Abstract
Impurity elements have been added to a commercial low carbon steel grade to simulate the levels that could arise as a result of increased scrap recycling during steel production. In this study, continuous cooling transformation (CCT) diagrams were constructed for the steels with varying levels of impurities, and it is shown that impurities suppress the phase transformation across a wide range of cooling rates. It was found that a step was formed in the start temperature curve, separating the reconstructive and displacive transformations. The influence of impurities on both the reconstructive transformation and displacive transformation are discussed. Additionally, Cu precipitates were observed using scanning transmission electron microscopy (STEM) in the highest impurity-containing steel after slow cooling (0.05°C/s), fast cooling (5°C/s) and interrupted cooling. It was found that the precipitation kinetics is in the following order: cementite within the secondary phase and cementite at secondary phase-ferrite interface> ferrite grain boundaries> ferrite grain matrix. Atom-probe tomography (APT) revealed Cu precipitates formed on the surface of cementite lamellae, but not within it. This work offers insights for the phase transformation control and precipitation regulation during the thermomechanical processing of low carbon steels containing impurity elements due to scrap recycling.
