Suppression of selective surface oxidation in DP steels by effective shielding of copper - Kinetic simulation and experimental validation

The high strength steels have become essential for the auto body parts owing to the combination of high strength as well as better crash resistance. The galvanizing and galvannealing of these high strength steels are required for longer performance life of the steels. The constituent alloying elements in the high strength steel e.g. manganese, silicon and chromium have a tendency to oxidize at the steel surface during the continuous annealing of the steel. The oxides greatly inhibit the wettability of steel with molten zinc during galvanizing. In the present study the application of prior copper coating on Dual Phase (DP) steel was shown to act as a diffusion barrier for the alloying elements during annealing. The kinetic simulation for diffusion of manganese and oxygen through the copper layer having an initial thickness of 200 nm showed that the oxidation front can be arrested at the sub-surface region near the steel-copper interface. The simulation results were verified with experimental annealing simulation and subsequent depth profiling by Glow Discharge Optical Emission Spectroscopy (GDOES) and Transmission Electron Microscopy (TEM)/Energy Dispersive Spectroscopy (EDS) line scanning. The copper coating was able to suppress the surface segregation and selective oxidation of manganese as oppose to the conventional DP steels. The galvanizing of the copper coated DP steels exhibited defect free zinc coating as compared to the regular DP steels having high number of bare spots. The simulation technique can be further extended to determine the thickness and type of pre-coating layers for high strength steels.