Effect of magnesium treatment on inclusions in 1215 free-cutting steel

Sulfide inclusions in 1215 free-cutting steel not only affect the machinability of steel, but also have a big influence on the anisotropy and the quality of steel. The control of sulfide inclusions is the key to improvement of steel quality. In the current study, Mg treatment was employed to control the morphology, size and distribution of inclusions to explore the effect of Mg on the inclusion modification. Through the high-temperature smelting experiment, a series of steel ingots with different Mg contents were prepared. Using the optical microscope, scanning electron microscope and non-aqueous solution electrolytic etching method, the distribution of inclusions and the two-dimensional as well as the three-dimensional morphology of inclusions in the steel were analyzed. Besides, thermodynamic calculation was conducted to figure out the modification mechanism of inclusions. The results show that, Mg has a strong deoxidation ability, which has a significant influence on the morphology and distribution of sulfide inclusions. When the Mg mass fraction increases from 0 to 0.000 6% and 0.001 7%, the morphology of inclusion changes from spherical, ellipsoidal (type I sulfide) to cluster, chain-like, coralloid (type II sulfide), and then to polyhedron, irregular block (type III sulfide). When the Mg mass fraction further increases to 0.002 7%, the effect of Mg is no more significant. MnS can not be generated in the molten steel. MnS mainly precipitates in the solid-liquid two-phase region during the solidification process, the corresponding precipitation temperature and solidification fraction of steel is 1 502.0 ℃ and 0.409, respectively. During the solidification process, part of MnS takes oxide inclusions as heterogeneous nucleation points, and forms complex inclusion with internal oxide and external sulfide. Once the Mg is added, Al₂O₃ inclusions are modified to dispersive distributed tiny MgO·Al₂O₃ inclusions, which has little possibility of aggregation and growing, providing more heterogeneous nucleation points for the precipitation of MnS. Therefore, the number density of inclusions is increased, while the average equivalent diameter is decreased.