The Causes and Effects of Preannealed Radiative Property Variations Across Full‐Hard Advanced High‐Strength Steel Coils

This study investigates the connection between surface topography and radiative property variations across full‐hard (unannealed and cold‐rolled) advanced high‐strength steel (AHSS) coils. Directional–hemispherical reflectivities of samples extracted from various locations on a full‐hard DP‐780 coil are measured using a Fourier transform infrared spectroscopy spectrometer at wavelengths between 0.5 and 20 μm. Surface topography analysis is performed using optical profilometry, optical microscopy, and scanning electron microscopy. Surface height profiles are generated using 3D depth mapping of optical images. The surface profiles are then used in a geometric optical approximation model, which correlates the radiative properties to surface topography variations. Significant differences in radiative properties and surface topography are observed along the length and width of the coil. These variations in radiative properties may explain the temperature excursions that cause nonuniformities in mechanical properties observed across AHSS coils produced on continuous galvanizing lines in the industry.