Investigating coating liquefaction and solidification of furnace-heated Al-Si coated 22MnB5 steel using laser reflectance

As part of the hot stamping process, aluminum-silicon (Al-Si) coated 22MnB5 steel blanks are austenitized using a roller hearth furnace, but the coating melts at similar to 577 degrees C, creating an intermediate liquid phase which, among other issues, shortens the lifetimes of the ceramic rollers and complicates blank transfer. Little is known about how this liquid regime is related to furnace process parameters, and how these parameters should be adjusted to minimize the impact of the liquid phase on hot stamping operations. This study pioneers laser reflectance measurements to infer the surface state of furnace-heated Al-Si coated steel coupons in situ. Time-resolved spectral reflectance measurements reveal multiple peaks, the first of which indicates the moment at which the coating liquefies. While the temperature corresponding to the start of the first peak is insensitive to heating rate, as the set-point of the furnace increases, the later peaks in the signal occur quicker but shift towards higher temperatures. These measurements are connected to the evolving surface microstructure via ex situ scanning electron microscopy and energy-dispersive X-ray spectroscopy of coupons extracted from the furnace at intermediate heating times.