Special casting powder for high-nitrogen nickel-saving high-toughness stainless steel

Abstract

The invention relates to the technical field of casting powder, in particular to special casting powder for high-nitrogen nickel-saving high-toughness stainless steel, which comprises the following raw materials: calcium oxide, core-shell fluoride nanoparticles, magnesium oxide, barium oxide, potassium carbonate, lithium carbonate and nano dispersed glass microspheres. According to the invention, the fluoride nanoparticles with core-shell structures are adopted, calcium fluoride is taken as a core, and silicon dioxide is taken as a shell, so that excessive volatilization of fluorine components can be inhibited at a high temperature, the thermochemical stability of slag is maintained, and uniform diffusion of nitrogen elements on a molten steel interface is promoted; the silicon dioxide shell layer forms a high-viscosity framework network in the melting process, so that the scouring resistance and the film forming continuity of the slag are improved; besides, the nano dispersed glass microspheres adjust the rheological property of the liquid slag, a multi-scale dispersed phase is introduced into the molten slag, the synergistic effect between viscosity and surface tension is optimized, a slag film can be evenly spread, a covering layer is formed, and therefore dissipation of nitrogen and interface segregation are effectively reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of protective slag, in particular to a special protective slag for high-nitrogen and nickel-reduced high-strength and high-toughness stainless steel. BACKGROUND

[0002] High-nitrogen and nickel-reduced high-strength and high-toughness stainless steel has high strength, high toughness and excellent corrosion resistance, and has been widely used in high-end equipment manufacturing, nuclear power and marine engineering fields; this kind of steel usually adopts nitrogen pressure protection or argon covering vacuum melting process during smelting to inhibit the escape of nitrogen and reduce the amount of nickel; however, in the smelting process of high-nitrogen steel, the high temperature of the molten steel and the high nitrogen partial pressure can easily lead to unstable nitrogen transfer between the molten steel and the slag system and fluctuation of the slag film structure, thereby affecting the cleanliness of the molten steel and the surface quality of the casting blank.

[0003] The commonly used protective slag for high-nitrogen stainless steel is mainly CaO-Al2O3-SiO2-MgO system, in order to improve the liquidity and nitrogen absorption stability, Li2CO3, BaO or F - fluxes are often added to reduce the viscosity and adjust the melting characteristics; however, the composition and structure of these protective slags are relatively simple, and at high temperature, the molten slag is uneven, the slag film is thick and thin or easy to break, which leads to easy volatilization of nitrogen element, ultimately weakens the protection effect of the molten steel surface, affects the stability of inclusion control, reduces the nitrogen content retention rate and damages the uniformity of the structure, therefore, we propose a special protective slag for high-nitrogen and nickel-reduced high-strength and high-toughness stainless steel. SUMMARY

[0004] The present application aims to provide a special protective slag for high-nitrogen and nickel-reduced high-strength and high-toughness stainless steel to solve the problems of the composition and structure of these protective slags being relatively simple, the molten slag being uneven at high temperature, the slag film being thick and thin or easy to break, leading to easy volatilization of nitrogen element, ultimately weakening the protection effect of the molten steel surface, affecting the stability of inclusion control, reducing the nitrogen content retention rate and damaging the uniformity of the structure.

[0005] The present application provides a special protective slag for high-nitrogen and nickel-reduced high-strength and high-toughness stainless steel, which comprises the following raw materials: calcium oxide, core-shell fluoride nanoparticles, magnesium oxide, barium oxide, potassium carbonate, lithium carbonate and nanodispersed glass microspheres; The core-shell fluoride nanoparticles are prepared by fluorination precipitation-sol coating method; The nanodispersed glass microspheres are prepared by sol-gel combined with spray drying and calcination process.

[0006] As preferred, the calcium oxide is 45-55 parts by weight, the core-shell fluoride nanoparticles are 2-6 parts by weight, the magnesium oxide is 2-4 parts by weight, the barium oxide is 2-4 parts by weight, the potassium carbonate is 0.8-1.5 parts by weight, the lithium carbonate is 0.8-1.2 parts by weight, and the nanodispersed glass microspheres are 8-12 parts by weight.

[0007] As preferred, the preparation process of the core-shell fluoride nanoparticles is as follows: The calcium nitrate and ammonium fluoride are mixed and added into deionized water at a mass ratio of 1:10, the molar ratio of calcium ions to fluoride ions is 1:2, and the reaction is stirred at a speed of 300-500 rpm for 30-60 min at 25-40℃, and the pH is adjusted to 7-9 with 0.1 mol / L ammonia solution to form a calcium fluoride nanosuspension; The calcium fluoride nanosuspension is dispersed in an ethanol / water mixed solvent at a volume ratio of 1:3-5, the volume ratio of ethanol to water is 3:1, tetraethoxysilane and 5-10% ammonia water based on the total volume of the solution are added, and the reaction is carried out at a stirring speed of 400-600 rpm for 2-4 h to form a silica coating layer; After the reaction is completed, centrifugation is carried out at a speed of 8000-10000 rpm for 10-15 min, the supernatant is discarded, and the nanosuspension is washed with anhydrous ethanol and deionized water alternately for 2-3 times; then it is dried at 80-100℃ for 6-8 h and calcined at 450-550℃ in air atmosphere for 1-2 h to obtain the core-shell fluoride nanoparticles.

