Cracking during Continuous Casting of Steels : Analysis tools and procedures to identify their causes on microalloyed steel slabs

Abstract: Transverse corner cracks are one of the most common but complex defects formed in microalloyed steels during continuous casting (CC). Such cracks are detrimental because it leads to a loss in productivity with a great impact on environment, energy and economics for several steel making companies all over the world. If cracks are detected, a supplementary process has to be applied to remove these cracks (e.g. grinding and scarfing). If cracks cannot be detected due to oxide scales covering the cracks, then they propagate during subsequent processing (e.g. hot rolling), promoting breakouts, which consequently results in material rejection and yield loss.This thesis focuses on the study of different phenomena occurring during the secondary cooling zone combining the effect of the process parameters together with the material behaviour, which assembles four scientific articles that explains and identifies some of the risks for cracking formation during CC of a HSLA steel slab. This information is key to avoid productivity losses and defect formation in the final cast product. These made possible to address research gaps and better understanding of the steel failure during CC. Additionally, this work proposes the development of strategies for reducing/preventing the formation of cracks during continuous casting of a specific HSLA alloy and caster machine.Crack susceptibility of the steel was determined through the so called hot ductility method, this made possible to identify the temperature range at which the steel is more susceptible to crack between 700-800 °C. It was found that one of the causes for failure of the steel at high temperatures is the combination of different ferrite morphologies being Widmanstätten ferrite the predominant phase, which is considered unfavourable to toughness and ductility. This temperature range was considered detrimental for cracking formation, which was demonstrated by including real temperature measurements with pyrometers and numerical modelling, that by avoiding the low ductility zone, the quality of the slabs could improve significantly. Outcomes of this work demonstrated that the control of the cooling is beneficial for the process in order to decrease the risk for crack susceptibility of the steel during processing.Oxide scale formation was considered another important factor that influences the cooling and final quality of steel slabs during CC. The formation of oxide films during cooling of the strand has industrial implications in terms of crack formation and the overall yield of the continuous casting machine since this reduces the cooling efficiency of the strand. Accurate insights in this particular work provides an understanding of oxide scale morphology, type and mechanisms leading to an accurate control of its formation during casting. Some of the findings revealed that oxidation rate and scale thickness is much higher under water vapour than in dry air. The presence of different oxides (i.e. wüstite, magnetite and hematite) change the oxidation behaviour due to micro-mechanical properties that shown higher plasticity for wüstite in comparison to magnetite and hematite. Furthermore, it was demonstrated that the oxide scale not only is influenced by temperature and environmental condition but by the surface of the substrate. These factors are considered one of the reason for uneven cooling during CC process, thus affecting the heat transfer coefficient resulting in undesired surface quality of steel slabs.