Continuous casting comprises thermal, mechanical and chemical processes running in a complex system that contains a number of elements, such as a solidifying steel strand, a mould with an oscillation mechanism, a withdrawal mechanism, a water cooling subsystem with nozzles, several control subsystems , etc. An external observer might see the process as robust and stable, but in reality there are fluctuations in the internal thermal and mechanical quantities, reflected in the structure and quality of the product. The research on unsteady behaviour of the quantities such as a solidifying strand temperature field, solid shell thickness and metallurgical length was conducted using an industrial diagnostic system DGS complemented with special measurement equipment and a thermal numerical model. Selected results of the monitoring and simulation of the non-standard process states are shown and analysed in the paper. Methods for determining the boundary conditions for the numerical model are also presented. The effect of the Leidenfrost phenomenon on the heat-transfer coefficient during water cooling by nozzles is also discussed. Since the determination of precise and immediate boundary conditions has technical limits, the model provides only smoothed values in time and space. As knowledge of the instantaneous state of the fluctuating process is a prerequisite for achieving quality and defect-free production, it is appropriate to complement the thermal numerical model by on-line monitoring of the machine's internal state. The results of the simulations are closely linked to the real process data.

Numerical models of solidification and cooling of continuously cast billets or blooms are used both in research and in operational conditions to predict solid shell thickness, metallurgical length, solidification rate etc. The numerical model must be verified according to real values of quantities. Although several different quantities can be used to verify the model, most often the models are verified by comparing the calculated and measured surface temperatures of the strand in the secondary and tertiary cooling zones.The casting process is influenced by a number of known and hidden parameters, often time-varying, which are reflected in the measured surface temperatures, but which cannot be incorporated into the model due to a lack of information to define the exact boundary conditions. For the purposes of model verification, it is therefore necessary to revise the measured data. It is not enough to use only mathematical methods to process data without knowledge of the casting process, because uncertainties and temperature fluctuations have different and often difficult to detect causes. The article deals with sources of temperature uncertainties and fluctuations and methods of extraction of relevant values from measured signals.

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The paper describes a determination of the cause of transverse internal cracks on a continuously cast slab using an analytical software tool, called LITIOS. It is a complex system based on long-term monitoring of the casting parameters and their impact on the quality of the slabs. Transverse internal cracks are characterised by identifying the possible causes of their origin. Using the LITIOS system, selected constant (invariable) and variable parameters of the casting were assigned to specific cracks and on their basis the causes of the cracks were determined. Keywords: continuous casting of steel, slab, casting parameters, quality prediction, crack, software tool lanek opisuje ugotavljanje vzrokov notranjih pre~nih razpok v kontinuirno litem slabu z uporabo analitskega programskega orodja LITIOS. To je kompleksen sistem, ki temelji na dolgotrajnem nadzoru parametrov pri litju in njihovem vplivu na kvaliteto slabov. Ocenjene so pre~ne notranje razpoke s prikazom mogo~ih vzrokov za njihov nastanek. Z uporabo sistema LITIOS so bile dodeljene izbrane konstante (nespremenljivke) in spremenljivi parametri litja dolo~eni razpoki in na tej osnovi so bili dolo~eni vzroki za nastanek razpok. Klju~ne besede: kontinuirno litje jekla, slab, parametri litja, napovedovanje kakovosti, razpoka, programsko orodje

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