On the degradation the endurance of silicon-rich TRIP800 steel after hot-dip galvanization

Incorporation of Advanced High Strength Steels in structural parts of vehicles should save weight and enable economy of fuel. A very efficient protection against corrosion is achieved by hot-dip galvanization, which consists in dipping the steel in a molten zinc bath at a temperature around 450 °C. Degradation of the fatigue toughness of several steels after galvanization has been reported in several previous studies. Presently, we present new results concerning the evolution of mechanical properties of a silicon-rich TRIP800 steel after galvanization. The most striking is a reduction of endurance by a factor of ten at 525 MPa after galvanization. In the present work, we have ascertained two potential explanations for this degradation: (1) Modification of the core microstructure of the TRIP steel as a consequence of heat-treatment and (2) the propagation of cracks formed within the coating intermetallic layers after galvanization into the steel substrate upon cyclic loading. We found that heat-treatment is not responsible for the drop of fatigue lifetime. Our results neither provide any evidence that the cracks pre-existing in the zinc coating could be the starting point of the fracture in steel.