12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Substandard advanced high strength steel (AHSS) mechanical properties produced through continuous galvanizing can often be traced back to temperature excursions during intercritical annealing, caused by errors in the pyrometrically-inferred temperatures used to control the furnaces. These errors originate from the changing surface state of the steel strip, in terms of roughness and oxide formation, which causes wavelength-dependent variations in spectral emissivity that are unaccounted for in the pyrometry measurement model. In recent years, there has been a research effort to investigate the correlation between surface state and spectral emissivity of AHSS during inter-critical annealing. It was found there exists significant spectral emissivity variations during annealing. In addition, it has been shown that the variation of spectral emissivity also depends on the annealing atmosphere, since this causes different types and morphologies of oxides to form. Consequently, the effect of annealing atmosphere on surface state and spectral emissivity of AHSS with different alloy compositions, and the actual correlation between surface state and spectral emissivity during inter-critical annealing need to be studied in detail to produce AHSS strip that conforms to accepted standards. This paper elucidates the effect of annealing atmosphere and material alloy components on both the surface state and radiative properties of DP980 coupons. AHSS samples having different alloy compositions (i.e. Mn/Si ratios) are heated in a continuous galvanizing simulator within an atmosphere consisting of 95% N2 and 5% H2. The measurements were performed at two dew points, 10°C and −30°C, using heating schedules similar to that of industrial annealing line. The surface state of the coupons, including oxide type and surface topography, are characterized using scanning electron microscope, X-ray diffractometer, 2-D surface profilometry and atomic force microscopy, while the radiative properties (e.g. spectral emissivity) are measured using an FTIR spectrometer. Wagner’s diffusion model is applied to delineate the level of internal or external oxide formation given the annealing atmosphere and alloy components. This study highlights how annealing atmosphere and alloy components affect oxide formation and surface topography, and the connection with the radiative properties of AHSS steel strip during galvanizing.