Production practices have clearly demonstrated that the incorporation of Direct Reduced Iron (DRI) has a profound influence on the productivity and yield of electric arc furnace (EAF) steelmaking. In recent years, China has witnessed a steady increase in the number of newly - built and operational large - scale EAF plants.
However, due to a variety of factors, the quality of scrap steel entering these plants is extremely poor, with densities ranging from 0.3 to 0.7 t/m³. Many EAFs are forced to add 3 - 4 times the amount of scrap steel to produce a single furnace of steel. The use of DRI can significantly reduce the number of feedings, thereby shortening the smelting cycle. Continuous addition of 20 - 50% DRI can substantially enhance productivity.
Moreover, with the implementation of oxy - fuel smelting, foamed slag, and scrap preheating technologies, as long as DRI replaces low - density scrap, the productivity of EAFs will increase. The yield of molten steel is closely related to the metallization rate, gangue content, and carbon content of DRI. To achieve a high yield, it is essential to add DRI with a higher metallization rate to the EAF or introduce a recarburizer to promote iron reduction.
The properties and quantity of slag also play a role in the yield of molten steel. Under the same alkalinity conditions, foamed slag can reduce the amount of slag, thereby improving the yield.
The carbon content of the electrodes consumed when using DRI is generally low. After adding DRI to the EAF, an appropriate amount of recarburizer is added to create a reducing atmosphere in the furnace, which reduces electrode oxidation and thus decreases electrode consumption. However, the carbon content in the molten steel is higher. At high temperatures, EAFs often employ foamed slag technology for submerged arc operation. At this time, the increased arc concentration raises the likelihood of electrode fracture. In general, electrode consumption does not increase due to the use of DRI.
When DRI is added in batches without changing the original feeding method, there is no increase in refractory material consumption. During continuous feeding, "splashing" occurs, opening up the slag surface and exposing the arc, which leads to a slight increase in refractory material consumption. After applying DRI, the FeO content in the slag is higher, and the C - O reaction time is longer, potentially increasing the chemical attack on refractory materials. However, through the foam slag process and adjustments to other parameters, refractory material consumption can be maintained at the original level.
The use of DRI increases the acid gangue content. To maintain the original slag alkalinity, flux consumption will obviously rise. Studies have shown that for every 1% increase in DRI, flux consumption increases by 1 kg/t. However, when DRI is used as a raw material, the content of [P] and [S] in the molten steel is low, and the slag basicity does not need to be excessively high, so flux consumption does not necessarily increase.
The addition of DRI leads to an increase in the energy consumption of EAF steelmaking, mainly for the following reasons:
The melting of DRI consumes energy. The lower the metallization rate of DRI, the higher the FeO content. The reduction reaction of FeO during EAF steelmaking is an endothermic reaction. At steelmaking temperatures, reducing 1 t of FeO requires approximately 800 kWh of electricity.
The gangue content in DRI, especially SiO₂, has a significant impact on energy consumption. The higher the SiO₂ content, the higher the power consumption. To maintain slag alkalinity, as the SiO₂ content increases, the amount of added quicklime inevitably rises, leading to an increase in slag volume. Melting 1 t of slag requires about 530 kWh of electricity, and both SiO₂ and quicklime need to be melted, consuming additional energy.
DRI with a high carbon content also affects power consumption. Since the reaction of [C] + [O] → CO in the molten pool is an exothermic reaction, if an appropriate amount of oxygen is blown, every additional 1 Nm³ of oxygen will reduce power consumption by 2 - 4 kWh.
When the continuous DRI addition method is adopted and the power supply is matched with the DRI addition rate (28 - 38 kg/MW·min for cold charging and 50 kg/MW·min for hot charging), the smelting time can be greatly shortened, and the EAF can operate at maximum input power, which is beneficial for increasing output. However, when DRI is added in batches, improper feeding (such as DRI being too concentrated or close to the furnace wall) can cause DRI to accumulate or stick to the furnace wall, significantly extending the melting time and increasing power consumption.
The charge temperature of DRI has a greater impact on power consumption and a lesser impact on the smelting cycle. When using fully cold - loaded sponge iron, power consumption will be 100 - 150 kWh/t higher than that of all - scrap steel smelting. If fully hot - loaded, power consumption is equivalent to that of scrap steel.
To reduce the energy consumption of EAF steelmaking when using DRI, various steel mills have adopted measures such as preheating the added DRI, while preventing secondary oxidation. In general, to minimize power consumption, it is advisable to use DRI with a high metallization rate and low SiO₂ content, appropriately increase carbon during the smelting process, and adopt hot and continuous charging systems.
Currently, DRI is mainly used in EAF applications at home and abroad to smelt high - quality products, such as oil casings and drill pipes in the petroleum industry; deep - drawn automotive sheets in the mechanical industry; special - purpose steel wires and steels like spring steel, bearing steel, steam turbine generator rotors, gun barrels, and steel materials for the aviation, aerospace, and atomic energy industries.
Since DRI does not contain residual elements, steel without residual elements can be directly produced, and the number of inclusions in the steel is significantly reduced. This improves the hot - rolling and cold - rolling properties of the steel, especially its tensile properties. The use of DRI can also significantly reduce the sulfur content in steel and control the shape of steel sulfur inclusions, thereby enhancing steel quality and changing its expansion and torsion resistance.
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