Measurements and Calculation of Interfacial Tension between Commercial Steels and Mould Flux Slags Jessica Elfsberg * , and Taishi Matsushita Division of Materials Process Science, Royal Institute of Technology S-100 44 Stockholm, Sweden * Corresponding author; e-mail: jessicae@kth.se Surface quality of continuously cast is strongly inﬂuenced by the interfacial tension between steel and mould ﬂux slag. The meniscus shape and the inclusion entrapment are directly determined by interfacial tension. To achieve a better understanding of the continuous casting process, the interface between four commercial steels and the mould ﬂuxes used at the continuous casting of each steel grade have been investigated. The situation at this interface is determined by the surface tension of steel and slag respectively and also by the mass transfer occurring across the interface. The surface tensions of the mould ﬂux slags have been measured by sessile drop method. The results indicate that the surface tension of mould ﬂux slags decreases with increasing temperature but does not vary so much within the present composition range. Interfacial tensions between steel samples and mould ﬂux slags have been measured in the same way with the aid of X-ray unit. Estimation of interfacial tension from the steel and slag composition was done by applying empirical models. The measured and the calculated values were in agreement. The interfacial tension was lower for higher alloyed steel grades according to both experiments and calculations though the inﬂuence of surface active elements is signiﬁcant. Keywords: continuous casting, interfacial tension, mould ﬂux, steel, surface tension Submitted on 30 September 2010, accepted on 22 November 2010 Introduction In continuous casting of steel, the surface quality is inﬂuenced by the conditions in the mould. There must be an appropriate cooling rate, sufﬁcient lubrication and a stable meniscus. Mould ﬂux properties are crucial factors for the surface quality of continuously cast metals. The mould ﬂux has several functions [1]:  Protect the steel from oxidation  Absorb inclusions  Provide thermal insulation  Control horizontal heat transfer  Lubricate the steel shell The interface between steel and mould ﬂux slag is continuously refreshed due to the steel and ﬂux ﬂows. The situation at the interface is most probably not equilibrium. Dynamic interfacial phenomena, for example Marangoni convection due to the concentration or temperature gra- dients, are likely to occur [2]. The steel ﬂow in a continuous casting mould may be turbulent and vortices may form at the interface between steel and mould ﬂux slag. The stability of the meniscus will control the surface quality by inﬂuencing the oscillation mark formation as well as the shell growth. The interfacial tension between metal and mould ﬂux slag is a very important factor determining the meniscus shape in the mould. The meniscus shape may also be inﬂuenced by the ﬂow in the liquid metal and by the pressure variations in the molten slag caused by the mould oscillation. There are many kinds of instabilities that can occur in the meniscus region. One example of instability is the change of the meniscus shape caused by reactions between steel and mould ﬂux. The changes are not only caused by an actual change of interfacial tension but also by mass transfer across the interface and a difference in electrical potential between steel and slag [3]. Possible reactions between steel and mould ﬂux are oxidation of elements in the steel, mainly aluminium, and loss of surface active elements as sulphur or phosphorus from the metal to the slag. In the present work, the interfacial tension between steel and slag has been measured. The systems were at a non-equilibrium state due to the reactions. As mentioned earlier, reactions could change the interfacial tension. The dynamic, non- equilibrium or apparent interfacial tension determines the dynamic shape of a sessile drop. Minaev [4] has earlier discussed how the mass transfer across a moving reactive interface changes the area size of the reacting surface as well as the width of the diffusion layer. The changes are caused by a change of interfacial tension, or vice versa. Anyway, mass transfer across an interface will change the interfacial tension and the mass transfer intensiﬁes. Emulsiﬁcation is described as the optimal state for efﬁcient mass transfer. Examples of moving reactive interfaces are growing bubbles or two- phase ﬂows. Even less dramatic changes of a surface, for example the shape of a drop will create moving reaction interfaces. Any deformation of a drop will thus not only change the contact area but also the diffusion layer thickness. This change will intensify the mass transfer which further changes the interfacial tension, and as the interfacial tension is changed, the mass transfer changes. The direction of the interfacial tension change by mass transfer across the interface depends on the elements that are transferred. If the concentration of surface active elements at DOI: 10.1002/srin.201000221 steel research int. 82 (2011) No. 4 404 ß 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.steelresearch-journal.com