Influence of microstructure evolution on hot ductility behavior of austenitic Fe–Mn–Al–C lightweight steels during hot tensile deformation

Abstract

Four alloys based on the Fe–30Mn–(8.5–12)Al–(1.0–1.3)C (wt%) system were prepared to investigate the effects of microstructure evolution and κ-carbide precipitation behavior on hot ductility behavior of austenitic lightweight steels. Hot tension tests were carried out at temperatures of 500–1230 °C using a Gleeble simulator. At high temperatures above 1000 °C, dynamic recrystallization occurred in all alloys, leading to high tensile ductility. At temperatures of 700–900 °C, the ductility decreased in all alloys due to the intragranular precipitation of κ-carbide, with increases in the amounts of Al and C contents then leading to a greater loss of ductility due to the formation of coarse intergranular κ-carbides. The addition of Cr and Mo suppressed the precipitation of κ-carbide, reducing the extent of ductility loss. At 500 °C, the ductility was recovered due to a reduction of inter-/intragranular κ-carbide precipitation and the development of slip bands caused by planar gliding of dislocations through κ-carbide shearing. The spacing among slip bands then became coarse with an increase in the Al and C contents, resulting from the coarsening of κ-carbide. Meanwhile, dynamic strain aging (DSA) behavior was observed in all alloys deformed at 500 °C. This occurred because the hot tensile tests were carried out under a high strain rate condition; therefore, the mobility of the dislocations was fast and thus solute atoms pinned the dislocations despite deformation at a high temperature. With a coarsening of κ-carbide, the extent of serration was reduced, resulted from the fact that the content of solute C decreased due to the greater precipitation of κ-carbide; i.e., the amounts of solute C atoms to cause the DSA behavior were reduced.