Connecting surface microstructure to the radiative properties of galvannealed steel

Accurate pyrometry during galvannealing is crucial for obtaining a coating with the desired properties; however, interpreting pyrometric data is complicated by dramatic changes in radiative properties as the coating transforms from a liquid metal to a solid intermetallic. This study explores the relationship between instantaneous surface state and radiative properties by correlating spectral reflectance measurements made on industrially processed galvannealed specimens with surface profilometry, XRD, and SEM characterization. The radiative properties of the galvannealed coating are approximately wavelength-independent over the near-infrared spectrum (1.5 – 2.5 µm), which is advantageous for ratio pyrometry. Between 2.5 and 15 µm, however, the radiative properties are sensitive to the formation of zeta-dominant microstructures on the surface and evolving surface roughness. These findings suggest the potential of inferring other industrially relevant surface parameters of galvannealed steel through in situ spectroscopy.