9th ECCC European Continuous Casting Conference – ECCC 2017

An improvement of surface quality of casting slab increases productivity of steel. The slab surface sometimes has transverse cracks along a grain boundary when the slab is bended and/or unbended around the transformation temperature from austenite phase to ferrite one. This defect is mainly caused by film-like ferrite and carbide and/or nitride precipitations around the grain boundary and alloying elements such as Al, B and Nb which have good affinity with carbon or nitrogen also raise the crack sensitivity much more. However, the adverse effect of each element on embrittlement behavior is not necessarily clear. In this study, the effect of boron on the hot ductility of steel has been investigated. It is found that the cooling rate of steel after solidification significantly affects on both the hot ductility of steel and the precipitating morphology of BN. The increase in cooling rate results in precipitating a few hundred nanometer-sized of BN on the grain boundary and worsening the hot ductility of steel. On the other hand, as lowering the cooling rate, BN precipitates on other inclusions such as Al2O3 which distribute anywhere in steel and the hot ductility of steel is improved.