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Ask A Scientist

What is steel made of?

Asked by: Nick Lewis
School: Maine- Endwell Middle School
Grade: 6
Teacher: Mr. Wagstaff
Hobbies/Interests: Nick enjoys playing football and baseball.
Career Interest: A defensive end for the NFL's Philadelphia Eagles

Answer from Roy T.R. McGrann

Associate Professor at Binghamton University

McGrann’s area of research is engineering design education. He received his Ph.D from the University of Tulsa and has interests in computer-aided design, structural fatigue analysis, sustainable engineering, history, and philosophy of science and technology.  

I won’t comment on the Eagles: I’m a Steelers fan. By the way, the Steelers are so named because Pittsburgh was the main center of steel production in the United States in the late 19th and early 20th centuries.  Steel was first made in large quantities around 1865.  During World War II, 95 million tons of steel were produced in Pittsburgh. It was fairly inexpensive to transport the main raw materials for steel to the steel mills on the rivers and railways that converged in Pittsburgh. 

The three principal ingredients of steel are: iron ore, limestone, and coal.  Iron ore is mostly iron and rust and impurities, such as sulfur and phosphorous. Limestone is melted with the iron ore, “pulling’ the impurities out.  The coal is in the melt also, as a source of heat and it adds the ingredient that, mixed with iron, makes steel. This ingredient is carbon.

Steel is a metal alloy.  An alloy is a mixture of two or more elements, either metals or non-metals.  Steel is mostly iron, which is a metal.  The non-metal, carbon, is present in a very small amount in steel, usually less than 1%.  The process of making steel involves adjusting the carbon so that only this small amount of carbon remains.

The first step to making steel is to make a mixture of carbon and iron that is called “pig iron.” It can have up to 5% carbon.  This is made by creating a hot mixture (around 3000°F and similar to molten lava) of the iron ore, limestone, and coal.  This is done in a very tall furnace, about the size of a six-story building.  The impurities float to the top and the cast iron is drained out the bottom of the furnace.  When it cools, the resulting iron is very brittle, meaning that if you place a lot of weight on it so that it is pulled, it does not stretch before it breaks. Instead, it just comes apart all of a sudden.  This was a big problem when using cast iron to make structures such as bridges.  If fact, one of the reasons steel was developed was that cast iron railroad bridges used to collapse without warning, when trains passed over.  With steel, if it is going to break, you can see it bend out of shape a little bit after each train passes over and replace it before it gets too deformed.

To make the ductile alloy, steel, the next step is to refine the pig iron into steel.  There are several processes to do this: oxygen furnaces, electric arc furnaces, and electric induction furnaces.  All of these processes involve “cooking” the cast iron so that the carbon can be separated from the iron, combined with other elements, and be removed.

The chemistry of the steel can be controlled during this process by adding other ingredients. For example, if a lot of chromium and nickel are added, stainless steel can be produced.

The chemical ingredients and processing of steel can be controlled to give different functional properties.  Steel is also completely recyclable.  Often, scrap steel is added during the refining process. Steel is one of the most versatile materials that we have.  Go Steelers!

Last Updated: 9/18/13