Effect of Si Content on the Microstructures and Toughness in Heat‐Affected Zone of Offshore Engineering Steels with Mg Deoxidation

Herein, the effect of Si content on the microstructures and toughness in the heat‐affected zone (HAZ) with the heat input of 100 kJ cm −1 for Mg‐deoxidized offshore engineering steels is presented. For the matrix, increasing Si content coarsens the lath bainites (LBs) in HAZ due to the higher A r3 and lower A r1 . Thus, the high‐angled grain boundary density (HAGBD) decreases from 0.88 to 0.53 μm −1 , and the effective grain size increases from 10.6 to 18.3 μm. M–A constituents are transformed from dotted and filmed ones to the stringer and blocky ones, with the area fraction increasing from 2.9% to 5.6%. Among them, the stringer M–A particles are likely to debond from the matrix, while the blocky M–A particles tend to initiate cracks because the interfacial energy of stringer particles is lower than that of blocky ones. Accordingly, increasing Si content from 0.0003 to 0.120 wt% results in both the increased crack initiation probability by more M–A constituents with the larger size and the lowered crack propagation barrier by the smaller HAGBD, thereby reducing the HAZ toughness at −40 °C from 202 to 58 J.