Effects of thermo-mechanical treatment and microalloying with Cr, Nb and Ti on phase transformation in C–Mn steel strips produced by compact strip production process

The C–Mn steel strips microalloyed with Cr, Nb, Ti was produced by compact strip production process and then heat-treated under different conditions. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and transmission electron microscopy were used to investigate phase transformations in the steel after different treatments. It was revealed that the phase transformations upon quenching were greatly affected by the austenization temperature and time. When the steel was annealed at 950 °C, carbides of Cr, Mn and Fe were dissolved dramatically, while carbides of Nb and Ti are relatively stable at this temperature. When the temperature increases to 1100 °C, the carbides of Nb were dissolved rapidly, while the carbides of Ti still show somewhat stable (partial dissolution). Annealing time influences both the amount and the shapes of carbides in the steel, which leads to different phase transformations in the following air cooling processes. Grain growth in the steel in annealing process strongly depends on the dissolution of carbides on grain boundaries. Additionally, a subsequent rolling after annealing treatment at 950 °C lead to obvious precipitation of carbides of Ti and Nb in the steel.