Regulation of local heat transfer in the secondary cooling zone of continuous casting to reduce stresses

Full text: In continuous casting processes the strand leaving the mould must be cooled intensively to freeze the liquid metal. In this secondary cooling zone water is used as quenching agent. In steel and copper casting processes water is sprayed onto the strand. In aluminum casting processes water coming out of the mould is poured over the strand and flows down vertically. Because of the high temperatures of the metal the Leidenfrost problem occurs. A vapor film forms between the metal surface and the liquid water, thus reducing the heat transfer. After the collapse of the vapor film, the surface is wetted and nucleate boiling occurs, leading to many times higher heat transfer. That is why the heat transfer in steel casting is regulated in a manner that the surface temperature is kept above 900 °C. In the presentation it will be shown in which way the impingement density, the drop size, the drop velocity and the nature of the metal influence the Leidenfrost temperature, the DNB temperature, the maximum heat flux and the total heat transfer. In addition, the quality of water is of special importance. The impact of salt concentration, dissolved gases, casting lubricants and solid particles is demonstrated. The differences in the application of pneumatic and hydraulic nozzles are discussed. An equation based on the analytical solution of the freezing process is developed to predict the heat transfer as a function of casting speed, strand dimensions and metal nature. key words: continuous casting, secondary water cooling, heat transfer, Leidenfrost temperature, analytical model