Determinação da distribuição de água no resfriamento secundário do lingotamento contínuo da Vallourec Tubos do Brasil e de seus efeitos sobre o comportamento térmico do aço

Abstract

The quality of the steel in the continuous casting machine is directly related to the thermal behavior during the solidification and cooling processes. The evaluation of the heat flux and temperature profiles is fundamental in the definition and optimization of the operational parameters and it depends on the characterization of the cooling conditions. In the present work, the water flux density of nozzles with flat jet and full cone jet was experimentally measured using an apparatus in industrial scale that reproduces the secondary cooling of the continuous casting of round billets of Vallourec Tubos do Brasil. An index of water flux density was defined to express the water distribution in both longitudinal and angular directions of the strand. A mathematical model for heat transfer and solidification for the continuous casting of round billets was developed applying the non-uniform water distribution approach. The mathematical model was validated by experimental measurements of the billet superficial temperature, performed at the industrial plant, and by the experimental estimation of the heat transfer coefficients of the secondary cooling zone. The results of both uniform and non-uniform water distribution approaches were compared. It was observed that the non-uniform water distribution approach is capable to represent more accurately the local thermal phenomena and that the water flux density distribution plays a more important role than the total water flow rate concerning the thermal behavior of the steel. Modifications in the configuration of the first zone of the secondary cooling were simulated, such as variation of the total water flow rate, usage of eight nozzles in the third cooling ring, reduction of the distance between the mold exit and the cooling rings, increase of the number of cooling rings and the usage of full cone jet nozzles instead of flat jet nozzles. The objective of these modifications was to decrease the superficial temperature variation in the first zone of the secondary cooling, which can be the main cause of cracks in the continuous casting and quality defects in the tube mill. The non-uniform water distribution approach applied on the mathematical model of heat transfer and solidification of steel is a useful tool to enhance the comprehension about the thermal behavior of the steel along the secondary cooling zone of the continuous casting and to identify operational parameters modifications that can improve the quality of the as-cast product.