New insights to the metallurgical mechanism of niobium in high-carbon pearlitic steels

Abstract

Two series of high-carbon pearlitic steels (68C-xNb and 75C-xNb) with different C and Nb contents were produced by hot-rolling. The effects of Nb on the microstructure and impact toughness of high-carbon pearlitic steels were investigated using scanning electron microscope, transmission electron microscope, electron backscattering diffraction and impact toughness tests. In particular, the nucleation and growth of pearlite were investigated through in-situ observations. It was found that the average pearlitic nodule was markedly refined from 10.2 μm to 6.0 μm with the addition of 0.026 wt% Nb in 68C steels, and it was refined from 13.7 μm to 6.7 μm with the addition of 0.014 wt% Nb in 75C steels. In situ observations determined that the refinement of the pearlitic nodule was due to the refinement of prior austenite grains and retardation of pearlite growth by Nb addition. The impact toughness of high-carbon steels was significantly improved by Nb addition without a decrease in strength because a finer pearlitic nodule was obtained. The maximum improvement ratios for 68C and 75C steels were 81% and 26%, respectively. In addition, according to the experimental results and theoretical calculations of the dissolution of Nb-containing precipitates, the refinement of prior austenite was mainly caused by the solute-dragging effect of Nb during isothermal holding at 1180 °C. Moreover, there was an optimum addition of Nb to 75C steels (less than 0.015 wt%); above this threshold, neither the prior austenite grains or pearlitic nodules were further refined, nor was the impact toughness further improved.