Structure and properties of free-cutting steels after deformation to high strains

Abstract The overall property changes in a high-sulphur free-cutting steel and a lead-bearing high-sulphur free-cutting steel have been determined by compounding the results of tensile tests after cold rolling the steels for differing amounts up to 98% reduction. The structural changes have been determined by optical and thin-foil transmission electron metallography, while the tensile fractures found after cold rolling have been examined by scanning electron microscopy. The high-sulphur steel showed a lower work-hardening rate than the equivalent plain carbon steel at moderate strains, but at high strains a tensile strength of 1700 MN m−2 was obtained. The MnS particles were more plastic than the matrix (plasticity ratio >1) and a fibre-reinforcing effect could give rise to the high strength. For the lead-bearing steel the work-hardening rate was lower than with the lead-free steel. The lead caused the sulphides to fragment and, hence, the final strength level achieved was much lower, but still greater, t...