mol / L.

[0010] Preferably, the modified nano-silicon carbide is prepared by the following method: Activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane were mixed at a mass ratio of 1:0.1-0.3, dispersed in anhydrous ethanol, and the pH was adjusted to 9-10 with triethylamine solution. The mixture was stirred at 300-400 rpm for 4-6 hours at 60-80℃ to obtain a mixture. The mixture was transferred to a sealed microwave reactor and irradiated at 100-120℃ and 200-400W for 20-30 minutes. After irradiation, it was centrifuged at 8000-10000 rpm for 5-10 minutes and washed with ethanol and deionized water 2-3 times in sequence. It was then vacuum dried at 50-60℃ for 10-12 hours to obtain modified nano-silicon carbide.

[0011] Preferably, the triethylamine solution has a mass concentration of 0.25%-0.5%.

[0012] Preferably, the method for preparing the protective slag for austenitic stainless steel is as follows: S1.1 Weigh the following raw materials in parts by weight: 20-28 parts by weight of calcium oxide, 4-8 parts by weight of aluminum oxide, 5-10 parts by weight of magnesium oxide, 30-38 parts by weight of silicon dioxide, 6-10 parts by weight of calcium fluoride, 3-5 parts by weight of modified cerium-lanthanum composite oxide, 2-4 parts by weight of modified nano silicon carbide, and 8-12 parts by weight of boron oxide-phosphorus pentoxide composite. S1.2 Mix silicon dioxide, calcium oxide, aluminum oxide and magnesium oxide, and melt them in a pre-melting furnace at 1400-1450℃ for 1-2 hours to form a pre-melted matrix glass phase; S1.3. The pre-melted matrix glass phase, modified cerium-lanthanum composite oxide, modified nano silicon carbide, calcium fluoride and boron oxide-phosphorus pentoxide composite are dry-mixed in a mixer at 500-600 rpm for 30-40 min; then deionized water is added at a solid-liquid ratio of 1:0.6, and the mixture is spray-dried into spherical particles to obtain a protective slag for austenitic stainless steel.

[0013] Preferably, in step S1.3, the inlet temperature of the spray dryer is 160-180℃ and the outlet temperature is 70-80℃.

[0014] Preferably, in step S1.3, the particle size of the spherical particles is 50-100 μm.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the protective slag used for this austenitic stainless steel, diethylenetriaminepentaacetic acid (DITA) is used as a structure directing agent to achieve atomic-level uniform mixing of cerium and lanthanum ions. Medium-temperature heat treatment causes controlled pyrolysis, ultimately yielding a precursor with fine grains, excellent dispersibility, and high reactivity. Under the high-temperature environment of continuous casting, cerium and lanthanum oxides can rapidly and uniformly melt into the slag, and due to their high surface energy, they preferentially combine with active oxygen in the slag to form a dense and stable composite phase barrier layer of lanthanum cerate in situ. This barrier layer can effectively prevent the diffusion of high-content active alloying elements such as Cr and Ni from 904 steel into the slag, inhibit interfacial oxidation reactions, reduce the penetration rate of corrosive media at the interface, and thus reduce defects such as excessive oxide scale and scale on the surface of the billet.

[0016] 2. In this protective slag for austenitic stainless steel, modified nano-silicon carbide forms a silicon carbide-silica core-shell structure through surface siloxane coating modification, which significantly reduces the direct reactivity of silicon carbide with molten steel and reduces its interference with the basic properties of the protective slag. At the same time, the SiO2 layer in the core-shell structure can preferentially react with FeO in the molten slag to form a stable silicate glass phase, effectively buffering the impact of FeO content fluctuations on key process parameters such as viscosity and fluidity of the protective slag, improving the compositional stability and performance consistency of the protective slag during continuous casting, and reducing problems such as lubrication failure and decreased inclusion absorption capacity caused by unstable protective slag properties. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0018] This invention provides a protective slag for austenitic stainless steel, comprising the following raw materials: calcium oxide, aluminum oxide, magnesium oxide, silicon dioxide, calcium fluoride, modified cerium-lanthanum composite oxide, modified nano-silicon carbide, and boron oxide-phosphorus pentoxide composite. The modified cerium-lanthanum composite oxide was prepared by complexation co-precipitation with diethylenetriaminepentaacetic acid followed by heat treatment in a medium-temperature inert atmosphere. Modified nano-silicon carbide is prepared by grafting silicon carbide with octaaminopropyl cage-like polysilsesquioxane.

