A Review of Mold Flux Development for the Casting of High-Al Steels
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MOLD flux has been widely used in the process of continuous casting, and plays important roles such as providing the thermal insulation, protecting the molten steel from oxidation, absorbing inclusions, lubricating the strand, and controlling heat transfer between the mold and steel shell.[1–4] With the development of advanced high strength steel (AHSS) that aims to reduce vehicle weight and improve mechanical properties of special steels, aluminum as an effective additive has been generally used for the new generation steel development, such as transformation induced plasticity (TRIP) steels and twin-induced plasticity.[5,6] However, with the addition of aluminum in the molten steel, it tends to react with the major component silica in the mold slag to form alumina in the traditional lime-silica-based mold flux during the process of continuous casting, which could in turn cause the picking up of alumina content in the slag system and deteriorate mold flux properties such as viscosity, crystallization and melting temperature.[7–11] The changes of mold flux properties would lead to unstable heat transfer and insufficient lubrication, which then results in severe surface defects like cracks and depressions.[12–14] Therefore, it is essential and eager to solve above issues. The development of new generation mold flux for the casting of high-Al steels was conducted intensively and it could be divided into two strategies. (1) Traditional reactive mold flux, it is conventional CaO-SiO2-based mold flux with low basicity and low viscosity. (2) Non-reactive mold flux, it is based on the new developed CaO-Al2O3 system, which already contains a certain WANLIN WANG, Lotus Professor, and BOXUN LU and DAN XIAO, Graduate Students, are with the School of Metallurgy and Environment, Central South University, Changsha 410083, P.R. China. Contact e-mail: [email protected] Manuscript submitted July 23, 2015. Article published online October 7, 2015. 384—VOLUME 47B, FEBRUARY 2016
number of alumina content. The objective of this review is to highlight the development of various mold flux systems for casting AHSS. The evolution of traditional CaO-SiO2-based mold flux was concluded first, and its disadvantages were also discussed here. Then the non-reactive mold flux was introduced and the latest developments were summarized. It was emphasized that the CaO-Al2O3-based mold flux shows the great potential to be applied to the casting AHSS. Moreover, the aim of this review is to provide technical guidance and strategy for the design and optimization of new generation mold flux system used for the casting of AHSS.
II.
TRADITIONAL REACTIVE MOLD FLUX
The traditional mold flux system is based on CaO-SiO2 mold slag system with low basicity and viscosity, where a series of fluxing agents like Na2O, Li2O, B2O3, etc. were added to optimize the mold flux properties to optimize the problem introduced by the slag/steel reaction during the process of high-Al steel casting. Intensive research has been conducted and summarized as following. Omoto et al.[15] designed a
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