In Situ Observation of MgO Inclusions in Liquid Iron-Aluminum Alloys
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INTRODUCTION
IN Al-killed steels, the reduction of MgO from slags by dissolved Al and transfer of Mg into liquid steels has been observed by several researchers.[1,2] In these works, dissolved Mg in steel modifies original Al2O3 inclusions to form spinel-type (MgAl2O4) inclusions. Mg pickup has been shown to rise with the increase of slag basicity and diffusion of Mg in molten steel has been shown to control the rate of slag-steel reaction. A recent investigation by Harada found that spinels were the final inclusions after MgO refractory reduction in Al-killed steel.[3] The spinel inclusions were considered to be in equilibrium with a spinel layer formed at the interface between the metal and MgO refractories. In all above slag/refractory reduction experiments, Al levels in the steels were below 0.1 wt pct. Park[4] and Zhang,[5] respectively, have elucidated the formation mechanism of spinel inclusions by thermodynamic calculations. According to these predictions, HAOYUAN MU, Ph.D. Student, TONGSHENG ZHANG, Postdoctoral Researcher, LIANG YANG, Exchange-Program Student, RICHARD J. FRUEHAN, Professor, and BRYAN A. WEBLER, Assistant Professor, are with the Center for Iron and Steelmaking Research, Materials Science and Engineering Department, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213. Contact e-mail: [email protected] RODRIGO R. XAVIER, formerly Exchange-Program Student with the Center for Iron and Steelmaking Research, Materials Science and Engineering Department, Carnegie Mellon University, is now Undergraduate Student with the Departamento de Engenharia Metalu´rgica e de Materiais, Universidade Federal de Minas Gerais, Av. Antoˆnio Carlos, 6627, Belo Horizonte, MG, Brazil. Manuscript submitted July 13, 2016. Article published online August 25, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
spinels formed when Al and Mg contents in steels fell into a certain region and inclusions could evolve to MgO as dissolved Mg content increased. With Al levels in steels nowadays upwards of 2 wt pct to achieve desired mechanical properties,[6] reduction of MgO from slag and refractory is accelerated and dissolved Mg in steel can increase to a relatively high level in a time period that is comparable to ladle treatment. MgO, instead of spinels, can form in high Al steels as a result of slag/refractory reduction. The formation of MgO inclusions in steel without metallic Mg addition has rarely been reported by other researchers. But the stability and behavior of MgO inclusions in such steels at steelmaking temperatures is worth studying because solid MgO inclusions are potentially detrimental to industrial practice as well as steel properties. This work discusses MgO stability at 1873 K (1600 °C) by analyzing in situ CLSM observation of the evolution behavior of MgO inclusions, as well as by comparing SEM/EDS inclusion analysis results before and after CLSM experiments.
II.
EXPERIMENTAL APPROACH
A. Materials Metal samples for in situ inclusion observation experiments were produced from an Fe-0.16 w
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