Effect Of Thermomechanical Processing And Heat Treatment On The Microstructure And Mechanical Properties Of Ultra-High Strength Steels

A martensite-based and an austenite-based Fe-Mn-Al steels were investigated aiming the pilot-scale production of steels that meet military specifications for Rolled Homogeneous Armor (RHA) and High-hardness Armor (HHA) plates. Each material underwent specific thermomechanical processes and the following hardness, Charpy V-notch (CVN) impact toughness at -40 °C and room-temperature tensile properties were correlated with the resulting microstructures. Specimens of the martensitic steel were austenitized at 1010 °C, quenched and tempered at 150, 175, 200, 225 and 250 °C for times up to four hours. As for the Fe-Mn-Al steel, its hot rolling schedule was design to promote strain accumulation in the as-rolled matrix, which is conducive to the precipitation of NiAl during annealing heat treatments. Specimens of this steel were annealed at 700, 750, 800 and 900 °C for times up to one hour. The results show evidence of tempered martensite embrittlement (TME) in the martensite-based steel, especially when tempered for four hours at the investigated temperature range. Regarding the Fe-Mn-Al steel, electron backscattered analyses (EBSD) revealed strain accumulation in the as-rolled austenitic matrix. Annealing at 750 °C for 30 min. strengthened the steel through intra-granular precipitation of NiAl. It was followed by a drop in the CVN impact toughness. Nevertheless, literature review showed that the investigated material outperformed similar austenite-based Fe-Mn-Al steels strengthened through k-carbide --Abstract, p. i