[0008] As preferred, the amount of tetraethoxysilane added is such that the molar ratio of silicon to calcium is 0.1-0.5:1.

[0009] As preferred, the D50 particle size of the core-shell fluoride nanoparticles is 20-100 nm.

[0010] As preferred, the preparation process of the nanodispersed glass microspheres is as follows: The tetraethoxysilane, calcium carbonate, sodium carbonate, and aluminum isopropoxide are mixed and dissolved in an ethanol / water mixed solvent at a solid-liquid ratio of 1:10-15, the volume ratio of ethanol to water is 2:1, the pH is adjusted to 2-4 with 1-3 mol / L nitric acid solution, and the homogeneous sol is obtained by stirring at 25-35℃ and 400-600 rpm for 1-2 h; The sol is ultrasonically dispersed for 10-20 min and then sent into a spray drying tower for drying, the inlet air temperature is 160-180℃, the outlet air temperature is 90-100℃, and the atomization pressure is 0.25-0.35 MPa to obtain a dry powder precursor containing a nanodispersed phase; Then the dry powder precursor is placed in a muffle furnace and calcined at 600-800℃ for 0.5-2.0 h, and cooled to room temperature in the furnace to obtain nanodispersed glass microspheres with a D50 particle size of 10-50 μm.

[0011] Preferably, the tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide are mixed in a molar ratio of silicon, calcium, sodium, aluminum of 1:0.6-0.8:0.2-0.4:0.05-0.10.

[0012] Preferably, the preparation process of the high-nitrogen and low-nickel type high-strength and high-toughness stainless steel special protective slag is as follows: S1.1, the following weight parts of raw materials are weighed: calcium oxide 45-55 parts by weight, core-shell fluoride nanoparticles 2-6 parts by weight, magnesium oxide 2-4 parts by weight, barium oxide 2-4 parts by weight, potassium carbonate 0.8-1.5 parts by weight, lithium carbonate 0.8-1.2 parts by weight and nanodispersed glass microspheres 8-12 parts by weight; S1.2, the calcium oxide, magnesium oxide, barium oxide, potassium carbonate and lithium carbonate are added to a high-speed mixer, stirred at 400-600 rpm for 10-15 min to uniformly mix the components; then the core-shell fluoride nanoparticles and nanodispersed glass microspheres are added, and continue to stir for 10-20 min to obtain a uniform mixture; The mixture is adjusted to a solid content of 60-70%, and the protective slag precursor particles are prepared by spray granulation; S1.3, the protective slag precursor particles are placed in a refractory crucible and pre-melted at 700-900℃ for 1h; then cooled to room temperature at a rate of 3-5℃ / min, crushed and sieved through a 100-200 mesh screen to obtain a high-nitrogen and low-nickel type high-strength and high-toughness stainless steel special protective slag.

[0013] The pre-melt treatment is aimed at removing the crystallization water in the raw materials, decomposing part of the carbonates, and preliminarily solid-phase reacting the components to form a eutectic precursor, laying a foundation for rapid melting in the subsequent smelting process.

[0014] Preferably, in S1.2, the atomization pressure of spray granulation is 0.25-0.35 MPa, the inlet air temperature is 150-180℃, and the outlet air temperature is 80-100℃.

[0015] Preferably, in S1.3, the pre-melt treatment gas is composed of nitrogen and carbon dioxide, wherein the volume ratio of nitrogen to carbon dioxide is 1:2-4; the gas flow is controlled at 100-300 mL / min.

[0016] Compared with the prior art, the beneficial effects of the present application are: This invention discloses a high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel protective slag. It employs core-shell structured fluoride nanoparticles, with calcium fluoride as the core and silica as the shell, constructing a stable fluoride slow-release structure. This structure can suppress excessive volatilization of fluorine components at high temperatures, maintain the thermochemical stability of the slag, and promote uniform diffusion of nitrogen at the steel-molten interface. The silica shell forms a high-viscosity skeletal network during melting, improving the slag's erosion resistance and film continuity. Furthermore, nano-dispersed glass microspheres regulate the rheological properties of the liquid slag. By introducing a multi-scale dispersed phase into the molten slag, the synergistic effect between viscosity and surface tension is optimized, enabling the slag film to spread rapidly and uniformly, forming a continuous, dense, and stable coating layer, thereby effectively reducing nitrogen escape and interfacial segregation. The synergistic effect of these two nanostructure materials gives the protective slag good fluidity, thermal stability, and interfacial reaction balance under high-temperature smelting conditions, significantly improving the surface protection effect and compositional stability of high-nitrogen, nickel-saving stainless steel. 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 high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag, comprising the following raw materials: calcium oxide, core-shell fluoride nanoparticles, magnesium oxide, barium oxide, potassium carbonate, lithium carbonate, and nano-dispersed glass microspheres; The core-shell fluoride nanoparticles were prepared by a fluorination precipitation-sol-gel coating method. The nano-dispersed glass microspheres were prepared by a sol-gel combined spray drying and calcination process.

