Deformation Behavior of Lightweight Fe-Mn-Al-C Steel

Abstract

Development of ultra high strength steels with good ductility and toughness has long been pursued in steel industries. As a result, especially for automotive applications, several advanced steels have been introduced such as interstitial free steels, dual phase steels, transformation-induced plasticity (TRIP) aided steels, etc. in addition, in response to the recent skyrocketing fuel cost and more strict regulations for safety and CO2 emission, automotive industries demand more advanced steels with light weight. At present, twinning-induced plasticity (TWIP) steels satisfy such requirements with the tensile strength of∼900MPa and ductility over∼50%. The extraordinary mechanical properties of TWIP steels are attributed to the fact that twins are dynamically formed during deformation of austenite with high Mn content due to relatively low stacking fault energy (SFE) of 20–40 mJ/m2, and these twins act as the effective obstacles for dislocation movement leading to high strain hardening. Recently, there is a growing interest in the so-called lightweight steels having excellent combinations of specific strength and ductility, which can meet the demands for energy conservation and environmental protection. Use of such lightweight steels in the transportation systems such as automobiles can significantly improve their performance, with concurrent reduction in fuel consumption and the emission of exhaust gases. There are basically two variants of lightweight steels

ferrite-base, and austenite-base steels depending on the type and amounts of alloying elements added, although most of the investigations have been carried out on austenite-base steels. These steels have rather large amount of C and therefore their microstructure usually contains κ- carbide particles as strengthening precipitates. Several recent investigations revealed that the Al addition up to ∼10 wt.% into the high Mn austenitic steels is beneficial for achieving not only remarkable weight savings but also mechanical properties comparable to or better than those of TWIP steels. These improvements mainly result from the SFE increase with increasing the Al content in the high Mn austenitic steels and the corresponding change of the deformation mode from TWIP to dislocation slip. It is also reported that the C addition into the high Mn–Al austenitic steels provides further strengthening by κ-carbides precipitation .As compared to austenite-base lightweight steels, however, the ferrite-base lightweight steels have not been received attention at all. Considering that the properties of steels are largely dependent on the types of constituent phases, it is of scientific and technological interests to study the deformation behavior and mechanical properties of lightweight steels with various matrix and second phases.