High-nitrogen nickel-saving stainless steel continuous casting protective slag containing nano-additives and its preparation method

Abstract

This invention relates to the field of protective materials for stainless steel continuous casting, and discloses a high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives and its preparation method. Addressing the issues of surface depressions, cracks, and discontinuous slag films that easily occur in the continuous casting of high-nitrogen, nickel-saving stainless steel, the protective slag of this invention comprises, by mass percentage, CaO, SiO2, Al2O3, MgO, BaO, Na2O, Li2O, B2O3, fixed carbon, and 0.20%~1.20% nano-composite additives; the nano-composite additives include nano-TiO2 attached to the surface of nano-h-BN. Through the synergistic design of the low-fluorine multi-element fluxing slag and the surface-loaded nano-composite additives, graded responsive slag formation and spreading are achieved, effectively solving the problem of discontinuous slag films. The slag layer is stabilized at 9~12mm, the surface quality qualification rate of the cast billet is increased to over 99.0%, and the grinding rate is reduced to below 3.5%.

Description

Technical Field

[0001] This invention relates to the field of stainless steel continuous casting protective materials, and in particular to high-nitrogen nickel-saving stainless steel continuous casting protective slag containing nano-additives and its preparation method. Background Technology

[0002] In continuous casting, the protective slag plays a crucial role in the crystallizer, performing essential functions such as heat insulation, inclusion adsorption, forming a lubricating film, and regulating heat transfer through the slag film's crystalline structure. The mineral composition, crystallinity, and layered structure of the protective slag are key factors in controlling the heat flux density of the crystallizer and preventing surface cracks in the cast billet. In recent years, high-nitrogen, nickel-saving stainless steels (such as 200 series J5 and other nickel-saving austenitic stainless steels) have achieved increased strength and reduced costs by increasing nitrogen content and decreasing nickel usage. However, the continuous casting process window for this steel grade is narrower than that of ordinary stainless steel, making it prone to problems such as nitrogen porosity, surface depressions, and a sharp increase in crack sensitivity at the continuous casting end. Existing protective slags for low-nickel, high-nitrogen austenitic stainless steels mostly follow the traditional CaO–SiO2–Na2O–F–C high-fluorine fluxing route, or employ a simple high crystallinity or low viscosity design. This singular design logic easily disrupts the balance between lubrication and heat transfer, leading to abnormal slag film structure that directly induces surface longitudinal cracks, depressions, and localized heat transfer instability.

[0003] Based on the above application status, the following two core technical problems that are closely coupled need to be solved in this field: First, how to overcome the problems of discontinuous liquid slag film and insufficient local lubrication in the continuous casting process of high nitrogen nickel-saving stainless steel, and to construct a rapid and uniform slag formation, spreading and penetration behavior in the crystallizer, thereby reducing the occurrence rate of surface depressions and cracks; Second, how to break the deep dependence of existing special protective slags on high fluorides (usually exceeding 4.5% or even as high as 12%), and to construct a solid slag film with a reasonable crystallization rate and fine grain microstructure while significantly reducing the fluoride content, so as to avoid the loss of control of heat flow peak or heat transfer instability caused by simply low fluorination. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives and its preparation method.

[0005] According to one aspect of the present invention, a high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives is provided. The chemical composition of the protective slag, by mass percentage, comprises: CaO 32.0%~42.0%, SiO2 24.0%~34.0%, Al2O3 4.0%~12.0%, MgO 1.5%~5.0%, BaO 1.5%~6.0%, Na2O 3.0%~8.0%, Li2O 0.3%~1.8%, and B2O3 1.0%~4.5%. 1.0%~3.0%, fixed carbon 1.0%~2.6%, nanocomposite additives 0.20%~1.20%, the balance being unavoidable impurities; the nanocomposite additives include nano-TiO2 and nano-h-BN, with the nano-TiO2 adhering to the surface of the nano-h-BN to form a composite structure. The content of each component is based on the final chemical analysis value of the finished protective slag. In actual batching, it is necessary to adjust the content according to the pre-melting process. The burn-off rates of volatile components such as Na2O and B2O3 are appropriately over-produced to ensure that the finished product reaches the target content. In this technical solution, CaO and SiO2 are controlled within the aforementioned ranges, and their sum is maintained at approximately 60% to 70%. This aims to construct the basic calcium silicate network structure of the protective slag. If CaO is too low or SiO2 is too high, resulting in insufficient basicity, the slag's ability to absorb Al2O3 inclusions in molten steel will decrease sharply; if CaO is too high, a high-melting-point dicalcium silicate phase will easily precipitate, deteriorating lubrication. The fluoride content is limited to a low range of 1.0% to 3.0% to break the dependence of traditional slag systems on high fluoride, fundamentally reducing the erosion of submerged entry nozzles. However, excessively low fluoride content leads to insufficient depolymerization of the silicon-oxygen network. Therefore, this invention provides synergistic compensation by introducing 1.0% to 4.5% B₂O₃, 3.0% to 8.0% Na₂O, 0.3% to 1.8% Li₂O, and 1.5% to 6.0% BaO. B₂O₃ can transform part of the three-dimensional network into oligomeric two-dimensional units, significantly reducing viscosity in the low-temperature region and inhibiting coarse crystallization. Li₂O and Na₂O further simplify the Si-O and BO structures, reducing the activation energy for high-temperature flow. BaO, as a network modifier, provides non-bridging oxygen and lowers the melting point. Without the aforementioned proportions (e.g., B₂O₃ below 1.0% or Li₂O below 0.3%), the melting and flowability cannot be brought back to the industrial range under low fluoride conditions. The fixed carbon content is controlled at 1.0%–2.6% to avoid excessive free carbon delaying liquid phase formation and exacerbating carbon-rich slagging. Crucially, the system utilizes 0.20%–1.20% of nanocomposite additives. Nano-TiO2, acting as a low-dose heterogeneous nucleation seed, lowers the nucleation barrier due to its lattice constant similar to silicates, promoting the attachment and growth of crystallization units on its surface. When attached to the 2D layered structure of nano-h-BN, it not only utilizes steric hindrance to prevent the dense, irreversible agglomeration of nano-TiO2 particles caused by van der Waals forces, but also leverages h-BN's excellent high-temperature stability and interlayer slip properties to compensate for the necessary microscopic spatial separation framework function at extremely low carbon content. If the nanocomposite additive content is below 0.20%, the heterogeneous nucleation effect is weak, and the slag film tends to vitrify, leading to excessively rapid heat transfer. If it is above 1.20%, the defects of bulk TiO2 promoting coarse crystals and increasing viscosity are easily repeated.

