Effect of hot rolling temperature on grain size and precipitation hardening in a Ti-microalloyed low-carbon martensitic steel

The grain refinement and strength improvement resulted from hot rolling temperature in Ti microalloyed low carbon martensitic steel was investigated in this study. Two different started hot rolling temperatures (950 °C and 1100 °C) but with the same reheating quenching process were applied to the steel. The microstructures and second precipitated particles were examined by scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-rays diffraction and phase analysis method. It was found that hot rolling could induce the precipitation of TiC. Moreover, the amount of the TiC in steel rolled at 950 °C is higher and the size of the precipitates is much finer than that in steel rolled at 1100 °C. Both the large deformation without recrystallization and the precipitates in steel rolled at 950 °C are more effective on the grain refinement after the reheating process, in which the effective grain size (EGS) can be refined to 1.4 μm. In addition, the steel rolled at 950 °C exhibits a much higher strength than that rolled at 1100 °C due to the additional dislocation strengthening and more precipitation strengthening