An effective strategy for manufacturing ultra-high purity AISI 316L stainless steel by vacuum C deoxidization pretreatment plus Mg–Ca composite treatment

Aiming at the special ultra-high purity (UHP) requirements of AISI 316L stainless steel used in semiconductor equipment, a strategy of vacuum C deoxidization pretreatment plus Mg–Ca composite treatment was proposed in this work. The effects of this strategy on cleanness and inclusion modification were systematically investigated by experimental observations and thermodynamic calculations. The results demonstrated that this strategy significantly reduced the O and S contents to 0.0005 wt% and 0.0007 wt%, respectively. The main inclusions after vacuum C deoxidization were large-sized Al2O3·SiO2 and MnS. After Mg treatment, the oxide inclusions were gradually modified into MgO·Al2O3 and MgO, while the sulfide inclusion was still MnS. After additional Ca treatment, the inclusions were further modified into finer CaO, MgO, CaO·MgO, and CaS. Moreover, this strategy remarkably decreased the number and size of the inclusions. The best level was that all inclusions were smaller than 2 μm, and the proportion of inclusions smaller than 1 μm was as high as 75.47%. Meanwhile, the evolution mechanism of inclusions during different treatments was clarified in this work. Finally, the optimum process for preparing UHP AISI 316L stainless steel was proposed as vacuum C deoxidation plus 0.002 wt% Mg and 0.0016 wt% Ca composite treatment.