Structure and Metallurgical Properties Formation of Semireduced Pellets

• Published in: Steel in translation 2022-10, Vol.52 (10), p.945-955
• Main Authors: Bersenev, I. S., Vokhmyakova, I. S., Borodin, A. V., Pigarev, S. P., Sivkov, O. G., Zagainov, S. A.
• Format: Article
• Language: English
• Subjects: Additives; Blast furnace gas; Chemistry and Materials Science; Compressive strength; Disintegration; Hematite; Low temperature; Materials Science; Metallurgy; Oxidation; Pellets; Reduction (metal working); Temperature effects
• ISSN: 0967-0912; 1935-0988
• DOI: 10.3103/S0967091222100023

The structure formation of semireduced pellets and their metallurgical properties are investigated. The reduction kinetics, reactivity, and metallurgical properties of the samples are considered for three types of pellets. An increase in the temperature of the reducing gas intensifies the reduction process at the first stages of oxygen removal (in the range φ = 5–25%). Later on, the effect of temperature is leveled due to the formation of fusible compounds on the pellets surface. The strength of pellets with different reduction values depends on the initial charge composition, as well as the additional introduction of calcium-containing additives, contributes to an increase in the strength of reduced pellets. The highest values of compressive strength are found in the pellets with a reduction value of 10%. The pellets with a reduction value of 20–40%, regardless of the temperature and the pellet type, are characterized by low compressive strength, which is apparently associated with the formation of new phases and residual content of the original hematite phase. An increase in the content of metallic iron in pellets contributes to an increase in compressive strength, which is observed starting from a reduction value of 50%. Low temperature disintegration indices (LTD) of the reduced pellets have high values, with the maximum difference reaching 2.5% relative units. An increase in the initial value of samples reduction from 50 to 70% contributes to an increase in the yield of whole pellets (LTD +6.3 ) and, accordingly, a decrease in the formation of fines (LTD –3.15 ; LTD –0.5 ). The softening onset temperature of the reduced pellets is 7% higher than that of the oxidized ones, which contributes to bringing the cohesive zone in the blast furnace closer to the hearth. With an increase in the reduction value, the tendency to secondary oxidation of semireduced pellets decreases, which is possibly due to the high content of FeO in the pellets at φ r = 70%. In unfluxed pellets, the reactivity is lower than that of fluxed ones by 2.4 times at φ r = 50% and by 1.6 times at 70%.