Steel is a kind of metal alloy that's made of iron and carbon. It's known for being incredibly strong and is often used to build the framework of skyscrapers. It's also helpful to have "nerves of steel."
Steel isn't just a kind of metal. A steel can also be the long, rough thing you sharpen knives on. Or if you're one of the Three Musketeers, you probably carry a steel, or a sword. People who are really brave have nerves of steel. And if you want to use steel as a verb, go right ahead: if there's something hard or unpleasant ahead, you'll need to steel yourself to get ready for it.