[0019] Activated silicon carbide: Silicon carbide with a particle size of 20-30nm was immersed in a mixture of concentrated sulfuric acid (mass concentration ≥98%) and hydrogen peroxide (volume ratio 3:1) and ultrasonically treated at 40℃ for 2h; hydrogen peroxide was added dropwise (completely added within 1h) to avoid excessive local concentration that would accelerate decomposition.

[0020] The octaaminopropyl cage-like polysilsesquioxane was purchased from Xi'an Qiyue Biotechnology Co., Ltd.

[0021] The preparation steps of the boron oxide-phosphorus pentoxide complex are as follows: Weigh the appropriate mass of boric acid and ammonium dihydrogen phosphate according to the B2O3 / P2O5 molar ratio of 1:1, place them in a high-temperature resistant crucible or pressure reactor with a sealed lid, and keep them at 300-400℃ for 1-2 hours; then raise the temperature to 900-1100℃ and keep them at 1.5-3 hours under sealed / pressure conditions; during this period, intermittent stirring is required to ensure that the melt is uniform and reacts fully to form a homogeneous B2O3-P2O5 eutectic complex; quickly pour the eutectic complex into deionized water for water quenching, then dry it at 100-120℃, pulverize it, and sieve it to obtain the boron oxide-phosphorus pentoxide complex.

[0022] Example 1: A protective slag for austenitic stainless steel was prepared through the following steps: S1.1 Weigh the following raw materials by weight: 20 parts calcium oxide, 4 parts aluminum oxide, 5 parts magnesium oxide, 30 parts silicon dioxide, 6 parts calcium fluoride, 3 parts modified cerium-lanthanum composite oxide, 2 parts modified nano silicon carbide, and 8 parts boron oxide-phosphorus pentoxide composite. S1.2 Mix silicon dioxide, calcium oxide, aluminum oxide and magnesium oxide, and melt them in a pre-melting furnace at 1400℃ for 2 hours to form a pre-melted matrix glass phase; S1.3. The pre-melted matrix glass phase, modified cerium-lanthanum composite oxide, modified nano silicon carbide, calcium fluoride and boron oxide-phosphorus pentoxide composite are dry-mixed in a mixer at 500 rpm for 30 min; then deionized water is added at a solid-liquid ratio of 1:0.6, and the mixture is passed through a spray dryer with an inlet temperature of 170℃ and an outlet temperature of 80℃ to produce spherical particles with a particle size of 80 μm, thus obtaining a protective slag for austenitic stainless steel.

[0023] The preparation method of the modified cerium-lanthanum composite oxide is as follows: Cerium nitrate and lanthanum oxide were mixed at a mass ratio of 1:0.5 and dissolved in a 0.05 mol / L nitric acid solution to form a cerium nitrate-lanthanum nitrate mixed solution. Ammonia water with pH 9 was added dropwise to the mixed solution, and the mixture was stirred at 400 rpm for 3 hours to generate cerium-lanthanum composite oxide. Diethylenetriaminepentaacetic acid was dissolved in deionized water to obtain a 5% (w / w) solution, and the pH was adjusted to 7 with 0.5 mol / L sodium hydroxide. The cerium-lanthanum composite oxide was mixed with a diethylenetriaminepentaacetic acid solution at a mass ratio of 1:8; the mixture was refluxed at 100°C and stirred at 200 rpm for 8 h; after the reaction was completed, the reaction solution was cooled to room temperature and the solid product was separated by filtration; the solid product was washed 4 times with deionized water to remove unreacted diethylenetriaminepentaacetic acid and byproducts. The washed solid product was dried at 120 °C for 12 h to obtain a precursor of diethylenetriaminepentaacetic acid-cerium lanthanum composite oxide; subsequently, it was heat-treated at 250 °C under nitrogen protection for 2 h to obtain a modified cerium-lanthanum composite oxide.

