Mechanical properties enhancement of a novel medium Mn-TRIP/TWIP assisted steel by dispersion of M2B-type borides particles

Abstract

The strain hardening behavior of a novel medium Mn TRIP/TWIP assisted steel which has been synergically reinforced by M2B-type boride particles was investigated in this work. As-cast material was thermo-mechanically processed by forging and hot rolling at 1150 °C. Hot rolled strip was divided into three parts, two of which were annealed at 800 and 900 °C, respectively, to generate three different microstructures, and therefore a range of different mechanical properties. The hot rolled sample showed a small grained austenitic microstructure reinforced by the dispersion of tetragonal M2B particles. This microstructure evolved to a similar coarsening degree after the heat treatments used, but while annealing at 800 °C produced a nearly 100%-austenite microstructure, about 13% of ferrite was observed after the treatment at 900 °C. EBSD studies showed that part of the tetragonal M2B-type boride particles transformed into the orthogonal M2B structure at 800 °C, as predicted by Thermo-Calc calculations. The effect of grain size, austenite stability and the structural variant of borides present were related to the mechanical properties determined by tensile test at room temperature. The differences observed for yield strength between as hot rolled and annealed samples were associated to a Hall-Petch grain size effect. On the other hand, differences in strain hardening behavior and elongation to failure observed among three samples were interpreted from perspectives of the TRIP/TWIP behavior of austenite and the deformation behavior of M2B-type borides. The presence of some non-shearable dual-phase M2B-type borides and an optimal rate of strain-induced transformation of austenite into martensite at 800 °C caused a significant enhancement of total elongation to failure (up to 60%) with no changes on the tensile strength from 1100 MPa, resulting in an ultra-high product of strength and elongation (66.0 GPa%).