The effects of inclusion size and chemical composition on acicular ferrite (AF) nucleation and possible nucleation mechanisms were discussed. With the use of low-carbon structure steel containing 13 ppm Mg, the chemical composition and size of inclusions were quantified by scanning electron microscopy combined with an energy dispersive spectroscopy system. Results showed that the complex inclusions MgO-MnS and MgO-Al2O3-MnS are the most effective inclusions for inducing AF nucleation in Mg-containing A36, and their probabilities of AF nucleation are 35.8% and 41.4%, respectively. Moreover, the inclusion size in the range of 1–2 μm exhibits the most remarkable ability to induce AF formation. AF nucleates at the interface of MgO and MgO-Al2O3 because of the low lattice misfit with AF. Complex inclusions, such as MgO-MnS and MgO-Al2O3-MnS, present a better ability of inducing AF formation than that of MnS due to the Mn-depleted zone.
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