No abstract

Prior cold deformation is known to influence the ferrite-to-austenite (α → γ) transformation in medium-manganese (Mn) steels that occurs during intercritical annealing. In the present study, a 7Mn steel with ultra-low residual carbon content and varying amounts of prior cold deformation was intercritically annealed using various heating rates in a dilatometer. The study was conducted using an ultra-low carbon steel so that assessments of austenite formation during intercritical annealing would reflect the effects of cold deformation on the α → γ transformation and Mn partitioning and not effect cementite formation and dissolution or paraequilibrium partitioning induced austenite growth from carbon. Increasing prior cold deformation was found to decrease the Ac1 temperature, increase austenite volume fraction during intercritical annealing, and increase the amount of austenite nucleation sites. Phase field simulations were also conducted in an attempt to simulate the apparent accelerated α → γ transformation with increasing prior cold deformation. Mechanisms for accelerated α → γ transformation explored with phase field simulations included an increase in the amount of austenite nucleation sites and an increased Mn diffusivity in ferrite. Simulations with different amounts of austenite nucleation sites and Mn diffusivity in ferrite predicted significant changes in the austenite volume fraction during intercritical annealing.

Experimental research on intercritical annealing of a cold‐rolled Fe–7Mn steel with ultralow carbon concentration is presented, providing insight into the efficacy of generating Mn‐enriched austenite during relatively short (≈1000 s) heat treatments compared with batch annealing, as well as the phase transformation mechanisms that can occur during intercritical annealing. It is shown through both bulk characterization and electron microscopy in conjunction with energy‐dispersive X‐ray spectroscopy that Mn partitioning to austenite occurs via diffusional, partitioning growth of austenite during these shorter treatments and that the intercritical temperature influences the level of Mn enrichment in austenite. Evidence of massive (partitionless) and diffusional (partitioning) transformations of ferrite to austenite occurring simultaneously in a single sample are also shown along with a thermodynamic rationale for this occurrence, which is suggested to be related to residual Mn‐banding that develops during solidification.

Double soaking (DS) is a thermal processing route intended to produce austenite–martensite microstructures in steels containing austenite‐stabilizing additions and consists of intercritical annealing (primary soaking), followed by heating and brief isothermal holding at an increased temperature (secondary soaking), and quenching. Herein, experimental dilatometry during DS of a medium‐manganese (Mn) steel with nominally 7 wt% Mn and an ultralow residual carbon concentration, in combination with phase‐field simulations of austenite formation during secondary soaking, is presented. The feasibility of maintaining heterogeneous Mn distributions during DS is demonstrated and insight is provided on the effects of the secondary soaking temperature and prior Mn distribution on the ferrite‐to‐austenite phase transformation during the secondary soaking portion of the DS treatment.

Microstructural changes during thermal processing of a medium manganese steel containing (in wt%) 0.19C and 4.39 Mn were evaluated in situ with a high energy X-ray diffraction system (HEXRD). Samples with an initial fully martensitic microstructure were heated to intercritical annealing (IA) temperatures of 600 or 650°C, held for 30 min, and cooled to room temperature. Diffraction data were analyzed to determine the variations in austenite and ferrite phase fractions and phase lattice constants throughout the ICA cycles. On heating, the 2 vol. pct of austenite present in the starting microstructure decomposed, and cementite precipitation then occurred. During isothermal holding, the austenite fraction increased, up to 20% for the sample annealed at 650°C. The measured austenite fractions were less than those calculated by Thermo-Calc for equilibrium conditions, indicating that the 30-min hold time was insufficient to achieve near-equilibrium conditions. Observed changes in lattice parameters during isothermal holding were interpreted to reflect composition changes due to redistribution of the C and Mn between austenite and ferrite. The results are discussed in relation to the potential for controlling austenite stability during ambient temperature deformation.