Unraveling the effect of tramp elements on phase transformations in steels by combining CALPHAD modeling and experiments

The transition from blast furnace to electric arc furnace steelmaking is a step toward enhancing circularity through increased scrap utilization, thereby reducing CO2 emissions. However, higher scrap use introduces tramp elements that may affect steel quality. This study investigates the influence of tramp elements on the phase transformation behavior of a low‐alloyed steel by combining modeling and experiments. Dilatometry and optical microscopy are employed to analyze phase transformations and microstructures, enabling the construction of continuous cooling transformation diagrams. Prior austenite grain size is measured with a high‐temperature laser scanning confocal microscope. To complement the experimental investigations, a computational modeling framework based on the CALPHAD method is performed using Thermo‐Calc. Experiments reveal that tramp elements shift phase transformations to longer times and lower temperatures, decreasing critical cooling rates and enhancing hardenability. Refinement of prior austenite grain size with increasing tramp element content indicates segregation effects at grain boundaries. To identify the decisive mechanism driving the altered phase transformations, CALPHAD modeling highlighted the crucial role of element segregation in lowering grain boundary (GB) energy. These findings suggest that the segregation of tramp elements is likely to be an important factor in controlling phase transformation kinetics in scrap‐based steels.