Partially recrystallized triplex-phase medium Mn steel strengthened by B2 precipitates

We report the annealing time-dependent microstructure–mechanical properties relationship of cold-rolled and annealed Fe–6Mn-0.05C–3Ni-1.5Al (wt.%) steel, containing Ni and Al to form NiAl B2 precipitates. The microstructures of the investigated materials after cold-rolling and intercritical annealing show a mixture of lath and equiaxed zones consisting of a triplex matrix phase (ferrite, austenite, and tempered martensite) with B2 precipitates. Recovery of the deformed martensite in the cold-rolled microstructure during annealing resulted in the formation of sub-boundaries and a continuous transition from partially-recrystallized tempered martensite to recrystallized ferrite. The investigated materials showed a decrease in strength and an increase in ductility and strain-hardening rate with increasing annealing time. A decrease in the fraction of tempered martensite, an increase in the fraction of recrystallized ferrite/austenite grains, and enhanced transformation-induced plasticity (TRIP) kinetics with increasing annealing time led to enhanced ductility and strain hardening of the materials. The mixed presence of less stable equiaxed austenite and more stable lath austenite resulted in sustained TRIP effect during tensile deformation and superior strain hardening capacity of the specimen annealed for 24 h. This study provides a novel microstructure design solution for medium Mn steels, and further optimization of the composition and processing will lead to the development of medium Mn steels with superior mechanical performance.