Effect of lanthanum on the precipitation and dissolution of NbC in microalloyed steels

The effect of lanthanum on the precipitation and dissolution of NbC in microalloyed steel was investigated using a combination method of thermo-mechanical experiment and the first-principle calculations. The interaction of niobium with lanthanum is attractive in the 2nn to 6nn shell, and that of carbon with lanthanum is relatively strongly attractive in 4nn(B) and 5nn coordination shells. These attractive interactions lead to an increase in the Nb and C solubility in fcc Fe, and a decrease in the chemical potential of these two solutes, thereby suppressing the formation of NbC. The strain-induced precipitation intensity was characterized using field emission scanning electron microscope (FE-SEM) and inductively coupled plasma optical emission spectrometry (ICP-OES). The experimental results reveal that after deformation NbC precipitated more in La free steel than in La addition steel. And since the relatively higher dissolution rate of NbC in La addition steel, the Zener pinning effect decreased slightly faster, leading to a higher coarsening rate of austenite grain during isothermally heating at 1200 °C.