Alteration of Phase Transformation Behavior and Prior Austenite Grain Structure Due to Elevated Tramp Element Concentrations in a Hypoeutectoid Steel

In order to reduce CO2 emissions, the steel industry shifts its production toward the usage of higher scrap contents. This transformation leads to an elevation of tramp element concentrations, which can alter the steel's response during heat treatments. In this work, the phase transformation behavior of a hypoeutectoid steel grade is studied. A reference alloy from the blast furnace route is compared to alloys with increased tramp element concentrations. Dilatometry experiments are performed to obtain information about phase transformations. Additionally, the microstructure is examined with hardness measurements, optical as well as scanning electron microscopy including electron backscatter diffraction. Moreover, in situ high-energy X-ray diffraction measurements deliver information of the microstructural evolution during the heat treatment and provide a precise overview of the evolved phases after cooling. Finally, the prior austenite grain size is determined with a high-temperature laser scanning confocal microscope and is correlated to the observed phase transformation alterations. The examination of microstructures reveals that an increase in tramp element contents facilitates the formation of displacive phases along with retained austenite. Furthermore, it is observed that tramp elements impede the growth of prior austenite grains by segregating to the grain boundaries at high temperatures, thereby inducing a solute drag effect.