OCR Text |
Show 15 The results of calculation with actually obtained oxygen content data in the two types of operation, long time waiting with sealing gas and holding with N2 Jet Heater, are shown in Fig.29, normalizing with a hypothesis that 1.0 of scale film is generated by 1.0% of oxygen partial pressure in 8 hours. The amount of scale in N2 Jet Heater operation is almost a half of that of conventional waiting operation, which supports the effects of N2 Jet Heater mentioned above. Fig.30 shows the changes of AI2O3 content in mold flux from the cast start. In the conventional waiting mode. AI2O3 in mold flux is quickly increased from the beginning reaching 13% at 40 meters of casting length. On the contrary, in the operation with used tundish held by N2 Jet Heater, AI2O3 in mold flux is saturated up to only 7% which can be deemed as the usual behavior. - Non-heated (O2 content: 1.00%] #- N2 heater (O2 content: 0.05%) 4 6 Waiting time (hr) Fig.29 Comparison of the scale formation speed 6 4 2 t 0 sjon-heatmg^, N2 heater j. (Main powder; Al2C>3=5.5%]) (Frontpowder ;Ai20-jt=3.7%) 0 5 10 15 20 25 30 35 40 45 50 Casting length (m) Fig.30 Change of AI2O3 content in mold flux'at initial stage of casting 4.5.2 Improvement in slab and final product qualities Fig.31 shows the surface quality of the first and second bottom ULCS (Ultra Low Carbon Steel) slabs with an index of defect number per unit surface area. In the casting campaign with used tundish held by N2 Jet Heater, the surface defects on the first and second slab are reduced, in comparison with the conventional operation, to 60% and 15%, respectively. Such a significant improvement enables to supply these slabs as materials for automobile outer panel sheets without surface conditioning. A comparison in sliver defects occurrence in hot rolled sheets is shown in Fig.32, focusing on the first and second bottom slabs of initial heat. In comparison with two conventional tundish waiting modes, the hot rolled products quality is remarkably improved with using the tundish held by the N2 Jet Heater. 1.0 0.8 0.6 0.4 0.2 0.0 \y y y y \ x x \y y y / -I X S X \y y y y X X X \y y y y •J \ \ s ' \y y y y x x x \y y y y •; \ x x ' \y y y y \ x x x ,y y y y - X X X • y y y y | X X X y y y y N2 heater (\lon- N2 heated X X X X y y / . s x x x x x x x y y y . s x x x ^ x x x (first slab) rtoh-' heated (second slab) Fig.31 Comparison of slab surface quality N2 Heater Non-heated Burner-heated Fig.32 Comparison of hot-coil surface quality (first and second slab) |