Atom-scale study on the action mechanism and stability control of Mg in shipbuilding-steel with Mg treatment

Mg treatment can effectively improve the weldability of shipbuilding steel with high heat-input. However, a stable control and improvement of Mg content in steel is the limiting factor for Mg treatment technology. In this study, the existing form and mechanism of Mg action in EH40 shipbuilding steel were systematically analyzed, and the multiple mechanism of Mg action and the methodology to increase Mg content were discussed at the atom-scale, by first principles and molecular dynamics. The experimental results showed that the Mg in the shipbuilding steel mainly exists in the inclusions and second-phase particles, which can refine the inclusions and strengthen the high temperature stability of second-phase particles. The first principles calculation showed that the Mg atom has an affinity for Si side of the γ-Fe-Si-Mg atom-system, and Mg atoms promote Ti and Nb precipitation, which contributes to the enrichment and stability strengthening of second-phase particles. MD analysis showed that the clusters of Si and Mg atoms may protect Mg atoms in molten steel when the temperature is above 950 °C, and promotes the formation of high-temperature resistant pinning particles. For the practical production of Mg treated shipbuilding-steel, the optimal Mg treatment involves two stages: feeding Mg-containing cored wire into the molten steel and adding SiMg alloy, meanwhile the temperature of molten steel should be kept stable to achieve a stable increase in Mg content.