A low-carbon boron-containing covering flux for G115 heat-resistant steel large round billet continuous casting

Abstract

A low-carbon boron-containing protective slag for G115 heat-resistant steel large round billet continuous casting, which has the following chemical components by weight percentage: CaO: 29-38%, SiO2: 26-34%, Al2O3: 5-8%, Na2O: 5-8%, NaF: 6-11%, Li2O: 2-4%, MgO <= 2%, Fe2O3 <= 1%, MnO: 3-6%, B4C: 1.8-2.7%, C <= 0.1%, and the balance is inevitable impurities, wherein the particle size of B4C is 45-75 mu m. The present application realizes precise boron supplementation while preventing carbon deposition, thereby meeting the stringent requirements of G115 continuous casting, and producing G115 large-size continuous casting round billet products of phi 600-phi 1200 mm to meet the requirements of CSTM 00017-2021 and Q / OAPD 2753-2022, Q / OAPD 2253-2022 and other standards.

Description

Technical Field

[0001] This invention belongs to the field of auxiliary materials technology in iron and steel metallurgy, and specifically relates to a low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets. Background Technology

[0002] With rapid economic development, the demand for energy is increasing. Thermal power generation is an important component of my country's energy mix, but it also brings a series of environmental pollution problems. Therefore, the need for high-efficiency, low-emission, high-parameter thermal power units is becoming increasingly urgent. Higher steam temperature and pressure parameters in coal-fired power generation result in lower coal consumption and fewer pollutant emissions, but also place higher demands on material performance. The P92 material used in 600℃ ultra-supercritical units is no longer sufficient for higher-parameter units; and to construct ultra-supercritical units with parameters of 630℃ or higher, the high-temperature resistance of materials must reach 650℃.

[0003] G115 is a novel martensitic heat-resistant steel (see Chinese patent CN103045962B). This heat-resistant steel adopts a composite strengthening principle, containing multiple strengthening elements such as W, Co, Cu, B, and N. Its high-temperature long-term aging stability, high-temperature creep strength, and oxidation resistance are significantly higher than P92. It is suitable for long-term use under high-pressure conditions at temperatures below 650℃. Comparative studies have found that G115's creep strength and oxidation resistance at 650℃ are superior to P92's performance at 600℃. The existing process route of ingot casting + (electroslag) + forging + pipe manufacturing is basically mature. G115 has also achieved its first engineering application in the world's first 630℃ ultra-supercritical demonstration project.

[0004] To improve yield and further refine the process, the G115 continuous casting process was developed. However, due to the high alloy content, low carbon content (0.060~0.100%), and boron content (0.008~0.022%) of G115, the mold flux plays a crucial role in continuous casting. It must possess functions such as preventing steel oxidation, absorbing inclusions, ensuring the supply of key components (such as carbon and boron), providing thermal insulation, lubrication, and heat transfer. Ordinary mold fluxes cannot meet these requirements.

[0005] Chinese patent CN103045962B mainly involves the composition design, strengthening concept, manufacturing method, and performance characteristics of G115, but the technology does not involve the continuous casting protective slag specifically for G115.

[0006] Chinese patent CN102000793A discloses a protective slag for continuous casting of large-section round billet pipeline steel, but does not involve a continuous casting protective slag specifically for G115.

[0007] Chinese patent CN106475536 discloses a mold flux for P91 continuous casting and its preparation method, but does not involve a continuous casting flux specifically for G115.

[0008] Chinese patent CN117900400A discloses a mold flux for high-speed, low-carbon steel continuous casting based on MgO replacing Li2O and low Na2O. It mainly involves the low-cost production of MgO replacing Li2O, but the carbon content is between 1.0% and 4.5%, and it does not contain boron, making it unsuitable for G115 continuous casting.

[0009] Chinese patent CN117182012A discloses a protective slag for continuous casting with a diameter of φ1000~1200mm. It mainly modifies the protective slag by increasing Al2O3 and MgO, but does not involve low carbon or boron content, and is not suitable for G115 continuous casting.

