Development of a multivariate spectral emissivity model for an advanced high strength steel alloy through factorial design-of-experiments

Abstract

Variations in the spectral emissivity of advanced high strength steels (AHSS) during intercritical annealing leads to errors in pyrometry measurements, which, in turn, cause thermal excursions that impact the mechanical properties of the steel. This paper presents an empirical approach for modelling the spectral emissivity of advanced high strength steel. Samples of two dual-phase steel (DP980) alloys, having Si/Mn ratios of 0.04 and 0.23, are heated within a galvanizing simulator in atmospheres of 95%/5% N2/H2 and dew points of 10°C and −30°C. The spectral hemispherical reflectance of the annealed samples was measured with an FTIR spectrometer. The variation of the spectral emissivity with dew point, alloy composition, pre-annealed surface state, and wavelength is analyzed using full factorial designs. The significant main and interaction effects vary across the spectral range, with the ratio of alloy components and pre-annealed surface state dominating at shorter and longer wavelengths, respectively. The predicted spectral emissivity values obtained from the model fitted for a three-channel pyrometer shows good agreement with the measurements. This study shows response surface methods (RSM) to be a viable approach for developing spectral emissivity models for pyrometry applications.