The characteristics of inclusions and the formation of acicular ferrite (AF) in Ti–Mg-Ca killed EH36 steel were researched. Typical inclusions in the Ca-free samples were MgO·Al2O3 + MnS and pure MgO·Al2O3. With the addition of Ca, MgO·Al2O3 was modified into Mg–Ca–Al–O , which promoted MnS precipitating on those inclusions. Mg–Ca–Al–O+MnS/MnS–CaS was the nucleus of AF. While pure MgO·Al2O3 was inert for nucleation of AF. Therefore, the percentage of AF increased to 21% in the steel with 0.0017% Ca. However, 0.0051% Ca led to a rise of CaS content in the inclusions. Probability of AF nucleation on the inclusions was reduced due to high lattice mismatch between CaS and α-Fe. Area fraction of AF in the steel with 0.0051% Ca decreased while that of ferrite side plates increased. It is appropriate when the content of Ca is 0.0017% in this study.
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