[0019] Calcium oxide (CAS No.: 1305-78-8, purity 99%) and ammonium fluoride (CAS No.: 12125-01-8) were both purchased from Hubei Xinrunde Chemical Co., Ltd.

[0020] Calcium nitrate (CAS No.: 10124-37-5) was purchased from Aladdin Biochemical Technology Co., Ltd.

[0021] Tetraethoxysilane (CAS No.: 78-10-4, purity 98%) was purchased from Shanghai Myriel Biochemical Technology Co., Ltd.

[0022] Magnesium oxide (CAS No.: 1309-48-4, purity SP, 98%), potassium carbonate (CAS No.: 584-08-7, purity AR, 99%), lithium carbonate (CAS No.: 554-13-2, purity AR, 98%), calcium carbonate (CAS No.: 471-34-1, purity AR, 99%), and sodium carbonate (CAS No.: 497-19-8, purity AR, 99.8%) were all purchased from Shanghai Yuanye Biotechnology Co., Ltd.

[0023] Barium oxide (CAS No.: 1304-28-5, purity AR) was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd.

[0024] Aluminum isopropoxide (CAS No.: 555-31-7, purity ≥99%) was purchased from Yangzhou Zhongtianli New Material Co., Ltd.

[0025] Example 1: A preparation process for a high-nitrogen, nickel-saving, high-strength and high-toughness protective slag for stainless steel, comprising the following steps: S1.1 Weigh the following raw materials in parts by weight: 45 parts calcium oxide, 2 parts core-shell fluoride nanoparticles, 2 parts magnesium oxide, 2 parts barium oxide, 0.8 parts potassium carbonate, 0.8 parts lithium carbonate, and 8 parts nano-dispersed glass microspheres. S1.2 Add calcium oxide, magnesium oxide, barium oxide, potassium carbonate and lithium carbonate to a high-speed mixer and stir at 400 rpm for 15 min to mix the components evenly; then add core-shell fluoride nanoparticles and nano-dispersed glass microspheres, and continue stirring for 20 min to obtain a uniform mixture. The mixture was adjusted to have a solid content of 60%, and then spray-granulated with an atomization pressure of 0.30 MPa, an inlet air temperature of 160°C, and an outlet air temperature of 80°C to obtain protective slag precursor particles. S1.3 Place the precursor particles of the protective slag in a refractory crucible and pre-melt them at 900℃ for 1 hour (the gas consists of nitrogen and carbon dioxide, with a volume ratio of nitrogen to carbon dioxide of 1:2; the gas flow rate is controlled at 200 mL / min); then cool them to room temperature at a rate of 4℃ / min, crush them and pass them through a 100-mesh sieve to obtain a high-nitrogen, nickel-saving, high-strength and tough stainless steel protective slag.

[0026] The preparation process of core-shell fluoride nanoparticles is as follows: Calcium nitrate and ammonium fluoride were mixed and added to deionized water at a mass ratio of 1:10, wherein the molar ratio of calcium ions to fluoride ions was 1:2. The mixture was stirred at 400 rpm for 60 min at 30 °C. The pH was adjusted to 8 with 0.1 mol / L ammonia solution to form a calcium fluoride nano suspension. Calcium fluoride nano-suspension was dispersed in an ethanol / water mixed solvent at a volume ratio of 1:3, wherein the volume ratio of ethanol to water was 3:1. Tetraethoxysilane (the molar ratio of silicon to calcium was 0.3:1) and ammonia water accounting for 8% of the total volume of the solution were added. The mixture was stirred at 400 rpm for 4 hours to form a silica coating layer. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 15 min, the supernatant was discarded, and the mixture was washed three times alternately with anhydrous ethanol and deionized water. Then it was dried at 80 °C for 8 h and calcined in air at 500 °C for 2 h to obtain core-shell fluoride nanoparticles with a D50 particle size of 65 nm.

[0027] The preparation process of the nano-dispersed glass microspheres is as follows: Tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide (molar ratio of silicon, calcium, sodium and aluminum is 1:0.6:0.2:0.05) were mixed and dissolved in an ethanol / water mixture at a solid-liquid ratio of 1:10, wherein the volume ratio of ethanol to water was 2:1. The pH was adjusted to 4 with 2 mol / L nitric acid solution and stirred at 30℃ and 400 rpm for 2 h to obtain a homogeneous sol. The sol was ultrasonically dispersed for 20 minutes and then sent to a spray drying tower for drying. The inlet air temperature was 160℃, the outlet air temperature was 90℃, and the atomization pressure was 0.30MPa, thus obtaining a dry powder precursor containing a nano-dispersed phase. The dry powder precursor was then placed in a muffle furnace and calcined at 650°C for 2.0 h, and then cooled to room temperature in the furnace to obtain nano-dispersed glass microspheres with a D50 particle size of 22 μm.

