Influence of TiN Additions on the Microstructure of a Lightweight Fe–Mn–Al Steel

The potential of different inclusions to act as heterogeneous nucleation sites for primary austenite during solidification of a lightweight Fe–30Mn–5.5Al–1.5C–1.2Si steel was analyzed by thermodynamic calculations and experimental heats. Thermodynamic simulations and lattice disregistry calculations were utilized to predict the stability and nucleation potential of different inclusions. TiN was considered as the grain-refining addition because of the success of this inoculant in other austenitic steel castings. Addition of TiN was performed through the use of a pre-made master alloy containing a large volume fraction of fine TiN inclusions. Experimental castings were produced from cylindrical phenolic resin-bonded sand molds with a bottom chill to introduce directional solidification. Additions of 0.5 and 1.5% of the TiN containing master alloy, up to 0.29 wt% Ti addition to the melt, did not yield detectable grain refinement of the as-cast grain structure when compared to the steel castings without additions. Scanning electron microscopy revealed that the inclusions present in the resulting castings consisted mainly of Ti(C,N) with up to a 0.4% area fraction, and this suggests that the original TiN inclusions were at least partially dissolved. Thermodynamic modeling predicted the equilibrium stability of Ti4C2S2 at temperatures above 1440 °C. Although this phase was not observed experimentally, a nanoscale interface layer of Ti4C2S2 or sulfur adsorption on the surface of the Ti(C,N) inclusions may be responsible for poisoning of the nuclei.