Utilization of Non-Metallic Inclusion to Induce Intragranular Acicular Ferrite Formation Contributing to “Oxide Metallurgy”: Effect of Ti/Al Content on the Microstructure Evolution

Abstract

Non-metallic inclusion is generally aimed to be removed during the refining process of steel production. The steelmakers always intend to produce clean steel to optimize the final product properties. However, the fine size inclusion is hard to remove completely; alternatively, it could be served as the nucleation site to induce the formation of intragranular acicular ferrite (IAF). This is an optimal microstructure with the “interlock” morphology and has been reported to be able to improve mechanical property, e.g., low-temperature impact toughness, according to the concept of “oxide metallurgy.” In this work, the low-alloy steels with different amounts of Ti and Al contents are prepared, and the inclusion characteristics (i.e., composition, size, distribution, etc.) are quantitatively investigated. Furthermore, high-temperature confocal laser scanning microscopy is applied to observe the IAF formation in situ with controlled isothermal holding and cooling conditions. The effect of nature of inclusions on IAF formation is investigated in the proposed steels. Subsequently, the theoretical model according to classical nucleation theory is utilized to evaluate the capability of different kinds of inclusions to induce IAF. Last but not least, the microstructure features in different steels are also investigated.