[0010] Chinese patent CN116967411A discloses a protective slag for continuous casting of low-carbon steel, but its carbon content is 2-4% and it does not contain boron, making it unsuitable for G115 continuous casting.

[0011] Chinese patent CN104624997A discloses a protective slag for continuous casting crystallizers of boron-containing steel, comprising the following raw materials in parts by weight: 3-7 parts glass powder, 40-55 parts premelted material, 27-35 parts wollastonite, 2 parts carbon black, 4-5 parts graphite, 5-9 parts sodium carbonate, 2.5-5.5 parts sodium fluoride, 2-4 parts bauxite, 2-5 parts lightly calcined magnesia, 5-8 parts boron carbide, and 1.2 parts binder. The basicity of the protective slag (CaO / SiO2) is 1.1-1.2, the melting point is 1030-1090℃, and the viscosity at 1300℃ is 0.15-0.25 Pa·s. This protective slag is designed for boron-containing steel with a boron content of 0.001-0.003%, while the actual boron content of G115 is generally 0.010-0.018%, which is much higher than the boron-containing steel involved in this patent. In addition, G115 has a low carbon content, while the protective slag has a carbon content of 2-5%, which can easily cause carbon increase. Its melting point is too low (1030-1090℃) and its viscosity is too high (0.40-0.44Pa·s), making it unsuitable for continuous casting of G115.

[0012] Chinese patent CN116586576A discloses a protective slag for low-carbon steel in continuous casting of large round billets, which has a carbon content of 11-17% and does not contain boron, making it unsuitable for G115 continuous casting.

[0013] Currently, existing protective slag technologies are only suitable for heat-resistant steels with low alloy content, such as P91 and P92, or boron-containing steels with low boron content. They also generally have a high carbon content (2-5%) and lack any specific function to address boron loss, thus failing to meet the requirements of G115. Therefore, to meet the special requirements of continuous casting of G115 steel with high alloy content (viscous molten steel, numerous and varied inclusions), low carbon content (within the peritectic steel range, large phase transformation volume shrinkage, and a high tendency to generate longitudinal cracks in continuously cast billets; ordinary protective slags can have a carbon content of 3-5%, easily causing carbon enrichment), and containing boron (boron is extremely reactive), there is an urgent need to develop a dedicated protective slag suitable for the continuous casting of large-size round billets of G115 steel. Summary of the Invention

[0014] The purpose of this invention is to provide a low-carbon boron-containing protective slag for continuous casting of large round billets of G115 heat-resistant steel, which can prevent carbon increase while accurately replenishing boron, thereby meeting the stringent requirements of G115 continuous casting and producing large-size continuous casting round billets of G115 with diameters of φ600~φ1200mm that meet the requirements of standards such as CSTM 00017-2021, Q / OAPD 2753-2022, and Q / OAPD 2253-2022.

[0015] To achieve the above objectives, the technical solution of the present invention is as follows: A low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets has the following chemical composition by weight percentage: CaO: 29~38%, SiO2: 26~34%, Al2O3: 5~8%, Na2O: 5~8%, NaF: 6~11%, Li2O: 2~4%, MgO≤2%, Fe2O3≤1%, MnO: 3~6%, B4C: 1.8~2.7%, C≤0.1%, with the balance being unavoidable impurities. The B4C particle size is 45~75μm.

[0016] Preferably, the CaO / SiO2 ratio is 1.1 to 1.3.

[0017] Preferably, the melting point of the protective slag is 1050~1150℃.

[0018] Preferably, the viscosity of the protective slag at 1300℃ is 0.15~0.25 Pa·s.

[0019] Preferably, the initial carbon content of all raw materials for the protective slag is ≤0.1%.

[0020] Preferably, the protective slag is double-vacuum packaged during transportation and filled with argon gas.

