How is steel made strong?

Steel gets strong when iron is mixed with the right small amount of carbon, plus sometimes other elements, so its inner crystal layers cannot slide easily. Heat and cooling can also lock those crystals into tougher arrangements, making iron much harder to bend or dent.

The neat part is that strength is tuned, not guessed. A bridge, a knife, and a train rail do not need the same steel. Steelmakers change carbon, add metals such as chromium or nickel, and use heat treatment to fit the job.