Mechanism-controlled thermomechanical treatment of high manganese steels

Abstract

Austenitic high manganese steels exhibit outstanding mechanical properties, such as high energy absorption, owing to various deformation-mechanisms such as dislocation slip, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Here, we show a novel thermomechanical treatment to manufacture a high manganese steel Fe–18Mn-0.3C (wt.-%) with excellent mechanical performance by combining these three deformation-mechanisms. This process of mechanism-controlled rolling resulted in ultra-high tensile strength of the high manganese steel up to 1.6 GPa, simultaneously with uniform elongations up to 15%.A thermomechanical process was developed to establish this combination of properties. Warm rolling was conducted at 200 °C, to suppress TRIP and activate TWIP as deformation mechanism. Thus, a high density of deformation twins and dislocations was introduced to the microstructure, avoiding martensite formation. During a subsequent recovery annealing at 520 °C or 550 °C, the dislocation density was reduced, yet the high density of deformation twins was preserved. The combination of warm rolling and recovery annealing resulted in an ultrafine microstructure with a high density of twins and moderate density of dislocations. The TRIP effect is predominant during plastic deformation at ambient conditions in the highly twinned microstructure. The resulting steel exhibits an ultra-high yield strength and sufficient ductility, favorable properties for lightweight construction in automotive or aerospace industry.