Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step. The models are simplified to show the idea; the numbers on screen come from the sources below.
Tap the buttons and slide the sliders.
Set the carbon, then push the layers of atoms. How much carbon stops them sliding, and when does it crack?
Carbon atoms sit in the gaps of the iron lattice and resist slip. Past about 2% carbon the metal is brittle cast iron.
Add a pinch of carbon, then push. Does it slide or stay put?
Drawing not to scale. Hardness figures: Britannica, slowly cooled steel.
Drop in loads of ore and coke. Where does the oxygen go, and what collects at the bottom?
Carbon and carbon monoxide strip oxygen from iron oxide; molten iron and slag separate at the bottom.
Tap the button to drop rock into the oven. Watch the hot iron drip!
Drag the blow timer. How fast does the carbon fall?
Pure oxygen burns carbon out as carbon monoxide gas over a roughly 20-minute blow.
Slide the timer and watch the carbon go away.
The blow curve is a rough illustration; the 4% start and ~20-minute length are real.
Mix carbon and chromium, then run the bend test and the rust test.
Vary carbon for strength and chromium (≥10.5%) for a self-healing passive film. Results are illustrations.
Pick a recipe. Does it bend? Does it rust?
Test results are illustrations, not measurements.
Tap the scrap pile to recycle a tonne. How much iron ore does it save?
Each tonne of scrap avoids about 1.4 t of iron ore and 1.5 t of CO₂ (worldsteel, a trade group).
Tap the button to melt old steel into new steel!
Tall buildings have bones made of steel. And steel starts as red, crumbly rock dug out of the ground!
Steel is mostly iron, with a tiny pinch of carbon. Iron alone is soft. The carbon gets stuck between the tiny bits of iron, so they can't slide. That makes it strong. But too much carbon makes it crack.
The red rock is iron stuck to oxygen. It's a lot like rust. A giant oven called a blast furnace pulls the oxygen away. Glowing, runny iron drips out!
That iron has too much carbon. So big furnaces blow in oxygen, and the extra carbon burns away in a shower of sparks. Now it's steel.
Old cars and cans can be melted and made into new steel, again and again. That means less digging for rock.
Most steel starts out as iron ore, a reddish rock full of iron stuck to oxygen. In a blast furnace, coke (a nearly pure form of carbon made from coal) pulls the oxygen off the ore, leaving liquid iron.3 That iron has soaked up about 4% carbon, which makes it brittle, so a basic oxygen furnace blows pure oxygen through it to burn most of the carbon away.4 The result is steel: iron with just a pinch of carbon. The other main way to make steel is to melt scrap steel in an electric arc furnace.13
So making steel is really about carbon: first you use carbon to get the oxygen out, and then you take most of that carbon back out again.
Steel is not an element. It's an alloy, a mixture of a metal with other elements. Steel is iron with a small amount of carbon, up to about 2%.1 Most steels have much less than that, often well under 1%.5 Many steels also get small amounts of other metals, like manganese, nickel or chromium, to give them special properties.1
| Material | Carbon | What it's like |
|---|---|---|
| Pure iron | almost none | Soft: in its pure form it's not much harder than copper1 |
| Low-carbon steel | about 0.25% | Strong and easy to shape, about twice as hard as pure iron (slowly cooled)1 |
| Higher-carbon steel | about 0.8–1% | Harder still, used for tools and springs1 |
| Cast iron / pig iron | 2–4.5% | Hard but brittle: it cracks instead of bending172 |
| Stainless steel | low, plus at least ~10.5% chromium | Resists rust619 |
Metals are made of tiny crystals, where atoms sit in neat, repeating layers.1 When you bend a piece of pure iron, layers of atoms can slip past each other, which is why pure iron is soft and easy to bend. Carbon atoms are much smaller than iron atoms, so they squeeze into the gaps between them. There they distort the layers and make it much harder for them to slip.121 Scientists describe this as blocking the movement of dislocations, tiny faults in the crystal that let layers slide.
According to Britannica, slowly cooled steel with 0.77% carbon is almost three times as hard as pure iron (a hardness of about 200 compared with 70 on one standard scale).1 But more isn't always better. Over about 2% carbon, iron becomes cast iron, which is brittle.17 Steelmakers can also change steel's hardness by heating it and cooling it quickly, which is called quenching, and then reheating it gently, which is called tempering.1
In the player, the atom drawing isn't to scale, and the "bend" and "rust" tests in the steel lab are simplified illustrations, not measurements.
Iron makes up about 5% of Earth's crust. But you almost never find it as shiny metal, because it's joined to oxygen in minerals called iron oxides.513 The most important are hematite and magnetite. Hematite's chemical formula, Fe2O3, is the same as dried rust.8 Pure hematite is about 70% iron, while real ores often contain 50–66%.5
Most iron ore is dug from huge open-pit mines, a lot of it in Australia and Brazil.13 In the United States, eight open-pit mines in Michigan, Minnesota and Utah shipped almost all the country's iron ore in 2024.10
The second source is scrap: old cars, cans, appliances and beams. In the US, electric arc furnaces, which mostly melt scrap, made about 72% of all raw steel in 2024.9 Worldwide, the trade group World Steel Association says about 70% of steel is made from ore through blast furnaces and oxygen furnaces, and about 30% in electric arc furnaces.13
Each tonne of pig iron takes about 1.6 tonnes of iron ore and about 450 kilograms of coke, according to the World Steel Association.13
People have made small amounts of steel for thousands of years. Early smiths heated iron ore in charcoal fires and hammered the spongy iron into shape. Steel was probably first made without anyone planning it, when extra carbon from the charcoal got into the iron.14 But making steel stayed slow and expensive for centuries.