[0021] The protective slag composition design features of this invention are as follows: 1. Boron is extremely chemically reactive at high temperatures, readily combining with oxygen and nitrogen in molten steel to form stable oxides (B₂O₃) and nitrides (BN). These compounds cause boron to lose its alloying properties. The addition of boron carbide (B₄C) to the protective slag in this invention has the following effects: I. Achieving Precise Boron Supplementation. The mechanism of boron carbide is to continuously and stably release boron into the molten steel within the temperature range of 1520~1650℃ during the continuous casting process of G115, thereby achieving precise boron supplementation and ensuring that the steel obtains an appropriate amount and uniformly distributed solid solution boron.

[0022] This invention, through research, discovered that adding excessive boron carbide to the protective slag negatively impacts its melting and lubrication properties, leading to drastic viscosity fluctuations in the later stages of casting and affecting the quality of the continuously cast billet. This invention controls the boron carbide (B4C) particle size to 45-75 μm (325-200 mesh), ensuring a slow, continuous, and uniform release of boron during continuous casting, avoiding the formation of localized boron-rich zones, and achieving precise boron replenishment for high-boron steel. This ensures that even with a low boron carbide (B4C) content of 1.8-2.7%, the boron replenishment requirements for G115 steel can be met. Overly coarse boron carbide (B4C) particles can lead to uneven release and potentially form localized boron-rich zones, while overly fine particles are easily carried away by the gas flow, reducing utilization.

[0023] II. Improved Crack Sensitivity. Boron carbide itself has excellent heat-insulating properties, which can increase the surface temperature of the billet and thus improve high-temperature plasticity. This directly avoids the risk of cracks forming in weak areas such as troughs of oscillation marks due to insufficient thermoplasticity. Boron carbide has a strong shielding effect, which can effectively prevent heat loss from the billet and increase the total heat of the billet. This not only helps to improve the overall heat-insulating performance of the protective slag, but also optimizes the lubrication effect of the billet, further reducing the risk of crack formation.

[0024] Compared with boron-containing materials such as ferroborone and boron oxide commonly used in steelmaking, boron carbide has significant advantages, especially in the specific high-temperature and dynamic process environment of continuous casting. The outstanding advantages of boron carbide are its functional durability, compositional stability, and impact on the quality of molten steel.

[0025] First, boron carbide has an extremely high melting point of 2350℃. During the melting process of the protective slag, it does not decompose immediately but releases active boron slowly and continuously under the influence of high-temperature molten steel. In contrast, ferroboron dissolves at only 1380℃, melting and releasing boron earlier and faster in the protective slag, making it unsuitable for use in protective slag and only suitable for the G115 smelting process. Boron oxide has an even lower melting point of only 450℃, dissolving in the slag even more rapidly, resulting in a concentrated and rapid release of boron. Therefore, the chemical stability and boron release mechanism of boron carbide are more suitable for use in protective slag, ensuring a continuous replenishment of boron in the meniscus region of the molten steel throughout the continuous casting process, providing a more stable and lasting effect against the boron consumed by oxygen and nitrogen.

[0026] Secondly, regarding the impact on the melting point and viscosity of the protective slag, boron carbide itself is infusible; its decomposition product, B₂O₃, acts as a flux. Therefore, its initial impact on the basic melting point of the protective slag is small, which is beneficial for maintaining the designed slag structure. Ferroboron, as a pre-melted alloy, directly lowers the melting point of the protective slag, potentially altering its designed melting performance. Boron oxide has a more drastic impact on the protective slag; as a strong flux, it significantly lowers the melting point and viscosity, potentially causing premature melting, loss of the insulating layer effect, or loss of viscosity control. Therefore, boron carbide is more suitable for use in protective slags, providing greater flexibility and stability for precise control of the protective slag formulation and avoiding damage to the slag's melting characteristics due to premature melting of the additives.

