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Industrial trials and theoretical analysis are performed to study the effect of addition sequences of calcium and sulfur on inclusions in a calcium‐treated and resulfurized Al‐killed steel. The evolution path of inclusions at each production step is Al2O3 → Al2O3–CaO–MgO → Al2O3–MgO and Al2O3–CaS–CaO → Al2O3–MgO–CaS with the calcium treatment before sulfur addition. Calcium treatment after the sulfur addition promotes the formation of (Mn, Ca)S complex sulfide inclusions and is beneficial to modify the shape of sulfide inclusions to spherical ones in the rolled steel, where the size of sulfide inclusions is smaller than that of the heat of calcium treatment before sulfur addition. The relative plasticity of (Mn, Ca)S complex inclusions has a linear relationship with the content of CaS. Thermodynamic calculation results show that the critical solute sulfur content corresponding to the formation of CaS in the molten steel decreases sharply with the increase of the calcium content. With the same amount of calcium in the steel, the calcium treatment after sulfur addition promotes the formation of small (Mn, Ca)S complex sulfide inclusions with lower deformability during the hot rolling process, which is helpful for the mechanical property of the calcium‐treated and resulfurized Al‐killed steel. [ABSTRACT FROM AUTHOR]

The effect of calcium addition on inclusions (oxides and sulfides) in heavy steel ingot is investigated via lab experiments and thermodynamic calculation. The characteristics (chemistry, size, morphology, and number) of inclusions in the steel samples are investigated by scanning electron microscope with an energy‐dispersive spectrometer. The three‐dimensional morphology of inclusions is investigated by the nonaqueous solution electrolysis method. Inclusions in the fast‐quenched steel are changed from Al2O3 to Al2O3–CaO, Al2O3–CaO–CaS, and CaS after Ca addition. After slow cooling, inclusions in the solid steels are changed from Al2O3, single MnS, and Al2O3–MnS complex inclusions to Al2O3–CaO–CaS–MnS and CaS–MnS. The evolution trajectory of inclusions is Al2O3→Al2O3–CaO→Al2O3–CaO–CaS→Al2O3–CaO–CaS–MnS. With increasing Ca content in the steel, the number density and proportion of single MnS inclusions decrease. The formation mechanism of inclusions after Ca addition is explained by thermodynamic calculation. [ABSTRACT FROM AUTHOR]

Based on the principle of minimum Gibbs free energy, a thermodynamic model is established to predict the composition of inclusions in the Al‐killed steel during the calcium treatment process. The Wagner model combined with coexistence theory is applied to calculate the Gibbs free energy of the steel‐inclusion system. The composition evolution of inclusions is accurately predicted through the current thermodynamic model which is widely applied to predict the modification of inclusions by the calcium treatment process with different contents of total oxygen (T.O), total sulfur (T.S), and total calcium (T.Ca) in the molten steel. Alumina inclusions can be modified to liquid calcium aluminate inclusions with appropriate T.Ca contents. Solid CaS or CaO inclusions can be formed as a result of the excessive calcium addition to molten steel, while the insufficient calcium addition can hardly modify alumina inclusions to liquid ones. The effect of the T.O and T.S contents on the liquid window is also discussed. The relationship between the value of T.S/T.O in the molten steel and the composition of inclusions is also studied. Based on the established thermodynamic model, a software for the precise control of inclusions is also developed to guide the calcium treatment operation timely in the accrual refining process. [ABSTRACT FROM AUTHOR]