Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The thermal aspect is dominant in semi-continuous direct chill (DC) casting. In steady state, 80% of the total energy contained inside the ingot is evacuated by secondary cooling. For this reason, water is often suspected as a potential cause for the issues related to ingot quality. In the secondary cooling zone, the ingot is cooled by jet impingement boiling. Opinions in the literature about the influence of water quality on the boiling curve are conflicting. This can be associated mainly to the difficulties that experimenters meet during the measurement of the boiling curve repeatedly. Furthermore, published works present the entire boiling curve rather than identifying what are the significant parameters in it. This understanding is nonetheless essential in order to improve the measurement repeatability by focusing on the most significant boiling parameters as well as to ensure that the measured variations of the boiling curve are those that have an impact on the process. Considering this lack of knowledge, a sensitivity study has been completed to compare the relative influence of four parameters that characterize the boiling curve (Leidenfrost temperature, film boiling heat flux, critical heat flux and the temperature that belongs to the critical heat flux) on three issues responsible for the diminution of the recovery rate (hot tearing, bleed-out, butt curl). The study has shown that except the critical heat flux temperature, every tested boiling parameter plays a significant role on the three recovery rate obstacles. Moreover, for every case, a mutual interaction exists between the three significant boiling parameters. The latter demonstrates that the variation of each of the three most important boiling parameters is relevant and needs to be measured following a modification of the water quality. This study is the first step in the framework of a project aimed at the conception of an appropriate device for measuring the variation of the cooling capacity of water and its role on ingot quality.