Steel is made by removing impurities from iron, primarily by controlling its carbon content. Iron straight from a blast furnace contains around 4% carbon, making it brittle. Reducing that carbon to below 2%, and typically well below 1%, transforms it into steel: stronger, more flexible, and far more useful. The methods for achieving this have evolved dramatically over the past 170 years, from blasting air through molten metal to injecting pure oxygen in processes that take less than an hour.
Why Carbon Content Matters
The difference between iron and steel comes down to carbon. Cast iron contains 2.1% carbon or more, which makes it hard but prone to shattering. Most steel contains less than 0.35% carbon, though specialty steels can go up to about 2%. Small amounts of other elements like manganese, silicon, and phosphorus are also present, but carbon is the defining ingredient. The entire history of steelmaking is essentially the story of finding faster, cheaper, and more precise ways to pull carbon and other impurities out of molten iron.
The Bessemer Process: Steel Goes Industrial
Before the 1850s, making steel was slow and expensive, limited to small batches for swords, springs, and tools. The Bessemer process, developed by Henry Bessemer in 1856, changed everything. The idea was deceptively simple: force air upward through molten pig iron inside a large pear-shaped container called a converter. As air passed through the liquid metal, oxygen reacted with impurities like silicon, manganese, and carbon, burning them off. The process was violent and spectacular, with flames and sparks shooting from the top of the converter, but it could turn several tons of pig iron into steel in about 20 minutes.
This was revolutionary. For the first time, steel could be produced cheaply and in large quantities, making it practical for railroad tracks, bridges, and building frames. The Bessemer process had limitations, though. It couldn’t remove phosphorus, a common impurity in European iron ore that made steel brittle. And because it relied on air (which is about 78% nitrogen), the finished steel sometimes absorbed nitrogen, which also weakened it. These shortcomings drove the development of better methods.
The Open Hearth Furnace
The open hearth process, also called the Siemens-Martin process, emerged in the 1860s and gradually overtook the Bessemer converter. Instead of blowing air through the metal from below, the open hearth used a wide, shallow furnace where molten iron sat in a pool and was heated from above by burning gas. The process was much slower, taking 8 to 12 hours per batch, but that slowness was actually an advantage. Steelmakers could test the metal during production and adjust the chemistry as needed, giving them far more control over the final product.
The open hearth also had a major practical benefit: it could melt and recycle scrap steel alongside fresh iron. Charges typically contained 60 to 65% liquid iron, with the rest made up of scrap metal. This made it economical for steelmakers who had growing supplies of old steel to reuse. The open hearth dominated global steel production for nearly a century, from the late 1800s through the 1950s.
Basic Oxygen Steelmaking: The Modern Standard
The process that replaced the open hearth, and still produces the majority of the world’s steel today, is basic oxygen steelmaking (BOS), developed in the late 1940s and early 1950s in Austria. It combines the speed of the Bessemer process with far better chemistry. Instead of blowing regular air through molten iron, BOS uses a lance to inject a stream of nearly pure oxygen into the metal from above.
Pure oxygen reacts with carbon in the iron to produce carbon dioxide and carbon monoxide gas, which escape from the furnace. It also oxidizes silicon and phosphorus. Steelmakers add limestone, which breaks down into calcium oxide during the heat. That calcium oxide bonds with the oxidized impurities to form a layer of slag that floats on top of the molten steel and can be skimmed off. The phosphorus, for example, reacts with oxygen and then combines with calcium oxide to form a stable compound that gets trapped in the slag.
A BOS converter can process a batch of around 300 tons of steel in 30 to 45 minutes. The raw materials are molten iron from a blast furnace plus some scrap steel. This speed and scale made the open hearth obsolete by the 1990s in most countries. The BOS route, starting from iron ore in a blast furnace, produces roughly 1.7 to 2.1 tons of CO2 per ton of crude steel, making it a major contributor to industrial carbon emissions worldwide.
Electric Arc Furnaces and Recycled Steel
The other major steelmaking route skips the blast furnace entirely. Electric arc furnaces (EAFs) use powerful electrodes to create arcs of electricity that generate intense heat, melting steel scrap directly. The primary raw material is recycled steel, though direct-reduced iron or even some blast furnace iron can be added when economically practical.
EAF steelmaking is energy-intensive, requiring an average of about 1.5 million BTUs of electrical energy per short ton. However, with modern techniques that optimize the process, electricity consumption can drop to around 360 kilowatt-hours per ton. Because EAFs melt existing steel rather than reducing iron ore with coal, their carbon footprint depends heavily on how the electricity is generated. In regions with clean power grids, EAF steel has a significantly lower environmental impact than the blast furnace route.
Today, EAFs account for roughly 30% of global steel production, with higher shares in countries like the United States where scrap supply is abundant. The split between the BOS and EAF routes largely reflects whether a country has more access to iron ore or to scrap metal.
Turning Steel Into Specialty Alloys
Once crude steel is produced through either route, it can be further refined by adding other elements. The most familiar example is stainless steel, which gets its corrosion resistance from chromium. Adding enough chromium creates a thin, invisible oxide layer on the surface that prevents rust. Nickel, typically added at 8 to 10%, changes the internal crystal structure of the steel from its usual arrangement to a different configuration that makes the metal more formable, weldable, and resistant to extreme temperatures. This combination of chromium and nickel is what makes stainless steel versatile enough for everything from kitchen sinks to surgical instruments.
Other alloying elements serve different purposes. Manganese increases strength and wear resistance. Vanadium and molybdenum improve performance at high temperatures. The base steelmaking process, whether BOS or EAF, produces the foundation. Alloying transforms that foundation into thousands of specialized grades, each tuned for a specific job.