<p>The effect of isothermal temperature on the nucleation and growth of carbide-free bainite was studied by in-situ observation of laser scanning confocal microscopy combined with electron backscatter diffraction technique. The results show that the nucleation position of bainite includes the original austenite grain boundary, the side of the pre-formed bainite lath, the intragranular inclusions, the end of the bainite lath and the sub-grain boundary. The growth rate of bainite lath formed at the grain boundary is the largest, and the growth rate of bainite lath formed at the inclusions in the crystal is the smallest. The variants in the same crystallography can nucleate on the same habit plane. Although the crystallography of different bainite laths belongs to the same Bain group or CP group, the growth rate is still significantly different. CP1 group (V1–V6) is mainly formed in the high temperature bainite transformation. V1/V2, V1/V4, V1/V8, V1/V10 and V1/V15 variants formed in the low-temperature bainite transformation help to increase the proportion of high-angle grain boundaries.</p>