[0024] The preparation method of modified nano-silicon carbide is as follows: Activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane were mixed at a mass ratio of 1:0.1, dispersed in anhydrous ethanol, and the pH was adjusted to 9 with a 0.25% triethylamine solution. The mixture was stirred at 400 rpm for 6 hours at 60°C to obtain the mixture. The mixture was transferred to a sealed microwave reactor and irradiated at 100℃ and 200W for 30 min. After irradiation, it was centrifuged at 10000 rpm for 10 min and washed three times with ethanol and deionized water. The modified nano-silicon carbide was obtained by vacuum drying at 60℃ for 12 h.

[0025] Example 2: A protective slag for austenitic stainless steel was prepared through the following steps: S1.1 Weigh the following raw materials by weight: 28 parts calcium oxide, 8 parts aluminum oxide, 10 parts magnesium oxide, 38 parts silicon dioxide, 10 parts calcium fluoride, 5 parts modified cerium-lanthanum composite oxide, 4 parts modified nano silicon carbide, and 12 parts boron oxide-phosphorus pentoxide composite. S1.2 Mix silicon dioxide, calcium oxide, aluminum oxide and magnesium oxide, and melt them in a pre-melting furnace at 1400℃ for 2 hours to form a pre-melted matrix glass phase; S1.3. The pre-melted matrix glass phase, modified cerium-lanthanum composite oxide, modified nano silicon carbide, calcium fluoride and boron oxide-phosphorus pentoxide composite are dry-mixed in a mixer at 500 rpm for 30 min; then deionized water is added at a solid-liquid ratio of 1:0.6, and the mixture is passed through a spray dryer with an inlet temperature of 170℃ and an outlet temperature of 80℃ to produce spherical particles with a particle size of 80 μm, thus obtaining a protective slag for austenitic stainless steel.

[0026] The preparation method of the modified cerium-lanthanum composite oxide is as follows: Cerium nitrate and lanthanum oxide were mixed at a mass ratio of 1:0.5 and dissolved in a 0.05 mol / L nitric acid solution to form a cerium nitrate-lanthanum nitrate mixed solution. Ammonia water with pH 9 was added dropwise to the mixed solution, and the mixture was stirred at 400 rpm for 3 hours to generate cerium-lanthanum composite oxide. Diethylenetriaminepentaacetic acid was dissolved in deionized water to obtain a 5% (w / w) solution, and the pH was adjusted to 7 with 0.5 mol / L sodium hydroxide. The cerium-lanthanum composite oxide was mixed with a diethylenetriaminepentaacetic acid solution at a mass ratio of 1:10; the mixture was refluxed at 100°C and stirred at 200 rpm for 8 h; after the reaction was completed, the reaction solution was cooled to room temperature and the solid product was separated by filtration; the solid product was washed 4 times with deionized water to remove unreacted diethylenetriaminepentaacetic acid and byproducts. The washed solid product was dried at 120 °C for 12 h to obtain a precursor of diethylenetriaminepentaacetic acid-cerium lanthanum composite oxide; subsequently, it was heat-treated at 280 °C under nitrogen protection for 4 h to obtain a modified cerium-lanthanum composite oxide.

[0027] The preparation method of modified nano-silicon carbide is as follows: Activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane were mixed at a mass ratio of 1:0.3, dispersed in anhydrous ethanol, and the pH was adjusted to 9 with a 0.25% triethylamine solution. The mixture was stirred at 400 rpm for 6 hours at 60°C to obtain the mixture. The mixture was transferred to a sealed microwave reactor and irradiated at 100℃ and 200W for 30 min. After irradiation, it was centrifuged at 10000 rpm for 10 min and washed three times with ethanol and deionized water. The modified nano-silicon carbide was obtained by vacuum drying at 60℃ for 12 h.

