Formation and evolution of inclusions in Si-Mn-killed 60Si2Mn spring steel under LF-RH process with calcium treatment

This study aims to elucidate inclusion evolution in Si-Mn-killed, calcium-treated 60Si2Mn spring steel under LF-RH process. High Si content combined with medium-high basicity refining slag elevates T.Al content to relatively high level via slag-steel reaction. Excessive calcium treatment of 60Si2Mn spring steel under low-O/high-S conditions predominantly forms pure CaS inclusions, while [O] preferentially combines with [Ca] yielding pure CaO. With rising oxygen, CaS formation progressively diminishes as [Si] and minor [Mg] participate in reactions, generating CaO-SiO 2 (-MgO) inclusions (Ca:Si molar ratio >2.0). During the RH refining process, the oxygen is transferred from the top slag to molten steel despite the low (FeO+MnO) content in top slag, which combines with oxygen-affinitive elements, resulting in the formation of new inclusions, increasing the amount of inclusions in melt. Thermodynamic calculations reveal low oxygen increments promote 2CaO·SiO 2 formation in over-calcium-treated steel. Beyond a critical oxygen threshold, extensive Al and Mg participation triggers transforms CaO-SiO 2 inclusions into CaO-SiO 2 -Al 2 O 3 (-MgO) liquid inclusions. During inclusion modification via calcium treatment, T.Al and Si exert significantly stronger influence on SiO 2 content of inclusions than T.Ca and T.O. At low Si (≤0.5%) and high T.Al (≥0.013%), inclusions evolves toward near-CaO-Al 2 O 3 compositions. Conversely, liquid CaO-Al 2 O 3 -SiO 2 (-MgO) inclusions are yielded.