Abstract: Combining theory and produce practice in Qian’an Steel and basing on the principle of RH decarburization and production data, the decarburization process was divided into three stages：rapid stage, slow stage and sluggish stage. The influence of vacuum pressure, lift gas flow rate, opportunity of oxygen blowing and scrape or alloy mixing， the immersion depth of snorkel，the stick slag on hot offtake on decarburization were researched. The importance of vacuum equipment accuracy control was emphasized. It is concluded that the forward vacuum process can increasing decarburization rate, the lowhigh control mode of lift gas flow rate are suitable，and exhausting rate in tube can be increased by cleaning slag in hot offtake regularly. After process optimization，the success rate of extralow carbon was increased from 50 % to more than 90 % and average carbon mass fraction in steel at the end of RH treatment was controlled below 0.001%.

Key words: RH vacuum refining, extra-low carbon steel, decarburization