[0028] Example 3: A protective slag for austenitic stainless steel was prepared through the following steps: S1.1 Weigh the following raw materials by weight: 24 parts calcium oxide, 6 parts aluminum oxide, 7 parts magnesium oxide, 34 parts silicon dioxide, 8 parts calcium fluoride, 4 parts modified cerium-lanthanum composite oxide, 3 parts modified nano silicon carbide, and 10 parts boron oxide-phosphorus pentoxide composite. S1.2 Mix silicon dioxide, calcium oxide, aluminum oxide and magnesium oxide, and melt them in a pre-melting furnace at 1400℃ for 2 hours to form a pre-melted matrix glass phase; S1.3. The pre-melted matrix glass phase, modified cerium-lanthanum composite oxide, modified nano silicon carbide, calcium fluoride and boron oxide-phosphorus pentoxide composite are dry-mixed in a mixer at 500 rpm for 30 min; then deionized water is added at a solid-liquid ratio of 1:0.6, and the mixture is passed through a spray dryer with an inlet temperature of 170℃ and an outlet temperature of 80℃ to produce spherical particles with a particle size of 80 μm, thus obtaining a protective slag for austenitic stainless steel.

[0029] The preparation method of the modified cerium-lanthanum composite oxide is as follows: Cerium nitrate and lanthanum oxide were mixed at a mass ratio of 1:0.5 and dissolved in a 0.05 mol / L nitric acid solution to form a cerium nitrate-lanthanum nitrate mixed solution. Ammonia water with pH 9 was added dropwise to the mixed solution, and the mixture was stirred at 400 rpm for 3 hours to generate cerium-lanthanum composite oxide. Diethylenetriaminepentaacetic acid was dissolved in deionized water to obtain a 5% (w / w) solution, and the pH was adjusted to 7 with 0.5 mol / L sodium hydroxide. The cerium-lanthanum composite oxide was mixed with a diethylenetriaminepentaacetic acid solution at a mass ratio of 1:9; the mixture was refluxed at 100°C and stirred at 200 rpm for 8 h; after the reaction was completed, the reaction solution was cooled to room temperature and the solid product was separated by filtration; the solid product was washed 4 times with deionized water to remove unreacted diethylenetriaminepentaacetic acid and byproducts. The washed solid product was dried at 120 °C for 12 h to obtain a precursor of diethylenetriaminepentaacetic acid-cerium lanthanum composite oxide; subsequently, it was heat-treated at 260 °C under nitrogen protection for 3 h to obtain a modified cerium-lanthanum composite oxide.

[0030] The preparation method of modified nano-silicon carbide is as follows: Activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane were mixed at a mass ratio of 1:0.2, dispersed in anhydrous ethanol, and the pH was adjusted to 9 with a 0.25% triethylamine solution. The mixture was stirred at 400 rpm for 6 hours at 60°C to obtain the mixture. The mixture was transferred to a sealed microwave reactor and irradiated at 100℃ and 200W for 30 min. After irradiation, it was centrifuged at 10000 rpm for 10 min and washed three times with ethanol and deionized water. The modified nano-silicon carbide was obtained by vacuum drying at 60℃ for 12 h.

[0031] Example 4: The difference between this example and Example 3 is that 3 parts by weight of modified cerium-lanthanum composite oxide are used.

[0032] Example 5: The difference between this example and Example 3 is that 5 parts by weight of modified cerium-lanthanum composite oxide are used.

[0033] Example 6: The difference between this example and Example 3 is that 2 parts by weight of modified nano-silicon carbide are used.

[0034] Example 7: The difference between this example and Example 3 is that 4 parts by weight of modified nano-silicon carbide are used.

[0035] Example 8: The difference between this example and Example 3 is that the cerium-lanthanum composite oxide and the diethylenetriaminepentaacetic acid solution are mixed at a mass ratio of 1:8.

[0036] Example 9: The difference between this example and Example 3 is that the cerium-lanthanum composite oxide and the diethylenetriaminepentaacetic acid solution are mixed at a mass ratio of 1:10.

[0037] Example 10: The difference between this example and Example 3 is that the activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane are mixed at a mass ratio of 1:0.1.

[0038] Example 11: The difference between this example and Example 3 is that the activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane are mixed at a mass ratio of 1:0.3.

