Operational features and development of a combined design for the roof of a 100-ton electric arc furnace

Abstract

The roof resistance of an electric arc furnace depends on the refractories used, the design features of the roof, and its operational conditions [I].One of the features for the roofs of high-tonnage furnaces consists of the marked cyclic changes in the temperature of the subroof space which occur in the campaign as a result of the periodic removal of the roof from the furnace during fettling, charging, and topping up.Timings of the roof in the removed conditions were carried out over 28 ~ears on 100-ton furnaces at the ChelyabinskMetallurgical Factory (ChMZ). It was established that in the fettling periods and the following charging periods the roof is located in the removed state for 9-39 rain, and in the topping up period from 3 to 9 min. The average removal time was, respectively, 22 and 5 min ( Table i).Analysis of the timings brought to light a number of features of the overall duration of the roof's dwell away from the furnace.As a rule, fettling firsttakes place, then charging, and then dressing in one or two stages.Of the total number of recorded dressings, in one stage there were 39% of cases, in two stages 43%, and none at all in 18% cases.Fettling commenced 1-5 min after removal of the roof and lasted from 2 to 17 min when working in one stage, and from 4 to 27 rain when working in two stages.The time gap between fettling and charging was up to 15 min. Charging was accomplished in 1-5 min.In order to determine the depth of cooling of the roof during its removal from the furnace in the period of fettling and charging measurements were made of the temperature of the working surface of the roof lining.Measurements were made by the contact method using Chromel--Alumel thermocouples in a section located at an equal distance from the center of the roof and roof ring.Results are shown in Fig. i.It is established that immediately after removing the roof the lining is rapidly cooled. During the first 10-13 min the temperature of the working surface of the roof is reduced to 640-700~ Then the cooling rate is reduced. About 25 min after removing the roof its working-surface temperature is 530~The final temperature to which the working surface may be cooled is determined by the length of time for which it stays in the removed state.Reducing the roof's removal time from the furnace, which can be accomplished as a result of undelayed fettling, reducing the duration of the dressings and the gap between them, will reduce the depth of cooling in the roof lining, on account of which there will be a reduction in the magnitude of the stresses acting in the lining, and, hence, an improvement in the roof resistance. Change in temperature ol working surface of removed roof in an electric arc furnace.East Institute of Refractories.Chelyabinsk Metallurgical Factory.

Experience with the use of mullite-corundum refractories in the roofs of electric-steel furnaces

“…[2,3]. In foreign plants, for electric furnace roofs extensive use is being made of high-alumina refractories and materials containing from 65 to 97% A1203.…”