Connecting surface microstructure to the radiative properties of galvannealed steel

Accurate pyrometry during galvannealing is crucial for achieving coating having the desired properties; however, a major challenge arises from changes in the coating's radiative properties as it transforms from a liquid metal to a solid intermetallic. This study explores the relationship between the surface state and radiative properties of the coating by correlating spectral reflectance measurements of industrially processed galvannealed specimens over the near-infrared (NIR) and mid-infrared (MIR) spectra with characterization by surface profilometry, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The radiative properties of the galvannealed coating are approximately wavelength-independent over NIR wavelengths (1.5–2.5 μm), which is advantageous for ratio pyrometry. Over MIR wavelengths (2.5–15 μm), however, the radiative properties are sensitive to the formation of intermetallic microstructures on the surface and evolving roughness. These findings suggest the potential of inferring other industrially relevant surface parameters of galvannealed steel through in situ spectroscopy.