[0039] This invention relates to a protective slag for austenitic stainless steel prepared by adding modified cerium-lanthanum composite oxide and modified nano-silicon carbide during the preparation of 904 super austenitic stainless steel. The performance indicators and testing standards for the 904 super austenitic stainless steel obtained using this protective slag are as follows: A section of 904 super austenitic stainless steel (20×10×3mm) prepared by protective slag from austenitic stainless steel was selected as the sample. The sample was ultrasonically cleaned with acetone for 10 minutes, rinsed with ethanol, and dried. The sample was vertically suspended in a high-pressure reactor lined with polytetrafluoroethylene (PTFE). Nitric acid solution (mass concentration ≥68%) was added, with a solution volume to sample surface area ratio ≥20 mL / cm². The reactor was sealed and heated to 121℃ (pressure 0.1 MPa), maintaining this temperature for 48 hours as one cycle. After each cycle, the nitric acid lost through volatilization was replenished through the reactor's replenishment port (to the initial volume based on the decrease in solution volume). The liner was replaced before the next cycle, for a total of 5 cycles. After each cycle, a new solution was used, and the sample mass loss was recorded. The sample was removed, washed sequentially with deionized water and ethanol, dried, and weighed. The corrosion rate was calculated using the formula. The low corrosion rate indicates that the protective slag effectively inhibits the reaction between the stainless steel surface and the corrosive medium, forming a dense physical or chemical barrier.

[0040] The fluidity of molten slag is one of the important indicators for evaluating the performance of protective slag, especially in the continuous casting process. Good fluidity can ensure that the protective slag uniformly covers the molten steel surface, reduce secondary oxidation of the molten steel, improve the stability of the continuous casting process and the quality of the final product. The performance indicators and test standards for protective slag used in austenitic stainless steel are as follows: The protective slag sample was placed in a platinum crucible and placed in a high-temperature furnace, heated to 1400-1450℃ at a heating rate of 10℃ / min; argon gas (flow rate 1-2L / min) was introduced to prevent oxidation, and the temperature was kept constant for 2 hours; the rotor of a rotational viscometer was inserted, and the viscosity of the slag was measured at 1400℃ and a rotation speed of 5 rpm; low-viscosity slag can reduce the frictional resistance between the molten steel and the crystallizer, improve the surface quality of the billet, and at the same time promote uniform heat transfer and reduce hot cracks.

[0041] The protective slag for austenitic stainless steel prepared in Examples 3-11 and the obtained 904 super austenitic stainless steel were tested according to the above standards, and the data are shown in Table 1: Table 1. Performance data of 904 super austenitic material and protective slag prepared in Examples 3-11

[0042] Examples 3 and 4-5 show that: when other components in the protective slag for austenitic stainless steel remain constant, and the weight percentage of modified cerium-lanthanum composite oxide continuously increases, the corrosion rate of 904 super austenitic stainless steel and the slag viscosity of the special protective slag both continuously decrease; in the modified cerium-lanthanum composite oxide, La... 3+ As a network modifier cation, its introduction can provide free oxygen (O2). 2- The modified cerium-lanthanum composite oxide breaks the Si-O-Si or Al-O-Si bonds in the SiO2-Al2O3 glass network of the protective slag matrix, reducing the degree of polymerization of silicon-oxygen anions and thus reducing the viscosity of the high-temperature slag. At the high temperature of continuous casting, the highly dispersed modified cerium-lanthanum composite oxide rapidly melts into the slag and preferentially combines with active oxygen (such as FeO and dissolved oxygen) in the slag, prior to elements such as Cr and Ni in the molten steel. This reduces the oxidizing power of the slag and generates a dense and stable rare earth-rich oxide layer (such as a complex oxide phase containing La and Ce) in situ at the steel / slag interface. This interface layer acts as a kinetic diffusion barrier layer, effectively blocking the diffusion of active elements from the steel into the slag and the transfer of oxygen into the steel, thereby inhibiting the occurrence of interfacial oxidation reactions, which manifests as a reduction in the corrosion rate.

