high carbon steel

A type of steel that has enough carbon in it to make it very tough, yet not so tough that it becomes brittle and inflexible. Sword blades made out of high-carbon steel tend to keep an edge well, but be flexible enough that you can bend them over 3" out of line and they will flex back true - if they are tempered properly. Stainless either won't bend or will bend and stay bent, for the most part. Unfortunately, carbon steel rusts easily and you have to keep it oiled. Gun oil works well for this. Note that the temper of a carbonized steel blade will determine how flexible it is - an improperly tempered carbon steel blade will be hard but brittle, and shatter easily.

It is also called spring steel. You can often find narrow pieces of it lying in the gutter because it is used to make the bristles for the brushes on street sweeper trucks.

In order for it to perform in a springy manner, it must usually be heat treated properly in order to minimize the number of large ferrite crystals and to encourage the formation of fine-grained pearlite.

Unlike alloys with less carbon, when machined or struck, it produces a lot of sparks. A common mistake made by people that are trying to start a fire with flint and steel is that they are not using a steel with a high enough carbon content.

It is used to make some, but not all, steel cables, and most rebar. Rebar may not be springy because its low cost does not justify much heat treatment.

Oh yeah, and its also pretty good for making springs.