Modification of non-metallic inclusions in high-strength steels containing increased Mn and Al contents

Purpose: The aim of the work is to determine an influence of the effectiveness of the modification of the chemical composition and morphology of non-metallic inclusions by rare earth elements in advanced high strength steels (AHSS) with increased Mn and Al contents. Design/methodology/approach: The effects of the modification of non-metallic inclusions were assessed in four thermomechanically rolled AHSS containing various Mn (3 and 5%) and Nb (0 and ~ 0.04%) concentration. The surface fraction, surface area and aspect ratio were determined at longitudinal sections of the sheets with a thickness of 3.3 mm. The chemical composition of particles was investigated using point analysis and mapping by means of EDS and WDS techniques. Findings: The refining treatment of a liquid steel has an important effect on the sulphur content in a steel, a surface fraction of non-metallic inclusions, their deformability during hot-rolling and morphology. On the other hand the addition of mischmetal does not affect an inclusion size. The chemical composition of particles is independent on the Mn content in a range investigated, i.e., from 3 to 5%. The steels with the addition of REE contain totally modified, fine oxysulfides of Ce, La and Nd whereas the steel not subjected to the refining treatment contains elongated MnS, complex MnS + AlN compounds and AlN particles decorated in the outside zone by MnS and Al₂O. Practical implications: The knowledge of the stereological parameters of non-metallic inclusions and their morphology are of primary importance for the steelmaking and hot-working technologies of steel products. Originality/value: An effectiveness of the modification of the chemical composition and morphology of non-metallic inclusions by REE in advanced high-strength steels with increased Mn and Al contents is addressed in the current study.