Interpreting the radiative properties of advanced high strength steel strip using a hybrid thin film/geometric optics model

Advanced high strength steel (AHSS) alloys derive their mechanical performance through a carefully controlled intercritical annealing process. Unfortunately, variations in spectral emissivity caused by selective oxidation and non-uniform surface roughness imparted by cold-rolling impact the accuracy of the pyrometrically-inferred temperatures used for process control. This study evaluates the spectral reflectance of cold-rolled and annealed AHSS in the visible and near-infrared spectra, with the objective of improving the reliability of pyrometry. Reflectance measurements reveal that both thin film interference and roughness significantly impact the radiative properties of AHSS over the wavelengths important to pyrometry; these results are interpreted empirically using electron microscopy and optical profilometry. Predictions using a thin film interference model and a hybrid model, which accounts for both wave interference and roughness effects, generally underestimate the reflectances of oxidized AHSS having rough states. For most of these samples, the weaker interference effect observed in the measured reflectances are attributed to the rugged and non-uniform oxide morphology.