Transformation of Oxide Inclusions in Type 304 Stainless Steels during Heat Treatment
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NON-METALLIC inclusions in stainless steels can have detrimental effects on properties such as strength, toughness, corrosion resistance, and surface finish.[1–5] Oxide inclusions can also affect the steel microstructure by serving as nucleation sites for solidification and austenite-to-ferrite transformation, and by pinning grain boundaries.[6–8] It was reported that fine MnO-Cr2O3 particles can effectively pin the austenite grain boundaries in Type 304 stainless steels.[9] The formation, removal, and modification of inclusions during the steel production processes of deoxidation, slag refining, calcium treatment, and reoxidation have been widely studied,[10–26] mostly for molten steel at steelmaking temperatures. Less attention has been paid to changes in oxide inclusions in solid steel during heat treatment, since in general little change would be expected. However, it was reported that MnO-SiO2 oxide inclusions may change to MnO-Cr2O3 during heat treatment of Type 304 stainless steel[27]; based on laboratory experiments, Kitamura et al. reported that the transformation of inclusions from MnO-SiO2 to MnO-Cr2O3 would be affected by the concentrations of YING REN is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China and also with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA, 15213. LIFENG ZHANG is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB). P. CHRIS PISTORIUS is with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University. Contact e-mail: [email protected] Manuscript submitted November 10, 2016.
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Mn, Si, and Cr in the steel.[28,29] However, the mechanism, thermodynamics, and kinetics of this inclusion transformation during heat treatment of 304 stainless steels are still unclear and are the focus of the current study. Several thermodynamic[30,31] and kinetic models[32–40] have been developed to predict the compositions of slag, steel, and inclusions in molten steel and to predict the precipitation of sulfides,[41–43] nitrides[44–46] and carbides[23,47,48] during solidification, cooling, and heat treatment. However, prediction of changes in oxide inclusions in solid steel during heat treatment has received little attention. In this work, such changes were studied experimentally and using a kinetic model. Isothermal heat treatment of Si-Mn-killed Type 304 stainless steels was carried out at different temperatures in the range of 1273 K to 1473 K (1000 °C to 1200 °C) under an argon atmosphere, and the resulting changes in inclusion composition and size were subsequently studied by scanning electron microscopy. The kinetic model used the thermodynamic package FactSage to predict the local equilibrium at the inclusion–steel interface at every time step. This is similar to
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