Evolution Mechanism of Inclusions in Medium-Manganese Steel by Mg Treatment with Different Aluminum Contents

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A typical representative of medium-manganese steel, Transformation Induced Plasticity (TRIP) steel, which has high strength and high plasticity, is the most potential automobile steel. However, the content of silicon in the conventional TRIP steel is more than 1 mass pct, which easily leads to the precipitation of stable oxides, such as Mn2SiO4, on the surface of the steel sheet. These oxides are detrimental to the coating ability of the TRIP steel and deteriorate the surface quality of the steel sheet.[1,2] Replacement of silicon by aluminum can improve the coating ability and surface quality of the TRIP steel sheet. On one hand, the addition of aluminum to the steel can reduce the density of steel, which can meet the lightweight requirements of automobile steel. Furthermore, aluminum addition can prevent the appearance of hydrogen cracks.[3] In

ZHE YU and CHENGJUN LIU are with the Key Laboratory for Ecological Metallurgy of Multimetallic Ores (Ministry of Education), Northeastern University, Wenhua Road, Heping District, Shenyang 110819, Liaoning, P.R. China and also with the School of Metallurgy, Northeastern University. Contact e-mail: [email protected] Manuscript submitted September 10, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS B

addition, the aluminum in steel can also increase the stacking fault energy of austenite.[4] Therefore, C-Mn-Al-TRIP steel, as a new type of steel, has a tendency to replace the traditional C-Mn-Si-TRIP steel. Generally, the aluminum content in medium-manganese TRIP steel ranges from 1.1 to 5.0 mass pct. In recent years, many studies have been carried out on the material properties of medium-manganese steel,[5–7] and the influence of non-metallic inclusions on the properties of medium-manganese steel has been noted. Park et al.[8] characterized the inclusions formed in high Mn-Al alloy steel with 10 to 25 mass pct Mn and 1 to 6 mass pct Al, and he found that the main inclusions were MnOÆAl2O3 and Al2O3. The Al2O3 inclusions proved to deteriorate the casting performance of molten steel, and reduce the mechanical properties of the steel.[9,10] Therefore, it is necessary to control the inclusions in the Mn-Al alloy steel. Magnesium treatment can modify the stable inclusions in steel from Al2O3 to spinel (MgOÆAl2O3) or pure MgO, which can improve the casting performance of molten steel.[11–13] However, the study of magnesium treatment on medium Mn and high Al steel has not been reported. In the present study, the medium-manganese steels containing different aluminum contents were prepared to reveal the evolution of inclusions in Fe-0.2C-10Mn-Al-Mg-O system. The samples under magnesium

treatment for different times were analyzed by the SEM–EDS. The confocal laser scanning microscope (CLSM) was used to observe the evolution process of Al2O3 inclusions in the magnesium-containing molten steel. It is well known that cluster-like Al2O3 are easily formed after aluminum deoxidation of molten steel, and the agglomeration behavior of Al2O3 inclusions has also been confirmed by in si

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