[0028] Example 2: The difference between this example and Example 1 is that the amount of tetraethoxysilane added makes the molar ratio of silicon to calcium 0.1:1.

[0029] Example 3: The difference between this example and Example 1 is that the amount of tetraethoxysilane added makes the molar ratio of silicon to calcium 0.5:1.

[0030] Example 4: The difference between this example and Example 1 is that ammonia water accounts for 5% of the total volume of the solution.

[0031] Example 5: The difference between this example and Example 1 is that ammonia water accounts for 10% of the total volume of the solution.

[0032] Determination of shell thickness: Disperse the sample with ethanol using ultrasound (1 mg / mL, 1-5 min), drop 1-2 μL onto a copper grid (carbon film), and dry at room temperature; obtain the particle cross-section by TEM (e.g., 200 kV) and measure the shell thickness (nm) of each particle.

[0033] Determination of specific surface area and particle size: Sample dispersion (as above) was measured using DLS to determine volume distribution (report D50); specific surface area (m²) was measured using BET (nitrogen adsorption). 2 / g).

[0034] Determination of high-temperature volatilization / controlled release behavior: Place 5-10 mg of dry powder into a TGA sample pan; introduce argon or a simulated melting atmosphere (CO / CO2 mixture can be used); set the heating program (e.g., 10 °C / min from room temperature to 1200 °C), and simultaneously use FTIR online to detect the escaping gas (monitor the mass spectrum peaks or infrared characteristics of fluorine-containing compounds / HF); obtain the volatilization onset temperature, volatilization peak temperature, and total F volatilization amount (mass%) from the mass loss curve and gas intensity curve.

[0035] Table 1 Performance data of core-shell fluoride nanoparticles

[0036] The results in the table show that when the molar ratio of silicon to calcium increases from 0.1 to 0.5, the average shell thickness increases from 10 nm to 45 nm, and the corresponding high-temperature volatilization behavior is significantly suppressed (the volatilization initiation temperature increases from about 580℃ to 720℃), the volatilization peak becomes higher, and the total F volatilization decreases from 25.0% to 6.0%. This indicates that a thicker and denser shell can effectively delay and suppress the high-temperature release of fluorides, thereby improving the thermochemical stability of the slag system and helping to maintain the nitrogen content of the liquid steel.

[0037] In contrast, the surface area value increased slightly when the silicon content was low (Example 2) (exposing more active core surfaces), resulting in significant volatilization at low temperatures. This indicates that the fluxing activity was released early but accompanied by a large volatilization loss, which is not conducive to high nitrogen protection.

[0038] Comparing Examples 1, 4, and 5, when the ammonia content was increased from 5% to 10%, the average shell thickness increased from 22 nm to 30 nm, while the specific surface area significantly increased (to 23 nm). 2 / g increased to 30m 2 / g), accompanied by a decrease in the volatilization initiation temperature (from 670℃ to 620℃) and an increase in the total volatilization (from 10.0% to 18.0%); this is mainly because the higher ammonia catalytic concentration accelerates the hydrolysis and condensation rate of tetraethoxysilane, leading to the rapid formation of a silica network, but with a loose structure and increased defects, resulting in a higher specific surface area and poorer sealing performance at the microscopic level, thus failing to effectively suppress the volatilization of fluorine.

[0039] A lower ammonia content (Example 4) facilitates slow, uniform shell growth, resulting in a denser and more uniform coating, while also ensuring controlled release and corrosion resistance.

[0040] The particle size distribution (D50) did not vary much among the groups, but a slight increase (70 nm) was observed in the high Si case (Example 3), which was due to the additional silica coating that increased the apparent diameter of the particles; overall, the narrower particle size was beneficial to the consistency of the behavior after slag feeding.

[0041] Example 6: A preparation process for a high-nitrogen, nickel-saving, high-strength and high-toughness protective slag for stainless steel, comprising the following steps: S1.1 Weigh the following raw materials in parts by weight: 55 parts calcium oxide, 6 parts core-shell fluoride nanoparticles, 4 parts magnesium oxide, 4 parts barium oxide, 1.5 parts potassium carbonate, 1.2 parts lithium carbonate, and 12 parts nano-dispersed glass microspheres. S1.2 Add calcium oxide, magnesium oxide, barium oxide, potassium carbonate and lithium carbonate to a high-speed mixer and stir at 400 rpm for 15 min to mix the components evenly; then add core-shell fluoride nanoparticles and nano-dispersed glass microspheres, and continue stirring for 20 min to obtain a uniform mixture. The mixture was adjusted to have a solid content of 60%, and then spray-granulated with an atomization pressure of 0.30 MPa, an inlet air temperature of 160°C, and an outlet air temperature of 80°C to obtain protective slag precursor particles. S1.3 Place the precursor particles of the protective slag in a refractory crucible and pre-melt them at 900℃ for 1 hour (the gas consists of nitrogen and carbon dioxide, with a volume ratio of nitrogen to carbon dioxide of 1:2; the gas flow rate is controlled at 200 mL / min); then cool them to room temperature at a rate of 4℃ / min, crush them and pass them through a 100-mesh sieve to obtain a high-nitrogen, nickel-saving, high-strength and tough stainless steel protective slag.

