Iron alloyed with carbon results in the substance that we know as steel. Other materials such as chromium and tungsten can also be used, but carbon is the most cost-effective. Depending on the amount mixed with the iron, carbon can affect the hardness, flexibility, and tensile strength of steel. In a modern steel making plant, the iron goes through three main processing units to reach a final state usable by other industries.
1. Produce Directly Reduced Iron (DRI)
Pure iron does not naturally occur. Iron is always in combination with either oxygen or sulfur, and these iron mineral ores first need to be purified. Direct reduction is the process of removing oxygen from iron in its solid state. This is a modern economical alternative that is more efficient and requires less heat. In a direct reduction furnace the iron is heated to only 800 to 1,050o C. Hydrogen and carbon monoxide derived from either coal or natural gas is then introduced, and this removes the oxygen from the heated ores. The DRI produced is purer that molten pig iron, which is the intermediate product in older smelting methods. The DRI is fed into an Electric Arc Furnace.
2. Produce molten steel and slag.
An Electric Arc Furnace (EAF) uses large electrodes that give out a powerful arc of electricity that can produce intense heat and melt DRI as well as metal scraps. The DRI and/or scrap metal is fed through the opened roof of the EAF. Some furnaces melt pure DRI, others melt scrap metal with some amounts of DRI for chemical balance. The electrodes are lowered into the load, gradually increasing in voltage. The melting process produces molten steel and slag. Slag floats to the surface of the molten steel and is mostly made up of oxidized metals and other impurities. A small solidified sample of the steel is taken through automatic lances that enter the EAF. This sample is tested for the right temperature and chemistry. Once this is achieved, the EAF is tilted in one direction to pour out the slag and then tilted in the opposite direction to remove the molten steel that flows out of a tap hole and onto a ladle.
3. Form the molten steel into billets.
A billet is a manufacturing term that generally refers to a cast semi-finished material. Steel producers shape and mold the molten steel into various dimensions depending on which industry they will supply. A continuous casting unit is one kind of machine that can mold molten steel into billets of any necessary shape. Cold water is used to solidify the molten steel as it runs through the rollers of the casting unit.
Steel is very much utilized today despite the presence of lightweight and cheaper materials such as aluminum or plastic. It is the major reinforcement material in buildings and other infrastructure construction. The majority of an automobile's parts are still made from steel, and other varieties such as stainless steel and flat carbon steel are used for major household or industrial appliances and tools.