Influence of refining process and utilization of different slags on inclusions, titanium yield and total oxygen content
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ORIGINAL PAPER
Influence of refining process and utilization of different slags on inclusions, titanium yield and total oxygen content of Ti‑stabilized 321 stainless steel Xing‑run Chen1,2 · Guo‑guang Cheng1 · Yu‑yang Hou1 · Jing‑yu Li1 · Ji‑xiang Pan2 Received: 22 May 2019 / Revised: 22 November 2019 / Accepted: 22 November 2019 © China Iron and Steel Research Institute Group 2020
Abstract Ti-stabilized 321 stainless steel was prepared using an electric arc furnace, argon oxygen decarburization (AOD) furnace, ladle furnace (LF), and continuous casting processes. In addition, the effect of refining process and utilization of different slags on the evolution of inclusions, titanium yield, and oxygen content was systematically investigated by experimental and thermodynamic analysis. The results reveal that the total oxygen content (TO) and inclusion density decreased during the refining process. The spherical CaO–SiO2–Al2O3–MgO inclusions existed in the 321 stainless steel after the AOD process. Moreover, prior to the Ti addition, the spherical CaO–Al2O3–MgO–SiO2 inclusions were observed during LF refining process. However, Ti addition resulted in multilayer CaO–Al2O3–MgO–TiOx inclusions. Two different samples were prepared by conventional CaO–Al2O3-based slag (Heat-1) and TiO2-rich CaO–Al2O3-based slag (Heat-2). The statistical analysis revealed that the density of inclusions and the T iOx content in CaO–Al2O3–MgO–TiOx inclusions found in Heat-2 sample are much lower than those in the Heat-1 sample. Furthermore, the TO content and Ti yield during the LF refining process were controlled by using TiO2-rich calcium aluminate synthetic slag. These results were consistent with the ion–molecule coexistence theory and FactSage™7.2 software calculations. When TiO2-rich CaO–Al2O3-based slag was used, the TiO2 activity of the slag increased, and the equilibrium oxygen content significantly decreased from the AOD to LF processes. Therefore, the higher TiO2 activity of slag and lower equilibrium oxygen content suppressed the undesirable reactions between Ti and O. Keywords 321 Austenitic stainless steel · Oxygen content · Inclusion · TiO2-rich CaO–Al2O3-based slag · Ladle furnace (LF) refining process · Ion–molecule coexistence theory
1 Introduction As a Ti-stabilized austenitic stainless steel (SS), 321 SS renders a set of unique advantages, such as superior high-temperature strength, adequate creep resistance, and excellent oxidation and corrosion resistance, compared to 304 stainless steel. Hence, 321 SS is widely used in heat exchangers, high-pressure pipes, engine turbines, aerospace exhaust manifolds, and nuclear industry [1–5]. Nevertheless, as a Ti-stabilized steel, 321 steel has several casting * Guo‑guang Cheng [email protected] 1
State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
Hongxing Iron and Steel Co., Ltd., Jiuquan Iron and Steel Group Corporation, Jiayuguan 735100, Gansu, China
2
and operational challenges such
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