Influence of Mold Flux on Initial Solidification of Hypo-Peritectic Steel in a Continuous Casting Mold

Continuous casting of hypo-peritectic steel was conducted with a pilot slab caster. Such experimental data as local heat flux, thickness of solidified shell or mold flux film, and dendrite primary arm spacing were obtained. On the basis of these experimental results, influence of mold flux on initial solidification in the mold was discussed. With mild cooling by crystallization of mold flux, local heat flux and solidification rate decreased in the mold. The changes in them quantitatively correspond to each other. Dendrite primary arm spacing increased with the mild cooling. Relationship between the arm spacing and cooling rate was established and cooling rate on quite initial stage of solidification was stimated. Cooling rate at 1 mm thickness of solidified shell was estimated as about 10 000-17 000 K/min and changed by mold flux. Unevenness of the solidified shell thickness becomes remarkable when the shell grows to be 1 mm thick. Relation between the unevenness and the cooling rate was discussed, and critical cooling rate against the uneven solidification was observed around 17 000 K/min. Thermal resistance of mold flux film was also evaluated and it was clarified that thermal resistance in the film is larger than that by air gap, and Crystallization in the film contributes to increase of both resistances. It is also considered that increase of casting speed makes air gap thinner, so reduction of radiation by crystallization of mold flux becomes more important in high speed continuous casting.