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.
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This work is financially supported by National Nature Science Foundation of China under grant No. 51774024
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