What Is The Role Of Niobium in Steel?

Niobium (Nb) can form interstitial intermediate phases such as NbC or NbN in steel. During recrystallization, the pinning of dislocations by NbC and NbN and the inhibition of grain growth greatly increase the recrystallization time. At higher than the critical temperature, the role of niobium on the recrystallization of solute drag mechanism, while at lower than the critical temperature, the precipitation pinning mechanism.
That is, when a grain boundary encounters a plasma point in the process of movement, part of the grain boundary will be cut off. If the grain boundary is to break free from the plasma point pinning, it will inevitably produce a new interfacial area, which in turn will increase the new interfacial energy, and thus the grain can only be coarsened by providing a greater driving force. The characteristic of niobium in steel is to increase the recrystallization temperature of austenite, expand the temperature range of the non-recrystallized zone, promote austenite grain deformation and defect "accumulation", and ultimately to achieve the purpose of ferrite grain refinement.

Niobium is one of the very important microalloying elements and is the element of choice for controlled rolling of steel. It is the most effective alloying element for grain refinement and has a significant strengthening effect. Its extremely strong effect on retarding austenite recrystallization and grain refinement in heat treatment processes such as controlled rolling and normalizing is one of the important advantages of niobium. A relatively low niobium content of about 0.03%, which is commonly used today, can play a significant role in controlled rolling and normalizing steels. In the rolling temperature below 950 ℃, each standard deformation after rolling does not occur after recrystallization, so that the elongated austenite grains due to the nucleation of the grain boundaries and subgranular area increases, it can phase into fine grains. Therefore, for low carbon manganese steel, through the addition of a small amount of niobium, can be achieved in the ordinary rolling mill to achieve the purpose of controlled rolling and grain refinement.