Strengthening of austenite by niobium during hot rolling of Microalloyed steel

Abstract

Austenite in niobium–microalloyed steels at finish rolling temperatures is significantly stronger than austenite in mild steel. The present study, utilizing laboratory scale rolling, plane strain compression testing, and optical and electron microscopy, was carried out to quantify the effects of the various strengthening mechanisms, namely grain refinement, solid solution strengthening, and retained work hardening, and to establish whether or not strain induced precipitation of carbonitrides has a direct influence on strength. The effects of grain size and of niobium in solid solution are small compared with that due to work hardening retained because of the retarding effect of precipitates on recrystallization between passes. In addition, precipitates of sizes less than ~2–3 nm (below the limit of observation on extraction replicas) have been found to increase the flow stress in laboratory scale rolling by up to 17%. This direct strengthening effect depends critically on the thermo mechanical history and is lost if precipitates have time to coarsen to observable sizes. The combined effects of these contributions to the strengthening are shown to account quantitatively for the flow stresses observed in experimental rolling and in industrial plate rolling of microalloyed steels.

MST/267