Interpretation of microstructure evolution and mechanical properties under aging treatments of a Fe–24Mn-0.6C–2Al-0.6V austenitic steel for cryogenic application

Abstract

Aging treatment is an effective way of strengthening high-Mn strip steels based on automotive industry. However, the role of this method in enhancing yield strength and cryogenic toughness (−196 °C) of high-Mn steels has still unclear so far. Besides, solute segregation has always been a vital factor affecting the mechanical properties of high-Mn steels, whereas it is still lacking in adequate attention. In the present work, the microstructural evolution, including solute macrosegregation, grain boundary segregation and precipitation, and the resultant mechanical properties under different aging treatment were elaborated. It was found that the banded segregation of C, Mn and V elements formed during aging treatment strongly affects the cryogenic toughness. Tridimensional EPMA analyses demonstrate that there exists dense banded segregation of C, V and Mn elements in longitudinal section and cross section for both aged steels (aging at 500 °C and 800 °C). However, more obvious banded segregation of C and V elements developed in the steel aged at 800 °C (800A), and more micron-scale VC carbides tend to precipitate at segregation band, which leads to the easy initiation and propagation of microcracks along the segregation band. Moreover, numerous nano-scale VC precipitates significantly increase the yield strength of 800A steel by 100 MPa, but there exist severer grain boundary segregation and precipitation after aging treatment, which enhances the twining stress by increasing local SFE of grain boundary regions. The twinning formation hence became harder, further deteriorating the cryogenic toughness of the 800A steel.