Fluoride containing mould fluxes may damage both the environment and casters, so the development of fluorine free mould fluxes is of great interest. The key issue for the development of fluorine free or low fluoride mould fluxes is to find a suitable crystal phase as a substitute for cuspidine. Some researchers have suggested that additions of TiO 2 to fluxes can help to form perovskite (CaTiO 3 ) as a suitable substitute for cuspidine. However, the rate of sticker breakouts was found to increase greatly after the use of the TiO 2 containing, fluorine free mould fluxes in the continuous casting of peritectic steel grades. One of the possible reasons for this phenomenon is the formation of titanium nitride and carbonitride during the melting process of mould flux. This possible explanation has been proved in this study using both thermodynamic calculation and laboratory experiments on TiO 2 containing, fluoride free mould fluxes.

Transverse corner cracks were observed frequently in Nb containing steel slabs produced by a vertical bending type slab caster. The hot ductility of the steel was measured with a hot tensile test, and the result indicated that the temperature at the slab corner fell into the embrittlement temperature range which resulted in the cracking problem. To ensure that the temperature of the slab was uniformly above the embrittlement temperature range during the straightening process, asymmetric secondary cooling nozzles have been implemented on the caster. After this modification, the temperature of the slab corner during the unbending operation was increased to above the embrittlement temperature range of the Nb bearing steel, and the incidence of transverse corner cracks was greatly reduced.

During the continuous casting of medium carbon steel slabs, stable liquid slag film performance is necessary to ensure that the process runs smoothly. In this paper, the effects of mould flux crystallisation behaviour on the properties of leftover liquid slag films were examined. First, the compositions of the leftover liquid slags of high fluorine, CaO-SiO 2 -TiO 2 and CaO-SiO 2 -Na 2 O based mould fluxes after a single crystalline phase was separated were calculated. Then, the viscosity, melting temperature, break temperature and degree of crystallisation of each leftover liquid flux were measured. The results revealed that the properties of leftover liquid slag films of CaO-SiO 2 -TiO 2 and CaO-SiO 2 -Na 2 O based mould fluxes are not as stable as those of high fluorine films after crystallisation. This paper provides a new method for developing fluorine free mould fluxes and for determining the most suitable type of crystalline phase that should be utilised in the continuous casting of medium carbon steel slabs.