Abstract: As for the IF steel making process of BOF+LF+RH+CC,the free oxygen content drops to a minimum level after deoxidization and LF refining,and right before RH decarburization.The RH decarburization reaction of steel with extreme low oxygen can be divided into four stages in terms of the apparent decarburization rate constant.Differed from traditional three-stage RH treatment,a transitional stage with medial decarburization occurs between the low and high decarburization stages.Based on the regular solution model,a decarburization model was developed to forecast oxygen content in the steel and FeO content in the top slag.It is found that there is no oxygen transferring between molten steel with extreme low oxygen and the top slag before RH blowing oxygen.After RH blowing oxygen,the oxygen content increases linearly,then oxygen transfers from the molten steel to the top slag which has a lower oxygen potential,and the FeO content increases consequently.Though the FeO content at the end of RH refining rises higher than the initial content,it is still fairly low enough to avoid reoxidation by the top slag to molten steel.