Abstract: In order to improve the inclusion control process and improve the quality and performance of cold-drawn steel tubes, the inclusions in the refining process of B18 cold-drawn steel tubes were systematically analyzed. The evolution of the purity of molten steel and the composition, morphology, particle size distribution and quantity of the microscopic inclusions during the refining process were studied. The results showed that the content of T.O and N in molten steel decreased significantly from LF to VD process, but the molten steel increased a little after entering tundish. The size of micro inclusions in LF to VD process was concentrated in 0-5 μm, and the total number of inclusions decreased with the refining process. In LF refining process, the main inclusions were Si-Mn-Al and Si-Ca-Al-Mg type inclusions and a small amount of CaS and MnS. In the process of 
VD refining, the Si-Mn-Al inclusions were transformed to Si-Ca-Al-Mg and CaS inclusions. The main inclusions in tundish steel were MgO-Al₂O₃-CaS and CaS、MnS inclusions.The large inclusions were SiO2-Al₂O₃, CaO-MgO-Al₂O₃, MgO-Al₂O₃-SiO₂ and other composite inclusions. The main sources of the large inclusions in tundish were deoxidation reaction products, slag involved and reaction products of molten steel and furnace lining.

Key words: cold drawn tube, cleanliness, inclusions, compound inclusions, molten steel reoxidation