An introduction to the production and properties of steel alloys
According to the World Steel Association, 1869.9 million tons of steel were produced in 2019. This represents a 3.4% increase in output from 2018 and is more than double the output in 1999. The world has an ever growing need for steel. It’s used in construction, industry, and manufacturing. Being both strong and inexpensive, it is ideal for all types of manufacture.
What is steel made of?
Iron, the major elemental component of steel, is one of the most plentiful elements in the earth’s crust. All steel alloys are primarily iron and 0.002–2.1 % carbon by weight. In this range, carbon bonds with iron to create a strong molecular structure. The resulting lattice microstructure helps achieve certain material properties, like tensile strength and hardness, that we rely on in steel.
Although all steel is made of iron and carbon, different types of steel contain different percentages of each element. Steel can also include other elements like nickel, molybdenum, manganese, titanium, boron, cobalt, or vanadium. Adding different elements to the “recipe” for a steel alloy affects its material properties. The method of manufacture and treatment of the steel further enhances those abilities.
One notable group of steel alloys contain chromium. All such alloys are known commonly as stainless steel.
How to make steel
At the most basic, steel is made by mixing carbon and iron at very high temperatures (above 2600°F).
Primary steelmaking creates steel from a product called “pig iron.” Pig iron is smelted iron, from ore, which contains more carbon than is correct for steel.
The steelmaker uses a system that bubbles oxygen through melting pig iron. This process creates equal oxidization throughout the molten metal. Oxidization removes excess carbon. It also vaporizes or binds impurities made of elements like silicon, phosphorus, and manganese.
Secondary steelmaking is done “in the ladle.” It is a process of refining and alloying steel. Secondary steelmaking can start by melting scrap, or continues a primary process. Elements can be added to get a specific alloy. The steelworker can also remove surface impurities (de-slagging). The ladle is heated and cooled to the temperatures needed for the necessary chemical processes.
In a foundry, steel is sand or investment cast into patterned shapes. In a steel mill, steel is cast by a continuous caster into raw building materials. Continuous casters create standardized raw steel shapes rather than near-finished parts. Raw steel will be machined or worked into final products. Steel mills commonly cast and form sheets, billets, bars, blooms, pipes, ingots, and wires.
A mill may also hot-roll or cold-roll steel during production. These processes create different shapes and finishes. Before shipment, the steel might be cut, spooled, or bundled before leaving the mill.
In foundry or mill, steel may be heat treated. Final steps like quenching, tempering, normalizing and annealing can shape the way the alloy behaves in an application.
Invention of steel
Archeologists have found the earliest steel at site dating from 4,000 years ago, in Turkey. Crucible steels, like the famous South Indian Wootz steel, were made consistently as early as the 4th century BC. However, until the mid-1800s, steelmaking was incredibly challenging.
Steel melts around 2700°F. Maintaining this high-heat was a challenge for ancient furnaces making crucible steel. Further, impurities are found in steel alloys, made of elements like silicon and manganese. Managing these still present a challenge. In ancient steelmaking, they made for long, multistep process. Founders would spend a long day heating, stirring, de-slagging, and re-heating their alloys. After the steel was cast, it then went to be worked by smiths. Being pounded on the anvil created final shapes. It also helped distribute and mitigate carbon variance, pores, or inclusions.
In 1856, Henry Bessemer took out a patent for a process to create steel in a new way. Using a Bessemer converter, rather than traditional melt vessels, allowed the steelmaker to bubble air through the molten metal. In reaction to the air, impurities would oxidize and off-gas. Oxidization also helped create and sustain the high heat necessary for steelmaking.
A process that once a full day in the foundry and more time in the forge was replaced with a 20-minute process that could create 5 tons of steel. Bessemer’s steel was also stronger, and higher quality, than most steelmakers could hope for. This innovation supported the Industrial Revolution.
Is steel magnetic?
Most steel is magnetic, but not all. Steel is mostly made from iron, and iron is magnetic. Ferromagnetism was first discovered in nature in “lodestones”—stones made of magnetite, an oxide of iron. Other elements are also ferromagnetic, like cobalt and nickel. These elements are also sometimes found in steel.
Stainless steels are famously non-magnetic, though all stainless steels contain iron and many contain nickel. Truthfully, only some stainless alloys are non-magnetic. Austenitic stainless steel—which contains nickel—is non-magnetic in most cases (although it can become very mildly magnetic when worked.) Other types, like ferritic or martensitic alloys, are stainless and magnetic.
Properties of steel
Steel is so commonly used because of its specific material properties combined with its relative low cost. Compared to other many other building and tool making materials (like wood, stone, concrete, or cast iron), alloys of steel offer:
- Hardness: resistance to indentation when pressed with gradually increasing pressure
- Toughness: when the material does deform, toughness describes how far it goes before fracturing
- Yield strength: resistance to changing shape while being pulled with gradually increasing pressure
- Tensile strength: a material’s ability to withstand being pulled before breaking
- Malleability: the ability to be shaped by hammering or pressing without breaking
- Ductility: the ability to be shaped without losing toughness—working metal often makes it more brittle, but ductile materials don’t embrittle through work as quickly.
The tested range of these properties varies between alloys, but as a whole, steel manages to be both harder and tougher (less brittle) than many other materials.
Types of steel
There are four major groupings of steel alloys: carbon, tool, alloy, and stainless steels.
- Carbon Steel—Mild, medium, and high carbon steels vary mostly by hardness and ductility. Mild or low carbon steels tend to be more ductile compared to other steels, but also offer lower hardness. On the other end of the range, high carbon steels are harder. However, high carbon steel usually has lower ductility.
- Tool Steel—High carbon steel with added elements like tungsten, vanadium, or molybdenum, heat treated and quenched to superior hardness, are used for tool steels.
- Alloy Steel—This family of steels generally refers to steels mixed with specific elements for extraordinary material properties, outside of those that commonly fall in other families. All steels are alloys and many have extra elements. However, alloy steels are unusual steels built for a specific application, and can range from value formulations to exotic alloys used for jet engines.
- Stainless Steel—These steels are alloyed with chromium to make them rust resistant through passivation.
Steel production: a story of recycling
One of the best features of steel (and other metals) is that scrap can become completely new, high-quality metal. The process of secondary steelmaking creates alloys as good as any that come from pig iron. Metal items can degrade from use, but the elemental chemistry of metal means that melting and alloying creates a completely new product.
Growth in steelmaking output therefore doesn’t require matching growth in smelting of new ore (although pig iron production remains a vital part of the steel supply chain.) Reclaiming and processing scrap steel means that yesterday’s car panel can be tomorrow’s I-beam.
With 98% of steel reclaimable, the metal is one of the world’s most recyclable products. Still, it is not without environmental challenge. Coke, a form of coal, is usually used as the carbon input to steelmaking. Additionally, the high energy required to melt or smelt and oxidization and other processes of production do create chemicals and carbon dioxide. Fortunately, there is a lot of research being done in the steelmaking sector to mitigate issues with production. Some include recycling carbon dioxide back into the steel itself, as the source of carbon, lowering need for other sources like coke.
With these technologies refined and implemented, steelmaking will continue to be one of the major industries of the future. It undergirds, drives, and builds our economy.