Influence of chemical composition on the microstructure and mechanical properties of multiphase TRIP-assisted steels, processed using thermal cycles compatible with continuous hot-dip galvanizing

Multiphase TRIP-assisted steels are particularly interesting for the automotive industry, as they exhibit an exceptional strength-ductility balance due to the combination of a multiphase microstructure and the mechanically-induced transformation of metastable austenite to martensite (i.e. the TRIP effect). The multiphase microstructure - and the retention of metastable austenite - is achieved through the combination of appropriate chemistry and heat treatment conditions, i.e. an intercritical anneal followed by an isothermal dwell in the temperature range for bainite formation. The present work focuses mainly on the effect of heat treatment conditions that are compatible with CGL process, i.e. a high bainitic dwell temperature, on the microstructure and mechanical properties of two TRIP-assisted steel grades differing by their silicon and aluminium contents. It has thus been established that it seems easier to retain austenite when Si is replaced by Al, and more particularly that it was possible to retain a significant amount of austenite in a mixed 0.5Al-1.0Si grade, and hence to observe a TRIP effect. On the other hand, a Si-alloyed grade did not exhibit a significant retention of austenite, possibly due to carbide precipitation. It has also be seen that the intercritical annealing temperature has a strong influence on the amount of austenite that can ultimately be retained after the bainitic dwell, possibly by affecting the C content in the intercritical austenite.