[0027] Third, regarding potential contamination of molten steel, boron carbide, a pure compound, contains little or no impurity elements (such as Si, Al, P, S, etc.) and will not introduce harmful impurities into the molten steel. Ferroboron, being an iron alloy, inevitably contains impurities such as silicon, aluminum, phosphorus, and sulfur. These impurities may be absorbed by the molten steel, affecting the cleanliness of high-purity steel grades like G115. Boron oxide is an oxide, but excessive or improper use may increase the oxidizing properties of the slag, posing a slight risk of re-oxidation to the molten steel. Therefore, ensuring the purity of molten steel is crucial for high-quality steels like G115. Boron carbide is a more suitable boron source.

[0028] Fourth, in terms of the operability and stability of protective slag, boron carbide has stable physicochemical properties, is easy to store, transport, and mix, and is not prone to clumping or deterioration. While ferroboron is also stable, its impurity content cannot be ignored. Boron oxide strongly absorbs moisture from the air and deliquesces, forming boric acid (H3BO3). This poses significant difficulties for the storage and mixing of protective slag, and may lead to increased hydrogen content in the molten steel due to moisture introduction, which is detrimental to quality. Clearly, boron carbide is more suitable for use as protective slag.

[0029] Finally, from a cost and efficiency perspective, boron carbide has a high boron content per unit (approximately 78%) and high utilization rate. Although the unit price may be higher, its overall cost-effectiveness is superior. Ferroboron has varying boron content per unit (typically 10%-20%) depending on the grade and contains a large amount of iron. Boron oxide has a relatively high boron content per unit (approximately 31%), but due to its instability and hygroscopicity, its actual effective utilization rate may be lower. Therefore, boron carbide has a greater advantage in terms of efficiency and stability.

[0030] 2. The role of other components in the protective slag: G115 is a typical peritectic steel. The primary function of the protective slag is to inhibit surface longitudinal cracks caused by the peritectic reaction. The protective slag must have strong crystallization ability to form stable and uniform high-resistance crystals. CaO and SiO2 will form crystals with high melting point and high thermal resistance (gunnerite 3CaO·2SiO2·CaF2). Therefore, the protective slag contains 29~38% CaO and 26~34% SiO2, which is the basis of the protective slag. CaO is the main alkaline oxide, which adjusts the alkalinity and improves the melting rate and fluidity of the protective slag. SiO2 is the main acidic oxide, which forms a silicate network structure with CaO and controls the viscosity and crystallization performance of the protective slag.

[0031] The main function of Al2O3 is to improve the stability of the protective slag, inhibit excessive crystallization of the protective slag in the crystallizer, and ensure lubrication performance. The Al content in G115 is 0.07~0.012%. The Al2O3 in the protective slag mainly acts as a stabilizer. An appropriate amount of Al2O3 can promote crystal nucleation, but an excessive amount will form high-melting-point spinel with MgO, which will deteriorate the lubricity of the protective slag. Therefore, the Al2O3 content in the protective slag is 5~8%.

[0032] Na₂O is a strong fluxing agent that significantly lowers the melting point and viscosity of the protective slag, improving its fluidity. The melting point of the G115 continuous casting protective slag needs to be between 1050 and 1150°C, slightly lower than the molten steel temperature, to ensure that the protective slag can melt in time to form a liquid slag layer. - NaF acts as a flux, introduced into the protective slag. It also helps reduce viscosity, but the amount added must be limited to avoid corrosion of the equipment. Li₂O is a strong flux; a small amount can significantly lower the melting point and viscosity, increasing the melting rate of the protective slag. Considering both melting point and viscosity, the content of Na₂O in the protective slag is 5-8%, NaF 6-11%, and Li₂O 2-4%. The viscosity of the protective slag (at 1300℃) is controlled at 0.15-0.25 Pa·s to ensure good fluidity, facilitating flow into the gap between the billet and the crystallizer to form a uniform slag film. Fe₂O₃ is an impurity and must be strictly controlled.

