Influence of rare earth addition on inclusions, microstructure, and toughness of the heat-affected zone in high-heat-input welded Mg–Ca treated shipbuilding steel

After the addition of rare earth (RE) elements to the Mg–Ca treated steel, the mean size of inclusions increases from roughly 1 μm to 2 μm, whereas the number density declines from 533 mm −2 to 174 mm −2 . In Mg–Ca treated steel, intragranular acicular ferrite (IAF) constitutes the predominant microstructure, occupying approximately 60% of the area. Following RE addition, the IAF fraction falls to 15%, while widmanstätten ferrite (WF) and bainitic ferrite (BF) become the dominant microstructures. After impact testing at −20 °C, the fracture mode changes from ductile–brittle fracture in the Mg–Ca treated steel to cleavage fracture in the Mg–Ca–RE treated steel, and the impact energy value decreases from 252 J to 33 J.