Effect of Nb and V on Austenite Grain Growth Behavior of the Cr-Mo-V Steel for Brake Discs

In order to improve the performance of the steel for brake discs, including strength, toughness and thermal fatigue resistance, Nb and V are added to the steel. The effect of Nb and V on austenite grain growth behavior of the Cr-Mo-V steel for brake discs was studied by analyzing the phase equilibria as well as the microstructures. The precipitation behaviors of precipitates were also investigated based on transmission electron microscopy and energy-dispersive spectroscopy. The results showed that V-rich M8C7 and NbC particles existed at the experimental temperatures. The abnormal grain growth in Nb-free steels was affected by the partial dissolution and coarsening of M8C7 particles. With increasing V content, the grains were refined, but the mixed grain phenomenon became more serious. There were many small NbC particles in Nb-bearing steel, so the grains were effectively refined. Furthermore, a prediction model, which calculates the austenite grain size considering precipitate pinning effect, was established, and the model was well consistent with the actual situation in the Nb-bearing steel up to 1,100 °C. The grains of the Nb-free steels were sensitive to the increments of temperature and time due to the rapid dissolution or coarsening of M8C7 particles.