An ultra low carbon Cu bearing steel: influence of thermomechanical processing and aging heat treatment on structure and properties

An ultra low carbon Cu bearing steel, micro alloyed with Nb was thermo-mechanically processed. Variation in microstructures and associated mechanical properties at different finish rolling temperatures was studied. Microstructures in hot rolled steels showed a mixture of acicular ferrite and bainitic ferrite with inter-lath or intra-lath precipitation of microalloying NbC/Nb(CN) precipitates. Second phase particles were observed, mostly at lath boundaries and were identified as retained austenite, twinned martensite islands or martensite austenite constituents. Variation in strength at different finish rolling temperatures was due to variation in volume fraction of bainitic ferrite and dislocation substructure in the matrix. Precipitation behaviour of Cu was studied by differential scanning calorimetry and transmission electron microscopy. At peak age hardening condition, coherent bcc Cu particles were formed and activation energy for this reaction was evaluated. At higher aging temperature, growth of fcc ε-Cu particles and recovery of structure occurred which decreased the strength value.