[0043] Furthermore, comparing Examples 3 and 6-7 reveals that: when other components in the protective slag for austenitic stainless steel remain unchanged, and the weight percentage of modified nano-silicon carbide increases from 2 parts (Example 6) to 3 parts (Example 3), and then from 3 parts (Example 3) to 4 parts (Example 7), the corrosion rate of 904 super austenitic stainless steel and the slag viscosity of the special protective slag exhibit a non-linear characteristic of first decreasing and then increasing. Specifically: When the amount was increased to 3 parts by weight (Example 3), the slag viscosity decreased from 0.28 Pa·s to 0.15 Pa·s. This was because the POSS-derived SiO2 core-shell layer on the surface of the modified nano-silicon carbide exhibited high interfacial activity under high-temperature melting conditions. In the early stage of high-temperature melting, these in-situ generated active SiO2, together with boron oxide and phosphorus pentoxide in the system, participated in structural rearrangement in local micro-regions, forming a low-melting-point borophosphosilicate multi-element eutectic system. This eutectic system acted as a fluxing agent in the slag, reducing the liquidus temperature and optimizing the slag's fluidity. However, when the amount was increased to 4 parts by weight (Example 7), the slag viscosity rebounded. This was because the effective volume fraction of insoluble solid particles (SiC particle centers) further increased. According to the principle of suspension rheology, excessive nano-solid particles would increase the probability of collisions between particles and produce local aggregation, forming a microscopic rigid framework network. At this time, the thickening effect and flow hindrance effect caused by solid particles began to dominate, partially offsetting the viscosity reduction effect brought by the eutectic system, thus leading to a moderate rebound in macroscopic viscosity.

[0044] A comparison of Examples 3 and 8-9 shows that as the mass ratio of cerium-lanthanum composite oxide to diethylenetriaminepentaacetic acid solution increases, the corrosion rate of 904 super austenite and the viscosity of the special protective slag exhibit a non-linear trend of first decreasing and then increasing. When the dosage of diethylenetriaminepentaacetic acid (DTPA) increases from 1:8 to 1:9, its role as a complexing and dispersing agent is enhanced, which can more effectively prevent particle agglomeration in subsequent processing, thereby preparing a precursor with better dispersibility and higher activity. This allows the modified rare earth oxides to play a more rapid role in the slag, effectively reducing viscosity and corrosion rate. However, when the dosage of DTPA is too high (e.g., 1:10), the excessive organic matter generates more carbon residue during heat treatment, which hinders the effective contact between rare earth oxides and slag, leading to a decrease in its effect and thus causing a rebound in viscosity and corrosion rate.

[0045] A comparison of Examples 3 and 10-11 shows that as the mass ratio of activated silicon carbide to octaaminopropyl cage-like polysilsesquioxane increases, the corrosion rate of 904 super austenite and the viscosity of the special protective slag exhibit a trend of first decreasing and then increasing. The amino groups of octaaminopropyl cage-like polysilsesquioxane and the hydroxyl groups (-OH) on the surface of activated SiC form a core-shell composite structure through chemical bonding. This structure not only enhances the interfacial bonding force but also improves the stability of the slag. In addition, the cage-like structure of octaaminopropyl cage-like polysilsesquioxane forms a nanoscale dispersed phase in the slag, effectively suppressing the flow shear stress of the slag at high temperatures, thereby reducing the viscosity. At the same time, the high proportion of SiC improves the high-temperature skeleton strength of the protective slag, and the SiO2 passivation layer formed on its surface exhibits higher stability in chlorine / sulfur-containing environments, effectively isolating the corrosive medium from the metal matrix, thereby reducing the corrosion rate of 904 super austenite.

[0046] Based on the above test results, considering the performance stability of industrial production, the controllability of raw material costs, and the overall benefits, Example 3 is selected as the optimal example. Comparative Example 1: The difference between this comparative example and Example 3 is that the modified cerium-lanthanum composite oxide was not used; instead, the cerium-lanthanum composite oxide was used directly.

[0047] Comparative Example 2: The difference between this comparative example and Example 3 is that modified nano-silicon carbide was not used; silicon carbide was used directly.

