Experimental study on transverse cracking in micro-alloyed steels

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Transverse surface cracking of micro-alloyed steels is still one of the most critical problems in steel industry affecting different production areas starting from continuous casting, passing by slab transportation to storage, and ending by hot rolling. It might lead to disturbances at the production lines, defect at the final products, or even an expensive and time consuming slab repair.Micro-alloyed steels with high amounts of Ti, Nb and V are more prone to cracking than other steels due to their high ability to produce detrimental precipitates in the form of carbides and nitrides on the grain boundaries of the matrix. These precipitates are believed to be one of the main reasons of low hot ductility during steel processing and consequently being responsible for cracking. These types of steels are most sensitive to crack formation during the slab bending/unbending in continuous casting within a temperature range between 600 °C to 1000 °C. One more effect that could lead to crack initiation, is the slab deformation within a temperature range slightly after ferrite formation, where most of the deformation is concentrated on a small amount of freshly transformed ferrite and might produce cracks under a small amount of applied deformation during straightening or even later on during transportation of the slabs.The effect of micro-alloying Ti on hot ductility is still complex and not clear. Ti effect in respect with TiN formation, size and matrix distribution, and Ti:N stoichiometric ratio under different conditions of cooling and straining, is to be studied through an integrated research program combining both physical simulation by deformation dilatometer ‘DIL 805 T’ and simulation by MatCalc calculations. Different grades with different chemical compositions are to be studied aiming to understand the effect of processing conditions like cooling, straining rates and austenite-ferrite transformation on the hot ductility of Nb- and Ti-micro-alloyed steels during solidification, bending and straightening in continuous casting shop. Dilatation tests for determining the ferrite start temperature are to be compared to MatCalc calculations. MatCalc will also play an important role in calculating the primary precipitates during solidification through Scheil-Guliver simulation modelling and studying the precipitation kinetics during cooling after solidification.Means of microstructure analysis by light optical microscope, scanning electron microscope and energy dispersive X-ray spectroscopy will be used to study the effect of micro-alloying elements and process condition on the type, position, size and distribution of the formed precipitates and compare it to the MatCalc simulation results.The main objective of the project is to draw a ductility map for the considered steels, showing under which conditions it would be most beneficial in processing such types of steels to avoid/minimize transverse surface cracking of steel slabs.