Why is this important?
Automakers rely on galvanized advanced high strength steel (AHSS) alloys for automotive lightweighting, in oder to increase the fuel efficiency of automobiles and extend the range of electric vehicles. The superior mechanical properties of AHSS alloys are derived from their engineered microstructure, which is realized through a carefully-controlled intercritical annealing (ICA) process. Unfortunately, temperature excursions during this process results in substandard steel, leading to millions of dollars in wasted product. These costs must be absorbed by the entire automotive manufacturing chain.
Thermal excursions are mainly caused by variations in radiative properties of the steel along the coil, which affect the pyrometers used to control the ICA furnace as well as the radiative transfer to the coil within the furnace. The steel industry must understand the origins of these variations in order to avoid thermal excursions.
The fraction of incident radiation that is absorbed by a surface at a given wavelength is defined by its spectral absorptance, αλ, while the fraction that is reflected is given by its spectral reflectance, ρλ = 1-αλ. The spectral emittance, ελ, defines the efficiency with which a surface emits radiation relative to that of a blackbody. These properties are related by Kirchhoff’s Law, ελ = αλ = 1-ρλ.
Radiative properties depend on the bulk electromagnetic properties of the substrate steel, the surface roughness relative to the wavelength, and the presence of an oxide layer. In the case of steel, there are two distinct regimes. At longer wavelengths radiative properties are dominated by surface roughness, which can be modeled using the geometric optics approximation (ray tracing) and diffraction-based theories. At shorter wavelengths a thin oxide layer on the surface causes a phase shift in the incident radiation, which superimposes with radiation reflected off the top of the oxide layer to produce coherent constructive and destructive wave interference patterns.
Characterizing the radiative properties of AHSS
Theoretical models are validated by analyzing the radiative properties, surface topography, and oxide state of AHSS steel coupons that are processed in a galvanizing simulator at McMaster University. Some of the steel coupons are kept in their as-received state, while others are polished to a mirror finish. The specular and directional-hemispherical reflectance is measured ex situ using a Bruker Invenio X FTIR with an integrating sphere. These measurements are interpreted with optical profilometry, ellipsometery, and optical and electron microscopy.
Parallel research is focused on developing more reliable pyrometry algorithms, as well as an apparatus for making in situ spectral emissivity measurements on steel coupons processed according to industrially-relevant conditions. Steel coupons are resistively-heated according to temperature schedules representative of ICA. Incandescence is measured using a four-channel pyrometer and a near infrared spectrometer.
Annealing atmosphere plays a particularly important role in continuous galvanizing. Since the steel is uncoated, a high purity, 95%/5% N2/H2 atmosphere is used to transform trace amounts of O2 into H2O, and to reduce any oxides that may form on the steel. The effectiveness of this reaction depends on the ambient moisture content and the temperature. Accordingly, both the atmosphere composition and dew point is controlled throughout the heating process.
These results are being used to develop a Bayesian pyrometry algorithm that combines the spectral incandescence measurements with information from EM theory and ex-situ reflectance measurements of AHSS coupons.