Influence of composition and temperature upon relativeplasticity index of silicate inclusions in steel

Abstract

A study of the effect of inclusion composition upon the deformability of inclusions relative to that of a (<0·03 wt-% C) steel matrix is described. The inclusions were a result of deoxidizing the steel melts with Mn-Si-Al alloys. Inclusion compositions were determined by microprobe analysis for both ‘as-cast’ and ‘as-rolled’ specimens. Hot rolling was performed on a two-high laboratory mill to a nominal 70% reduction in height in six passes, at rolling temperatures of 600°–1300°C with interpass reheating. Inclusion deformation was measured by optical projection microscopy. Inclusions within the low-liquidus-temperature region of the MnO-SiO₂-Al₂O₃ ternary system have the lowest deformable/non-deformable transition temperatures. Large, dispersed precipitate phases within the inclusions had little effect on the ‘relative-plasticity index’ for the degree of matrix deformation employed in this study. Inclusions which remained glassy during rolling had their transition temperatures controlled by their SiO₂ content. Precipitation of a second phase from the as-cast glassy inclusion often prevented deformation at temperatures where plastic deformation would otherwise have been expected.