Formation Characteristics And Microstructure Of The Oxide Scale On Slab Surface During Continuous Casting

During the continuous casting process, from the secondary cooling to air cooling zone, a large amount of scale has been formed on the slab surface, which causes significant loss of metal and difficulty of equipment maintenance. Simultaneously, due to residual element enrichment, reticular cracks formed on the interface between scale and metal substrate will also cause the slab "hot shortness". However, for a long time, the problem of slab surface oxidation during the continuous casting process haven not got enough attention, not only the relevant basic research work is unsufficient, but few control measures are adopted in practical production.Based on the conditions of continuous casting process, by combining the thermodynamic calculation and high temperature experiment research, with the aid of metallographic microscope, SEM-EDS and XRD, and other detection analysis methods, the phase composition of scale on slab surface, formation characteristics and microstructure were studied, and the results are as follows.Within the secondary cooling temperature, the stable phases are iron oxide compound with lower partial pressure of water vapor, while FeO or FeO and iron oxide compound are stable phases with higher partial pressure of water vapor.Within the air cooling temperature, no effect of amount and kinds of alloying elements on the phase composition has been found. The stable oxide phase are Fe2O3 inside oxide scale layer.The thickness of scale increases with the increment of ambient oxygen potential and temperature. The thickness of ferritic stainless steel scale is much bigger than those of 45# steel and 20CrMnTi steel under the same conditions, due to the antioxidation of chromium element on the formation of oxide scale.For 20CrMnTi steel and 45# steel, the compactness of external scale layer is higher than that of inner scale layer, the crack can be formed easily at the joint of inner oxide layer and substrate. In addition, the compactness of scale formed at lower temperature is higher than that at higher temperature.The scale layer of 20CrMnTi steel and 45# steel consists of FeO, Fe3O4, and Fe2O3 layer from interior to exterior. Among them, the thickness of FeO layer is the biggest and the that of Fe2O3 layer is the thinnest. The ratio of high oxide is relatively high for 45# steel. However, the scale of ferritic stainless steel mainly consists of Fe2O3/Fe3O4, with middle layer of FeCr2O4. FeCr2O4 spinel can increase the binding force between scale and matrix, and thus reduces descaling performance.As for 20CrMnTi steel and ferrite stainless steel, the enrichment of both Cr and Si elements at the junction of oxide and substrate can be noted, while the distribution of Mn element is uniform.