Evolution of oxide–CaS complex inclusions during protective atmosphere electroslag remelting

Abstract

The inclusions in the consumable steel electrode and electroslag remelted steel were characterized using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The evolution mechanism of oxide–sulfide complex inclusions during electroslag remelting (ESR) was elucidated based on inclusion experimental identification and thermodynamic calculation. The results show that the combination of protective atmosphere and deoxidation operation during ESR lowers the total oxygen content from 0.0017% in the electrode to 0.0008% in the ingot. The number proportion of the inclusions smaller than 3 μm in the steel greatly increases after ESR. The inclusions in the steel electrode are two oxide–sulfide complex types of CaS+CaO–Al2O3–SiO2–MgO containing about 3% MgO and CaS+CaO–Al2O3–SiO2–MgO containing about 11% MgO. SiO2 in the original oxide inclusions that had not been removed in ESRR process was reduced by soluble aluminum in liquid steel, and the products remain in the ESR process until in remelted ingot. The CaO–Al2O3–SiO2–MgO inclusions with uniform elements distribution, which contain about 1%MgO and about 2%SiO2, in the ingot are newly formed oxide inclusions in the ESR. CaS inclusions in the steel electrode were removed during the ESR through dissociating into soluble calcium and sulfur in liquid steel, and in the way of reacting with Al2O3 in liquid oxide inclusions. The shell-type CaS around low-melting-temperature oxide inclusion generated as a result of the reaction between CaO in the oxide inclusion and dissolved aluminum and sulfur in liquid steel during solidification of liquid steel in the ESR process. The shell-type CaS around high-melting-temperature oxide inclusion is the reaction products of enriched soluble Ca and S during solidification of liquid steel. Patch-type CaS in the oxide–sulfide complex inclusion precipitated from the complex inclusion melt during the cooling of liquid steel in the ESR process.

Keywords

• non-metallic inclusion
• sulfide
• electroslag remelting
• ultralow oxygen
• sulfide capacity