Effect of Al on the Formation of IAF in Al–Ti–Mg Deoxidized and RE-Treated Steel

Abstract

To clarify the effect of aluminium on the formation of intragranular acicular ferrite (IAF) in Al–Ti–Mg deoxidized and rare earth-treated steels with different S content, the thermodynamic calculation and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) were used to study the distribution and composition of inclusions . The results showed that the addition of Al led to the increase of mean size of effective inclusions. With the content of Al increasing from 0.0027 to 0.0086 wt%, the main inclusions in the low S steels changed from La₂O₃ to LaAlO₃ and the proportion of IAF decreased rapidly. However, the inclusions in the high S samples were mainly composed of La₂O₂S or LaAlO₃ and MgO with MnS precipitating on the surface of them. As the Al content increased from 0.0030 to 0.0097 wt%, the main composition of core evolved from La₂O₂S to LaAlO₃, but the proportion of IAF slightly changed. As the Al content approached 0.03 wt%, the proportion of IAF decreased rapidly and a plenty of upper bainite or widmanstatten were discovered.