Characterization of the Three-Dimensional Morphology and Formation Mechanism of Inclusions in Linepipe Steels

The characterization of the three-dimensional (3D) morphology and modification of inclusions in linepipe steel plates were investigated in the present work. The deoxidation products, Al₂O₃ inclusions, were modified into deformable calcium aluminate inclusions by calcium treatment in order to efficiently prevent nozzle clogging. The effect of sulfur content on the morphological characteristics and chemical composition of inclusions by calcium treatment were analyzed by extracting inclusions from steel matrix using non-aqueous electrolyte and automatic scanning electron microscopy connects with energy-dispersive spectrometer. The evaluation of the effect of calcium treatment was proposed using the 3D morphology of inclusions. Meanwhile, the modification and control of inclusions in the linepipe steel were studied by thermodynamic calculation on calcium treatment with the strategy of reducing the size of inclusions before calcium treatment, and controlling the contents of [Al], [Ca], and total oxygen in a reasonable narrow range during calcium treatment. The modification and transformation process of alumina inclusions was revealed as Al₂O₃ → MgO-Al₂O₃ → CaO-MgO-Al₂O₃-CaS (after calcium treatment) → CaO-Al₂O₃-CaS. A kind of high-melting MgO-Al₂O₃ core was wrapped by the low-melting component of CaO-Al₂O₃ indicating an incomplete reduction of MgO and Al₂O₃ by calcium.