Abstract：In this paper, high temperature laser confocal microscopy was used to observe in situ the evolution of microstructures of traditional ship plate steel and Mg treated ship hull steel during welding thermal cycle, and the phase transformation of steel during continuous cooling was studied. The results show that the austenite size of Mg-bearing hull structural steels is small, the grain size is obviously refined and a large number of acicular ferrites appear in the grain after holding at 1250 C for 100s, 300s and 500s. It is shown that Mg-containing oxide particles can pin austenite grain boundaries and hinder their movement. In the continuous cooling process, a large number of acicular ferrite nucleation can be observed in Mg-containing hull structural steels, which further refines the structure. In addition, the statistical results of inclusions show that the inclusions are refined and increased effectively after Mg treatment, and the proportion of 0.5-3.5 um inclusions which play the role of oxide metallurgy reaches 82.8% of the total amount.
2020, Vol. 55 
