Discussion on the relationship between steel grades and continuous casting slag protectants

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Continuous casting slag protection technology, as one of the key technologies in continuous casting production, has a crucial impact on the smooth progress of this process as well as on the quality of the cast billets. In particular, surface defects in the cast billets are basically formed within the mold, and they are directly related to the slag protection used. In recent years, this technology has, in practical applications such as the development and widespread use of hollow particle slag, played a significant role in improving the quality of cast billets and stabilizing the continuous casting production process. At the same time, research on the basic properties of slag, such as its lubrication and heat transfer characteristics, has also received continuous attention. I. Design requirements for the properties of fluxes based on different steel grades: Steels with different compositions have distinct characteristics of their molten steel as well as differences in their solidification patterns, which in turn determine the requirements regarding the properties of the fluxes used. 1. Low-carbon steel: First, the w(C) in the steel is less than 0.08% or 0.06%. These steels exhibit good mechanical properties at high temperatures; there are no significant volume changes due to phase transformations during solidification, and they have low sensitivity to internal stresses and cracks. Therefore, they are usually produced at higher drawing speeds in order to improve productivity. Based on the solidification characteristics and quality requirements of low-carbon steel itself, the design primarily takes into account the lubrication and consumption of slag. A higher pulling speed requires an increase in the heat flow to the mold in order to accelerate the solidification of the molten steel and prevent sticking and metal leakage. This necessitates that the crystallization temperature of the slag be low while its solidification temperature be moderate, so as to ensure that the slag used in the molds for low-carbon steel remains in an amorphous state at temperatures above 950°C, thereby minimizing the risk of sticking and metal leakage. During high-speed pouring, in order to allow sufficient amount of liquid slag to flow into the area between the molten metal flow and the inner surface of the mold, ensuring good lubrication and adequate consumption, the viscosity of the slag is typically chosen to be in a lower range. Furthermore, in such steel grades, the primary ferrite shell has low segregation and high strength; moreover, the vibration marks on the cast billet are deep. Therefore, a slag with good heat-retaining properties should be used to raise the temperature of the primary shell at the meniscus, which helps to mitigate the risks associated with overly deep vibration marks. Therefore, for continuous cast low-carbon steel to meet the above requirements, it is necessary to design a slag with certain heat transfer properties, good thermal insulation properties, good non-metallic absorption capacity, good lubricating properties, and stable performance. 2. Medium carbon steel: During the solidification of medium carbon steel, a δ→γ phase transformation occurs, resulting in significant volume contraction. This type of steel is highly sensitive to cracks and prone to surface cracks, especially at high drawing speeds. Avoiding longitudinal and transverse cracks is the primary concern; therefore, the focus of designing fluxes for medium-carbon steel should be on controlling the heat flow from the ingot to the mold, limiting the heat flux in the mold, and ensuring that the flux has a high thermal resistance. Therefore, a slag with a high solidification temperature and a high crystallization temperature should be selected; the \"air gaps\" in the crystalline film help to slow down the heat transfer rate of the slag, which in turn helps to reduce the thermal stress generated during the cooling of the cast slab. 3. High-carbon steel: This type of steel is characterized by poor thermal strength ； The casting temperature and casting speed are low ； At the same time, it is prone to bonding and steel leakage. High-carbon steel tends to stick, which is related to the small solidification shrinkage of the initially formed shell. Therefore, the focus of designing flux for high-carbon steel should be on ensuring lubrication. To this end, the viscosity and solidification temperature of this flux should be low, and its tendency to glassify should be high, in order to ensure good lubricating properties; however, the poor thermal strength of high-sulfur steel also needs to be taken into account, requiring appropriate adjustment of the thermal resistance of the flux. Furthermore, due to the low liquidus temperature of high-carbon steel, the casting temperature is lower compared to other types of steel; therefore, the design of the flux must take this temperature into account. To prevent the molten steel from freezing, high-carbon steel requires flux with good thermal insulation properties and a low bulk density, and the carbon content can be somewhat higher, even reaching around 20%. 4. Special steels: The composition of the molten steel in special steels varies greatly. The formulation of the fluxes used for these types of steel is relatively complex, and it is often tailored according to the intended use of the steel and the potential defects that may occur. Examples include stainless steel, silicon steel, as well as steels containing Nb, V, Ti, and other elements. II. Design requirements for the properties of mold flux based on continuous casting process parameters 1. Pulling speed The pulling speed of a continuous casting machine is one of the important process parameters in continuous casting production. 　 As the drawing speed increases, the consumption of flux decreases. The consumption of the slag protectant is a measure of the average amount of liquid slag that penetrates into the gap between the casting slab and the mold, and therefore it serves as an important process control parameter; its value is generally required to be above 0.3 kg/m2. An increase in the pulling speed leads to a reduction in the consumption of the slag shield; insufficient consumption of this slag shield results in poor lubrication and heat transfer conditions in the cast slab. Therefore, when designing a slag shield for high-speed continuous casting, it is necessary to increase its melting speed, reduce its viscosity and solidification temperature, in order to improve the flow properties of the liquid slag and meet the requirements regarding its consumption. At the same time, to increase the melting rate, it is necessary to reduce the packing density, lower the carbon content and increase the carbonate content, as well as select appropriate raw materials with suitable physical properties. 　 As the pulling speed increases, the heat flow to the crystallizer increases. This is because as the drawing speed increases, the residence time of the molten steel in the mold decreases, the temperature of the shell increases, and the thickness of the solidifying shell thinns. As a result, the static pressure of the molten steel more easily causes the shell to come into contact with the walls of the mold, which facilitates heat transfer and leads to an increase in the heat flux density as the drawing speed rises. Therefore, for high-speed continuous casting fluxes, the solidification temperature and crystallization temperature should be appropriately reduced to decrease the thickness of the slag film and ensure good heat transfer in the mold; at the same time, the critical heat flux values for different steel grades must also be taken into account. 2. Cross-sectional shape of the billet 　The difference between square billets and slab billets lies first in the ratio of the surface area to the volume within the mold cavity, that is, the specific surface area. The slab sizes are 220mm×1500mm; the thin slabs represented by 2 and 3 have sizes of 100mm×1000mm and 50mm×1300mm, while the small square billets have sizes of 160mm×160mm and 130mm×130mm. The drawing speed for both slabs and small square billets is approximately 1.5 m/min. In summary, as the specific surface area of the cast ingot increases, the consumption of flux per unit area (kg/m2) decreases sharply, and the specific surface area of slabs is smaller than that of square billets. Due to the smaller specific surface area of slabs, the consumption of flux is rapid, which in turn requires a faster melting speed; therefore, the flux designed for slabs has a faster melting speed than that designed for billets. Furthermore, billets are not very sensitive to the viscosity of the flux used; therefore, high-viscosity fluxes are often employed to reduce slag inclusions and erosion of the submerged entry nozzles. This is because billets have a large specific surface area, resulting in lower requirements for flux consumption (kg/m2), which makes it easier to meet these requirements during the continuous casting process. Secondly, due to the large fluctuations in the liquid level in the width direction during slab continuous casting, it is necessary for the protective slag to melt at a rapid rate, so as to create a sufficient thickness of slag layer that can cover the entire surface of the molten steel. Additionally, slab continuous casters are mainly used to produce low- and medium-carbon steels, while billet casters are used to produce not only medium-carbon steels but also a wide range of high-carbon steels.