[0006] Preferably, the chemical composition of the protective slag, by mass percentage, is further reduced to: CaO 36.0%~40.0%, SiO2 26.0%~30.0%, Al2O3 6.0%~10.0%, MgO 2.0%~4.0%, BaO 2.5%~5.0%, Na2O 4.5%~7.0%, Li2O 0.6%~1.3%, B2O3 1.5%~3.5%. The composition of the protective slag consists of 1.2%~2.5% fixed carbon, 1.2%~2.2% nanocomposite additives, and 0.30%~0.80% CaO to SiO2 by mass ratio of 1.25~1.50. The mass percentage range of each component and the CaO / SiO2 mass ratio range jointly define the composition; the actual composition must satisfy both conditions simultaneously. The basic basicity (CaO / SiO2 mass ratio) is strictly controlled within 1.25~1.50, with the optimal phase region falling within the symbiotic region of wollastonite and calcium silicate. This range ensures sufficient thermal resistance for crystallization while preventing tearing of the slag film due to excessive growth of a single crystal phase. Al2O3 and MgO are preferably selected at 6.0%~10.0% and 2.0%~4.0%, respectively, to maximize the system's compositional buffering capacity. This prevents drastic shifts in high-temperature viscosity and transition temperature after continuous absorption of inclusions floating in molten steel, thereby extending the service stability of the slag film.

[0007] More preferably, the protective slag further comprises the following components by mass percentage: MnO 0~1.5%, Fe2O3 ≤0.8%; the raw material for CaO is selected from at least one of calcite, limestone, cement clinker, and pre-melted material; the raw material for SiO2 is selected from at least one of wollastonite, glass powder, and quartz powder; the raw material for Al2O3 is selected from at least one of calcined bauxite and industrial alumina; The raw materials are selected from at least one of fluorite and cryolite. MnO and Fe2O3, as transition metal oxides, can effectively absorb infrared thermal radiation within the crystallizer through electronic transitions in their d-orbitals, providing auxiliary radiation thermal resistance control before the crystallization film is fully formed. Strictly limiting the Fe2O3 content to below 0.8% is to prevent abnormal fluctuations in the redox potential of the slag caused by iron ions under high-frequency oscillations, thus avoiding corrosion of the copper plate. The selection of specific inorganic mineral raw materials ensures the phase stability of the introduced components and the controllability of the batching cost.

[0008] In the specific structure of the nanocomposite additive, preferably, nano-TiO2 accounts for 65%~85% of the total mass of the nanocomposite additive, and nano-h-BN accounts for 15%~35% of the total mass of the nanocomposite additive; the primary particle size of the nano-TiO2 is 20~80 nm; the lateral dimension of the nano-h-BN sheets is 50~300 nm, and the thickness is 10~80 nm; the nano-TiO2 is attached in a dot-like manner to the surface of the nano-h-BN sheets to form composite secondary particles. From the perspective of crystallization kinetics, the probability of heterogeneous nucleation is exponentially positively correlated with the effective interfacial area of ​​the nucleating agent. Nano-TiO2 with a lateral dimension of 20~80 nm has an extremely high specific surface area, but its simple addition is prone to severe dense irreversible agglomeration due to strong van der Waals forces, losing its nucleation activity; the nano-h-BN sheets with a lateral dimension of 50~300 nm provide an excellent two-dimensional flat carrier, using its high specific surface energy to anchor TiO2 to form dot-like attachment, and the steric hindrance effect prevents the dense aggregation between TiO2 particles. In the subsequent post-loading process, multiple TiO2-anchored h-BN particles are assembled into composite secondary particles of 0.5~5μm through the bridging effect of the binder. These secondary particles have a loose satellite structure rather than a dense agglomerate, and each primary particle inside still retains an accessible active surface. When the protective slag undergoes high-temperature melting in the crystallizer, the organic binder decomposes, and the secondary particles disintegrate, releasing well-dispersed primary nanounits, which play an efficient heterogeneous nucleation role at the solid-liquid interface. If the TiO2 content is less than 65%, there are insufficient activation nucleation sites per unit volume; if it is higher than 85%, the h-BN surface will be completely coated or even multilayered, weakening the high-temperature lubrication and framework separation effect provided by h-BN itself through hexagonal lattice cleavage.

[0009] Furthermore, the fixed carbon is composed of carbon black and flake graphite; by mass percentage, the carbon black accounts for 35% to 60% of the total fixed carbon, and the flake graphite accounts for 40% to 65% of the total fixed carbon. Due to its large specific surface area and high reactivity, carbon black burns rapidly mainly in the lower temperature range, providing initial porosity in the powder layer and preventing premature sintering of the protective slag. Flake graphite, with its high crystallinity and high oxidation activation energy, can exist for a longer period in the higher temperature range, maintaining the "skeleton" isolation structure within the slag layer and ensuring that the liquid slag can continuously and stably penetrate the meniscus. The blending of the two in a specific ratio successfully achieves seamless slag formation rates across the entire temperature range.

[0010] Furthermore, the protective slag is in the form of hollow or porous granular powder, with a closed porosity of 10%~25% and a loose packing density of 0.75~1.00 g / cm³. 3In the cross-section of the protective slag particles, the continuous phase is a pre-melted glassy matrix of CaO-SiO2-Al2O3, and the dispersed phase includes carbon black, graphite, and composite secondary particles formed by the aforementioned nanocomposite additives. The hollow or porous structure significantly reduces the apparent thermal conductivity of the particles and increases the heat-receiving specific surface area. When the protective slag comes into contact with high-temperature molten steel, heat can be rapidly and uniformly transferred to the interior of the particles, avoiding the "raw core" phenomenon where the outer surface of traditional solid particles melts while the interior remains cold. If the closed porosity is less than 10%, the granulation slag formation rate is significantly slowed; if it is greater than 25%, the slag layer structure is too loose, easily leading to instantaneous large-area collapse and drastic fluctuations in the thickness of the molten slag layer. The pre-melted glassy matrix, as the continuous phase, completely eliminates local micro-component segregation caused by differences in the latent heat of melting of different minerals.

[0011] According to another aspect of the present invention, a method for preparing the above-mentioned high-nitrogen nickel-saving stainless steel continuous casting protective slag containing nano-additives is provided, comprising the following strictly sequential steps: S1, preparation of pre-melted glassy base material: inorganic raw materials other than fixed carbon, nano-composite additives and granulation binder are mixed uniformly, pre-melted and rapidly cooled to obtain glassy pre-melted material, which is then dried and pulverized to obtain pre-melted powder; S2, post-loading of nano-composite additives: nano-TiO2 and nano-h-BN are dispersed in a composite binder to obtain a uniform slurry, which is then sprayed into the pre-melted powder for mixing, so that the nanocomposite material adheres to the surface of the pre-melted material to form a post-loaded pre-melted material; S3, introduction and granulation of a low-carbon melting rate regulation system: fixed carbon raw materials are mixed with the post-loaded pre-melted material, sprayed into a composite granulation liquid, granulated and dried to obtain the continuous casting protective slag. The core of this process route lies in the "post-loading" mechanism. If the nanocomposite additive is pre-melted in S1 in the same furnace, TiO2 will completely dissolve and enter the silica glass network, completely losing its heterogeneous nucleation effect at the solid-liquid interface. Through the post-loading treatment in S2, the nanocomposite particles are forced to target and position themselves on the surface of the pre-melted matrix particles. When the protective slag begins to melt in the crystallizer, these nanoparticles can play a role in controlling the crystallization behavior at the melt boundary at the first moment.

