Effects of Mg–Ca treatment and Ca treatment on impact toughness and morphology of sulfides in 45MnVS non-quenched and tempered steel

An industrial experiment was conducted at a certain steel plant in China to compare and analyze the effects of Ca treatment and Mg–Ca treatment on inclusions in 45MnVS non-quenched and tempered steel. Through scanning electron microscopy-energy dispersive scanning analysis of the morphology and composition of inclusions, as well as Aspex quantitative analysis of their quantity, type and size, the formation mechanism of MnS–oxide (MnS inclusions with oxide cores) was intensively studied. The influence of sulfide morphology on the impact properties of steel was also analyzed. The results show that the quantity percentage of spindle-shaped sulfides in Ca-treated steel is 19.99%, and that in Mg–Ca-treated steel is 35.38%. Compared with Ca-treated steel, there are more MnS–oxide inclusions in Mg–Ca-treated steel. Controlling the content of Ca and Mg in the oxide core of MnS–oxide inclusion above 10 wt.% and the area ratio below 5 would contribute to the formation of spindle-shaped inclusions after rolling. The mismatch between MnS and oxides decreases with the increase in MgO content in the oxides, which is beneficial to nucleation and precipitation of MnS with this type of oxides as the core. Under the same deformation conditions, the size of sulfide does not affect its aspect ratio. Under the experimental conditions, the inclusion containing a certain amount of MgO can enhance its sulfur capacity, facilitating the formation of composite sulfides. The transverse impact energy of Ca-treated steel is 25.785 J, and that of Mg–Ca-treated steel is 32.119 J. Compared with the traditional Ca-treatment, Mg–Ca treatment can increase the number of spindle-shaped sulfides in the steel, thereby improving the transverse impact toughness of the steel and reducing the anisotropy of the mechanical properties of the material.