12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Arcelor Mittal Cleveland Hot Dip Galvanizing line produces 260,000 metric tons of Galvannealed AHSS products (GA) annually for automotive customers. New AHSS grades and thickness gauges are under constant development for automotive applications to meet the future automotive fuel efficiency standards. The use of alloy elements such as Manganese, Silicon and Aluminum in Dual Phase and TRIP chemistries can lead to challenges to many aspects of the continuous galvanizing process, including galvannealing or alloying.
Under galvannealing, or under alloy, of AHSS is one of the major quality concerns. Based on several case studies at the industrial coating line, the present work reviewed various mechanisms that have caused under alloy on both Dual Phase and TRIP steel. Coated steel samples with under alloyed appearances were examined by SEM/EDS, XRD and GDS. Metallography results showed that non-uniform zinc coating and silicon segregation at the edge were two major contributing factors for edge under alloy on TRIP and Dual Phase steel. It was found that unpickled hot band scales and micro flaps were responsible for the underalloyed streaks and patches in Dual Phase steel. It is suggested that selective internal oxidation in the radiant tube furnace can help galvannealing of AHSS, to a limited extent. DFF pre-oxidation can also enhance galvannealing of AHSS but it can lead to other processing issues such as low surface roughness and high dew point in radiant tube furnace. Both internal oxidation and DFF pre-oxidation practices could produce abnormal heavy coating buildup on strip in zinc pot during line stop which could lead to safety and quality issues. It is believed that when tensile strength for dual phase is over 1180 MPa, ferrite volume fraction plays a critical role in the galvannealing process. The high resistivity and low magnetic permeability of the steel could potentially lead to traditional galvannealing furnace tripping off causing steel not alloyed. The investigations concluded that the homogeneity of alloy elements, incoming steel substrate cleanliness, pot equipment reliability, steel microstructure and galvanneal inductor design are all important aspects to ensure full galvannealing of AHSS.