The effect of trace vanadium addition on mechanical properties, microstructure and austenite phase characteristics in a 3.9% Al-Delta TRIP steel

Abstract

This study aimed to examine the effect of adding 0.12 wt% vanadium on the properties of delta TRIP steel through microstructural and mechanical investigations. The delta ferrite phase was observed in all annealing temperatures and samples. Carrying out bainitic transformation at 350 °C for 6, 10, and 15 min resulted in the stability of the austenite phase and the formation of ferritic bainite phase after heat treatment. The presence of vanadium led to the formation of MC precipitate phases and reduced the remaining austenite percentage from 14% to 6% by weight in the heat-treated sample. The low percentage of austenite in vanadium-containing steel has weakened the TRIP effect in strengthening. On the other hand, the presence of MC precipitates has compensated for the reduction in strength from the TRIP strengthening process through precipitation hardening and reducing the grain size of the delta ferrite phase. The resulting properties (UTS: 866 MPa and El: 41.3%) have led to the creation of a formability index of 35 GPa% in vanadium-containing steel, which is better than the mechanical properties (UTS: 857 MPa and El: 37.4%) and the formability index of vanadium-free steel (32 GPa%). These factors make this steel a suitable option for automotive industry applications. Based on these features make this steel a suitable candidate for automotive industries. Microstructural investigations revealed that the addition of vanadium reduces the grain size of the delta ferrite phase. Typically, the austenite phase forms around the ferritic bainite phase and delta ferrite grain boundaries. During tensile testing, it transforms into martensite through the TRIP process.