[0012] Preferably, in step S1, the mixing time is 8-15 minutes; the pre-melting treatment temperature is 1380-1450℃, and the time is 20-35 minutes; the rapid cooling is performed by water quenching or steel strip rapid cooling; the particle size D50 of the pre-melted powder is 50-120 μm. The pre-melting temperature range of 1380-1450℃ ensures complete dissociation of the silicate and aluminate mineral phases to form a uniform liquid phase, while avoiding… Significant burn-off of volatile components such as F⁻ and Na₂O is observed. If the temperature is below 1380℃, the high-melting-point phase will not be completely dissolved, becoming uncontrolled, coarse primary crystal nuclei. Pulverizing these components to a D50 of 50–120 μm provides the optimal powder rheological basis for subsequent droplet encapsulation and granulation. It should be noted that although 1380–1450℃ can control volatilization losses within an acceptable range, F⁻, Na₂O, and B₂O₃ will still exhibit a relative burn-off rate of approximately 8%–20%. Therefore, in the actual feeding of the S1 step, it is necessary to back-calculate the target composition of the downstream final product and over-compensate for the above three volatile components. The specific over-compensation coefficient is determined through a small-scale pre-melting-chemical analysis calibration test.

[0013] More preferably, in step S2, the composite binder is a lithium borate-sodium silicate composite binder with a solid content of 2%~6%; the process of dispersing and preparing a uniform slurry includes: first ultrasonic dispersion for 15~30 minutes, then high-shear dispersion at 3000~6000 rpm for 5~15 minutes; the mixing is low-temperature mixing in a plowshare mixer or a high-speed mixer for 5~10 minutes; the particle size of the composite secondary particles in the formed post-loaded pre-melted material is 0.5~5μm. The use of lithium borate-sodium silicate composite binder not only provides excellent room-temperature adhesion, but it is also a beneficial fluxing component of the protective slag, without introducing any foreign impurity phases. The ultrasonic cavitation effect combined with high-shear hydrodynamic force completely tears apart the van der Waals aggregation network between nanoparticles, allowing the primary nanoparticles to be anchored on the h-BN sheets in a highly dispersed state. The 0.5–5 μm composite secondary particles assembled after being bridged by a binder are essentially satellite-like assemblies formed by the loose aggregation of multiple nano-primary units through organic-inorganic bridging, rather than dense sintered masses. After the binder bridges decompose under the high-temperature conditions of the crystallizer, these secondary particles rapidly disintegrate into well-dispersed nano-primary units, effectively performing nucleation and lubrication functions at the solid-liquid front of the molten slag. Low-temperature mixing avoids premature induction of chemical reactions, ensuring that the aforementioned composite secondary particles exhibit a discrete satellite structure distribution on the macroscopic slag particle surface.

[0014] Further, in step S3, the composite granulation liquid is a starch-carboxymethyl cellulose composite granulation liquid with a solid content of 3%~8%, wherein the mass ratio of starch to sodium carboxymethyl cellulose is 3:1; the granulation is carried out by disc granulation or high-intensity stirring granulation, and the moisture content of the material is controlled to reach 8%~14% after the composite granulation liquid is sprayed in; the particle size of the granulated particles is 0.3~1.2mm; the drying treatment temperature is 180~240℃, and the time is 20~40 minutes. Starch-carboxymethyl cellulose provides strength to the granules while allowing for clean combustion in the furnace to form micropores. The granulation moisture content of 8%~14% is a key critical window for inducing internal hollowing of the particles using capillary tension. The final drying temperature is strictly limited to 180~240℃, which effectively eliminates free water and macromolecular bound water, giving the particles excellent resistance to crushing and pulverization during transport. Simultaneously, it resolutely prevents early low-temperature oxidation of fixed carbon and nano-h-BN, ensuring that all functional components of the formulation are fully incorporated into the crystallizer's operating environment. Since the liquid-solid content and injection volume during granulation can be determined by back-calculation from the water content endpoint, the amount of organic carbon introduced typically does not exceed 0.3% of the total mass of the finished protective slag. This portion of organic carbon is rapidly burned off under the high-temperature conditions of the crystallizer and will not remain in the slag film to interfere with mineral crystallization behavior.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention achieves a graded slag formation and spreading process by synergistically designing a low-fluorine multi-element fluxing slag, surface-loaded nanocomposite additives, and hollow granulation technology. This effectively solves the problems of surface depressions, cracks, and discontinuous liquid slag films that are prone to occur in high-nitrogen nickel-saving stainless steel. This invention uses trace amounts of nano-h-BN to replace part of the high-temperature skeleton function of free carbon, and, combined with a hollow or porous particle structure, reduces the total fixed carbon content to 1.0%~2.6%, maintaining controlled and stable seepage while ensuring rapid slag formation. In practical industrial applications, the liquid slag layer of this protective slag can be stabilized at 9~12 mm, and the slag consumption per ton of steel can be controlled at 0.28~0.42 kg / t. This can improve the surface quality qualification rate of high-nitrogen nickel-saving stainless steel billets to over 99.0%, and reduce the grinding rate to below 3.5%.

[0016] This invention significantly reduces fluorine dependence while successfully locking in an ideal crystallization and heat transfer control window. This invention will... The fluorine content is strictly controlled within a low range of 1.0% to 3.0%, significantly reducing the environmental and equipment burden caused by immersion nozzle erosion and fluorine volatilization. Simultaneously, the low-temperature viscosity reduction and crystal inhibition effects of B2O3, the network depolymerization effect of Li2O / Na2O, and the melting point reduction effect of BaO, combined with the controlled heterogeneous nucleation effect of nano-TiO2, precisely compensate for the heat flow control gap caused by the decrease in fluorine concentration. Using nano-TiO2 with a primary particle size of 20–80 nm dispersed on the surface of h-BN as a low-dose heterogeneous nucleating agent, its large interfacial area induces the formation of fine and uniform titanium-containing composite crystalline phases and silicate microcrystals, avoiding the coarse CaTiO3 crystallization and viscosity surge problems easily induced by traditional high TiO2 content. The resulting slag film possesses both sufficient crystalline phase thermal resistance and continuous liquid phase lubrication capability, effectively preventing uncontrolled peak heat loss, resulting in more uniform growth of the high-nitrogen nickel-saving billet shell near the meniscus, and significantly reducing the probability of abnormal oscillation marks and adhesion. Attached Figure Description

[0017] Figure 1 This is a SEM image of the protective slag of the present invention; Figure 2 This is a SEM image of the nanocomposite additive of the present invention. Detailed Implementation

[0018] The present invention will be further described in detail below with reference to specific embodiments. However, the scope of protection of the present invention is not limited to these embodiments. Any changes or equivalent substitutions that do not depart from the concept of the present invention are included within the scope of protection of the present invention.

