Effect of substitution of Si by Al on the microstructure and mechanical properties of bainitic transformation-induced plasticity steels

The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity (TRIP) steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3rd generation advanced high strength steels (AHSS). It is shown from the dilatometry study, microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than 450 °C whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450 °C. Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450 °C while Si-added steel has a better strength-elongation combination at temperatures lower than 450 °C. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite (MA) blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.