Grain growth behavior of niobium microalloyed 20MnCr5 steel and the effect of boron

Abstract Grain growth behaviors of niobium free, 0.021 wt. % niobium microalloyed and 0.024 wt. % niobium‐0.0022 wt. % boron microalloyed 20MnCr5 steels are investigated in this paper. Optical microscopy and field‐emission scanning electron microscopy were used to analyze the prior austenite grain. Transmission electron microscopy and energy dispersive spectroscopy were used to investigate the morphologies and compositions of the precipitates, respectively. The results show that adding niobium reduces the size of prior austenite grain. The addition of boron to niobium microalloyed steel, however, causes grain coarsening. Thermodynamic and kinetic calculations were performed to explain the mechanism of grain growth behavior. The results indicate that the addition of niobium and boron causes the difference in solution temperatures of aluminum nitride and niobium carbonitride precipitates. The addition of boron inhibits the precipitation of niobium carbonitride and aluminum nitride and causes precipitates coarsening. The precipitation observation backs up the calculation results.