PURPOSE: To make progress of decarbonization while restraining rephosphorization at tapping stage in a converter and to reduce the decarbonizing cost by tapping off molten steel having less than the specific value of C concn. at blowing-out in the converter under non-deoxidizing condition together with the specific quantity of converter slag into a ladle in refining of a low C steel.

CONSTITUTION: The molten steel having <0.15% C concn. by wt.% at blowing-out in the converter is tapped off into the ladle under non-deoxidizing condition together with 3-25kg/molten steel T of the converter slag. Then, in the case the C concn. at blowing-out in the converter is high, as this has tendency to rephosphorize, this is limited to the above value, which does not rephosphorize. Successively, at the time of lowering O concn. with deoxidation, as the reaction of C+O→CO(gas) does not progress, and deoxidation is not further executed, this is tapped off under undeoxidizing condition. Further, in accordance with increase of the flowing-out converter slag quantity, decarburizing quantity at tapping off is increased, and in the case of tapping off the slag at lower limit, the decarburization is increased by about 20% and at upper limit, this is increased by about 40% but at the time of exceeding the upper limit, the decarburization is not increased to more than 40%. Further, according to increase the flowing-out converter slag, the phosphorus concn. in the ladle is lowered, but after deoxidation, the rephosphorization is developed and in the above range of the slag quantity, the phosphorus concn. after deoxidation can be controlled to ≤90% of the phosphorus at blowing-out in the converter.