Sulphide shape control

Abstract

In the rolling of sheet and strip in modern mills, deformation takes place mainly in the longitudinal direction. As the extension increases, there is a deterioration in the transverse and through-thickness toughness parameters, attributable mainly to the plastically deformable inclusions, such as manganese sulphides. The form of the inclusion in the wrought product is a function of its morphology in the cast steel and its plasticity. The effect of composition on both factors has been investigated. Increasing the aluminium or sulphur content of the steel promotes the formation of Type II manganese sulphides, which are precipitated as extensive arrays of rods in an interdendritic eutectic distribution, and which deform, as an array, to produce highly elongaled stringers. Mildly deoxidized steels contain globular, isolated, Type I sulphides, which do not form the particularly harmful stringers, but which are still highly deformable. Cerium and calcium additions successfully modified the initial morphology of the sulphide inclusions to an isolated globular form, and also decreased their plasticity, whereas titanium additions of up to 0·12% were ineffective in these steels at the manganese levels involved. The electron microstructure of deformed α-MnS, α-Al₂O₃, and Y-phase inclusions have been investigated. Evidence of recovered substructures, recrystallization and precipitation within manganese sulphides, and Ti₃S₄ precipitation within V-phase inclusions is presented.