OCR Text |
Show 14 The heat balance of the N2 Jet Heater system is shown in Fig.26. Due to the sensible heat recovery of nitrogen gas which is exhaled out of tundish, 9 8 % of the overall thermal efficiency is achieved. As a result, the heating efficiency which is defined as "Heat transferred to tundish divided by Heat of fuel gas combustion" is almost equivalent to that of the conventional tundish burner system. 4.4.2 Sealing capability As shown in Fig.27 in which A- and B- point correspond to those in Fig.21, the oxygen content of tundish inside atmosphere can be maintained steadily low, namely around 300ppm. Conclusively, the tundish can be kept non-oxidizing atmosphere with the N2 Jet Heater. [14.5%| Fis.26 Heat balance 5000 E 4000 Q. - 3000 | 2000 § 1000 CM n o ° of N2 Jet Heater -4- I ki*jMj»JJ_LjaJU ' 1 J~i_j- j A-po int " B-point • i , , 10 20 30 40 50 60 70 80 90 Time (min) Fig.27 Change of O2 content in tundish held by N2 Jet Heater 4.5 Effects of N 2 Jet Heater on steel qualities 4.5.1 Improvement in reoxidation of molten steel The change in initial heat's metal chemistries between the end of vacuum degassing and at the timing when tundish is filled up at teeming start can indicate the oxidation magnitude of used tundish. Using the index, the changes in total oxygen content of molten steel is compared in Fig.28. In comparison with the long waiting with sealing gas, the total oxygen pick-up is reduced by 5 0 % . This vouches that the reoxidation of remains in used tundish is effectively suppressed by N 2 Jet Heater. Assuming that the remaining skull is oxidized by the oxygen component of tundish atmosphere generating a uniform scale layer, the thickness of scale film (X) is given by the following equation preferred in equation (3)) 1.0 X=a((P02)°-5 exp(-16600/T))05 x(t)0-5 (3) where, PO2 : Oxygen partial pressure in tundish (%) T : Temperature (K) t : Time (sec) a : Constant 0.8 I X CD I 0.6 I- 0.2 0.0 IE A(0)=Difference of O concentration of tundish and the previous process 777 N2 heater Non-heated Heated Fig.28 Comparison of (O) pick up in tundish |