Effect of rare earth and magnesium treatment on inclusions in ultra-high purity 316L stainless steel

This study investigated the effects of rare earth(RE) and magnesium treatments on non-metallic inclusions in ultra-high purity (UHP) 316L austenitic stainless steel, aiming to enhance steel cleanliness to meet the stringent requirements for semiconductor equipment materials. Traditional aluminum deoxidation processes produce alumina, which is prone to shedding during service and contaminates the entire semiconductor processing system. Although rare earth and magnesium are considered potential alternative deoxidizers, systematic studies on their combined treatment process, particularly the influence of addition sequence on inclusions in UHP 316L stainless steel, remain insufficient. Using a VIM+VAR duplex process, three deoxidation routes were designed, RE (Y) treatment only, Mg-Ce sequential treatment, and Ce-Mg sequential treatment. The effects of each process, under industrial production conditions on the size, distribution, type, and rating of inclusions were systematically examined. Inclusion characteristics and their evolution at various processing stages were evaluated using metallographic microscopy, SEM-EDS, and Thermo-Calc thermodynamic simulations. The results demonstrate that in VIM+VAR smelting of UHP 316L stainless steel, the Ce-Mg sequential deoxidation sequence is the most effective strategy for high-level inclusion control. This approach results in the lowest inclusion distribution density and smallest average equivalent diameter in VIM+VAR samples, with no large inclusions (≥8 μm) observed. The fine Type D inclusion rating is below 0.5, meeting industry standards for UHP 316L stainless steel. In contrast, the Mg-Ce treatment shows poor inclusion control due to severe premature magnesium loss, which limits effective synergy with rare earth. The proper RE-Mg addition sequence fully utilizes the strong deoxidation capability of rare earth elements and the bubble flotation and stirring effects of magnesium vapor, providing a key process route for achieving high cleanliness control in UHP 316L stainless steel.