Abstract
With the development of clean steel technology, the control of non-metallic inclusions in steel is of increasing importance. Magnesium–calcium treatment can effectively balance the castability of molten steel and the control on inclusion size, which is an inclusion modification approach with application prospect. In view of this, how three addition methods (i.e., adding Mg before Ca, adding Mg after Ca, and adding Mg together with Ca) influenced the modification effect of inclusions in liquid iron was experimentally studied, and how these inclusions evolved with time was discussed in this paper. The results demonstrated that despite the sharp difference in their inclusion evolution, composite inclusions with a magnesium aluminate spinel (MAS) core and an outer CaO–Al2O3–MgO layer were formed by all the three addition methods, with the average inclusion size of 1–2 μm. Furthermore, thermodynamic calculation was adopted to reveal the transformation relationship between MAS and calcium aluminate in each of the three addition methods, and clarify the formation and disappearance mechanisms of the intermediate product CaS in the process of Mg–Ca treatment. The thermodynamic calculation results agreed well with the experiment data.
Highlights
Steel cleanliness plays a vital role in determining the properties of steel, such as toughness, ductility, formability, corrosion resistance, and surface quality
The results demonstrated that adding magnesium into liquid iron could modify the inclusions extremely fast, which was mentioned in authors’ previous research
To explore the dynamic evolution mechanisms of inclusions in the process of Mg–Ca treatment, experiments were performed in this study on Al-deoxidized molten pure iron, and the following conclusions were drawn through systematic analysis on inclusions: (1) The modification effect of inclusions in iron was influenced by the addition methods, so different evolution rules were presented for the three methods
Summary
Steel cleanliness plays a vital role in determining the properties of steel, such as toughness, ductility, formability, corrosion resistance, and surface quality. Non-metallic inclusions essentially reflect steel cleanliness, and the defects of steel products are closely related to them. Evidence can always be found to confirm that almost all typical defects of steel are directly associated with inclusions, such as surface defects of cold rolled sheet, fatigue fracture of bearing steel, drawing break of cord steel, and fin crack of can steel. In-depth studies on inclusions are necessary to develop clean steel. Inclusions primarily come from the by-products of deoxidation in steelmaking. Since no deoxidation method can take the place of precipitation deoxidation on a large scale so far, inclusions remain a permanent research topic in the field of steelmaking in the foreseeable future
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