Inclusion Modification and Evolution during Reheating of Ca-Treated Ti–Al–Mg Deoxidized High-Strength Low-Alloy Steels with Two Different Ti/Al Content Ratios

Abstract

This study investigated the evolution mechanism of inclusion characteristics during reheating and their effects on acicular ferrite nucleation in Ti–Al–Mg deoxidized and calcium-treated high-strength low-alloy steels. The findings indicate that regulating the Ti/Al content ratio in steel may facilitate the formation of dominant TiO_x core inclusions rather than dominant Al₂O₃, thereby influencing the calcium treatment's effectiveness on inclusion modification. Via reheating, the compositions of Ti₂O₃–MgO–Al₂O₃ core inclusions in high Ti/Al content ratio (5.50) steel change to Ti₂O₃–MgO, while (Mn, Ca) S shell with wrapped distribution become MnS and CaO–Al₂O₃ with short-arc, mosaic distribution due to their Ostwald aging. This implies that Al and O ions in core inclusions diffuse to surface layer, reacting with the decomposed Ca of CaS, forming more stable CaO–Al₂O₃. However, Al₂O₃–MgO–CaO core inclusions and MnS shell compositions in steel with low Ti/Al content ratio (0.50) remain almost unchanged during heating. Furthermore, the MnS and CaO–Al₂O₃ shell precipitates with multiple points in steel with high Ti/Al content ratio are more conducive to acicular ferrites nucleation with more branches, which are attributed to the joint action of the Mn-depleted mechanism, minimum mismatch mechanism and strain-induced mechanism, yielding a higher toughness than that at low Ti/Al content ratio.