Effect of tramp elements on the microstructural evolution of a ferritic-pearlitic steel

Pursuing the reduction of CO ₂ emissions, steel industry shifts towards a scrap-based electric arc furnace production that results in elevated levels of tramp elements. The effect of these elements on material properties and microstructure is not understood in detail in advanced microstructural complex steels. This study employs methods to investigate the microstructure of a typical cold headed wire-rod steel, micro-alloyed with vanadium, from blast furnace production and a trial alloy with intentionally increased tramp element concentrations. Light optical microscopy is performed to determine grain sizes of the ferritic-pearlitic microstructure and their phase fractions. The interlamellar spacing of pearlite is explored with scanning electron microscopy. Additionally, the regularity of pearlite lamellae is assessed with a machine learning approach. Thermally etched austenite grains are observed with a high-temperature laser scanning confocal microscope at 1000 °C. A comparison to recalculated prior austenite grains from electron backscatter diffraction patterns is performed. The microstructure after cooling is analyzed by LePera etching and phase maps are obtained by electron back scatter diffraction. The results show that tramp elements do not have an influence on the grain size and phase fractions of the ferritic-pearlitic microstructure in the studied alloy. However, it seems that regular growth of the lamellae is impeded in the trial alloy. Moreover, smaller prior austenite grain sizes are observed and displacive constituents are significantly pronounced, if tramp elements are increased in the alloy.