Steel alloys are steel with small amounts added of one or more other metals. Many different steel alloys exist for a wide range of uses. Like a cook flavoring a dish with a pinch of this spice and a pinch of that, a modern metallurgist will add bits of this and that metal to basic steel in order to get just the right properties.
All steel starts as iron. Iron itself changes a culture because it is much harder than the metals that were used before it (copper, lead, bronze). Yet, iron can still be worked with a hammer. Iron atoms will slip and slide around each other when subjected to force, unless a few atoms of carbon are introduced. The carbon jams up this motion; hence, steel.
Various metals can also be added to steel to change its properties. The properties of interest are usually hardness, strength, toughness, and corrosion resistance as well as durability in extreme environments such as intense heat. For example, a support beam will need strength, a bolt toughness, and a chisel hardness.
Some of the common alloying metals are as follows:
Aluminum to give strong magnetism
Bismuth improves machinability
Chromium increases corrosion resistance
Copper increases corrosion resistance
Manganese increases hardness and reduces brittleness
Molybdenum increases toughness
Nickel increases corrosion resistance and toughness
Tungsten increases the melting point
Vanadium increases strength and toughness
For extreme-environment steels, see superalloys.
There is no perfect steel alloy. The design always involves tradeoffs, such as hardness vs. brittleness or toughness vs. ductility. Also, other solutions are at the designer's disposal. For example, a strong steel beam can be encased in a hard steel jacket to achieve both properties where they matter. For corrosion resistance, other choices include coating the steel with zinc or simply painting.
Once upon a time, steel was a concept, then a reality, and now steel is a large category of industrial metals.