Effect of Oxides on Characteristics of MnS and Transverse Mechanical Properties in Commercial Al-Killed Non-Quenched and Tempered Steel

In this paper, the characteristics of tensile fractures and the typical inclusions in as-cast blooms and as-rolled bars are carefully investigated to figure out the causes of different transverse tensile properties in two commercial non-quenched and tempered steels. In low Ca bearing steel (3 ppm), MgO–Al2O3 (MA) + MnS are main inclusions and MA oxides express high ability for MnS heterogeneous nucleation. Consequently, the MnS inclusions in as-rolled bars are larger in size and smaller in number, the transverse ductility is satisfied due to the high space distance between the MnS inclusions. However, the transverse ductility of high Ca bearing steel (9 ppm) is much poor because MnS inclusions are agminated in distribution, causing severe stress concentration. The calculation results show that CaO in CaO–MgO–Al2O3 oxides (CMA) would react with Al and S in molten steels, suppressing the formation of MnS inclusions on surface of CMA oxides. An integrated illumination including the evolution processes of typical oxides in molten steels, the generation behaviors of MnS during the solidification, the deformation behaviors of sulfides and the relationship between sulfides and crack initiation have been conducted based on observation results and thermodynamic calculation.