Decarburization Efficiency in EAF With Hot Metal ... - Steel Library

More documents

Recommendations

Info

Comparison of the performance results of the Shagang and Tianjin furnaces clearly indicates that the hot metal charging method has rather significant influence. Slag door charging is a less favorable solution, mostly due to the uneven mixing pattern of the liquid bath in the furnace, reduced reaction area and lower efficiency of the available chemical energy utilization. For the same share of hot metal in the charge, the specific energy consumption is about 20 kWh/ton higher, with a 2-minute power-on time increase and consequent loss of productivity. Figure 6 Figure 5 CONSO injectors layout and melting zone distribution (Tianjin EAF No. 2). CONSO injector profiles during melting (a) and superheating (b) phases (40% hot metal). Evaluation of Decarburization Efficiency The furnaces presented in this paper were originally designed for a reference charge configuration with a maximum 40% hot metal. The average decarburization rates of 0.12% C/minute were calculated to be achieved with average specific oxygen injection rates of 300 Nm 3 /hour/m 2 of liquid bath surface, corresponding to four CONSO lances operated at 2,000 Nm 3 /hour each. Considering that with 40% hot metal, the carbon content in the liquid bath is about 2% (1.7% C from hot metal and 0.3% C from scrap), the required continuous (a) (b) 66 ✦ Iron & Steel Technology A Publication of the Association for Iron & Steel Technology
Figure 7 In-blow sample carbon content (250 heats from commissioning phase) oxygen lancing time with the earlier defined decarburization rate should be about 16 minutes. Since the calculated average power-on time was 25 minutes, about 7 minutes were reserved for burner operation and oxygen injection at subsonic flowrates. According to melting process simulations, all heats could be completed without extra power-off time for additional decarburization. Decarburization rates achieved in practice proved to be better than the earlier assumed values. Figure 6 shows the results of the in-blow carbon sampling from the early days of the Shagang EAF operation. In some cases, the results indicate slightly higher carbon values, which can be explained by inconsistency in the carbon and silicon contents in hot metal, which deviated the in/ out oxygen balance. Nevertheless, thanks to a correctly defined moment of sampling, sufficient time was always reserved for eventual oxygen blow pattern update with oxygen flowrate increase, if required. However, in most of the cases, the final phase of superheating was done with a reduced oxygen injection rate to avoid too-low carbon at tapping. The CONSO injection system operation fully confirmed expectations regarding high decarburization efficiency. It was also concluded that the capacity of the system can be further increased using higher supersonic oxygen flowrates. Facing shortage and inconsistency of scrap supplies, the EAF users were forced to increase the share of hot metal in the charge — initially to 50% and later even to 70%. New melting programs with higher specific oxygen flowrates were established for such occasions. Precise evaluation of the actual decarburization rates is an extremely difficult task for several reasons. Among them, the experienced variations in hot metal parameters, i.e., carbon, silicon and manganese contents, as well as its temperature at the moment of pouring into the EAF, play a key role. Diversified hot metal deliveries from different blast furnaces additionally increased the deviation range of the starting conditions. In view of that, the following evaluation procedure was adopted: • Starting carbon content was calculated based on the composition of furnace charge components. Figure 8 Decarburization results. • Start of decarburization time was identified as the moment when intense flames were seen (spontaneous carbon “ignition”). • Decarburization time was counted until the time of the first sampling, when end carbon value was analyzed. • Actual decarburization rates were calculated from carbon content difference and recorded time. Figure 8 shows the results from about 1,300 heats produced with variable hot metal share in the charge. Available results demonstrate the fact that — except for earlier identified factors — the actual decarburization rates also depend on the initial carbon content in the liquid bath. Some analogies to the decarburization reaction in VOD or AOD processes can be noted at this point. Interdependence between carbon content and decarburization intensity is a precious observation which can be used for setting up the process conditions for a high share of hot metal in the furnace charge. It can be assumed that the decarburization rate can be very high, providing that sufficient oxygen injection density is ensured. For higher carbon contents in the charge, the deviation of the actual decarburization rates is limited. This can be explained easily by the fact that available oxygen Figure 9 Decarburization rates by sample-to-sample method. AIST.org January 2012 ✦ 67
Page 1 and 2: Decarburization Efficiency in EAF W
Page 3 and 4: Table 2 Data for the Shagang and Ti
Page 5: more intensively used as full-power
Page 9: Evaluation of the EAF productivity

Decarburization Efficiency in EAF With Hot Metal ... - Steel Library

Create successful ePaper yourself

Delete template?

Save as template?