Effect of Ti/N Ratio on the Phase Transformation, Secondary Cracks, and Low-Temperature Toughness in Heat-Affected Zone of Shipbuilding Steel Plates with Mg Deoxidation

This paper aims to reveal the relationship between the phase transformation, secondary cracks and toughness of the heat-affected zones of shipbuilding steel plates with Mg deoxidation based on microstructural and crystallographic perspectives for the steels, with Ti/N ratios of 2.70, 3.00, 4.57, and 5.67, termed as TN27, TN30, TN46, and TN57, respectively. The microstructure of TN30 mainly consists of intragranular acicular ferrites and polygonal ferrites, which can effectively inhibit crack initiation and propagation. In contrast, the microstructures of TN27, TN46, and TN57 are mainly composed of polygonal ferrites, ferrite side plates, and upper bainites. The numbers and average sizes of secondary cracks first decrease and then increase by increasing the Ti/N ratio from 2.0 to 5.7. The higher proportion of high-angle grain boundaries, elevated kernel average misorientation values, and smaller effective grain size in TN30 steel contribute to inhibiting crack propagation. The impact toughness of the heat-affected zone in TN30 is significantly higher than those in the other three steels, decreasing from 183 J in TN30 to 135 J, 139 J, and 49 J in TN27, TN46, and TN57, respectively.