Effect of Galvanizing Heat Treatment on the Microstructure and Mechanical Properties of a 6Mn-1.5Si Third Generation Advanced High Strength Steel

Third Generation Advanced High Strength Steels (3G-AHSS) are promising candidates for automotive weight reduction, but must be compatible with the thermal processing capabilities of the continuous galvanizing process (CGL) in order to be successfully employed in automotive applications. The effects of starting microstructure, intercritical annealing temperature (IAT) and holding time at IAT on the volume fraction of retained austenite and mechanical properties were studied for a prototype 3G-AHSS with 0.2wt% C-6wt% Mn-1.5wt% Si. Increasing the IAT and holding time at IAT increased the vol% retained austenite for tempered martensite starting microstructure samples with the maximum retained austenite observed for 710qC IAT and 600s holding time. In the case of martensitic starting microstructure samples, a decreasing trend in vol% retained austenite was observed after 360s and 60s holding time at the lower and higher IAT, respectively. The corresponding microstructures were analyzed with SEM and the conditions favouring high retained austenite vol% were chosen for uniaxial tensile testing. Martensitic starting microstructure samples exhibited a very high UTS u % elongation product, in the range of 3G-AHSS, for the lower IAT and short holding time unlike tempered martensite starting microstructure samples where higher IAT and long holding time were required. The best heat treatment conditions tested which are compatible with the continuous galvanizing process were found to be a 675qC IAT with a 120s holding time for the martensitic starting microstructure sample.