Basic steel refers to steel produced in a furnace lined with a basic, rather than acidic, substance. The vast majority of steel produced in industrial societies is made in this way. Insulating the furnace structure from the furnace heat is a central challenge in steel production. Removing impurities from iron mixtures is another challenge in producing steel. Since the 1950s, modern methods of insulating furnace walls with low-pH, or low-acid, substances have been used.
Steel is primarily iron, but contains a small amount of other elements in the mixture. This is desired because it can improve the material properties of the substance, namely the hardness. Carbon is the most common element added to steel, but manganese, chromium, and tungsten are also used. These additions to the base iron are called alloying materials. Steel is much stronger than pure aluminum or iron by themselves, but it must be manufactured in particular ways.
During the Second Industrial Revolution, the Bessemer process for mass-producing steel was discovered and broadly applied. The chief innovation in the process was the ability to remove impurities from molten iron by blowing air through it. Excess silicon, manganese, and carbon are oxidized by the air and can then escape from the substance.
To keep the furnace structure from melting during steelmaking, which can involve very high temperatures, the container is lined with a refractory material. Refractories are substances that retain their physical properties at temperatures higher than 1,000 degrees Fahrenheit (537.77 degrees Celsius). In basic steel production, refractories with a low pH are used. In the Bessemer process, the mineral dolomite was a common low-pH refractory used to line the container when making basic steel.
In 1952, a new steelmaking procedure known as the Linz-Donawitz, or LD, process was developed. Rather than using the Bessemer technique of blowing air through the molten iron, pure oxygen is used instead. Increased oxidation of the iron allows impurities to be removed more effectively. Modern furnaces can convert 350 tons of iron into steel in less than 40 minutes. The LD process of making basic steel also produces less air pollution than the Bessemer process.
Basic refractories allow the removal of sulfur and phosphorous impurities more effectively than acidic refractories. Once oxygen is blown through the iron, impurities either leave as a gas or form a slag that floats to the surface. Oxidized carbon escapes as either carbon monoxide or carbon dioxide. Other impurities form new compounds that separate from the mixture, leaving behind basic steel.