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Show ead goes slightly up during burning. The heat efficiency for molten steel is approximately 52 % and heating speed is from 0.4 to 1.6 k/min. (Fig.8) Effective heat efficiency ie.E is defined as equation(l~ Hat + Href E = (1) H in Effective heat efficiency is higher with an increase of both lance height and heat input, (Fig. 9) because an incre~sed heat input decreases the relative value of heat loss to lance cooling water and finally reaches the efficiency of only heat loss to exhaust gas. Fig. 10 shows the heat balance for burning in RH vessel in a vacuu~ Maximum heat efficiency for molten steel is obtained at a certain lance height (4 meters from molten steel surface at Hirohata's RH). 2)Effects on steel Quality Fuel burning during treatment might cause contamination of steel in chemical compositions of [C] and [H] due to CO2 and H2 in exhaust gas. Laboratory scale experiments are done to investigate [C] pick-up of molten steel in a vacuum melting pot. Gas blowing index ie. V, is Vo V, = (2) (0.16X (X/d)-I. 5) [C] pick-up speed increases with increase of V, which indicates gas velocity at the surface of molten steel. (Fig.ID On the assumption that transport velocity of [C] per unit reaction area may be constant under the same condition of gas blowing, [C] pick-up speed in production scale is estimated to be 0.01 ppm/min at the gas blowing index V I =1 to 20 Nm/sec Fig. 12 shows the actual [C] and [H] pick-up measured, almost negligible effects be i ng conf i rmed. 5. Operational results of RH-MFB 5-1 Operating methods The oxygen blowing and burner heating methods in the RH-MFB system are outlined in Fig. 13. The basic operating methods are as follows: (l~perating method during RH treatment For extra-low carbon steel, first the oxygen top blowing is performed to promote decarburization by controlling the free oxygen in molten steel, second. aluminum reheating to control the temperature of molten steel and third, burner heating during alloying treatment to also control the temperature. For deoxidized steel, aluminum reheating by oxygen top blowing and burner heat ing during alloying treatment only to control the temperature of molten steel are performed. 4 |