Microstructure evolution and compressive properties of a low carbon-low alloy steel processed by warm rolling and subsequent annealing

A low carbon-low alloy steel was processed by warm rolling with reductions range from ~30% to ~70% followed by annealing at 450°C. Then, the microstructural evolution was characterized by Field Emission Scanning Electron Microscopy (FE-SEM), Electron Backscatter Diffraction (EBSD), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and compressive testing under strain rates of 1.0 × 10⁻³–2.0 × 10³ s⁻¹ was carried out. Microscopy analyses showed that ultrafine-grained structures with high-density dislocations and more and finer M₃C carbides by comparison with the tempered steel were achieved after warm rolling. Subsequent annealing promoted the further precipitation of finer carbides and led to dislocation recovery as well as a slight coarsening of grains. Compressive testing results indicated that the yield strengths of the warm rolled steels at different strain rates were significantly increased by ~40–70% compared with the as-received sample, which was mainly attributed to a combination of dislocation strengthening, grain boundary strengthening and precipitation strengthening. After annealing, the yield strength decreased slightly due to a dislocation recovery and a slight increment of the grain sizes. In addition, the influence of microstructure evolutions including dislocation densities, grain sizes and carbide precipitations during warm rolling and subsequent annealing on the strain rate dependence of strength for steels was also analyzed.