Radiometric properties of galvannealed steel

Abstract

Galvannealed steel has a wide application in automobile industry due to its superior corrosion resistance, weldability, and paint adhesion. The intermetallic phases in galvanneal coatings are connected to these desired properties and strongly influenced by the heating schedule during the manufacturing process. However, the coating transformation during the galvannealing process affects radiometric properties of galvanneal coating and complicates inline pyrometry. Inaccurate temperature readings and lack of inline surface state monitoring methods lead to deficient properties and increase scrap rates during production. A good understanding of the radiometric properties of galvannealed steel helps improve inline pyrometry and potentially enables inferences of coating surface state based on radiometric measurements, but currently there is a lack of in-depth studies on radiometric properties of galvannealed steel in literature. This thesis fills the knowledge gap by first correlating the radiometric properties of industrially galvannealed steels with their surface morphology and phase composition through ex-situ analysis. The ex-situ data gives insight into pyrometry improvement and inline surface state monitoring with galvannealed steel, which are further verified with in-situ study. Results from in-situ radiometric experiments on lab-simulated galvannealing process at different wavelength ranges further confirm previous findings from the ex-situ analysis. The radiometric properties of galvannealed steel rapidly change during production due to the coating transformation. The 1.5–2.5 μm wavelength range, part of the near-infrared (NIR) region, is ideal for pyrometry due to the consistent radiometric behavior between different surface state of galvanneal coatings. The optimal wavelength range of inline surface state monitoring is the mid-infrared (MIR), or 2.5–15 μm, due to the strong correlation between galvanneal surface state and radiometric properties observed in this range.