[0042] The preparation process of core-shell fluoride nanoparticles is as follows: Calcium nitrate and ammonium fluoride were mixed and added to deionized water at a mass ratio of 1:10, wherein the molar ratio of calcium ions to fluoride ions was 1:2. The mixture was stirred at 400 rpm for 60 min at 30 °C. The pH was adjusted to 8 with 0.1 mol / L ammonia solution to form a calcium fluoride nano suspension. Calcium fluoride nano-suspension was dispersed in an ethanol / water mixed solvent at a volume ratio of 1:3, wherein the volume ratio of ethanol to water was 3:1. Tetraethoxysilane (the molar ratio of silicon to calcium was 0.3:1) and ammonia water accounting for 8% of the total volume of the solution were added. The mixture was stirred at 400 rpm for 4 hours to form a silica coating layer. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 15 min, the supernatant was discarded, and the mixture was washed three times alternately with anhydrous ethanol and deionized water. Then it was dried at 80 °C for 8 h and calcined in air at 500 °C for 2 h to obtain core-shell fluoride nanoparticles with a D50 particle size of 65 nm.

[0043] The preparation process of the nano-dispersed glass microspheres is as follows: Tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide (molar ratio of silicon, calcium, sodium and aluminum is 1:0.7:0.3:0.08) were mixed and dissolved in an ethanol / water mixture at a solid-liquid ratio of 1:12, wherein the volume ratio of ethanol to water was 2:1. The pH was adjusted to 4 with 1 mol / L nitric acid solution and stirred at 30℃ and 400 rpm for 2 h to obtain a homogeneous sol. The sol was ultrasonically dispersed for 20 minutes and then sent to a spray drying tower for drying. The inlet air temperature was 160℃, the outlet air temperature was 90℃, and the atomization pressure was 0.30MPa, thus obtaining a dry powder precursor containing a nano-dispersed phase. The dry powder precursor was then placed in a muffle furnace and calcined at 650°C for 2.0 h, and then cooled to room temperature in the furnace to obtain nano-dispersed glass microspheres with a particle size of 25 μm.

[0044] Example 7: The difference between this example and Example 6 is that tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide are mixed in a molar ratio of silicon, calcium, sodium and aluminum of 1:0.6:0.2:0.05.

[0045] Example 8: The difference between this example and Example 6 is that tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide are mixed in a molar ratio of silicon, calcium, sodium and aluminum of 1:0.8:0.4:0.10.

[0046] Determination of nanophase dispersibility and glass phase homogeneity: Microsphere powder was ultrasonically dispersed in ethanol (1 mg / mL, ultrasonic for 2 min), dropped onto a copper grid (with carbon film), and dried at room temperature; the cross-section of the microspheres (or ultrathin sections) was observed by TEM (≥200 kV) to confirm the nanophase size; the powder was placed in an XRD sample holder, and Cu-Kα, 2θ scan was performed from 5 to 80° to determine whether it was mainly amorphous (broad arc) or had crystalline phase peaks (such as CaCO3, carbonates or crystalline oxides), and its content was quantified.

[0047] Rheological property determination: Take 10-20g of sample and place it in a high-temperature crucible, which is installed in the sample chamber of the rheometer; in a protective atmosphere (inert reducing atmosphere), heat to 1400℃ at 10℃ / min, and perform isothermal viscosity-time observation in the target working temperature range (1200-1300℃) (hold for 30-60min); record the viscosity-temperature curve (Pa·s).

[0048] Table 2 Performance data of nano-dispersed glass microspheres

[0049] Example 7 (Si:Ca:Na:Al=1:0.6:0.2:0.05) exhibits the smallest nanophase size (12nm) and the highest amorphous (glass) content (88%), while having a narrow particle size distribution (D50=22μm). This indicates that the sol hydrolysis / condensation kinetics and spray drying conditions of this formulation are more conducive to the formation of a uniform and dense amorphous network. This structure is beneficial for maintaining uniform rheological properties and good erosion resistance during melting. However, its viscosity is relatively high (0.60 Pa·s at 1250℃), indicating that its fluidity is weak and higher temperatures or longer times are required to quickly spread into a continuous slag film.

[0050] In contrast, Example 8 (Si:Ca:Na:Al=1:0.8:0.4:0.10) exhibits a larger nanophase size (20 nm) and a lower glass phase content (78%) due to its higher alkali metal and aluminum content, suggesting a greater susceptibility to local crystallization or nanophase aggregation. It also exhibits the lowest working viscosity (0.35 Pa·s) at 1250 °C, indicating excellent rapid spreading and low-temperature flow characteristics under smelting conditions. However, its higher surface porosity (15%) leads to a decrease in the compactness and erosion resistance of the slag film, and the lower glass phase content increases the risk of crystallization / brittle phase formation.

[0051] Example 6 shows a balance in various indicators: the nanophase size, amorphous content and viscosity are all in the middle range, indicating that it takes into account both melt flowability and slag film density.

