Abstract: The thermal regime and structural control between continuous caster offline and a reheat furnace has a significant impact on the cracking problem occurring during hot charging or hot rolling. The effect of the charging process on the characteristics of austenite transformation and sizes of reheated grains was experimentally analyzed using thermal simulation. The in situ observation results using a high-temperature confocal laser scanning microscope shows that film-like ferrite and Widmanstätten ferrite are present along the grain boundary with amounts of retained austenite when hot charged at 700℃ in the dual phase region for J55 steel, and the size and location of the austenitic grain remain constant after reheating to 1200℃. When warm charged at 600℃ in the single phase region, large amounts of ferrite and pearlite are observed, and austenite grains are clearly refined after reheating to 1200℃. It is also found that the austenitic grain can only be visibly refined when the transformed ferrite fraction is higher than 70% for SS400 steel hot charged at different temperatures using a muffle furnace. The grain refinement is more obvious when the transformed ferrite fraction is higher than the critical value.