High Temperature Refractory-Steel Interface Reactions During Melting and Casting of High Manganese and Aluminum Steels and Elimination of Phosphorus Pick-Up
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Age hardenable high manganese and aluminum austenitic steels with nominal composition of Fe-28-30%Mn-6-9%Al-0.6-1%%C-1-1.56%Si-0.5Mo are being considered as a possible replacement for quenched and tempered Cr and Mo steels for use in military ballistic armor as well as in the automotive and mining industries. These steels have an advantage over more traditional high strength steels with up to 18% less density and exceptional combinations of strength and toughness. However phosphorus is considered as impurity for this steel and should be kept below 0.006%P because of a deleterious effect on toughness. Unfortunately it is very hard for foundries to achieve these low phosphorus levels, even when extremely pure charge materials are used. The source of phosphorus during steelmaking is unknown and a critical challenge for industrial foundries. Phosphorus containing monolithic refractory materials have been identified as the possible source of extraneous phosphorus. Two different experiments were conducted to investigate the interaction between two different commonly used phosphate and silicate bonded monolithic refractories and a Fe-Mn-Al-C steel during melting and melt transfer. The first experiment was designed to study phosphorus pick-up during melting and long holding times, > 30 min at 1590°C. The second experiment involved short holding times in a ladle (<30 sec) during melt transfer operations in the foundry. Laboratory scale experiments showed that these steels can pick-up more than 0.14% P from phosphate bonded monolithic refractory and this causes heavy grain boundary k-carbide precipitation and up to 7% of a brittle phosphorus rich eutectic phase. Whereas steel remelted in contact with silicate bonded refractory showed almost a 100% austenite matrix and no change in phosphorus content. In the ladle melt transfer operation experiment, negligible increase in phosphorus content and higher Si (0.97%) and C (1.13%) was found in steels poured from phosphate bonded monolithic refractory lined hand-ladle in comparison to silicate bonded monolithic refractory lined hand-ladle poured steels that had Si and C content of 0.74% and 0.97% respectively. Furthermore comparatively higher aged hardening rate and solution treated Charpy V notch, CVN, breaking energy was demonstrated by phosphate bonded monolithic refractory lined hand-ladle poured steels. However regardless of refractory type, aging at 530oC for 10 hours decreased the CVN breaking energy down to an average 5 J. The results of these studies show that dissolved aluminum in molten Fe-Mn-Al-C steels will react with the P2O5 in commonly used refractories that coat the induction furnace and ladles during melting and phosphorus will revert into the melt. The amount of picked up phosphorus is dependent on the processing condition such as holding time. The results of these studies show that phosphorus bonded monolithic refractories should be avoided during melting of Fe-Mn-Al-C steels.