PURPOSE: To permit preliminary refining without decreasing the contents of C and Mn in molten iron in the stage of dephosphorizing, desulfurizing and preliminary refining of molten metal by increasing the oxygen in a preliminary refining agent or the oxygen component in the carrier gas for the preliminary refining agent according to the decrease in the content of phosphor in the molten iron.

CONSTITUTION: Molten ion of <0.25% Si is dephosphorized and desulfurized by preliminary refining. In this case, ore powder of oxides of Fe and Mn as a dephosphorizing agent is blown into the molten iron by gaseous oxygen or a carrier gas consisting essentially of oxygen, whereby the phosphorus is oxidized and slagged off. Carbonates of alkali metals such as Na2CO3, or oxides and halides of alkaline earth metals such as CaO, CaF2, CaCl2 are used as a desulfurizing agent. The amt. of the desulfurizing aget to be used is large for the beginning and when the P in the molten iron decreases to 0.03W0.05%, the content of ores of Fe and Mn as the dephosphorizing agent or the content of O2 in the carrier gas is increased to increase oxidizing powder, whereby the molten iron is dephosphorized effectively.