Abstract
The main role of Rare Earth (RE) elements in the steelmaking industry is to affect the nature of inclusions (composition, geometry, size and volume fraction), which can potentially lead to the improvement of some mechanical properties such as the toughness in steels. In this study, different amounts of RE were added to a niobium microalloyed steel in as-cast condition to investigate its influence on: (i) type of inclusions and (ii) precipitation of niobium carbides. The characterization of the microstructure by optical, scanning and transmission electron microscopy shows that: (1) the addition of RE elements change the inclusion formation route during solidification; RE > 200 ppm promote formation of complex inclusions with a (La,Ce)(S,O) matrix instead of Al2O3-MnS inclusions; (2) the roundness of inclusions increases with RE, whereas more than 200 ppm addition would increase the area fraction and size of the inclusions; (3) it was found that the presence of MnS in the base and low RE-added steel provide nucleation sites for the precipitation of coarse niobium carbides and/or carbonitrides at the matrix–MnS interface. Thermodynamic calculations show that temperatures of the order of 1200 °C would be necessary to dissolve these coarse Nb-rich carbides so as to reprecipitate them as nanoparticles in the matrix.
Highlights
The chemical composition, population density, and morphology of non-metallic inclusions in metals are among the key factors determining the steels’ quality [1,2,3,4]
Once the melt reached to 1650 ◦ C, the amounts of alloying elements were adjusted and the chemical composition was measured by using the Optical Emission Spectrometry (OES) technique on site
The value of E was calculated from the standard deviation (σ) and the number of measured inclusions (n) according to American
Summary
The chemical composition, population density, and morphology of non-metallic inclusions in metals are among the key factors determining the steels’ quality [1,2,3,4]. These issues have become the leading subjects in the field of steelmaking processes in the last few decades. Despite various roles of RE in steels, the main use of RE in steels concerns the shape control of inclusions, especially MnS particles during the hot deformation processes [5,6,7,8,9]. It has been suggested that the addition of these elements results in a considerable change in inclusion composition and generally leads to the formation of several constituents such as oxysulphides (Ce2 O2 S, La2 O2 S), oxides (Ce2 O3 , La2 O3 ) and sulfides (Ce2 S3 , La2 S3 ) [10,11]
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