Warm extrusion of TRIP steels: Process control and tensile properties

TRIP steel rods were warm extruded to reductions of area of 40%, 60% and 80%. Temperature increases on the order of 150 °C were compensated for by adjusting the billet pre-heat temperature to maintain a final extrusion temperature of 450 °C. The 40% and 60% reductions were each performed in on extrusion pass, while the 80% reduction required a two-step sequence (60% followed by 50%). Extrusion constants were determined as 340 MN m^-2 (50 klbf in^-2) for hot extrusion. 1030 MN m^-2 (150 klbf in^-2) for single-pass warm extrusion and 2070 MN m^-2 (300 klbf in^-2) for the second (50%) warm reduction. Tensile tests indicated that a high austenite stability resulting from too low an average working temperature during warm extrusion caused a loss of tensile ductility owing to insufficient working hardening. The austenite stability could be decreased, however, by tempering at 590 °C to remove some carbon from solid solution, giving tensile properties equivalent or superior to those obtained by warm rolling. Difficulties in composition and/or temperature control during the processing of TRIP steels can thus be compensated for by a simple final heat treatment.