That changed in 1856, when the English inventor Henry Bessemer patented a way of blowing air through molten pig iron.5 The oxygen in the air burned out carbon and other impurities, and those reactions made the metal even hotter. Flames and sparks shot out of the converter "like a volcano".14 A Bessemer converter could make 30 tons of steel in about half an hour.16 Bessemer wasn't the only one with the idea: the American William Kelly had a similar air-blowing process, and the metallurgist Robert Mushet worked out that you should blow the carbon out and then add back just the right amount.14
Cheap steel quickly replaced iron in railways, bridges and buildings. Steel frames made the first skyscrapers possible.14 Bessemer himself had seen that pure oxygen would work even better than air, but that only became practical in the 20th century, once cheap oxygen was available. That's the basic oxygen furnace used today.4
Ordinary steel rusts because iron reacts with oxygen and water and turns back into iron oxide, the same kind of stuff it was mined as.8 Rust is porous and flaky, so it doesn't protect the metal underneath, and the rusting keeps going.22
Stainless steel contains at least about 10.5% chromium (usually 10–30%).619 The chromium reacts with oxygen in the air to form an extremely thin, invisible layer of chromium oxide. This "passive film" seals the surface, and if it's scratched it re-forms in the air.197 The most common kind, called 304 or 18/8, has 18% chromium and 8% nickel.624
Steel can be melted and made into new steel over and over. Even the trade group World Steel Association says it can be recycled forever without losing its properties.15 Every steel plant uses some scrap: up to 100% in electric arc furnaces and up to about 30% in the blast furnace route.15 In the US, the overall scrap recycling rate has been 80–90% for the past decade. Nearly all old cars are recycled, and about 98% of structural steel from buildings.11 Recycling one ton of steel saves about 1.1 tons of iron ore, according to the USGS.11 The World Steel Association puts it at 1.4 tonnes of ore and 1.5 tonnes of CO2 saved for every tonne of scrap used.15 (In the player's recycling toy, the running totals use these trade-group figures, so treat them as rough.)
Scrap can't supply all the steel we need yet, though. The world makes far more steel now than it did decades ago, when today's scrap was new.12 Want to know more about recycling in general? See why recycling is important.
Making steel from ore uses a lot of coal. Coal supplies about 75% of the steel industry's energy and raw carbon.23 The International Energy Agency says iron and steel directly produce about 2.6 billion tonnes of CO2 a year, around 7% of all CO2 from the world's energy system.12 Britannica puts steelmaking at about 8% of global greenhouse-gas emissions, more than any other industry.3 (The two numbers differ because they measure slightly different things.)
Engineers are working on cleaner ways to make steel. One is to use hydrogen instead of coke to pull oxygen off the ore, which leaves water instead of CO2. The iron is then melted with electricity.1523 Using more scrap and more low-carbon electricity also helps.12 These methods are still being scaled up, and the IEA says progress needs to speed up a lot.23 Coal itself formed from ancient plants: see how coal is formed.
Most steel is magnetic because it's mostly iron. But some stainless steels, including the common 18/8 kind, are not magnetic, because of how their atoms are arranged.6 Find out why in how magnets work. And for another material made by melting rock, see how glass is made.
Iron from a blast furnace, called pig iron, contains about 4% carbon, which makes it brittle. Steelmakers blow pure oxygen through the molten iron, and the oxygen burns most of that carbon away as gas. When the carbon is down to the right amount, usually well under 1%, the iron has become steel.
Steel comes from two places: iron ore dug out of the ground, and old scrap steel. Iron ore is rock full of iron oxide, mined mostly in big open-pit mines. Around the world about 70% of steel is made from ore in blast furnaces and oxygen furnaces, and about 30% in electric arc furnaces that mainly melt scrap.
Steel is iron with a small, carefully controlled amount of carbon, up to about 2%. Pure iron is fairly soft. The carbon atoms squeeze between the iron atoms and stop layers of atoms from sliding, which makes steel much stronger and harder. With more than about 2% carbon, it becomes brittle cast iron.
Stainless steel is iron with at least about 10.5% chromium and low carbon. The most common kind, called 304 or 18/8, has 18% chromium and 8% nickel. The chromium forms an invisible oxide skin on the surface that keeps out air and water and repairs itself if scratched.
Steel itself can be recycled over and over without losing its strength, and iron is about 5% of Earth's crust. The US Geological Survey estimates world iron ore resources at more than 800 billion tons. Scrap alone can't meet all demand yet, because the world uses more steel today than it made decades ago.
Nature almost never makes steel. On Earth, iron is nearly always locked up with oxygen in minerals like hematite, so it has to be smelted. The first iron people used came from meteorites, which contain metallic iron and nickel. Early smiths likely made steel without planning to, when charcoal fires added carbon to iron.
Yes, the US makes most of the steel it uses. In 2024 it produced about 81 million metric tons of raw steel and imported roughly 13% of what it used, according to the USGS. About 72% of US steel is made in electric arc furnaces that melt mostly scrap.