Simulated Rod Rolling of Interstitial Free Steels.

Abstract

Simulations of the rod rolling of a Ti-Nb interstitial free (IF) steel were carried out in the ferrite (840, 770, 700, 600, 500°C) and austenite (950°C) ranges. Typical industrial pass strains and temperatures were employed, but the rates were much lower because of the limitations of the experimental apparatus. To allow for this difference, strain rates interpass times were used. The mean flow stresses for ferrite rolling at 700°C were observed to be similar to those developed in the austenite at 950°C. Industrial schedules at 950°C, as well as rate corrected ones at this temperature and at 840°C, led to mean flow stress envelopes which displayed peaks and valleys. This is shown to result from the periodic triggering of dynamic recrystallization during deformation, with the subsequent occurrence of rapid metadynamic softening during the interpass intervals. Microstructural examination of the material indicated that dynamic recrystallization took place in the ferrite at 840°C, whereas dynamic recovery was operative at the lower deformation temperatures. The final mechanical properties of the material were evaluated after the simulations, by means of tensile tests on hollowed out torsion specimens. These showed that yield strengths of up to 420 MPa can be reached by the warm working of IF steels. Thus the low temperature rod rolling of IF steels has distinct promise for the production of highly ductile materials, with controlled levels of yield strength ranging from 180 to 420 MPa. In this way, the high price of IF steels can be offset by the economies resulting from low temperature reheating which include lower scale losses, and by the possible elimination of subsequent cold drawing/heat treatment/pickling processes.