From the many results of reports, (I)-(IV), the mechanism of decarbonization was considered, firstly in a high-frequency induction furnace in which slag did not exist and secondly in an open hearth furnace in which slag existed. The following results were obtained:
(1) In a high-frequency induction furnace in which slag did not exist, the inclination of moving velocity of molten steel by stir under free surface was small, and the boundary layer could be neglected. But there existed thin layer in which C was gradually decreased by reaction of oxygen during a flow along a free surface.
(2) Above the free surface, there existed Fe-O reaction phase. This phase was a thin layer in which existed (a) vapour of iron (this vapour pressure (1mmHg, 1600°C) decreased gradually as becoming more distant from the free surface.), (b) FeO molecules and FeO drops formed by association of FeO molecules, (e) oxygen moleeules, partial pressure of which was lower than above, and (d) CO and CO2 molecules which were reaction products of C and gaseous oxygen; and this phase showed resistance to encroach of oxygen to molten steel.
(3) In a high-frequency induction furnace in which slag did not exist, the decarbonization occurred mainly at the above-stated free surface, through which oxygen from gas phas and C in molten steel reacted, and reaction product, CO escaped immediately to gas phase. And so, the rate of decarbonization was controlled by arriving velocity of gaseoup oxygen when arriving velocity of C was higher and byarriving velocity of Cwhenarriving velocity of C was lower.
(5) In an open hearth furnace, oxy supplied through slag, and decarbonization. proceeded only by formation and escape of Co bubbles. Accordingly, in a high-frequency induction furnace, C-O relation was along Vacher & Hamilton's curve, but on the contrary, C-O relation was upper than Vacher & Hamilton's curve in the open hearth furnace.
(6) In the open hearth furnace, the tate determined steps were (a) movement of oxygen through the boundary layer between slag and molten steel, and (b) diffusion and escape of CO through the boundary layer between molten steel and gas phase of CO bubbles or crevices in the hearth.