Influence of Hot Rolling Practice and Furnace Residence Time on the Strength and Toughness of Normalised Nb-Ti-V Structural Steel Plate

Abstract

The influence of moderate thermomechanical rolling and heat treatment time on the microstructure and mechanical properties of normalised Ni-containing Nb-Ti-V microalloyed plate steels was investigated. Conventional rolling (CR) and low austenitic temperature rolling (TM) schedules were applied to 25 and 55mm thick plates. Predicted plate thermal profiles were used to simulate normalising of 25 and 55mm thick plates at the mid-thickness. As expected, the microstructure after TM rolling was finer due to austenite sub-structure development below the no-recrystallisation temperature which increases the amount of ferrite nucleation sites. The tensile strength of the CR plates was generally higher due to a larger pearlite volume fraction resulting from slow cooling from a relatively coarse austenite. In 25mm plate, the presence of acicular ferrite after CR resulted in a higher yield strength than that obtained after TM rolling. Furnace residence times of 0.75min/mm and longer allowed for plates to be completely normalised – characterised by continuous, well-defined polygonal ferrite grain boundaries. Short furnace residence times restricted the plate temperature to below the end of the ferrite-to-austenite transformation (Ac3), where untransformed ferrite is retained from the as-rolled microstructure and is characterized by poorly defined grain boundaries. Rolling practice and normalising residence time had little to no effect on impact toughness, but significantly influenced the normalised yield strength of thicker plate.