The effects of Ca content on intragranular acicular ferrite nucleation in the CGHAZ of Al-Ca-Ti deoxidised shipbuilding steel

This study reveals the effects of Ca content (0.0002–0.0025 wt%) on the CGHAZ toughness of Al-Ca-Ti steels. Increasing the Ca content transforms inclusions from Ti 2 O 3 -MnS to CaO·6Al 2 O 3 -CaTiO 3 -MnS(-CaS), raising the effective inclusion density from 72.11 to 114.35 mm −2 . First-principles and lattice mismatch calculations reveal the superior lattice matching between CaTiO 3 and MnS, facilitating Mn-depleted zone formation around Ca-containing inclusions. Moreover, CaO·6Al 2 O 3 -CaTiO 3 -MnS(-CaS) exhibits lower lattice mismatch with ferrite (<6%) compared to Ti 2 O 3 -MnS, significantly promoting primary IAF nucleation. Consequently, with increasing Ca content, the primary IAF length grows from 7.7 to 11.3 μm, and the secondary IAF count increases from 2.1 to 3.7 per inclusion, enhancing CGHAZ impact toughness from 18 to 162 J.