Effect of continuous galvanizing heat treatments on the microstructure and mechanical properties of Al-Si TRIP steels

TRIP-assisted steels are ideal for lightweight automotive applications due to their high strength and ductility. In order to be used by the automotive industry, galvanizing is essential to provide corrosion protection. One of the challenges in galvanizing TRIP steels is that the conventional heat treatments used to obtain the required properties are not compatible with the higher temperatures required for hot dip galvanizing. Heat treatments were performed with higher than nominal bainite hold temperatures compatible with hot-dip galvanizing for two high Al - low Si TRIP steels. Two different intercritical annealing temperatures were used for each steel to obtain 35% and 50% intercritical austenite. Five different bainite hold times were used ranging from 4 seconds to 120 seconds. The amount of retained austenite in each sample was measured using X-ray diffraction. Tensile testing was conducted to determine the effect of alloy composition, intercritical annealing temperature and bainite hold time on the tensile strength and uniform elongation. The results of the processing parameters on the mechanical properties and microstructural evolution are presented. Copyright © 2007 MS&T'07®.