Abstract: In the current study, evolution characteristics of inclusions were evaluated using SEM with automatic scanning feature in 8620RH nonquenched and tempered steel samples during the “EAF→LF→VD→CC”production process. Cleanliness of the steel in producion process were evaluated, combined with the detection of total oxygen and nitrogen of the steel. The results show that type of oxide was Al2O3-CaO-MgO, mainly consisted of Al2O3. After adding sulfur operation, content of CaO in inclusions decreased, and the amount of sulfides and oxysulfides increased obviously. Content of MgO increased gradually during the production process, and number density of inclusions with high MgO content in the end of each production steps was all higher than that in the beginning. Few large spinel inclusions were tested in steel samples. High cleanliness was achieved in 8620RH microalloyed forging steel dioxidized by aluminium，and w(T.O)was 10×10-6 in slab. Number densities of oxides in billets and rolled products are lower than 10 per mm2. Most sulfides adhere to small oxides and grow around them. Most sulfides were adhered to oxides so that it contributed to improvement of machinability, but there are still some largesize sulfides in rolling samples. Proper increase on T.O content could be adopted so that sulfides could percipitate on the tiny oxides and improvement on machinability can be achieved.

Key words: inclusions, 8620RH microalloyed forging steel, cleanliness in production process