ANALYSIS OF THE SOURCE OF TRANSVERSE CRACKS IN CONTINUOUSLY CAST SLABS OF TITANIUM BEARING, MEDIUM CARBON STEELS

摘要

The aim of the present work was to analyze the potential sources of transverse cracks in continuously cast slabs of titanium bearing, medium carbon steels. A statistical evaluation of the inspected slabs revealed that medium carbon steels are more prone to this type of defect. Additionally, for the same carbon content range, steels with titanium exhibited a higher frequency of defects. Analyses of slab samples of these steels showed that transverse cracks were mostly located in the slab corners, running along the oscillation marks path. Analyses of metallographic samples and evaluation of fracture surfaces confirmed that cracks propagate along columnar grain boundaries. TEM analyses in samples taken from the affected region revealed the presence of small cuboidal TiN particles, which may affect the steel ductility. A critical review of previously published data showed that hot ductility is impaired when the volume fraction of precipitates is higher and their size is smaller. Coarse austenitic grains, as usually obtained in medium carbon steels, increase the concentration of precipitates at the grain boundaries. Additionally, higher cooling rates, as normally obtained at the slab surface, promote finer precipitates. Therefore, these steels might be particularly sensitive to crack propagation when subjected to strain (like in the unbending process). Although this propagation occurs after material is completely solid, some perturbations in the mold were also detected during casting of these grades (higher mold temperature variability and TiO2 pick up in the mold slag). Local fluctuations in mold heat transfer and lubrication may also promote defect initiators at the slab surface.