[0033] MnO is an effective flux that significantly reduces the melting point, solidification temperature, and high-temperature viscosity of the protective slag. It helps form a stable and uniform liquid slag film within the crystallizer, providing excellent lubrication, preventing the billet from sticking to the crystallizer, and effectively inhibiting the precipitation of high-melting-point crystals in the protective slag. This promotes the formation of more glassy phase in the slag, which is beneficial for maintaining the lubrication and heat transfer stability of the slag film. MnO reduces the thermal conductivity of the protective slag, helping to control the heat flux density within the crystallizer and preventing excessively rapid cooling of the billet shell. Therefore, 3-6% MnO is added to the protective slag to fully utilize its fluxing and anti-undesirable crystallization effects, while avoiding excessive dilution and deterioration of lubrication performance due to excessive content.

[0034] 3. To meet the requirements of creep strength and microstructural stability under high-temperature service conditions of 630~650℃, carbon content is a key factor. Therefore, the carbon content of G115 steel is controlled between 0.060~0.100%. Any externally introduced carbon, even in trace amounts, may disrupt the precise alloy composition design of G115, increase carbide precipitation, and affect tensile, impact, and other mechanical properties, as well as weldability. During continuous casting, the protective slag comes into direct contact with the high-temperature molten steel, posing a risk of carbon dissolving from the protective slag into the molten steel, thus increasing carbon content. Therefore, the carbon content of the G115-specific protective slag of this invention must be strictly limited to ≤0.1%. All raw materials used to prepare this protective slag (such as lime, quartz sand, etc.) must undergo rigorous chemical analysis to ensure extremely low initial carbon content. During the manufacturing process of the protective slag, a clean production environment and processes must be adopted to avoid any steps that may introduce carbon contamination. The finished protective slag should be double-vacuum packaged and filled with high-purity argon gas for protection to isolate it from air and moisture, preventing accidental carbon increase during storage and transportation.

[0035] 4. Lubricating properties of the protective slag. This can be achieved by adjusting the basicity (CaO / SiO2) and adding fluxes (Na2O, F). - The flux (containing Li₂O) controls the viscosity and crystallization properties of the protective slag, enabling it to form a uniform and stable liquid slag layer within the crystallizer. This effectively lubricates the cast billet, reduces friction, and prevents sticking and leakage. The basicity of this protective slag is 1.1~1.3, classifying it as a high-basicity protective slag. It has a high CaO content and is dominated by network modifiers. 2+ Ions break numerous Si-O-Si bonds, depolymerizing the complex network into simple isolated islands of [SiO4]. 4- Small units such as tetrahedrons or dimers simplify and loosen the structure, allowing ions to flow more easily, thus reducing viscosity. This is further enhanced by the addition of fluxes (Na₂O, F₂). -The addition of Li₂O will further reduce viscosity. High-basicity slag has a simple ionic structure, facilitating atomic rearrangement into crystals. The high CaO content creates thermodynamically favorable conditions for the precipitation of high-melting-point calcium-containing crystals (such as wollastonite CaO·SiO₂ and lanceolite 3CaO·2SiO₂·CaF₂). Therefore, high-basicity protective slag exhibits strong crystallization ability and a high crystallization rate.

[0036] 5. In terms of heat transfer control, the crystallization properties of the protective slag directly affect the heat transfer of the billet. By controlling the Al2O3 content and basicity, the crystallization tendency of the protective slag can be adjusted so that it forms an appropriate crystal layer in the crystallizer, and the heat transfer rate of the billet can be uniformly controlled to avoid surface cracks of the billet due to uneven heat transfer.

[0037] 6. In terms of stability, the addition of Al2O3 and MgO helps to improve the chemical stability of the protective slag, reduce compositional fluctuations during continuous casting, and ensure the long-term stability of the protective slag performance.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows: In the continuous casting process of steel, the protective slag plays a crucial role. This invention, specifically designed for G115 steel, addresses this issue by creating a dedicated protective slag for the G115 continuous casting process. By strictly controlling the carbon content to ≤0.1%, it avoids carbon enrichment in the molten steel. The addition of a very small amount of B4C achieves stable boron replenishment, excellent heat preservation, and optimized lubrication, meeting the boron replenishment requirements of high-boron-content G115 steel and solving the cracking problem in G115 and other steel grades. This provides strong support for improving the internal and surface quality of the cast billet. Furthermore, by adding an appropriate amount of MnO, the melting point, solidification temperature, and high-temperature viscosity of the protective slag can be lowered, thereby improving its lubricity and heat transfer stability.

