Design of microalloyed steel hot rolling schedules by torsion testing: Average schedule vs. real schedule

Abstract

The hot torsion simulator has been extensively used as a means to understand the microstructure evolution of different steel grades during hot rolling. The test is suitable to simulate 'real' industrial schedules as well as schedules designed to obtain information regarding the intrinsic properties of the materials. For example, it is common to apply 'average' schedules, in which deformation per pass, interpass time, strain rate and cooling rate are kept constant, to determine the characteristic temperatures Ar3, A r1 and Tnr, (start and finish of the austenite transformation and no recrystallization temperatures) of steels. In this work, both a 'real' schedule simulating a rolling schedule in a reversing mill and an 'average' schedule were applied to a series of Ti and Nb microalloyed steels. In general, the steels exhibited somewhat different behaviours for the different thermomechanical schedules, e.g. the pancaking temperature region is easily detectable after an 'average' schedule, while for the 'real' schedule some softening can be detected in the pancaking region, which is strongly dependent on the strain and interpass time. Moreover, the paper analyzes a new approach to stress-strain curves, which is used to better understand the sequence of events which take place during rolling and their dependence on rolling parameters. © 2010 ISIJ.