Formation Mechanism Of Fe-Al Alloy Interfacial Layer And Ultrafine Al₂O₃ Inclusions During Al-Deoxidation

The large size alumina inclusions produced in the Al-deoxidation steel which seriously affect the microstructure and properties of the steel.it is impossible to completely remove alumina inclusions and eliminate the harmful effects by floating removal?calcium treatment and magnesium treatment.In recent years,a lot of research shows submicron to micron-sized inclusions can induce ferrite nucleation,nano-sized inclusions can pin the grain boundaries,refine the grain and structure of the steel,and improve the toughness.Therefore,on the basis of removing as much as possible,it is important to study the dispersion of ultrafine alumina inclusions?submicron and nano?in steel for improving the performance of steel.In order to investigate the formation mechanism of alumina particles in early stages of Al-deoxidation reaction,the samples were prepared by Al-deoxidation for Fe-O?90 ppm?melt in confocal laser scanning microscope?CLSM?in paper.The inclusions in samples were observed and analyzed by field emission scanning electron microscope?FESEM?with energy dispersive spectrometer?EDS?.The results suggests that the size of alumina inclusions is between 15 nm and 150 nm and the shapes of alumina inclusions are irregular in various samples,there is still a significant difference in size between inclusions.Large-sized nano-inclusions have particle sizes between 60 and 150 nm,and small-sized nano-inclusions have particle sizes below 30 nm.The size and number density of alumina inclusions increase with the increase of temperature of deoxidation while decrease with the increase of cooling rate.Based on the EDS results and Fe-Al phase diagram,it is found that the Al-deoxidation reaction zone is mainly composed of the FeAl,Fe₂Al₅ and FeAl₃phase.The average size of alumina particles in the FeAl phase are larger than that of in the Fe₂Al₅ phase.Alumina particle is not observed in the FeAl₃ phase by FESEM.The formation mechanism of three different Fe-Al phases is related to the diffusion of iron and aluminum atoms and reaction kinetics during heating and holding temperature.At low temperatures,the diffusion coefficient of iron atoms is low,forming a FeAl₃ phase,and the formation mechanism is reacted by a diffusion process.At high temperature,the diffusion coefficient of iron atoms is high,and Fe₂Al₅ and FeAl phases are formed.The formation mechanism is the dissolved aluminum and dissolved oxygen is reacted to precipitate in the cooling process.The FeAl₃ phase can be formed at a relatively high temperature is due to the poor iron zone in the reaction zone.The observation and estimation based on the growth mechanisms of inclusions indicate that the growth of nanoscale alumina with large size is controlled by the diffusion growth and Brownian collision in high temperature melt,while nanoscale alumina with small size are secondary precipitate phase in the process of solidification which is not controlled by the Brownian collision.The formation mechanism of alumina particles in various Fe-Al phases is related to the formation mechanism of various Fe-Al phases and O diffusion.The transition process of various Fe-Al phases in the reaction zone can well reflect the formation process of alumina particles in the early stages of Al-deoxidation.