Precipitation Strengthening in Austenitic High Mn Steels

Abstract

Austenitic high Mn steels usually show high work hardening rate and accordingly excellent combination of ultimate tensile strength and ductility that are suitable for automotive applications. However, in spite of such excellent tensile properties, these austenitic high Mn steels usually show low yield strength as compared to other conventional ferritic high strength steels. Therefore, there is a great need for improving yield strength of austenitic high Mn steels without sacrificing other properties. In the present study, effects of carbide former Ti, Nb, and V on the microstructure and tensile properties were investigated. Amount of each strengthening effect of the steels were analysed including solid solution strengthening, grain boundary strengthening and precipitation strengthening. Addition of Ti, Nb and V increased its yield strength. Their strengthening effects were mostly from grain refinement and precipitation strengthening. Among the addition of Ti, Nb and V, V addition in Fe-18Mn-0.7C steel shows best strengthening effect among the steels. Also, Fe-16Mn-0.8C-0.5Si-0.5V steel was developed to obtain high yield strength which is similar yield strength of hot press forming (HPF) steel. The alloy was fabricated and their microstructure and tensile properties were evaluated after annealing of cold rolled sheets. It shows that the cooling rate after annealing as well as alloy composition have a large effect on precipitation behavior of carbides and accordingly tensile properties. The results show that there is a precipitation of fine VC with a small amount of Fe3C during annealing and an increase in annealing time decreases strength due to coarsening of both carbides and grain sizes despite an increase in the volume fraction of VC. Utilization of slow air-cooling instead of water-quenching promotes an additional precipitation of fine VC and Fe3C, resulting in a large increase in strength that are much higher than those of water-quenched specimen as well as conventional high Mn steels.