Mechanisms of inclusion evolution and intra-granular acicular ferrite formation in steels containing rare earth elements

Xiaoxuan DENG , Min JIANG and Xinhua WANG (School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China)

摘 要：Inclusion characteristic and microstructure of rare earth （RE） elements containing steel were evaluated with scanning electron microscopy with energy dispersive spectroscopy （SEM-EDS）, element-mapping, optical microscopy （OM）, and automated feature analysis （AFA） option equipped with ASPEX PSEM. Factsage was used to calculate the equilibrium inclusion composition. Based on the calculation, an inclusion evolution mechanism was proposed. Furthermore, line scanning analysis was used to elucidate the intra-granular acicular ferrite （IAF） nucleation mechanism. The result showed that two different inclusions exist in sample steel: （Mn-Al-Si-Ti-La-Ce-O） +MnS complex inclusion and isolated MnS inclusion. Almost all nucleation sites for IAF are complex inclusions, while single MnS inclusion cannot induce IAF. A possible formation mechanism of complex inclusion is proposed based on calculated results using Factsage, which agrees well with experimental results. A Mn-depletion zone （MDZ） which exists adjacent to the （Mn-Al-Si-Ti-La-Ce-O） +MnS complex inclusion can account for the IAF formation. However, the low volume fraction （1.49×10-7） of effective inclusion may result in only 10% （volume fraction） IAF.

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