Due to the importance of the physical and chemical properties of the mold flux used in the production of high-quality steel, in particular the suppression of surface defects on steel sheets, steelmaking engineers have attempted to develop new types of mold flux. This paper presents the results of research on entrapment of mold flux and on heat transfer between the mold and the solidified shell. The authors have been developing a mold flux with non-Newtonian fluid properties using nitrogen. That is, the viscosity of the molten mold flux is low at a high shear rate to reduce the friction between the mold and the solidified shell, but is high at a low shear rate to prevent mold flux entrapment. In order to approximate the properties of mold flux as a non-Newtonian fluid, nitride is added to the conventional flux to adjust the silicate network structure through the reaction between nitrogen and calcium. The contact angle of the non-Newtonian mold flux, which represents the wettability between the mold and the solidified shell, is low in comparison with that of the normal mold flux without nitrogen. It is suggested that the non-Newtonian mold flux increases the heat transfer between the mold and the solidified shell. A casting test was carried out using this non-Newtonian mold flux, and the results showed that entrapment of mold flux decreased and heat transfer increased, as assumed.