PURPOSE: To obtain a heat insulating material for steel making having an extremely outstanding heat insulating characteristic and handling property without requiring calcination by composing this heat insulating material of the reduced slag generated in steel making stages, a sp. gr. control agent, a binder and a collapsing agent.

CONSTITUTION: This heat insulating material for steel making is prepd. by contg. the reduced slag which is generated in the steel making stages and is heretofore disposed by discarding, the sp. gr. control agent for controlling the bulk sp. gr. of the heat insulating material, the binder for physically binding both and the collapsing agent for accelerating cleavage and powdering of the heat insulating material. The heat insulating material described above does not degrade working environment even if a pretreatment, such as granulation, is not executed. The reduced slag described above consists mainly of the reduced slag generated by floating in the furnace during the course of refining of steel making and is preferably used at 25 to 75wt.% of the heat insulating material. Active carbon, coal-component and calcined matter of paper sludge, etc., are used as the sp. gr. control agent. The entire surface of the molten steel is covered with the heat insulating material by controlling its bulk sp. gr. to ≤0.8. The binder is adequately water glass, ethyl silicate, etc., and the collapsing agent is talc, vermiculite, etc.