Ultrahard steel

Chō Kōshitsu Suchīru (超硬質スチール) is a material that is used in the making of blades capable of cutting and piercing the skin of Titans.

Uses
Swords made of ultrahard steel are the main weapon used in the fight against the Titans. These one edge blades are included as part of the vertical maneuvering equipment, which is the only known usage for the hardened steel. They are the only portable weapons that are strong enough to pierce the body of a Titan; Cannons are able to wound a Titan's body, but they are very slow and inaccurate. Firearms can destroy the Titan's eyes, but it is very hard for them to pierce a Titan's skin effectively enough to kill it. Therefore, blades are used two at a time to cut the nape of the Titans neck with accuracy. They can also be used to cut or pierce almost any other part of a Titan's body such as tendons, fingers, or eyes. They are rather lightweight so that soldiers are able to carry several blades at a time without being seriously weighed down or affecting their movements.

However, they do have some limitations. After continued usage, the blades wear down and lose their sharp edge, which may result in the blades breaking after too much usage. Also, since a soldier has to "amputate" the nape of the Titan's neck, a single blade is difficult to use before being grabbed or having another matter interfere, so two blades are needed to kill a Titan effectively with less risk. The blades may break if they are used against something extremely hard, such as the hardened skin of the Female Titan or the  Armored Titan. For that reason, soldiers wear many blades with their vertical equipment so that they are able to replace them easily whenever necessary. Levi Ackerman has on occasion thrown these blades into Titans from longer ranges.

Creation
The steel is created in the factories of production cities, where it is melted down and forged into the metal needed for the blades. The metal cannot be produced in any other place.

The blades were created by Angel Aaltonen, a first-class weapons craftsman who collaborated with the Survey Corps 50 years before the fall of Wall Maria. He was also the creator of the vertical maneuvering equipment.

Trivia

 * Although the blades are meant to be used against Titans, they are an effective weapon against humans, too. Levi Ackerman used these blades to kill a person as he evaded Kenny Ackerman. The momentum that the vertical equipment gives to the user may assist in the strike of the blade.
 * Given that the swords are referred to as "flesh paring" blades, this could imply the swords have a low angle at the edge, roughly 15-20 degrees.
 * Due to the blades shattering upon impact with hardened substances such as hardened Titans, or when Mikasa's blades chipped after killing an Abnormal Titan, this could imply that the blades are made of a very high carbon steel or a high chromium content stainless steel. Adding too much carbon to a blade can make them very hard, but very brittle (meaning unable to bend and cope with higher, structural stress), and chromium also makes the steel very hard and rust-resistant, but very brittle, as well.
 * Although this is not the correct purpose, the blades can be launched like a giant tomahawk. When launched, they are strong enough to kill a person, or blind both the eyes of a Titan.
 * The blades seem to have segments on it reminiscent of a box-cutter knife blade.