Critical Strain for Complete Austenite Recrystallisation during Rough Rolling of C-Mn Steel and Nb-Ti-V Microalloyed Steel

Abstract

The influence of roughing strain on the extent of austenite recrystallisation in plain carbon steel and Nb-Ti-V microalloyed steel was investigated. Reheating and roughing simulations were conducted on a deformation dilatometer using industrial heating, soaking times, strain and strain rates. The roughing schedules comprised of varying the pass strain magnitude within a typical roughing temperature range. The double stroke method was used to determine the austenite softening fraction. The austenite grain size, prior to and after rolling, was measured on quenched specimens. Grain refinement was achieved in both steels after all rolling schedules. An applied pass strain of 0.15 was sufficient to completely soften the austenite after the first pass and produced the finest recrystallised grain size. This was attributed to sufficient nucleation sites and driving force for recrystallisation compared to lower strains. Partial recrystallisation occurred after the first pass due to the relatively coarse initial grain size. The steel chemistry played an insignificant role in controlling the recrystallisation kinetics at high roughing temperatures. The through-thickness strain distribution calculated from FEM simulations showed that, for a given applied strain, a similar magnitude of deformation is achieved at the centre of a hot-rolled plate.