Original Article

Effect of Mg treatment on inclusion characteristics in U71Mn heavy rail steel

¹ Center for Advanced Solidification Technology of Shanghai University, State Key Laboratory of Materials for Advanced Nuclear Energy, Shanghai 200444, PR China
² Inner Mongolia Baotou Steel Union Co, Ltd., Baotou 014010, PR China

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Received: 30 March 2026
Accepted: 22 June 2026

Abstract

To investigate the effect of Mg on non-metallic inclusions in U71Mn heavy rail steel, an industrial-scale Mg treatment trial was conducted. The influence of Mg on inclusions was analyzed by using a metallographic microscope, a scanning electron microscope, an inclusion 3D etching instrument, and FactSage software. The results showed that after Mg treatment, the large and irregular MnS and CaO-Al₂O₃-SiO₂ inclusions were transformed into small, near-spherical composite inclusions consisting of MgO or MgO·Al₂O₃ cores surrounded by MnS. After Mg treatment, the inclusion density in the bloom increased from 31–56 mm⁻² to 41–62 mm⁻², while the equivalent diameter decreased from 2.8–3.7 μm to 2.3–3.5 μm. The proportion of large-sized inclusions in the steel was significantly reduced, and the inclusions were more uniformly distributed in the steel matrix. After Mg treatment, the deformation degree of composite inclusions during rolling decreased, resulting in a more equiaxed shape of the inclusions in the rail. Compared with the untreated steel, the aspect ratios of inclusions in the rail head, web, and base after Mg treatment decreased to 4.12, 5.27, and 3.50, respectively, while their average sizes were reduced to 7.5, 13.6, and 7.1 μm, respectively. Mg treatment effectively improved the cleanliness of heavy rail steel and reduced both the size of inclusions and their deformation during rolling.