The results of the study of the influence of the thermophysical properties of the welding flux obtained by processing man-made waste of electric steelmaking (metallurgical slag) on the structure and properties of welded butt joints of thin-sheet low-carbon steel, with automatic arc welding on ceramic linings, are presented. Welding modes have been established using the developed flux, contributing to the achievement of seam sizes according to GOST8713–79, C4 joints, compliance with the mechanical properties of joints close to the base metal and ensuring a minimum level of welding deformations and stresses.