Regular Article

Inclusions evolution during the LF refining process of 439 ultra-pure ferritic stainless steel

¹ State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 10083, PR China
² Hongxing Iron & Steel Co., Ltd., Jiuquan Iron and Steel Group Corporation, Jiayuguan 735100, PR China

^* e-mail: chengguoguang@metall.ustb.edu.cn

Received: 14 May 2019
Accepted: 30 August 2019

Abstract

The morphology, composition, size, and number of inclusions in 439 ultra-pure ferritic stainless steel samples were analyzed using an automatic scanning electron microscope combined with an energy-dispersive X-ray spectrometer. In addition, the appropriate contents of titanium, aluminum, and calcium were analyzed through the coupling of thermodynamics calculation and experimental results. CaO-Al₂O₃-MgO inclusions existed in the 439 steel before Ti additions in the ladle furnace (LF) refining process. After Ti addition in the LF refining process, the inclusions were transformed into CaO-Al₂O₃-MgO-TiO_x inclusions. The evolution of these inclusions was consistent with thermodynamic calculation, which indicated that when the Al, Ca, and Ti contents were within a reasonable range, Ca treatment could significantly modify the aluminate and spinel to form CaO-Al₂O₃-MgO liquid inclusions. In addition, the compositions of inclusions after the addition of titanium were mostly located in the Al₂O₃-TiO_x stable phase. The collision of the CaO-Al₂O₃-MgO liquid inclusions and Al₂O₃-TiO_x inclusions resulted in the modification of the CaO-Al₂O₃-MgO-TiO_x inclusions. The compositions of most inclusions were located in the liquid zone. The control range of the aluminum, calcium, and titanium contents was obtained: logAl% ≥ 1.481logTi% − 0.7166, Ca% ≥ 34.926(Al%)³− 3.3056(Al%)² + 0.1112(Al%) − 0.0003.