[0019] like Figure 1 and Figure 2 As shown, the high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives provided by this invention comprises, by mass percentage: CaO 32.0%~42.0%, SiO2 24.0%~34.0%, Al2O3 4.0%~12.0%, MgO 1.5%~5.0%, BaO 1.5%~6.0%, Na2O 3.0%~8.0%, Li2O 0.3%~1.8%, and B2O3 1.0%~4.5%. The composition consists of 1.0%~3.0%, fixed carbon 1.0%~2.6%, and nanocomposite additives 0.20%~1.20%, with the balance being unavoidable impurities (mainly trace amounts of K2O, P2O5, TiO2, SO3, etc. introduced from the raw materials, generally not exceeding 2.5% in total). The above component contents are based on the final chemical analysis values ​​determined by X-ray fluorescence spectroscopy and wet chemical analysis of the finished protective slag. During actual feeding in step S1, since the pre-melting temperature is between 1380~1450℃, Volatile components such as Na2O and B2O3 will experience varying degrees of burn-off; therefore, over-feeding is necessary based on the pre-calibrated volatilization loss coefficient. Specifically, through small-scale pre-melting tests, the actual volatilization rate of each batch of mineral raw materials is measured at the target pre-melting temperature and holding time. The feed amount is then calculated using the formula "target content ÷ (1 - volatilization rate)". For example, if the target content is 2.0% and the standard volatility loss rate is 15%, then the actual feed content should be calculated based on fluorite. The input amount should be approximately 2.35% (2.0% ÷ 0.85). The overmixing method for Na2O and B2O3 is similar, with an overmixing range generally between 8% and 20%. The component contents listed in subsequent examples refer to the finished product content confirmed by chemical analysis, not the raw material input values.

[0020] In this invention, CaO and SiO2 synergistically construct the basic calcium silicate network, accounting for approximately 60% to 70% of the total composition, forming the main framework of the protective slag. CaO serves as the alkalinity regulator and inclusion absorber, while SiO2 acts as a network-forming body to stabilize the glassy matrix. BaO, as a large ionic radius network modifier, provides more non-bridging oxygen and significantly lowers the melting point of the system, working with Na2O and Li2O to depolymerize the silicon-oxygen network under low-fluorine conditions. B2O3, by forming a mixed coordination structure of [BO3] triangles and [BO4] tetrahedra, suppresses the precipitation of coarse primary crystals in the medium- and low-temperature regions. By controlling the fluorine content within a low fluorine window of 1.0% to 3.0%, and with the synergistic compensation of B2O3, Li2O and BaO, the system can maintain an industrially acceptable melting temperature and viscosity range while significantly reducing volatile fluorides.

[0021] The key raw materials used in the examples and their sources are as follows: CaO is made from calcite with a particle size of less than 0.075 mm (produced in Huangshi, Hubei Province, CaO content 54.2%) and CaO-SiO2 pre-melted material (self-made, glassy state) as needed; SiO2 comes from wollastonite (produced in Xinyu, Jiangxi Province, SiO2 content 51.8%) and high-purity quartz powder (≥99.0%, produced by Hongyuan Mining, Lingshou County); Al2O3 is made from calcined bauxite (Yangquan, Shanxi Province, Al2O3 content 85%) and industrial alumina (Aluminum Corporation of China AO-2 type); MgO is introduced from calcined magnesite powder; BaO is introduced from analytical grade BaCO3; Na2O and Li2O are introduced from soda ash (≥99.5%) and lithium carbonate (≥99.5%), respectively; B2O3 is introduced from borax (Na2B4O7·10H2O) and boric acid (H3BO3) in proportion. The carbon black is derived from 97% grade fluorite powder (CaF2≥97%) supplemented with a small amount of cryolite (Na3AlF6); MnO and Fe2O3 are used as auxiliary materials for regulating radiation thermal resistance, introduced by MnO2 (chemically pure) and Fe2O3 (chemically pure); the carbon black used is conductive carbon black produced by the acetylene process (Cabot Corporation VXC-72R, specific surface area approximately 250 m²). 2 / g), the flake graphite is high-purity flake graphite from Qingdao Heilong Graphite Company (carbon content ≥99.5%, average particle size 50μm); the nano TiO2 is rutile nano titanium dioxide (primary particle size 20~80nm, specific surface area ≥80m²) sold by Shanghai Aladdin Reagent Company. 2 / g); Nano-h-BN is selected from layered hexagonal boron nitride (lamellar transverse size 50~300nm, thickness 10~80nm, purity ≥99.0%) produced by Ningbo Moxi Technology Co., Ltd.; Lithium borate-sodium silicate composite binder is prepared by mixing lithium borate (LiBO2) and water glass (modulus 2.8) at a mass ratio of 1:3 and diluted to the required solid content; Starch-carboxymethyl cellulose composite granulation solution is prepared by mixing soluble starch and sodium carboxymethyl cellulose (viscosity 1000~1200 mPa·s) at a mass ratio of 3:1 and dissolving in deionized water to prepare an aqueous solution with a solid content of 3%~8% for later use.

[0022] The preparation process strictly follows a three-stage sequential process: pre-melted base material preparation, post-loading of nanocomposite additives, and immobilized carbon incorporation and granulation. In the pre-melting stage, all inorganic raw materials except for immobilized carbon, nanocomposite additives, and granulation binders are dry-mixed for 10 minutes in a V-type mixer. The amounts of Na2O and B2O3 were over-mixed according to the pre-calibrated volatility loss coefficients and transferred to a silicon molybdenum rod resistance furnace. The mixture was held at 1380–1450℃ for 20–35 minutes to fully melt and homogenize the system. It was then rapidly cooled by water quenching to form a glassy molten mass. After drying, it was pulverized in a ball mill, with the particle size D50 controlled at 50–120 μm. Samples were then taken for chemical analysis to confirm the actual content of each component in the pre-melted material. In the post-loading stage, nano-TiO2 and nano-h-BN were added to the lithium borate–sodium silicate composite binder in a predetermined ratio. The mixture was first dispersed in a water bath ultrasonic bath for 15–30 minutes, then sheared at 3000–6000 rpm for 5–15 minutes in a high-speed disperser to obtain a uniformly dispersed slurry. This slurry was then atomized and sprayed into the pre-melted powder through a dual-fluid nozzle and mixed at low temperature in a plow mixer for 5–10 minutes to obtain a post-loaded pre-melted material with 0.5–5 μm composite secondary particles adhering to its surface. In the granulation process, carbon black and flake graphite are premixed in the required proportion and then mixed again in the pre-loaded pre-melted material. Then, a starch-carboxymethyl cellulose composite granulation liquid with a solid content of 3% to 8% is sprayed in to stabilize the moisture content of the material between 8% and 14%. After granulation by a disc granulator or a high-intensity stirring granulator, the material is dried in a belt dryer at 180 to 240°C for 20 to 40 minutes to obtain the target product.

