Abstract: The characteristics such as particle size distribution， average particle size， total amount and chemical composition of FH40 shipbuilding steels formed by Mg， Zr， MgZr combined treatment were investigated by the quantitative metallographic and SEMEDS methods. The results showed that the average particle size decreased while the total amount of inclusions increased after Mg， Zr， MgZr treatment. With 0024%Mg addition， the average mass fraction of MgO was 1257% in the inclusions， which was not fully modified； With 0072%Mg addition， the average mass fraction of MgO was 3558%， in the inclusions which meant that Al2O3 completely translated into MgO·Al2O3， and the size of inclusions decreased obviously. With 003% Zr addition， the main inclusions of the ingot were mainly Al2O3+MnS， AlZrO+MnS. The Zrcontaining inclusions were all found after MgZr combined treatment with the amount of the Mg and Zr additions changed as follow low Mg+high Zr， high Mg+low Zr and high Mg+high Zr. Especially， ZrO2 inclusions wrapped with MgO·Al2O3were found in the high Mg+high Zr treated steel with the apparent tendency to be aggregated.

Key words: shipbuilding steel, inclusions, magnesium, zirconium, modification