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, than that of the equivalent plain carbon steel. The results obtained are discussed in relation to the high-strain work-hardening mechanisms found in plain carbon steel, and to the roles played by MnS and Pb in improving the machinability of steel.