[0023] The protective slag obtained in this invention was characterized and evaluated using the following methods. Chemical composition was determined using X-ray fluorescence spectrometry (PANalytical Axios type) according to GB / T 21114, with the F⁻ content verified by ion-selective electrode method (GB / T5195.1); fixed carbon content was determined by high-frequency inductively coupled infrared absorption according to GB / T 2007. Melting temperature was determined using the hemispherical point method on a melting point apparatus (Beijing Anton Paar CMD-6000 type) according to YB / T 4163, with a heating rate of 15℃ / min. Viscosity at 1300℃ was determined using the rotating column method on an RTW-14 melt property analyzer according to GB / T 25139, with a column rotation speed of 200 r / min; simultaneously, the viscosity-temperature curve was recorded, and the inflection temperature of the system was obtained from its inflection point. Crystallinity was determined using the slag-thin-film method. Slag equilibrated at 1300℃ for 10 minutes was poured into a copper mold and allowed to cool naturally, producing thin films with a diameter of 20 mm and a thickness of 2 mm. The crystalline phase content was determined by X-ray diffraction (Rigaku SmartLab type, Cu Kα), and grain size and morphology were observed using a scanning electron microscope (Hitachi SU8010 type). The loose packing density was determined according to GB / T 1479.1, and the closed porosity was calculated from the ratio of true density to apparent density. The industrial application test was conducted on a 120t slab continuous casting machine at a certain factory. The steel used for casting was J5 type 200 series nickel-limiting austenitic stainless steel (C 0.10%, Cr 17.5%, Mn 7.8%, Ni 1.2%, N 0.18%), with a cross-sectional size of 200mm×1280mm. The casting speed was 1.05~1.15 m / min, and the casting time for each heat was approximately 45 minutes. Each type of protective slag was continuously cast for 5 heats. The thickness of the liquid slag layer (iron wire insertion-rapid cooling method), slag consumption per ton of steel, vibration mark depth (average of 30 measurements on the surface of the slab using a micrometer), indentation rate (percentage of indentations deeper than 0.3mm per meter of slab length), surface qualification rate (GB / T 22638 slab surface quality rating qualification rate), and grinding rate (percentage of grinding weight to the total weight of the slab). Finally, the average value of each indicator was taken from the 5 heats. Example 1

[0024] Based on the chemical analysis values ​​of the finished product, the chemical composition of the protective slag in this embodiment is as follows: CaO 39.5%, SiO2 29.2%, Al2O3 8.5%, MgO 3.2%, BaO 3.8%, Na2O 6.0%, Li2O 1.0%, B2O3 2.8%. 2.0%, MnO 0.70%, Fe2O3 0.25%, fixed carbon 1.8%, and nanocomposite additives 0.55% (the remaining approximately 0.7% is trace amounts of K2O, P2O5, and other impurities introduced from the raw materials, totaling 100%). The mass ratio of CaO to SiO2 is 1.35. The nanocomposite additives consist of 75% nano-TiO2 and 25% nano-h-BN. The fixed carbon comprises 45% carbon black and 55% flake graphite. S1: The above inorganic raw materials (excluding fixed carbon, nanocomposite additives, and granulation binders) are mixed in a V-type mixer for 12 minutes according to the overmixed feed amount. The feed ratio was overmixed to 2.35% with a measured volatilization loss rate of 15%, Na2O to 6.82% with a loss rate of 12%, and B2O3 to 3.11% with a loss rate of 10%. The mixture was then transferred to a silicon molybdenum rod resistance furnace and held at 1420℃ for 28 minutes. After water quenching, drying, and pulverizing, a pre-melted powder with a D50 of approximately 85μm was obtained. S2: Chemical analysis confirmed that the content of each component met the target design values. Nano-TiO2 and nano-h-BN were added to a lithium borate solution with a solid content of 4% at a mass ratio of 75:25. In the sodium silicate composite binder, after ultrasonic dispersion for 20 minutes, a uniform slurry was obtained by shearing at 4500 rpm for 8 minutes. This slurry was then sprayed into pre-melted powder and mixed in a plow mixer for 7 minutes to obtain a post-loaded pre-melted material. S3: Carbon black and flake graphite were premixed and added to the post-loaded pre-melted material for homogenization. A starch-carboxymethyl cellulose composite granulation solution with a solid content of 5% was sprayed in, controlling the moisture content at 11%. Granulation was performed in a disc granulator to obtain particles with a diameter of 0.3~1.2 mm. The particles were then dried at 210℃ for 30 minutes to obtain the final protective slag. The prepared protective slag has a porous structure with a closed porosity of approximately 18% and a loose packing density of 0.85 g / cm³. 3 . Example 2

[0025] Based on the chemical analysis values ​​of the finished product, the chemical composition of the protective slag in this embodiment is: CaO 42.0%, SiO2 33.0%, Al2O3 6.5%, MgO 2.8%, BaO 2.8%, Na2O 5.5%, Li2O 0.6%, B2O3 1.8%. The composition consists of 1.3% CaO, 0.50% MnO, 0.20% Fe2O3, 1.3% fixed carbon, and 0.25% nanocomposite additives (the remaining approximately 1.45% is trace amounts of impurities such as K2O and P2O5 introduced from the raw materials, totaling 100%). The mass ratio of CaO to SiO2 is 1.27. The nanocomposite additives comprise 65% nano TiO2 and 35% nano h-BN, while the fixed carbon is composed of 35% carbon black and 65% flake graphite. Inorganic raw materials are weighed according to the overmixed amounts of each component, pre-melted at 1380℃ for 30 minutes, water-quenched, and then pulverized to a D50 of approximately 110μm. Chemical analysis confirms that the target content has been achieved. The solid content of the composite binder used in the post-loading stage is 2%, ultrasonically dispersed for 25 minutes, and then sheared at 3000 rpm for 15 minutes. The moisture content of the granulation is controlled at 8%, using disc granulation, and the drying temperature is 180℃ for 35 minutes. The resulting particles had a closed porosity of approximately 13% and a loose packing density of 0.78 g / cm³. 3 This embodiment generally falls within the region of the broad composition range of the present invention where the fluxing components (BaO, B2O3, Li2O, etc.) are relatively low and the CaO+SiO2 main phase is relatively high. The basicity of 1.27 is within the broad range defined by the independent terms, and is used to examine the process feasibility of the system under low nano-dosage and low multi-component fluxing conditions. Example 3

