Abstract: In order to achieve precise control of oxygen content in LF refining process of 1215MS steel，thermodynamic calculations were carried out on the oxygen balance reaction between slag and steel．On the basis of combining production practice in a certain factory，the refining process control techniques such as weak deoxidation of molten steel，the refining slag composition，and modified inclusions in calcium treatment were analyzed and studied．Corresponding improvement measures have been proposed and good results have been achieved．The results showed that by retaining 0.050%-0.060% carbon mass fraction at the end of the converter process，step-by-step weak deoxidation was adopted during the tapping process，controlling the aluminum mass fraction in molten steel to be 0.002 5%-0.004 5%，and the oxygen mass fraction to be 0.003 0%-0.005 0%．LF refining process adjusted the composition of refining slag w(FeO+MnO) to be 5%-7%，w (Al₂O₃) to be 18%-20%，basicity to be 2.0-2.5，Mannesmann index to be 0.10-0.15，and calcium treatment controled the endpoint calcium mass fraction of molten steel to be 0.001 0%-0.002 0% at the end of refining．The mass fraction of dissolved oxygen at the refining endpoint could be precisely controlled to be 0.005 0%-0.006 0%，the total oxygen mass fraction of the casting billet could be controlled to be 0.012%-0.014%，the proportion of sulfide inclusions with a length to width ratio no more than 4  increased from 53.2% to 84.8%.

Key words: high sulfur free cutting steel, oxygen content, weak deoxygenation, composition of refined slag, sulfide modification