Continuous galvanizing of a C-Mn-Si-Mo complex phase ultra high strength steel for automotive anti-intrusion applications

From the perspective of crashworthiness, complex phase ultra high strength steels are ideal candidates for automotive anti-intrusion applications. These steels must be protected from corrosive environments in order to maintain the long-term integrity of the structures involved, for which continuous galvanizing is amongst the most cost-effective solutions. Several challenges have to be overcome in order to process the above steels in the continuous galvanizing line while achieving the minimum target tensile strength of 1250MPa. In the present research, a steel chemistry was designed specifically for processing in the continuous galvanizing line using a relatively lean composition based on carbon, manganese and silicon with additional hardenability being provided by molybdenum addition. The target tensile strength of 1250MPa was achieved using both intercritical and austenitic annealing for 120s followed by cooling to room temperature at 20 ^°Cs ^-1. The steels were successfully reactively wet by the Zn(Al,Fe) bath using a 95%N 2-5%H 2, -30°C dew point process atmosphere, resulting in a high quality adherent coating.