[0039] This invention uses a low-carbon, boron-containing protective slag to maintain the content of key elements such as C and B in molten steel. Combined with the control of other components, the basicity of the protective slag is controlled at 1.1~1.3 and the viscosity at 0.15~0.25 Pa·s, which provides good lubrication, heat transfer and stability for the continuous casting of G115 steel, reduces the generation of longitudinal cracks on the surface of large-size continuous casting billets of G115 steel and improves the quality of the billets.

[0040] Compared to Chinese patent CN104624997A (which contains 5-8 parts boron carbide, and calculations show its proportion in the formulation system is relatively high, significantly higher than the 1.8-2.7% of this invention), the boron carbide content in the protective slag of this invention is significantly reduced. Combined with the control of boron carbide particle size, precise boron supplementation is achieved, avoiding the impact of excessive boron carbide addition on the melting point and viscosity of the protective slag, making it more suitable for continuous casting of G115 steel. Research has found that adding too much boron carbide negatively affects the melting and lubrication properties of the protective slag, causing drastic fluctuations in viscosity during the later stages of G115 casting, thus affecting the quality of the continuously cast billet. Detailed Implementation

[0041] The present invention will be further described below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0042] The composition of the protective slag used in the embodiments of the present invention is shown in Table 1, with the remainder being unavoidable impurities.

[0043] The comparative example used P91 protective slag to produce G115 continuous casting billets. The chemical composition of this protective slag by weight percentage was: SiO2: 26.38%, CaO: 31.76%, Li2O: 0.58%, Al2O3: 4.06%, Na2O: 8.83%, F - 7.56%, MgO: 2.21%, Fe2O3: 0.69%, C: 8.31%, volatile matter: 9.62%.

[0044] Using the protective slag described in the embodiments and comparative examples of this invention, G115 steel was produced by continuous casting. The results of the horizontal low-magnification inspection of the continuous casting billets are shown in Table 3. As can be seen from Table 3, the G115 continuous casting billets produced by the protective slag of this invention have no central cracks, surface cracks, or central porosity. The shrinkage cavity is grade 1, and the general porosity is grade 0.5, which is better than the G115 continuous casting billets produced by using P91 protective slag, whose central cracks and central porosity are both grade 0.5, shrinkage cavities are grade 2, and general porosity is grade 1.0. The comparison shows that the protective slag of this invention can significantly improve the quality of the continuous casting billets, especially the quality of the core of the continuous casting billets.

[0045] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

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Claims

1. A low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets, characterized in that, Its chemical composition by weight percentage is as follows: CaO: 29~38%, SiO2: 26~34%, Al2O3: 5~8%, Na2O: 5~8%, NaF: 6~11%, Li2O: 2~4%, MgO≤2%, Fe2O3≤1%, MnO: 3~6%, B4C: 1.8~2.7%, C≤0.1%, with the balance being unavoidable impurities. The particle size of the B4C is 45~75μm.

2. The low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets as described in claim 1, characterized in that, The CaO / SiO2 ratio is 1.1~1.

3.

3. The low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets as described in claim 1, characterized in that, The melting point of the protective slag is 1050~1150℃.

4. The low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets as described in claim 1, characterized in that, The viscosity of the protective slag at 1300℃ is 0.15~0.25 Pa·s.

5. The low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets as described in claim 1, characterized in that, The initial carbon content of all raw materials used in this protective slag is ≤0.1%.

6. The low-carbon boron-containing protective slag for continuous casting of G115 heat-resistant steel large round billets as described in claim 1, characterized in that, The protective slag is double-vacuum packaged during transportation and filled with argon gas.

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