Direct strip casting (DSC) is an innovative near-net-shape process for the formation of thin steel sheets which drastically simplifies the conventional process, allowing significant cost and energy savings. It has thus been taken to the industrial stage in the past few decades. Materials produced by DSC undergo extreme cooling rates, resulting in highly non-equilibrium conditions of solidification and further phase transformations. The current challenge is to be able to predict the final microstructure under such processing conditions as a function of the steel composition.
Nano-bainitic steels have been increasingly studied as they offer unprecedented mechanical properties but their upscaling to industrial applications has not been possible due to the high processing costs of these alloys using conventional steel processing methods. The aim of the current project is to explore the use of DSC to process nano-bainitic steels by understanding the phase transformations and the effect of minor alloying additions on the microstructure and resulting mechanical properties of these steels. Five compositions were carefully selected to study the influence of chemistry on the microstructure development. The microstructure characterizations were carried out by means of SEM, EDS and XRD analysis. Carbon, silicon and chromium were found to have a significant effect on the final microstructure and mechanical properties of both the as-cast specimens and those which underwent coiling treatments: higher alloying elements contents resulted in coarser martensitic and widmanstatten ferrite laths in the as-cast microstructures, and finer nano-bainitic laths in the coiled specimens. Increasing the composition also led to higher amounts of retained austenite in both sets of samples (up to 20 vol% in the as-cast samples and 50 vol% in the coiled specimens).
An accurate tuning of the chemistry of DSC nano-bainitic steels can therefore allow control of the final microstructure and resulting mechanical properties of these grades.