Relationship between crystallographic structure of complex inclusions MgAl₂O₄/Ti₂O₃/MnS and improved toughness of heat-affected zone in shipbuilding steel

Abstract

In accordance with the minimum degree of disregistry mechanism in oxide metallurgy, the intragranular acicular ferrite (IAF) generated by microalloying elements in austenite was studied. Herein, the effect of Mg treatment on the microstructure and toughness of the heat-affected zone (HAZ) in shipbuilding steel was investigated. Mg treatment produced inclusions that influenced the formation of acicular ferrite in the microstructure. This refined the HAZ microstructure and improved its toughness. Electron backscatter diffraction was used to determine the oxides of titanium and the MgO·Al₂O₃ or MgAl₂O₄ complex inclusions that induced the formation of IAF. MnS precipitated on MgAl₂O₄ on a specific habit plane and in a specific direction. MnS had a specific orientation relationship with MgAl₂O₄, i.e., ${\{100\}}_{{\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}}$//{100}_MnS. The 35-mm-thick plate obtained in the industrial test after welding at a welding heat input of 120 kJ/cm had an average impact absorbed energy of 282.7 J at − 40 °C and 2 mm from the weld joint in the HAZ. The two-dimensional disregistry index between inclusions can be used as the basis for controlling their distribution and adsorption force. Microalloy addition in the order of Al–Mg–Ti is key to obtaining abundant dispersion and fine nucleation in austenite.