Enhancing Mechanical Properties of Hot Wrought Steel by Microalloying and Optimizing Heat Treatments

Novel methods to increase performance of steels without increasing production costs are always sought after. In this study, a chemistry–process–structure–property paradigm was employed to modify rolled homogeneous armor (RHA) steels that contain no nickel (Ni) or chromium (Cr) and instead, microalloying with niobium (Nb). Characterization of 0.02 and 0.05 wt.% Nb additions was targeted in tandem with optimized heat treatments to improve performance without detrimentally increasing carbon equivalence. Designed alloys were cast in a vacuum induction melting furnace and thermo-mechanically processed. Optimal heat treatment conditions were determined with computational simulation software, JMatPro, for materials properties, which resulted in an exploration of three tempering temperatures, 400, 500, and 600 °C. Microstructures were investigated by optical and electron microscopy, where Nb additions were determined to reduce average grain area of ferrite, bainite, and martensite by as much as 75, 59, and 58%, respectively. Hardenability was characterized by Jominy end-quench testing and showed a slight increase in average hardness for Nb-bearing alloys due to the precipitation of NbC. Compression and tension tests revealed minimal strain rate sensitivity and a nearly 30% increase in strength in comparison with reference RHA materials. Impact tests showed a moderate increase in energy absorption of up 61%. Fractography of the failed specimens highlighted MnS precipitates as well as NbC and Mo₂C carbides. Overall, results showed small (≤ 0.05 wt.%) additions of Nb accompanied by an optimized heat treatment resulted in a modified, less-expensive RHA with similar performance to typical Ni/Cr-RHA.