Non-uniform distribution and strengthening effect of Cu precipitates enclosed in austenite during intercritical annealing in a medium Mn steel

Abstract

The evolution of Cu particles which form in a Cu-bearing medium Mn steel, whose composition is Fe-0.16C-5.62Mn-2.01Cu-1.95Ni-0.65Al-0.49Si in mass%, was investigated with focus on Cu particles in the reversed austenite. Almost all Cu particles were formed in the martensite matrix during heating to annealing temperature, and calculation of phase boundaries and miscibility gaps in the multi-component alloy indicated that Cu particles in annealed martensite were probably incorporated into reversed austenite with the migration of α/γ interface during annealing. DICTRA simulation and STEM-EDX analysis revealed that the partitionless mode of austenite growth prevailed at an earlier stage and it switched to a mode accompanying alloy partitioning. Cu particles in the ferrite (or annealed martensite) matrix partially dissolved and the rest coarsened during annealing while Cu particles in austenite were on the way to total dissolution, but the rate was slow. As a result, the size and volume fraction were greater near the α/γ interface in austenite and they were opposite in the center of reversed austenite. During deformation, retained austenite provided a remarkable TRIP effect. Cu particles in the ferrite matrix caused strengthening significantly and those in austenite could have a function of delaying the decreasing in strength due to slow diffusivity of alloy elements and dissolution rate.