Key Takeaways & Executive Findings
- •• 3D X-ray microscopy reveals that increasing slag basicity from 0.52 to 1.03 and temperature from 1423 K to 1573 K reduces viscosity and accelerates bubble floating rates. • Longer holding times (10 to 30 s) increase bubble floating distance and decrease the volume fraction and average equivalent sphere diameter of bubbles solidified in mold flux. • Higher basicity, temperature, and holding time enhance the removal rate of bubbles, particularly for larger bubbles, improving mold flux performance. • The findings provide critical insights for optimizing low basicity mold flux formulations for high-Mn high-Al steel continuous casting, potentially reducing defects and improving process stability.
Abstract
During the continuous casting process of high-Mn high-Al steels, various types of gases such as Ar need to escape through the top of the mold. In which, the behavior of bubbles traversing the liquid slag serves as a restrictive link, closely associated with viscosity and the thickness of liquid slag. In contrast to two-dimensional surface observation, three-dimensional (3D) analysis method can offer a more intuitive, accurate, and comprehensive information. Therefore, this study employs a 3D X-ray microscope (3D-XRM) to obtained spatial distribution and 3D morphological characteristics of residual bubbles in mold flux under different basicity of liquid slag, different temperatures, and different holding times. The results indicate that as basicity of slag increases from 0.52 to 1.03, temperature increases from 1423 to 1573 K, the viscosity of slag decreases, the floating rate of bubbles increases. In addition, when holding time increases from 10 to 30 s, the bubbles floating distance increases, and the volume fraction and average equivalent sphere diameter of the bubbles solidified in the mold flux gradually decreases. In one word, increasing the basicity, temperature, and holding time leading to an increase in the removal rate of bubbles especially for the large. These findings of bubbles escape behavior provide valuable insights into optimizing low basicity mold flux for high-Mn high-Al steels.
1. Introduction
Mold flux plays key roles during the continuous casting process, such as isolating air, absorbing inclusions, lubricating strand, and controlling heat transfer between the mold and steel shell. During the continuous casting of high-Mn high-Al steels, [Al] and [Mn] in the steel are easy to react with SiO2 in the slag [1–2], causing changes in composition of slag, inducing the deterioration performance of the mold flux, which will affect the lubrication and heat transfer ability between the slab and the mold flux [3]. In severe cases, the breakout accident may occur. Therefore, peoples focus their main mind on reducing these reactions [4–5].
However, the behavior of bubbles escaping from the mold flux is also worthy of attention. During the continuous casting of high-Mn high-Al steels, various gases are blown in or produced. For example, the argon are blown into the mold through the nozzle, which can prevent the nozzle from blocking, homogenize the composition of molten steel, protect the casting, and promote the floating of inclusions [6]; the volume fraction of air in the hollow ball slag is more than 60%; CO and CO2 are generated during the combustion of graphite and carbon black in the carbon-rich layer [7]; SO2 and CO2 are generated because of decomposition of sulfate and carbonate [8–9]. These gases need to escape through the slag, always with a small part captured in the slag and entering into the slag channel. If the gases are too much, the mold flux will be overturned, and then [Al] in molten steel will react with the oxygen in the air to form Al2O3, which may also change the composition of slag [10]. Besides, the bubbles in the slag film will change the thermal conductivity of slag, act as the core of heterogeneous nucleation during the solidification of liquid slag and promote crystallization, and then affect the radiation heat transfer and conduction heat transfer [11–12].
The structure of mold flux is divided into powder layer, sintering layer, and liquid slag layer. Lots of connected holes in the powder layer and sintering layer, therefore, the resistance of gases passing through connected holes is negligible. Although the thickness of the liquid slag layer is thin, usually 7–15 mm, the resistance of gases passing through liquid slag is the limiting stage of gases escape, and excessive resistance will lead to slag turning and level fluctuation [13–14]. The bubbles contact each other and grow up during rising upward in the liquid layer, but the sizes of which usually small when solidified in the liquid firm. Generally, it is necessary to observe by optical microscope or electron microscope. However, whether optical microscope or electron microscope, only obtains the two-dimensional (2D) images of bubbles on the surface of the sample and takes a lot of time to obtain the statistical data of a large number of bubbles, and the three-dimensional (3D) distribution of bubbles cannot be observed [15–16]. Therefore, an intuitive, accurate, comprehensive, and efficient experimental method is needed to study the escape behavior of bubbles in mold flux.
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Qiang Liu, Xiang Li, Shen Du, Ming Gao, Yanbin Yin, Jiongming Zhang (2025). Investigation of bubbles escape behavior from low basicity mold flux for high-Mn high-Al steels using 3D X-ray microscope. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-2896-6
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Frequently Asked Questions
What is the main objective of this study?
The study aims to investigate the escape behavior of bubbles from low basicity mold flux used in high-Mn high-Al steel continuous casting, using 3D X-ray microscopy to analyze the spatial distribution and morphology of residual bubbles under varying slag basicity, temperature, and holding time.
How does slag basicity affect bubble escape?
Increasing slag basicity from 0.52 to 1.03 decreases slag viscosity, which accelerates the floating rate of bubbles, thereby enhancing their removal from the mold flux.
What is the role of temperature in bubble behavior?
Raising the temperature from 1423 K to 1573 K reduces slag viscosity, leading to faster bubble floating and improved bubble removal efficiency.
How does holding time influence bubble removal?
Extending holding time from 10 to 30 seconds increases the distance bubbles float and decreases the volume fraction and average equivalent sphere diameter of bubbles solidified in the mold flux, indicating better bubble removal.
Why is 3D X-ray microscopy used in this study?
3D X-ray microscopy provides an intuitive, accurate, and comprehensive method to observe the three-dimensional distribution and morphology of bubbles, overcoming the limitations of traditional 2D surface observation techniques.
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