On the basis of chemical composition, steels can be grouped into three major classes: CARBON STEELS, LOW-ALLOY STEELS, and HIGH-ALLOY STEELS.
All steels contain a small amount of incidental elements left over from steelmaking. These include manganese, silicon, or aluminium from the deoxidation process conducted in the ladle, as well as phosphorus and sulphur picked up from ore and fuel in the blast furnace.
Copper and other metals, called residuals, are introduced by scrap used in the steelmaking furnace. Because all these elements together normally constitute less than 1% of the steel, they are not considered alloys.
Carbon steels are by far the most produced and used, accounting for about 90% percent of the world’s steel production.
Carbon steel is often simply a steel alloy containing anything from 0.12% to 2% carbon.
The various uses of carbon steel depend on the amount of carbon added to the alloy; for instance, carbon steel with the lowest amount of carbon is called wrought iron and is used for things like fencing.
MEDIUM-CARBON STEEL, also called mild steel, is the type of steel that is used for structural purposes in buildings and bridges.
HIGH-CARBON STEEL is used for springs and steel wires。
ULTRA-HIGH-CARBON STEEL has approximately 1.25–2.0% carbon content. Steels that can be tempered to great hardness. This grade of steel could be used for hard steel products, such as truck springs, metal cutting tools and other special purposes like (non-industrial-purpose) knives, axles or punches.
Ultra high carbon steel is brittle and very hard, and can’t be cold-worked. It’s used to make extremely hard components like blades, cutting tools and large machine parts, hot water radiators, industrial castings and metal lamp posts.
The more carbon there is in steel, the harder and stronger it gets when heat treated. But it also becomes less ductile, which means it loses strength when deformed and isn’t as malleable. With or without heat treatment, a higher carbon content in steel makes the metal less weldable, and the more carbon there is, the lower the melting point.
OTHER THAN CARBON STEEL, THERE ARE THE FOLLOWING TWO CATEGORIES OF STEEL BY CHEMICAL COMPOSITION:
LOW-ALLOY STEELS have up to 8% alloying elements. Applications for low-alloy steels range from military vehicles, earthmoving and construction equipment, and ships to the cross-country pipelines, pressure vessels and piping, oil drilling platforms, and structural steel.
HIGH-ALLOY STEELS are defined by a high percentage of alloying elements. Stainless steel is high-alloy steel which contains at least 12 percent chromium.
High alloy steels are expensive to manufacture, and shaping such steels is more difficult than with ordinary steels. They are also more difficult to shape with machine tools, and harder to weld.
But depending on which particular combination of alloys is present, a high alloy steel may be extremely hard, making it ideal for use where it is subject to wear, such as the cleats and pads of a bulldozer. Or it might be extremely resistant to shock loads, meaning it’s well suited to being used where the steel is hit again and again with a sudden hard blow, as for instance in machinery that is used to crush stone into gravel.