Effect of Sulfides and Sulfide Morphology on Anisotropy of Tensile Ductility and Toughness of Hot-Rolled C-Mn Steels

Abstract

The effects of sulfur content (0.004 or about 0.013 pct) and sulfide morphology (stringered or globular) on anisotropy of tensile ductility and Charpy V-notch (CVN) shelf energy were investigated in a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels. The effect of sulfide inclusions on fracture strain or CVN shelf energy correlated with a single parameter,P, regardless of inclusion shape, stringered or globular, or test direction, longitudinal, transverse, or through-thickness. The parameterP was defined as the total projected length of inclusions per unit area on a plane parallel to the fracture plane. The fracture strain and CVN shelf energy decreased with an increase inP. The magnitude ofP was directly proportional to the volume fraction of inclusions and inversely proportional to the inclusion dimension perpendicular to the fracture plane. The lower tensile ductility and CVN shelf energy in the 0.2 as compared with the 0.1 pct carbon steels was a consequence of the greater pearlite content in the former steels. This greater pearlite content had no apparent effect on the work-hardening rate,H, but decreased the strain-rate sensitivity of the flow stress,M, and the strain-rate sensitivity of the work-hardening rate,B. The decrease inM andB with increasing pearlite content is in accord with a decrease in tensile ductility according to recent models of neck development in tension tests. It appears that the decrease in tensile ductility with increasing pearlite content is a result of enhanced localized shearing, which promotes the coalescence of voids nucleated at second phases.