[0048] Comparative Example 3: This comparative example differs from Example 3 in that the boron oxide-phosphorus pentoxide complex was not added.

[0049] Table 2 Performance data of 904 super austenitic material and protective slag prepared in Example 3 and Comparative Examples 1-3

[0050] A comparison of Example 3 and Comparative Example 1 shows that when cerium-lanthanum composite oxide is used directly without modification, the viscosity of the slag in the protective slag for austenitic stainless steel increases. The unmodified cerium-lanthanum composite oxide has high chemical activity and easily reacts with components such as FeO and SiO2 in the slag to generate high-melting-point compounds (such as cerium silicates or lanthanum iron oxides). These compounds further increase the viscosity of the slag, hindering the uniform spreading of the liquid protective slag on the surface of the molten steel, resulting in incomplete local coverage and weakening the protective function of the oxide film. In addition, the high viscosity of the slag makes it difficult to effectively adsorb inclusions such as Al2O3 and sulfides in the molten steel, causing inclusions to aggregate and be obstructed from floating, ultimately remaining in the billet and forming defects.

[0051] A comparison of Example 3 and Comparative Example 2 shows that: when silicon carbide is used directly without modified nano-silicon carbide, the corrosion rate of 904 super austenite increases; ordinary silicon carbide particles, due to their high surface energy, are prone to agglomeration, resulting in uneven distribution in the protective slag matrix; this uneven distribution makes it difficult for the protective slag to form a continuous and dense anti-corrosion layer, thus increasing the Cl content in the molten slag. - S 2- Corrosive media can more easily penetrate the surface of stainless steel, damaging its surface passivation film. Unmodified silicon carbide particles have a large number of active sites on their surface, which can easily react with components such as FeO and SiO2 in the slag to form low-melting-point eutectic, further weakening the chemical stability of the protective slag. Although the introduction of unmodified SiC has a slight viscosity reduction effect due to its physical filling effect (compared to Comparative Example 1), the chemical instability and corrosion promotion effect it causes result in a much greater negative impact on the overall performance (especially corrosion resistance) than its slight improvement on viscosity.

[0052] A comparison of Example 3 and Comparative Example 3 shows that: without the addition of the boron oxide-phosphorus pentoxide complex, the viscosity of the protective slag for austenitic stainless steel increases; when the protective slag lacks the boron oxide-phosphorus pentoxide complex, the high-viscosity slag cannot uniformly cover the stainless steel surface, resulting in local areas being exposed to Cl-containing substances. - / S 2- In corrosive media, the oxide film is destroyed more quickly; in addition, the uneven heat conduction inside the slag with excessively high viscosity increases the temperature fluctuation of the molten pool, thereby inducing stress corrosion cracking of the stainless steel surface structure.

[0053] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A protective flux for austenitic stainless steel, characterized in that, The raw materials include: calcium oxide, aluminum oxide, magnesium oxide, silicon dioxide, calcium fluoride, modified cerium-lanthanum composite oxide, modified nano-silicon carbide, and boron oxide-phosphorus pentoxide composite. The modified cerium-lanthanum composite oxide was prepared by complexation co-precipitation with diethylenetriaminepentaacetic acid followed by heat treatment in a medium-temperature inert atmosphere. Modified nano-silicon carbide is prepared by grafting silicon carbide with octaaminopropyl cage-like polysilsesquioxane.

2. The protective slag for austenitic stainless steel according to claim 1, characterized in that, The composition includes 20-28 parts by weight of calcium oxide, 4-8 parts by weight of aluminum oxide, 5-10 parts by weight of magnesium oxide, 30-38 parts by weight of silicon dioxide, 6-10 parts by weight of calcium fluoride, 3-5 parts by weight of modified cerium-lanthanum composite oxide, 2-4 parts by weight of modified nano silicon carbide, and 8-12 parts by weight of boron oxide-phosphorus pentoxide composite.

