A flux is mainly used for lowering the melting point. For example, if we add “adequate” salt (as flux) over ice, the melting point of the ice is lowered to form a liquid/slush mixture in a solution. Likewise, in steelmaking, a fluxing agent is desired between oxides to form a fluid creamy slag that is determined through phase diagrams which display the temperature behavior and the amount of material required.
Viscosity behavior of the slag is one of the most important parameters that should be considered in steelmaking. Steelmaking takes place at temperatures around 1600oC. However, the melting points of some oxide components are much higher than 1600oC: CaO (2613oC), MgO (2852oC), Al2O3 (2072oC), MnO (1945oC) and SiO2 (1713oC).
When these oxides are combined, a slight natural fluxing also occurs between them. However, more performant fluxes are needed to form a liquid slag. But this is not enough. A good or optimum slag formation is desired: that is mostly a creamy slag, where refractory is protected and requirements of a clean steel are performed through improved desulfurization capacity of the slag.
A hard and stiff (highly basic) or a too fluid (highly acidic) slag formations are definite problem indicators revealing that all metallurgical requirements are not fully met and there may be a threat for accelerated refractory wear. Thus, a good creamy slag is described as: fluid enough to effectively refine the steel, but not very fluid to secure that a minimum interaction occurs with the refractory.