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Modern steels are made with varying combinations of alloy metals to fulfill many purposes.  Carbon steel, composed simply of iron and carbon, accounts for 90% of steel production.  High strength low alloy steel has small additions (usually < 2% by weight) of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase.  Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections. Stainless steels and surgical stainless steels contain a minimum of 10% chromium, often combined with nickel, to resist corrosion (rust).  Some stainless steels are magnetic, while others are nonmagnetic.

Some more modern steels include tool steels, which are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening.  This also allows the use of precipitation hardening and improves the alloy's temperature resistance.  Tool steel is generally used in axes, drills, and other devices that need a sharp, long-lasting cutting edge.  Other special-purpose alloys include weathering steels such as Cor-ten, which weather by acquiring a stable, rusted surface, and so can be used un-painted.

Many other high-strength alloys exist, such as dual-phase steel, which is heat treated to contain both a ferrite and martensic microstructure for extra strength.  Transformation Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize amounts of austentite at room temperature in normally austentite-free low-alloy ferritic steels.  By applying strain to the metal, the austentite undergoes a phase transition to martensite without the addition of heat.  Maraging steel is alloyed with nickel and other elements, but unlike most steel contains almost no carbon at all.  This creates a very strong but still malleable metal.  Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy.  Eglin Steel uses a combination of over a dozen different elements in varying amounts to create a relatively low-cost metal for use in bunker buster weapons.  Hadfield steel (after Sir Robert Hadfield) or manganese steel contains 12–14% manganese which when abraded forms an incredibly hard skin which resists wearing. Examples include tank tracks, bulldozer blade edges and cutting blades on the jaws of life.  A special class of high-strength alloy, the superalloys, retain their mechanical properties at extreme temperatures while minimizing creep.  These are commonly used in applications such as jet engine blades where temperatures can reach levels at which most other alloys would become weak.

Most of the more commonly used steel alloys are categorized into various grades by standards organizations.  For example, the American Iron and Steel Institute has a series of grades defining many types of steel ranging from standard carbon steel to HSLA and stainless steel.  The American Society for Testing and Materials has a separate set of standards, which define alloys such as A36 steel, the most commonly used structural steel in the United States.

Though not an alloy, galvanized steel is a commonly used variety of steel which has been hot-dipped or electroplated in zinc for protection against rust.