Regular Article

Study on the behaviour of DS-Class inclusions in advanced bearing steel

¹ Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China
² Iron and Steel Research Institutes of Ansteel Group Corporation, 68 Jianguo South Road, Tiedong District, Anshan, Liaoning 114009, PR China

^* e-mail: whjyeah@163.com

Received: 8 April 2018
Accepted: 5 September 2018

Abstract

The source and generated mechanisms of DS-size inclusions in bearing steel were studied by sampling systematically and using ASPEX, SEM and EDS to analyse the morphology, composition and amount of non-metallic inclusions larger than 13 µm. Two kinds of typical DS-size inclusions were found in the refining process: type I is CaO-MgO-Al₂O₃-SiO₂ distributed evenly and wrapped by CaS; type II is composite inclusion with MgO-Al₂O₃ as the core, CaO-Al₂O₃ and CaO-SiO₂ as the inner layer, covered by CaS. Based on the FactSage and thermodynamic calculations, the DS inclusions formation mechanisms were drawn. There are two formation mechanisms of the type II inclusions: one is the solid phases will precipitate from the matrix of type I inclusions as the temperature drops; another is that when the reaction time is not sufficient, the MgO∙Al₂O₃ spinel core will not be transformed completely, and the evolution can be summarized as: Al₂O₃ → MgO-Al₂O₃ → CaO-MgO-Al₂O₃ → surrounded by CaO-Al₂O₃(SiO₂) → covered by CaS.