Fundamental research is required to support the commercialization of 3rd Generation Advanced High Strength steels (3G AHSS). Mid-manganese 3G AHSS steels can contain up to 11wt% manganese and are expensive if traditional ferroalloying practices are used; reductive alloying is a promising alternative. This study has researched the fundamental science behind possible processing methods. Silicothermic reduction of MnO from slag was studied. The reaction is fast but can be blocked by a stagnant layer of SiO bubbles cutting the rate of reaction by one order of magnitude. A theoretical model for mixed mass transport control was tested against original experimental data. Across nine datasets, the mass transfer coefficient for metal species, kMetal, was 2.3∙10-4m/s and the slag mass transfer coefficient, kSlag, was 6.7∙10-4m/s. In real industrial systems, gas blockage should not have an effect because stirring will dislodge these bubbles. Carbothermic reduction is dramatically different and has been qualitatively documented in this work. The reaction occurs in two stages: the first approximately three times faster than the second. The first stage is characterized by internal CO nucleation and growth and is rate-limited by the formation and growth of these CO bubbles. The second stage occurs along the metal interface and shows that the slag and metal are essentially separated by an intermediary gas phase. This reaction is controlled by decomposition of metal oxides at the gas-slag boundary, decomposition of CO2 at the gas-metal boundary, and transport of CO2 across the gas bubble; this mechanism is nearly identical to the carbothermic reduction of FeO. Reductive alloying can be utilized with the silicothermic reduction process to obtain high levels of manganese in steel but the carbothermic reduction may be too slow to be a viable process. / Thesis / Doctor of Philosophy (PhD) / 3rd Generation Advanced High Strength steels (3G AHSS) are a promising opportunity to produce steels with improved mechanical properties. These steels are alloyed with up to 11wt% manganese; traditional alloy additions are added as ferroalloys which may not be the most economical solution to achieve the required concentrations of manganese. Reductive alloying is a potential method for achieving high concentrations of manganese in the metal. By adding manganese oxide to slag, and reductants like carbon or silicon to the molten metal, manganese can be reduced from slag to metal. This work has determined the kinetics (rate of reaction) during the silicothermic and carbothermic reduction of manganese oxide from slag. The silicothermic reduction of manganese oxide is fast and can achieve high levels of manganese in the metal. The carbothermic reduction is much slower with questionable viability.