Structures made chiefly or largely from the material known as steel might just be humankind's most prominent additions to Earth's landscape.

If all life on Earth were teleported elsewhere, and a group of aliens happened to investigate, the most durable and imposing objects they would find that had clearly not arisen from natural geological processes would contain steel: skyscrapers, bridges, heavy machinery and essentially anything required to withstand strong forces over time.

You perhaps have some knowledge of where steel "comes from" and what it "is." If nothing else, you certainly know what it generally looks, feels and perhaps even sounds like in certain instances.

If you think of steel as a metal, that's natural, but steel is in fact classified as an alloy or a blend of different metals. In this case, almost all of the primary metal is iron no matter the specific recipe, but as you'll see, even small amounts of carbon can change steel's properties significantly.

Prepare to learn a great deal about what can rightfully be called the most important material in the history of construction and engineering,

As you no doubt know from having seen, heard and been in contact with your share of the stuff, steel is known foremost for its durability, hardness and toughness. In some cases, it is renowned for its shininess, too.

What these qualities translate to in quantifiable physical terms is a very high melting point (about 1,510 °C, higher than most metals; copper's, for example, is nearly 500 degrees cooler) and a very high density (7.9 g/cm³, almost eight times that of water).

Steel is harder and stronger overall than its so-called parent element, iron. Yet it is extremely flexible and known for its high tensile strength (i.e., its ability to withstand applied loads, or forces, without losing its shape).

The tensile strength of all types of steel is high compared to other materials but varies significantly between types of steel. At the low end, values are approximately 290 N/mm²; at the high end, tensile strength is as high as 870 N/mm².

• One square millimeter (mm²) is only one-millionth of a square meter. This means that steel can have a tensile strength of 870 million newtons per square meter — equal to a mass of 88.8 million kilograms, or 195.7 million pounds (97,831 tons), on Earth!

If you have ever used a cast-iron skillet, you may have noticed how remarkably sturdy (or at least heavy) it seemed. When iron is the sole or nearly sole component of something like a pan, it is more brittle than steel.

But for most everyday cooking temperatures (which seem "hot," but are nowhere near smelting-furnace-like), the functional difference between iron and steel may not be readily apparent, even if they usually look somewhat different.

Most of the steel produced today is simply called carbon steel, or plain carbon steel, even though it may contain metals besides iron and carbon, such as silicon and manganese.

The amount of steel variation may not look significant on the surface, because carbon never makes up more than 1.5 percent of steel. However, when you consider that this small fraction can itself range by a factor of 10 (0.15 percent to 1.5 percent), you begin to appreciate the physical impact this can have.

Steel can be divided into different categories using a number of criteria. Those used by scientists (who are often concerned more with the properties of things than with actually using them) are often different from those whose chief concern is the types of end products made from steel.

Mechanical: As noted, the tensile strength of steel can range between 290 N/m² and 870 N/m². Adding carbon to steel makes it harder because of the way the carbon atoms in effect disperse themselves among the iron atoms in a way that makes dislocations of material very difficult, forming "grains" of Fe₃C. This also makes steel more brittle than iron, so converting iron to steel, despite the manifest advantages of the latter, does not come at zero practical cost.

Steel that is classified on the basis of its mechanical properties starts with "Fe," and what follows is 1) E and the minimum yield stress value is the steel is classified mainly on this basis_, or 2) just the value of the tensile strength if this is the primary classification trait. (_Yield stress is a measure of resistance to mechanical deformation.)

• For example, "Fe 290" is steel with a tensile strength of 290 N/mm2. while "Fe E 220" is steel with a yield stress of 220 N/mm². 

Chemical: Plain carbon steels that vary from 0.06 percent carbon to 1.5 percent carbon are divided into the following types depending on their specific carbon content.

1. Dead mild steel — up to 0.15

   percent

   carbon 2. Low carbon or mild steel — 0.15

   percent

   to 0.45

   percent

   carbon 3. Medium carbon steel — 0.45

   percent

   to 0.8

   percent

   carbon 4. High carbon steel — 0.8

   percent

   to 1.5

   percent

   carbon  

Stainless steel is a type of steel that gets its name from its resistance to oxidation (rusting) as well as to corrosion, as that which might occur from the application of a strong acid. It was invented in 1913 by the British metallurgist Harry Brearley, who discovered that by adding the metal chromium to steel in high amounts (13 percent), the chromium would react with oxygen in air to form a self-renewing protective film around the object.