[0026] Based on the chemical analysis values ​​of the finished product, the chemical composition of the protective slag in this embodiment is as follows: CaO 36.5%, SiO2 26.5%, Al2O3 9.5%, MgO 3.8%, BaO 4.8%, Na2O 6.8%, Li2O 1.3%, B2O3 3.5%. The composition consists of 2.5% CaO, 1.0% MnO, 0.50% Fe2O3, 2.2% fixed carbon, and 1.0% nanocomposite additives (the remaining approximately 0.5% is trace impurities, totaling 100%). The mass ratio of CaO to SiO2 is 1.38. The nanocomposite additives comprise 85% nano-TiO2 and 15% nano-h-BN. The fixed carbon is composed of 60% carbon black and 40% flake graphite. Inorganic raw materials were weighed according to the over-mixing dosage, pre-melted at 1450℃ for 22 minutes, water-quenched and pulverized to a D50 of approximately 65μm. Chemical analysis confirmed that the target content was achieved. The composite binder used in the post-loading stage had a solid content of 6%, was ultrasonicated for 30 minutes, and then sheared at 6000 rpm for 5 minutes. The granulation moisture content was controlled at 14%, using strong stirring granulation, and drying at 240℃ for 25 minutes. The resulting particles had a closed porosity of approximately 24% and a loose packing density of 0.95 g / cm³. 3 This embodiment examines the region in the broad composition range of the present invention where both the fluxing component and the nano-additive are relatively high, the basicity is moderate but the multi-component fluxing effect is enhanced, in order to observe the overall performance when the nano-dosage and fluxing strength increase simultaneously. Example 4

[0027] Based on the chemical analysis values ​​of the finished product, the chemical composition of the protective slag in this embodiment is as follows: CaO 40.0%, SiO2 30.0%, Al2O3 8.5%, MgO 3.0%, BaO 3.5%, Na2O 5.5%, Li2O 0.9%, B2O3 2.5%. The composition consists of 1.8% CaO, 0.55% MnO, 0.20% Fe2O3, 1.7% fixed carbon, and 0.45% nanocomposite additives (the remaining approximately 0.9% is trace impurities, totaling 100%). The mass ratio of CaO to SiO2 is 1.33. The nanocomposite additives comprise 80% nano-TiO2 and 20% nano-h-BN. The fixed carbon is composed of 50% carbon black and 50% flake graphite. Inorganic raw materials were weighed according to the over-mixing dosage, pre-melted at 1410℃ for 25 minutes, water-quenched and pulverized to a D50 of approximately 80μm, and chemical analysis confirmed compliance. In the post-loading stage, the composite binder liquid had a solid content of 3.5%, was ultrasonicated for 20 minutes, and sheared at 5000 rpm for 10 minutes. The granulation moisture content was 11%, using disc granulation, and drying at 220℃ for 30 minutes. The resulting particles had a closed porosity of approximately 17% and a loose packing density of 0.86 g / cm³. 3 This embodiment falls within the preferred component range, with the amount of nanocomposite additives used falling within the lower half of the preferred range (0.30%~0.80%), primarily to verify the performance retention at lower nano-dosage levels. Example 5

[0028] Based on the chemical analysis values ​​of the finished product, the chemical composition of the protective slag in this embodiment is as follows: CaO 42.0%, SiO2 31.0%, Al2O3 7.5%, MgO 2.5%, BaO 3.0%, Na2O 5.0%, Li2O 0.8%, B2O3 2.0%. The composition consists of 1.3% CaO, 0.55% MnO, 0.20% Fe2O3, 1.4% fixed carbon, and 0.40% nanocomposite additives (the remaining approximately 2.35% is trace impurities, totaling 100%). The mass ratio of CaO to SiO2 is 1.35. The nanocomposite additives comprise 70% nano-TiO2 and 30% nano-h-BN. The fixed carbon is composed of 40% carbon black and 60% flake graphite. Inorganic raw materials were weighed according to the over-mixing dosage, pre-melted at 1400℃ for 30 minutes, water-quenched and pulverized to a D50 of approximately 90μm, and chemical analysis confirmed compliance. In the post-loading stage, the composite binder liquid had a solid content of 3%, was ultrasonicated for 18 minutes, and sheared at 4000rpm for 12 minutes. Granulation with a moisture content of 10% was performed using disc granulation, and drying at 200℃ for 35 minutes. The resulting particles had a closed porosity of approximately 15% and a loose packing density of 0.82 g / cm³. 3 This embodiment further reduces alkalinity while maintaining a moderate level. With a fixed carbon level, the study focuses on the supporting ability of nanocomposite additives on crystallization behavior and liquid slag layer stability under low fluorine and low carbon conditions. Example 6

[0029] Based on the chemical analysis values ​​of the finished product, the chemical composition of the protective slag in this embodiment is as follows: CaO 38.0%, SiO2 27.5%, Al2O3 9.0%, MgO 3.5%, BaO 4.2%, Na2O 6.5%, Li2O 1.2%, B2O3 3.2%. The composition consists of 2.2% CaO, 0.80% MnO, 0.35% Fe2O3, 2.0% fixed carbon, and 0.70% nanocomposite additives (the remaining approximately 0.85% is trace impurities, totaling 100%). The mass ratio of CaO to SiO2 is 1.38. The nanocomposite additives comprise 78% nano-TiO2 and 22% nano-h-BN. The fixed carbon is composed of 55% carbon black and 45% flake graphite. Inorganic raw materials were weighed according to the over-mixing dosage. Pre-melting was performed at 1430℃ for 26 minutes, followed by water quenching and pulverization to a D50 of approximately 70μm. Chemical analysis confirmed compliance. In the post-loading stage, the composite binder had a solid content of 5%, was ultrasonicated for 25 minutes, and sheared at 5500rpm for 9 minutes. Granulation was carried out with a moisture content of 13%, using vigorous stirring. Drying was performed at 230℃ for 22 minutes. The resulting particles had a closed porosity of approximately 22% and a loose packing density of 0.93 g / cm³. 3 This embodiment is at the upper limit of the preferred range, with the amount of nanocomposite additive taken to the preferred upper limit of 0.70%, forming a parameter gradient with Example 1, in order to observe whether a critical problem of over-crystallization will occur after the nano dosage is further increased.

[0030] Comparative Example 1 The formulation of Example 1 was followed, but the nanocomposite additive was completely removed, and its proportion was added to CaO and SiO2 in a proportional manner (the final chemical analysis value of the product is CaO 39.8%, SiO2 29.5%, and all other components remain unchanged, with a total of 100%). The process flow remained the same. The pre-melting, post-loading (spraying only blank binder liquid), and granulation steps were performed exactly as before to eliminate the interference of process differences on the results, and to focus on examining the actual contribution of the nanocomposite additive, the core component, to the crystallization and heat transfer behavior of the system.

[0031] Comparative Example 2 The formulation and process of Example 1 were followed, but 0.55% of the nano-TiO2 / nano-h-BN composite additive was replaced with an equal mass of micron-sized rutile TiO2 powder (D50 approximately 2 μm, produced by Asia Pacific Chemicals) introduced separately and applied in step S2 using the same slurry-spray process, with all components totaling 100%. This comparative example was used to verify the direction of change in system performance after the particle size was reduced from nano to micron and after the removal of the h-BN two-dimensional carrier.

