Abstract: Taking 700 XL hot rolled automobile beam steel as the research object, the influence of different coiling temperature and cooling rate on the phase transition of iron oxide sheet was simulated by Gleebe thermal simulation equipment. The morphologies of the scale were observed by SEM, EBSD and X-ray diffraction. The results show that the thickness of the scale increases with the increase of coiling temperature, and the slower the cooling rate after coiling, the thicker the thickness of iron oxide. When coiling at high temperature of 600℃,the oxide layer is mainly composed of Fe₃O₄ and FeO, without eutectoid reaction. When coiling at low temperature of 500℃, FeO is easy to decompose. The slower the cooling rate is, the higher the degree of eutectoid reaction of FeO is.The iron oxide is mainly composed of proeutectoid Fe₃O₄, eutectoid structure and some residual FeO.