[0052] Example 9: A preparation process for a high-nitrogen, nickel-saving, high-strength and high-toughness protective slag for stainless steel, comprising the following steps: S1.1 Weigh the following raw materials in parts by weight: 50 parts by weight of calcium oxide, 4 parts by weight of core-shell fluoride nanoparticles, 3 parts by weight of magnesium oxide, 3 parts by weight of barium oxide, 1.2 parts by weight of potassium carbonate, 1.0 part by weight of lithium carbonate, and 10 parts by weight of nano-dispersed glass microspheres. S1.2 Add calcium oxide, magnesium oxide, barium oxide, potassium carbonate and lithium carbonate to a high-speed mixer and stir at 400 rpm for 15 min to mix the components evenly; then add core-shell fluoride nanoparticles and nano-dispersed glass microspheres, and continue stirring for 20 min to obtain a uniform mixture. The mixture was adjusted to have a solid content of 60%, and then spray-granulated with an atomization pressure of 0.30 MPa, an inlet air temperature of 160°C, and an outlet air temperature of 80°C to obtain protective slag precursor particles. S1.3 Place the precursor particles of the protective slag in a refractory crucible and pre-melt them at 900℃ for 1 hour (the gas consists of nitrogen and carbon dioxide, with a volume ratio of nitrogen to carbon dioxide of 1:3; the gas flow rate is controlled at 200 mL / min); then cool them to room temperature at a rate of 4℃ / min, crush them and pass them through a 100-mesh sieve to obtain a high-nitrogen, nickel-saving, high-strength and tough stainless steel protective slag.

[0053] The preparation process of core-shell fluoride nanoparticles is as follows: Calcium nitrate and ammonium fluoride were mixed and added to deionized water at a mass ratio of 1:10, wherein the molar ratio of calcium ions to fluoride ions was 1:2. The mixture was stirred at 400 rpm for 60 min at 30 °C. The pH was adjusted to 8 with 0.1 mol / L ammonia solution to form a calcium fluoride nano suspension. Calcium fluoride nano-suspension was dispersed in an ethanol / water mixed solvent at a volume ratio of 1:3, wherein the volume ratio of ethanol to water was 3:1. Tetraethoxysilane (the molar ratio of silicon to calcium was 0.3:1) and ammonia water accounting for 8% of the total volume of the solution were added. The mixture was stirred at 400 rpm for 4 hours to form a silica coating layer. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 15 min, the supernatant was discarded, and the mixture was washed three times alternately with anhydrous ethanol and deionized water. Then it was dried at 80 °C for 8 h and calcined in air at 500 °C for 2 h to obtain core-shell fluoride nanoparticles with a D50 particle size of 65 nm.

[0054] The preparation process of the nano-dispersed glass microspheres is as follows: Tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide (molar ratio of silicon, calcium, sodium and aluminum is 1:0.7:0.3:0.08) were mixed and dissolved in an ethanol / water mixture at a solid-liquid ratio of 1:12, wherein the volume ratio of ethanol to water was 2:1. The pH was adjusted to 4 with 1 mol / L nitric acid solution and stirred at 30℃ and 400 rpm for 2 h to obtain a homogeneous sol. The sol was ultrasonically dispersed for 20 minutes and then sent to a spray drying tower for drying. The inlet air temperature was 160℃, the outlet air temperature was 90℃, and the atomization pressure was 0.30MPa, thus obtaining a dry powder precursor containing a nano-dispersed phase. The dry powder precursor was then placed in a muffle furnace and calcined at 650°C for 2.0 h, and then cooled to room temperature in the furnace to obtain nano-dispersed glass microspheres with a particle size of 25 μm.

[0055] Example 10: The difference between this example and Example 9 is that 2 parts by weight of core-shell fluoride nanoparticles are used.

[0056] Example 11: The difference between this example and Example 9 is that 6 parts by weight of core-shell fluoride nanoparticles are used.

[0057] Example 12: The difference between this example and Example 9 is that 8 parts by weight of nano-dispersed glass microspheres are used.

[0058] Example 13: The difference between this example and Example 9 is that 12 parts by weight of nano-dispersed glass microspheres are used.

[0059] Determination of nitrogen retention rate and inclusion content in molten steel: The initial nitrogen content of the base material / molten steel was measured (using a combustion-based nitrogen analyzer), and the initial value (N) was recorded. initial ); Smelting is carried out and samples are taken at the target time point (such as the end of heat preservation) to determine the nitrogen content (N) of the molten steel. final ), calculate nitrogen retention rate (%) = (N final / N initial )×100%; Cast the sample and take a standard metallographic sample, polish and scan to characterize the inclusions, and record the inclusion number density (numbers / cm³). 2 ) and size distribution (average diameter).

[0060] Table 3. Performance data of special protective slag for high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel and high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel.

[0061] As can be seen from the table, when the core-shell particles increased from 2 parts by weight to 6 parts by weight, the high-temperature viscosity decreased significantly (from 0.50 Pa·s to 0.36 Pa·s), indicating that the slag fluidity increased after melting, making it easier to spread and form a uniform slag film on the surface of the molten steel. At the same time, the total F volatilization decreased (from 20% to 12%), indicating that the higher content of core-shell particles helps to delay F release, thereby improving thermal stability and nitrogen retention effect. The nitrogen retention rate of the molten steel increased accordingly (from 88% to 94%).

