Ultrahard steel

Super hardened steel is a material strong enough to cut and pierce the skin and the hard mass of muscles of the Titans.

Uses
Swords made of super hardened steel are the main weapon in the fight against the titans. These one edge blades are included as part of the 3DMG, which is the only known usage for the hardened steel. They are the only man-portable weapons that are strong enough to pierce the body of a titan; canons can pierce a Titan's body but they are very slow and inaccurate and firearms can destroy titan's eyes, but it is unknown if they can pierce effectively a Titan's muscles. The blades are used two at a time to cut with accuracy the nape of the titans but they can also be used to cut or pierce almost any part of a titan's body like tendons, fingers of even the eyes. They are quite lightweight because even when the soldiers where a lot of blades with them, their weight does not affect the movement of the 3DMG.

However it has some limitations. After continue usage, the blades weak down and lose their sharp edge, which may even end up breaking after using them too much. Also since the soldier has to "amputate" the nape of the titan, a single blade is practically useless so two of this blades are need to kill effectively, but a single blade still can be used in some situations. Also the blades may break if they cut something extremely hard, like the hardened skin of the Female Titan or the Armored Titan. That is the reason why soldiers wear a lot of blades with their 3DMG so that they can replace them easily.

Creation
The steel is created in the factories inside the walls, where the giant and powerful industrial havens melt the stone and create the metal needed for the blades. The metal cannot be produced in any other place.

The blades were created by Angel Aaltonen, a weapon smith which collaborated with the Survey Corps 50 years before the fall of Wall María. He also was the creator of the 3DMG

Trivia

 * Althought the blades are meant to be used agains the titans, they are so strong and light that they can be an effective weapon against humans too. Levi used this blades to cut a person in half but it may also be helped by the momentum speed that the 3DMG gives to the user.