The sharpness of a katana depends on various factors, including the quality of the steel used in its construction and the skill of the swordsmith. While different types of steel can be used to create katana blades, two traditional Japanese steels are often highly regarded for their sharpness:

1. Tamahagane: Tamahagane is a type of steel traditionally used in Japanese sword-making. It is produced through a labor-intensive process that involves smelting iron sand and charcoal in a tatara furnace. Tamahagane is known for its high carbon content and purity, which contributes to its excellent sharpness and edge retention.
2. Hitachi Yasuki Steel: Hitachi Yasuki Steel, specifically their Yasuki Hagane series, is another widely recognized steel used in Japanese sword-making. It is a modern variation of tamahagane, produced by the Hitachi Metals company. Yasuki Steel is renowned for its consistent quality, high carbon content, and exceptional sharpness.

Both Tamahagane and Hitachi Yasuki Steel offer excellent qualities for katana blades, including sharpness, durability, and the ability to hold a keen edge. However, it's important to note that the expertise of the swordsmith and their craftsmanship play a crucial role in shaping and heat treating the steel, ultimately determining the final sharpness and performance of the katana.