PRECIPITATION BEHAVIOR OF CARBIDE DURING HEATING PROCESS IN Nb AND Nb-Mo MICRO-ALLOYED STEELS

As an important carbide forming element, Nb plays an important role in steel. Precipitated Nb can restrain the austenite grain growth during soaking process and provide precipitation strengthening after gamma/alpha phase transformation. Precipitated or dissolved Nb can inhibit recrystallizaton of deformed austenite. Recently, both Nb and Mo are added in steel to enhance the role of Nb. However, these kinds of researches mostly focused on continual cooling process of gamma/alpha transformation or isothermal process during tempering, and precipitation behavior of MC-type carbide in steel containing Nb and Mo during reheating process and the effect of Mo on precipitation of NbC in ferrite were rarely reported. Therefore, in this work, precipitation behaviors of MC-type carbide and the synergis-tic effect of Nb and Mo in steel containing Nb or Nb-Mo during reheating process at the heat rate 20 degrees C/min were investigated by means of Vickers hardness test, SEM, HRTEM and DSC. The results show that both Nb and Nb-Mo steels have hardness peaks at 300 and 700 degrees C, which are attributed to the precipitation of epsilon-carbide and MC-type carbide, respectively. The MC-type carbide precipitates at about 650 degrees C during reheating process, which is in a good agreement with the nose temperature of MC-type carbide calculated by Avrami equation. (Nb, Mo)C particle forming in Nb-Mo steel during precipitation has a small mismatch with ferrite matrix compared with NbC, leading to the decrease of interfacial energy. Thus, the precipitation kinetic of MC-type carbide in Nb-Mo steel is faster than that in Nb steel, which results in the denser and finer MC-type carbide and higher precipitation strengthening effect.