Precipitation behavior of nanoscale cementite in 0.17% carbon steel during ultra fast cooling (UFC) and thermomechanical treatment (TMT)

Motivated by the unique effects of ultra fast cooling (UFC) on strengthening in hypoeutected steels (Bin et al., Mater. Sci. Eng. A 575 (2013) 189–198), we describe here the extraordinary benefit of combining ultra fast cooling and thermo-mechanical treatment (TMT) in 0.17 wt% carbon steel. At optimal condition (UFC stop temperature 500 °C, holding time of 60 min), the thermomechanical treatment after UFC enables uniform precipitation of nanoscale cementite in the bainitic lath microstructure, leading to significantly high yield strength (675 MPa)-elongation (19%) combination. The study also demonstrates that the UFC stop temperature has an important effect on the microstructure and associated mechanical properties. When the UFC stop temperature is decreased from 660 °C to 500 °C, the microstructure changes from a two phase pro-eutectoid ferrite – degenerate pearlite to bainite microstructure with nanoscale precipitation of cementite.