Investigating the evolution of the aluminum-silicon coating on 22MnB5 steel during heating

Abstract

To combat greenhouse gas emissions and improve fuel economy, automotive manufacturers have searched for methods to reduce vehicle weight. As such, hot stamping has become an increasingly popular sheet metal forming technique, due to its ability to produce exceedingly strong parts that can downgauged while maintaining sufficient strength to protect passengers in the event of a collision. In this process, steel blanks are heated in a roller hearth furnace to approximately 930°C to austenitize the grain structure before they are automatically transferred to a die where they are simultaneously quenched and formed into the final part shape. Rapid cooling converts austenitic grains to martensite, achieving ultimate tensile strength values greater than 1500 MPa.

The blanks are often coated with a protective aluminum-silicon (Al-Si) layer to prevent oxidization and decarburization at elevated temperatures. However, the coating melts around 577°C and undergoes a temporary liquid phase before iron-rich intermetallics diffuse from the base steel and solidify the coating. The molten aluminum can pollute and degrade the furnace rollers, leading to their failure. The state change also dramatically alters the radiative properties of the blanks, complicating thermal modelling attempts. Furnace operators have attempted to minimize the impact of coating liquefication through trial-and-error adjustments to the heating parameters of the blanks, but without a clear understanding of the transformation behaviour of the coating, these attempts fail to attain an optimal solution.

This work documents several measurement techniques employed to characterize the evolution of the Al-Si coating during heating. In-situ laser-based reflectance measurements revealed that the liquefication of the coating is comprised of multiple reaction steps at different temperatures. An expansion of this technique captured time-resolved roughness profiles of the blanks, and a quantitative assessment of coating transformation was acquired. Ex-situ spectral reflectance and in-situ spectral emittance measurements were also performed to correlate changes in radiative properties to the developmental stages of the coating. Additionally, inflection points in the radiative spectra were linked to the morphology of an oxide layer or the diffusion of intermetallic compounds. Finally, the chemical evolution of the surface was captured through Raman spectroscopy measurements, documenting the Raman spectra of several intermetallic compounds unpublished in literature. These results expand the knowledge base on the Al-Si coating, benefitting hot stamping practitioners and furnace operators alike.