[0032] Comparative Example 3 The formulation and raw material system of Example 1 were used exactly (all components total 100%), but the process was modified so that the nanocomposite additive was directly fed and melted together with other inorganic raw materials in the S1 pre-melting stage, omitting the S2 post-loading process, and directly entering the granulation stage in S3. This comparative example examines the impact of the timing of the introduction of the nanocomposite additive, i.e. the "post-loading" mechanism, on the performance of the final protective slag, and verifies the irreplaceability of the S2 process.

[0033] Comparative Example 4 The formulation was designed to simulate the traditional protective slag used for nickel-saving austenitic stainless steel: by mass percentage, CaO 39.0%, SiO2 33.0%, Al2O3 5.5%, MgO 2.5%, BaO 1.0%, Na2O 8.0%, Li2O 0.4%, B2O3 0.8%. The composition is 5.5% fluorine, 3.0% fixed carbon, 0.3% MnO, and 0.2% Fe2O3 (the remaining approximately 0.8% is impurities, totaling 100%), containing no nano-components, and prepared using a one-step mixing and granulation process (without pre-melting). This comparative example represents the current conventional high-fluorine technology in the field, used to contrast the overall effectiveness of the low-fluorine nanocomposite solution of this invention.

[0034] The key melting, flow, and crystallization parameters of the protective slags prepared in each embodiment and comparative example, as well as the application results on the J5 type nickel-saving austenitic stainless steel slab continuous casting machine, are summarized in the table below.

[0035]

[0036] As shown in the table above, the protective slag prepared in Examples 1 to 6 of this invention maintains a stable slag layer thickness of 9.4~11.6 mm on the slab casting machine, a stable slag consumption of 0.32~0.40 kg / t per ton of steel, a slab surface qualification rate of over 99.0%, a grinding rate reduced to below 3.5%, and a sinkhole incidence rate of no more than 1.2%. In contrast, the four comparative examples all showed a significant decrease in surface qualification rate, a doubling of grinding rate, and a sinkhole incidence rate exceeding 3%. Among them, Examples 1 and 6 showed the most outstanding overall performance, indicating that the above-mentioned preferred component range and the preferred basicity range of CaO / SiO2 1.25~1.50 can indeed further improve the system performance.

[0037] Analysis of the microscopic crystallization behavior shows that the solid slag film obtained in this embodiment of the invention exhibits a fine-grained structure dominated by wollastonite-cubic silicate symbiosis, with an average grain size controlled within a narrow range of 3-5 μm, and the grains are uniformly dispersed in the glassy matrix. This morphology is closely related to the high interfacial density heterogeneous nucleation effect brought about by the dot-like loading of nano-TiO2 on the h-BN surface. The small lattice mismatch and low interfacial energy between nano-TiO2 and the silicate phase can significantly reduce the nucleation barrier, allowing a large number of tiny nuclei to germinate simultaneously in the early stage of slag film formation, eventually forming a fine and uniform crystalline phase structure through mutual competition. As mentioned earlier, the 0.5~5μm composite secondary particles formed by bridging with binder in the post-loading process are loose satellite-type assemblies, and the active surfaces of the individual nano-primary particles within them are not embedded. Under the high-temperature conditions in the crystallizer, the organic-inorganic binder bridge rapidly decomposes, and the secondary particles immediately disintegrate. The released nano-TiO2 / h-BN primary units play a nucleation role at the solid-liquid front of the molten slag in a highly dispersed state. Therefore, its effect is far superior to that of the micron-sized TiO2 powder in Comparative Example 2, which loses its effective interfacial area due to true dense agglomeration. This fine-grained distribution in the slag film constructs a dense and continuous grain boundary network, effectively scattering the thermal radiation in the crystallizer. This results in the slag film exhibiting controlled radiation-conduction composite thermal resistance, avoiding peak heat flow near the meniscus, thereby suppressing strain accumulation and surface crack initiation in the early stage of solidification of nickel-saving austenitic stainless steel. Meanwhile, the interlaminar shear slip characteristics of h-BN sheets at high temperatures contribute to the slag film with additional "soft lubrication", forming a gradient lubrication structure together with the liquid phase film, so that the oscillation depth is maintained at a shallow oscillation level of 0.27~0.36mm.

[0038] The low-fluorine matrix, under the synergistic fluxing effect of B2O3, Li2O, Na2O, and BaO, still maintains a viscosity of 1300℃ within the range of 0.155–0.225 Pa·s and a transition temperature of 1075–1135℃. This viscosity-temperature window precisely matches the moderate casting speed of 0.9–1.2 m / min for nickel-saving high-nitrogen steel. The reversible configuration between [BO3] and [BO4] provides different degrees of network depolymerization effect in different temperature ranges, thereby regulating the flow activation energy of the melt; Li + As the network-modifying ion with the smallest radius, it strongly polarizes the Si–O bond and weakens the long-range network strength, along with Ba. 2+ The provided dual regulatory mechanism of "high polarization of small cations – high depolymerization of large cations" in the formation of a large number of non-bridging oxygens enables the system to even... Even when reduced to 1.2%~2.5%, it can still be used stably in industrial applications. In addition, the relay combustion of carbon black and flake graphite in different temperature ranges allows the slag layer to form interconnected pores as rapid slag-forming channels in the low-temperature range, while in the medium and high-temperature range, it relies on the isolation framework provided by flake graphite to delay premature liquid phase connection. This maintains a clear three-layer structure of upper powder slag – middle sintered slag – lower liquid slag, avoiding premature collapse of the slag layer structure.

[0039] After completely removing the nanocomposite additive in Comparative Example 1, the crystallization rate dropped sharply from 58.3% in Example 1 to 22.1%, the average grain size increased from 3.2 μm to 18.6 μm, the slag film tended to vitrify, and the radiative thermal resistance of the copper plate in the crystallizer was significantly insufficient, causing the heat transfer peak to shift forward and resulting in a surge in the billet concavity rate to 4.8%. This, from the opposite perspective, confirms the irreplaceable role of the nanocomposite additive as a heterogeneous nucleation center in regulating the crystal phase structure and constructing a reasonable thermal resistance distribution. In Comparative Example 2, the nano-TiO2 / h-BN composite was replaced with an equal amount of micron-sized TiO2. Due to the increase in TiO2 particle size by about 25 to 100 times, the specific surface area decreased sharply, and the effective nucleation sites per unit mass decreased drastically. In addition, the lack of h-BN sheet support as an anti-agglomeration barrier caused the micron-sized TiO2 particles to undergo dense and irreversible agglomeration during the slurry preparation stage. Instead, they became the preferred precipitation sites for coarse CaTiO3, resulting in increased viscosity and coarser crystalline phase. Consequently, the liquid slag layer was less than 8.5 mm thick, and the grinding rate reached 7.4%. Comparative Example 3 kept the formula unchanged but pre-melted and fed the nanocomposite additive in the S1 stage. TiO2 completely dissolved in the melt above 1400℃ and entered the silicon-oxygen network to become a network modifier, losing its heterogeneous nucleation ability at the solid-liquid interface. h-BN partially decomposed into B2O3 and N2 in the molten pool and escaped, resulting in the near disappearance of the active nucleating phase in the final slag film. The crystallinity was only 33.8% and the grains were coarse, which conversely proved the irreplaceable role of the post-S2 loading process in locking the functional sites of the nanophase. Comparative Example 4 followed the existing high-fluorine traditional technology route. Although the crystallinity reached 71.2%, its main crystal phase was coarse fluorapatite and wollastonite coarse grains (average size 22.4μm), with a loose structure and poor thermal stability. In addition, the excessively high This exacerbates the erosion and volatilization loss of the submerged entry nozzle, causing slag consumption per ton of steel to surge to 0.62 kg / t. The surface depression and grinding problems of the billet are even more severe than those of this invention. It is evident that relying solely on high-fluorine design or high crystallinity alone cannot simultaneously meet the complex process requirements of continuous casting of low-fluorine environmentally friendly and high-nitrogen nickel-saving steel grades. The system constructed in this invention, which combines low-fluorine multi-element flux with surface-loaded nanocomposite additives, can achieve a true balance between process window, surface quality, and environmental burden.

