Laboratory Study on Evolution Mechanisms of Non-metallic Inclusions in High Strength Alloyed Steel Refined by High Basicity Slag

Laboratory study was carried out to discuss evolution mechanisms of non-metallic inclusions in high strength alloyed steel refined by high basicity slag, aiming at formation of lower melting temperature inclusions to improve anti-fatigue properties of steel.It is found that steel/slag reaction time had great effect on inclusion types, compositions and shapes. With reaction time extended from 30 to 180 min, solid MgO–Al2O3 and MgO-based inclusions were finally changed into CaO–MgO–Al2O3 system inclusions lower melting temperature (<1773 K). While shapes of inclusions varied in the route blocky/angular→near spherical→spherical.Al2O3/MgO·Al2O3/MgO and MgO/MgO·Al2O3/CaO·2Al2O3 stability diagram were obtained by thermodynamic calculation. The results indicated that MgO and MgO·Al2O3 inclusion would be formed at early stage of steel-slag reaction because activity of Mg is much larger than that of Ca in steel. However, with increase of Ca activity, solid MgO·Al2O3 and MgO inclusions would be inevitably and gradually transferred into complex liquid inclusions even dissolved [Ca] is as low as 0.0002%. Thus, SEM-EDS mappings of CaO–MgO–Al2O3 system inclusions are characterized by high melting temperature solid MgO·Al2O3 or MgO-based inclusion cores surrounded by lower melting temperature CaO–Al2O3 outer surface layers, which would be softer during hot rolling and therefore be helpful to improve anti-fatigue properties of steel.Model was established to elucidate change mechanisms of inclusions. Transferring kinetics of inclusions was discussed qualitatively to analyze velocity controlled steps. It is found that diffusion of Mg and Ca in solid inclusion core and the formed CaO–Al2O3 outer surface layer would be probably the limited step during evolution of inclusions. However, further work should be done to discuss evolution kinetics of inclusions quantitatively.