Katana-makers use two types of tamahagane ... After the smith hammers all slag from the tamahagane, he heats the hard, high-carbon steel and shapes it into a long, U-shaped channel.

made up of Hard Carbon steel, later on after restoration the overall length including hilt is ≈ 76 cm (30 in). This was a perfectly curved cold steel. By the way this katana was too much rusty ...