Steel two-slag purifying method for ultra-cleaning welding

The invention relates to a steel two-slag purifying method for ultra-cleaning welding, including the following steps successively: 1) melted iron and waste steel are added in an initial electric purifying furnace, and oxygen blowing smelting is carried out; 2) slag begins to be changed when the temperature of molten steel is heated till 1500 DEG C, P removing effect is improved, oxygen blowing heating is carried out, and steel comes out when the temperature of molten steel reaches to 1600 to 1620 DEG C, the content of C element is 0.02% to 0.05% and the content of P element is below 0.003%; 3)alloy deoxidant is added in ladle to carry out LF refining, and active lime is used as main slagging material to carry out initial S removing; 4) after 20 to 30 minutes of refining, the slag begins to be changed when the content of S is reduced to below 0.015%, the secondary slag is refined and active lime matched with calcium carbide is used as main slagging material, Ca lines are added to strengthen S removing effect, so that the content of sulphur in steel is reduced to below 0.003%.