An Extraordinary Reversed Austenite Transformation Behavior in a Cu-Rich Medium Manganese Steel

Abstract

A new Intercritical Annealing-Tempering (IAT) treatment was proposed to process Fe-4.5Ni-5Mn-2Al-4Cu (all in wt.%) ultra-low carbon steel and to investigate the intrinsic precipitation behaviors and microstructural evolution that correlated with the heat treatments. It was shown that the austenite volume fraction increased from 12.4% to 18.5% after long-term low-temperature aging process by reversion. In particular, a type of retained austenite with a very large aspect ratio was found in prior austenite grain boundaries (PAGBs) through electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) characterizations. Furthermore, more acicular oriented austenite in PAGBs was also observed after the aging process. This austenite morphology was elucidated via the growth of reversed austenite before coalescing on impingement and grain boundary migration which was also verified by a modified numerical simulation method based on the segregation engineering mechanism. The austenite stability in PAGBs was attributed to further substitutional alloying elemental partitioning, leading to acicular austenite evolution by gradually growing and coalescing without the loss of coherency and mother martensite matrix hardening induced mechanical stabilization.

Keywords: Medium manganese steel, Grain boundary segregation, Austenite reversion, Nanoscale precipitates, Austenite morphology