Abstract: In this paper , the change of inclusions in 40Cr cold heading steel during LF refining was systematically studied by a variety of analysis methods , it was found that the area of inclusions and total O in steel changed consistently during LF refining , inclusion’s area ratio in steel was reduced from 1.41% to 0.45% after LF refining ; And LF refining could effectively remove the large inclusions whose equivalent diameter ECD>10μm , most of the equivalent diameter of inclusions are fall in the range of 2 ~ 5μm . Moreover, when high basicity and highly reductive slag was used in LF refining, combining with calcium treatment technology, it could effectively transfer Al₂O₃ inclusions to MgO-Al₂O₃ system、CaO-Al₂O₃ system and CaO-MgO-Al₂O₃ system inclusions, therefore, the casting performance and cold-flow properties of steel were improved; But calcium treatment process is necessary to be controlled more reasonably in order to reduce the liquid steel slag and secondary oxidation and improve cleanliness of molten steel.

Key words: High strength cold heading steel, LF refining, Calcium treatment, Inclusions, Change behavior