Thermo-mechanically controlled processed ultrahigh strength steel: Microstructure, texture and mechanical properties

A low-carbon microalloyed steel containing high Ni and Cu content has been developed and subjected to thermo-mechanical processing by varying the finish rolling temperature (FRT∼850-750 °C) and cooling rates (air cooling and water quenching). Microstructures of air cooled samples consist of granular bainite and lath or plate-like bainite, whereas, water quenched samples exhibit a mixture of lower bainite and lath martensite. A refinement in microstructure has been noticed with the decrease in FRT and increase in cooling rate. Transmission electron microscopy demonstrates the presence of coarse (Ti, Nb)C precipitates (~90-160 nm) and fine Cu precipitates (<20 nm). Macro-texture and micro-texture results reveal the dominance of Goss and rotated Goss texture components, which strengthened with the decrease in FRT and increase in cooling rate. The proposed steel composition and TMCP schedule have offered YS ∼ 1000 MPa, UTS ∼ 1400 MPa, total elongation greater than 10% maintaining a low YS: UTS ratio (0.68-0.80). Such a satisfactory combination of tensile properties achieved in as-cooled or as-quenched conditions (without the need of any tempering treatment) makes the steel suitable for automotive application.