Traditionally, the steel used in the crafting of katana swords is a type of high-carbon steel known as tamahagane. Tamahagane is produced through a traditional Japanese process of smelting iron sand (satetsu) with charcoal (sumi) in a tatara furnace. This process results in a material that is rich in carbon content and has a varying composition of other elements.

Tamahagane steel is characterized by its unique grain structure and properties. It consists of a combination of hard, high-carbon steel called "hagane" and softer, low-carbon steel called "kawagane." The differential hardening process, known as "yakiba," is applied during the sword's forging and heat treatment, resulting in a hardened cutting edge and a softer, more flexible spine (mune).

However, it's important to note that traditional tamahagane steel production is a labor-intensive and time-consuming process. In modern times, other types of steel are often used in the production of katana swords. These steels can include various types of high-carbon steels or tool steels that offer comparable properties and performance.

It's worth mentioning that the specific type and quality of steel used in a katana can vary depending on the skill and preferences of the swordsmith, as well as the intended purpose of the sword. A well-crafted katana requires careful selection of steel and meticulous forging techniques to achieve the desired balance of strength, sharpness, flexibility, and durability.