Microstructural evolution of intercritically annealed medium-manganese steels

Abstract

Intercritical annealing (IA) of medium-Manganese (Mn) steels is an AHSS design concept for the production of formable sheet steel. The microstructure after IA includes retained austenite (5-40 volume pct), which forms from an initial microstructure of either cold-rolled ferrite or martensite, and is stabilized to room temperature via C and Mn enrichment. Retained austenite is essential to the elevated strength-elongation, and can be engineered with equilibrium modelling and judicious selection of alloy composition and heat treatment.Long IA times (i.e. batch annealing) are expected to result in near-equilibrium microstructures with uniform Mn distributions within austenite. Relatively short IA times (<1000 s), however, may be more attractive due to energy conservation and more flexible processing routes. With shorter IA times, the phase transformation mechanism associated with austenite growth is of interest, as the sluggish diffusion of Mn in austenite is likely to result in the retention of any Mn gradients in austenite which develop during austenite growth. This thesis primarily focuses on elucidating austenite formation mechanisms during IA, and investigating the efficacy of generating Mn enrichment in austenite during relatively short IA treatments. In particular, the effect of cementite on the austenite formation mechanism, and the kinetics of Mn and carbon (C) partitioning to austenite, were investigated. Additional work focused on the effect of prior cold deformation on austenite growth and Mn redistribution during double soaking (DS) heat treatments. Scanning transmission electron microscopy with energy dispersive X-ray spectroscopy, transmission kikuchi diffraction, and field emission scanning electron microscopy were utilized for microstructural characterization, while in situ high energy X-ray diffraction, ex-situ X-ray diffraction, and dilatometry were used for bulk assessments. Phase field simulations using MICRESS®, and one dimensional diffusional simulations with the DICTRATM module of Thermo-Calc®, were also conducted for austenite formation and Mn partitioning. Austenite growth was found to be controlled predominantly by Mn diffusion. C-diffusion controlled kinetics were inhibited due to Mn enrichment in cementite, which stabilized cementite and caused dissolution to be controlled via Mn diffusion. Formation of film-like austenite from martensitic microstructures did not require the preservation of initial austenite films during heating. Phase field simulations predicted Mn-partitioning or massive transformation of austenite during DS, depending on the secondary soaking temperature. Experimental results for DS treatments were consistent with phase field simulations, which indicated that a bimodal Mn distribution can be maintained in a fully austenitic microstructure during DS treatments.