The influence of intergranular oxidation on surface crack formation in continuous casting of steel

High-temperature oxidation phenomena play an important role in steel processing. What is mostly underrated is the importance of internal oxidation in casting processes, namely the continuous casting process. To investigate the impact of intergranular oxidation on surface defect formation, experiments for two cooling strategies and time sequences for a conventional slab caster were conducted. As the influence of silicon on high-temperature oxidation is well known and its effect on surface ductility is marginal silicon was chosen as an alloying element to provoke intergranular oxidation. The methods used were the In-Situ Material Characterization by Bending test (IMC-B), which provides the investigation of the susceptibility to surface crack formation by 3-point bending under oxidizing testing conditions and simultaneous thermal analysis for the well-controlled study of high-temperature oxidation phenomena. The results show that during a cooling cycle supporting highly oxidizing conditions, silicon favors the formation of a low-melting eutectic (FeO-Fe2SiO4) at the interface, infiltrating the steel along the austenite grain boundaries. The intergranular oxidation formed has a depth of less than 50 μm but leads to a stress concentration during a subsequent tensile deformation. In consequence, cracks may easily nucleate and propagate along austenite grain boundaries. A change in the steel composition by reducing the silicon content to almost zero or a less harmful temperature sequence reduces intergranular oxidation and subsequently the susceptibility to crack formation.