Effect of Zr addition on inclusions, micro-structures and toughness in the heat-affected zone of Mg deoxidised ship steel after high heat input welding

After Zr addition in the Mg deoxidised steel, the total area fraction of ductile micro-structures in the heat-affected zone (HAZ) decreases from 85% to 40%. In Mg steel, complex inclusions induce intragranular acicular ferrite (IAF) with a probability of 67%, while in Mg-Zr steel, the probability of inducing IAF drops to 53%. In the Mg-Zr steel, many larger inclusions with the size greater than 5 μm generate to decrease the HAZ toughness, which are the ZrN-dominated complex inclusions and the inclusions aggregating sulphide, oxides and carbon nitrides. The impact toughness at −20 °C after the high heat input welding (HHIW) of 400 kJ/cm decreases from 143 J for Mg steel to 23 J of Mg-Zr steel.