The effects of chromium on γ-austenite to α-ferrite phase transformation in Nb Microalloyed steel was observed using ultra high-temperature confocal laser scanning microscopy. It is indicated that the starting temperature of the γ→α phase transformation decreases with increasing the Cr content. The hot ductility of Nb microalloyed steel is improved by adding 0.12wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which leads to the reduction of grain boundary ferrite thickness. The proportion of high angle grain boundaries is increased by adding chromium. In particular, the proportion is up to 48.7% when the Cr content is 0.12wt%.The high angle grain boundaries hinder the crack propagation and improve the ductility of Nb microalloyed steel.