The effect of continuous galvanizing thermal cycle on the microstructure and mechanical properties of two multiphase TRIP-assisted steels

Multiphase TRIP assisted-steels are particularly attractive for the automotive industry, as they exhibit an exceptional strength-ductility balance which is attained through the combination of a complex microstructure and of a TRIP effect, i.e. the mechanically induced transformation of metastable retained austenite. This multiphase microstructure - and particularly the retention of metastable austenite - is obtained through the combination of appropriate chemistry and processing conditions, i.e an intercritical annealing followed by an isothermal hold in the temperature range for bainite formation. It has been established that this second step is important in controlling austenite retention and hence the mechanical properties. In the present work, the effect of heat treatment cycles compatible with the continuous hot dip galvanizing process, namely a high bainitic dwell temperature, on the microstructure and mechanical properties of two multiphase TRIP-assisted steel grades - one Si-alloyed grade and one mixed Si-Al grade - has been studied. It was shown that bainite formation in the Si-alloyed grade was too slow to bring about the retention of a significant amount of austenite down to room temperature. The mixed AlSi grade, on the other hand, exhibited faster bainite formation kinetics under heat treatment conditions compatible with the CGL process, such that it is possible to retain a significant amount of austenite. The partial substitution of silicon by aluminum appears thus a promising path for the production of galvanized TRIP-assisted steels.