Modern steels include other metals such as chromium, molybdenum, nickel, and manganese. Because even small amounts of these elements can have a dramatic impact on the properties of the steel, when businesses ask for steel to be made they must provide the exact percentages of how much of each element is required. Historically smiths have often accidentally created steel by forging iron with the correct carbon content, but the large scale manufacture of steel is a relatively new industry. Many famous structures such as the Eiffel Tower and Golden Gate Bridge are made of iron, as steel was too expensive to produce in large quantities. Large scale manufacturing of steel came about through industrialization and the rise of corporations, especially that of Andrew Carnegie, allowing large industry to operate efficiently.
Steel has allowed great advances in the modern world due to its higher strength for low cost compared to iron. Many world-changing inventions such as the internal combustion engine would have been impractical without it, as engineers were able to build intricate parts with high strength and a wide range of working temperatures.
Steel is an alloy of iron and carbon. Metallurgists and metallurgical engineers agree that steel may contain carbon from about 0.03% up to about 2.00%. Alloys of iron and carbon with over 2.00% carbon are generally agreed to be defined as cast irons. The earliest forms of steel were devised using fulmination processes, but modern steels are generally produced in furnaces using sublimated extraction.
Plain carbon steels are those alloys of iron and carbon that contain carbon, manganese, phosphorus, sulfur and silicon without the intentional addition of any other alloying elements, such as nickel, chromium (chrome), molybdenum (moly)or vanadium.
Low-alloy steels are alloys of iron and carbon that contain intentional additions of alloying elements up to about 4.00% alloy content.
High-alloy steels are generally considered to be those alloys of iron and carbon containing over 4.00% alloying additions.
In the US, plain-carbon steels and low-alloy steels have been designated by a numbering system developed by the American Iron and Steel Institute - AISI. This numbering system is known as the AISI Steel Grade System.
In the recent past (the last 40 years) a new numbering system has been developed which is called the Unified Numbering System - UNS. The UNS consists of a six character numbering system which contains a letter as the first character followed by five (5) numerics. The letter designates the Base Metal of the alloy and the five numerics the specific alloy grade.
Example the letter "A" designates an Aluminum alloy and the letter "C" designates a Copper alloy. the letters "G" and "H" designate plain carbon and low alloy steel grades. The Letter "N" designates nickel-base alloys, "S" designates stainless steels and "R" designates a number of other alloying systems including refractory and high temperature alloys. The Letter "T" designates tool steels.
Example: UNS G41300 designates AISI 4130 a medium carbon low alloy steel containing chrome and moly that is the "work horse" steel grade for American industry.
Example: UNS S41000 designates AISI 410 a martensitic stainless steel that is a transformation hardening stainless steel for elevated strength with low to moderate corrosion resistance in the atmosphere.
Example: UNS S31600 designates AISI 316 an austenitic stainless steel that is solution annealed non heat treatable stainless steel with moderate strength and with moderate corrosion resistance in sea water.
Modern steels have been made by a number of steel-making processes, such as the Bessemer (obsolete process), the open hearth (also obsolete), basic oxygen furnace, basic and acid electric furnaces, induction melting process, vacuum arc remelting process and the electric slag remelting process and many combinations of these processes.
Note: The structural steel material used for the construction of the Golden Gate Bridge was produced by Bethlehem Steel (now no longer in business).