The accuracy with which the solidification and cooling of a continuously cast billet is investigated depends on the setting of the boundary conditions of the numerical model of the temperature field. An in-house numerical model of the 3D temperature field of a concast billet had been used. This model enables the analysis of the temperature field of the actual blank as it passes through the zero-, primary-, secondary- and tertiary-cooling zones, i.e. through the entire caster. This paper deals with the derivation of transfer phenomena under the cooling nozzles of the secondary zone. These phenomena are expressed by the values of the heat transfer coefficients (HTCs). The dependences of these coefficients on surface temperature and other operational parameters must also be given. The HTCs beneath the nozzles are given by the sum of the forced convection coefficient and the so-called reduced convection coefficient corresponding to heat transfer by radiation. The definition of the boundary conditions is the most difficult part of the numerical and experimental investigation of the thermokinetics of this process. Regarding the fact that on a real caster, where there are many types of nozzles (with various settings) positioned inside a closed cage, it is practically impossible to conduct measurement of the real boundary conditions. Therefore, an experimental laboratory device was introduced in order to measure the cooling characteristics of the nozzles. It simulates not only the movement, but also the surface of a blank and for the necessary range of water flow in the operation and the casting speeds. The transfer phenomena beneath the water cooling nozzles are presented on a simulated temperature field for a real 150×150 mm steel billet under different operational conditions. This is ensured by the correct process procedure: real process → input data → numerical analysis → optimization → correction of process. The presented model is a valuable computational tool and accurate simulator for investigating transient phenomena in caster operations, and for developing control methods, the choice of an optimum cooling strategy to meet all quality requirements, and an assessment of the heat-energy content required for direct rolling.

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