Abstract
To reveal the effect of adding order of magnesium and sulfur on the evolution of inclusions in Al-killed free-cutting steel, both deoxidized experiments and thermodynamic calculations were carried out in this paper. The samples, which were extracted from the liquid steel at different time after magnesium and sulfur addition, were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The results showed that the adding order had a significant influence on the formation and evolution of inclusions. In the case of magnesium added before sulfur, MgO formed immediately after magnesium addition and then transformed to spinel accompanied by MeS (Mg0.9Mn0.1S) after sulfur addition. In the case of sulfur added prior to magnesium, MgO and MeS precipitated simultaneously after magnesium addition. During the equilibrium solidification, the transformation of MgO to spinel was calculated to take place before MnS precipitated from the saturated liquid steel.
Highlights
Non-metallic inclusions have a great influence on the mechanical properties and machinability of steel
Jiang et al [8] quantitatively studied the modification of sulfide inclusions by calcium in a resulfurized alloy steel, and the results showed that the proportion of MnS
In the case of magnesium added before sulfur, MgO forms around the magnesium bubbles after magnesium addition, as shown in Equation (13)
Summary
Non-metallic inclusions have a great influence on the mechanical properties and machinability of steel. Manganese sulfide is usually used to improve machinability in free-cutting steel as the stress concentration source [1,2,3,4]. The deformation of manganese sulfide along with the rolling direction deteriorates the mechanical properties of the steel [5,6]. The control of the sulfide inclusions morphology is vital for balancing the mechanical properties and machinability of free cutting steel. Experimental results of Luyckx et al [7] verified that the rare earth additives could be used to achieve the control of sulfide morphology in aluminum killed steels. The degree of sulfide shape control was improved by increasing the zirconium content or reducing the manganese content of the steel
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