[0062] However, an excessively low core-shell particle content (2 parts by weight) leads to high slag viscosity, poor fluidity, and is accompanied by higher F volatilization and nitrogen loss. The number and average size of inclusions also increase, reflecting insufficient protection of the molten steel by the protective slag.

[0063] When the content of nano-dispersed glass microspheres is low (8 parts by weight, Example 12), the high-temperature viscosity is slightly higher (0.46 Pa·s), the total F volatilization is slightly higher (17%), the nitrogen retention rate of the liquid steel is slightly lower (90%), and the inclusion number density increases, indicating a decrease in melting uniformity and slag film density.

[0064] As the microsphere content increased to 12 parts by weight (Example 13), the high-temperature viscosity was moderate (0.39 Pa·s), the total F volatilization decreased (13%), the nitrogen retention rate of the steel liquid increased (93%), and the inclusion density decreased. This indicates that the glass microspheres enhanced the uniformity and erosion resistance of the slag through the multi-scale dispersed phase, enabling the slag film to form a more continuous and dense protective layer on the surface of the high-nitrogen nickel-saving steel.

[0065] Based on the above measurements and comprehensive evaluation, Example 11 was selected as the optimal example. Comparative Example 1: The difference between this example and Example 11 is that no nano-dispersed glass microspheres were added.

[0066] Comparative Example 2: The difference between this example and Example 11 is that calcium fluoride is used instead of core-shell fluoride nanoparticles.

[0067] Comparative Example 3: The difference between this example and Example 11 is that glass powder is used instead of nano-dispersed microspheres.

[0068] Table 4. Performance data of high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag and high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel.

[0069] As can be seen from the table, Example 11 has a moderate high-temperature viscosity, the lowest F volatility, the highest nitrogen retention rate in the molten steel, and the fewest inclusions. This indicates that under the synergistic effect of core-shell fluoride nanoparticles and nano-dispersed glass microspheres, the protective slag has good fluidity and uniform melting in smelting, and can form a dense slag film, thereby effectively reducing nitrogen escape and inclusion generation.

[0070] The high-temperature viscosity (0.35 Pa·s) of Comparative Example 1 (without added nano-dispersed glass microspheres) at 1250°C was slightly lower than that of Example 11 (0.36 Pa·s). This data shows that the main function of nano-dispersed glass microspheres is not simply to reduce the viscosity of molten slag. In fact, too low viscosity may lead to an excessively thin slag film that is easily eroded and broken by the steel flow, which is not conducive to the formation of a stable protective layer.

[0071] The key role of nano-dispersed glass microspheres lies in their function as a multi-scale dispersed phase, which can significantly improve the viscoelasticity and spreading wettability of molten slag. The solid particles they provide during the melting process increase the cohesion and structural stability of the molten slag, enabling the slag film to maintain good fluidity while possessing excellent film-forming continuity and density. This explains why, although Comparative Example 1 has a slightly lower viscosity, its slag film structure is loose (manifested as a high inclusion number density) and its protective performance is poor (manifested as a low steel liquid nitrogen retention rate).

[0072] Comparative Example 2 (using calcium fluoride instead of core-shell particles) showed a higher total F volatilization, a lower nitrogen retention rate in the liquid steel, and an increased number of inclusions. This fully demonstrates that calcium fluoride volatilizes rapidly at high temperatures, failing to achieve the slow-release function of fluorine and thus leading to nitrogen retention failure. In contrast, the core-shell fluoride nanoparticles used in this invention effectively inhibit the premature volatilization of the calcium fluoride core by virtue of the physical barrier effect of their silica shell.

[0073] Comparative Example 3 (using glass powder instead of microspheres) showed a slight decrease in fluidity, a slight increase in the volatilization of F at high temperature, and lower steel liquid nitrogen retention rate and inclusion quantity than Comparative Example 1. This indicates that ordinary glass powder lacks a nano-dispersed phase and cannot provide uniform melting and multi-scale support.

[0074] Comprehensive analysis shows that the synergistic optimization of the slag film structure by core-shell fluoride nanoparticles and nano-dispersed glass microspheres significantly improves the thermal stability, controlled release, and slag film density of the special protective slag for high-nitrogen nickel-saving steel, thereby improving the nitrogen retention rate of the liquid steel and reducing inclusions.

[0075] 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 high-nitrogen, nickel-saving, high-strength and high-toughness protective slag for stainless steel, characterized in that, The raw materials include: calcium oxide, core-shell fluoride nanoparticles, magnesium oxide, barium oxide, potassium carbonate, lithium carbonate, and nano-dispersed glass microspheres; The core-shell fluoride nanoparticles were prepared by a fluorination precipitation-sol-gel coating method. The nano-dispersed glass microspheres were prepared by a sol-gel combined spray drying and calcination process.

2. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel protective slag according to claim 1, characterized in that, The composition includes 45-55 parts by weight of calcium oxide, 2-6 parts by weight of core-shell fluoride nanoparticles, 2-4 parts by weight of magnesium oxide, 2-4 parts by weight of barium oxide, 0.8-1.5 parts by weight of potassium carbonate, 0.8-1.2 parts by weight of lithium carbonate, and 8-12 parts by weight of nano-dispersed glass microspheres.

3. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 2, characterized in that, The preparation process of the core-shell fluoride nanoparticles is as follows: Calcium nitrate and ammonium fluoride are mixed and added to deionized water at a mass ratio of 1:10, wherein the molar ratio of calcium ions to fluoride ions is 1:

2. The mixture is stirred at 300-500 rpm for 30-60 min at 25-40℃. The pH is adjusted to 7-9 with 0.1 mol / L ammonia solution to form a calcium fluoride nano suspension. Calcium fluoride nano-suspension was dispersed in an ethanol / water mixed solvent at a volume ratio of 1:3-5, wherein the volume ratio of ethanol to water was 3:

1. Tetraethoxysilane and ammonia water accounting for 5-10% of the total volume of the solution were added, and the mixture was reacted at a stirring speed of 400-600 rpm for 2-4 hours to form a silica coating layer. After the reaction was completed, the mixture was centrifuged at 8000-10000 rpm for 10-15 min, the supernatant was discarded, and the mixture was washed 2-3 times alternately with anhydrous ethanol and deionized water. Then it was dried at 80-100℃ for 6-8 h and calcined in air at 450-550℃ for 1-2 h to obtain core-shell fluoride nanoparticles.

4. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 3, characterized in that, The amount of tetraethoxysilane added is such that the molar ratio of silicon to calcium is 0.1-0.5:

1.

5. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 3, characterized in that, The core-shell fluoride nanoparticles have a D50 size of 20-100 nm.

6. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 2, characterized in that, The preparation process of the nano-dispersed glass microspheres is as follows: Tetraethoxysilane, calcium carbonate, sodium carbonate and aluminum isopropoxide are mixed and dissolved in an ethanol / water mixed solvent at a solid-liquid ratio of 1:10-15, wherein the volume ratio of ethanol to water is 2:

1. The pH is adjusted to 2-4 with 1-3 mol / L nitric acid solution and stirred at 25-35℃ and 400-600 rpm for 1-2 hours to obtain a homogeneous sol. The sol was ultrasonically dispersed for 10-20 minutes and then sent to a spray drying tower for drying. The inlet air temperature was 160-180℃, the outlet air temperature was 90-100℃, and the atomization pressure was 0.25-0.35MPa, to obtain a dry powder precursor containing a nano-dispersed phase. The dry powder precursor was then placed in a muffle furnace and calcined at 600-800℃ for 0.5-2.0h, and then cooled to room temperature in the furnace to obtain nano-dispersed glass microspheres with a D50 particle size of 10-50μm.

7. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 6, characterized in that, The tetraethoxysilane, calcium carbonate, sodium carbonate, and aluminum isopropoxide are mixed in a molar ratio of silicon, calcium, sodium, and aluminum of 1:0.6-0.8:0.2-0.4:0.05-0.

10.

8. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 2, characterized in that, The preparation process of the high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag is as follows: S1.1 Weigh the following raw materials in parts by weight: 45-55 parts by weight of calcium oxide, 2-6 parts by weight of core-shell fluoride nanoparticles, 2-4 parts by weight of magnesium oxide, 2-4 parts by weight of barium oxide, 0.8-1.5 parts by weight of potassium carbonate, 0.8-1.2 parts by weight of lithium carbonate, and 8-12 parts by weight of nano-dispersed glass microspheres. S1.2 Add calcium oxide, magnesium oxide, barium oxide, potassium carbonate and lithium carbonate to a high-speed mixer and stir at 400-600 rpm for 10-15 min to mix the components evenly; then add core-shell fluoride nanoparticles and nano-dispersed glass microspheres, and continue stirring for 10-20 min to obtain a uniform mixture. The mixture was adjusted to have a solid content of 60-70%, and then spray granulation was performed to obtain protective slag precursor particles. S1.3 Place the precursor particles of the protective slag in a refractory crucible and pre-melt them at 700-900℃ for 1 hour; then cool them to room temperature at a rate of 3-5℃ / min, crush them and pass them through a 100-200 mesh sieve to obtain a high-nitrogen, nickel-saving, high-strength and tough stainless steel protective slag.

9. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 8, characterized in that, In step S1.2, the atomization pressure of spray granulation is 0.25-0.35 MPa, the inlet air temperature is 150-180℃, and the outlet air temperature is 80-100℃.

10. The high-nitrogen, nickel-saving, high-strength and high-toughness stainless steel special protective slag according to claim 8, characterized in that, In step S1.3, the gas used for pre-melting is composed of nitrogen and carbon dioxide, with a volume ratio of nitrogen to carbon dioxide of 1:2-4; the gas flow rate is controlled at 100-300 mL / min.

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