Influence of Nb microaddition on a microstructure of low-alloyed steels with increased manganese content

The present study is a first step of a project to obtain thermo-mechanically processed fine-grained increased Mn content TRIP steels with large fractions of retained austenite. Two 0.17C-3Mn-1.6Al-0.2Si-0.2Mo steels with and without Nb microaddition were produced in a vacuum induction furnace. The influence of Nb microaddition on a macrostructure, a grain size and hot-working behavior were examined. The steels are characterized by a slight macrosegregation of Al in the as-cast state, minimized for a Nb-microalloyed steel. After hot forging refined bainitic-martensitic structures with large fractions of gamma phase obtained. The steel microalloyed with Nb has finer granules of retained austenite at comparable fractions of this phase. The force-energetic parameters of hot-working were determined in an uniaxial hot-compression test at temperatures of 1150 and 950 degrees C and strain rates from 0.1 to 10s(-1). The Gleeble 3800 thermomechanical simulator was used. The hot-working behaviour of the investigated steels is challenging because of higher flow stresses and epsilon(max) strains compared to conventional TRIP steels with lower Mn contents.