Investigation of alloying element effects on phases in Fe-Mn-Al-C low-density steels for automotive applications

The κ-carbides are a type of secondary phase that precipitates inside lightweight Fe-Mn-Al-C alloys, and it influences crucially the toughness and yield strength of a mechanical piece. This research’s purpose is to analyze and comprehend the influence of chemical composition and cooling rate on the precipitation of such phase. This research deeply analyzes the influence in change in chemical composition and in cooling and ageing regimes from the metallurgical point of view, bringing a highlight on the development of microstructure after annealing and ageing treatments. By means of metallographic analyses and dilatometric tests, the development of phases inside the microstructure of several steel samples was properly tracked. [OS1.1] The steel samples utilized for this work were classified by casting methods and chemical composition. Part of the samples were molten in a vacuum furnace and casted in a metal mould, while others were molten in an open induction furnace and casted in a sand mould. All the samples underwent groove rolling after casting to achieve an even distribution of the grains. After this first step, several metallographic images of them were taken to compare the effect of subsequent treatments with respect to the initial state. The heat treatments were performed aiming to achieve a complete austenitic microstructure and an even distribution and growth of intragranular κ-carbides to ensure the best possible mechanical properties. This study found a direct relationship between content of Carbon and Aluminum and the phase fraction of precipitates, finding the addition of Manganese influencing in a noticeable way only the fraction of austenite inside the microstructure. The relationship between cooling rate and precipitation of secondary phase particles resulted in a decrease in fraction of κ-phase with the increase in cooling rate. The fraction of carbides was greatly improved by decreasing the ageing temperature and increasing the ageing time. These results have been related to the diffusion of Manganese and Aluminum atoms from the matrix that determines the growth of carbides inside the austenite grains or at the grain boundaries.