[0040] The above embodiments are only used to illustrate the technical solutions of the present invention in detail. Any modifications, equivalent substitutions or parameter adjustments made by those skilled in the art without departing from the concept and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-nitrogen, nickel-saving protective slag for continuous casting of stainless steel containing nano-additives, characterized in that, The chemical composition of the protective slag, by mass percentage, includes: CaO 32.0%~42.0%, SiO2 24.0%~34.0%, Al2O3 4.0%~12.0%, MgO 1.5%~5.0%, BaO 1.5%~6.0%, Na2O 3.0%~8.0%, Li2O 0.3%~1.8%, B2O3 1.0%~4.5%, 1.0%~3.0%, fixed carbon 1.0%~2.6%, nanocomposite additives 0.20%~1.20%, balance being unavoidable impurities; The nanocomposite additive includes nano TiO2 and nano h-BN, and the nano TiO2 is attached to the surface of the nano h-BN to form a composite structure.

2. The high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives according to claim 1, characterized in that, The chemical composition of the protective slag, by mass percentage, includes: CaO 36.0%~40.0%, SiO2 26.0%~30.0%, Al2O3 6.0%~10.0%, MgO 2.0%~4.0%, BaO 2.5%~5.0%, Na2O 4.5%~7.0%, Li2O 0.6%~1.3%, B2O3 1.5%~3.5%, 1.2%~2.5%, fixed carbon 1.2%~2.2%, nanocomposite additives 0.30%~0.80%; The mass ratio of CaO to SiO2 in the protective slag is 1.25 to 1.

50.

3. The high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives according to claim 1 or 2, characterized in that, The protective slag also includes the following components by mass percentage: MnO 0~1.5%, Fe2O3 ≤0.8%; The raw material for CaO is selected from at least one of calcite, limestone, cement clinker, and pre-melted material; the raw material for SiO2 is selected from at least one of wollastonite, glass powder, and quartz powder; the raw material for Al2O3 is selected from at least one of calcined bauxite and industrial alumina; The raw materials are selected from at least one of fluorite and cryolite.

4. The high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives according to claim 1, characterized in that, In the nanocomposite additive, the mass of nano-TiO2 accounts for 65% to 85% of the total mass of the nanocomposite additive, and the mass of nano-h-BN accounts for 15% to 35% of the total mass of the nanocomposite additive; The primary particle size of the nano-TiO2 is 20~80nm; the lateral dimension of the nano-h-BN sheets is 50~300nm, and the thickness is 10~80nm; the nano-TiO2 is attached in a dotted manner to the surface of the nano-h-BN sheets to form composite secondary particles.

5. The high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives according to claim 1, characterized in that, The fixed carbon is composed of carbon black and flake graphite; by mass percentage, the carbon black accounts for 35% to 60% of the total fixed carbon, and the flake graphite accounts for 40% to 65% of the total fixed carbon.

6. The high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives according to claim 1, characterized in that, The protective slag is in the form of hollow or porous granular powder, with a closed porosity of 10%~25% and a loose packing density of 0.75~1.00 g / cm³. 3 On the cross-section of the protective slag particles, the continuous phase is a CaO-SiO2-Al2O3 pre-melted glassy matrix, and the dispersed phase includes carbon black, graphite, and composite secondary particles formed by the nanocomposite additives.

7. A method for preparing a high-nitrogen, nickel-saving stainless steel continuous casting protective slag containing nano-additives as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Preparation of pre-melted glassy matrix: Inorganic raw materials other than fixed carbon, nanocomposite additives and granulation binder are mixed evenly, pre-melted and rapidly cooled to obtain glassy pre-melted material, which is then dried and pulverized to obtain pre-melted powder. S2, Post-loading of nanocomposite additives: Nano TiO2 and nano h-BN are dispersed in a composite binder to prepare a uniform slurry. The uniform slurry is then sprayed into the pre-melted powder for mixing, so that the nanocomposite adheres to the surface of the pre-melted material to form a post-loaded pre-melted material. S3. Introduction and granulation of low-carbon melting rate regulation system: The fixed carbon raw material is mixed with the post-loaded pre-melted material, sprayed into the composite granulation liquid, and after granulation and drying, the continuous casting protective slag is obtained.

8. The preparation method according to claim 7, characterized in that, In step S1, the mixing time is 8-15 minutes; The pre-melting treatment is performed at a temperature of 1380~1450℃ for 20~35 minutes. The rapid cooling is performed by water quenching or rapid cooling with steel strip; the particle size D50 of the pre-melted powder is 50~120μm.

9. The preparation method according to claim 7, characterized in that, In step S2, the composite adhesive is a lithium borate-sodium silicate composite adhesive with a solid content of 2% to 6%. The process of dispersing and preparing a uniform slurry includes: first ultrasonic dispersion for 15-30 minutes, and then high shear dispersion at 3000-6000 rpm for 5-15 minutes; The mixing is carried out at low temperature in a plow-type mixer or a high-speed mixer for 5 to 10 minutes; the particle size of the composite secondary particles in the resulting post-loaded pre-melted material is 0.5 to 5 μm.

10. The preparation method according to claim 7, characterized in that, In step S3, the composite granulation liquid is a starch-carboxymethyl cellulose composite granulation liquid with a solid content of 3% to 8%, wherein the mass ratio of starch to sodium carboxymethyl cellulose is 3:1; the granulation is carried out by disc granulation or high-intensity stirring granulation, and the moisture content of the material is controlled to reach 8% to 14% after the composite granulation liquid is sprayed in; The particle size obtained by granulation is 0.3~1.2mm; The drying process is carried out at a temperature of 180~240℃ for 20~40 minutes.

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