PURPOSE: To practically use converter-slag as pig making-raw material and to reduce the cost in the process of manufacturing pig and steel, by the following method in which the phosphorus in moten pig enriched by utilizing converter- slag in pig making-process is removed, before the pig is charged into a converter.

CONSTITUTION: For the molten pig tapped from a blast furnace, dephosphorizing treatment is carried out before the pig is charged into a converter, thereby it is able to controll the enriched phosphorus in the molten pig corresponding to the used amount of converter-slag. In order to dephosphorize the molten pig, desiliconizing treatment is achieved before the dephosphorization of the molten pig tapped from a blast furnace. This is essential to elevate the dephosphorizing efficiency. The desiliconizing treatment may be carried out on a casting bed, in a mixer or a ladle. Then, soda ash is preferred to caustic lime for dephosphorizing the molten pig, because amount of generated slag and the loss of Fe and Mn are decreased, and additionally the high efficiency for dephosphorizing and desiliconizing is obtained. Next, the dephosphorized molten pig is charged into the converter.