The steel industry is comprised of Electric Arc Furnace (EAF) and integrated steel producers. There are significant differences between these steel making processes.
EAF steelmakers, often referred to as mini-mills, produce steel by melting scrap. The integrated steel mills produce steel from iron ore utilizing the blast furnace and basic oxygen furnace processes.
In the integrated process, the blast furnace is provided with energy in the form of coke which is mixed with iron ore pellets/sinter and limestone to constitute a furnace burden. Liquid iron, containing approximately 4% dissolved carbon is tapped from the bottom of the furnace into hot-metal cars and transferred to the basic oxygen furnace (BOF). The metallic charge to the BOF consists of 60-70% hot metal from the blast furnace and 30-40% steel scrap. The primary function of the BOF is to refine the liquid iron to steel by reducing the carbon to a prescribed level, generally less than 0.5%, as well as removing impurities such as silicon, sulfur, and phosphorus. This is accomplished by injecting oxygen into the bath and the addition of fluxes.
With EAF steel making, normally 100% steel scrap is charged to the furnace. In the process, oxygen is also injected and fluxes added to control carbon content and remove impurities. As more high quality steel products are being produced by EAFs, steelmakers are beginning to supplement the scrap charge with direct reduced iron (DRI) and iron carbide.
Because the EAF process is more energy efficient and less capital is required, production of steel by EAFs has increased significantly over the past quarter of a century. In 1996, approximately 39% of the total of 103 million tons of steel shipped was made in EAFs with the balance produced by the blast furnace/basic oxygen process. The total energy required to make one ton of liquid steel in an EAF is about 640 kwh or 6.4 million Btu/ton which is only a fraction of the 19m Btu/ton required using the bast furnace/basic oxygen furnace method.
After melting and refining, the liquid steel is transported to a continuous casting machine where it is cast and solidified. However, a few ignot mold shops still exist. Continuous casting machines produce semi finished sections (billets, blooms, and slabs). Ignots require an additional rolling operation to produce the semi finished shapes. Final steel products, strip, sheets, bars, rods, plates, tubes, etc. are manufactured from the semi finished sections using a variety of rolling mills and processes.