Abstract: The effect of Mg treatment on secondary phase particles, microstructure and toughness in the HAZ of steel plate were reviewed, and the combined effect of w(Ti)/w(N), Al, Nb and B on microstructure and toughness of the Mg treatment steel plates were discussed. When the Mg mass fraction was 27×10^-6, the volume fraction of IAF in HAZ had the highest value of 55.4 %, and the size of effective inclusions to promote the growth of IAF was concentrated at about 2 μm. With the increase of Mg content, TiN nanoparticles in steel were incrementally refined, and the value of the low-temperature impact toughness in HAZ zone increased linearly with the increase of the volume density of TiN nanoparticles. For ferritic shipbuilding steel plate, Mg treatment promoted the growth of intragranular acicular ferrite. After adding Mg mass fraction with 49×10^-6, the impact toughness of the heat affected zone at -40 ℃ increased from 20 J to 167 J. For bainite marine engineering steel plate, after adding Mg mass fraction with 42×10^-6, the secondary phase particle dissolution amount at high temperature became large, the pinning force decreased, and the grain size was coarsened. The toughness of HAZ at -40 ℃ decreased from 201 J with w(Mg)=2×10^-6 to 58 J with w(Mg)=42×10^-6. The optimum w(Ti)/w(N) was maintained at about 3.42. At this time, the number of precipitated particles in the secondary phase increases and the size decreased, which enhanced the pinning effect on grain boundaries. The addition of Al would reduce the HAZ toughness of the steel plate and the impact toughness at -20 ℃ was reduced from 201 J to 75 J when the Al mass fraction was 200×10^-6. After adding Nb,the brittle structure in steel was increased. The HAZ impact toughness at -40 ℃ decreased from 127 J to 58 J after adding Nb mass fraction with 160×10^-6 to Mg treatment steel plate. B would improve the hardenability of the steel, and the segregation at the grain boundary inhibits the formation of grain boundary ferrite. The impact toughness at -40 ℃ increased from 40 J to 141 J when 22×10^-6 w(B) was added to the Mg treatment steel plate.

Key words: magnesium treatment, oxide metallurgy, secondary phase particles, toughness, heat affected zone