Influence of Ti/N Ratio on Inclusions, Microstructures and Toughness in Heat‐Affected Zone of Shipbuilding Steel Plates with Mg Deoxidation after High Heat Input Welding

The inclusions, microstructures and toughness in heat‐affected zone (HAZ) of the shipbuilding steel plates with Mg deoxidation and the different Ti/N ratios were studied after the high heat input welding of 400 kJ/cm. With increasing the Ti/N ratio from 3.00 to 5.67, the average size of inclusions increases, the mass fraction of total TiC inclusions increase from 2.8% to 5.0%, and the average size of MgO‐TiN‐MnS increases from 2.82 μm to 5.49 μm, while the number density that can induce intragranular acicular ferrite (IAF) nucleation decreases from 87 mm^‐2 to 62 mm^‐2. These changes in inclusions are detrimental to induce the IAF nucleation. The HAZ microstructures are composed of the IAFs, polygonal ferrites (PFs) and upper bainites (UBs). With increasing the Ti/N ratio, the area fraction of the IAFs decrease from 44% to 14%, and the area fraction of UBs increases from 1.8% to 9.5%. The HAZ fracture mode changes from ductile fracture to brittle fracture after the high heat input welding of 400 kJ/cm, and the HAZ toughness decreases from 183.3 J to 48.7 J at‐20 °C.

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