Carbon Concentration and the Use of Direct-Reduced Iron in Ironmaking and Steelmaking

Direct reduced iron (DRI), as a primary feedstock in ironmaking and steelmaking, must maintain strength and integrity during processing, transportation, and use to avoid any operational issue and loss of materials. Mechanical properties and structural changes of DRI during production should be understood. The first part of this study was designed to find associations among physical properties, structural changes, and inner phases of direct reduced iron developed during reduction and carburization.

The effects of gas composition and the extent of reduction on the compressive strength of DRI were investigated. Gas mixtures containing different proportions of hydrogen, carbon monoxide, water vapor, carbon dioxide, and methane were tested. The structure of laboratory and industrial DRI was examined and correlated with gas composition and mechanical properties. After reduction, structural changes accompanying reduction and carburization were investigated with scanning electron microscopy, swelling was measured, and phases were quantified by X-ray diffraction. The major strength loss occurred in the first reduction step, when hematite grains transformed into porous magnetite. Carburization after reduction had a minor effect on strength, except when extensive precipitation of elemental carbon caused lower strength, similar to the metal dusting corrosion mechanism. Higher strengths were obtained when water vapor was added to the reducing gas.

With the growing interest and production today, a better understanding of DRI melting behavior in continuous and batch processes is required. DRI carbon content and its forms such as cementite, graphite, or amorphous carbon can have an impact on DRI melting temperature and melting behavior. In the second part of this study, the effect of carbon bonding and carbon concentration on DRI melting behavior was investigated. This study will help steelmakers to select the optimal DRI type and composition for their steelmaking operation.

The concentration and chemical bonding state of carbon in DRI might affect DRI melting temperature and rate. The effects of carbon bonding state and concentration were evaluated with high-temperature confocal microscopy, differential scanning calorimetry, and by monitoring the carbon monoxide generation rate from reactions between DRI pellets and a laboratory slag-steel melt. In industrial steelmaking, DRI melting is likely controlled by heat transfer; the concentration and bonding state of carbon play secondary roles.