Evolution of oxide inclusions and cleanliness control in Al-killed GCr15SiMn bearing steel during secondary refining process

Abstract

The evolution of oxide inclusions and cleanliness control in Al-killed GCr15SiMn bearing steel was investigated using a combination of industrial tests and thermodynamic calculations during the secondary refining process. Three different refining conditions were employed in the LF process. With varying ranges of slag basicity (about 5.6–5.7, 4.8–4.9 and 3.9–4.3) and Al content (0.028–0.035 mass%, 0.023–0.025 mass% and 0.014–0.02 mass%), the total oxygen content in the steel after VD refining was measured as 0.0019 mass%, 0.0017 mass% and 0.0008 mass%, respectively. Throughout the refining process, the inclusions underwent a transformation from MgO-Al₂O₃ (or Al₂O₃) into CaO-Al₂O₃(-MgO), and eventually to CaO-Al₂O₃-SiO₂ inclusions. Furthermore, it was observed that under high Al content and slag basicity refining conditions, a significant number of inclusions in VD treatment were formed and transformed into CaO·Al₂O₃·SiO₂ inclusions enriched with 60 mass% CaO. This indicated that when the proportion of CaO in calcium aluminates exceeded 45 mass%, there was an increased risk of larger-sized inclusions. Conversely, refining conditions with a slag basicity of 3.9–4.3 and an Al content of 0.014–0.02 mass% demonstrated effective inclusion control, resulting in lower number density, smaller sizes within 10 μm and an average composition located in the liquid region. This study carries important practical significance for the smelting of high cleanliness GCr15SiMn bearing steel.