Structural transformation of oxide scale of 700-MPa grade hot rolled high strength steel

Due to exposure to air during rolling processes, a layer of oxide scale always coats the surface of the hot-rolled steel plates. During the subsequent cooling processes, the FeO in the oxide scale undergoes a eutectic reaction. The formation of a lamellar structure (Fe+Fe₃O₄) during this reaction is influenced by different cooling methods. The addition of alloying elements, however, also affects the eutectic reaction. The final oxide scale, hence, varieswith different compositions. For 700-MPa grade high strength steels, poor control of iron oxide scale is detrimental to the surface quality; such surface defects as iron oxide scale shedding, surface red rust, pit, are incurred. These defects, however, affect the overall performance of the steel. Consequently, the improvement of the surface quality of hot-rolled steel by controlling the iron oxide scale, without compromising the mechanical properties, has attracted the interest of many researchers. In this paper, the effect of cooling temperature and cooling rate on the structural transformation of tertiary oxide scale during hot-rolling was studied. A sample of 700 L steel grade was used. The study was carried out by the thermogravimetric analysis (TGA). The results show a "nose temperature" range of 450‒500 ℃ for the 700 L eutectoid transformation. The FeO shows the shortest incubation period of eutectoid transformation, hence, is prone to eutectoid transformation, forminga large number of eutectoid phase (Fe+Fe₃O₄). Addition of alloying elements such as manganese (Mg), niobium (Nb), and titanium (Ti) to the 700 L steel lead to grain refinement in the steel. It also increases the amount of diffusion channel of ions that participate in the eutectoid phase transformation. Consequently, the eutectoid transformation is delayed, and the eutectoid " C” curve shifts to the left. This is comparableto the eutectoid transformation rule of oxide scale on the surface of other steel grades.