A number of types of stainless steel are in use today:

• Martensitic stainless steels contain 12 to 14

  percent

  chromium and 0.12 to 0.35

  percent

  carbon and were the first stainless steel developed. These steels are magnetic and can be hardened by treating them with heat. These are used in hydraulic pumps, steam pumps, oil pumps and valves, among other engineering equipment.  
  
  * Ferritic stainless steels have a greater amount of chromium (16 to 18

  percent) and about 0.12

  percent

  carbon. These steels are more corrosion-resistant than martensitic stainless steels, but have little capacity to be hardened with the use of heat. These stainless steels are used primarily in forming and pressing operations owing to their high resistance to corrosion.
  
  * Austenitic stainless steels contain a large amount of both chromium and nickel; many variations in precise chemical composition exist, but the most widely used consist of 18

  percent

  chromium and 8

  percent

  nickel, with carbon kept to a minimum. They resist corrosion very well at the cost of not being heat-treatable to any appreciable extent. These steels are used in pump shafts, frames, sheathing and everyday components such as screws, nuts and bolts.

You have already seen how alloys can make an already useful material better, or perhaps more to the point, more specialized. How does this process work at the molecular level?

Most pure metals, though many seem hard, are actually too soft by themselves to be used in heavy manufacturing. (One notable exception is the automotive industry, where steel is left mostly unalloyed and contains almost pure iron.) But mixing in other metals can produce outstanding results.

For example, nickel and chromium are corrosion-resistant and are known for their inclusion in surgical instruments made from stainless steel. If an alloy with a higher magnetic permeability is desired for use in steel magnets, cobalt is an excellent choice.

Manganese is used in larger-scale projects such as heavy-duty railway crossings owing to its considerable strength and hardness. Finally, molybdenum is able to maintain its strength at unusually high temperatures even by the standards of metals and is used in precision applications such as high-speed drill tips.

• When larger ions are added to the existing steel lattice, this disrupts the lattice in such a way that it makes it more difficult for adjoining "layers" to slide past each other, which increases the steel's hardness. Adding smaller atoms can have the same effect via a different form of mechanical disruption to the iron crystal lattice structure.

Among the many desirable properties of steel is that it is environmentally friendly. It may not always look that way with large steel structures dotting the skyscape in often unpleasing locations, but its great durability means that, for example, it will not degrade into something toxic and leach unseen into groundwater and other areas. Renewable energy sources (e.g., solar, wind and hydro power) make ample use of stainless steel.

• Steel is now the most-recycled material on Earth; although it is heavy, its magnetic properties make it easier to recover from streams and other places than other forms of waste. It can reduce CO₂ emissions.

Compared to other materials, steel requires a low amount of energy when constructing relatively lightweight steel elements, and it can be shaped into various forms. It gives better shape and edge than iron which is used to make weapons.

Steel, as noted, is used in the automobile industry. Think of the number of cars on the roads of just your own city during rush hour, all of them with bodies, doors, engines, suspensions and interiors consisting largely of steel.

• On average, 50 percent of a car is made of steel.

Apart from its role in passenger vehicles, steel is used in the production of farm vehicles and machines.

Most of the appliances in modern homes, such as refrigerators, televisions, sinks, ovens and so on are made of "plain" steel. Also, those with a yen for spending time in the kitchen are keenly aware of stainless steel's role in fine cutlery. Stainless steels notably lend themselves to the easy maintenance of a sterile environment, which is one of the qualities that makes it a good choice for surgical instruments and implants.

Because it lends itself to the easy formation of welds, steel, more than just making up the invisible framework of modern structures, has become featured in its own right in examples of contemporary architecture. So-called "mild" steel is used for everyday building construction, especially in areas where high winds are a feature of the local climate.

Steel itself is an alloy and by definition has no chemical or molecular formula, regardless of type. It is nevertheless useful to examine some of the important reactions that take place in the steel-making process.

The combustion of iron plus scrap steel, or in come cases scrap steel alone, involves a number of different reactions. Some of the important ones are:

2 C + O₂ → 2 CO
Si + O₂ → SiO₂
4P + 5 O₂ → 4 P₅O₂
2 Mn + O₂ → 2 MnO

The CO (carbon dioxide) is a waste product, but the rest is added to lime to continue the steel-making process by forming slag.