Influence of dew point and galvannealing furnace temperature on the surface oxidation and coating behavior of DP450 steel

This study investigates the microstructural, surface, and mechanical properties of DP450 steel under varying annealing and galvannealing conditions, emphasizing the effects of dew point and galvannealing furnace (GAF) temperature. Microscopic analyses showed a predominantly ferritic matrix with secondary martensite and bainite phases within DP450. Surface oxidation was examined at dew points of −25 °C, 0 °C, and +15 °C. Lower dew points favored external selective oxidation of manganese and chromium, while higher dew points promoted internal oxidation, affecting coating quality. Depth profiling confirmed that dew point variations influenced oxide layer thickness and composition. Galvannealed coatings mainly consisted of delta and zeta intermetallic phases, with Fe diffusion and alloying strongly depended on dew point and GAF temperature. Low temperatures and dew points caused insufficient alloying, heterogeneous coatings, increased zeta phase, and surface irregularities. Raising dew point and temperature enhanced Fe diffusion, producing uniform delta phase coatings with smoother surfaces. Mechanical tests linked coating friction and powdering behavior to phase composition and surface morphology. Optimizing dew point and GAF temperature balances oxidation and Fe–Zn reaction kinetics, resulting in coatings that meet automotive standards for Fe content, phase ratios, and mechanical performance.