Growth Rate, Microstructure and Phase Composition of Oxide Scales for Three Typical Steels in Simulated Continuous Casting Process

Abstract

Growth rate, microstructure and phase composition of scale layer formed during oxidation in 56%H₂O-9%O₂-N₂ and following continuous cooling in ambient air were experimentally investigated by means of optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) for 45, 20CrMnTi and TTS443M steels respectively, to examine the effects of strand surface temperature and steel composition on the scale formation in simulated continuous casting process. The growth rates were found to be approximately parabolic and the calculated activation energy of TTS443M steel is much higher than those of the two other steels. For 45 and 20CrMnTi steels, the scale layers were looser and a distinct gap formed at the scale-substrate interface at higher strand surface temperature. The dominant phases within the scale were iron oxides except for FeO • Cr₂O₃ phase simultaneously existing in the oxide scale of 20CrMnTi steel. On the other hand, the scale layer formed on TTS443M steel was compact and tightly attached to the steel surface. At both lower and higher strand surface temperature, iron oxide was main phase in external layer of the scale, while chromia was dominant in inner layer with an appreciable Cr enrichment.