3. The protective slag for austenitic stainless steel according to claim 2, characterized in that, The modified cerium-lanthanum composite oxide is prepared as follows: Cerium nitrate and lanthanum oxide are mixed at a mass ratio of 1:0.5 and dissolved in nitric acid solution to form a cerium nitrate-lanthanum nitrate mixed solution; ammonia water with pH 9-10 is added dropwise to the mixed solution, and the mixture is stirred at 300-400 rpm for 2-3 hours to generate cerium-lanthanum composite oxide. Cerium-lanthanum composite oxide was mixed with diethylenetriaminepentaacetic acid solution at a mass ratio of 1:8-10; the mixture was refluxed at 80-100℃ and stirred at 150-200 rpm for 6-8 hours; after the reaction was completed, the reaction solution was cooled to room temperature and the solid product was separated by filtration; the solid product was washed 3-4 times with deionized water to remove unreacted diethylenetriaminepentaacetic acid and byproducts. The washed solid product was dried at 110-120℃ for 10-12h to obtain a precursor of diethylenetriaminepentaacetic acid-cerium lanthanum composite oxide; subsequently, it was heat-treated at 250-280℃ under nitrogen protection for 2-4h to obtain a modified cerium-lanthanum composite oxide.

4. The protective slag for austenitic stainless steel according to claim 3, characterized in that, The concentration of the nitric acid solution is 0.05-0.1 mol / L.

5. The protective slag for austenitic stainless steel according to claim 3, characterized in that, The diethylenetriaminepentaacetic acid solution is prepared by dissolving diethylenetriaminepentaacetic acid in deionized water to obtain a solution with a mass concentration of 5-10%, and adjusting the pH to 6-7 with sodium hydroxide at a concentration of 0.5-1 mol / L.

6. The protective slag for austenitic stainless steel according to claim 2, characterized in that, The method for preparing the modified nano-silicon carbide is as follows: Activated silicon carbide and octaaminopropyl cage-like polysilsesquioxane were mixed at a mass ratio of 1:0.1-0.3, dispersed in anhydrous ethanol, and the pH was adjusted to 9-10 with triethylamine solution. The mixture was stirred at 300-400 rpm for 4-6 hours at 60-80℃ to obtain a mixture. The mixture was transferred to a sealed microwave reactor and irradiated at 100-120℃ and 200-400W for 20-30 minutes. After irradiation, it was centrifuged at 8000-10000 rpm for 5-10 minutes and washed with ethanol and deionized water 2-3 times in sequence. It was then vacuum dried at 50-60℃ for 10-12 hours to obtain modified nano-silicon carbide.

7. The protective slag for austenitic stainless steel according to claim 6, characterized in that, The mass concentration of the triethylamine solution is 0.25%-0.5%.

8. The protective slag for austenitic stainless steel according to claim 2, characterized in that, The preparation method of the protective slag for austenitic stainless steel is as follows: S1.1 Weigh the following raw materials in parts by weight: 20-28 parts by weight of calcium oxide, 4-8 parts by weight of aluminum oxide, 5-10 parts by weight of magnesium oxide, 30-38 parts by weight of silicon dioxide, 6-10 parts by weight of calcium fluoride, 3-5 parts by weight of modified cerium-lanthanum composite oxide, 2-4 parts by weight of modified nano silicon carbide, and 8-12 parts by weight of boron oxide-phosphorus pentoxide composite. S1.2 Mix silicon dioxide, calcium oxide, aluminum oxide and magnesium oxide, and melt them in a pre-melting furnace at 1400-1450℃ for 1-2 hours to form a pre-melted matrix glass phase; S1.

3. The pre-melted matrix glass phase, modified cerium-lanthanum composite oxide, modified nano silicon carbide, calcium fluoride and boron oxide-phosphorus pentoxide composite are dry-mixed in a mixer at 500-600 rpm for 30-40 min; then deionized water is added at a solid-liquid ratio of 1:0.6, and the mixture is spray-dried into spherical particles to obtain a protective slag for austenitic stainless steel.

9. The protective slag for austenitic stainless steel according to claim 8, characterized in that, In S1.3, the inlet temperature of the spray dryer is 160-180℃ and the outlet temperature is 70-80℃.

10. The protective slag for austenitic stainless steel according to claim 8, characterized in that, In S1.3, the particle size of the spherical particles is 50-100 μm.

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