Abstract: The shipbuilding steel with Mg treatment has excellent performance for large heat input welding, and the stable pinning effect of the particles is the key for the preventing of the grains coarsening. For conforming the behavior of pinned particles for Mg treated DH36 shipbuilding steel under high temperature, the high temperature confocal experiments and the welding thermal simulation experiments had been designed and performed. The results showed that, when the temperature was lower than 1 170 ℃，the pinned particles in steel were mainly the Ti & Nb composite particles like stars with small size and TiN particles. For higher temperature and longer high temperature duration, the Ti & Nb composite particles lost the pinning effect for solving in the steel, and the austinite grain size had grown from 5 μm to nearly 50～60 μm. The new steady pinned particles were mainly the Mg & Ti composite particles, and the stability was decided by the [Mg] content in the steel.

Key words: oxide metallurgy, shipbuilding steel, pinned particle, Mg treatment, grain refining