Cleanliness and Control of Inclusions in Al-Deoxidized Bearing Steel Refined by Basic Slags during LF-VD-Ar Bubbling

Abstract

Cleanliness and control of inclusions in Al deoxidized bearing steel were studied by industrial trials, in which three basic slags were used in the LF-VD-Ar bubbling refining process. With basicity (mass ratio of CaO/SiO₂) about 3.9–4.2, 5.2–6.5 and 6.9–7.1 while Al₂O₃ about 30.9–32.5 mass%, 37.2–40 mass% and 29.8–30.4 mass%, T.O content in steel after Ar bubbling was 0.0006 mass%, 0.0007 mass% and 0.0004 mass%, respectively. During the refining, inclusions experienced the evolution from Al₂O₃ into spinel and finally into CaO–MgO–Al₂O₃. By comparison, inclusions were more desirably controlled when slag basicity and Al₂O₃ contents were about 6.9–7.1 and 29.8–30.4 mass%, with lowest number density, smaller sizes within 20 µm and average composition in liquid region. Particularly, after Ar-bubbling, pick-ups in the number density of inclusions were observed for the three heats of trials and large inclusions (even exogenous inclusions over100 µm) were often seen in heat 2. The obtained results prompted the risk of large inclusions in bearing steel in Ar bubbling, which were detrimental to fatigue lives of bearing.